CeZrO4 Catalyst During a Low

Oct 24, 2007 - CenTACat and School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast BT9 5AG, Northern Ireland, United Kingdo...
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J. Phys. Chem. C 2007, 111, 16927-16933

16927

Deactivation Mechanism of a Au/CeZrO4 Catalyst During a Low-Temperature Water Gas Shift Reaction A. Goguet,† R. Burch,† Y. Chen,† C. Hardacre,*,† P. Hu,† R. W. Joyner,† F. C. Meunier,† B. S. Mun,‡ D. Thompsett,§ and D. Tibiletti† CenTACat and School of Chemistry and Chemical Engineering, Queen’s UniVersity Belfast, Belfast BT9 5AG, Northern Ireland, United Kingdom, AdVanced Light Source, Berkeley Lab, 1 Cyclotron Road, Berkeley, California 94720, and Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, United Kingdom ReceiVed: June 7, 2007; In Final Form: September 4, 2007

On-stream deactivation during a water gas shift (WGS) reaction over gold supported on a ceria-zirconia catalyst was examined. Although the fresh catalyst has very high low temperature (99.9% pure and used without further purification, and the steam was fed by passing the gases through a temperature controlled water saturator. Within the quartz reactor, the catalyst bed was held between two plugs of quartz wool with the temperature in the reactor controlled by a Eurotherm 2604. The reaction composition was measured using a PerkinElmer Clarus 500 gas chromatograph fitted with a HayeSep N (80-100 mesh) column. The water and hydrogen were analyzed by thermal conductivity detection (TCD), while carbon monoxide, methane, and carbon dioxide were analyzed both by TCD and by a flame ionization detector fitted with a methanator. Typically, 30-150 mg of catalyst was loaded into the reactor and exposed to the stated WGS mixture at the desired temperature and a flow rate of 100-120 cm3 min-1 without any pretreatment. To examine the effect of annealing, the catalyst was treated in N2 at 200 °C for 24 h at a flow rate of 200 cm3 min-1. To examine the effect of reduction, the catalyst was treated in 8.1% H2 in N2 at 200 °C for 16 h at a flow rate of 200 cm3 min-1. To examine the effect of hydrolysis, the catalyst was treated in 6.3-19% H2O in N2 at 200 °C for the desired time at a flow rate of 200 cm3 min-1. In each case, the activity of the catalyst before and after treatment was measured to examine the deactivation. For the deactivation studies, 2.0% CO, 2.4% CO2, 19% H2O, and 8% H2 in N2 were used as the feed mixture to simulate the gas stream exiting a reformer used for stationary applications where there is typically a high steam/carbon ratio. TPO Characterization. Typically, after exposure to the WGS mix, the catalyst was first cooled down to room temperature in N2 flow, then exposed to the oxidizing mixture (1% O2/He, total flow rate 50 cm3 min-1) for 20 min before being ramped up to the desired temperature and held at this temperature for 60 min. After purging in N2 for 10 min, the WGS mixture was reintroduced, and the activity of the catalyst was tested to evaluate if any catalytic activity was recovered. DRIFTS Characterization. The catalyst was characterized using in situ DRIFTS at different stages of deactivation. The experimental setup is described in detail elsewhere.5 Typically, ca. 66 mg of catalyst was loaded in the reactor, and the spectrum of the catalyst at room temperature was recorded. The mirror recorded under an Ar feed prior to the experiment was used as the background. The IR data are reported as log 1/R, which provides a linear representation of the band intensity against sample surface coverage for strongly absorbing media, where the sample reflectance is R ) I/I0.19 Importantly, the DRIFTS reactor used has been modified such that the residence time distribution is Gaussian, indicating that it can be considered as a plug flow reactor. Therefore, the spectra obtained may be considered as representative of the surface of the catalyst under realistic reaction conditions.

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Figure 1. CO conversion measured over 2% Au/CeZrO4 under WGS conditions at 200 °C. Feed: 2.0% CO, 2.4% CO2, 19% H2O, and 8% H2 in N2; total flow rate 120 cm3 min-1; catalyst mass 30 mg.

High-Pressure XPS. High-pressure XPS was performed on station 9.3.2 at the Advanced Light Source (ALS), Berkeley Laboratory, under 400 mTorr total pressure with either a ratio of CO/CO2/H2O/H2 of 1:1:1:1 or H2O/CO of 9:1 using a photon energy of 450 eV. The binding energy of the Au 4f peaks was referenced with respect to the Zr 3d5/2 peak at 182.4 eV from the CeZrO4 support.20 The samples were pressed into a silver plate to limit the amount of charging. DFT Calculations. The total energy calculations were carried out using the SIESTA code21,22 based on the standard DFTslab approach with the generalized gradients approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional and a double-ζ plus polarization basis set. Troullier-Martins normconserving pseudo-potentials were used to describe the ionic cores. The energy cutoff for the real space grid was 150 Ry. The localization radii of the basis functions were determined from an energy shift of 0.01 eV. Spin polarization was included whenever necessary. Recent computational studies of ceria15 using the SIESTA code gave good results for the bulk and surface properties of ceria. CeO2(111) was modeled by a p(3 × 3) unit cell with nine layers in which the top three layers were relaxed. A (2 × 2 × 1) k-point sampling of the surface Brillouin zone was used. An Au10 cluster was built using the following steps: (i) placing an Au10 cluster with a fcc structure and top and bottom layers as (111) facets on CeO2(111); (ii) performing molecular dynamics calculations on the whole system; and (iii) carrying out structure optimization.23 A Au10 cluster on the OH-terminated CeO2(111) surface was calculated in a similar way. Finally, the effect of water dissociation on the structural change of the Au cluster was investigated. Results Activity Tests. A typical deactivation profile is reported in Figure 1. After an initial fast deactivation phase over the first 7 h, the catalyst decreased in activity at a constant rate of 2.65% h-1. Rapid thermal deactivation of this catalyst has been observed previously for this catalyst above 250 °C, which was not influenced by the gas environment.15 Therefore, to examine whether the deactivation at 200 °C was due to a slower rate of thermal deactivation, the catalyst was heated to 200 °C in N2 for 24 h before the catalyst was re-exposed to the WGS reaction mixture. If the deactivation was due to thermal deactivation, heating the catalyst in the absence of the WGS mixture would result in a continued loss in activity. However, no significant change in the activity observed before and after N2 treatment

Deactivation Mechanism of a Au/CeZrO4 Catalyst

Figure 2. CO conversion measured over 2% Au/CeZrO4 under WGS conditions at 200 °C. Feed: 1.4% CO, 2.4% H2O in N2; total flow rate 120 cm3 min-1; catalyst mass 30 mg (black). After 21 h exposure to the starting WGS mixture, the water concentration was increased to 6.3% (green), 8.2% (red), and 12.7% (blue). Following 9 h exposure to a high water content WGS mixture, the initial feed (i.e., 1.4% CO, 2.4% H2O in N2) was reintroduced.

Figure 3. Rate of catalyst deactivation as a function of the water content of the WGS mix for a 2% Au/CeZrO4 catalyst at 200 °C.

with the catalyst also maintaining a similar deactivation rate indicated that thermal deactivation is not the predominant cause of the loss in activity. Similar results were also obtained after exposing the catalyst to 8% H2/N2 flow at 200 °C for 16 h, indicating that over-reduction of the catalyst, as proposed for Pt/CeO2,2 is unlikely to have a significant role in the deactivation of the 2%Au/CeZrO4 catalyst. In contrast, treating the catalyst in 19% H2O/N2 at 200 °C for 15 h resulted in a substantial deactivation with a drop in CO conversion of ∼10%. Figure 2 illustrates the effect of water concentration on the catalyst deactivation rate. As found with the full WGS mixture, on-stream deactivation was observed with a reaction composition of 1.4% CO + 2.4% H2O. After 20 h on-stream, the water concentration was increased to 6.3, 8.2, and 12.7%, while the amount of nitrogen in the feed was decreased to maintain the CO concentration and total space velocity. As expected, the CO conversion increased because the reaction is positive with respect to the water.1 However, the rate of deactivation also increased with an increasing water concentration. Moreover, it was clear that on reducing the water concentration back to 2.4%, the initial condition, a significant additional loss of activity had occurred. The effect of water is shown clearly in Figure 3, where a linear increase in the rate of deactivation during long-term exposure to the WGS conditions was found as the water concentration was raised. TPO. To examine whether gradual carbon deposition contributes to the deactivation of the 2%Au/CeZrO4 catalyst under

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Figure 4. CO2 flux (red line) during the TPO of the 2% Au/CeZrO4 catalyst after 130 h reaction under full WGS conditions at 200 °C. The temperature profile is shown by the black line. No CO flux was observed.

Figure 5. In situ DRIFT spectra of 2% Au/CeZrO4 (a) during 72 h reaction under WGS conditions at 150 °C (feed: 2.5% CO, 7% H2O, 13% H2 in Ar) and the catalyst following reaction being purged in Ar for 10 min and exposed to (b) 7% H2O in Ar and (c) 7% H2O/1% O2/Ar mixture.

WGS conditions, the activity of the catalyst was compared before and after a number of TPO treatments. The TPO experiments were performed after 130 h reaction under the full WGS conditions, which corresponded to a loss of >40% of the initial activity of the fresh catalyst. The TPO profile up to 500 °C is shown in Figure 4. Three features are observed: a sharp CO2 peak at room temperature and two broad peaks at ∼90 and ∼200 °C. The first sharp peak is thought to be due to the oxidation of the carbonyl species, while the broad peaks may be attributed to the removal of carbon-containing species such as formates and carbonates.17 This removal of carboncontaining species at low temperatures under oxidizing conditions is supported by corresponding DRIFTS studies shown in Figure 5. After 72 h reaction under full WGS conditions at 150 °C, the catalyst was purged in Ar for 10 min and exposed to 7% H2O in Ar. Under these conditions, the carbonyl band at 2097 cm-1 disappeared, and a decrease in the bands between 1600 and 1200 cm-1 associated with the removal of the carbonate species was observed. On addition of 1% O2 to the mixture at 150 °C, a further decrease of the intensity of the bands in this region was observed due to the removal of bulk-like carbonates. Interestingly, under these conditions, the 2132 cm-1 band associated with the electronic transition over

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Figure 6. CO conversion measured over 2% Au/CeZrO4 at various times on-stream under WGS conditions at 125 °C. Feed: 2.5% CO, 7% H2O, 13% H2 in Ar.

reduced ceria also disappeared completely, indicating that the ceria was fully reoxidized. Importantly, the activity of the catalyst following TPO up to 230 °C was not found to increase as compared to that found prior to the oxidation treatment, showing that the deactivation due to carbon-containing adsorbed species is unlikely. TPO performed at higher temperatures is not informative in the elucidation of the deactivation mechanism below 250 °C due to the problem of thermal deactivation. For example, after treating the catalyst under oxidizing conditions up to 500 °C, the catalyst activity dropped by 20%; however, this loss in activity is mainly associated with the thermal deactivation of the catalyst above 250 °C reported previously.15 In Situ DRIFTS. In situ DRIFTS analysis of the 2% Au/ CeZrO4 catalyst was carried out at 125 °C with 2.5% CO + 7% H2O + 13% H2 in Ar.5,24-26 This setup allows the correlation of DRIFTS spectra as a function of the sample activity. The CO conversion observed over a fresh sample was initially very high (ca. 80%) but dropped sharply within ca. 20 min before exhibiting a slower decrease (Figure 6). It should be noted that this initial conversion probably includes some CO oxidation coupled with a reduction of the catalyst rather than through the WGS reaction. The corresponding DRIFT spectra revealed the presence of carbonate species from the very beginning of the experiment (Figure 7a). Formates and an intense carbonyl band at 2097 cm-1 were then gradually formed. A carbonyl band at this wavenumber has been assigned to CO adsorbed on metallic gold.27,28 It must be stressed that no carbonyl bands were observed at short times on-stream, while CO(g) was clearly observed over the sample (see Figure 7b). This is likely due to the fact that the gold is initially in the +3 oxidation state and does not strongly adsorb CO. In the presence of CO(g), the cationic gold is rapidly reduced,29 leading to a high initial CO conversion, as opposed to WGS activity, and thereafter, the carbonyl bands are observed. The evolution of the formate and carbonyl signals was followed as a function of time, as both these surface species have often been proposed as main or minor reaction intermediates1 (Figure 8). The surface concentration profile of each species is similar. Initially, the buildup of the concentration of each band was observed and was followed by a gradual decrease in intensity with time on-stream, which was similar to the gradual decrease in the CO conversion as seen previously in Figure 4. It is important to note that from this data, it is not possible to distinguish between reaction mechanisms as this requires both transient kinetic and quantitative information.24 Given that the WGS reaction is first-order in CO, the amount of adsorbed CO may be used as a probe for the surface state of the gold particles. Therefore, the fact that there is a loss in surface adsorbed CO that correlates with the deactivation is a

Figure 7. (a) In situ DRIFT spectrum of 2% Au/CeZrO4 as a function of time on-stream under WGS conditions at 125 °C. Feed: 2.5% CO, 7% H2O, 13% H2 in Ar. (b) Expanded region in more detail between 2000 -and 2500 cm-1. A mirror signal was used as reference.

Figure 8. Relative intensity of formate and carbonyl DRIFTS band measured over 2% Au/CeZrO4 as a function of time on-stream under WGS conditions at 125 °C. Feed: 2.5% CO, 7% H2O, 13% H2 in Ar. The regions 2900-2800 and 2100-2000 cm-1 were used to calculate the intensities of the formate and carbonyl bands, respectively.

significant observation and indicates that the structure of the gold particles is markedly modified during deactivation, resulting in a decrease in CO chemisorption ability. Although a decrease is also found in the formate band during reaction on-stream, this is not a good probe for deactivation of the catalyst. Recent quantitative results from steady-state isotopic kinetic analysis combined with DRIFTS (SSITKA-DRIFTS) on this 2 wt% Au/

Deactivation Mechanism of a Au/CeZrO4 Catalyst

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Figure 10. Side view and top view (inset) of final state of water dissociation (light brown atoms are Ce; red atoms O; yellow atom Au; and white atoms hydrogen).

Figure 9. (a) Side view and top view (inset) of the geometrical structures of Au10/CeO2(111). (b) Side view and top view (inset) of the geometrical structures of the Au10 cluster on an OH-terminated CeO2(111) surface (light brown atoms are Ce; red atoms O; yellow atom Au; and white atoms hydrogen).

CeZrO4 catalyst under WGS conditions have shown that, although the formates exchange on a time scale similar to that observed for the production of CO2 in the gas phase, the rate of decomposition of this surface species only accounts for a fraction of the rate of CO2 being formed. Therefore, the formates are, at best, only a minor intermediate and may be a spectator.30 DFT Results. From in situ EXAFS characterization of this catalyst under WGS conditions, the active form of gold is thought to be comprised of small metallic gold clusters in intimate contact with the oxide support.15 Therefore, to understand the effect of water on the proposed oxide supported gold clusters, DFT calculations were performed on an Au10 cluster on a clean and hydroxylated CeO2(111) surface. Figure 9a shows the optimized structure of the Au10 cluster when this was first placed on a perfect CeO2(111) surface. It can be seen that the first layer of the Au10 cluster binds to oxygen on the surface and that the average distance between the Au atoms in the first layer of Au10 and the O atoms on CeO2(111) is 2.20 Å (i.e., similar to the distance of a single Au atom on the top of a surface oxygen on this surface).15 In comparison, the average distance of Au-OH is 3.04 Å for the Au10 cluster on the OH-terminated CeO2(111) surface. This destabilizing effect of hydroxyl on the binding of the gold cluster to the oxide support is illustrated in Figure 9b. This increase in distance between gold and surface oxygens is found for all the interfacial gold atoms in the cluster with the exception of the Au atom in the center of the first layer, where only a small change as compared to the Au10/CeO2(111) is observed.

As water dissociation is a key step in the WGS reaction, the effect of water dissociation on the structure of the Au10 cluster was also investigated. To this end, water adsorption on an O vacancy was calculated (∼0.6 eV), and the final state of water dissociation at the interface between Au10 cluster and CeO2(111) surface was optimized. Figure 10 illustrates the water dissociation on an O vacancy at the metal-oxide interface. Following reaction and formation of the surface OH, the distance between (H)Au‚‚‚OH is 3.28 Å. Figure 10 clearly shows that the water dissociation results in the Au atom being pushed away from the surface, leading to a weakening of the gold-support interaction. The hydroxylation of the surface is feasible according to DFT calculations either as a consequence of the reaction of surface oxygen atoms with dihydrogen to form surface hydroxyls or via water dissociation, depending on reaction conditions. For example, in the former exothermic reaction, the energy gain is approximately 1.4 eV. High-Pressure XPS Results. The model proposed previously was that under WGS conditions, the gold exists as small (∼1 nm) hemispherical metallic particles in full contact with the CeZrO4 support.15 The active sites are at the gold-oxide interface, and deactivation occurs as the gold particle gradually loses contact with the support. With increasing temperature, the gold detaches from the support, and in the extreme case, the gold nanoparticle becomes a sphere, which is in only point contact with the support. This model is difficult to test; however, it is consistent with high-pressure XPS results taken at 400 mTorr using full WGS conditions. These data showed that as the temperature is increased between 150 and 300 °C, there is a 16% decrease in the relative intensity of the Au 4f peaks as compared to the Zr 3d peaks in a ratio of Au 4f7/2/Zr 3d5/2 of 0.067 to 0.056, respectively. No change in the binding energy was observed with, in all cases, the Au 4f7/2 feature appearing at 84.2 eV. This binding energy is in the range characteristic of small gold particles, with a small final state shift as compared to the accepted value of bulk gold of 83.7 eV. The change in intensity may be rationalized quantitatively by the transformation from gold hemispheres to spheres. Assuming that the attenuation of the X-ray photoelectron flux as it passes through the gold particle is governed by a Beer-Lambert relationship and characterized by a mean free path λ of 6-10 Å31 for electrons of kinetic energy ∼370 eV, the Au 4f peaks would be expected to decrease between 10 and 20%.32 This is in good agreement with the decrease of 16% obtained in the XPS experiments and is consistent with the model just proposed and reported previously for the thermal deactivation.15 In contrast, no change

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Figure 11. Schematic of the proposed (a) structure of gold under WGS conditions showing the site for CO adsorption and (b) change in structure of the Au/CeZrO4 catalyst above and below 250 °C during WGS operation (light brown atoms are Ce; red atoms O; yellow atom Au; and white atoms hydrogen).

in the Au 4f7/2/Zr 3d5/2 intensity ratio was observed in the XPS, changing the gas composition from the full water shift mixture to a H2O/CO ratio of 9:1. Given that the deactivation is found to be dependent on the overall water pressure in the system, the fact that the catalyst does not change significantly at these low overall pressures is not surprising. Discussion The results obtained with the on-stream deactivation catalytic tests before and after exposure to hydrogen or nitrogen clearly show that the loss in activity is not due to a modification of the catalyst structure induced by hydrogen or by a slow thermal dewetting of the gold, which would be kinetically limited at temperatures lower than 250 °C. Furthermore, the fact that TPO experiments performed at or below 230 °C did not lead to catalyst reactivation indicates that the accumulation of carboncontaining species is not likely to be the main mechanism involved in the low-temperature catalyst deactivation unless the species are very strongly bound. However, we think that it is unlikely since TPO performed until 500 °C did not show any additional features and that DRIFTS results showed that at temperatures as low as 150 °C, the visible surface species were readily decomposed. These results are in contrast with reports that the accumulation of formates/carbonates over the goldceria based catalyst led to deactivation.6,8,9 This may be associated with the difference in the supports used in each case as recent results have shown that ceria/zirconia supports appear to be less sensitive to the accumulation of such species.33 The most important effect on the deactivation rate of the Au/CeZrO4 catalyst below 250 °C is the concentration of water either on its own or during the WGS conditions. The steady-state results clearly demonstrate the detrimental effect of water with increasing concentrations of water leading to higher deactivation rates.

From the DFT calculations, the effect of the water is thought to be associated with a dewetting of the gold atoms from the oxide surface. Therefore, during the low-temperature WGS mechanism, the gold on the surface of the support would progressively become detached through a water-assisted dewetting mechanism. The nanoparticle being no longer anchored, the intimate contact between the support and the gold particle, which governs the redox properties of the support and the adsorptive properties of the gold particles, is diminished, and the low-temperature WGS activity is progressively lost. Interestingly, a similar effect has recently been reported for gold supported on TiO2, where for the reduced and oxidized (110) surfaces, small clusters of gold are found, whereas on the hydrated surface, larger aggregates are found due to a weakening of the gold-support interaction.34 This deactivation mechanism is consistent with the DRIFTS results, which show a decrease in the surface CO concentrations during deactivation. As the gold cluster becomes detached, the adsorption characteristics of the gold will be modified. Changes will occur both in terms of the type and in terms of the number of adsorption sites on the gold particle at positions remote from the interface. In general, CO adsorption is found to be weak on metallic gold supported on an inert oxide, such as silica, even at defect sites.35 Therefore, the fact that CO is observed over Au/CeZrO4 indicates that the nature of the metal-support interaction is critical in determining the strength of CO chemisorption. The significance of this result is that it gives us a probe for monitoring the special gold sites that can adsorb CO sufficiently strongly to be visible in DRIFTS at elevated temperatures. The previously reported EXAFS results showed that the gold did not undergo significant metal sintering.15 Therefore, the loss of these special sites when the catalyst deactivates during the WGS reaction or after exposure to water is best interpreted in terms of a breakage of the gold-support

Deactivation Mechanism of a Au/CeZrO4 Catalyst link at the metal-oxide interface. This then results in detachment of the gold. Figure 11a shows a schematic of the proposed gold particle on the surface of the oxide under WGS conditions. The interfacial gold atoms (Auδδ+) are thought to be those that are able to adsorb CO at elevated temperatures. This is supported by our DFT calculations, which show that CO chemisorption is enhanced on the Au site that is near the interface of AuCeO2 as compared to CO chemisorption on the Au cluster without support. An important feature of the model is that the metal cluster is bound to the surface of the oxide via gold atoms (Auδ+) in cerium vacancies in the oxide lattice. It is the removal of these Auδ+ anchor points that is responsible for the dewetting of the gold cluster. While the formation and stability of the gold clusters is essential to achieve and maintain a high WGS activity, the oxide support in the vicinity of the metal is also critical. Wang et al. have shown that the WGS activity over ceria supported gold and copper is dependent on the formation of oxygen vacancies close to the metal cluster.36 These interfacial/ vacancy sites are proposed to be responsible for water dissociation, which may lead to the formation of the surface hydroxyl groups that lead to the deactivation of the catalyst. Therefore, for the Au/CeZrO4 catalyst, with both the onstream deactivation and the deactivation observed on raising the temperature above 250 °C,15 a similar mechanism may be proposed. In both cases, the important effect is the loss of the metal-support interaction either by thermal activation or by hydrolysis. Many studies have indicated that for the gold to remain active, an intimate contact between the metal and the support must be maintained.37 Figure 11b shows a schematic of the proposed change in structure above and below 250 °C. Conclusion Using a combination of experimental and theoretical methods, a mechanism for the deactivation of Au/CeZrO4 catalysts during a low-temperature WGS reaction has been proposed. Hydrolysis of the interface between the gold and the support reduces the metal-support interaction and causes the metal nanoparticle to detach from the surface. The intimate contact between the support and the gold is essential for high WGS activity, and the deactivation rate is directly proportional to the concentration of water in the reaction mixture. Over-reduction of the catalyst and site blocking by formates and carbonates are not thought be significant in the deactivation on-stream in the present case. The proposed deactivation mechanism is consistent with that described previously15 to explain thermal deactivation at elevated temperatures. Acknowledgment. We acknowledge the EPSRC for funding under the CARMAC Project and the ALS for beamtime. Supporting Information Available: XPS data showing the effect of temperature on the Au 4f peaks. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Burch, R. Phys. Chem. Chem. Phys. 2006, 8, 5483. (2) Zalc, J. M.; Sokolovskii, V.; Loffler, D. G. J. Catal. 2002, 206, 169.

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