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J. Phys. Chem. C 2007, 111, 17044-17051
CO Oxidation on Gold in Acidic Environments: Particle Size and Substrate Effects Brian E. Hayden,* Derek Pletcher, Michael E. Rendall, and Jens-Peter Suchsland School of Chemistry, UniVersity of Southampton, Southampton SO17 1BJ, UK ReceiVed: June 15, 2007; In Final Form: August 2, 2007
The electrooxidation of carbon monoxide on titania- and carbon-supported gold nanoparticles of mean diameters 500 mV less positive than that where bulk gold oxide is formed and, indeed, the formation of this oxide strongly inhibits CO oxidation. The literature, however, implies that the mechanism at gold is similar to that at other precious metals, but adsorbed CO and adsorbed OH species are present only at very low coverages. This may implicate specific, low-coverage sites on the gold surface as critical to the oxidation reaction; it also suggests that the intermediates are both formed and react very rapidly.16-18 Several groups have reported studies of CO oxidation in acid solution at single-crystal gold surfaces and,19-23 although the voltammetry is similar to that of polycrystalline gold, the conclusions are somewhat different. Blizanac et al.23 provide evidence that oxidation of the gold surface begins at potentials as low as +0.3 V vs SHE and highlight competition for surface sites between “OH” and anions of the electrolyte.22 Also, infrared spectroscopy of the surface confirms the adsorption of CO, although there is disagreement about the extent of adsorption.19-23 Weaver et al.19,20 conclude that the coverage is never above 0.1 monolayer in acid solution, while Blizanac et al. report almost full monolayer coverage as well as well-formed CO stripping peaks on cyclic voltammograms.23 The differences between the data at polycrystalline and single-crystal surfaces are very surprising and an explanation must await further studies. We report here electrochemical studies of CO oxidation on carbon- and titania-supported Au nanoparticles as well as
10.1021/jp074651u CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007
CO Oxidation on Gold in Acidic Environments
Figure 1. Schematic of the electrochemical array cell. CE: counter electrode compartment. (WE: working electrode compartment, RE: reference electrode compartment, GS: glass sinter, and GS/GI: gas inlet with glass sinter)
polycrystalline Au for comparison. The investigation uses a novel combined combinatorial physical vapor deposition (PVD) and electrocatalyst screening methodology to elucidate both support as well as particle-size effects on the reaction.24-26 Bulk gold was investigated using rotating disc electrode voltammetry. We have recently published a short communication on the electrochemical oxidation of CO on supported Au nanoparticles.27 We are not aware of other work on supported Au, although some papers report studies at Pt nanoparticles.28-35 2. Experimental Section 2.1. Sample Preparation. The preparation of the samples has been described in detail elsewhere.24-26 In brief, the electrochemical screening arrays were micro-fabricated on silicon by Applied Microengineering Ltd., UK. Each array consisted of 10 × 10 gold electrodes (1 mm × 1 mm), each with individual electrical contact tracks to external contacts at the edges of the silicon nitride-capped silicon wafer (31.8 mm × 31.8 mm). The deposition of C and TiOx substrate layers and gold nanoparticles was carried out in a purpose-built molecular beam epitaxy system modified for high throughput synthesis of graded-component, thin film materials.26 The gold electrodes were first coated with a uniform layer of carbon (typical thickness 30-60 nm) or TiOx (typical thickness 60100 nm and where x ) 1.96-1.99) by rotation of the substrate during deposition.24 Then gold was deposited with a graduated flux of Au atoms across the diagonal or vertical (dependent on the chamber and hence on the source position) of the array, controlled through a shutter in the PVD system.26 The array data were compared to measurements at bulk gold (5 mm disc prepared by melting Alfa Aesar Premion gold foil, 99.9985%) and TiOx/Au-coated Ti disc electrodes (rotating disc electrode setup, Pine Instruments). The coating of the Ti discs (diameter 5 mm) with TiOx and Au particles also followed procedures described earlier.24 2.2. Electrochemical Measurements. The electrochemical experiments with the TiOx/Au arrays were carried out either in a new three-compartment glass cell with a water jacket (see Figure 1) or with the C/Au arrays in the previously described cell.25 The new cell had a counter electrode CE compartment (CE gold gauze, Alfa Aeser 99.99%) which was separated from the working electrode WE compartment by a glass sinter. The reference electrode (RE) was mounted in a Luggin capillary whose tip was placed a few millimeters from the array. A specifically designed socket allowed precise positioning of the array to ensure electrical contact. The array was sealed with a Viton gasket to the working electrode compartment of the glass cell. The glass cell was cleaned by repeated boiling in ultrapure
J. Phys. Chem. C, Vol. 111, No. 45, 2007 17045
Figure 2. (a) Steady-state polarization for the carbon monoxide oxidation on a polycrystalline gold electrode. The insert shows the Levich plot at 1.2 V for the sweep to more positive potentials. The measurement was taken at ν ) 20 mV s-1 in CO-saturated 0.5 M HClO4. The dashed voltammogram shown is a base voltammogram of the same electrode in an argon-deaerated electrolyte under identical conditions. (b) Tafel presentation of the positive scan at 900 rpm. Dashed gray line is the Tafel fit with a slope of 118 mV dec-1.
water, followed by a concentrated perchloric acid rinse, and finally washed with ultrapure water. The experimental reference electrode was a commercial mercury/mercuric sulfate electrode (MSE-0.5 M H2SO4, Sentec). All potentials presented in this report are however reported against the reversible hydrogen electrode RHE (T ) 298 K, 0.5 M HClO4,). The MSE was calibrated before experiments against a commercial hydrogen electrode (Hydroflex, Gaskatel GmbH) in the working electrolyte (0.5 M HClO4), and the potential was typically 0.697 ( 0.009 V. The electrochemical responses of the 100 electrodes in the array were measured simultaneously using a potentiostat, 100 channel current followers, and data acquisition cards and a PC together with software written in the laboratory; this instrumentation was described earlier.28 The RDE measurements were carried out in a standard three-electrode/two-compartment, glass cell with a Luggin capillary. It also had a water jacket. Measurements on TiOx/Au and bulk gold were undertaken at a temperature of 298 K by pumping water from a thermostatically controlled water bath (Grant). Both cyclic voltammograms (50 mV s-1) and potential step experiments were carried out at the arrays. The activity of the nanoparticulate samples was assessed using a potential step sequence. The potential was held initially at 0 V (TiOx/Au) and 0.05 V (C/Au) for 45 s to give a baseline in the absence of CO oxidation and then stepped to 0.2 V (only TiOx/Au), 0.3, 0.4, 0.5, and 0.6 V (90 s each); the current was recorded at the end of the pulse. At the RDE, steady-state voltammograms were recorded between 0 and 1.8 V (bulk gold) at a scan rate of ν ) 20 mV s-1. The rotation rates were 400, 900, 1600, and 2500 rpm. Solutions of 0.5 M aqueous perchloric acid were prepared using ultrapure water (ELGA Ultrapure, 18.2 MΩ cm, total carbon content +0.45 V at both the C-supported nanoparticles and bulk gold. The other striking difference is observed at potentials positive to the onset of gold oxidation. At the C-supported gold nanoparticles, the voltammograms again show the features associated with inhibition of the CO oxidation by oxide formation and reactivation on the reverse scan. This is identical to the behavior obtained at bulk gold (Figure 3). In contrast, at the Au on the TiOx support, inhibition of CO oxidation does not occur at high anodic potentials. The apparent insensitivity to CO oxidation to anodic potentials can only be a result of an alternative mechanism to the oxidation (which does not rely on clean gold sites) or modified redox behavior of the titania-supported gold particles. We believe that it is likely to be due to the latter, that is, changes in the electrochemistry of the Au/AuO couple of gold nano-
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Figure 4. Cyclic voltammograms on a TiOx/Au 10 × 10 electrode array in CO-saturated 0.5 M HClO4 at 298 K. The voltammograms were measured at a scan rate of 50 mV s-1.The particle sizes of gold range from 2 to 6.3 nm along the diagonal (indicated).
Figure 5. A comparison of the cyclic voltammetry in CO-saturated electrolyte (0.5 M HClO4) of gold supported on carbon (gray) and titania (black) for four different particle sizes (indicated). The measurements were made at a temperature of 298 K, and the scan rate was ν ) 50 mV s-1.
particles on a TiOx substrate, and this will be the subject of another publication.39 Suffice it to report here that the Au/AuO couple becomes less reversible at small nanoparticles, and hence it appears more difficult to form the oxide on small gold nanoparticles. Such changes in redox behavior are not seen at the C-supported gold nanoparticles. The oxidation of CO at the carbon-supported samples was examined further by comparing the currents for CO oxidation
at the different particle sizes, at +0.8 V during the potential sweep experiment presented in Figure 5. The current densities were recalculated using the real area of gold exposed to the electrolyte (as calculated from TEM data).24 The resulting plot of CO oxidation activity as a function of particle size is shown in Figure 6, the large number of data points arising from the use of the electrode arrays. It is clear that there is a particlesize effect; below 3 nm, there is a steep decrease in activity. A
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Hayden et al.
Figure 6. Particle-size-dependent specific activity at 0.8 VRHE for the CO oxidation at gold nanoparticles supported on carbon (current data extracted from cyclic voltammetry shown in Figure 5), using TEMdetermined surface areas to correct the real surface area currents.
Figure 7. Potential step experiments in CO-saturated 0.5 M HClO4 on selected electrodes out of a (10 × 10) TiOx/Au array. The potentials are indicated on the graph; also the particle diameters are shown on the right-hand side. The measurement was taken at 298 K.
similar “particle-size effect” was found for oxygen reduction on both C- and TiOx-supported Au nanoparticles.25 The lower CO oxidation activity at the 2.5 nm Au particles on carbon explains the slight difference in the cyclic voltammetry reported in Figure 5. At the larger nanoparticles, Figure 5b-d, the CO oxidation peak is fully formed before the gold oxide formation commences and deactivation occurs. The difference in potential between the maximum in the CO oxidation peak, and the commencement of deactivation, diminishes as the nanoparticles become smaller. At the smallest nanoparticles, Figure 5a, a larger overpotential is required to drive CO oxidation, and deactivation commences before the oxidation is fully mass-transfer-controlled. In addition, at the bigger particle sizes (e.g., 7.5 and 4 nm) a second peak was observed at around 1.4 V, associated with the charge transferred in the formation of AuO (see Figure 3 for bulk gold). This peak decreased as the gold surface area decreased, and at particle sizes below 4 nm it was visible only as a shoulder. At the TiOx/Au surfaces, Figure 5a-d, CO oxidation takes place over a wider range of potentials. This is particularly evident for the smaller supported particles, with significant oxidation at low potentials. At more positive potentials there are no signs of deactivation (positive scan) or hence reactivation (reverse scan). A small AuO reduction peak is seen on the reverse scan at the larger nanoparticles, as in the case of C-supported Au. While it is apparent that toward the limit of larger particle size, CO oxidation behavior tends to that expected for Au, an “activated” oxidation process is also taking place. Further evidence of a more complex oxidation comes from our attempts to identify a linear region in the Tafel plots for the new CO oxidation reaction occurring at low overpotentials: The Tafel slopes have been found to be significantly higher than 120 mV per decade. The current densities over the range 0.2-0.5 V are very large compared to those at C/Au or bulk gold, and it can be seen in Figure 5b that the first oxidation process contributes ∼50% of the peak current density.27 In view of unusual CO oxidation activity at these low potentials, the activity was investigated using a potential step technique whereby the potential was stepped more positive at 90 s intervals. Figure 7 shows the results from three electrodes in an array with TiOx/Au surfaces with Au nanoparticles of different mean diameters. It can be seen that activity depends strongly on particle size and it does not simply increase with particle size. The activity increases as the potential is made more positive and at each potential the
activity remains almost constant for the 90 s, with some initial decrease as a result of mass transport limitation. The variation of the CO oxidation activity with particle size is further illustrated with the data at +0.3 and 0.5 V in Figure 8. The large number of data points resulting from the “combinatorial approach” employing the 10 × 10 arrays provide impressive statistics in identifying the particle-size dependence. The dependence of particle size is highly peaked. Below ∼3 nm, the activity decreases rapidly. Note that this was also the case for CO oxidation on carbon-supported Au nanoparticles. The peak in activity for the titania-supported nanoparticles results from the overlap of the deactivation with a steady increase in activity as particle sizes decrease in the range 7-3 nm. We suggest that the apparent maximum in activity observed at 3 nm results from two distinct contributions. There is an “intrinsic” deactivation (observed on both carbon and titania) for small Au particles. In the case of titania-supported Au, there is a unique activation of the Au particles for CO oxidation. 4. Discussion Bulk gold electrodes show very well formed and steep responses for the oxidation of CO in 0.5 M HClO4, and the voltammetry at both stationary and rotating disc electrodes; the reaction becomes mass-transport-controlled positive to the peak/ wave (Figures 2 and 3). The oxidation occurs at substantially less positive potentials than those for the formation of monolayer gold oxide and, indeed, the oxidation of CO is inhibited once a full monolayer of gold oxide is formed. On the negative-going scan, full reactivation of the CO oxidation reaction occurs at potentials where only a very small fraction of the gold oxide surface is reduced, as evidenced in argon-purged electrolyte. This suggests that only a few active gold sites are required to oxidize CO efficiently, at least at these high over-potentials. Surfaces with only a small fraction of Au sites are very active for CO oxidation. The mechanism for the oxidation of CO on gold, however, remains unclear. We are not aware of any equivalent earlier study of CO oxidation on polycrystalline gold, but we would stress that the results we observe are very similar to those for single-crystal gold surfaces.23 In the case of carbon-supported Au surfaces, CO electrooxidation catalysis is similar to that observed on bulk gold for nanoparticles >3 nm. The voltammetry is similar in that the onset of the oxidation takes place at the same overpotential, and oxidation of the supported gold strongly inhibits the reaction
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Figure 8. Titania-supported gold particle-size-dependent specific activities (TEM-corrected) at the potential steps of 0.3 and 0.5 V vs RHE at 298 K. The x-error bars indicate the standard deviation of the particle-size distribution, and the y-error bars are the standard deviation of the current density across one diagonal at one (10 × 10) array (worst case).
(Figure 5). At particle sizes
