bridging the gap between surface science and industrial catalysis

4
Bridging the Gap between Surface Science and Industrial Catalysis Frederic C. Meunier* Laboratoire de Catalyse et Spectrochimie, ENSICAEN-CNRS-University of Caen, 14050 Caen, France T he enormous impact of heteroge- neous catalysis on the quality of our daily life is often underestimated. Transportation fuels, plastics and fibers, fer- tilizers, pollution control systems, food pro- cessing, and fragrances are all classical ex- amples of products or processes in which heterogeneous catalysis plays a role in re- ducing manufacturing or operation costs and limits waste production. The under- standing of the structureactivity relation- ship of catalytic materials has always been complicated by the complexity and nonuni- form structure of catalyst particles, thereby hampering catalyst development. Surface science techniques and the investigation of single-crystal surfaces have been crucial in unraveling many aspects of reaction mech- anisms, and the 2007 Nobel Prize awarded to Gerhard Ertl was an acknowledgment of these outstanding achievements. 1 How- ever, industrial catalysts are only remotely related to the perfect surfaces traditionally studied by surface science techniques. Typi- cal commercial materials consist largely of a mixture of phases that can differ in stoichi- ometry, bulk and surface structure, particle shape and size, defects, impurities, and in- teractions with a supporting substrate (to different extents), which can induce struc- tural and electronic modifications to the ac- tive phase. Usually, the support itself also presents a complex nonhomogeneous surface. Nanoparticles as Model Catalytic Systems. The study of model well-defined metal nanopar- ticles has highlighted important features of heterogeneous catalytic reactions and has been a major step toward understanding in- dustrial catalysts. 2,3 For example, Au-based materials were shown to exhibit markedly different catalytic properties for the oxida- tion of CO with O 2 depending on the size of Au nanoclusters supported on a single- crystal TiO 2 (110) surface. The critical size for the Au nanoclusters was 3.5 nm diam- eter, under which high catalytic activity was observed. 4 Lambert and co-workers re- cently reported the preparation of Au nano- particles using various synthetic techniques and deposition on a range of inert sup- ports for styrene oxidation with molecular oxygen, for which the critical size for the Au particles was 2 nm. 5 Somorjai and co-workers investigated the structureactivity relationship of 12 nm Pt particles supported on silicon wafers and stabilized with tetradecyltrimethylam- monium bromide (TTAB), an emulsifier act- ing as a stabilizing agent. 6 The Pt nanocrys- tals exhibited mostly either cuboidal or cuboctahedral shapes; the cubic particles displayed only Pt(100) planes at the surface, while both Pt(100) and Pt(111) planes were present in the case of cuboctahedral crys- tals (Figure 1). The samples were tested for a structure-sensitive reaction, benzene hy- drogenation. Only cyclohexane was ob- served in the case of the cubic particles, while both cyclohexane and cyclohexene were detected in the case of the cuboctahe- dral crystals. The intrinsic activity (that is, See the accompanying Article by Tsang et al. on p 2547. *Address correspondence to [email protected]. Published online December 23, 2008. 10.1021/nn800787e CCC: $40.75 © 2008 American Chemical Society Understanding the structureactivity relationships of catalytic materials has always been complicated by the complexity and nonuniform structures of catalyst particles, thereby hampering catalyst development. ABSTRACT Understanding the structureactivity relationships of catalytic solids has always been hampered by the complexity and nonuniform structures of catalyst particles. Materials based on well- defined colloidal metal particles are ideal model solids to investigate such structureactivity relationships. A new paper by Tsang et al. in this issue indicates that highly selective ,- unsaturated aldehyde hydrogenation Pt-based catalysts can be obtained following the decoration of Pt nanocrystals with a second metal. The effects of the particle size, substrate shape, and electronic modifications were related to the sample activity. The possibility of depositing these tailor- made colloidal particles onto conventional supports opens exciting new routes toward rational catalyst design and ultraselective catalytic processes. PERSPECTIVE www.acsnano.org VOL. 2 NO. 12 2441–2444 2008 2441

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Bridging the Gap between SurfaceScience and Industrial CatalysisFrederic C. Meunier*

Laboratoire de Catalyse et Spectrochimie, ENSICAEN-CNRS-University of Caen, 14050 Caen, France

The enormous impact of heteroge-neous catalysis on the quality of ourdaily life is often underestimated.

Transportation fuels, plastics and fibers, fer-

tilizers, pollution control systems, food pro-

cessing, and fragrances are all classical ex-

amples of products or processes in which

heterogeneous catalysis plays a role in re-

ducing manufacturing or operation costs

and limits waste production. The under-

standing of the structure�activity relation-

ship of catalytic materials has always been

complicated by the complexity and nonuni-

form structure of catalyst particles, thereby

hampering catalyst development. Surface

science techniques and the investigation of

single-crystal surfaces have been crucial in

unraveling many aspects of reaction mech-

anisms, and the 2007 Nobel Prize awarded

to Gerhard Ertl was an acknowledgment of

these outstanding achievements.1 How-

ever, industrial catalysts are only remotely

related to the perfect surfaces traditionally

studied by surface science techniques. Typi-

cal commercial materials consist largely of

a mixture of phases that can differ in stoichi-

ometry, bulk and surface structure, particle

shape and size, defects, impurities, and in-

teractions with a supporting substrate (to

different extents), which can induce struc-

tural and electronic modifications to the ac-

tive phase. Usually, the support itself also

presents a complex nonhomogeneous

surface.

Nanoparticles as Model Catalytic Systems. The

study of model well-defined metal nanopar-ticles has highlighted important features ofheterogeneous catalytic reactions and hasbeen a major step toward understanding in-dustrial catalysts.2,3 For example, Au-basedmaterials were shown to exhibit markedlydifferent catalytic properties for the oxida-tion of CO with O2 depending on the size ofAu nanoclusters supported on a single-crystal TiO2(110) surface. The critical sizefor the Au nanoclusters was �3.5 nm diam-eter, under which high catalytic activitywas observed.4 Lambert and co-workers re-cently reported the preparation of Au nano-particles using various synthetic techniquesand deposition on a range of inert sup-ports for styrene oxidation with molecularoxygen, for which the critical size for the Auparticles was �2 nm.5

Somorjai and co-workers investigatedthe structure�activity relationship of �12nm Pt particles supported on silicon wafersand stabilized with tetradecyltrimethylam-monium bromide (TTAB), an emulsifier act-ing as a stabilizing agent.6 The Pt nanocrys-tals exhibited mostly either cuboidal orcuboctahedral shapes; the cubic particlesdisplayed only Pt(100) planes at the surface,while both Pt(100) and Pt(111) planes werepresent in the case of cuboctahedral crys-tals (Figure 1). The samples were tested fora structure-sensitive reaction, benzene hy-drogenation. Only cyclohexane was ob-served in the case of the cubic particles,while both cyclohexane and cyclohexenewere detected in the case of the cuboctahe-dral crystals. The intrinsic activity (that is,

See the accompanying Article by Tsanget al. on p 2547.

*Address correspondence [email protected].

Published online December 23, 2008.10.1021/nn800787e CCC: $40.75

© 2008 American Chemical Society

Understanding the

structure�activity

relationships of catalytic

materials has always been

complicated by the complexity

and nonuniform structures of

catalyst particles, thereby

hampering catalyst

development.

ABSTRACT Understanding the

structure�activity relationships of

catalytic solids has always been

hampered by the complexity and

nonuniform structures of catalyst

particles. Materials based on well-

defined colloidal metal particles are

ideal model solids to investigate such

structure�activity relationships. A new

paper by Tsang et al. in this issue

indicates that highly selective �,�-

unsaturated aldehyde hydrogenation

Pt-based catalysts can be obtained

following the decoration of Pt

nanocrystals with a second metal. The

effects of the particle size, substrate

shape, and electronic modifications were

related to the sample activity. The

possibility of depositing these tailor-

made colloidal particles onto

conventional supports opens exciting

new routes toward rational catalyst

design and ultraselective catalytic

processes.

PERSPECTIV

E

www.acsnano.org VOL. 2 ▪ NO. 12 ▪ 2441–2444 ▪ 2008 2441

the rate normalized to the numberof surface atoms) observed wasalso different from that calculatedfrom the combination of the activ-ity of Pt(100) and Pt(111) surfacesdetermined over single-crystals. ThePt atoms at the surface of the nano-particles exhibited 3-fold higher ac-tivity as compared to the Pt atomspresent over the correspondingsingle-crystal surface. These resultsstress that significant differencescan be expected between the activ-ity measured over single-crystal sur-faces and that obtained over metalnanoparticles, albeit as large as 12nm.

Co-Decorated Pt Nanocrystals forSelective Hydrogenation. In an articlepublished in this issue, Tsang et al.describe the preparation of nonsup-ported Pt particles made via a col-loid technique with various sizesranging from 24 nm down to 2.8nm, each sample presenting a nar-

row size distribution, en-abling thorough investiga-tion of any potential size-activity relationship.7 The2.8 nm diameter particlescorrespond to a metal dis-persion of �35%, meaningthat about a third of the Ptatoms are located at thesurface of the particles.Similar or even higher dis-persion is usually sought forcommercial catalysts, forobvious activity versus ma-terial cost reasons. The se-lective hydrogenation of�,�-unsaturated aldehydeswas used as a model reac-tion, in which the preferen-tial hydrogenation of thecarbonyl function to yieldan unsaturated alcohol wastargeted, while the hydro-genation of the CAC wouldbe prevented. The largerparticles exhibited thehighest selectivity to theunsaturated alcohol, whichseemed to level off at�85% with increasing par-ticle diameter. In contrast, aselectivity lower than 50%

was measured for all samples withparticle diameter lower than 5 nm.

Tsang et al.7 decorated 4.8 nm-large Pt nanoparticles (typical metaldispersion ca. 20%) with cobalt di-rectly in the colloidal suspensionused to prepare the metal particles,using cobalt acetyl acetonate as co-balt precursor.7 This technique islikely to lead to a homogeneous dis-tribution of Co, as opposed to tradi-tional methods, such as incipientwetness impregnation. Interest-ingly, the authors showed that theCo precursor decomposition wasactually catalyzed by the platinumand that the Co uptake was limited,possibly only to Pt sites with a lowcoordination number. The Co deco-ration led to an outstanding and un-precedented increase of the selec-tivity to the unsaturated alcohol,which was above 99% for some ofthe Co�Pt samples. It is also inter-esting to note that the Co decora-

tion had essentially no effect onthe catalytic activity of Pt particleswith diameters larger than 15 nm.

Tsang et al.7 carried out IR stud-ies using carbon monoxide as aprobe molecule to cast light on thenature of the Pt sites present in theparent and Co-decorated samples.7

Unusual Pt-multicarbonyl specieswere observed in the case of theparent material, along with morecommon linear and bridged carbo-nyl species. This observation wasprobably related to a high concen-tration of low-coordination Pt at-oms forming defective sites. Such ahigh concentration of defects islikely related to the fact that the Ptparticles were not subjected to anythermal treatment following colloidpreparation, contrary to traditionalpreparation routes. In the case ofthe Co-decorated sample, the Pt-multicarbonyl band was absent,supporting the analysis that the Cospecifically decorated Pt sites with alow coordination number.

The particle size-dependent ef-fect of the Co decoration, whichwas not observed in the case of thelarger Pt particles, was attributed tothe electron donation effect of theCo atoms, which modified the Ptelectronic structure and created apolarized interface. This assumption

Figure 1. TEM images of TTAB-stabilized particles of platinumwith (a) mostly cubic shape (average size: 12.3 � 1.4 nm) and(b) mostly cuboctahedral shape (average size: 13.5 � 1.5 nm).Reprinted from ref 6. Copyright 2007 American Chemical Soci-ety.

Tsang et al. indicated

that decoration with

other metals led to a

range of selectivity

varying in a volcano-

shape manner as a

function of position in

the periodic table, with

Co being the optimum

promoter.

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VOL. 2 ▪ NO. 12 ▪ MEUNIER www.acsnano.org2442

was supported by the red-shift ofthe vibration of the CO linearly ad-sorbed on Pt present on flat sur-faces and by X-ray photoelectronspectroscopy analysis of the Co�Ptmaterials.7 These electronic andelectrostatic effects (the latter ofwhich would influence the adsorp-tion mode on and reactivity of thepolar substrate) will naturally begreater in the case of the smaller Ptparticles, while larger crystals will beessentially unaffected.

The work reported by Tsang etal.7 emphasizes that catalysts basedon colloidal metal particles are idealmodel solids to investigatestructure�activity relationships ofmetals.7 This is especially importantin the case of small nanoparticles(�5 nm) such as those produced bytheir methods, since those relatewell to the size of traditional sup-ported metal particles. The effectsof particle size and geometry andthe electronic modifications by het-eroatoms on the material activitycan then be independently exam-ined. To this end, Tsang et al. indi-cated that decoration with othermetals led to a range of selectivityvarying in a volcano-shaped man-ner as a function of position in the

periodic table, with Co be-ing the optimum promoter.More detailed work will beuseful in defining the posi-tion of the metal promoteratoms, possibly usingatom-resolved scanningtunneling microscopy(STM) studies, whichproved crucial in determin-ing the structure of hy-drodesulfurizationalumina-supported MoS2

catalysts modified by Co.8

The deposition of thesehighly selective Co�Pt par-ticles onto carbon-basedsupports led to catalystsexhibiting the same out-standing selectivity and ac-tivity.7 The effect of more reactivesupports can also be investigated,to unravel the details of potentialstrong metal�support interactions(SMSIs) and discriminate betweensize/shape effects on the one handand support interactions on theother (Scheme 1). In fact, elucidat-ing the exact role of oft-misnamed“inert” supports represents a lastingchallenge for the catalysis commu-nity; Lambert and co-workers em-phasized this point in their recentreport.5 Understanding the effect ofthe support is paramount, since alarge number of applications ofnoble metals involve the use oflarge surface area and chemicallyreactive metal oxides, especially inthe automotive and petrochemicalindustry. An understanding of therole of novel active catalytic coat-ings present in metal-oxidecore�shell catalysts, also devel-oped by Tsang and co-workers,9

would also lead to the rationalmodification of the metallic core.Composites based on cerium oxide,showing essentially no accessible Ptmetal at the surface of the “metal-in-ceria” catalyst,10 present interest-ing catalytic properties that havebeen ascribed to electronicallymodified ceria.11

Other advanced methods forthe preparation of metal nanoparti-cles with narrow size distributions in

the gas or liquid phase have re-cently been described. The prepara-tion of Au particles as small as 1.5nm was reported and based on 55-atom Au “magic” clusters.5 Caps etal. proposed a new method basedon laser vaporization of binary alloyrods to prepare pure Au or Au-containing alloy particles with nar-row particle sizes as low as 1.5 nm.12

Somorjai and co-workers showedthat the epitaxial growth of binaryPd/Pt nanocrystals with specificshapes was possible.13 All thesemethods can lead to the prepara-tion of unsupported metal particlesthat can be deposited subsequentlyon various carriers as required forinvestigations of well-definedmodel catalysts.

The materials prepared by thetechniques mentioned here areextremely useful when used asreference samples to unravel theprevailing structures of the sup-ported metal particles under reac-tion conditions. This would beparticularly appropriate for reac-tions involving CO, such as COoxidation, the Fischer�Tropschsynthesis, and the water�gasshift reaction, since CO is a verysensitive molecular probe. Theuse of operando IR spectroscopywould allow comparison of thesignal due to the absorption bythe carbonyl present at the cata-

Scheme 1. The availability of well-defined metal nanoparticlesshould allow better understanding of the modification of thecatalytic activity and selectivity depending on the features ofthe metal nanoparticles and the materials obtained followingdeposition on mostly inert well-defined supports or strongly in-teracting carriers. The activity of metal core�support shellcomposites could also be unraveled in greater detail.

Understanding the

effect of the support is

paramount, since a

large number of

applications of noble

metals involve the use

of large surface area and

chemically reactive

metal oxides, especially

in the automotive and

petrochemical industry.

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lyst surface with that on the

model materials, thereby lifting

the ambiguity of band assign-

ments and revealing the true na-

ture of the metal active sites un-

der reaction conditions.14 In

addition to traditional supports,

the nanoparticles could also be

supported on carbon nanotubes,

which offer well-defined and

smooth surfaces for the prepara-

tion of dispersed nanoparticles.

Thus, extremely well-defined

model catalysts including a metal-

lic active phase and an inert sup-

port would be available for funda-

mental studies.15 In particular,

these model materials should en-

able a better direct comparison of

experimental and computational

data pertaining to the

structure�activity relationships

of supported metals.3,16,17

CONCLUSIONS ANDPROSPECTS

The work by Tsang et al.7 epito-

mizes the often elusive target of

catalyst developers that is the ra-

tional design of optimized cata-

lytic materials through the un-derstanding of therelationship between catalyststructure and activity (Scheme2).7 Such an achievement waspossible because of the devel-opment of controlled synthe-sis methods for the prepara-tion of nanometer-sized metalparticles exhibiting similarshapes and narrow size distri-butions, relevant to those ofconventional supported metalcatalysts. The increasing num-ber of synthetic methods forthe preparation of well-defined nanoparticles for awide range of metal and al-loys stimulated by the devel-opments of nanoscience andadvanced characterizationmethods opens an exciting fu-ture for the heterogeneous ca-talysis community.

REFERENCES AND NOTES

1. Bowker, M. The 2007 Nobel Prize inChemistry for Surface Chemistry:Understanding NanoscalePhenomena at Surfaces. ACS Nano2007, 1, 253–257.

2. Bell, A. T. The Impact ofNanoscience on HeterogeneousCatalysis. Science 2003, 299, 1688–1691.

3. Huang, W.; Ji, M.; Dong, C. D.; Gu, X.;Wang, L. M.; Gong, X. G.; Wang, L. S.Relativistic Effects and the UniqueLow-Symmetry Structures of GoldNanoclusters. ACS Nano 2008, 2,897–904.

4. Valden, M.; Lai, X.; Goodman, D. W.Onset of Catalytic Activity of GoldClusters on Titania with theAppearance of NonmetallicProperties. Science 1998, 281, 1647–1650.

5. Turner, M.; Golovko, V. B.; Vaughan,O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson,B. F. G.; Lambert, R. M. SelectiveOxidation with Dioxygen by GoldNanoparticle Catalysts Derived from55-Atom Clusters. Nature 2008, 454,981–983.

6. Bratlie, K. M.; Lee, H.; Komvopoulos,K.; Yang, P.; Somorjai, G. A. PlatinumNanoparticles Shape Effects onBenzene Hydrogenation Selectivity.Nano Lett. 2007, 7, 3097–3101.

7. Tsang, S. C.; Cailuo, N.; Oduro, W.;Kong, A. T. S.; Clifton, L.; Yu, K. M. K.;Thiebaut, B.; Cookson, J.; Bishop, P.Engineering Preformed Cobalt-Doped Platinum Nanocatalysts for

Ultraselective Hydrogenation. ACSNano 2008, 2, 2547–2553.

8. Lauritsen, J. V.; Kibsgaard, J.; Olesen,G. H.; Moses, P. G.; Hinnemann, B.;Helveg, S.; Nørskov, J. K.; Clausen,B. S.; Topsøe, H.; Lægsgaard, E.;Besenbacher, F. Location andCoordination of Promoter Atoms inCo- and Ni-Promoted MoS2-BasedHydrotreating Catalysts. J. Catal.2007, 249, 220–233.

9. Yeung, C. M. Y.; Yu, K. M. K.; Fu, Q. J.;Thompsett, D.; Petch, M. I.; Tsang,S. C. Engineering Pt in Ceria for aMaximum Metal-SupportInteraction in Catalysis. J. Am. Chem.Soc. 2005, 127, 18010–18011.

10. Yeung, C. M. Y.; Meunier, F.; Burch,R.; Thompsett, D.; Tsang, S. C.Comparison of New MicroemulsionPrepared “Pt-in-Ceria” Catalyst withConventional “Pt-on-Ceria” Catalystfor Water�Gas Shift Reaction. J.Phys. Chem. B 2006, 110,8540–8543.

11. Hardacre, C.; Ormerod, R. M.;Lambert, R. M. Platinum-PromotedCatalysis by Ceria: A Study ofCarbon Monoxide Oxidation overPt(111)/CeO2. J. Phys. Chem. 1994,98, 10901–10905.

12. Caps, V.; Arrii, S.; Morfin, F.;Bergeret, G.; Rousset, J.-L. Structuresand Associated Catalytic Propertiesof Well-Defined NanoparticlesProduced by Laser Vaporisation ofAlloy Rods. Faraday Discuss. 2008,138, 241–256.

13. Habas, S. E.; Lee, H.; Radmilovic, V.;Somorjai, G. A.; Yang, P. ShapingBinary Metal Nanocrystals throughEpitaxial Seed Growth. Nat. Mater.2007, 6, 602–607.

14. Daly, H.; Ni, J.; Thompsett, D.;Meunier, F. C. On the Usefulness ofCarbon Isotopic Exchange for theOperando Analysis ofMetal�Carbonyl Bands by IR overCeria-Containing Catalysts. J. Catal.2008, 254, 238–243.

15. Javey, A. The 2008 Kavli Prize inNanoscience: Carbon Nanotubes.ACS Nano 2008, 2, 1329–1335.

16. Nigam, S.; Majumder, C. COOxidation by BN-Fullerene Cage:Effect of Impurity on the ChemicalReactivity. ACS Nano 2008, 2, 1422–1428.

17. Gao, Y.; Shao, N.; Zeng, X. C. AbInitio Study of Thiolate-ProtectedAu102 Nanocluster. ACS Nano 2008,2, 1497–1503.

Scheme 2. Features for the rational design of catalystsbased on model materials.

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