Oxidative, reductive, infrared and catalytic studies ofsupported rhodium catalystsCitation for published version (APA):Vis, J. C. (1984). Oxidative, reductive, infrared and catalytic studies of supported rhodium catalysts. TechnischeHogeschool Eindhoven. https://doi.org/10.6100/IR114107

DOI:10.6100/IR114107

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https://doi.org/10.6100/IR114107https://doi.org/10.6100/IR114107https://research.tue.nl/en/publications/oxidative-reductive-infrared-and-catalytic-studies-of-supported-rhodium-catalysts(8f684b51-ac04-4cb6-92cf-fe586f382f97).html

"It unfortunately often occurs that names are

mistaken for explanations, and people deceive

themselves with the believe that, for instance,in

attributing chemical decompositions to affinity,

attraction, contact-force, catalysis etc., they

explain them."

A.W. Williamson, 1851.

Dit proefschrift is goedgekeurd door de promotoren

Prof.dr. R. Prins

en

Prof.dr. V. Ponec.

OXIDATIVE, REDUCTIVE, INFRARED AND

CATALYTIC STUDIES OF

SUPPORTED RHODIUM CATALYSTS

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL EINDHOVEN, OP GEZAG VAN DE RECTOR ~~GNIFICUS, PROF.DR. S.T.M. ACKERMANS, VOOR EEN COf'.1MISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE VERDEDIGEN OP

VRIJDAG 24 FEBRUARI 1984, TE 14.00 UUR

DOOR

JAN CORNELIS VIS

GEBOREN TE OUD VOSSEMEER

Contents.

page

1.

1.1. Future needs of chemical industry 1

1.2. The importance of metal catalysts 2

1.3. Oxidation and reduction properties of supported

metals

1.4. CO adsorption and I.~.-spectroscopy

1.5. Synthesis gas reactions and their mechanism

1.6. References

2. Reduction and oxidation of Rh/Y-Al 2 g 3 ~~~~·~-~~ catalysts as studied by Temperature Programmed

Reduction and Oxidation

2.1. Introduetion

2.2. Experimental

2.3. Results

2.3.1. Hydragen chemisorption

2.3.2. TPR and TPO of Rh/Y-Al 20 3 2.3.3. TPR and TPO of Rh/Ti0 2

2.4. Discussion

2.5. Conclusions

2.6. References

3. The and

Y-Al 223 as studied with Temperature Programmed Reduction-Oxidation and Transmission Electron

l"licroscopy

3.1. Introduetion

3.2. Experimental

3.3. Results

3. 3 .1. Hydragen

3.3.2. TPR/TPO

3.3.3. TPR/TPO

chemisorption

of the RA-series

of the RT-series

3.3.4. TEM measurements

3.4. Discussion

3.5. Conclusions

3.6. References

4

4

9

12

15

17

20

23

25

26

31

32

36

39

43

45

47

53

57

60

61

4.

and Ti0 2 catalysts at elevated pressures

4.1. Introduetion

4. 2.

4.3. Results

4. 3 .1. Rh/La 2o 3 4.3.2. Rhjy-Al 2o3 4.3.3. Rh/Ti0 2

4.4. Discussion

4.5. References

5. an in-situ infrared cell and

measurements

5.1. Introduetion

page

66

68

73

79

82

83

85

5.1.1. General introduetion 88

5.1.2. of a new in-situ I.R.-cell 90

5.1.3. CO on Rh, as studied with I.R.-spectroscopy 91

5.2. Experimental 92

5.3. Results and discussion 93

5.4. Conclusions 101

5.5. References 103

6.

6 .1. Introduetion 106

6. 2. Experimental 109

6. 3. Results

6. 3.1. The structure of rhodium af ter reduction 114

6.3.2. The structure of rhodium af ter co admission 121 6.4. Discussion

6.4.1. The structure of rhodium af ter reduction 127

6.4.2. The structure of rhodium af ter co admission 130 6.5. Conclusions 133

6.6. References 134

General Discussion

Surrunary

Acknowledgements

Samenvatting

Dankwoord

Curriculum Vitae

List of publications

page

140

144

147

148

151

153

154

Chapter 1 page

Chapter 1. lntrod u ct ion

1.1. Future needs of chemical industry.

The role of chemistry in nowaday society is more

than most people think or wish. The chemical

industry might be responsible for a major part of

environrnental pollution, it also answers to numerous

material needs of mankïnd. And last but not least, if

solutions have to be found for pollution problems, it will have to be chemistry

To a great extent this depends on crude oil as

a raw material and energy carrier. Through various

processes the crude oil is separated in various fractjons

of hydrocarbons, which are thereafter mainly used as

fuels. The very light fractions (containing 1, 2 or 3

carbon atoms and therefore denoted as Cl, C2 and C3

fractions) serve as raw material for many processes,

producing alcohols, aldehydes, polymers and so on.

The of chemical industry upon society, and the

vulnerability of this industry through its dependenee on

crude oil became very clear during the oil crisis of

1973. Since that time the of crude kept

Only a couple of years ago crude oil prices were

predicted to reach $ 100 per barrel in 1990 (1973 $ 2.60). The chemical industry was forced to meet this

challenge by returning to a cheaper and more abundant raw

material: Coal. Coal can be to give a mixture

of hydrocarbons resembling the heavy oil fractions. And

with known refining and processes they can be

converted to useful bulk raw materials and fuels. 'Coal

can also be ~asified. It can react with steam under

certain conditions and with certain catalysts to yiel.d a

mixture of carbon monoxide and hydrogen, which is in a 1

to 1 mixture known as synthesis gas {syngas). Syngas can

be seen as the new raw material that would take the place

1

Chapter 1 page

of crude oil in a foreseeable future, and the chemistry

involving the conversion of syngas to bulk chemieals

forms, with some other reactions, the socalled c1 chemistry.

Current projections predict however that the price of

crude oil will reach only $ 50 in 1990 (current price

about $ 30), half of the earlier level. This has narrowed the economie advantages presented by c1 chemistry for the

and nineties. In potential it is still

promising, but a more cornprehensive analysis of economie

and technical factors is required to decide whether

c1 chemistry projects will pay off in the near term or only later. This thesis deals with some of the

chemical fundamentals of the conversion of syngas, and of

some of the catalysts involved in it.

1.2. The irnportance of roetal catalysts.

Numerous processes in the manufacturing of crude oil

involve roetal catalysts, because they are able to

dissociate hydrogen and to transfer it to reacting

molecules. Metal catalysts contain mostly noble or

transition metals such as platinum (Pt), palladium (Pd),

cobalt (Co), rhodium (Rh) or nickel (Ni). The conversion

of syngas is an excellent example of a reaction proceding

with metallic elements. In fact this conversion proceeds

with all the metals mentioned above and some ethersas

well, iron being the best known example from the Fischer-

Tropsch process.

An important item in syngas conversion is whether the

oxygen atom from carbon monoxide is retained in the

molecule, leading to products such as methanol, glycol,

acetic anhydride etc., or removed in the form of water or

carbon dioxide, leading to hydrocarbons. Rhodium then

takes a rather special place arnong the catalytic active

2

Chapter 1 page

metals, since it seems to be able to catalyse processes

in either direction. Recently, interest in rhodium has

increased further because it is an irreplaceable compound

in the three-way catalyst, applied in the USA for

cleaning exhaust gases of automobiles. Plans exist to

introduce this three-way catalyst on cars in Europe too,

and this will undoubtedly promate the research on rhodium

catalysts very much. However, this thesis is mainly

directed towards studies of rhodium catalysts in relation

with cl chemistry.

As mentioned above,

place on the roetal

many catalytic conversions take

surface. So it is of interest to

create as large a surface- to volume ratio as possible

for rhodium, especially in view of the availibility and

price of Rh. Rh as RhCl3.xHi} casts about f 40/g; as a

comparison, the price of gold is f 37/g. The ordinary way

to create this Rh surface is to bring the roetal onto a

high surface area support, such as silica-alumina's,

alumina, silica, etc. These kinds of supports are very

porous, so we can apply the roetal as a salt in an aqueous

solution, which is suck up into the pores by capillary

farces. Once we have established the pare-volume of a

certain support, we can dissolve the wanted amount of

roetal salt in that particular amount of water and add it

to the support. It can be assumed that after careful

drying the roetal salt is present within the pores of the

support.

Now we have to bring the catalyst into an active state.

This is in certain cases the metallic state, which means

we have to reduce the catalyst. In some cases it may be

necessary to heat or to oxidize the catalysts first

(calcination), in order to remave for instanee ligands

from the roetal salt. This means that the oxidation-

reduction behaviour of the catalytically active roetal is

of vital interest.

3

Chapter 1 page

1.3. Oxidation and reduction properties of supported

metals.

An excellent way of Temperature Programmed

TPR/TPO experiments a

studying these processes is

Reduction and Oxidation. In our

certain amount of catalyst is

placed in a horizontal, cylindrical quartz reactor,, held

in an electrical furnace. A mixture of Hz/Ar or o2/He can flow through the catalyst bed, while the temperature of

the oven can be varied continuously by a temperature

controller. The ingoing gasstream (of known composition)

is being compared with the outgoing gasstream. If there

is a difference in composition (which means consumption

of one of the components in the reactor), this leads to

an electrical signal from the detector comparing the

gasses, and this signal, integrated over time, is

proportional to the amount of gas consumed. This signal

is also recorded, and this recording is the so-called

TPR- or TPü-profile the reader will encounter many times

in the Chapters to come. A schematic representation of

the TPR/TPO apparatus is given in Chapter 2. TPR and TPO

experiments are the main subject of Chapters 2 and 3.

1.4. CO adsorption and I.R.-spectroscopy.

In hydracarbon and oxygenate synthesis from CO and H2

the bonding between CO and the metal catalyst plays an

important role. We will give a short description of this

process, following the one given by Coulson (1).

In the CO molecule there are ten valenee electrons,

occupying the following molecular orbitals in order of

increasing energy: 3v, 4v, lpiy, lpiz and Sa. The bonding

in the molecule is provided for by the 3a and the two lpi

orbitals. Two electrons occupy the non-bonding 4a

orbital, made from the oxygen 2s and 2px orbitals and

4

Chapter 1 page

a

-QjQ

b

therefore concentrated around the oxygen nucleus. The

last two electrons occupy the 5o orbital formed from the carbon 2s and 2px orbital. They form the lone pair on the

carbon atom, directed away from the oxygen atom. The

charge density contours of these orbitals are shown in

Fig. 1.

0.1

0.05

0.15 Z(nm) -0.10 --0.05

X(nm) t

Figure 1. Charge density contours of the 30(a), 40(b), 1~ or 1~ (c) and So(d) orbitals of CO. From ref. (15). y

z

In the bonding with the metal this so lone is donated to an empty metal orbital, forming a dative a-bond. Then there is back donation from a filled metal d-

orbital to the Lowest Unoccupied Molecular Orbital of the

carbon monoxide molecule, which is the pi*-antibonding

orbital. This back donation to an antibonding orbital

causes the carbon-oxygen bond to weaken a little. The

amount of weakening will be a measure for the electron

donating capacities of the adsorbing metal atom. Since CO

has a small dipole moment (0.1 , with the negative

charge on the C-atom due to the directional properties of

the 5 o orbi tal) the CO stretching vibration is infrared

active and so we can study the weakening of the CO bond

d

5

Chapter 1 page

upon adsorption by a metal by means of IR-spectroscopy.

Of course under favourable circumstances.

-0----0 \

\... \ \ I I

'',\~:'./'

-- ::.-:s~0~- -/ .

c::~--- --- --·~~ -

Figure 2. The electrical "image" resulting from a positive charge above-, a dipale vibrating parallel with- and a äipole vibrating perpendicular to a metal surface.

Pearce and Sheppard (2) discuss an important phenomenon

first mentioned by Francis and Ellison (3). They argue

that in the case of IR-studies of species, adsorbed upon

a metal surface, it is necessary that the IR-radiation

makes a large angle with the metal surface (near grazing

incidence), because at the metal surface itself there is

a knot in the standing-wave field (reflection) and

absorption cannot be seen there. Only at grazing

incidence the resulting field (in- plus out- going) will

have sufficient intensity.

Another point of interest is that a perfect metal

surface should be perfectly polarizable and this leads to

what is called the metal-surface selection rule (2).

Their line of reasoning is that the electric field lines

6

Chapter 1 page

from a charge above a roetal surface

roetal surface, will be directed

going towards this

as if there were an

opposite charge of equal size under the roetal surface at

equal distance.

Likewise, a dipole

virtual dipole of

the roetal surface.

above a roetal surface will induce a

equal size but opposite signs, under

As can be seen from Fig. 2 the

vibrations of a dipole parallel to the surface will be

annihilated by its mirror image under the surface, while

for a dipole vibrating perpendicular to the surface its

mirror image will reinforce it.

For IR-radiation of for instanee 2000 cm-1, the

wavelength is 5 ~m. Since this exceeds sufficiently the

thickness of an adsorbed layer, and also the spacing

between dipole and image-dipole, the physics described

above predict that only dipole vibrations perpendicular

to a roetal surface are infrared-active.

In recent years the contribution of infrared studies to

the understanding of adsorption processes at solid

surfaces has increased substantially, a.o. by the

introduetion of Fourier Transform Infrared Spectroscopy

(usually indicated as FT-IR spectroscopy). This

development had been initiated by the invention of the

interferometer by Michelsen in the late nineteenth

century (4, 5). Interference of light had been recognized

long before that time, but with the Michelson

interferometer it became possible to separate the two

interfering beams in such a way that their relative path

differences could be varied precisely. Michelsen already

realized that the visibility curves of his interference

patterns contained speetral information, but it was Lord

Raleigh who recognized that the interferogram was related

to the spectrum of the radiation passing through the

interferometer through the mathematica! operatien known

as the Fourier Transformation (4, 6). The interferometer

7

Chapter 1 page

played an important role in spectroscopy during the

following years, but the technology required for the full

Fourier Transformation was lacking at the time. The

astronomist Fellgett recognized the possibilities of the

interferometer in increasing the intensity of the signal

on the detector compared with the grating spectro-

photometer, where slits have to be used to create the

energy dispersion characteristic for the latter

instrument. By an interferometer, data from all speetral

frequencies are measured simultanuously. The resulting

reduction in measurement time is now known as Fellgett's

advantage (4, 7). It can be expressed more mathematically if one realises that the signal-to-noise ratio increases

for longer measurement times

Fellgett

in

a lso

proportion

was the

with the

first to square root of time.

actually perferm a

calculate a spectrum.

numerical Four~er Transformation to

Another advantage of the interferometer is that the

"throughput" of radiation,

flow through the apparatus

thanks to the absence

Jaquinot advantage (4, 8).

a measure for the total energy

per second, is also greater

of slits. This is known as the

The real break-through

spectroscopy came with the

for mid-infrared FT-IR

application of the Cooley-

Tukey fast Fourier Transferm algorithm to interferometry,

and the simultaneous development of fast, dedicated

minicomputers. Spectra that took hours to measure and

still considerable time to calculate, could now be

plotted several seconds after starting the measurement of

an interferogram. Because of these reasons, since 1968

FT-IR spectroscopy gained an important place in the study

of heterogeneaus catalysis. We started our infrared

investigations with the development of an in situ

infrared cell, and the first results obtained therewith

are reported in Chapter 5.

8

Chapter 1 page

l.S. Synthesis gas reactions and their mechanism.

We tried to point out in this introduetion so far the

importance of the synthesis of certain chemieals from

carbon monoxide and hydrogen apart from and together with

the broad supply of chemieals from natural sources,

whether or not after refining. We explained how these

syntheses take place over supported metal catalysts, how

these catalysts are made and how one can study their

oxidation and reduction behaviour. Subsequently we dealt

with the interaction of carbon monoxide with these metal

catalysts and_ how we can study this interaction with FT-

IR spectroscopy. What is left to introduce is a view on

the theories of how the syntheses of chemieals over these

catalysts are thought to take place.

For this we refer only to the outstanding review of

Biloen and Sachtler on the subject (9) and we restriet us

here to their conclusions.

The conversion of synthesis gas to hydrocarbons and

alcohols can be described by three overall reactions:

n CO + 2n H2 CnH2n + n H20 [1]

m CO + ( 2m+l) H2 -- CmH2m+2 + m H20 [2] p co + 2p H2 - c p 1H2P_ 1CH20H + p-1 H20 [3] In the presence of the product water the watergas shift

reaction can occur:

The products are mainly unbranched parafins, olefins and

alcohols, and the chain length distribution of the

molecules usually obeys the so-called Schulz-Flory

9

Chapter 1 page

distribution, implying that the molecules are formed by

step-wise chaingrowth propagation, followed by a singular

termination step. This concept involves the presence on

the surface of chains Yn and insertable monomers X, leading to the following reaction scheme:

x - À. etc. [5] The subscript of the product P denotes the number of

carbon atoms in the product molecule. The Schulz-Flory

distribution means that the ratio Pn+l over Pn (denoted

as a) is constant. This implies that the insertable

monomer X is a Cl species. Biloen and Sachtler suggest it

might be a -CH2- carbenic species, a conclusion arrived

at after analysis of the literature concerned. However,

some writers still prefer a cautious CHx notation. The

initiating species YQ (and Yl) are also Cl species. The

activation energy for the formation of methane is usually

found to be the same as for the formation of higher

hydrocarbons, so it is assumed that the rate-determining

step in both processes is the same. Since hydragenation

is known to be much faster than the Fischer-Tropsch

reaction, the termination step cannot be rate-

determining. And since the formation of methane does not

involve the propagation step, this one can be ruled out

too. Biloen and Sachtler show that in the overall

reactions

co - CH -x

- CH -x

[6]

[7]

in both cases the first step is a fast one. This implies

10

Chapter 1 page

that the conversion of -CHx- to -CH2- is rate

determining. Biloen and Sachtler propose the Fischer-

Tropsch propagation is a cis migration on the metal atom:

[8]

The terminatien steps, hydrogenation and S-H-elimination,

lead to parafins and olefins, respectively.

Rhodium catalysts are known to produce alcohols as well

as hydrocarbons in the CO hydrogenation. Since insertion

of CO in an alkyl group is thermodynamically favourable

in the case of rhodium (10), one is tended to formulate a

reaction mechanism for the formation of alcohols in which

an insertion step of CO plays a key role. However, no

definitive mechanism explaining the formation of

oxygenated products has been given in literature yet. It

should be remarked here that some related reactions are

known which make some speculations plausible. For

instanc;the Monsanto process, the conversion of methanol

to acetic acid (11). This process is catalysed by a

mononuclear rhodium complex. The oxidation state of the

rhodium in the complex changes from I to III and back

through oxidative addition of reactants and reductive

eliminatien of products. Polyols can be formed also in a

homogeneous process, and not only by rhodium catalysts

(12,13). Watson and Somorjai roughly distinguished three

temperature areas in which hydrogenation of CO would lead

mainly to the products indicated (14):

T

Chapter 1

T >600 K CH _... I x s

page

[11]

It is speculated that steps involving insertion of oxygen containing building blocks take place on oxidic sites, those not being able to dissociate CO because the electrens to donate to the pi*- antibonding orbital of the carbon monoxide. It is generally accepted that

of hydrocarbons takes place over metallic formation rhodium ensembles. performance of some

Catalytic catalysts

studies concerning in the conversion

the of

synthesis gas are presented in Chapter 4. Some overall kinetic parameters of the processes described above have been determined.

In Chapter 6 we come back to the special example of the interaction between carbon monoxide and a rhodium-alumina catalyst. There has long been a controversy in literature as to what processes took place during this interaction, and we used a number of complementary techniques to elucidate this problem. Since this Chapter will be published as such, we refer to its own introduetion for a review of the relevant literature.

Chapter 7 presents a general discussion of the results presented in this thesis and some suggestions for further research.

1.6. References.

1. coulson, C.A. in "Valence", University Press, London 1961.

2nd Ed., Oxford

2. Pearce, H.A. and Sheppard, N., Surface Sci. 59, 205 {1976).

12

Chapter 1 page

3. Francis, S.A. and Ellison, A.H., J.Opt.Soc.Am., 49,

131 (1959).

4. Griffiths, P.R., in "Chemical Infrared Fourier

Transform Spectroscopy", John Wiley and Sons, New

York 1975.

5. Miche1son, A.A., Phil.Mag. Ser. 5, 31, 256 (1891).

6. Lord Ra1eigh, Phil.Mag. Ser. 5, 34, 407 (1892).

7. Fe11gett, P., in "Proceedings of Aspen International

Conference

A.T. Stair

1970.

on Fourier Spectroscopy",

and D.J. Baker, Eds.),

(G.A. Vanasse,

AFCRL-71-0019,

8. Jaquinot, P., 17e Congres du GAMS, Paris, 1954.

9. Bi1oen, P. and Sacht1er, W.M.H., in "Advances in

Cata1ysis", Vol. 30, p 165 (D.D. Eley, H. Pines and

P.B. Weisz, Eds.), Academie Press Inc., New York

1981.

10. Kuh1man, E.J. and Alexander, J.J.,

Chemistry Reviews, 33, 195 (1980).

Coordination

11. Pau1ik, F.E. and Roth, J.F., J.C.S.Chem.Oomm., 1578

(1968).

12. De1uzarche, A., Fonseca, P., Jenner, G. and

Kienneman, A., Erdoeh1 und Koh1e, 32, 313 (1979).

13. Keim, w., Berger, M. and Sch1upp, J.J., J.Cata1., 61, 359 {1980).

13

Chapter 1 page

14. Watson, P.R. and Sornorjai, G.A., J.Catal., 74, 282

(1982).

14

Olapter 2 page

Chapter 2

The reduction-oxidation behaviour of supported Rh/y-

Al203 and Rh/Ti02 catalysts as studied with Temperature

Programmed Reduction and Oxidation.

Summary.

Careful preparatien of Rh/Al20 3 catalysts leads to

ultradisperse systems (H/Rh>l.O). TPR shows that these

catalysts are almast completely oxidized during

passivation. Identical preparatien of Rh/Ti02 catalysts

leads to less disperse systems (H/Rh=0.3), exhibiting two

reduction peaks in TPR. These peaks are due to the

reduction of smal!, wel! dispersed Rh20 3 particles, and

of large, bulklike Rh2o 3 particles. In all cases

reduction of Rh2o 3 is complete above 450 K. Tio 2 is

partly reduced by a roetal catalysed proces above 500 K.

2.1. Introduction.

Over the past years it has become clear

catalysts take a special position in

that rhodium

the field of

supported transition roetal catalysts, because they are

able to produce hydrocarbons as wel! as oxygenated

products (alcohols, aldehydes, acids) from synthesis gas

(1-10). Various workers have tried to influence the

selectivity and activity of the rhodium catalysts via a

special p .. :eparation (1,2), via additives (3-5) or mixed

oxides (8-10), and where this worked out in the wanted

direction of higher production of oxygenates at least two

of them gave as the reasen the presence of stabilized Rh+

15

Chapter 2 page

in the surface (8,10). Same authors claimed the presence

of isolated Rh+ sites in monometallic Rh catalysts on the

basis of IR evidence (Worley et al. (11-13), Primet

(14)), while others

metallic rafts with

(15)).

found the Rh to be present as

electron microscopy (Yates et al.

In all cases it seems obvious that the support plays an

important role in either bringing or keeping the roetal in

a certain state of (un)reactivity. A special example of

such a roetal support interaction has been discovered by

Tauster et al. (16,17) and is now known as Strong Metal

Support Interaction: Supported metals such as Pt and Rh

loose their capability for chemisorption of H2 , co- and NO if they have been reduced at high temperatures (e.g. 773

K) on supports such as Ti0 2 and V20 5. Normal

chemisorption behaviour can be restored by oxidation at

elevated temperatures, followed by low temperature

reduction, at for instanee 473 K (16). SMSI has been

related to the occurrence of lower oxides of the support

(18, 19), but the exact nature of the interaction still

remains unc1ear.

Many of the above mentioned phenomena have to do with

one camman property: The oxidation-reduction behaviour of

supported Rh cata1ysts. We decided to study two systems,

representative for many of the ones referred to above:

2.3 wt% Rh/Al2o 3 and 3.2 wt% Rh/Ti02 . Via sintering (see Experimental) we have induced a variatien in partiele

size (dispersion) in order to answer the fo11owing

questions:

- how is oxidation-reduction influenced by partiele size

- how is oxidation-reduction influenced bu the support

used

-does partiele size show any effect upon SMSI.

Befare we come to the experimental techniques, we have

to introduce one last item: Passivation. Since it is

16

Chapter 2 page

obvious that reduced systems cannot simply be stared in

air, we passivate and stabilize them by applying a layer

of oxygen upon the metal particles in a controlled

{see Experimental). Some authors have already

way

paid

attention to the state catalysts are in after starage in

air. Thus Burwell jr. et al. used Wide Angle X-ray

Scattering, Extended X-ray Absorption Fine Structure

(EXAFS), hydragen chemisorption and hydrogen-oxygen

titration to characterize their supported Pt and Pd

catalysts {20-23).

We will show that a good insight in all these matters

can be gained with the aid of Temperature Programmed

Reduction and Oxidation (TPR and TPO), supported by

chemisorption measurements. TPR as a characterization

technique was presented by Jenkins et al. in 1975 {24,

25) and has been used extensively in the past few years,

as becomes obvious from a recent review by Hurst et al.

(26). The technique allows one to obtain (semi)quan-

titative information about the rate and ease of reduction

of all kinds of systems, and once the apparatus has been

built, the analyses are fast and relatively cheap. We

used an aparatus as described by Boer et al. (27), which

enabled us to extend the analyses to Ternperature

Prograrnmed Oxidation, and to gather information about the

rate and ease of oxidation as well.

2.2. Experirnental.

Tio2 (anatase, Tioxide Ltd., CLDD 1367, surface area 20

m2/g, pore volume 0.5 cm3/g) and y-AlzOJ (Ketjen, 000-

1.5E, surface area 200 m2/g, pore volume 0.6 cm3/g) were

impregnated with ·an aqueous salution of RhC13 .xH20 via

the incipient wetness technique to prepare a 2.3 wt% Rh/Al 2o 3 catalyst and a 3.2 wt% Rh/Tio 2 catalyst, as was established spectrophotornetrically afterwards.

17

Chapter 2 page

catalysts were dried in air at 355, 375 and 395 K for 2

hrs successively, followed by direct pre-reduetion in

flowing H2 at 473, 773 or 973 K for one hour. Prior to

removing the catalysts from the reduction reactor they

were passivated at room ternperature by replacing the

hydrogen flow by nitrogen and subsequently slowly adding

oxygen up to 20 %. Then the catalysts were taken out of the reactor and stored for further use.

Programmer

Vent.

Figure 1. Schematic representation of the TPR/TPO apparatus.

The TPR/TPO apparatus used is schematically represented in Fig. 1: A 5% H2 in Ar or a 5% 0 2 in He flow (300

ml/hr) can be directed through a microreactor. The

temperature of the reactor can be raised or lowered via

linear programming. H~ or o2 consumption is being monitored continuously by means of a Thermal Conductivity

Detector (TCD). A typical sequence of actions is as fellows:

- the passivated or oxidized sample is flushed under Ar flow at 223 K

- Ar is replaced by the Ar/H 2 mixture, causing at least

an apparent H2 consumption (first switch peak)

- the sample is heated under Ar/H 2 flow with 5 K/min to

873 K

18

Chapter 2 page

- after 15 min at 873 K the sample is caoled down with 10 K/min to 223 K

- the reduced sample is flushed with Ar Ar flow is replaced by the Ar/H~ mixture once again, now causing only an apparent H2 consumption (second

switch peak).

A similar sequence is followed during TPO. The switch peak procedure deserves sorne closer attention. The strong

signa! we cal! the first switch peak is mainly due to the

displacement of Ar by Ar/H2 in the reactor, but in sorne cases real hydragen consumption takes place, even at 223

K. Therefore we repeat the whole procedure after the TPR

has been perforrned: In that case the catalyst has been

reduced and caoled down to 223 K, and as a consequence is

covered with hydrogen. Then we replace the Ar/H 2 by pure Ar, resulting in a negative TCD signa!. Subsequently we switch back to Ar/H2 • Since we do nat expect any hydragen consumption frorn the reduced, hydragen covered sample at

this time, the resulting switch peak will be due solely

to the displacement of Ar by Ar/H 2. Sa the difference between the first and second switch peak reveals the real

hydragen consumption at 223 K, if there is any.

The reactions that might take place during TPO and TPR

are:

4 Rh 0.75) [1]

[2]

The quantities between brackets are the amounts of

hydragen or oxygen consumption per rhodium expected for

reduction of bulk Rh2o3 or formation of this very materal (apart from chemisorption of any kind).

Chernisorption measurements were carried out in a

conventional volumetrie glass apparatus after reduction

19

Chapter 2 page

of the passivated catalysts at 473 or 773 K in flowing H

for 1 hr followed by evacuation at 473 K for one hr.

After H2 admission at 473 K desarptien isotherms were

measured at room temperature. As desarptien became only

noticeable at pressures below 200 torr we believe that

the chemisorption value above that pressure is

representative of monolayer coverage (cf. Frennet et al.

(28)). The H/Rh values thus obtained for the various

systems are preseuted in Table 1.

H/Rh T(pre-red)

RT(LT) (l) (K) RA

473 1. 70 0.37

773 1. 53 0.29

973 1.23 0.12

(1): reduced in situ at 473 K. (2): reduced in situ at 773 K.

RT(HT)( 2 )

0.08

0.05

0.01

Table 1. Hydrogen chemisorption of Rh/y-Al 2 o 3 (RA) and Rh/Ti0 2 (RT) catalysts.

2.3. Results.

2.3.1. Hydragen chemisorption.

The hydragen chemisorption data as given in Table 1

were obtained for Rh/Al2o3 (RA) after reduction of the passivated catalyst in situ at 473 K. Rh/Ti0 2 (RT) was

also reduced in situ at 773 K, to induce SMSI behaviour.

The catalysts will be denoted from now on as RA 773

(Rh/Al2o3 , pre-reduced at 773 K), RT 973 (Rh/Ti0 2, pre-

20

Chapter 2 page

reduced at 973 K), etc., that is de code refers to the

temperature of reduction of the dried, impregnated

cata1yst (see Experimenta1). The results are represented

graphically in Fig. 2, showing the value of H/Rh as a

2.0

1.5

1.0

0.5

473 673 873 1073

Temp. pre-red.(K)

Figure 2. Hydragen chemisorption as a function of pre-reduetion temperature; a) Rh/Y-Alz03, reduced in situ at 473 K. b) Rh/Ti0 2 , reduced in situ at 473 K. c) Rh/Ti02, reduced in situ at 773 K.

function of pre-reduetion temperature (i.e. reduction

prior to passivation; reduction prior to chemisorption

was at either 473 or 773 K, as indicated). The systems

discussed here are represented by the separate dots.

Va1ues for Rh/Al2o

3 range from 1.70 to 1.00, for non-

SMSI Rh/Ti0 2 from 0.37 to 0.12 and for SMSI Rh/TiC2 from

0.10 to o.oL

21

Chapter 2 page 22

a

b

c

d

273 473 673 ~ 873 T(K)

Figure 3. 2. 3 wt% Rh/Y-Alz03 catalyst; a) TPR of passivated catalyst, pre-reduced

at 473 K. b) TPR of passivated catalyst, pre-reduced

at 773 K. c) TPO following TPR of the catalysts. d) TPR following TPO of the catalysts.

Chapter 2 page

The Temperature Programmed Reduction profiles of the

passivated RA 473 and RA 773 catalysts are shown in Fig.

3 a and b. The horizontal axis shows the temperature and

the vertical axis the hydragen consumption (in arbitrary

units).

RA 473 shows a maximum H2 consumption at 330 K, and

some further reduction above 473 K. RA 773 shows a single

consumption peak around 273 K, foliowed by desarptien of

H2 . The pre-reduetion had apparently been complete, and

passivatien of this ultra disperse Rh catalyst had led to

dissociative oxygen chemisorption, but nat full

oxidation, since the net H2 consumption mounted up to 1.0

H2/Rh. Apparently the pre-reduetion at 473 K (RA 473) had

nat been complete, and the oxygen chemisorbed on this

system was harder to remave than from the other ones (RA

973 showed an identical TPR profile as RA 773). The

subsequent TPO, Fig. 3c, which was identical for all

three systems, confirms these observations: 02

consumption starts at 223 K, in the switch peak,

continues when the temperature ramp is started, reaches a

maximum around 290 K, and then decreases very slowly

towards higher temperatures. Integration of the 02

consumption signa! proved to be difficult, due to the

small sample sizes (typically 50-75 micromale of metal)

and the small thermal conductivity of o2 but still mounted up toa rather satisfying 0.6-0.7 o2/Rh (0.75 was expected, since chemisorbed H2 had been removed by a heat

treatment in between TPR and TPO). The equality of the

TPO profiles of the RA 473, RA 773 and RA 973 catalysts

is in accordance with the fact that all these catalysts

have been brougt up to 873 K during the TPR run and it is

also nat surprising to find that the TPR profiles of the

23

Chapter 2 page

completely oxidized systems (following TPO) are identical

too (Fig. 3d). one single peak is observed around 360 K,

corresponding with

which agrees with the

bulk Rh 2o3 showed

a H2 consumption of about 1.5 H2/Rh, reduction of Rh 2o3 • Unsupported

a reduction peak at 400 K in our

apparatus, while bulk Rh roetal only started to become oxidized above 870 K.

R3 473 673

Figure 4. 3.2 wt% Rh/Ti02 catalyst;

~ T{K)

a

b

c

873

a) TPR of ~assivated catalyst, pre-reduced at 473 K.

b) TPO following TPR of the catalysts. c) TPR following TPO of the catalysts.

24

Chapter 2 page

2.3.3. TPR and TPO of Rh/Ti~.

The TPR profile for the passivated RT 473 catalyst is

shown in Fig.' 4a. The most striking feature is that H2 consumption starts already at 223 K in the switch peak,

that is immediately when H2/Ar is being flushed through the reactor. Keeping in mind·the much lower H/Rh value of

this catalyst compared to the Rh/Al 2o3 series, we think that passivatien here has caused only the formation of an

outer layer of oxide on the relatively large roetal

particles. If the remaining metallic care could be

reached by the hydragen molecules, these molecules could

be able to dissociate and provide atomie hydragen for an easy reduction of the oxide layer at low temperatures.

There is also some H2 consumption just above 473 K, as

for the corresponding Rh/Al203 catalyst, indicating that

also for Rh/Ti02 the pre-reduetion at 473 K had not been

complete. It can be seen that the Ti02 support is

reducible also, leading to H consumption around 573 and

700 K. The H2 consumption at 273 K amounts to about 0.4

H2 /Rh.

The TPR profiles for RT 773 and RT 973 are similar, but

since the Rh surface area decreases with increasing pre-

reduetion temperature (cf. Table L) the amount of

passivatien oxygen decreases also, and so does,

consequently, the H2 consumption at 223 K in the TPR

profiles. Also the consumption just above 473 K has

disappeared as a consequence of the higher pre-reduetion

temperature.

25

Chapter 2 page

Fig. 4b shows the TPO profile of a reduced RT catalyst.

The three TPO profiles of RT 473, 773 and 973 are alike:

Three "areas" of oxygen consumption show up, Which we

attribute respectively to chemisorption (which is the

only phenomenon on Rh/Alz0 3 ), corrosive chemisorption and

thorough oxidation. We will come back to this assignment

in the Discussion part.

The TPR profiles of the oxidized samples of the RT

series are identical again (Fig. 4c) and show two clearly

divided Hz consumption maxima, at 325 and 385 K.

Oonsumption in the first peak is about 1.3 Hz/Rh, in the

second one about 0.3 Hz/Rh. Taken together the Hz

consumptions come close enough to the expected value of

1.5 Hz/Rh to attribute them both to reduction of Rhz03·

The peak at 385 K is assigned to bulklike Rh 2o3 particles (note that the peakmaximum for unsupported Rh zO 3 is at

400 K) and the one at 325 K to a better dispersed ~0 3 phase. Furthermore the possibility of some support

reduction taking place here as well, in advance of the

support reduction around 573 and 700 K, cannot be

excluded.

2.4. Discussion.

Hydrogen chemisorption has been used through the years

by many workers to characterize metal surfaces (7, 9, 10,

14, 16, 20-23, 29, 30) and when attempts were made to

calculate metal surface areas from hydrogen chemisorption

data, a hydrogen to metal stoechiometry of one was used.

On the other hand, if CO was involved, some authors (31)

chose a stoechiometry of one, while others (14) mentioned

higher stoechiometries. We are of the opinion that if one

accepts a metal atom, such as Rh, to adsorb two or more

CO molecules, one should not reject the idea of the same

atom adsorbing more than one hydrogen atom. So we think

26

Chapter 2 page

that although our experimental H/Rh results exceed unity

(Table 1), they are real, and that the hydrogen

chemisorbed does not exceed a monolayer (cf. Ftennet et

al. (28)), and is all bound to the metal. We dit not try

to make a distinction between socalled "reversible" and

"irreversible" adsorption, because we believe that in the

thermodynamic sense there is no such distinction. In

following the procedures that lead to that supposed

distinction one merely encounters the physical

restrictions of ones apparatus, such as pumping speed and

conductivity of tubing.

As described in the Experimental section we admitted

hydrogen at 473 K. This was simply done to accelerate

adsorption but did not effect the ultimate amount of

adsorption, as was checked via the measuring of

adsorption isotherms at room temperature (32). This

leaves us with chemisorption values above unity, and

therefore, like some other authors (21, 30), we find it

impossible to calculate a partiele size or a dispersion

from these data, since there is no particular

stoechiometry value to prefer. We imagine these Rh

particles could be raftlike as suggested by Yates et al.

(15) (although they were dealing with only 0.5 wt% Rh/Al 20 3), where the edgeatoms could have the possibility

of adsorbi.ng more than one hydrogen atom. "Sintering" of

these particles -H/Rh decreases from 1.7 to 1.0 upon

reduction at 973 K- would then mean that the number of

edgeatoms decreases, for example by growing of the rafts

or even formation of (hemi)spherical particles. Still we

are dealing with ultradispersed systems. We estimate that

in all cases the Rh partiele size does not exceed 1 nm.

Fbr the RT series the H/RH values as established after

473 K reduction are much lower, which was to be expected

taking into account the difference in surface area

between Al 20 3 and TiO 2 and the similar me tal loadings.

27

Chapter 2 page

Here also we see a decrease in the H/Rh values as a

result of an increase in the pre-reduetion temperature.

But in this case this is simply due to growth of the

metal particles, resulting in a decreased metal surface

area. The H/Rh values after 773 K reduction show evidence for SMSI behaviour. In this case it means they are that

small that they are of the order of magnitude of the

experimental error. Therefore we cannot, at this point,

draw any quantitative conclusion about a relationship

between SMSI behaviour and partiele size. \'ie knew in

advance this would be difficult, since both partiele

growth and increasing SMSI behaviour lead to less

hydrogen chemisorption, and so the limits of experimental

accuracy might prohibit to make a distinction between the

two effects. It is evident, though, that RT 773 and RT 973 must have been in the SMSI state after pre-reduction, but they showed normal chemisorption behaviour after

passivation

chemisorption

following the

state.

and re-reduction at 473 K in the apparatus. This means that the passivation,

pre-reduction, must have destroyed the SMSI

Our TPR and TPO results also prove that passivation is

rather drastic. Fbr the small metal particles on Alzo 3

TPR of the passivated samples differs fram that of the

oxidized ones only in the position of the peak, that is

the ease of reduction. And from the shape of the TPO

pattern we can understand that a long passivation time

(like storage in air) comes close to a real oxidation.

But it is very clear that no matter what the initia!

state of the Rh was, passivated or oxidized, reduction is

complete above 400 K.

Worley and coworkers studied the oxidation state of Rh

on various supports, and starting from various Rh

precursors, applying IR specroscopy upon CO adsorption

(11-13). Apart from IR absorptions attributed to CO

28

Chapter 2

adsorbed on metallic Rh,

which they attributed

they

to CO

page

found some absorptions

adsorbed on isolated Rh+

sites. Fbr 2.2 wt% Rh catalysts they found these isolated

Rh+ sites to be more abundant for Al2o 3 than for Ti02 as

a support (13), and more abundant for RhCl3 than for

Rh(N0 3) 3 as a precursor (12, 13). They concluded that one

gets the worst reduction of Rh (they reduced at 673 K)

when using Al 2o 3 as a support, and when starting from

RhC1 3 as a precursor. From the TPR evidence presented

here, we conclude that the systems they have studied must

have been completely reduced prior to admission of 00,

and therefore we prefer another explanation. One obtains

the best dispersion of metallic Rh particles on Al203,

and when starting from RhC1 3 . Upon co adsorption the smaller particles break up and create the isolated

dicarbonyl species that were attributed to Rh+l by Worley

et al. EXAFS proof for this explanation will be published

elsewhere (33).

Our findings for Rh/Ti02 go very well along with the

results published by Burwell et al. for Pd and Pt on SiO and Al2o 3 (20-23). Upon passivatien the larger metal

particles farm an oxide skin, the formation of which can

beseen almast literallyin Fig.4b, the TPO of Rh/TiOz.

The small metal particles on Alz03 show only one tailing

chemisorption peak, but for Rh/Ti0 2 oxygen chemisorption

is followed at higher temperatures by corrosive

chemisortion and finally, around 700 K, by thorough

oxidation. Apparently oxygen diffusion through the oxide

layer is a strongly hindered process. Attribution of the

intermediate temperature region of oxidation to corrosive

chemisorption is supported by the results of a TPO we did

on a 3 wt% Rh/Sio2 (Grace Silica, S.P. 2.-324,382, 290 m2/g, H/Rh=0.4). This TPO is shown in Fig. Sa, and

exhibits two oxygen consumtion areas, one around 273 K

which is due to oxygen chemisorption, and one around 500

29

Chapter 2 page

K due to corrosive chemisorption. All of the Rh on Si02 is in a well dispersed form, as was confirmed by the

subsequent TPR of this Rh/Si02 catalyst, which showed

only one hydrogen consumption peak at 335 K. This

reduction peak corresponds to the first peak observed for

Rh/Ti0 2 at 325 K (cf. Results).

The oxidation peak around 700 K in the TPO of Rh/Ti02

is attributed to the formation of rather large, bulklike

Rh20 3 particles, the reduction of which is observed as a

separate H 2 consumption maximum at 385 K in TPR (Fig.

4c). That the large particles oxidize at higher

temperature than the small particles and reduce at a

higher temperature as well, has been proven by a rather

simple experiment, shown in Figs. Sb and c. First a TPO

is run with a reduced Rh/Ti0 2 system, up to 673 K.

Subsequently a TPR is performed which demonstratee that

the reduction peak at 385 K has completely disappeared.

Thus the fraction of the Rh which reduces at 385 K

oxidizes above 673 K, and vice versa.

/ ----,/ ~

~ \_

273 473 673 ........ T(K}

a

b

c

873

Figure 5. a) TPO of a reduced 3.0 wt% Rh/SiOz catalyst. b) TPO up to 673 Kof a reduced 3.2 wt%

Rh/Ti02 catalyst. c) Subsequent TPR.

30

Chapter 2 page

That one can distinguish between a well dispersed phase

and a bulklike phase of Rh2o3 on a· support has been noticed before by Yao et al. (34), although they used Al 20 3 as a support and had to oxidize for 12 hrs at 973 K

to create the bulk phase.

The fact that part of the Ti02 support is being reduced

as well, by a metal-assisted process, is in agreement

with findings for Pt/Ti02 (35, 36). Reoxidation of the support takes place during TPO,

exceeds the expected o2/Rh value of but is apparently hidden in the

since 02 consumption

0.75 in all cases,

TPO profile by the stronger 0 2 consumption caused by the Rh oxidation.

2.5. Cbnclusions.

The 2. 3 wt% Rh/A~ o3 catalysts proved to be "ultradisperse" with H/Rh values ranging fran 1. 7 to l.O.

The catalysts behaved accordingly in TPR and TPO: Easy

reduction and fast oxidation were observed to such an

extent that even a mild passivation led to almast

complete oxidation.

The 3.2 wt% Rh/Ti02 catalysts were much less dispersed

(H/Rh 0.37-0.12) and showed evidence of two distinct

forms of Rh (and Rh2 o3 ), appearing as two reduction peaks in TPR (reduction of well dispersed and bulklike Rh2o 3) and three oxidation areas in TPO (oxygen chemisorption

and corrosive chemisorption of well dispersed Rh, and

thorough oxidation of bulklike Rh).

A start was made in investigating the influence of

partiele size upon oxidation-reduction behaviour, but we

were not able yet to establish a relationship between

SMSI behaviour and partiele size. All Rh/Ti0 2 samples

could be brought into the SMSI state.

We shall continue this search

dispersion, via the me tal content

by varying the

and by varying the

31

Chapter 2 page

reduction procedure.

This investigation has shawn unambiguously that the

TPR-TPO technique is a very powertul tool in discriminating between the various ways in which oxygen

can react with a metal, and so TPR-TPO allows a careful

analysis of the state of dispersion of the metal on the

catalyst to be made.

2.6. References

1. Ichikawa, M., Bull.Chem.Soc.Jap. 51, 2268 (1978).

2. Ichikawa, M., Bull.Chem.Soc.Jap. 51, 2273 (1978).

3. Leupold, E.I., Schmidt, H.-J., Wunder, F., Arpe, H.-

J. and Hachenberg, H., E.P. 0 010 295 Al.

4. Wunder, P.A., Arpe, H.-J., Leupold, E.I. and Schmidt,

H.-J., Ger. Offen. 28 14 427.

5. Bartley, W.J. and Wilson, T.P., Eur.Pat.Appl. 0 021

443.

6.

7.

s.

9.

Castner, D.G., Blackadar, R.L. and Somorjai, G.A. I

J. Catal. 66, 2 (1980).

Watson, P.R. and Somorjai, G.A. I J.Catal. 72, 347

( 1981).

Watson, P.R. and Somorjai, G.A. I J .Catal. 74, 282

(1982).

Ichikawa, M. and Shikakura, K. in "Proceedings of the 7th International Congres on Catalysis", Tokyo 1980

(T. seyana and K. Tanabe, Eds.), part A, p. 925,

32

Chapter 2 page

E1sevier, Amsterdam.

10. Wi1son, T.P., Kasai, P.H. and E11gen, P.C., J.Cata1.

69, 193 (1981).

11. Rice, C.A., Wor1ey, s.o., Curtis, c.w., Guin, J.A. and Tarrer, A.R., J.Chem.Phys. 74, 6487 (1981).

12. Wor1ey, s.o., Rice, C.A., Mattson, G.A., Curtis, c.w., Guin, J.A. and Tarrer, A.R., J.Chem.Phys. 76, 20 (1982).

13. Wor1ey, s.o., Rice, C.A., Mattson, G.A., Curtis, c.w., Guin, J.A. and Tarrer, A.R., J.Phys.Chem. 86, 2714 (1982).

14. Primet, M., J.C.S.Farad.Trans.II, 74, 2570 (1978).

15. Yates, O.J.C., Murre11, L.L. and Prestridge, E.B.,

J.Cata1. 57, 41 (1979).

16. Tauster, S.J., Fung, s.c. and J.Am.Chem.Soc. 100, 170 (1978).

Garten, R.L.,

17. Tauster, S.J., Fung, s.c., Baker, R.T.K. and Hors1ey, J.A., Science 211, 1121 (1981).

18. Baker, R.T.K., Prestridge, E.B. and Garten, R.L.,

J.Catal. 50, 464 (1979) ..

19. Huizinga, T. and Prins, R., J.Phys.Chem. 85, 2156

{1981).

20. Uchijima, T., Herrmann, J.M., Inoue, Y., Burwe11 jr.,

R.L., Butt, J.B. and Oohen, J.B., J.Cata1. 50, 464

33

Chapter 2 page

(1977).

21. Kobayashi, M., Inoue, Y., Takahashi, N., Burwell . ,

R.L., Butt, J.B. and Cbhen, J.B., J.Catal. 64, 74

(1980).

22. Nandi, R.K., Georgopoulos, P., Cohen, J.B., Butt,

J.B. and Burwell jr, R.L., J.Catal. 77, 421 (1982).

23. Nandi, R.K., Molinaro, F., Tang, C., Oohen, J.B.,

Butt, J.B. and Burwell jr., R.L., J.Catal. 78, 289

(1983).

24. Robertson, S.D., McNicol, B.D., de Baas, J.H., Kloet,

s.c. and Jenkins, J.W., J.Catal. 37, 424 (1975).

25. Jenkins, J.W., McNicol, B.D. and Robertson, S.D.,

Chem.Tech 7, 316 (1977).

26. Hurst, N.w., Gentry, S.J., Jones, A. and McNicol,

B.D., Catal.Rev. Sci.Eng. 24, 233 (1982).

27. Boer, H., Boersma, N.F. and

Rev.Sci.Instr. 53, 349 (1982).

Wagstaff, N.,

28. Crucq, A., Degols, L., Lienard, G. and Frennet, A., ·

Acta Chim. Acad.Sci.Hung. 1982, 111.

29. Sinfelt, J.H. and Yates, D.J.C., J. Catal. 10, 362

(1968).

30. Wanke, S.E. and Dougharty, N.A., J.Catal. 24, 367

(1972).

33. Van 't Blik, H.F.J., van Zon, J.B.A.D., Huizinga, T.,

34

Chapter 2 page

Vis, J.C., Koningsberger, D.C. and Prins, R.,

J.Phys.Chem. 87, 2264 (1983).

34. Yao, H.C., Japar, s. and She1ef, M., J.cata1. 50, 407 (1977).

35

Chapter 3 page

Chapter 3

The Morphology of Rhodium Supported on Ti0 2 and Alz 03 as

Studied with Temperature Programmed Reduction-Oxidation

and Transmission Electron Microscopy.

Supported Rh/Al2o 3 and Rh/Ti0 2 catalysts with varying

roetal loadings have been investigated with chemisorption

and temperature programmed reduction and oxidation.

Hydragen chemisorption shows that all the rhodium on

Al2o 3 is well (H/Rh > 1 for loadings < 5 wt% and H/Rh > 0.5 up to 20 wt%), dispersion on Tio 2 is much

lower. TPR/TPO shows this is due to the growth of two

different kinds of rhodium /Rh2o 3 on Ti02 ; one kind

easily reduced/oxidized, showing high dispersion, the

ether kind harder to reduce/oxidize, showing lower

dispersion. TEM has shown that the first kind of Rh 2o 3 consists of flat, raftlike particles, the secend kind of

spherical particles.

3.1. Introduction.

Over the past years rhodium has been gaining importance

in catalytic chemistry. Not only is rhodium widely

recognized as the best catalyst to promate the reduction

of NO in three way catalysts (1-3), it also takes a

special place in the conversion of synthesis gas, since

its product range can include oxygenated products

(alcohols, aldehydes, acids) besides hydrocarbons (4-11).

Various workers have tried to influence the selectivity

36

Chapter 3 page

and activity of the supported rhodium catalysts in syngas conversion via a special· preparatien (4,5), via additives

(6-8) and via control over the oxidation state of the rhodium in the catalysts (9-11).

Ichikawa deposited rhodium-carbonyl clusters on various supports (4, 5) and after pyrolysis of the clusters, he found a large range of selectivities towards oxygenates in the hydragenation of co. He explained this by the acid-bas& properties of the supports (12).

Somorjai c.s. tried to hydrogenate CO over unsupported rhodium and rhodium foil, and found only hydrocarbons,

unless they preoxidized the metal (9). H2 /co atmosphere led to the reduction to rhodium metal and the production

of hydrocarbons, whereas Rh203.5H20 was better resistant towards reduction and produced oxygenates for a long time ( 10).

Where the influence of additives or mixed oxides worked

out into the right direction of enhanced oxygenate

production, some workers traeed this to the presence of rhodium ions in the surface (11,13). The supposed presence of rhodium ions has been a point of discussion

for quite some time. Several authors claimed it to be

present in mono-metallic rhodium catalysts. W?rley et al.

investigated a.o. a 0.5 wt% Rh/Al203 catalyst via infrared spectroscopy of adsorbed CO (14-16), and ascribed several infrared bands to CO molecules bound to

isolated Rh(I) sites. This is in contrast with earlier findings by D.J.C. Yates et al. (18). They investigated

some Rh/Al 2o 3 catalysts with electron microscopy and found rhodium to be present as metallic rafts. They did

notmention any isolated Rh(I) sites, but, as Worley and

coworkers stipulated themselves, the two groups used very

different methods of preparatien of the samples.

In all cases it seems obvious that the support plays an

important role in either bringing or keeping the metal in

37

Chapter 3 page

a certain state of (un)reactivity. A special example of

such an interaction between metal and support has been

described by Tauster et al. (19, 20) and is now known as

Strong Metal Support Interaction (SMSI). Supported noble

and transition metals such as Ft, Rh, Ru etc. are

normally capable of chemisorbing a.o. H2 and CO. But if

they are supported on oxides as Ti0 2 , V205 and Nb205, and

if they have been reduced at high temperatures (e.g. 773

K), this chemisorption capability is greatly diminished.

This SMSI phenomenon has been related to the occurrence

of lower oxides of the supports (21), although these are

known to be formed at lower temperatures than necessary

to cause SMSI (22) and the exact nature of the

interaction still remains unclear. The SMSI state can be

destroyed according to Tauster c.s. by oxidation at

elevated temperatures, followed by

reduction (473 K): this procedure

chemisorption behaviour (19).

low ternperature

restores normal

All of the above rnentioned phenomena have to do with

one common property: The oxidation-reduction behaviour of

supported rhodium catalysts. We therefore decided to

study a nurnber of Rh/Al20 3 and Rh/Ti02 catalysts with

varying rnetal loadings (see Experimental). A1203 was

chosen because it is known as a support giving good

dispersions and stable catalysts, and Ti02 because it is

known to exhibit SMSI. We varied the metal loading to

create a variatien in partiele size, to see if and how

oxidation-reduction and SMSI behaviour are influenced by

partiele size.

Befare we come to the experimental techniques we used,

we want to introduce aother item: passivation. It is

obvious that reduced catalyst systems cannot simply be

removed from the reduction reactor and then be stared in

air for later u se~ we stabil i ze the me tal surface by

applying a layer of oxygen upon the metal particles in a

38

Chapter 3 page

controlled way (see Experimental): We passivate the

catalysts. Although a simple low temperature reduction is

sufficient to remove the passivatien oxygen again (as

will be shown), some authors have given attention to the

state the catalysts are in after storage in air. One can

see this as a prolonged passivation, but without the

precautions we take to prevent uncontrollable effects

upon the first contact between air and the reduced roetal

catalyst. So Burwell Jr. et al. used Wide Angle X-ray

Scattering, Extended X-ray Absorption Fine Structure,

hydragen chemisorption and hydrogen-oxygen titration to

characterize their supported Pt and Pd catalysts (23-26),

and they found their catalysts to be oxidized to a great

extent after prolonged storage in air.

We will show that a good insight in all these matters

can be gained with the aid of Temperature Programmed

Reduction and of Temperature Programmed Oxidation (TPR

and TPO), supported by chemisorption measurements. TPR as

a characterization technique was presented by Jenkins,

Robertson and McNicoll in 1975 (27,28) and has been used

extensively in the past few years. The development has

been reviewed by Hurst et al. (29). The technique allows

one to get (semi)quantitative information about the rate

and ease of reduction of all kinds of systems, and once

the apparatus has been built the analyses are fast and

relatively cheap. We used an apparatus as described by Boer et al. (30), which enabled us to extend the analyses

to Temperature Programmed Oxidation, and to gather

information about the rate and ease of oxidation as well.

3.2. Experimental.

Tio 2 (anatase, Tioxide Ltd., CLDD 1367, surface area 20

m2/g, pore-volume 0.5 cm3/g) and Y-Al203 (Ketjen, 000-

l.SE, surface area 200 m2/g, pore-volume 0.6 cm3/g) were

39

ctmpter 3 page

wt% Rh

2.3

4.6

8.5

H/Rh

1.53

0.96

0.81

0.67

0.54

Table 1. Hydragen chemisorption of

the Rhjy-Al203 (RA) catalysts.

11.6

20.0

H/Rh Table 2. Hydrogen chemi-wt% Rh

LT( 1 J HT(ZJ sorption of the Rh/TiOZ

(RT) catalysts. 0.3 1.10 0.00

0.7 0.61 0.01

1.0 0.41 0.01

2.0 0.35 0.01

3.2 0.22 0.02

8.1 0.12 0.04

( 1 ) : reduced in situ at SZ3 K. ( 2) : reduced in situ at 773 K.

impregnated with aqueous so1utions of RhCl3.xH20 via

the incipient wetness technique to prepare the catalysts.

Their characteristics are presented in Table 1 and Table

2. The catalysts will be denoted from now on as RT

(Rh/Ti02 ) and RA (Rh/Alz03 ) catalysts, foliowed by the

metal loading. After impregnation the catalysts were

dried in air at 355, 375 and 395 K for 2 hours

successively, foliowed by direct pre-reduetion in flowing

Hz at 773 K for one hour. Prior to removing the

from the reduction reactor they were

temperature by replacing the Hz flow by Nz and

40

Chapter 3 page

subsequently slowly adding 0 up to 20 %. Then the catalysts were taken out of the reactor and stared for

further use.

In the TPR-TPO apparatus used a 5 % H2 in Ar or a 5 %

o 2 in He flow can be directed through a microreactor, which is connected to a temperature programmer. H2 or 02

consumption is being monitored continuously by means of a

Thermal Oonductivity Detector (TCD). A typical sequence

of experiments is as fellows:

- the passivated or oxidized sample is flushed under Ar

at 223 K

- Ar is replaced by the Ar/H2 mixture, causing at least

an apparent H consumption (first switch peak)

the sample is heated under Ar/H 2 flow with 5 K/min to

873 K

after 15 min at 873 K, the sample is caoled down with

10 K/min to 223 K

- the reduced sample is flushed with Ar

- Ar flow is replaced by the Ar/H2 mixture once more, now

causing only an apparent H consumption (second switch

peak).

An identical sequence is followed during TPO, so the

final oxidation temperature in TPO is also 873 K, unless

stated otherwise.

The switch peak procedure deserves some closer

attention. The streng signal we call the first switch

peak is due mainly to the displacement of Ar by Ar/H2 in

the reactor, but in some cases real hydragen consumption

might take place, even at 223 K. Therefore we repeat the

whole procedure after the TPR has been performed: In that

case the catalyst has been reduced and caoled down to 223

K, and as a consequence it is covered by hydrogen. Then

we replace the Ar/H 2 by pure Ar. Subsequently we switch

back to Ar/H2 . Since we cannot expect any hydragen

consumption from the reduced, hydragen covered sample at

41

Chapter 3 page

this time, the resulting second switch peak will be due

solely to the displacement of Ar by Ar/H2· So the difference between the first and second switch peak

reveals the real hydragen consumption at 223 K, if there

is any.

The reactions that might take place during TPO and TPR

are:

4 Rh 0.75) [1]

The quantities between brackets are the hydragen or

oxygen consumptions in TPR or TPO expected for reduction

of bulk Rh 2o 3 or formation of this very material (apart from chemisorption of any kind). In a standard experiment

a TPR is done on a passivated catalyst, followed by TPO

(on the now reduced catalyst), followed by TPR (on the

now oxiclized catalyst).

1.2

• 0.8

0.4

4 8 12 16 20

wt'l. Rh~

Figure 1. Hydragen chemisorption of Rh/y-Al 2 0 3 catalysts as a function of roetal loading. T(red) is 773 K.

42

Chapter 3 page

Chemisorption measurements were carried out in a

conventional glass apparatus after reduction of the

passivated catalysts at 773 K in flowing Hz for 1 hour,

followed by evacuation at 473 K for one hour. After

hydragen admission at 473 K desorption isotherms were

measured at room temperature. Fbr the RT series 773 K

(High Temperature} reduction will induce SMSI, and so the

RT catalysts were also reduced at 523 K (Low Temperature)

to study their normal chemisorption behaviour. In

measuring the desorption isotherms desorption became only

noticeable at pressures below 200 torr (l torr 133.3

N/m2} so we believe that the chemisorption value above

that pressure is representative of monolayer coverage

(cf. Frennet. c.s. (31}).

Transmission Electron Microscopy was carried out on a

Jeol 200 ex top entry stage microscope. Photographs were taken at a magnification of 430,000 times, and then

enlarged further photographically to a final magnifi-

cation of 1,290,000 times. TEM measurements were done on

samples pre-oxidized at 900 K. This temperature was

chosen because, as TPO measurements will show, for some

samples this temperature is necessary to cause total

oxidation. Samples were prepared by applying a slurry of

the catalyst in alcohol onto a coal coated capper

and evaporating of the alcohol.

Metal loadings were established for the passivated

samples spectrophotometrically.

3.3. Results.

3.3.1. Hydragen Chemisorption.

The hydragen chemisorption data as given in Table 1 for

the RA series (Rh/Al2o 3 } are represented graphically in Fig. 1. In a similar way the data from Table 2, for the

43

Chapter 3

o.a

0.6

0.4

0.2

2 4 8 wt% Rh..._

s:: a:: ...... :I:

0.004

0.002

page

Figure 2. Hydragen chemisorption of Rh/Ti0 2 catalysts as a function of metal loading. T(red) is 473 k (LT) or 773 K (HT).

RT series (Rh/Ti0 2 ), are presented in 2. Fbr

Rh/Al 2o 3 , the H/Rh value drops below 1.0 somewhere around

5 wt % loading but H/Rh is still above 0.5 at 20 wt % (which catalyst had to be prepared via two successive

impregnation and drying steps). Fbr Rh/Tio 2 reduced at

523 K H/Rh decreases much faster with increasing metal

loading, and drops below 1.0 before the metal lóading

reaches 0.5 wt %. Of course we must keep in mind ~he 10 times larger surface area of the alumina, but it is

obvious that anatase is less capable of stabilizing small

rhodium particles than alumina. Fbr Rh/Ti02 reduced at

773 K, some hydrogen chemisorption is still measurable at

high metal loadings, but the measured values are of the

order of magnitude of experimental error.

The attentive reader will have noticed by now that the

catalysts must have been already in the SMSI state after

44

Chapter 3 page

the pre-reduetion at 773 K. The fact that they do show normal chemisorption behaviour after 523 K reduction implies that the passivation treatment they received has nullified SMSI.

The reduction-oxidation behaviour of a selected number of these catalysts, RA 2.3, 4.6 and 20.0 and RT 0.3, 1.0, 3.2 and 8.1, will be presented below.

3.3.2. TPR/TPO of the RA series.

The Temperature Programmed passivated RA catalysts

Reduction is shown

profile in Fig.

of 3.

the The

horizontal axis shows the temperature, the vertical axis hydro~en consumption in arbitrary units.

) ~ ~

473 673 873 T(K)~

Figure 3. Temperature Programmed Reduction ot passi-vated 2.3 wt% Rh/y- Al 03.

All three catalysts show a hydrogen consumption maximum below 300 K, followed by a slight desorption. Hydrogen consumption decreases from 1.33 H2/Rh for RA 2.3 (which is almost enough to account for the reduction of

stoechiometric Rh 20 3), to 0.49 H2/Rh for RA 20.0 (average

45

O'l.apter 3 page

oxidation state of the rhodium in this case was +1).

V r---r---r----.

lÎ r----. ---~ 1'"--..

--v- ! ['--._ 273 473 673 !173

T(K)....,_ Figure 4. Temperature Programmed Oxidation of reduced

Rh/y-Al 20 3 ; 2.3 wt% (a), 4.6 wt% (b) and 20.0 wt% (c).

The subsequent TPO profiles show more difference. For

all three catalysts Fig. 4a, b, c), oxygen consurnp-

tion starts at 223 K, that is in the switch peak. For RA

2.3 the oxygen consurnption rises at the beginning of the

temperature ramp, to reach a maximum at 300 K, and to

fall down slowly towards higher temperatures. Total

oxygen consumption mounts up to 0.65 o2/Rh. The behaviour of RA 20.0 (Fig. 4c) is quite different. After some

oxygen consumption i.n the switch peak, oxygen consumption

46

Chapter 3 page

keeps a low level for several hundreds degrees of

628 K. o2/ 4.6 was an

temperature raise, to reach a maximum at

rhodium is 0.70. The behaviour of RA

intermedia te one (Fig. 4b). Integration of the oxygen

consumption signal proved difficult, because of the small

therrnal conductivity of 0 2 , and due to the small sample

sizes (typically 50-75 micromale of metal).

lJ \ i'.. -273 473

i

i

673

i

673 T(K)__"._

Figure 5. TPR of oxidized 20.0 wt% Rh/y-Al 2 o 3 •

The TPR profile of the oxidized catalysts is shown in

Fig. 5, and is alike for all three of them. One sharp and

well-defined peak at about 340 K, with an H2/Rh value

ranging frorn 1.3 (RA 20.0) to 1.6 (RA 2.3). In all three

cases we assurne we have to do with the reduction of

supported Rh2o 3• Unsupported Rh203 in showed a reduction peak at 400 K, while

our apparatus

bulk rhodium

roetal only started to becorne bulk-oxidized above 870 K.

3.3.3. TPR/TPO of the RT series.

Fig. 6 shows the TPR profiles of the passivated

47

Chapter 3

) ( ~

\

\

273

I

473 673 873 T(K)~

page

Figure 6. TPR of passivated Rh/Ti0 2 ; 0.3 wt% (a), 1.0 wt% (b), 3.2 wt% (c) and 8.1 wt% (d).

48

Chapter 3 page

catalysts RT 0.3, RT 1.0, RT 3.2 and RT 8.1. With

increasing rnetal loading, in other words decreasing H/Rh

values, we see how the reduction peak disappears into the

switch peak at 223 K. Fbr RT 0.3 the reduction peak is

still cornpletely separated from the switch peak, with a

maximum at 273 K and an H2 / rhodium of 0.56, for RT 8.1

the reduction peak has merged with the switch peak and

the hydragen consumption has d~opped to H2/Rh =0.25. We

believe the explanation for this lies in the fact that

--~

/ I-

--- - -----""' 1'---

_.-/ ~ L

273 473 673 873 T(K)~

Figure 7. TPO of reduced Rh/Ti0 2 ; 0.3 wt% (a), 1.0 wt% (b), 3.2 wt% (c) and 8.1 wt% (d).

49

Chapter 3 page

with decreasing H/Rh (and for RT 1.0 the H/Rh ratio is

nat more than 0.41, still lower than for RA 20.0) the

metal particles keep a metallic care

Hydragen molecules can diffuse

upon passivation.

through the outer

passivated layer of the particle, reach the metallic care

and dissociate there to provide atomie hydragen for an

easy reduction of the oxide layer at low temperature.

We find a support for this idea in Fig. 7 showing the

TPO profiles of the respective catalysts. All catalysts

show some oxygen consumption as soon as the temperature

ramp has been started. The two low-loaded catalysts, RT

0. 3 and RT l. 0 ( 7a and 7b) have an ear 1 y consumption

maximum around 300 K, likeRA 2.3, but also show distinct consumption around 600 K, while RA 20.0 had a maximum

around 620 K. In Fig. 7c, RT 3.2, we can distinguish

three areas of oxygen consumption apart from the switch

peak: Around 350 K, 600 K and a new maximum at 770 K.

This consumption at 770 K is dominant in Fig. 7d, the TPO

of the reduced RT 8.1 catalyst. At first glance we can

attribute the low temperature oxygen consumption to

chemisorption and the high temperature oxygen consumption

to thorough oxidation, but we will come back to this in

the Discussion. Anyway, it is this last observation, the

high temperature needed for thorough oxidation, that

confirms the presence of a metallic care in larger

particles after passivation. Oz/Rh is abaut 0.7 for all

RT catalysts. In Fig. 8 we see

catalysts, and a

here. RT 0.3 and RT

maximum, 330-340

the TPR profiles of the oxidized

very interesting phenomenon shows up

1.0 show only one clear consumption

K, Hz/Rh=l.20, like the RA catalysts

(Fig. 5). But RT 3.2 and RT 8.1 have a secend consumption

maximum at 385-400 K, abaut the temperature where

unsupported bulk Rh2 0 3 reduces. Taken bath peaks

together, H2/Rh is about 1.70 for RT 3.2 and 1.41 for RT

8.1, so bath peaks can without any doubt be ascribed to

50

Chapter 3

(\

_)

""' _.... 1----v---

lJ" v--... -

\..

v \ ----...._

_./ 1--..__

11

'--IJ ) \

273 473 673

Figure 8. TPR of oxidized Rh/Ti0 2 ; 0.3 wt wt% ( b) , 3. 2 wt% ( c) and 8. 1 wt%

873 T(K)....,_

page

(a), 1. 0 (d).

51

Chapter 3 page

the reduction of Rh2o 3. To be more specific, the low temperature TPR peak must belang to well-dispersed, ill-

defined Rh 2o 3, which is also easily formed, while the high temperature TPR peak belongs to the reduction of

bulklike, crystalline Rh2o 3, which can only be formed by the high temperature oxidation. This was proven earlier

(32) by halting a TPO experiment at a temperature

intermediate between the two maxima at 600 and 770 K. A

subsequent TPR proved to miss the second reduction peak,

so the 770 K TPO peak is connected to the 400 K TPR peak,

and the low temperature TPR and TPO peaks must, as a

consequence, be connected too.

I ./ \ ----- !'---. 273 473 673 873

I

I v--~ ~ --373 S73 773 973

T (K)......_

Figure 9. 1.0 wt Rh/Ti0 2 , wet reduced; TPR of oxidized system (a) and TPO of reduced system (b). Note the temperature axis in TPO.

52

Chapter 3 page

In all cases, af ter a slight desorption of H2, H2 conswnption rises again above baseline, shawing two maxima around 600 and 740 K. We think this must be attributed to reduction of the support in the neighbourhood of the metal; tli.e bare support does not reduce below 800 K.

3.3.4. TEM measurements.

To make the difference between the R~o3 reducible at 330 K and the Rh 2o 3 reducible at 400 K more clear, we

reduced part of the impregnated RT 1.0 batch at 773 K

without drying, a method already used by Burwell c.s.

(24) to prepare catalysts with law dispersions. After

oxidation at 900 K for 1 hr we performed a TPR, shown in

Fig. 9a. One single reduction peak for Rh203, at 410 K

was observed. The subsequent TPO, shown in Fig. 9b, run

this time up to 973 K, shows that oxidation proceeds

readily only at about 900 K. The H/Rh value of the catalyst, after the 900 K oxidation and a 523 K reduction in situ, was 0.20.

We examined this

comparable RT 1.0

catalyst,

(Fig.

RT

8b,

Transmission Electron Microscopy

prepared by applying a slurry of

l.O(wet), and the

7b) catalyst with

(TEM). Samples we re

the at 900 K oxidized

catalysts in absolute alcohol on a coal coated grid, and

evaporating the alcohol. TEM micrographs are shown in

Fig. 10. Fig. lOa shows the bare support, anatase, as

received from the manufacturer. Clearly visible is that

the Ti0 2 particles (in the order of magnitude of 50 nm in

diameter) are covered with very tiny seed-crystals of

TiC2, which apparently, due to the poor sintering

capacities of these kinds of oxides, did not get the

chance to grow into larger particles. In Fig. lOb (RT

1.0) we find them back as conglomerates (clustered

together in the impregnation and drying steps of the

53

Chapter 3

1oo.&

F i gure 10 a. T r a n sm i ss io n e lectro n mi cr o rap h of anatase T i 0 2

page 54

Chapter 3 page

Figure 10 b. Transmission electron micrographo f 1. 0 wt% Rh z03/ Ti0 2 , H/ Rh = 0.4

55

Olapter 3 page

Figure 10 c. Transmi s sion electron mLcrograph of 1. 0 wt% Rh 2 0JITi0 2 , H/Rh = 0 . 2

56

3 page

on the Ti02 surface show the uniform density

and srnooth outline characteristic for heavy roetal oxide

, diameter ranging frorn 1-6 nrn. The uniform

colour of the particles is an indication that they are

actually flat (raftlike). In Fig. lOc (RT l.Owet) the

Ti02 conglornerates are no langer visible. Apparently they

got the chance during the high ternperature reduction in

the presence of water to spread over the surface of the

large particles. The particles we do see, are Rh2o3 . The lighter contours of the particles indicate

that they are spherical. Their diameter is about 7 nrn.

3.4. Discussion.

Hydragen chernisorption has been used through the years

by rnany workers to characterizernetal surfaces (10, 12,

13, 17, 19, 23-26, 33, 34) and when atternpts were made to

ca1culate roetal surface areas frorn hydragen chernisorption

data, a hydragen-metal stoechiornetry of one was a1ways

used. On the other hand, if CO was involved, sorne authors

(35) chose a stoechiornetry of one, while others (18)

preferred stoechiornetries. Frorn Figs. 1 and 2 it

becornes clear though, that a1so H/Rh values can exceed

unity. Fbr Rh/Ti02 this occurs for roetal loadings below

0.5 %, and for Rh/Al2o 3 even for roetal loadings up to 5 %. Although in this study only two cata1ysts occur with

an H/Rh va1ue above unity, we have reported about other

ultra-dispersed systerns elsewhere (32, 36). And in our

opinion, if one accepts a roetal atorn, such as rhodium, to

adsorb two or more CO molecules, one should shou1d not

reject the idea of that same atorn adsorbing more than one

hydragen atorn. So we think that a1so in those cases where

H/Rh exceeds unity, the hydragen chernisorbed does not

exceed a rnono1ayer (cf. Frennet (31)) and is all bound to

the rnetal. A consequence of this is the irnpossibility to

57

Chapter 3

calculate a partiele size

highly dispersed systems,

page

fram chemisorption data for

since there is no way of

calculating the number of rhodium surface atoms. Fbr

larger, well defined particles as w found in RT l.O(wet),

with an H/Rh of 0.2, the calculated partiele size is 6 nrn

which is in good accordance with our TEM measurements

(Fig. lOc).

The H/Rh values measured for Rh/Ti02 after reduction at

773 K (Fig. 2), that is with rhodium in the SMSI state,

seem to show a tendency to increase, but the values

measured are still that small that we do not want to draw

any conclusions frorn that without further study.

Fram all TPR/TPO measurements presented here the

following picture comes up. Rhodium on a support can

occur in either a dispersed farm, or in a bulklike farm.

The dispersed farm is easy to reduce to the metal, giving

a reduction peak in TPR around 340 K. It is also easily

oxidized, which manifests itself in TPO's such as Figs.

4a, 7a and 7b, and from the fact that upon passivatien

the oxidation state of the Rh can almast reach +3 (cf.

Fig. 3, H2/Rh=l.33). That reaction with oxygen is

vehement, even at room temperature, shows also in the

fact that prolonged passivatien breaks SMSI (if not, one

would not have seen any hydragen consumption at all after

the low temperature reduction, since the RT catalysts

would still be in the SMSI state induced by the 773 ~

pre-reduction). The exact assignrnent of the three TPO

peaks is difficult since the literature does not provide

much material about the mechanism of oxidation of

support

Chapter 3 page

follawing logarithmic rate equations, and leading to an

oxide film with a thickness from 2 (38) to 100 (39, 40)

rum. Finally, Wagners oxidation theory (41) describes how

the oxidation process goes on, rate-controlled by volume

diffusion of the reacting ions and/or of electrens

through the growing oxide scale, leading to parabalie

rate equations. We can imagine that analogous models can

describe the oxidation processes in supported metal

particles as well, dependent on their sizes.

It is very clear that no matter how the rhodium is

present on the support, dispersed or not, oxidized or

passivated, reduction to metallic rhodium is complete at

400 K, that is