isokinetic behavior in the gas phase hydrogenation of nitroarenes over au/tio2: application of the...

Isokinetic behavior in the gas phase hydrogenationof nitroarenes over Au/TiO2: application of the selectiveenergy transfer model

Mark A. Keane • Ragnar Larsson

Received: 1 February 2012 / Accepted: 13 March 2012 / Published online: 10 April 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract The gas phase selective hydrogenation of a series of nitroarenes

(nitrobenzene, p-chloronitrobenzene, p-bromonitrobenzene, p-nitroaniline, p-nitro-

toluene, p-nitrophenol and p-nitroanisole) has been examined over Au/TiO2 (0.3 %

w/w Au, mean Au particle size = 3.9 nm). Compensation behavior is demonstrated

with an associated isokinetic temperature (Tiso) of 558 ± 32 K. We account for this

response in terms of the selective energy transfer (SET) model where the occurrence

of resonance between catalyst and reactant vibrations generates the activated

complex. An analysis of the stepwise variation of the activation energies has

identified a critical vibrational frequency of 853 cm-1, which is close (±2 cm-1) to

the reference value for nitro-group (in-plane symmetric O–N–O bending and

stretching) vibrations. Application of SET suggests activation of weakly adsorbed

nitroarene (at the support or metal/support interface) by excitation of the nitro-group

via IR radiation from a strongly adsorbed surface nitroarene component. The

excited nitroarene is then attacked by reactive hydrogen supplied by the Au sites to

generate the respective aromatic amine with 100 % selectivity. Agreement of the

SET predicted Tiso with the experimental value requires the incorporation of a term

due to C–N torsional entropy resulting from distortion of the O–N–O plane.

Keywords Isokinetic temperature � Compensation behavior � Selective energy

transfer model � Catalytic hydrogenation � Nitroarenes � Au/TiO2

M. A. Keane (&)

Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University,

Edinburgh EH14 4AS, Scotland, UK

e-mail: [email protected]

R. Larsson

Chemical Engineering II, University of Lund, P O Box 124, 221 00 Lund, Sweden

123

Reac Kinet Mech Cat (2012) 106:267–288

DOI 10.1007/s11144-012-0440-6

Introduction

Two phenomena, namely ‘‘compensation’’ and ‘‘isokinetic’’ behavior, in heteroge-

neous catalysis continue to attract significant research activity. The compensation

effect is manifested by a linear relationship between ln A (the Arrhenius pre-

exponential factor) and Eexp (the experimentally determined activation energy) [1,

2]. A value for the isokinetic temperature (Tiso) can be extracted from the resultant

compensation plot. The term ‘‘compensation’’ has been introduced to reconcile the

seemingly paradoxical situation where an increase in activation energy is

accompanied by an increase in the pre-exponential factor [1–3]. Compensation

phenomena in heterogeneous catalysis are observed for experimental measurements

involving a series of related reactions or catalysts and are often associated with

structure sensitive systems [2]. The isokinetic response arises where the Arrhenius

plots (ln k vs. 1/T) intersect at a common temperature. Isokinetic and compensation

effects are by no means synonymous and the establishment of a compensation

response does not necessarily mean that an isokinetic relationship is operative.

While the compensation effect can be regarded as an empirical relationship,

isokinetic behavior may be reconciled on the basis of chemical theory [3, 4]. In

earlier publications, we established an isokinetic response for the catalytic

hydrodehalogenation [5] and hydrodeoxygenation [6] of a series of benzene

derivatives over a common (Ni/silica) catalyst and hydrodechlorination of

chlorobenzene promoted by a family of carbon and oxide supported Ni catalysts

[7, 8]. We have demonstrated that the experimentally determined Tiso can be

accounted for using the selective energy transfer (SET) model [9]. The basis for the

SET model is the occurrence of a state of resonance between a vibrational mode of

the catalyst system and a complementary vibrational mode of the reacting molecule.

Our SET analysis [5–8] demonstrated resonance between the catalytic Ni–H

vibration and out-of plane C–H vibrations of the aromatic reactants with a

transferral of resonance energy from the catalyst to generate the ‘‘activated

complex’’. In every case, the Tiso predicted from the SET model was very close to

the value determined experimentally. We have now extended that work and report

here an SET analysis of the catalytic hydrogenation of a range of substituted

nitroarenes over Au/TiO2.

Aromatic amines are extensively used as intermediates in the manufacture of

pesticides, herbicides, pigments, pharmaceuticals and cosmetic products [10, 11].

Existing synthesis routes, involving Fe promoted reduction in acid media (Bechamp

reaction) or liquid phase hydrogenation using transition-metal catalysts, fall short in

terms of sustainable processing due to the generation of significant amounts of toxic

waste and low overall product yields [12, 13]. Reaction selectivity is critical and the

formation of azo- and azoxy-benzene by-products must be avoided to ensure clean

production of the target amine [14]. Moreover, a high selectivity in terms of –NO2

group reduction is difficult to achieve in the presence of other reactive substituents

(e.g., –Cl, –CH3 and/or –OH), as has been noted for the hydrogenation of aliphatic

[15] and aromatic [16] nitro-compounds in both gas [17, 18] and liquid [19, 20]

phase operation. We have previously demonstrated exclusive –NO2 group reduction

in gas phase hydrogenation over supported Au [17, 21–25]. In this paper, we

268 M. A. Keane, R. Larsson

123

examine the catalytic action of Au/TiO2 to promote the selective hydrogenation of a

series of substituted nitroarenes, establishing an isokinetic response, which we

relate, using the SET approach, to the critical reactant/catalyst interaction that leads

to reaction.

Experimental

Materials and catalyst preparation

The TiO2 (Degussa, P25) support was used as received. A 0.3 % w/w supported Au

catalyst was prepared by standard impregnation of the TiO2 support with a HAuCl4(Aldrich, 25 9 10-3 g cm-3, pH 2) solution. The slurry was heated (2 K min-1) to

353 K and maintained under constant agitation (600 rpm) in an ultra pure ([99.99 %,

BOC) He purge. The solid residue was dried in a flow of He at 383 K for 3 h, sieved

into a batch of 75 lm average diameter and stored under He in the dark at 277 K. Prior

to use in catalysis, the samples were activated in 60 cm3 min-1 H2 at 2 K min-1 to

623 K, which was maintained for 1 h. After activation, the catalyst was passivated in

1 % v/v O2/He at 298 K for off-line analysis.

Characterization analyses

The Au content (accurate to within ±2 %) was measured by inductively coupled

plasma-optical emission spectrometry (ICP-OES, Vista-PRO, Varian Inc.) from the

diluted extract of aqua regia. Temperature programmed reduction (TPR) analysis

reported elsewhere [21] has established that the precursor was reduced to zero

valent Au post activation at 623 K. BET area (reproducible to within ±3 %) was

measured in a 30 % v/v N2/He flow using the commercial CHEMBET 3000

(Quantachrome Instrument) unit where N2 (99.9 %) served as the internal standard.

A powder X-ray diffractogram was recorded on a Bruker/Siemens D500 incident X-

ray diffractometer using Cu Ka radiation. The sample was scanned at a rate of

0.02� step-1 over the range 20� B 2h B 90� (scan time = 5 s step-1) and the

diffractogram identified using the JCPDS-ICDD reference standards, i.e., anatase

(21-1272), rutile (21-1276) and Au (04-0784). The rutile:anatase ratio was

determined by XRD according to the method described by Fu et al. [26], i.e.,

% Rutile ¼ 1

½ðA=RÞ � 0:884þ 1� � 100 ð1Þ

where A and R represent the peak areas associated with the main reflections for

anatase (2h = 25.3�) and rutile (2h = 27.4�), respectively. Transmission electron

microscopy analysis employed a JEOL JEM 2011 HRTEM unit with a UTW energy

dispersive X-ray detector (EDX) detector (Oxford Instruments) operated at an

accelerating voltage of 200 kV, using Gatan DigitalMicrograph 3.4 for data

acquisition/manipulation. Samples for analysis were prepared by dispersion in

acetone and deposited on a holey carbon/Cu grid (300 Mesh). A total Au particle

Isokinetic behavior in the gas phase hydrogenation of nitroarenes 269

123

count of 720 was used to obtain the surface area-weighted mean Au diameter (dp),

which was calculated from

dp ¼P

i nid3iP

i nid2i

ð2Þ

where ni is the number of particles of diameter di. The size limit for the detection of

gold particles on TiO2 is ca. 1 nm.

Catalytic procedure

Reactions were carried out under atmospheric pressure, in situ immediately after

activation, in a fixed bed vertically mounted continuous flow glass reactor

(l = 600 mm, i.d. = 15 mm) over the temperature range 393 K B T B 573 K.

The catalytic reactor, and operating conditions to ensure negligible heat/mass

transport limitations, have been fully described elsewhere [27] but some features,

pertinent to this study, are given below. A preheating zone (layer of borosilicate

glass beads) ensured that the nitroarene reactants were vaporized and reached the

reaction temperature before contacting the catalyst. Isothermal conditions (±1 K)

were maintained by thoroughly mixing the catalyst with ground glass (75 lm).

The temperature was continuously monitored by a thermocouple inserted in a

thermowell within the catalyst bed. The nitroarene (as a butanolic solution) was

delivered, in a co-current flow of H2 (99.999 %), via a glass/Teflon air-tight

syringe and a Teflon line, using a microprocessor controlled infusion pump

(Model 100 kd Scientific) at a fixed calibrated flow rate, with an inlet –NO2 molar

flow over the range 0.01–0.11 mmol�NO2h�1, where the molar Au to inlet molar

–NO2 feed rate ratio spanned the interval 2 9 10-2–27 9 10-2 h. The H2 content

was at least 150 times in excess of the stoichiometric requirement, the flow rate of

which was monitored using a Humonics (Model 520) digital flowmeter;

GHSV = 2 9 104 h-1. The reactor effluent was frozen in a liquid nitrogen trap

for subsequent analysis, which was made using a Perkin-Elmer Auto System XL

gas chromatograph equipped with a programmed split/splitless injector and a

flame ionization detector, employing a DB-1 50 m 9 0.20 mm i.d., 0.33 lm film

thickness capillary column (J&W Scientific), as described elsewhere [28]. All the

nitroarene reactants were supplied by Sigma-Aldrich (C98 %) and used without

further purification. Adherence to pseudo-first order reaction kinetics has been

demonstrated elsewhere [17, 21, 22]. In a series of blank tests, passage of each

nitroarene in a stream of H2 through the empty reactor or over the support alone,

i.e., in the absence of Au, did not result in any detectable conversion. The specific

rate constants extracted from a pseudo-first order treatment are given in Table 1

for each nitroarene reactant. All the data presented have been generated in the

absence of any significant catalyst deactivation where each catalytic run was

repeated (up to five times) using different samples from the same batch of

catalyst: the measured rates did not deviate by more than ±5 %.


123

Table 1 Collation of the experimental kinetic data

Reactant 103/T (K-1) Ln kexp (mol mAu-2 h-1) Eexp (kJ mol-1) Ln A

Nitrobenzene 2.545 -12.92 72.37 9.291

2.481 -12.34

2.364 -11.33

2.283 -10.53

2.208 -9.81

2.114 -9.02

2.049 -8.54

1.988 -8.02

1.912 -7.50

1.876 -7.11

1.825 -6.77

1.776 -6.20

1.745 -5.70

p-Chloronitrobenzene 2.545 -11.00 52.10 4.857

2.481 -10.63

2.364 -10.12

2.283 -9.55

2.208 -9.01

2.114 -8.37

2.049 -7.80

1.988 -7.46

1.912 -7.18

1.876 -7.03

1.825 -6.79

1.776 -6.27

1.745 -5.89

p-Nitroaniline 2.364 -16.62 156.74 27.990

2.283 -15.17

2.208 -13.70

2.114 -11.67

2.049 -10.59

1.988 -9.45

1.912 -8.13

1.876 -7.10

1.825 -6.41

1.776 -5.66

1.745 -5.04

p-Bromonitrobenzene 2.481 -10.00 47.08 3.936

2.364 -9.47

2.283 -9.03

2.208 -8.67


123

Table 1 continued

Reactant 103/T (K-1) Ln kexp (mol mAu-2 h-1) Eexp (kJ mol-1) Ln A

2.114 -8.11

2.049 -7.55

1.988 -7.22

1.912 -6.99

1.876 -6.80

1.825 -6.45

1.776 -6.01

1.745 -5.88

p-Nitrotoluene 2.421 -13.13 91.86 13.731

2.364 -12.54

2.283 -11.39

2.208 -10.52

2.114 -9.52

2.049 -8.80

1.988 -8.19

1.912 -7.43

1.876 -7.15

1.825 -6.56

1.776 -5.89

1.745 -5.48

p-Nitrophenol 2.364 -14.44 120.60 19.832

2.283 -13.37

2.208 -12.33

2.114 -10.84

2.049 -9.88

1.988 -8.83

1.912 -7.91

1.876 -7.22

1.825 -6.80

1.776 -6.10

1.745 -5.40

p-Nitroanisole 2.364 -13.45 102.58 15.758

2.283 -12.45

2.208 -11.60

2.114 -10.12

2.049 -9.37

1.988 -8.76

1.912 -8.00

1.876 -7.30

1.825 -6.88

1.776 -6.16

1.745 -5.75


123

Results and discussion

Catalyst characteristics

The Au loading, BET surface area, support (TiO2) composition and Au dispersion/

particle diameter/surface area are given in Table 2. XRD analysis is consistent with

a mixture of anatase and rutile forms of TiO2 where anatase:rutile = 5:1, which is

characteristic of Degussa P25 [29] for thermal treatment up to 923 K [30]. The

representative TEM images provided in Fig. 1 serve to illustrate the nature of the

metal dispersion where the Au particles exhibit a pseudo-spherical morphology,

suggesting a small area of contact at the interface between the metal crystallites and

the TiO2 support. This is in agreement with published literature [31] demonstrating

that Au/TiO2 prepared by impregnation is characterised by a metal-support contact

angle [90�. The catalyst is characterized by a narrow (\1–8 nm) Au particle size

distribution, as shown by the histogram in Fig. 1.

Analysis of the experimental nitroarene hydrogenation data

The gas phase hydrogenation of nitrobenzene and substituted nitrobenzenes

bearing –Cl, –NH2, –Br, –CH3, –OH and –OCH3 in the para-position over Au/

TiO2 was 100 % selective in generating the corresponding amine product. Such

reaction exclusivity is unique when compared with catalytic systems tested to

date [19, 32] and represents a critical advancement in the clean production of

aromatic amines. The published work has focused on liquid phase batch

hydrogenation processes where the use of solvents/hydrogen donors/acid–base

promoters and by-product (toxic azo and azoxy-benzenes) formation necessitates

multiple downstream separation and treatment units [33–36]. A move from

inefficient batch to continuous operation has now been highlighted by the

pharmaceutical/fine chemical sector as the #1 priority to ensure sustainable

manufacture through process intensification [37]. Moreover, economies of scale

favor continuous processes for large throughput. The raw kinetic data given in

Table 1 are presented as a conventional compensation plot, i.e., ln A versus the

Table 2 Au loading, BET surface area, support composition and metal phase characteristics associated

with the activated Au/TiO2 catalyst

Au loading (% w/w) 0.3

BET surface area (m2 g-1) 51

TiO2 composition (%)a 84 anatase, 16 rutile

Au particle size (dp, nm)b 3.9

Au particle size range (nm) \1–8

Au dispersion (%) 26

Au surface area (m2 gNi-1) 82

a Based on XRD analysis (see Eq. 1)b Surface area weighted mean size (see Eq. 2)


123

experimentally determined activation energy (Eexp), in Fig. 2. The linear

relationship is consistent with compensation behavior where Tiso can be extracted

from the slope (0.219 mol kJ-1) according to [2]

5 nm

2 nm

2 nm

<1

1.1-

2.0

2.1-

3.0

3.1-

4.0

4.1-

5.0

5.1-

6.0

6.1-

7.0

7.1-

8.0

0

20

40

% A

u p

arti

cles

in r

ang

e

Au particle diameter (nm)

Fig. 1 Representative TEM images of Au/TiO2 and associated Au particle size distribution


123

Tiso ¼ 1=R slopeð Þ ð3Þ

and equals 548 K. Applying the approach recommended by Linert and Jameson [4],

i.e., obtaining Tiso from the intersection of the Arrhenius lines associated with the

raw experimental data, it can be seen from the entries in Fig. 3 that there is a

0

10

20

30

ln A

40 80 120 160

Eexp kJ mol-1

Fig. 2 Compensation plot for the hydrogenation of nitrobenzene (closed circle), p-chloronitrobenzene(open square), p-nitroaniline (closed diamond), p-bromonitrobenzene (open circle), p-nitrotoluene(closed inverted triangle), p-nitrophenol (open triangle) and p-nitroanisole (closed square) over Au/TiO2

1.6

-10

-12

-8

-6

-4

-14

-16

-181.8 2.0 2.2 2.4 2.6

1000/T K-1

ln k

exp

mol

mA

u-2

h-1

Fig. 3 Arrhenius plots for the hydrogenation of nitrobenzene (closed circle), p-chloronitrobenzene (opensquare), p-nitroaniline (closed diamond), p-bromonitrobenzene (open circle), p-nitrotoluene (closedinverted triangle), p-nitrophenol (open triangle) and p-nitroanisole (closed square) over Au/TiO2


123

common intersection point where 1000/T is ca. 1.8 to give a Tiso of ca. 556 K, which

is close to that obtained from the slope of the Compensation plot (Fig. 2). A more

detailed analysis of the seven Arrhenius plots given in Fig. 3 reveals a total of 21

intersection points which, taking the mean and standard deviation, gives a Tiso of

558 ± 32 K (see Table 3), that we adopt from this point onwards as a basis for

model development. The analysis thus far has established that the hydrogenation of

the seven nitroarene reactants over a common Au/TiO2 catalyst exhibits both

conventional compensation behavior and an isokinetic response.

Application of the SET model

The SET model considers the existence of resonance between a vibrational mode of

the catalyst and a vibrational mode of the reacting molecule, which transforms the

reactant towards the ‘‘activated state’’ [9]. Where m represents the wave number of

the vibration mode of the reactant and x is the wave number of the energy source

(the catalyst), it is possible to calculate Tiso according to

Table 3 Determination of Tiso from the abscissa associated with the point of intersection of two

Arrhenius lines (a;b) and associated value of ln kiso; see Fig. 3

Reactant Equation of the linear fit a;b 1000/Tiso

(K-1)

Tiso (K) Ln kiso

(mol mAu-2 h-1)

Nitrobenzene (1) y = 9.273 - 8.6998x 1;2 1.816 550.7 -6.526

p-Chloronitrobenzene (2) y = 4.8466 - 6.2617x 1;3 1.843 542.6 -6.761

p-Nitroaniline (3) y = 27.967 - 18.843x 1;4 1.757 569.2 -6.012

p-Bromonitrobenzene (4) y = 3.0330 - 5.6613x 1;5 1.898 526.9 -7.239

p-Nitrotoluene (5) y = 13.729 - 11.048x 1;6 1.818 550.1 -6.543

p-Nitrophenol (6) y = 19.912 - 14.551x 1;7 1.782 561.2 -6.230

p-Nitroanisole (7) y = 15.751 - 12.335x 2;3 1.838 544.1 -6.662

2;4 1.519 658.3 -4.665

2;5 1.856 538.8 -6.800

2;6 1.817 550.4 -6.530

2;7 1.795 557.1 -6.393

3;4 1.823 548.5 -6.384

3;5 1.827 547.3 -6.459

3;6 1.878 532.5 -7.420

3;7 1.877 532.8 -7.401

4;5 1.818 550.1 -6.358

4;6 1.798 556.2 -6.245

4;7 1.771 564.7 -6.092

5;6 1.765 566.6 -5.771

5;7 1.571 636.5 -3.627

6;7 1.878 532.5 -7.415

Mean Tiso (K) 558 ± 32

Wave number (cm-1) 776 ± 45


123

Tiso ¼ NhcR�1 m2 � x2� �

x�1 �p=2� arctg 0:5mx m2 � x2� �� 1

n o�1

ð4Þ

where N represents the Avogadro number, c the velocity of light, h the Planck

constant and R the gas constant. As the catalyst in effect donates energy to attain the

activated state of the reacting molecule, the expression ‘‘heat bath’’ has been applied

to the donating system [4]. At the maximum efficiency of resonance energy transfer,

x = m [9] and

Table 4 Nitrobenzene vibration frequencies taken from the compilation by Shlyapochnikov et al. [38]

IR frequency cm-1 Symmetry type (potential energy distribution %)

1095 m B2 (63) CC str. ? (35) H as. bend.

1068 m B2 (60) CC str. ? (38) H as. bend.

1022 m B1 (62) H o.o.p. ? (23) ring tors. A

1004 vw A1 (65) CC str. ? (20) H. as. bend. ? (13) ring i.p. def. A

977 B1 (83) H o.o.p. ? (13) ring tors. B

853 s A1 (5) ONO bend. ? (16) NO2 str. ? (13) CC str.

793 s B1 (50) H o.o.p. ? (29) NO2 o.o.p. ? (18) CN o.o.p.

704 vs B1 (54) NO2 o.o.p. ? (43) H o.o.p. ? (2) CN o.o.p.

681 s A1 (49) ring i.p. def. ? (23) ONO bend. ? (12) CN str.

675 s B1 (63) ring tors. ? (27) H o.o.p.

612 vw B2 (88) ring i.p. def.

532 w B2 (63) NO2 as. bend. ? (15) CN as. Bend ? (15) CC str.

Only wave numbers between 1,100 and 500 cm-1 are included and the assignments are those used by the

authors in [38]

m medium strength, vw very weak, s strong, vs very strong, w weak, str. stretching, as. antisymmetric,

bend. bending, o.o.p. out-of-plane, tors. torsional, i.p. in plane, def. deformation

Table 5 Infrared frequencies

reported for a vibration in the

843–872 cm-1 range for the

seven nitroarene reactants

Reactant IR frequency (cm-1) References

Nitrobenzene 853 [38]

852 [39]

p-Chloronitrobenzene 850 [40]

855 [41]

p-Nitroaniline 846 [42]

843 [43]

p-Bromonitrobenzene 852 [40]

851 [41]

p-Nitrotoluene 859 [41]

p-Nitrophenol 872 [44, 45]

p-Nitroanisole 853 [46]

Mean value/RMS 853 ± 2


123

Tiso ¼ Nhc=2Rm ¼ 0:719m ð5Þ

Using the Tiso reported above (558 K), the calculated value of the reactant vibra-

tional mode (m) is then 776 cm-1. From an overview of the available data [38] on

the vibrational spectra of nitrobenzene (Table 4), there is no recorded vibration at

this wave number. However, we can flag the signal at 853 cm-1, which is common

(mean = 853 ± 2 cm-1, see Table 5) to the nitroarene reactants considered in this

study [38–46]. Vibrations at this wave number have a high absorption intensity and

can be related to O–N–O bending and NO2 stretching. Both atomic motions are

associated with substitution reactions at the O–N–O moiety, as discussed below (see

‘‘Resonance effects’’ and ‘‘Mechanistic considerations’’ sections). Setting

x = 853 cm-1, application of Eq. 4 delivers the Tiso dependence on m shown in

Fig. 4. At this juncture, it should be noted that the minimum value Tiso = 613 K

(corresponding to m = 853 cm-1) is higher than the experimentally determined Tiso

(=558 ± 32 K).

Resonance effects

The very essence of the SET model as applied to catalysis is that there is a set of

‘‘resonators’’ belonging to the catalyst with a frequency that is close to a critical

vibration of the reacting molecule. Taking m = 853 cm-1 as the putative reactant

vibration, it is interesting to note that TiO2 (the support used in this investigation)

has been reported to exhibit an IR wave number at 850 cm-1 (in the case of anatase

[47]) or 806 cm-1 (in the case of rutile [48]). As the support used has an 80 %

anatase content (Table 2), the 850 cm-1 value is the more appropriate and suggests

the possibility of full resonance with the 853 cm-1 O–N–O bending/stretching

500

600

700

800

900

Tis

o K

600 800 1000 1200400

ν cm-1

Fig. 4 Calculated Tiso from Eq. 4 as a function of the frequency of the vibration of the reacting molecule(m) for an assumed value of the heat bath frequency = 853 cm-1


123

vibration. It should also be noted that neutron diffraction studies on rutile [49] have

generated a frequency value of 25.24 ± 0.34 9 1012 cps (841 cm-1), which is

close that recorded for anatase from IR data. These findings point to an activation of

the nitroarene reactant on the support with the donation of (resonance-)energy to

activate the O–N–O moiety. Corma et al. [50] have demonstrated, by in situ FTIR

analysis of Au/TiO2, that nitrobenzene can adsorb on both the metal and support

where the interaction is weaker on metallic gold relative to the TiO2 surface but is

enhanced on highly uncoordinated Au atoms [51]. Syomin et al. [52] have studied

the adsorption of nitrobenzene on Au(111) and recorded a 853 cm-1 adsorption

from 4 up to 100 monolayers. Blaser et al. [32], taking an overview of the available

literature, have proposed –NO2 activation at the Au–TiO2 interface to account for

accelerated selective nitro-group hydrogenation. From a consideration of heat of

adsorption, Corma and co-workers [51] have suggested that strong –NO2 interaction

with Ti (42 kcal mol-1) generates surface molecules that act as ‘‘spectators’’ and

are not reacted. We propose here that the strongly adsorbed nitroarene interacts with

the support leading to (near) resonance with the anatase/rutile vibration and the

853 cm-1 vibration of the adsorbate. Consequently, energy is transferred from the

TiO2 support to the adsorbate. A further transferral of resonance energy by IR

radiation from the adsorbed nitrobenzene to the 853 cm-1 vibration of those

reactant molecules that are not strongly adsorbed will serve activate the NO2 group,

which is then attacked by hydrogen activated at Au sites. While the nature of H2-Au

interaction is still not well understood, the consensus that emerges suggests a greater

facility for hydrogen chemisorption on smaller Au particles (B10 nm) [53, 54] that

bear a higher number of defects, i.e., edges and corners [55, 56], with a

consequential increase in specific hydrogenation rates for smaller particle sizes [57,

58]. From a consideration of the Au particle size histogram presented in Fig. 1, it

can be noted that the supported Au phase is in the nano-size range (\10 nm) that is

critical for catalytic activity in hydrogen mediated processes. The hydrogenation of

nitroarenes has been proposed to proceed via a nucleophilic mechanism [17, 20, 59],

where a weak nucleophilic agent (hydrogen) reacts with the activated –NO2 group.

There is evidence [60, 61], based on EELS analysis, for an induced dipole that

results from H2 adsorption on Au. Our SET analysis identifies the symmetric O–N–

O bending/stretching vibration as critical where an increase in the positive charge

on N and the N–O bond length can render the NO2 substituent more susceptible to

hydrogen attack.

Analysis of the activation energies

In the SET treatment, the activation energy (Eexp) can be quantized in that a specific

number of vibrational quanta must be transferred from the catalyst to the reactant in

order to arrive at the transition state [62, 63]. The enthalpy of activation (DH#) is

then the sum of these quanta. In earlier applications of the SET model [7, 8], we

established a stepwise variation in the experimental activation energy with one

common least term defining the step. Taking the approach applied previously, we

first estimate this least common term from the differences in the series of

experimental activation energies (Eexp), including a correction for RTmean where


123

Tmean represents the mean temperature for the dataset: RTmean = 4.02 kJ mol-1 in

this case. The RTmean term accounts for the influence of a pre-exponential factor that

is proportional to T, e.g., kT/h in the Eyring equation. The consecutive differences

between the Eexp - RTmean values are given in Table 6 where this series can be

expressed as a multiple of a least common term [64]. Taking 10 kJ mol-1 as a first

approximation for this common term, each entry in Table 6 (column 4) was divided

by this estimated value and the numerical result taken to the nearest whole number,

which we denote as n0 A more precise value of the common term (E0) can be

obtained [8] from

E0 ¼X

D Eexp � RTmean

� �=X

n0 ð6Þ

and equals 10.18 kJ mol-1 (Table 6). The parameter E0 is related to the critical

reactant vibrational frequency (m), which when excited results in conversion as

described below (‘‘Mechanistic considerations’’ section). The activation energy

corresponds to the sum of the vibrational quanta of the specific vibration mode in

the reacting molecule that deforms the molecule towards the structure of the

‘‘activated state’’, where [8, 9]

Eexp � RT þ Q ¼ nmþ mx0n2 ð7Þ

and Q is a term representing the heat of adsorption. Equation 7 follows from

spectroscopic theory where the vibrational energy (G(n)) of a particular vibrational

mode [64, 65] is given by

G nð Þ ¼ m nþ 1=2ð Þ þ mx0 nþ 1=2ð Þ2 ð8Þ

The parameter x0 is the anharmonicity constant and we identify the enthalpy of

activation with the vibrational energy above zero state.

Table 6 Analysis of consecutive differences of the experimentally determined activation energies:

RTmean = 4.02 kJ mol-1 based on highest and lowest reaction temperature

Reactant Eexp

(kJ mol-1)

Eexp - RTmean

(kJ mol-1)

D(Eexp - RTmean)

(kJ mol-1)

n0 (Eexp -

RTmean)/E0

n

Nitrobenzene 72.37 68.35 20.27 2 6.71 7

p-Chloronitrobenzene 52.10 48.08 104.64 10 4.72 5

p-Nitroaniline 156.74 152.72 109.66 11 15.00 15

p-Bromonitrobenzene 47.08 43.06 44.78 4 4.23 4

p-Nitrotoluene 91.86 87.84 28.74 3 8.63 9

p-Nitrophenol 120.60 116.58 18.02 2 11.45 11

p-Nitroanisole 102.58 98.56 30.21 3 9.68 10P

356.3 35

E0 =P

(Eexp - RTmean)/P

n0 = 10.18 kJ mol-1


123

The significance of E0

Let us first consider the difference between two enthalpies of activation (Eexp - RT ?

Q) with vibrational quantum numbers ni and nj. We can express the associated

D(Eexp - RT ? Q) by [6]

D Eexp � RTmean þ Q� �

= ni � nj

� �¼ mþ mx0 ni þ nj þ 1

� �ð9Þ

where this ratio is dependent on the choice of vibrational quantum numbers in the

second term. Taking the expression for E0 in Eq. 6 and including a contribution due

to the heat of adsorption (Q), where n0 in Table 6 takes the form (ni - nj), we can

approximate E0 by

E0 ¼ mþ mx0 ni þ nj þ 1� �

meanð10Þ

According to Eq. 7, the enthalpy of activation is a linear function of n where x0 = 0

and the coefficient of this dependence equals the frequency of the vibration (m) that

results in reaction. The empirical relationship between Eexp – RTmean and n is linear

(see Fig. 5) and can be forced through the origin, suggesting that the anharmonicity

is negligible, i.e., mx0 = 0. It therefore follows from Eq. 10 that E0 = m. Moreover,

as the line passes through origin, Q must approach zero, meaning that the reacting

molecules are weakly adsorbed on the surface. This finds support in Corma’s

assertion [51] that strong surface interactions do not lead to reaction, generating

spectator species (see ‘‘Resonance effects’’ section). We should note that previous

SET analyses of catalytic hydrogenolysis systems [5–8] have also established weak

Eex

p –

RT

mea

n k

J m

ol-1

4 8 12 16n

40

80

120

160

Fig. 5 Dependence of Eexp - RTmean on the calculated n values (see Table 6) for the hydrogenation ofnitrobenzene (closed circle), p-chloronitrobenzene (open square), p-nitroaniline (closed diamond), p-bromonitrobenzene (open circle), p-nitrotoluene (closed inverted triangle), p-nitrophenol (open triangle)and p-nitroanisole (closed square) over Au/TiO2. The linear fit for the seven points is given by Eexp -RTmean = -1.83 ? 10.295n; forcing the fit through the origin gives Eexp – RTmean = -0.90 ? 10.203n


123

reactant/catalyst interaction with a low associated heat of adsorption. The slope of

the straight line (forced through the origin) in Fig. 5 is 10.2 kJ mol-1, which cor-

responds to 853 cm-1 and supports the identification of the O–N–O bending/

stretching mode (853 ± 2 cm-1) as the critical vibration for all the nitroarene

reactants that have been considered (Table 5). The inference that the anharmonicity

of the bending/stretching vibration is small (close to zero) is supported by obser-

vations in literature, e.g., quasi-harmonic systems [66] and E-symmetry bending

vibrations [67]. Moreover, the in-plane bending vibration of nitrogen dioxide is

characterized by a very small second order term, in contrast to other vibrations in

that molecule [68].

On the difference between calculated and experimental Tiso

We must attempt to account for the difference between the experimental

Tiso = 558 ± 32 K and the value (=613 K) obtained from Eq. 4, given the good

agreement obtained in previous catalytic studies [5–8, 69]. The basis for the SET

estimation of Tiso is given by [9]

lnk ¼ lnZ þ x m2 � x2� ��1 �p=2� arctg 0:5mx m2 � x2

� ��1� �n o

�X

DEi=hc� E=RT ð11Þ

where DEi represents the energy difference between the vibration mi and mi?1 and Zincludes all contributions to the rate constant (k) that are not dependent on vibra-

tional SET. The rate constant can be written [70]

k ¼ kT=hð Þ eDS#=R� �

e�DH�=RT� �

ð12Þ

where the factor kT=hð Þ eDS#=R� �

corresponds to Z in Eq. 11. The deviation in the

calculated Tiso for the present series of reactions may arise from a unique entropy

term in addition to Z. If this is the case, then we must take into consideration a ‘‘sub-

factor’’ of Z that contains the relevant entropy

Entropy term ¼ lnZ þ S##=R ð13Þ

where the Z term, as before, is not related to vibrational resonance effects. The

inclusion of S## allows for a critical SET related entropy contribution that underpins

nitroarene interaction with the catalyst leading to reaction. The degree of reactant

activation determines the reaction energy (E) where the phenomenological relation

for the isokinetic effect is given by

lnk ¼ lnZ þ DH#=R 1= Tiso � 1=Tð Þð Þ ð14Þ

This means that when the reaction temperature (T) = Tiso, ln k is independent of the

value of the activation energy. Modification of Eq. 11 gives


123

lnk ¼ lnZ þ S##=Rþ x m2 � x2� ��1 �p=2� arctg 0:5mx m2 � x2

� ��1� �n o

�X

DEi=hc� DH#=RT ð15Þ

that when combined with Eq. 13 gives

S##=Rþ x m2 � x2� ��1 �p=2� arctg 0:5mx m2 � x2

� ��1� �n o

DH#=Nhc� �

¼ DH#=RTiso ð16Þ

where DH# is taken as equivalent to NP

DEi as DEi is one quantum step of the

activation [9]. We can reformulate the expression for Tiso as

1=Tiso ¼ S##=DH#

þ R=Nhcð Þx m2 � x2� ��1 �p=2� arctg 0:5mx m2 � x2

� ��1� �n o

ð17Þ

and consequently

Tiso¼ 1= S##=DH#þ R=Nhcð Þx m2�x2� ��1 �p=2� arctg 0:5mx m2�x2

� ��1� �n oh i

ð18Þ

or

Tiso ¼ 1=hS##=DH# þ 1= Nhc R�1 m2�x2

� �x�1

�

�f�p=2�arctgð0:5mxðm2�x2Þ�1Þg�1�i

ð19Þ

The expression given in bold font matches Eq. 4 where the calculated Tiso = 613 K,

taking x = m = 853 cm-1 (see Fig. 4). Adopting nitrobenzene as a representative

reactant, we can write

Tiso ¼ 1= S##=72370þ 1=613ð Þ� �

ð20Þ

where, applying the experimentally determined Tiso (=558 ± 32 K), we obtain

S## = 12 ± 8 J mol-1 K-1. The S## term for each nitroarene reactant was

Table 7 Estimation of the entropy term (S##) for the seven nitroarene reactants

Reactant Eexp (kJ mol-1) S## (J mol-1 K-1)

Nitrobenzene 72.37 12

p-Chloronitrobenzene 52.10 8

p-Nitroaniline 156.74 24

p-Bromonitrobenzene 47.08 7

p-Nitrotoluene 91.86 14

p-Nitrophenol 120.60 18

p-Nitroanisole 102.58 16

Mean value/RMS 15 ± 6


123

calculated in an analogous fashion and the results are presented in Table 7, giving a

mean value of 15 ± 6 J mol-1 K-1. These values are in good agreement with the

entropy data calculated by Lewis et al. [71] and discussed by Ercolani [72]. Tor-

sional motion around the C–N bond can accommodate O–N–O bending/stretching

through activation (by resonance), facilitating attack by hydrogen (activated at

surface Au sites), as discussed below.

Mechanistic considerations

The analysis above suggests that the activation energy for selective substituted

nitroarene hydrogenation is built up by quanta of a specific vibration (853 cm-1).

This vibration is assigned to a bending/stretching motion of the reacting nitro

group. We can therefore postulate that nitro group activation is rate determining.

A stepwise hydrogenation mechanism, on the other hand, can involve the

formation of nitroso- (–NO) and hydroxylamine (–NHOH) intermediates [32].

Makaryan and Savchenko [73] have proposed that the amine is produced via a

disproportionation of the hydroxylamine. In batch liquid phase operation, a

condensation reaction involving nitroso- and hydroxyl-amine has been proposed

to generate an azoxy intermediate that undergoes subsequent reduction to azo-

and hydrazo-products [32, 50, 74]. There was no detectable azoxy-, azo- or

hydrazo-component in the product streams generated in this gas phase study.

This differs from liquid phase reaction over Au/TiO2 where azo-formation from

nitrobenzene was observed and the results of time-resolved IR analysis suggested

that the conversion of hydroxylamine was rate determining in the production of

aniline [32]. We must note that the reported infrared spectrum of N-

phenylhydroxylamine [75] does not show any absorption at or near 853 cm-1.

From a consideration of the SET treatment, N-phenylhydroxylamine is therefore

not involved in the activation process that drives the reaction forward. SET

analysis suggests that the surface reactive species are not strongly adsorbed (low

associated Q value). This does not discount the involvement of a strongly

interacting nitroarene, which can serve as an energy source in the resonance

process that is the basis of the SET model. We propose that the activated

nitroarene reacts with hydrogen atoms generated at surface Au sites. Selective

energy transfer from the bending/stretching vibration of strongly interacting

spectator molecules to the nitro group in the reacting molecules generates the

‘‘vibrationally activated’’ aromatic. Agreement of the SET determined Tiso with

the experimental value requires the introduction of a term for C-NO2 entropy

associated with bond torsion. We envisage interaction of the activated nitro

group at H–Au or H–Au–TiO2 interface sites with the simultaneous formation of

two N–H bonds and loss of both oxygens that react with surface hydrogen. This

arrangement implies a transition state with a five-point binding of N, which is

possible by mixing sp2 hybridization with sp3 and a distortion of the O–N–O

plane. Rotation around the C–N bond resulting in this distortion requires the

inclusion of a torsional entropy term, as presented in Eq. 13.


123

Conclusions

The kinetics of the gas phase (393 K B T B 573 K) hydrogenation of nitrobenzene

and para-substituted (–Cl, –NH2, –Br, –CH3, –OH and –OCH3) nitrobenzenes have

been measured over a common 0.3 % w/w Au/TiO2 (mean Au particle

size = 3.9 nm) catalyst. In every case, the catalyst delivered 100 % selectivity in

terms of –NO2 reduction to give the corresponding amine. Compensation behavior

has been established with an associated Tiso = 558 ± 32 K. The SET model has

been used to analyse the observed stepwise variation of activation energy for the

seven nitroarene reactants. These energies can be expressed in terms of the

vibrational quanta of a vibration at 853 cm-1 that is associated with symmetrical

bending/stretching of the NO2 group. Taking this as the critical nitroarene vibration

that leads to conversion, application of SET generates a calculated Tiso = 613 K

that differs from the experimentally determined value. This disagreement can be

resolved by the introduction of an extra term due to torsional entropy

(15 ± 6 J mol-1 K-1). This suggests that nitroarene activation involves a twisting

of the C–NO2 plane to facilitate hydrogen attack. Vibrational resonance at

853 cm-1 is attributed to weakly adsorbed nitroarene reactants that receive

vibrational energy from strongly bound spectator molecules on the TiO2 support

with attack by hydrogen supplied by the supported Au nano-particles.

Acknowledgments We note the contribution of Dr. F. Cardenas-Lizana and X. Wang to this work.

EPSRC support for free access to the TEM/SEM facility at the University of St Andrews is also

acknowledged.

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