DOI: 10.1126/science.1240148
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341 Science  et al.Matteo Cargnello Role for Ceria Catalysts Control of Metal Nanocrystal Size Reveals Metal-Support Interface
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Control of Metal Nanocrystal Size Reveals Metal-Support Interface Role for Ceria Catalysts
Matteo Cargnello,1,2Vicky V.T.Doan-Nguyen,2Thomas R.Gordon,3Rosa E.Diaz,4Eric A.Stach,4Raymond J.Gorte,5Paolo Fornasiero,1*Christopher B.Murray 2,3*
Interactions between ceria (CeO 2)and supported metals greatly enhance rates for a number of important reactions.However,direct relationships between structure and function in these catalysts hav
e been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts.We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel,palladium,and platinum nanocrystals.We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts,clarifying the pivotal role played by the support.T
he properties of heterogeneous catalysts are often determined by the synergy be-tween support (typically metal oxides)and supported phases (typically metal nanoparticles).Ceria (CeO 2)is an example of an “active sup-port ”that can greatly increase rates for reactions involving redox steps,such as CO oxidation and the water-gas shift (WGS)reaction (1,2),by com-parison to “inert,”nonreducible supports such as alumina (3,4).The observed enhancement is assumed to result from active sites at the metal-ceria interface,because rates can be much greater than the sum of the rates over ceria and the metal individually (1).Evidence that the oxygen atoms migrate from the support to the metal particles has come only from model systems (5,6)not oper-ating under industrially relevant reaction con-ditions.Understanding size-activity relations for ceria-based catalysts is important for improving catalyst performance.The turnover rate for CO oxidation is thought to be independent of metal particle size (7,8),so this reaction is an ideal probe for studying the role that the metal-support
interface plays by measuring changes in rates upon varying the concentration of interfacial sites.Chemical methods for preparing metal nano-particles on high-surface-area supports typically lead to large or asymmetric metal particle size distributions,which prevent definitive correlation between particle size and activity (for example,subsets of particles could be completely inactive or disproportionately more active).Monodisperse metal particles tested under realistic reaction con-ditions are critical for understanding the relation between catalytic activity and specific particle size (9,10).
Here,we used monodisperse,size-tunable metal nanocrystals (NCs)of Ni,Pd,and Pt to demonstrate the role of the metal-support inter-face in ceria-based systems.The relative fraction of interfacial sites was varied for both ceria and alumina supports,and the role of ceria in enhanc-ing CO oxidation rates under realistic conditions was revealed (scheme S1)(11).
W e prepared monodisperse Ni,Pd,and Pt NCs by thermally decomposing metal(II)acetylaceton-ates in a benzyl ether solution in the presence of oleylamine (OLAM),trioctylphosphine (TOP),and,in some samples,oleic acid (OLAC)(table S1)(11).By varying the surfactant concentration and reaction temperature,various NC sizes for each metal (small,medium,and large)were ob-tained.Figure S1shows transmission electron microscopy (TEM)images of Ni (4to 12nm),Pd (2.5to 6.3nm),and Pt NCs (1.6to 2.9nm)that were quantitatively obtained with particle size distributions below 6%,without any post-
synthetic size-selective precipitation processes.The uniformity in size of the NCs was confirmed by small-angle x-ray scattering (fig.S2)(11)and by the fact that they formed large areas of three-dimensional (3D)hexagonal close-packed super-lattices with single domains exceeding several micrometers (12,13)(Fig.1).High-resolution TEM (HRTEM)studies (Fig.1,G to I)provided evidence of the overall crystallinity of the sam-ples,although x-ray diffraction patterns showed that the presence of defects (visible ,Fig.1G)broadened the diffraction peaks (fig.S2)(11),in agreement with previous studies (14).The monodisperse Ni,Pd,and Pt NCs were adsorbed from toluene solutions onto both alu-mina and ceria supports,and heating the ma-terials in air at 300°C completely removed the organic capping agents.Low metal loadings (0.5weight percent)and high-surface-area supports (~100m 2g −1for alumina and ~60m 2g −1for ceria)mimicked real catalyst formulations and ensured that the NCs were well separated and resistant to particle sintering.We examined these sam-ples by means of TEM and CO chemisorption.Because the high electron density of ceria makes the determination of size distributions by TEM particularly difficult (especially in the case of Ni and Pd)(15),the particle sizes and distribu-tions were initially determined by analyzing the alumina-based systems (fig.S3).We confirmed that particle sizes and distributions obtained on the alumina samples were also representative of the ceria-based counterparts through TEM an-alysis of the Pt/CeO 2samples (fig.S3,L to N).Strong Z-contrast between Pt and ceria in high-angle annular dark-field scanning TEM (HAADF-STEM)images l
et us confirm that the particle size and shape did not change upon deposition and calcinations of the particles on this support.The Z-contrast for the Ni/CeO 2and Pd/CeO 2samples was less strong,and thus we also used electron energy loss spectroscopy (EELS)to map the individual Ni and Pd NCs (fig.S4)(11)to measure size distributions.For Pd and Pt catalysts,there was little change in the NC sizes after deposition and calcination (fig.S5)(11).For Ni,we observed the formation of hollow spheres,likely caused by the Kirkendall effect (fig.S3,A to C)(11,16).Nonetheless,even in this case,the very narrow size dispersion was main-tained and there was no Ni metal loss during this process (fig.S5)(11).Furthermore,it is apparent from the particle size distribution measurements [histograms in fig.S5(11)]that the particles
1
Department of Chemical and Pharmaceutical Sciences,ICCOM-CNR,Consortium INSTM,University of Trieste,34127Trieste,Italy.2Department of Materials Science and Engineering,University of Pennsylvania,Philadelphia,PA 19104,USA.3
Department of Chemistry,University of Pennsylvania,Philadel-phia,PA 19104,USA.4Center for Functional Nanomaterials,Brookhaven National Laboratory,Upton,NY 11973,USA.5
Department of Chemical and Biomolecular Engineering,University of Pennsylvania,Philadelphia,PA 19104,USA.*Corresponding author.E-mail:pfornasiero@units.it (P.F.);cbmurray@sas.upenn.edu (C.B.M.)
Fig.1.As-prepared nanocrystals.(A to C )TEM images of hexagonal close-packed 3D assemblies of (A)Ni NCs,(B)Pd NCs,and (C)Pt NCs.(D to F )Magnified images.(G to I )High-resolution TEM images showing distinct NCs.
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supported on ceria had a slightly wider size dis-tribution:The distributions for the small,medium,and large sizes were completely separated for the Pt/Al 2O 3,whereas there was a slight overlap in the distributions for the Pt/CeO 2.There was no overlap in the particle size distributions for the Pd and Ni samples.We also conducted environ-mental TEM (ETEM)experiments by heating the samples in situ in air to 300°C under condi-tions otherwise similar to the calcination process (fig.S6)(11).The images show restructuring at the metal-ceria interface with the particles adhering to the ceria surface,but this neither changes the overall particle size nor the particle shape used for the calculation of the fraction of particular sites (see below).We also took into account the slight variability of the metal-ceria interface in our calculations by performing care-ful HRTEM studies (fig.S7)(11).The CO chem-isorption experiments provided information on the total population of accessible metal sites (table S2)(11)and confirmed that the trends in metal particle size are retained after deposi-tion and calcination.All of the above data confirm that particle sizes and distributions are maintained in the final catalysts (Fig.2).HRTEM analysis indicates that the larger par-ticles maintained their original cuboctahedral morphology,and suggests that smaller particles spread over both supports into shapes that re-semble a cubo-octahedron truncated along the {100}direction.
The data obtained by conventional and aberration-corrected TEM and by CO chemisorp-tion were used to prepare a physical model of the particles.In the case of Pd,the modeled par-ticles are shown in Fig.2D.The models were used to quantify the number of atoms with par-ticular coordination environments,such as cor-ner and perimeter sites at the interface with CeO 2and surface atoms that are not in direct contact with the support.Previous reports made use of the entire size distribution to develop a physical model of the particles (17,18),but in our case,extremely narrow distribution of NC sizes and shapes allowed the use of average values.The different particle sizes and shapes differ in terms of length of the metal-support interface,so we could directly analyze the impact of this parameter for CO oxidation.For the catalytic tests,the metallic phase of the NCs was ensured by a mild prereduction (fig.S8)(11),and TEM char-acterization of the catalysts after reaction did not show any change in size or shape of the particles.To ensure that neither mass nor thermal diffu-sion limitations affected the results,we used high space velocities and diluted each catalyst with inert support materials (8,11,19)(fig.S9).Reac-tion orders for CO and O 2were measured on the series of small Pt samples that gave the highest volumetric CO oxidation rates (fig.S10)(11).On the alumina-supported Pt,in excess CO,the re-action orders were ~–1in CO and ~1in O 2,in agreement with the previous values (3);hence,O 2activation is inhibited by adsorbed CO on the
metal particle surface (8).In the case of the ceria sample,the reaction orders were ~0in CO and slightly positive in O 2,implying that a second reaction mechanism must be active (20).The results of CO oxidation on ceria-and alumina-supported metals under lean conditions (excess oxygen;see supplementary materials)are reported as kinetic plots in Fig.3and light-off curves in fig.S11(11).
The metals deposited on ceria had higher cat-alytic rates than their alumina-supported counter-parts,as evidenced by the much lower temperatures needed to completely oxidize CO (fig.S11)(11).The apparent activation energies (E a )for the ceria-based catalysts (fig.S12)(11)were in the range of 40to 70kJ mol −1.Alumina-supported samples showed higher apparent E a values of 50to 150kJ mol −1.These values are in agree-ment with results from other studies (3).No-tably,we found similar activation energies for all ceria-supported catalysts,implying that a sim-ilar mechanism must be operative regardless of the metal.
The alumina-based catalysts exhibited rates that were independent of metal particle size when normalized to the metal surface area (Fig.3A),as determined by CO chemisorption (8).However,
the ceria-based catalysts displayed a strong size-dependent activity,with normalized reaction rates decreasing with increasing NCs size for all three metals studied (Fig.3B)(11).The alumina-supported N
Cs were essentially saturated by CO,which likely limited any effects of these intrin-sically different sites on the alumina-supported metals.By contrast,the zero-order rate in CO observed for ceria-based catalysts is a result of reaction between CO adsorbed on the metal and O 2provided by the ceria,so that the CO on the metal is unable to suppress the rate of O 2adsorp-tion onto ceria (21,22).Despite the large number of elegant theoretical and experimental studies of oxygen spillover from ceria to Pt on model sin-gle crystals under low pressures (6),no reports have overcome the so-called “material and pressure gap ”(23,24)by experimentally demonstrating —under realistic working conditions —the involve-ment of ceria lattice oxygen in the oxidation of CO,and thus addressing the role of the metal-ceria interface.
HRTEM allows us to build a model of the shape of the supported NCs as they form faceted solids on the support interface.To determine whether the important variable that is altered by particle size is the surface-to-volume ratio or
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Fig.2.Heat-treated nanocrystals.(A to C )HRTEM images of Pd/CeO 2catalysts after calcination at 300°C and reduction at 150°C:small (A),medium (B),and large (C)samples.(D )Physical models prepared to describe the particles.Blue,orange,and gray colors indicate corner,perimeter,and surface atoms,respectively;red and white are oxygen and cerium atoms of the ceria support.
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Fig.3.Kinetic data.(A and B )Arrhenius-type plots for CO oxidation over (A)Al 2O 3and (B)CeO 2samples,where a difference in the 1000/T scale should be noted.16AUGUST 2013VOL 341
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the perimeter-to-surface ratio in the metal NCs,we used the model described above to calculate the fraction of atoms located at the various sur-face sites (surface atoms not in contact with ceria,and perimeter or corner atoms at the metal-support interface;see Fig.2D)(17,18,25,26).We analyzed the scaling relation in this frame-work with the use of the particle shapes obtained by HRTEM (Fig.2).For any regular solid other than a sphere,the number of surface sites per volume is proportional to the diameter (d )as ~d −1,that of the edge sites to ~d −2,and that of the vertices to ~d −3.For this reason,the model was robust,in that the relations did not drastical-ly change when particles of slightly d
ifferent geometries were used.We then plotted in the same graph the fraction of sites with a partic-ular position as a function of particle size for all nine ceria-based samples (Fig.4).The slight scatter in the graph arose because we compared metals with dissimilar lattice constants and slightly different shapes.These results showed a scaling of d –0.9T 0.1for the surface atoms,and of d –1.9T 0.2and d –2.6T 0.1for perimeter and corner atoms in direct contact with the support,respectively.We then collected the turnover fre-quency (TOF)values of the CeO 2-based catalysts at 80°C (a convenient temperature to test all the catalysts under kinetic conditions)and plotted the data on the same graph.The TOFs for all nine samples showed a dependence of the diameter as d –2.3T 0.2,implying that the metal atoms at the nexus of the metal,support,and atmosphere were the active sites for this reaction and that the larger surface-to-volume ratio of small particles translates to an increased boundary length and higher activity.The value of the slope implies that the corner atoms were the most active sites overall,most likely because of their lower co-ordination number compared to the other perim-
eter atoms,as observed in other systems (17,18).The slight deviation from the expected trend for the small and medium Pt samples might be an effect from some very small particles that were detected by high-resolution STEM (fig.S3)(11)that contributed more to the observed reactivity.Nonetheless,we conc
lude that the perimeter atoms were the active sites for CO oxidation on ceria-based catalysts.It may be fortuitous,but the TOF for our Pt small sample (~0.2s −1at 80°C)is similar to that reported for single-site Pt/FeO x catalysts (0.3s −1at the same tempera-ture)(27).Despite the different nature of the systems,this further corroborates the validity of our approach.
The trend in catalytic activity (size depen-dence)was not influenced by the reaction en-vironment.Similar results were obtained from stoichiometric,lean (excess of oxygen),or rich (excess of CO)conditions (figs.S13and S14)(11).The apparent activation energies are in the range of 40to 70kJ mol −1for all the samples and conditions (fig.S15)(11).This experiment conclusively shows that CO oxidation by group VIII metals deposited on CeO 2is size-dependent,with a direct participation in the reaction of metal atoms at the perimeter and ceria surface oxygen,and that Ni in contact with ceria exhibits rates similar to those of Pd or Pt.Our results demonstrate a robust method to explore the role of interfacial sites in catalysis,and demonstrate that the use of size-selected nano-particles can successfully identify catalytically active sites.
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26.T.Janssens et al .,Top.Catal.44,15–26(2007).27.B.Qiao et al .,Nat.Chem.3,634–641(2011).Acknowledgments:We thank M.Graziani,T.Montini
(University of Trieste),B.Diroll,and K.Bakhmutsky (University of Pennsylvania)for discussions and help.Supported by University of Trieste through FRA project and COST Action CM1104(M.C.and P.F.);the U.S.Department of Energy ’s Advanced Research Projects Agency,Energy (ARPA-E)grant DE-AR0000123(V.V.T.D.-N.);NSF through the Nano/Bio Interface Center at the University of Pennsylvania,grant DMR08-32802(T.R.G.);Air Force Office of Scientific Research Multidisciplinary University Initiative grant FA9550-08-1-0309(R.J.G.);and a Richard Perry University Professorship (C.B.M.).Aberration-corrected EM (R.E.D.and E.A.S.)was carried out at the Center for Functional Nanomaterials,Brookhaven National Laboratory,which is supported by the U.S.Department of Energy,Office of Basic Energy Sciences,under contract DE-AC02-98CH10886.ived the idea for the study.M.C.and V.V.T.D.-N.synthesized the metal NCs.M.C.prepared the catalysts and collected the catalytic data.R.E.D.performed TEM,STEM,and ETEM characterization with help from V.V.T.D.-N.and T.R.G.E.dinated all TEM studies.T.R.G.prepared the physical model of the NCs.R.J.G.,P.F.,and C.B.M.supervised the project.M.C.wrote the draft and all authors commented on the data and the manuscript.
Supplementary Materials
/cgi/content/full/science.1240148/DC1Materials and Methods Scheme S1Figs.S1to S15Tables S1and S2
6May 2013;accepted 24June 2013Published online 18July 2013;10.1126/science.1240148
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Fig.4.Model analysis.Calculated number of sites with a particular geometry (surface and perimeter or corner atoms in contact with the support)as a function of diameter and TOF at 80°C of the nine ceria-based samples.
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