Revisiting the Au Particle Size Effect on TiO2‑Coated Au/TiO2Catalysts in CO Oxidation ReactionQi Yao,† Chunlei Wang,† Hengwei Wang, Huan Yan, and Junling Lu*
Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Centerof Chemistry for Energy Materials (iChEM), CAS Key Laboratory of Materials for Energy Conversion, University of Science andTechnology of China, Hefei, Anhui 230026, P. R. China
ABSTRACT: It is well-known that the particle size of Au nanoparticlesenormously affects the catalytic activity of supported gold catalysts in manyreactions. The origin of the Au particle size effect is still widely debated. In thiswork, we precisely deposited different thicknesses of ultrathin TiO2 overcoatsonto three Au/TiO2 catalysts with Au particle sizes of 2.9 ± 0.6 (Au/TiO2-S), 5.0± 0.8 nm (Au/TiO2-M), and 10.2 ± 1.6 nm (Au/TiO2-L) using atomic layerdeposition (ALD). High-resolution transmission electron microscopy illustratedthe Au nanoparticles on these three samples were encapsulated by the TiO2overcoat. X-ray photoelectron spectroscopy measurements showed that the Aunanoparticles remained at metallic state after applying TiO2 ALD overcoat.Diffuse reflectance infrared Fourier transform spectroscopy measurements of COchemisorption further revealed that the TiO2 overcoat preferentially decorated atthe low-coordination sites of Au nanoparticles and broadly tuned the populationof these sites accessible for participating in reactions. In CO oxidation reaction, the TiO2 coated Au/TiO2-S catalysts strikinglydemonstrated considerably higher activities than the uncoated Au/TiO2-M and Au/TiO2-L catalysts, even though the formerones contained significantly less amount of CO adsorption sites due to the TiO2 overcoating. Our work shows direct evidencethat CO adsorption on the low-coordination Au sites is not the rate-determining step, and the Au particle size effect in COoxidation is NOT related with either the number of the low-coordination Au sites or the changes in oxidation states. Size-relatedchange in the length of perimeter sites at the Au−TiO2 interface could certainly play a role in the Au particle size effect.
1. INTRODUCTIONGold has long been considered as the noblest metal.1−4 Gold istherefore not expected to be a good catalyst. Nonetheless,nanosized gold particles are discovered to be catalytic active in anumber of reactions including low-temperature CO oxidation,water gas shift, partial oxidation of hydrocarbons, reduction ofnitrogen oxides, and selective hydrogenation of alkenes.5−9 It isgenerally agreed that the catalytic activity of gold catalystsgreatly depends on the size of the gold nanoparticles, as well asthe nature of the support material and the preparation method.For instance, Haruta et al. demonstrated that the activity ofsupported Au catalysts for CO oxidation increases sharply withdecreasing Au particle size below 5 nm.10
Decreasing particle size inevitably results in an increase of thenumber of low-coordination sites and the total length ofperimeter sites at the metal−support interfaces. It is hard toseparate these three factors from each other, which makes itextremely difficult to correlate the catalytic activity with theirindividual roles. As a consequence, the origin of the Au particlesize effect on catalytic activity is widely debated and noconsensus has been reached. Several explanations wereproposed: (i) size-related change in the number of low-coordination Au sites.11−18 For example, Overbury et al.11
investigated a set of Au/TiO2 catalysts with the Au meanparticle size in the range of 2−10 nm in CO oxidation, and theysuggested that the decrease in activity with increasing particle
size beyond 2 nm is controlled by the population of low-coordinate sites, rather than by size-dependent changes inoverall electronic structure of the nanoparticle. Freund et al.found that the CO adsorption properties on monolayer islandsof gold do not have different adsorption properties to bulk gold.Thus, they suggested that the exceptional activity of goldnanoparticles for the low-temperature CO oxidation reaction isprobably due to the presence of highly uncoordinated atoms.17
Nørskov and co-workers examined the particle size dependenceusing many of the earlier approaches and comparing existingpublished activity data for Au catalysts on various supports.They more generally concluded that lower coordination sitesprovide more stable adsorption sites and play the major role indecreasing the gold particle size. Effects related to theinteraction with the support may also contribute, but to aconsiderably smaller extent.13,16 (ii) Size-related change in thelength of perimeter sites at the Au−TiO2 interface.6,19,20 In thiscase, the interfacial perimeter around the Au metal particles issuggested to be the sites for molecular oxygen adsorption andactivation, and then reacts with CO which is delivered from Au.The total length of the perimeter sites was proposed to governthe catalytic activity. (iii) Quantum size ef fect. By employing
Received: December 29, 2015Revised: April 12, 2016Published: April 14, 2016
scanning tunneling microscopy/spectroscopy, Goodman et al.showed that the structure sensitivity of gold clusters supportedon titania for CO oxidation is related to a quantum size effectwith respect to the thickness of the gold islands; islands withtwo layers of gold are most effective for catalyzing the oxidationof CO.21 (iv) Strain ef fect. On the basis of self-consistentdensity functional calculations, Mavrikakis found that anexpansive strain induced by the mismatch at the Au−supportinterface can increase the reactivity of gold surfaces, and thisstrain effect increases rapidly with decreasing particle size.22 (v)Cooperative ef fects between metallic and cationic Au species.23−26
Inspired from the inverse oxide/metal model catalyst, werecently proposed a strategy of atomically precisely applyingoxide overcoating onto supported metal catalyst to extend themethodology of inverse model catalyst into a real catalystsystem using atomic layer deposition (ALD);27 therein, themetal particle size is kept unchanged during oxide ALDovercoating, while the length of metal−oxide interfaces isprecisely tuned by the coverage of oxide overcoat. As aconsequence, the individual role of metal−oxide interfaces incatalytic reactions becomes possible to be separately inves-tigated. Once metal catalysts with different particle sizes areused for oxide ALD overcoating, the metal particle size effect isalso possible to be individually addressed. It might be worthy tomention that the metal particle size effect cannot be addressedon the inverse oxide/metal model catalyst system, where metalsingle crystals are often used.28−36 In our previous study, weclearly demonstrated that the activities of TiO2 ALD over-coated Au/Al2O3 catalysts in CO oxidation showed a volcano-like behavior as a function of TiO2 ALD cycles, providing directevidence that the catalytic activities of TiO2 overcoated Aucatalysts strongly correlate with the total length of perimetersites at the Au−TiO2 interface,27 while the Au particle sizeeffect was not addressed there.To understand the Au particle size effect, here we further
applied TiO2 ALD overcoating onto three Au/TiO2 catalystswith different particle sizes, so that the number of exposed low-coordination sites and the length of perimeter sites are preciselytuned by varying ALD cycles, while maintaining their ownparticle sizes. The Au/TiO2 catalyst with a larger Au particlesize was synthesized by increasing the Au loading, instead ofusing high temperature calcination to avoid the possiblechanges in the interactions between the Au particles andTiO2 support.37 Comparing the catalytic activity changes ofthese Au/TiO2 catalysts by TiO2 overcoat, we clearly illustratedthat the Au particle size effect in CO oxidation is NOT relatedwith the number of the low-coordination Au sites or thechanges in oxidation states. Size-related change in the length ofperimeter sites at the Au−TiO2 interface might largelycontribute to the Au particle size effect.
2. EXPERIMENTAL SECTION2.1. Au/TiO2 Catalyst Synthesis. The three Au/TiO2
catalysts were prepared using the deposition−precipitation(DP) method with urea.38,39 Titania Degussa P25 (BET surfacearea = 45 m2/g, nonporous, 70% anatase and 30% rutile, purity>99.5%) was used as the support. HAuCl4·4H2O (SinopharmChemical Reagent Co, Ltd., Au content, 47.8%) was used as theAu precursor. Typically, either 3 or 0.6 g of TiO2 was added to100 mL of aqueous solution, which contained HAuCl4 (4.2 ×10−3 M) and urea (0.42 M) at the initial pH of 2. The mixturewas vigorously stirred at 348 K for 12 h in the absence of lightto avoid the gold precursor decomposition. Next, the
suspension was centrifuged and thoroughly washed withdeionized water for at least six times to remove chlorineresidual. Finally, the obtained material was dried at 343 K in airovernight and further calcined at 523 K in 10% O2 in He fordifferent times to obtain the catalyst. The gold contents in Au/TiO2 catalysts were determined by an inductively coupledplasma atomic emission spectrometer (ICP-AES).
2.2. TiO2 ALD Overcoating. TiO2 ALD overcoating onAu/TiO2 catalysts was performed in a viscous flow reactor(GEMSTAR-6 Benchtop ALD, Arradiance) at 423 K byalternatively exposing titanium isopropoxide (TTIP, 99.7%,Sigma-Aldrich) and Millipore water for different ALD cycles.The TTIP precursor was heated to 353 K to get a reasonablevapor pressure, while the water source was kept at roomtemperature.40−42 The timing sequence of TiO2 ALD was 10,200, 8, and 250 s for TTIP exposure, N2 purge, water exposure,and N2 purge, respectively. The resulting catalysts are denotedas xc-Au/TiO2-S, xc-Au/TiO2-M, and xc-Au/TiO2-L for thesamples with relative small, medium, and large sizes of Auparticles, respectively (the number of ALD cycles, x = 10, 20,and 50).
2.3. Characterization. High-resolution transmission elec-tron microscopy (HRTEM) measurements were performed ona JEOL-2010 instrument operated at 200 kV. X-ray photo-electron spectroscopy (XPS) measurements were taken on aThermo-VG Scientific Escalab 250 spectrometer equipped withan aluminum anode (Al Kα = 1486.6 eV). The binding energiesin XPS spectra were referenced to the C 1s binding energy at284.8 eV. Ultraviolet−visible (UV−vis) diffuse reflectancespectroscopy measurements were carried out on a ShimadzuDUV-3700 spectrophotometer with a diffuse reflectance cell.These three facilities, as well as ICP-AES, belong to theInstruments’ Center for Physical Science, University of Science& Technology of China.Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) measurements of CO chemisorption were carriedout on a Nicolet iS10 spectrometer, equipped with an MCTdetector and a low temperature reaction chamber (PrayingMantis Harrick). After loading a sample to the reactionchamber, the sample was first calcined at 523 K in 10% O2 inHe for 30 min and then reduced in 10% H2 in He for another30 min. A background spectrum was collected after cooling thesample to room temperature in He. Subsequently, the samplewas exposed to 10% CO in He at a flow rate of 20 mL/min forabout 15 min until saturation. To avoid any variations causedby the decline of CO chemisorption peak during He purge,14
here we collected the DRIFT spectra of all samples during theflow of 10% CO in He. The CO chemisorption spectra wereobtained by subtracting the CO chemisorption DRIFTspectrum of TiO2 to remove the gas phase CO. All DRIFTspectra were collected with 256 scans at a resolution of 4 cm−1.To perform quantitative analysis of chemisorbed CO, the IRspectra were transformed into the Kubelka−Munk unit for abetter linear relation between the IR band intensity and theadsorbate concentration.20,43,44
2.4. Catalytic Activity. The activities of xc-Au/TiO2-S, xc-Au/TiO2-M, and xc-Au/TiO2-L catalysts in low-temperatureCO oxidation reaction were conducted using a fixed-bedtubular quartz reactor at atmospheric pressure. The innerdiameter of the reactor was about 0.8 cm. For the reaction test,50 mg of uncoated Au/TiO2-S catalyst was diluted by 1 g offine quartz chips (60/80 mesh) to avoid any hot spots. Athermal couple protected by a one-end closed quartz-tube was
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directly inserted into the catalyst bed to measure the sampletemperature. For other catalysts, the amount of catalyst wasadjusted to keep the same Au content. After a catalyst wasloaded to the quartz reactor, the catalyst was first calcined in10% O2 in He at a flow rate of 40 mL/min at 523 K for 1 h toremove any carbonaceous impurity. Next, the reaction gasconsisting of 2% CO and 8% O2 balanced in He was fed to thereactor at a total flow rate of 50 mL/min. The reactionproducts were analyzed by an online gas chromatograph (Fuli,GC-9790II) with a thermal conductivity detector. The COconversion was calculated based on the ratio of the COconsumed to the CO fed. For the kinetic studies of the reactionorders on the xc-Au/TiO2-S catalysts at 273 K, the total flowrate was kept constant at 50 mL/min, while either varying theconcentration of CO from 1% to 4% by keeping the O2concentration at 6%, or varying the concentration of O2 from2% to 8% by keeping the CO concentration at 2%.
3. RESULTS AND DISCUSSION
3.1. Characterization of TiO2 Overcoated Au/TiO2Catalysts. Three Au/TiO2 catalysts with different particlesizes were synthesized using the DP method.45 The Au loadingsof these three samples were determined to be 2.5, 12.5, and 2.5wt %, respectively, using ICP-AES. According to HRTEM, the
particle sizes were 2.9 ± 0.6 (Au/TiO2-S), 5.0 ± 0.8 nm (Au/TiO2-M), and 10.2 ± 1.6 nm (Au/TiO2-L), respectively, seenin Figure 1. Here, Au/TiO2-S and Au/TiO2-M were obtainedthrough a calcination step at 523 K in 10% O2 in He for 2 h,while the Au/TiO2-L sample was obtained through acalcination step at 523 K in 10% O2 in He for 8 h. TiO2ALD was performed to uniformly deposit TiO2 ultrathinovercoating layers onto these three samples for different ALDcycles, which were denoted as xc-Au/TiO2-S, xc-Au/TiO2-M,and xc-Au/TiO2-L, respectively (the number of ALD cycles, x= 10, 20, and 50). On 50c-Au/TiO2-S, xc-Au/TiO2-M, and xc-Au/TiO2-L, HRTEM images clearly show that the thickness ofthe TiO2 overcoat was about 1.5 nm (Figure 1b,e,h), indicatingan average growth rate of ∼0.3 Å/cycle (about 0.1 monolayer),in good agreement with the literature results.46 No visiblechanges in the Au particle sizes were observed on these threesamples after applying TiO2 overcoating at 423 K (Figure1c,f,i), which might benefit from the short period of ALDcoating process and low temperatures.According to the XP spectra shown in Figure 2, the binding
energy of spin−orbit split Au 4f7/2 was nearly constant at 84.3eV, with a separation of 3.7 eV from Au 4f5/2 after applyingTiO2 ALD overcoat, which implies that the Au nanoparticlesare present in metallic state and the TiO2 overcoat does notconsiderably change the electronic properties of Au nano-
Figure 1. HRTEM images of Au/TiO2-S, Au/TiO2-M, and Au/TiO2-L catalysts with and without TiO2 ALD overcoat. (a) Au/TiO2-S, (b) 50c-Au/TiO2-S, and (c) the particle size distribution of these two catalysts; (d) Au/TiO2-M, (e) 50c-Au/TiO2-M, and (f) the particle size distribution ofthese two catalysts; (g) Au/TiO2-L, (h) 50c-Au/TiO2-L, and (i) the particle size distribution of these two catalysts.
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particles, similar to the case of TiO2 overcoated Au/Al2O3catalysts,27 and the Au@TiO2 core−shell structured catalystsreported in the literature.47,48 Meanwhile, the gradual decreaseof the intensity of Au 4f binding energy as increasing thenumber of TiO2 ALD cycles confirms the successful depositionof TiO2 on Au nanoparticles.Figure 3 shows the UV−vis spectra of the xc-Au/TiO2-S, xc-
Au/TiO2-M, and xc-Au/TiO2-L samples. The broad absorptionband observed at around 300 nm corresponds to the band−band transition of the bare TiO2 (P25) support,
40,49 while thebroad band at approximately 542, 546, and 547 nm on Au/TiO2-S, Au/TiO2-M, and Au/TiO2-L, respectively, is attributedto the localized surface plasmon resonance (LSPR) of the Aunanoparticles.50−52 The LSPR of Au nanoparticles had a redshift after applying TiO2 ALD overcoat, and the red shiftincreased with increasing TiO2 ALD cycles. Our results areconsistent with the previous work, where Van Duyne et al.demonstrated that the LSPR red shift on Ag nanoparticles
increases as the alumina overcoat thickness increased and theshift depends on the morphology of Ag nanoparticles.53 Hanand co-workers also reported a LSPR red shift of 68 nm after aTiO2 shell is attached onto the bare 50 nm Au nanoparticles toform an Au@TiO2 core−shell structure.51 The red shift wasattributed to the changes in the dielectric constant of theenvironment surrounding the metal nanoparticles.47,51,53−55 Onthe other hand, the observed LSPR red shift in our case againsuggests the formation of new Au−TiO2 interfaces afterapplying TiO2 ALD overcoat.With CO as the probe molecule, DRIFTS measurements
were carried out to examine the accessibility of embedded Aunanoparticles to reactant. As shown in Figure 4a, the uncoatedAu/TiO2-S sample showed a strong CO peak at 2103 cm−1,assigned to linear CO on low-coordination neutral Ausites.56−58 After the application of a TiO2 ALD overcoat, theintensity of CO peak decreased dramatically and continueddecreasing with increasing TiO2 ALD cycles, similar to ourrecent observation on TiO2 coated Au/Al2O3 catalysts.
27 After
Figure 2. XP spectra of as-prepared xc-Au/TiO2-S (a), xc-Au/TiO2-M(b), and xc-Au/TiO2-L (c) catalysts in the Au 4f region.
Figure 3. UV−vis diffuse reflectance spectra of as-prepared xc-Au/TiO2-S (a), xc-Au/TiO2-M (b), and xc-Au/TiO2-L (c) catalysts.
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50 cycles, the CO peak became very weak along with a ∼4 cm−1
red shift, which is likely due to a weaker dipole−dipoleinteraction between CO molecules induced by the TiO2
overcoat “dilution”. Similar phenomenon was also observedon the TiO2 coated xc-Au/TiO2-M and xc-Au/TiO2-L samples(Figure 4b,c), while such red shift of CO peak position seemsto be less pronounced on the TiO2 coated Au/Al2O3
catalysts.27
In our previous study, HRTEM measurements of TiO2
overcoated Au nanocrystals and AFM measurements on aTiO2 overcoated Au film both provided solid evidence of anisland growth of TiO2 ALD on Au surface. DRIFTS COchemisorption measurements on TiO2 overcoated Au/Al2O3
catalysts further revealed that the TiO2 overcoat preferentiallynucleates and blocks the low-coordination sites of Aunanoparticles.27 Here, a similar trend of decrease of CO peakintensity with increasing TiO2 ALD cycles was observed onTiO2 overcoated Au/TiO2 catalysts, implying that thenucleation of TiO2 ALD on the surface of Au nanoparticles isnot affected by the support materials.Figure 4d shows the normalized CO peak intensity to the
Au/TiO2-S sample by the Au loading. Clearly, the CO peakintensity decreased by about 3 orders after 50 cycles of TiO2
ALD overcoating, and so did the population of exposed low-coordination sites, since they are nearly proportional to eachother.20,43,44 As a consequence, preferential decoration of low-coordination sites of Au nanoparticles by TiO2 ALD overcoatprovides an effective way to vary the number of low-coordination sites and the total length of perimeter sites in abroad range without changing the particle size, and thus, theirindividual roles in catalytic reactions become possible to beseparately investigated.3.2. Catalytic Activity. The xc-Au/TiO2-S, xc-Au/TiO2-M,
and xc-Au/TiO2-L catalysts were evaluated in CO oxidationreaction. On the uncoated Au/TiO2-S catalyst, 100% CO
conversion was achieved at about 293 K, while it was about 352and 428 K on xc-Au/TiO2-M, and xc-Au/TiO2-L samples,respectively (Figure 5). The catalyst activities were in the trendof Au/TiO2-S > Au/TiO2-M > Au/TiO2-L, which is attributedto its relative smaller Au particle size.10,11,13,21 Up to 20-cycleTiO2 ALD overcoating, the activity only showed a slightdecrease on these three catalysts, and considerable decreases inactivity were only observed after applying 50-cycle TiO2
overcoating; therein, the temperatures for achieving 100%CO conversion were shifted to about 338, 398, and 462 K on50c-Au/TiO2-S, 50c-Au/TiO2-M, and 50c-Au/TiO2-L, respec-tively. Such observation is partially different from our previousresults, where the activity of Au/Al2O3 catalysts after applyingTiO2 overcoat was first dramatically improved and reached themaximum at 20 cycles, and then gradually decreased withfurther increasing TiO2 ALD cycles. The volcano-like behaviorsuggested the Au−TiO2 interfacial perimeter sites are the activesites, because the total length of the perimeter sites created bythe TiO2 overcoat on Au nanoparticles is expected to firstincrease and reach to a maximum at a certain number of TiO2
ALD cycles and then decrease by further increasing the numberof ALD cycles due to the gradual encapsulation.27
Applying TiO2 ALD overcoat onto Au/TiO2 must generatenew Au−TiO2 interfacial perimeter sites. However, we did notobserve a visible increase in activity (Figure 5). This is likelybecause the original highly active Au−TiO2 (P25) interfaces areburied by the TiO2 ALD overcoat, and the new formed Au−TiO2 (ALD) interfaces are less active. To prove this hypothesis,we synthesized a TiO2 ALD film supported Au catalyst, by firstdepositing a TiO2 ALD film with a thickness of ∼2.2 nm ontoan alumina substrate, followed by the loading of Au with theDP method (Au/TiO2(ALD)). The Au particle size on thisAu/TiO2(ALD) sample was about 3.0 ± 0.43 nm (Figure 6),very close to that on the Au/TiO2-S sample. However, thisTiO2 ALD film supported Au catalyst showed a significantly
Figure 4. DRIFT spectra of CO chemisorption on xc-Au/TiO2-S (a), xc-Au/TiO2-M (b), and xc-Au/TiO2-L (c) catalysts. (d) Normalized CO peakareas on xc-Au/TiO2-S, xc-Au/TiO2-M, and xc-Au/TiO2-L to the uncoated Au/TiO2-S catalyst.
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lower activity than Au/TiO2-S (P25) (Figure 7). Given that theAu−TiO2 interfacial perimeter sites are the active sitesaccording to our previous work,27 it seems that the Au−TiO2(ALD) interfaces are less active than the Au−TiO2 (P25)interfaces. This is not surprising since the crystallinity andphase of TiO2 might have a large influence on the catalyticactivity of Au/TiO2 catalyst.
59−62
Kinetic studies of the reaction orders on the xc-Au/TiO2-Scatalysts were also carried out at 273 K. Therein, the COreaction order was measured by keeping the O2 concentrationat 6%, while varying CO from 1 to 4%, and the O2 reactionorder was measured by keeping the CO concentration at 2%,while varying O2 from 2 to 8%. We found that the CO reactionorder continuously decreased from 0.4 ± 0.01 to 0 ± 0.02 withincreasing TiO2 ALD overcoat cycles, while the O2 reactionorder first increased from 0.2 ± 0.01 to about 0.35 on xc-Au/TiO2-S (x = 10 and 20) and then deceased to 0.17 ± 0.02 on50c-Au/TiO2-S (Table 1). The continuous decrease of the COreaction order is in line with the gradual loss of the populationof exposed low-coordination Au sites, indicated by DRIFTS
Figure 5. Catalytic activities of xc-Au/TiO2-S (a), xc-Au/TiO2-M (b),and xc-Au/TiO2-L (c) catalysts in CO oxidation reaction.
Figure 6. HRTEM image of the Au/TiO2 (ALD) catalyst (a) and theAu particle size distribution (b).The average size of Au nanoparticleswas about 3.0 ± 0.43 nm.
Figure 7. Compaison of the catalytic activities of Au/TiO2-S and Au/TiO2 (ALD) catalysts in CO oxidation reaction.
Table 1. Kinetic Studies of the CO and O2 Reaction Ordersand the Activation Energy on the Uncoated and TiO2Coated Au/TiO2-S Catalysts with Different TiO2 ALDCycles
CO chemisorption (Figure 4). Similarly, first increase and thendecrease of the O2 reaction order also seems to fall in the sametrend with the changes of total length of the perimeter sites bythe TiO2 overcoat. The apparent activation barrier on theuncoated Au/TiO2-S sample was 26.5 ± 0.7 kJ/mol (Table 1),similar to the literature,11,19,63 while slightly higher activationbarrier was found on the Au/TiO2-S samples with TiO2overcoat (a barrier of 35.3 ± 0.9 kJ/mol on 50c-Au/TiO2-S),again supporting that the Au−TiO2 (ALD) interfaces are lessactive than the Au−TiO2 (P25) interfaces.The stability of ultrathin TiO2 overcoated Au nanoparticles
under reaction conditions was further examined on the Au/TiO2-S, 10c-Au/TiO2-S, and 20c-Au/TiO2-S catalysts byDRFITS CO chemisorption measurements. Here, we firstrecorded the CO chemisorption spectrum on an as-preparedsample; next, we exposed the sample to a mixture of reactiongases (2% CO, 8% O2, balanced in He at a total flow rate of 50mL/min) at room temperature for about 3 h. After that,another CO chemisorption spectrum was collected. As shownin Figure 8, there were no any visible changes in the CO peakintensity on the three samples after running the CO oxidationreaction for 3 h, which strongly suggests that the ultrathin TiO2overcoat on Au nanoparticles and the uncoated Au/TiO2sample were rather stable under current reaction conditions.This is not surprising since all the catalysts were pretreated at523 K before the reaction test and the current reactionconditions are rather mild.To understand the relation between the number of exposed
low-coordination Au sites and activity, we first calculated themon the uncoated Au/TiO2-S sample based on the often-usedtruncated octahedron Au nanoparticle model16 and the amountof catalyst used in the reaction test. Next, the population ofexposed low-coordination Au sites on the rest of samples wasthen normalized according to the normalized CO peakintensity shown in Figure 4c. In CO oxidation at 298 K, weobserved that a dramatic decrease of the population of exposedlow-coordination Au sites by even two orders did not cause anysignificant loss of activity on the 20-cycle TiO2 coated catalysts,until a decrease by about 3 orders on the three Au catalysts with50-cycle TiO2 overcoat, on which CO conversion decreasedonly by about half (Figure 9). Diminishing the population ofthe exposed low-coordination Au sites to order of magnitudethrough TiO2 ALD overcoat while preserving the activitystrongly suggests that the rate-determining step in COoxidation is not the CO adsorption step, consistent with ourprevious work.27 This suggestion is also in line with theproposal by Shaikhutdinov et al. in which they proposed thatthe support effects on the activity of Au catalysts in COoxidation must arise from the interaction of oxygen rather thanCO with these catalysts since the CO interaction with goldnanoparticles on various supports is very similar.18,64
Very surprisingly, we noticed that the CO conversion on 50c-Au/TiO2-S was 47% at 298 K, impressively larger than that onthe uncoated Au/TiO2-M (22%) and Au/TiO2-L (13%), eventhough the population of exposed low-coordination Au siteswas about 2 orders of magnitude less than the latter twosamples (Figure 9). Obviously, higher activity of 50c-Au/TiO2-S than Au/TiO2-M and Au/TiO2-L is due neither to the size-related changes in the number of low-coordination Au sites, norto the changes in oxidation states confirmed by XPS (Figure 2).One might argue that the DRIFTS technique might havecertain deviations from the linearity between the surfacecoverage and peak intensities. For instance, Van Every and
Griffiths reported that the band intensities of chemisorbed COon a 3% Rh/Al2O3 catalyst in the Kubelka−Munk spectra arelinear with surface coverage over a low coverage range but showa negative deviation from linearity at high coverage.65 Thiseffect is due either to a nonlinear response of the Kubelka−Munk function with analyte concentration or possibly to achange in the absorptivities of adsorbates with increasingcoverage. However, such “huge” difference by more than 2orders of magnitude overwhelms any other relative smalluncertainties of the DRIFTS technique.The Au−TiO2 interfacial perimeter sites are identified as the
active sites, according to our previous work.27 Nevertheless, for
Figure 8. DRIFT spectra of CO chemisorption on the as-prepared andused catalysts after 3 h reaction at room temperature. (a) Au/TiO2-S;(b) 10c-Au/TiO2-S; (c) 20c-Au/TiO2-S catalysts.
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the TiO2 overcoated Au catalysts, it could be a challenge toquantitatively determine the total length of the perimeter sitesat the Au−TiO2 interfaces as a function of TiO2 ALD cycles.Therefore, it is still not clear whether the higher activity of 50c-Au/TiO2-S than Au/TiO2-M (or Au/TiO2-L) is solely due tothe size-related change in the length of perimeter sites at theAu−TiO2 interface. Moreover, the contributions from thequantum-size effect21 or strain effect22 cannot be excluded out,based on our results.
4. CONCLUSIONSIn conclusion, preferential decoration of low-coordination sitesof Au nanoparticles by TiO2 ALD overcoat allows separatelyvarying the population of low-coordination sites and the totallength of perimeter sites in a broad range without changing theparticle size, so that these three bundled factors, for the firsttime, become possible to be separately investigated. TiO2 ALDovercoating was applied to three Au/TiO2 catalysts withdifferent particle sizes of 2.9 ± 0.6, 5.0 ± 0.8, and 10.2 ± 1.6nm. XPS showed that the Au nanoparticles maintained at themetallic state after overcoating. DRIFTS CO chemisorptionmeasurements revealed that TiO2 preferentially decorates at thelow-coordination sites of Au nanoparticles and broadly tunesthe population of these sites accessible for participating inreactions. In CO oxidation, the CO conversion on 50c-Au/TiO2-S was 47% at 298 K, impressively larger than theuncoated Au/TiO2-M (22%) and Au/TiO2-L (13%), eventhough the population of the exposed low-coordination Au siteswas about 2 orders of magnitude less than the latter ones. Thisresult suggests that the Au particle size effect in CO oxidation isobviously NOT due either to the size-related changes in thenumber of low-coordination Au sites, or to the changes inoxidation states confirmed by XPS. The perimeter sites at theAu−TiO2 interface are identified as the active sites; thus, size-related change in the length of perimeter sites could certainlyplay a role in the Au particle size effect. Nonetheless, thecontributions from the quantum-size effect and strain effectcannot be excluded out, based on our results.
Author Contributions†Q.Y. and C.W. contributed equally to this work.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSThis work was supported by the One Thousand Young TalentsProgram under the Recruitment Program of Global Experts, theNSFC (51402283 and 21473169), the Fundamental ResearchFunds for the Central Universities (WK2060030014,WK2060190026 and WK2060030017), and the startup fundsfrom University of Science and Technology of China.
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