structure–activity relationship of au/zro2 catalyst on formation of hydroxyl groups and its...

Journal ofMaterials Chemistry A

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aEdgewood Chemical Biological Center,

21010-5424, USA. E-mail: christopher.j.kar

5513; Tel: +1 (410) 436-5704bCenter for Nanophase Materials Sciences, O

TN 37831, USAcComputer Science and Mathematics Divisi

Ridge, TN 37831, USAdDepartment of Materials Science and En

Institute, Drexel University, 3141 Chestnut

† Electronic supplementary information (textural properties of titania supports, TEprepared at pH 3.0 and 9.0, catalyst acttest apparatus. See DOI: 10.1039/c3ta0008

Cite this: J. Mater. Chem. A, 2013, 1,6051

Received 7th January 2013Accepted 26th March 2013

DOI: 10.1039/c3ta00081h

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Structure–activity relationship of Au/ZrO2 catalyst onformation of hydroxyl groups and its influence on COoxidation†

Christopher J. Karwacki,*a P. Ganesh,b Paul R. C. Kent,bc Wesley O. Gordon,a

Gregory W. Peterson,a Jun Jie Niud and Yury Gogotsid

The effect of changes in morphology and surface hydroxyl species upon thermal treatment of zirconia on

the oxidation activity of Au/ZrO2 catalyst was studied. We observed using transmission Fourier transform

infrared (FTIR) spectroscopy progressive changes in the presence of monodentate (type I), bidentate

(type II) and hydrogen bridged species (type III) for each of the thermally treated (85 to 500 �C)supports consisting of bare zirconia and Au/ZrO2 catalysts. Furthermore, structural changes in zirconia

were accompanied by an increase in crystal size (7 to 58 nm) and contraction of the supports porosity

(SSA 532 to 7 m2 g�1) with increasing thermal treatment. Deposition of gold nanoparticles under similar

preparation conditions on different thermally treated zirconia resulted in changes in the mean gold

cluster size, ranging from 3.7 to 5.6 nm. Changes in the surface hydroxyl species, support structure and

size of the gold centers are important parameters responsible for the observed decrease (>90%) in CO

conversion activity for the Au/ZrO2 catalysts. Density functional theory calculations provide evidence of

increased CO binding to Au nanoclusters in the presence of surface hydroxyls on zirconia, which

increases charge transfer at the perimeter of the gold nanocluster on zirconia support. This further

helps in reducing a model CO-oxidation reaction barrier in the presence of surface hydroxyls. This work

demonstrates the need to understand the structure–activity relationship of both the support and active

particles for the design of catalytic materials.

1 Introduction

Gold supported metal oxide catalysts have been studied exten-sively following Haruta's and Hutching's respective discoveriesof low temperature oxidation of CO and the hydrochlorinationof ethyne to vinyl chloride.1,2 Catalysts, such as nanostructuredAu/ZrO2, represent the next generation in innovative materialsthat can be tailored to a wide range of commercial and indus-trial applications. For example, in respiratory protection foremergency responders, ambient temperature catalysts canprovide efficient conversion of carbon monoxide and

5183 Blackhawk Rd., APG, Maryland

[email protected]; Fax: +1 (410) 436-

ak Ridge National Laboratory, Oak Ridge,

on, Oak Ridge National Laboratory, Oak

gineering, A. J. Drexel Nanotechnology

Street, Philadelphia, PA 19104, USA

ESI) available: Adsorption equilibrium,M images of gold particles on zirconiaivity at pH 3.0–9.0 and description of1h

Chemistry 2013

formaldehyde to carbon dioxide. While it is broadly acceptedthat the intrinsic activity of small gold clusters are due to a highsurface concentration of coordinatively unsaturated sites, thereis growing evidence that the dominant activity is manifested atthe interface involving the metal oxide support. For example,recent work by Yate's research group observed dual catalyticsites for the chemisorption of CO on Au and TiO2, indicatingelectronic states unique to the surface structure of each metalwithin the reaction zone perimeter.3

Despite important theoretical and experimental advances,little attention has been devoted to a systematic understandingof the structure–activity relationships of specic supports, suchas ZrO2, and their effect on CO oxidation. Moreover, publisheddata shows for many supports a wide variety of preparationpathways, such as precursor type, pH, spatial binders, reducingagents and temperature treatment before and aer depositionof gold clusters.4–10 Compilation of these data not only provesdifficult in establishing accurate reaction mechanisms, butlimits the ability to develop effective design strategies forimproving catalyst performance.

In this paper we focus on the temperature sensitive proper-ties of zirconia to demonstrate the effect of changes in surfacehydroxyl species, crystal size, porosity and deposition of goldnanoparticles on CO oxidation activity for Au/ZrO2 catalysts.

J. Mater. Chem. A, 2013, 1, 6051–6062 | 6051

http://dx.doi.org/10.1039/c3ta00081h

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Crystalline zirconia and the amorphous form Zr(OH)4, providesimportant chemical properties suitable for CO oxidation inambient environments. However, all previous studies haveshown a much lower activity of Au supported on zirconia,compared to Au on titania.11

Due to the polymorphic nature of zirconia, numerouscompositions can exist, resulting in not only changes in struc-ture, but also in the type and number of active surface groups.12

Earlier work by Carter and Christensen,13 Yamaguchi,14

Southon15 et al., reported that amorphous zirconia undergoesstructural transformations between temperatures of 200 to1200 �C resulting in low index compositions of monoclinic(major) and tetragonal (minor) phases with crystal sizes rangingfrom about 7 to 60 nm.15,16 The growth in crystal size isaccompanied by a decrease in porosity resulting in reducedsurface area (>90%) and increased pore width.4,17 In two sepa-rate studies we reported on the concentration and type ofsurface hydroxyl groups on zirconia (treated at 175 �C) using 1HMAS-NMR and XPS (O1s), and showed how the change inhydroxyl concentration (terminal vs. bridging –OH) can affectthe supports activity for the reaction of sulfur dioxide.18,19

For effective CO oxidation, the preferred state of zirconiasurface, as well other oxides, is one which promotes formationof surface terminal and bridged hydroxyls through waterdissociation and can readily form oxygen vacancies with undercoordinated Zr4+ cations.12,20,21 The formation of hydroxyls(OH and O2

�) at the Au-oxide perimeter is critical for adsorptionof CO and its conversion to reaction intermediates, such ashydroxycarbonyl.22 In the absence of hydroxyl groups there isincreased coarsening of Au clusters23 and marked regression inCO conversion activity (and CO2 yield) based on the observedaccumulation of carbonate species on gold clusters.24 Further-more, anion vacancies provide a source of donor electrons foradsorbedmolecular oxygen, enabling formation of the activatedspecies (O2

�) which are critical for the conversion of CO toCO2.25 These properties although vital to the activity of manyoxidation reactions appear to be prominent on structures con-sisting of reduced crystal size and defect-rich surfaces.26 Forexample, Li et al.,24 showed signicant reduction in catalyticactivity under water gas shi (WGS) conditions for a low surfacearea ZrO2 (19–128 m2 g�1) where the particle size increasedfrom 7 to 55 nm (at constant Au size 3 nm) over a temperaturerange of 450 to 750 �C.

This paper represents part of an on-going program aimed atdeveloping reactive materials for the oxidation of CO and otherhydrocarbons at ambient temperatures. In this work we inves-tigated the structure–activity relationships that would enableoptimal development of oxidation catalysts for applications,such as respiratory protection and energy conversion. Meetingthis objective involved preparing and evaluating a number ofgold supported catalysts. One of these materials, gold sup-ported on a lower thermally treated zirconia, yielded a superiorlevel of CO oxidation activity. We report for the rst time COoxidation activity of Au/ZrO2 catalysts where bare zirconiasupport is thermally treated prior to the addition of Au nano-clusters (�4.4 nm diameter). Systematic changes in the prop-erties of the zirconia support, particularly surface hydroxyls,

6052 | J. Mater. Chem. A, 2013, 1, 6051–6062

crystal size and porosity, play an important role in the deposi-tion of active gold particles, which ultimately determine theactivity of the Au/ZrO2 catalysts.

2 Materials and experimental2.1 Materials

Au nanoparticles (NP) were prepared by a method similar toComotti et al.27 using AuCl3$HCl (Colonial Metals) assayed at49.4 wt% gold. A 0.008 M stock solution was prepared, fromwhich different aliquots were obtained depending on thedesired gold loading and further diluted with 150 ml (18 mega-ohm deionized DI water). Gold loadings reported in this studyare 2.0 wt%. For a 2.0 wt% gold loading on zirconia (0.5 g),7.5 ml of stock gold solution was diluted to 150 ml with DIwater. The pH of the diluted gold chloride solution ranged from2.5 to 3.0. NaOH (0.1 and 1.0 M) and HCl (0.1 M) was used toadjust to a desired pH from 3.0 to 9.0. Polyvinyl alcohol (PVA,Sigma-Aldrich, 25 000 MW) was used as a spatial ligand tominimize gold colloid size at a concentration of 1.5 PVA: 1 Au byweight. Typically a 1.0 ml of PVA (100 wt% in water) was used for2.0 wt% preparation. The gold solution was rapidly stirred atroom temperature for 4 h followed by slow drop wise addition of120 ml NaBH4 (0.5 M solution in 2-methoxyethyl ether). Theaddition of NaBH4 spontaneously reduced gold colloid to Au NPwhile producing a clear bright magenta solution. The solutionwas immediately added to the catalyst.

Zirconia used in this study was a XZO-631 manufactured byMEL (grade, Flemington, New Jersey). In the as-received statethe material consists of a composition of amorphous zirconiumhydroxide and small crystalline zirconium dioxide. For consis-tency we refer to all materials discussed in this paper as ZrO2 orzirconia. Samples of ZrO2 were dried or calcined at 85, 200, 500and 900 �C for 6 h and identied as: ZrO2-85, ZrO2-200, ZrO2-500and ZrO2-900, respectively. The as-received ZrO2 containedapproximately 0.24–0.28 g g�1 water, which was determined byweight loss aer drying at 85 �C. The temperature treatedzirconia was added to 100 ml of DI water and dispersed by rapidstirring for about 1–2 h. A freshly prepared solution containingAu NP (described above) was added drop wise to differentthermally treated zirconia in solution over a period of 3 h, fol-lowed by continued stirring for 12 h at room temperature. Thefreshly prepared catalyst was ltered and washed with approx-imately 2 L of DI water. The collected wet cake consisting of AuNP deposited on ZrO2 was dried at 85 �C. For catalysts preparedat pH 6 to 9.0, the ltrate was clear and colorless. For catalystsprepared under acidic conditions, the ltrate was clear-magentawhich increased in intensity at lower pH preparations. Themagenta color was an indication of residual Au NP which didnot deposit on zirconia. The zirconia data are compared to twotitania supports, Ti-anatase and Ti-isopropoxide. We havestudied numerous other commercial and lab-made titaniasamples, but only provide the data for two diverse supports thatshowed higher performance than some others and that repre-sent various structures and morphologies of titania. Ti-anataseis a low porosity anatase obtained from Sigma-Aldrich (nano-powder, 25 nm). Ti-isopropoxide was prepared in our laboratory

This journal is ª The Royal Society of Chemistry 2013


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by the hydrolysis of titania isopropoxide (Sigma-Aldrich) in awater–alcohol solution at room temperature, ltered, washedand dried at 85 �C.

2.2 Methods

XRD. X-Ray diffraction measurements were performed usingstandard procedures using a Philips diffractometer with XCel-erator detector, CuKa, a scan range from 20 to 120 2q and aspinning sample stage. All samples were ground in an agatemortar and placed on zero background sample slides. Wherepossible, the mean particle size of ZrO2 was estimated using theScherrer equation.

TEM. Transmission electron microscopy images wereobtained on a JEOL 2100 microscope operated at 200 kV.Sample Preparation: Aggregate zirconia particle size rangedfrom about 10 to 100 microns. Sample particles were sonicatedin 1 ml methanol for 10 min and transferred to carbon grids bya micro-liter pipette.

Nitrogen adsorption equilibria. BET surface areas and poresize distributions were obtained by nitrogen adsorptionisotherms at 77 K using a Quantachrome Autosorb-1. A relativepressure range of 10�4 to psat ¼ 1 was measured for theadsorption branch followed by a desorption branch to indicatehysteresis. All samples (ZrO2 and Au/ZrO2) were outgassed at85 �C under vacuum. BET specic surface area (SSA) was derivedfrom a linear region of the isotherm between relative pressuresfrom 0.03 to 0.3. Pore size distributions were calculated using aDFT adsorption equilibria kernel designed for metal oxidesurfaces.28

Transmission Fourier transform infrared (FTIR) spectros-copy. The high vacuum FTIR cell used for this study is virtuallyidentical to the one described in detail previously by Yates andco-workers.29,30 Briey, the particles under study were pressedinto a self-supporting disk by using an ultra-ne tungsten grid.The grid is suspended in the turbo-pumped vacuum chamber(base pressure 10�8 Torr) and is resistively heated in situ. Thesmall mesh size of the grid enables intimate thermal contactbetween the powder and the tungsten, whose temperature ismeasured by a spot-welded thermocouple. The ZrO2 and Au/ZrO2 samples are thus exposed to the same environment andthermal history, enabling the direct comparison.

The ZrO2 and Au/ZrO2 catalyst are mounted vertically bypressing them into 7 mm spots on the grid, which is mountedon a Z-translation stage that places the spots in the beam pathof the FTIR spectrometer. A clean spot of the grid is used as areference for FTIR spectra. Transmission FTIR spectra (256scans) were acquired at 2 cm�1 spectral resolution using aBruker Tensor 27 spectrometer equipped with an external liquidnitrogen MCT detector (Infrared Associates). The gold coatedoptics used to direct and focus the beam through the samplespot were housed in N2 purged optics enclosures. Differentiallypumped KBr windows were used to allow the IR light to passinto and out of the high vacuum chamber. Occasionally, somegas-phase water was observed due to humidity changes in theroom. In these cases, this interference was manually subtractedout by using reference spectra collected with the same


equipment and settings. Samples were held in high vacuum fora week prior to beginning experiments. All thermal pretreat-ments were performed by heating from room temperature at arate of 1 �C s�1 until the target temperature is reached, where itis held for 5 min prior to cooling back to room temperatureprior to collecting spectra. For each spectrum discussed here, anew reference spectrum was collected of the mesh aer eachthermal treatment.

XPS. X-ray photoelectron spectroscopy data were measuredusing a Perkin-Elmer Phi 570 ESCA/SAM system. O1s bindingenergies were referenced to the C 1s photoelectron peak at284.6 eV.

Catalyst activity. Catalyst samples were evaluated understeady state plug-ow conditions. See ESI for description of thetest apparatus (Fig. S3†). Typical test used 100 mg of catalystloaded as a packed bed in a 10 mm glass tube supported on aquartz frit. Temperature experiments were run from 100 �C atatmospheric pressure using a exible resistor wrapped aroundthe reactor tube. Under humid test conditions the catalyst wasloaded in the as-received state with no pre-humidication. Fordry tests, the catalyst was exposed to clean air at 100 �C for 1 hprior to testing. The reaction mixture consisted of 1200 ppm COmixed in air (compressed gas cylinder, purity 99.9%, 21 vol%O2/79 vol% N2) and water to achieve 25% relative humidityreferenced at 25 �C. Under dry conditions the same CO–airmixture was used with no water added. Catalyst testing in theabsence of molecular oxygen was performed in pure nitrogen(compressed gas cylinder, purity 99.9%). The gas phaseconcentrations of CO, CO2 and water were measured down-stream of the reactor tube by a Fourier transform infrared(FTIR) detector (MKS, 10 M pathlength). A by-pass loop wasused to establish the CO feed concentration. Flow rate to thereactor tube was 100 ml min�1, producing a space velocity of25 400 h�1 and CO feed rate of 0.075 mmol s�1. All conversionyields are reported as %CO2, (ppm-CO2/ppm-CO � 100) andreaction rate constant data reported as mol-CO-mol�1-Au-s�1.The moles of gold reported for the specic reaction rate datawere calculated based on the total gold loading of 2 wt% for a100 mg catalyst sample.

2.3 Computational details

All studies were performed using the electronic density func-tional theory with PBE-GGA exchange correlation functionalswith PAWs31,32 as implemented in the Vienna Ab-initio Simula-tion Package (VASP).33 The bare t-ZrO2 (110) surface (case (c))was constructed using a supercell of bulk tetragonal ZrO2 tocreate four layers of symmetric oxygen-terminated periodic slab(i.e. four layers of Zr). The x–y dimensions of the supercell wereabout 2 nm. A 3 nm vacuum layer to minimize self-interactionseparated the slabs. Zirconium terminated t-ZrO2 (110) surface(case (b)) was created by removing the oxygen layers from eitherend of this slab. The cubic zirconia (001) surface (case (a)) wasconstructed using a 4 � 4 supercell of bulk c-ZrO2 with fourlayers of zirconium. Relaxing the atoms reduces the symmetryto tetragonal. All geometry relaxations were performed with thepositions of the central layer held xed at the ideal bulk value

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and relaxing the remaining atoms. To perform structuralrelaxations, a conjugate-gradient algorithm was used with asingle k-point (the point) and a plane-wave cutoff of 300 eV.Sufficient numbers of empty levels were included in all thecalculations to achieve rapid convergence. Atomic forces wereminimized to 0.01 eV A�1. Adsorption (binding) energies ofdifferent species were calculated using this general formula:�Eads(surface+species) ¼ Etotal(surface+species) � (Etotal(surface) + Etotal(species)), whereEtotal(system) is the total energy of the relaxed system. A structurallyoptimized nearly spherical Au15 nanoparticle was relaxed onthis zirconia surface to obtain its binding energy. To obtain thecharge-density difference showed in Fig. 10, a ‘high-precision’calculation was performed in VASP for the optimized geometryin the presence of a single hydroxyl, with and without the OH(i.e. Dr(r) ¼ rCO/Au/(zirconia+OH)(r) � rCO/Au/(zirconia)(r)). A Badercharge analysis34 was performed to obtain the total charge of thegold nanoparticles. Nudged elastic band calculations wereperformed to obtain the diffusion barriers of CO and O2 on Au15(Fig. 10) and reaction barriers (Fig. 11). Tangential forces wereconverged to�0.3 eV A�1 or less. Six images were used to obtaindiffusion barriers, while up to twelve images were used to locatethe CO-oxidation barrier.

3 Results and discussion3.1 Structure and surface characterization

All bare zirconia were subjected to different temperature treat-ments prior to the deposition of gold. These samples are iden-tied as ZrO2-85, -200, -500 and -900. This approach eliminatedhigh temperature effects known to inuence Au NP propertiesand the growth of clusters once deposited on the support. It wasanticipated that changes in the properties of the zirconia couldinuence the deposition characteristics of Au NP, such ascluster size and surface loading fraction. The as-receivedamorphous zirconia was dried for 4 h in air at 85 �C in a staticoven and represents the baseline material designated as ZrO2-85. Similarly, as-received zirconia was temperature treated in airat 200–900 �C for 4 h in a static oven.

High resolution TEM images for the ZrO2-85, ZrO2-200 andZrO2-500 samples are shown in Fig. 1(A)–(C) and clearly show atrend in crystal growth inuenced by temperature treatment. At85 �C (panel A), the ZrO2-85 material consists primarily ofzirconium hydroxide (Zr(OH)4) and a small fraction of zirconia(ZrO2) crystals with a mean diameter of 7.0 nm. XRD diffractionmeasurements on ZrO2-85 (Fig. 2) show broad peaks of lowintensity (Cu Ka radiation), in the region of 2q¼ 25–35� and 45–55�, which is characteristic of amorphous zirconia. The broadnature of the XRD peaks did not allow estimation of the crystalsize using the Scherrer method. Thermal treatment of zirconia200 �C shows a signicant increase in cluster formation incomparison to the ZrO2-85, although the growth in crystal sizewas small. TEM image of a ZrO2-200 sample (Fig. 1, panel B)shows a high fraction of globular clusters consisting primarilyof (111) surfaces with a mean diameter of 8.5 nm. The higherlevel of ordering is not discernible in the XRD diffractionpatterns of zirconia until above 200 �C (Fig. 2). At a temperatureof 500 �C, ZrO2 is transformed to the monoclinic phase (major)

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and tetragonal phase (minor), with characteristic diffractionpeaks at 2q ¼ 24.2, 28.2, 31.5, 34.5, 49.5, and 50.2 representingthe monoclinic phase.10,16,35 Analysis of XRD diffraction dataand TEM images show a dramatic growth in crystal orderingand size. For the ZrO2-500 and ZrO2-900 supports, the meancrystal size determined from XRD diffraction data (Scherrermethod) was 19.5 nm and 58 nm, respectively.

Fig. 1(D) and (E) shows the TEM images of the Au–ZrO2-85,Au/ZrO2-200 and Au/ZrO2-500 catalyst samples (each preparedat pH 6.4. See Section 3.2 and ESI† for discussion on prepara-tions at different pH) at high magnication with histograms ofparticle size distributions (n particles ¼ 81, 104 and 81,respectively). The observed Au NPs (2.0 wt%) show (Fig. 1(G)–(I))a distribution of spherically shaped crystalline clusters with(111) orientation and a mean particle size of 3.7, 4.0 and 5.6 nmfor the Au–ZrO2-85, Au/ZrO2-200 and Au/ZrO2-500, respectively.Thermal treatment of the zirconia from 85 to 200 �C resulted inapproximately 8 percent increase in gold particle size, followedby a 40 percent increase on thermal treatment of zirconia from200 to 500 �C. Recall, the deposition of gold particles occurredunder similar preparation conditions with only changes to thezirconia support. The observed increase in gold particle size islikely due to the reduction in porosity (see Table 1 and Fig. 3)and decrease in surface hydroxyl species which are known to beimportant in the stabilization of gold clusters.36

Table 1 shows the textural properties derived from thenitrogen isotherms on zirconia thermally treated at 85, 200, 500and 900 �C. Plotted nitrogen sorption isotherms for the ZrO2-85,ZrO2-200 and ZrO2-500 samples are provided in Fig. S1.† Thecalculated BET-SSA (SBET) show a marked decrease in surfacearea ranging from 532 to about 7 m2 g�1 as the temperatureincreases from 85 to 900 �C samples, respectively. In compar-ison, the TiO2-anatase-85 and TiO2-isopropoxide-85 BET-SSAwas 206 and 577 m2 g�1, respectively (see Table S1†).

Furthermore, micropore volumes (Vmicro) decreased from0.48 (ZrO2-85) to 0.057 (ZrO2-900) ml g�1. Mean pore sizedimensions, derived from a DFT-SiO2 adsorption kernel (Fig. 3),show a signicant increase in pore width, Dp, ranging from2.5 nm (ZrO2-85) to 7.0 nm (ZrO2-500). Due to the very low BET-SSA and micropore volumes obtained for the ZrO2-900 a DFTpore size distribution was not performed. The textural proper-ties of the Au/ZrO2-85, -200, -500 and -900 were measured, butnot reported, because these data showed negligible change inporosity compared to the bare zirconia supports.

Fourier transform infrared (FTIR) spectroscopy. Thermaltreatments were performed on the ZrO2 and Au/ZrO2 samples asdescribed above from 85 to 550 �C to obtain an understandingof the changes to the samples over this treatment range.Measured spectra from 85 to 200 �C showed no discerniblechanges, which consisted of a broad signal indicative ofadsorbed molecular water and associated hydroxyls. For thisreason, only spectra for the 200 to 550 �C thermal treatmentsare shown in Fig. 4 and 5. Surface hydroxyls on ZrO2 have beenstudied by infrared spectroscopic techniques previously.12,37,38

Previously, researchers have assigned IR signals to isolatedmonodentate “type I” hydroxyls (3770 cm�1), bidentate “type II”hydroxyls (3670 cm�1) and tridentate “type III” hydroxyls



Fig. 2 XRD diffraction data for zirconia thermally treated at 85, 200, 500 and900 �C. (m ¼ monoclinic, t ¼ tetragonal).

Table 1 Textural properties of zirconia

Material ID SBET (m2 g�1) DPa (nm) Vtotal (ml g�1)

ZrO2-85 532 2.5 0.48ZrO2-200 451 3.1 0.52ZrO2-500 108 7.0 0.21ZrO2-900

b 7 — —

a Pore diameter calculated by NLDFT silica-equilibrium adsorptionkernel. b Pores size and total pore volume not calculated due to verylow BET-SSA.

Fig. 1 TEM images of ZrO2 supports (panels A–C) and Au/ZrO2 catalysts (panels D–E) and histograms (panels G–I) of gold cluster size distribution, where the baresupport was thermally treated prior to the deposition of gold at 85, 200 and 500 �C.


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(3400 cm�1), however, there is precedence in the literature thatindicates that the “type III” hydroxyl is really a second type ofbidentate bridged species. The true nature of these species hasbeen obscured due to the fact that there is a broad signal due tohydrogen bonded or associated OH groups.


ZrO2 surface groups. In our study, we only observed asomewhat dened peak at 3647 cm�1 superimposed on thebroad hydrogen bonding peak centered approximately at3350 cm�1 for ZrO2 thermally treated to 200 �C (Fig. 4 panel A).This indicates that there are type II and possibly type IIIhydroxyls but no evidence is observed for type I species aerthermal treatment to 200 �C. In addition, peaks in the 2940–2300 cm�1 range signals the existence of adventitious carbonsuch as hydrocarbons and possibly formates or bicarbonatespecies. In the nger print region (Fig. 4 panel B), broad andoverlapping peaks near 1560, 1450, 1380, 1260, 1155, and1040 cm�1 signal the existence of multiple carbonate species.39

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Fig. 3 Plotted data shows pore size distribution for ZrO2 thermally treated at 85,200 and 500 �C. The ZrO2-900 sample is not shown due to adsorption volume.Data derived from a DFT-SiO2 adsorption kernel.


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Obscured peaks near 1560 and 1380 cm�1 are likely due to theasymmetric and symmetric (C–O) stretches of bidentatecarbonates and polydentate carbonates, based existence ofpeaks within this region with the characteristic separation of200–240 cm�1.40 It is also possible that there is a hydrogen

Fig. 4 IR spectra of ZrO2 at different thermal treatments (200–550 �C). Panel A shydroxyls). Panel B shows carbonate region.

Fig. 5 IR spectra of Au/ZrO2 at different thermal treatments (200–550 �C). Panel

6056 | J. Mater. Chem. A, 2013, 1, 6051–6062

carbonate, based on the potential d(OH) peak at 1260 cm�1,however, this is unlikely as there is no band observed near1600 cm�1.41 Furthermore, there is the possibility of the exis-tence of a bidentate formates species due to the bands at 1560and 1380 cm�1, however the broad shape of the peaks preventssure identication.

Upon treatment to 250 �C (Fig. 4 panel A), a small shoulder(clearly seen in difference spectra) grows in at 3778 cm�1 whilethere is a loss in intensity at 3664 cm�1 and some loss ofintensity in the hydrogen-bonded OH region. This signals anincrease in type I hydroxyls with simultaneous loss of type IIhydroxyls. The CH stretching and nger print regions showsharpening peaks, indicating that these surface species arebecoming slightly more homogeneous.

Heating to 300 �C, minor changes are observed in the CHand carbonate regions, but signicant reduction is observed inthe hydrogen bonded and type II OH regions. A small shouldergrows in at 3778 cm�1, which is assigned to a type I on-tophydroxyl. In the direct spectra shown here, the peak is verysmall, however its existence is conrmed more clearly withdifference spectra. In addition to the appearance of type Ihydroxyls, there is a small decrease in the intensity in the blue

hows hydroxyl region with inset of zirconia at 450 �C (magnified type I, II and III

A shows hydroxyl region. Panel B shows carbonate region.




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shied region of the peak at 3647 cm�1, signifying the potentialloss of type II hydroxyls or their conversion to type I hydroxyls.In addition, there is signicant intensity loss in the hydrogenbonded OH peak centered at 3350 cm�1.

Aer treatment to 350 �C, the type I hydroxyl at 3778 cm�1 ismore clearly observed, in addition to further loss of type II andhydrogen bonded hydroxyls. The increase in the type I hydroxylband is likely due to the removal of hydroxyls to which thosetype I hydroxyls were formerly hydrogen bonded to. Further-more, loss of intensity in the CH stretching region signalsdesorption or decomposition of CH containing species, whilesome carbonate peaks reduce in intensity. Thermal treatment to400 �C results in the further depletion of type II and hydrogenbonded hydroxyls, in addition to some loss of CH containingspecies as well as carbonates.

A dramatic change occurs to the sample upon heating to450 �C (Fig. 4 panel A and inset). There is some loss of type I andtype II hydroxyls at 3764 and 3655 cm�1. Almost all of theobscuring hydrogen bonded OH groups have disappeared,leaving only the type I hydroxyls (3764 cm�1), type II hydroxyls(�3655 cm�1), and a third type of hydroxyl demonstrated by anoverlapping peak near 3580 cm�1. This peak could be due totype III hydroxyls, as the peak position is far blue shied fromthe broad OH-bonded peak previously seen near 3400 cm�1.While this type III hydroxyl has most oen been assigned to atridentate OH group,37 some authors suggest this peak is due toa hydrogen bonded species.38 In addition, almost all signal inthe CH stretching region disappears, suggesting almostcomplete removal of hydrocarbon or bicarbonate species fromthe surface. Signicant reductions in signal are seen in thengerprint region (Fig. 4 panel B), resulting in an ill-denedbroad peak with peak intensity near 1440 cm�1. This suggeststhat any hydrogen carbonates have been eliminated, and thatmore thermally stable carbonates remain. This agrees withprevious results.38 Further heating to 500 and 550 �C results inlittle changes in the OH species, other than a slight decrease inthe number of type I and type II hydroxyls, demonstrating theirstability. No further signicant change is observed elsewhere inthe spectrum, suggesting no major changes in surface speciesbetween 450 and 550 �C.

Au/ZrO2 surface groups. Aer pretreatment in vacuum to200 �C, the spectrum of Au/ZrO2 is nearly identical to ZrO2

(Fig. 5 panel A), with the exception of a slightly more prominenttype II hydroxyl peak at 3642 cm�1. However, a small new peakis observed at 3073 cm�1 which is likely due to an alkene,aromatic, or cyclic hydrocarbon species due to the reaction ofthe polyvinyl alcohol used during preparation. The other peaksin the hydrocarbon and ngerprint regions (Fig. 5 panel B) aresimilar in shape and intensity, other than the slight red shiingof a peak to 1440 cm�1. This leads to the conclusion that theparticles have a variety of carbonates and possibly formates onthe surface. Heating to 250 �C results in the loss of type II andhydrogen bonded OH groups as evidenced in reduced intensityin the peaks at 3642 cm�1 and the broad feature at 3500–3400 cm.1 In addition, a minute feature is visible at 3777 cm�1

which is suggestive of the existence of type I hydroxyls, howeverit is too weak to be condent. The peak at 3073 cm�1 increases


in intensity, suggesting that it is due to a decompositionproduct of residual polyvinyl alcohol.

As with the ZrO2 sample, there are changes in the CHstretching region and loss of intensity in the ngerprint regionindicative of elimination of small amounts of carbonate.

Treatment through 300, 350, and 400 �C results in theelimination of a majority of the type II and hydrogen bondedOH groups as remained aer treatment to 200 �C (Fig. 5 panelA). The spectra look very similar to those of ZrO2 with theexception of the remaining peak at 3073 cm�1, which hasincreased in intensity slightly. In the ngerprint region (Fig. 5panel B), the peaks at 1535 and 1431 cm�1 suggest bidentatecarbonate as a major species. Like the ZrO2 sample, heating to450 �C results in the most dramatic change to the sample. Thevery weak signal for type I hydroxyls (3777 cm�1) completelydisappears, leaving only a small peak assigned to type IIhydroxyls (3664 cm�1). Unlike for ZrO2, no type III hydroxyl(�3600 cm�1) is observable. The CH peak at 3073 cm�1 hasdiminished but is still observable, while the other CH stretchingmodes near 2800 cm�1 decrease by ca. 90%. The features in the1760–900 cm�1 have become more convoluted, resulting in avery broad peak in the 1300–1600 cm�1. Heating to 500 and550 �C, results in no observable changes in the infrared spectra,as also observed for ZrO2. Generally we conclude that calcina-tion leads to an overall loss of terminal and bridging OH specieswith thermal treatment of the zirconia support.

3.2 Catalytic activity of Au supported on ZrO2

CO oxidation activities of Au/ZrO2 catalysts were determinedbased on the oxidation of CO in humid and dry air. The activ-ities were ranked based on the calculated CO2 yield (%) and rateof CO oxidation reaction (or CO2 formation) expressed as mol-CO-mol�1-Au-s�1. CO oxidation activities of zirconia weredetermined at various acid–base treatments ranging from pH 3to 9. Fig. S2† shows TEM images of gold nanoparticles onzirconia (Au/ZrO2-85) prepared at pH 3.0 and 9.0. There is clearevidence the size of the gold particles increased in the range of15 to 30 nm (mean 22 nm) at pH 3.0 (Fig. S2† panel A) and 8 to20 nm (mean 12 nm) at pH 9.0 (Fig. S2† panel B) preparations.The increase in gold particle size is a contributing factorin reduced CO conversion activity compared to catalystsprepared at pH 6.4. See Fig. S3† for comparison of catalystactivities (%CO2 yield at 500 s exposure to a CO–air–watermixture) for the pH 3.0, 6.4 and 9.0 preparations.

CO reaction data (plotted as %CO2 yield) for the Au/ZrO2-85(Fig. 6) show the maximum yield (66.1%, avg n ¼ 5) occurs atabout pH 6.4 under humid conditions (25% RH), followed by33% and 16% at pH 9 and 3, respectively. The effect of pHtreatment on catalyst activity is likely related to the ratio andchemical state of charged ions in solution (Cl�, Na+, OH�, PVA,and water) containing colloidal Au clusters and charge on thesurface of the zirconia. For ZrO2 the point of zero charge (pzc) isabout 6.4.42,43 Numerous preparations performed in this studyshowed that maximum CO conversion was achieved when thesolution pH and surface charge of the ZrO2 were equivalent.Under these conditions an ionic neutral environment likely

J. Mater. Chem. A, 2013, 1, 6051–6062 | 6057


Fig. 6 Comparison of CO conversion activities of Au/ZrO2 catalysts and ZrO2 (nogold) at different pH preparations. Note all ZrO2 supports were temperaturetreated at 85 �C. All catalysts shown contain gold loadings at 2.0 wt%. Feed: 1200ppm CO, 21 vol% O2, 79 vol% N2, 25% RH, space velocity 25 400 h�1, sample wt100 mg, catalyst temperature 100 �C.


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provided for improved dispersion of gold clusters on thezirconia surface.

Comparison is made for different Au/ZrO2 catalyst samples(catalyst preparation at pH 6.4) where the zirconia was ther-mally treated over a temperature range of 85 to 900 �C. Fig. 7shows the highest catalytic activities measured at a bedtemperature of 100 �Cwere obtained for the lowest bare zirconiathermal treatments. The %CO2 yield of the Au/ZrO2-200 andAu/ZrO2-500 catalysts diminished to 14.8 and 5.4%, respec-tively, from a maximum of 66.1% for the Au/ZrO2-85 catalyst.Fig. 7 also shows the catalytic activity of Au/TiO2-anatase-85 andAu/TiO2-isopropoxide-85 catalysts prepared under similarconditions to the zirconia material. Both titania were selecteddue to their very different porosities, BET-SSA equal to 206(TiO2-anatase) and 577 m2 g�1 (TiO2-isopropoxide) (see TableS1†). For both catalysts, the CO conversion activities were

Fig. 7 CO conversion activities of Au/ZrO2 catalysts where the zirconia hasundergone different temperature treatments (85, 200, 500, 900 �C). Comparisonis made to a Au/TiO2 catalyst (Au/TiO2-anatase-85) prepared at pH 6.4 andtemperature treated at 85 �C. All catalysts shown contain gold loadings at 2.0 wt%. Feed: 1200 ppm CO, 21 vol% O2, 79 vol% N2, 25% RH, space velocity25 400 h�1, sample wt 100 mg, catalyst temperature 100 �C.

6058 | J. Mater. Chem. A, 2013, 1, 6051–6062

similar with values ranging from to 17.4 to 24.7 %CO2 yield (at500 s run time), yet signicantly lower than the Au/ZrO2-85catalyst. The marked difference between zirconia and titaniasupports, even with similar BET-SSA, suggests a differentcomposition of hydroxyl species.

Table 2 provides a summary of CO conversion rates at 100 �Cand %CO2 yields referenced at 500 s under humid and dryconditions for the Au–ZrO2 catalysts. For the Au/ZrO2 samplescalcined at 85, 200 and 500 �C (pH 6.4), we observed a dramaticdecrease in CO conversion rates from 0.0454 to 0.0038 mol-CO-mol�1-Au-s�1. Furthermore, when tested with dry air the Au/ZrO2 catalyst activity decreased to 33.8 and 14.8% CO2 yield forsamples temperature treated at 85 �C and 200 �C, respectively,about 50% of the performance observed under humid condi-tions. The observed decrease in catalytic activity for the dry testconditions indicates a gradual consumption of surface hydroxylgroups that otherwise would be replenished if adsorbed waterwas present.

Unlike previously reported zirconia materials (and titania),where loss of surface hydroxyl groups and reduced porosityresulted from high temperature calcinations, the predomi-nantly hydroxylated form of amorphous zirconia (ZrO2-85)shows signicant CO oxidation activity. The formation ofsurface hydroxyl groups by way of water dissociation is animportant factor which we think is vital in applicationsrequiring highly efficient catalytic materials. In the absence ofmolecular water, such as observed in the dry CO conversionsmeasurements (Table 2), the catalyst activity appears to bedependent on the available hydroxyl groups which areconsumed and not replenished. This process is known to bereversible with the addition of molecular water which canrapidly dissociate and re-protonate surface oxygens. Further-more, the porosity of the catalyst support plays an importantrole by providing a high surface area and access to active siteswhile enabling higher CO turnover rates and conversions. Byevaluating various crystalline forms of zirconia, it has beenshown that water interacts differently.44,45 For the most stableforms of zirconia, water is entirely dissociated, which is likely

Table 2 CO conversion yield and CO2 rate for Au/ZrO2 catalystsa

Catalyst ID

pHCO2 yieldat 500 s (%)

CO conversionrate (mol-CO-mol�1-Au-s�1)

During Audeposition 25% RH Dry 25% RH Dry

ZrO2-85 5.6 (no gold) 2.0 <1.0 0.0018 0.002Au/ZrO2-85 3.0 16.2 2.0 0.014 0.002Au/ZrO2-85 6.4 66.1 33.8 0.0454 0.024Au/ZrO2-85 9.0 33.0 8.5 0.0291 0.07Au/ZrO2-200 6.4 14.8 7.2 0.0072 0.005Au/ZrO2-500 6.4 5.4 2.3 0.0038 0.002Au/ZrO2-900 6.4 <1.0 <1.0 <0.005 <0.002

a Feed: 1200 ppm CO, 21 vol% O2, 79 vol% N2, 25% RH space velocity25 400 h�1, sample wt 100 mg, catalyst temperature 100 �C. Allcatalysts shown contain gold loadings at 2.0 wt%. Reported values aremean of n ¼ 5.




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inuenced by interaction through hydrogen bonding betweenhydroxyl groups adsorbed on neighboring cationic sites. Thespecic interactions involving water with polymorphic struc-tures are likely responsible for the differences in catalyticperformance that we observed in this study.

3.3 Theoretical calculations

To further investigate the observed increase in low-temperaturecalcined zirconia with surface hydroxyls we performed plane-wave density-functional-theory based calculations.33 A fewdifferent surfaces were chosen for comparison: (a) a zirconiumterminated (001) cubic c-ZrO2 surface (b) a zirconium termi-nated (110) tetragonal t-ZrO2 surface and (c) an oxygen termi-nated (110) t-ZrO2 surface. The spherical Au15 nanoparticles(�0.62 nm in size, Fig. 8) spread out and ordered on thezirconium terminated surfaces of both c-ZrO2 (001) and t-ZrO2

(110) (cases (a) and (b)) suggesting that the terminations aremore important than the actual phase of zirconia. Changing the

Fig. 8 (a) Zirconium terminated symmetric zirconia slabs lead to spreading ofgold nanoparticles due to their high surface energies. This reduces CO binding togold destroying its catalytic role. (b) The low-energy oxygen terminatedsymmetric surface supports spherical gold nanoparticles, keeping their role asactive catalytic sites. (Atom color index: red (O), grey (Zr) and orange (Au).Simulated unit cell shown in black rectangle).


termination to an oxygen terminated surface (case (c)) Au15maintained a nanoparticle structure upon relaxation, suggest-ing that the oxygen terminated surface is important for theadsorption of three-dimensional Au NP to zirconia, indepen-dent of its phase. This also shows how one can tailor thenanoparticle morphology on an oxide surface. The bindingenergy of the nanoparticle is 0.56 eV (case (c)), slightly largerthan what was reported for Au-nanowires on similar oxygen-terminated zirconia surfaces.46 A single CO molecule adsorbedto this spherical Au15-nanoparticle with adsorption energy of1.17 eV (at the v1-site in Fig. 9(a)). The C]O bond-length is1.17 A while the Au–C bond-length is 1.94 A. In contrast, the CObinding energy on the gold nanoparticle, which had spread outon zirconium surfaces, was very weak (�0.34 eV). This suggeststhe importance of low-surface energy47 oxygen terminatedmetal-oxide surfaces for gold catalysis.

To investigate the role of hydroxyl, we adsorbed a singlehydroxyl radical to the surface (case (c) above) at �1 nm fromthe nanoparticle (see Fig. 10). While the atomic positions of theAu15 and the adsorbed CO are weakly perturbed by the presenceof the surface OH, the CO binding energy increases substan-tially to 1.28 eV. The stronger CO adsorption is due to signi-cant charge transfer to interfacial oxygen atoms between goldand zirconia (Fig. 10) rendering the gold nanoparticle moreelectropositive (i.e. 0.3e lower Bader charge34). The strongcorrelation of the interfacial electronic structure to COadsorption due to a single surface OH not only stresses itsimportance to the catalytic activity of gold-nanoparticles onzirconia, as previously found for titania supported gold,49,50 butalso shows its sensitivity to environmental conditions, sup-porting the current experimental ndings as well as recentsimilar ndings for hydrogenation reactions.51

In recent calculations of CO-oxidation on unsupported goldclusters, a stronger CO adsorption site in gold is shown to yielda lower reaction barrier for CO-oxidation.52 As such, an increasein CO adsorption energy is a necessary rst step to explain anincreased CO-oxidation rate seen in our experiments. To furtherquantify the role of hydroxyl on CO-oxidation, energy barrier fora model reaction was computed. For this, adsorption energiesof CO and O2 were initially computed on different gold andzirconia sites, and the two strongest adsorption sites are shownin Fig. 9. For CO, the strongest adsorption occurs at the v2-siteof Au15, with adsorption energy of 1.22 eV. This is much higherthan the adsorption of CO on the zirconia surface, which is only0.1 eV. Further, the diffusion barrier for CO to migrate from v1-to v2-site on Au15 is only 0.31 eV (Fig. 9(a)). Adsorption of O2 isalso stronger on Au15 (1.79 eV) than on zirconia and is similar toadsorption at the Au-zirconia interface. But O2 diffusion barrieris as high as �1.1 eV, suggesting that CO-diffusion on gold isrequired to initiate the reaction. This also suggests a slightlydifferent mechanism for the CO-oxidation on Au/ZrO2 than onAu/TiO2

48 where CO on TiO2 was seen to take part in the reactiondue to the ease of surface diffusion compared to diffusionon Au, which was calculated to be as high as 0.76 eV.

Fig. 11 shows the energy barrier for a model CO-oxidationreaction starting from the lowest-energy CO and O2 adsorptioncongurations on Au15. The barrier is signicantly lower

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Fig. 9 Diffusion barrier for CO and O2 between their two lowest energy configurations on Au15. CO diffusion barrier is much lower than that of O2, and comparable tosurface diffusion on TiO2.48

Fig. 10 Charge density difference plot with and without adsorbed surface OH.The box shows strong charge transfer from Au15 to interfacial oxygen atoms.(Red/blue are charge-density isosurfaces at �0.001e A�3). (Atom color index:same as Fig. 8, white (H), blue (C)).

Fig. 11 CO-oxidation barrier is �0.2 eV lower in the presence of a single surfaceOH-group. The transition state changes from OCO/O to OC/OO type withhydroxyl.


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(�0.2 eV) in the presence of surface hydroxyl. Further, thetransition state is also quite different. Without hydroxyl, thetransition state has a O–O bond of 1.83 A, a C/O bond of 1.38 Aand a OCO angle of 125.3�. With the hydroxyl, the transition

6060 | J. Mater. Chem. A, 2013, 1, 6051–6062

state is a conguration with a O–O bond of 1.44 A, a C/O bondof 2.43 A and an OCO angle of 98.33�. As such, without hydroxyl,the formation of an OCO/O intermediate is seen, while withthe hydroxyl, the rate is governed by the formation of the OC/OO bond. Both of these congurations appear as transitionstates in previous calculations53 of CO-oxidation on gold nano-wires on zirconia. The difference in energy between the twostates were �0.1 to 0.2 eV, with the OCO/O state lower inenergy than the OC/OO state for a t-ZrO2/(101) surface. Assuch, shi of the OCO/O conguration from being a transitionstate to a low-energy intermediate state in the presence of asurface hydroxyl is a strong indication that the CO-oxidationreaction proceeds much more easily with surface-OH.

4 Conclusions

This work shows for the rst time the effect of changes insurface structure and composition of hydroxyl species onzirconia and the activity of Au/ZrO2 catalysts to oxidize CO. Inthis work the bare zirconia had undergone thermal treatmentprior to the deposition of gold, thus preventing coarsening ofgold clusters. We have shown by transmission FTIR spectra andcoupled with CO conversion activity measurements, thathydroxyl species consisting primarily of monodentate (type I),bidentate (type II) and hydrogen bridged species (type III)decrease in abundance with thermal treatment of the zirconiasupport. We show with progressive thermal treatment of thebare zirconia a dramatic reduction in porosity and growth incrystal size. The combined effect of reduced hydroxyl speciesand porosity appears to have an effect on stabilization of goldclusters which increased from 3.7 to 5.6 nm over a temperaturerange of 85 to 500 �C. The chemical properties of the hydroxyltype-I, II and III groups are shown to be related to the temper-ature sensitive transformation of amorphous zirconia to itspolycrystalline forms. Theory provides evidence to thisphenomenon by predicting increased CO binding to Au nano-particles in the presence of surface hydroxyls, a necessary stepto obtain higher CO-oxidation rates.52 The oxygen-terminatedsurface is shown to support spherical nanoparticles comparedto a zirconium terminated surface due to its low surface energy




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and this is shown to strongly inuence CO adsorption to gold.The role of surface hydroxyls is shown to increase the chargetransfer at the interface of the nanoparticle and zirconiamaking the nanoparticle more electropositive to which the CObinds more strongly. A model CO-oxidation reaction on Au15shows a �0.2 eV lower barrier in the presence of surface OH-group, with a signicant change in the transition state. Withfurther work in elucidating the relationship of surface hydroxylgroups in different conformations and structure of metaloxides, improved strategies for tailoring properties of functionalcatalysts can be developed.

Acknowledgements

This work was supported by the US Army Research DevelopmentEngineering Command, Aberdeen Proving Ground, MD, theArmy Research Office, Research Triangle Park, NC and the JointScience and Technology Office. PG and PRCK (DFT calculations)were supported as part of the Fluid Interface Reactions, Struc-tures and Transport (FIRST) Center, an Energy FrontierResearch Center funded by the U.S. Department of Energy,Office of Science, Office of Basic Energy Sciences. Computationsused resources of the National Energy Research ScienticComputing Center, which is supported by the Office of Scienceof the U.S. Department of Energy under Contract no. DE-AC02-05CH11231.

References

1 M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem.Lett., 1987, 2, 405–408.

2 G. J. Hutchings, J. Catal., 1985, 96, 292–295.3 I. X. Green, W. Tang, M. Neurock and J. T. Yates, Science,2011, 333, 736–739.

4 J. D. Grunwaldt, C. Kiener, C. Wogerbauer and A. Baiker,J. Catal., 1999, 181, 223–232.

5 A. Knell, P. Barnickel, A. Baiker and A. Wokaun, J. Catal.,1992, 137, 306–321.

6 S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A. Bruckner,J. Radnik, H. Hofmeister and P. Claus, Catal. Today, 2002,72, 63–78.

7 C. M. Wang, K. N. Fan and Z. P. Liu, J. Am. Chem. Soc., 2007,129, 2642–2647.

8 Y. Azizi, C. Petit and V. Pitchon, J. Catal., 1995, 269, 26–32.9 M. Date, M. Okumura, S. Tsubota and M. Haruta, Angew.Chem., Int. Ed., 2004, 43, 2129–2132.

10 V. Idakiev, T. Tabakova, A. Naydenov, Z. Y. Yuan and B. L. Su,Appl. Catal., B, 2006, 63, 178–186.

11 J. D. Grunwaldt, M. Maciejewski, O. S. Becker, P. Fabrizioliand A. Baiker, J. Catal., 1999, 186, 458.

12 Z. Y. Ma, C. Yang, W. Wei, W. Huai and Y. Sun, J. Mol. Catal.A: Chem., 2005, 227, 119–124.

13 A. Christensen and E. A. Carter, Phys. Rev. B: Condens. MatterMater. Phys., 1998, 58, 8050.

14 T. Yamaguchi, Catal. Today, 1994, 20, 199–217.15 P. D. Southon, J. R. Bartlett and B. B. Nissan, Chem. Mater.,

2002, 14, 4313–4319.


16 J. Li, J. L. Chen, W. Song, J. L. Liu andW. J. Shen, Appl. Catal.,A, 2008, 334, 321–329.

17 P. D. Southon, J. R. Bartlett and B. B. Nissan, Chem. Mater.,2002, 14, 4313–4319.

18 G. Mogilevsky, C. J. Karwacki, G. W. Peterson andG. W. Wagner, Chem. Phys. Lett., 2011, 511, 384–388.

19 G. W. Peterson, J. A. Rossin, C. J. Karwacki and G. Glover,J. Phys. Chem. C, 2011, 115, 9644–9650.

20 H. H. Kung, M. C. Kung and C. K. Costello, J. Catal., 2003,216, 425–432.

21 K. Qian, W. Zhang, H. Sun, J. Fang, B. He, Y. Ma, Z. Jiang,S. Wei, J. Yang and W. Huang, J. Catal., 2011, 277, 95–103.

22 M. C. Kung, R. J. Davis and H. H. Kung, J. Phys. Chem. C,2007, 111, 11767–11775.

23 G. Veith, A. Lupini, S. Pennycook and N. Dudney,ChemCatChem, 2010, 2, 281–286.

24 A. Li, N. Ta, W. Song, E. Zhan and W. J. Shen, Gold Bull.,2009, 42.

25 U. Landman, B. Yoon, C. Zhanga, U. Heizb and M. Arenz,Top. Catal., 2007, 44.

26 J. Soria, J. Sanz, I. Sobrados, J. M. Coronado,M. D. Hernandez-Alonso and F. Fresno, J. Phys. Chem. C,2010, 114.

27 M. Comotti, C. Della Pina, R. Matarrese andM. Rossi, Angew.Chem., Int. Ed., 2004, 43, 5812–5815.

28 P. I. Ravikovitch and A. V. Neimark, Langmuir, 2006, 22,11171–11179.

29 A. Panayotov and J. T. Yates, J. Phys. Chem. C, 2007, 111,2959–2964.

30 C. N. Rusu and J. T. Yates, J. Phys. Chem. B, 2000, 104, 12292–12298.

31 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater.Phys., 1999, 59, 1758–1775.

32 P. E. Blochl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994,50, 17953–17979.

33 G. Kresse and J. Furthmuller, Phys. Rev. B: Condens. MatterMater. Phys., 1996, 54, 11169–11186.

34 W. Tang, E. Sanville and G. Henkelman, J. Phys.: Condens.Matter, 2009, 21.

35 Y. Zhang, et al., J. Mol. Catal. A: Chem., 2010, 316, 100–105.

36 G. M. Veith, A. R. Lupini, S. J. Pennycook and N. J. Dudney,ChemCatChem, 2010, 2, 281–286.

37 A. A. Tsyganenko and V. N. Filimonov, J. Mol. Struct., 1973,19, 579–589.

38 J. Nawrocki, M. Rigney, A. McCormick and P. W. Carr,J. Chromatogr., A, 1993, 657, 229–282.

39 B. Bachiller-Baeza, I. Rodriguez-Ramos and A. Guerrero-Riz,Langmuir, 1998, 3556–3564.

40 C. Morterra, G. Cerrato and F. J. Ferroni, J. Chem. Soc.,Faraday Trans., 1995, 91, 125.

41 K. Pokrovski, K. T. Junk and A. T. Bell, Langmuir, 2001, 17,4297–4303.

42 J. Nawrocki, M. P. Rigney, A. McCormick and P. W. Carr,J. Chromatogr., A, 1993, 657, 229–282.

43 W. Hertl, Langmuir, 1989, 5, 96.

J. Mater. Chem. A, 2013, 1, 6051–6062 | 6061



Dow

nloa

ded

by V

ande

rbilt

Uni

vers

ity o

n 13

/05/

2013

10:

50:5

4.

Publ

ishe

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ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

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081H

View Article Online

44 A. Ignatchenko, D. G. Nealon and K. Humphries, J. Mol.Catal. A: Chem., 2006, 256, 57–74.

45 A. Vittadini, A. Selloni, F. P. Rotzinger and M. Gratzel, Phys.Rev. Lett., 1998, 81, 2954–2957.

46 Z. P. Liu, C. M. Wang and K. N. Fan, J. Am. Chem. Soc., 2007,129.

47 A. Eichler and G. Kresse, Phys. Rev. B: Condens. Matter Mater.Phys., 2004, 69.

48 I. X. Green, W. J. Tang, M. Neurock and J. T. Yates, Science,2011, 333, 736–739.

6062 | J. Mater. Chem. A, 2013, 1, 6051–6062

49 P. Ganesh, P. R. C. Kent and G. M. Veith, J. Phys. Chem. C,2011, 2, 2918–2924.

50 Z. X. Yang, R. Q. Wu and D. W. Goodman, Phys. Rev. B:Condens. Matter Mater. Phys., 2000, 61, 14066–14071.

51 B. Q. Xu, X. Zhang and H. Shi, J. Catal., 2011, 279,75–87.

52 Y. Gao, N. Shao, Y. Pei, Z. Chen and X. C. Zeng, ACS Nano,2011, 7818–7829.

53 Z. P. Liu, C. M. Wang and K. N. Fan, J. Am. Chem. Soc., 2007,129, 2642–2647.



structure–activity relationship of au/zro2 catalyst on formation of hydroxyl groups and its...

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