Abstract
Highly active and stable bimetallic Au–Pd catalysts have been extensively studied for several liquid-phase oxidation reactions in recent years, but there are far fewer reports on the use of these catalysts for low-temperature gas-phase reactions. Here we initially established the presence of a synergistic effect in a range of bimetallic Au–Pd/CeZrO4 catalysts, by measuring their activity for selective oxidation of benzyl alcohol. The catalysts were then evaluated for low-temperature WGS, CO oxidation, and formic acid decomposition, all of which are believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the introduction of Pd to a monometallic Au catalyst resulted in a significant decrease in catalytic activity. Furthermore, monometallic Pd was more active than Pd-rich bimetallic catalysts. The nature of the anti-synergy was probed by several ex situ techniques, which all indicated a growth in metal nanoparticle size with Pd addition. However, the most definitive information was provided by in situ CO-DRIFTS, in which CO adsorption associated with interfacial sites was found to vary with the molar ratio of the metals and could be correlated with the catalytic activity of each reaction. As a similar correlation was observed between activity and the presence of Au0* (as detected by XPS), it is proposed that peripheral Au0* species form part of the active centers in the most active catalysts for the three gas-phase reactions. In contrast, the active sites for the selective oxidation of benzyl alcohol are generally thought to be electronically modified gold atoms at the surface of the nanoparticles.
Highlights
Since the early reports of the high activity of gold as a catalyst for the hydrochlorination of acetylene[1] and low-temperature CO oxidation,[2] this precious metal has been shown to catalyze a wide range of reactions, including the low-temperature watergas shift (WGS) reaction,[3] the selective hydrogenation of nitro arenes,[4] and the upgrading of hydrocarbons.[5]
We have shown that the introduction of Pd into Au catalysts intended to enhance activity and allow the catalysts to be operated at lower temperatures that favor increased stability is very detrimental to the catalytic activity for WGS, formic acid decomposition (FAD), and CO oxidation
This anti-synergy showed a dependence on nanoparticle size, as revealed by the ex situ techniques, in situ CODRIFTS appears to detect the relative populations of the metal−support periphery adsorption sites and the adsorption sites on top of the metal particles
Summary
Since the early reports of the high activity of gold as a catalyst for the hydrochlorination of acetylene[1] and low-temperature CO oxidation,[2] this precious metal has been shown to catalyze a wide range of reactions, including the low-temperature watergas shift (WGS) reaction,[3] the selective hydrogenation of nitro arenes,[4] and the upgrading of hydrocarbons.[5]. No significant synergy was observed in those catalysts, regardless of the preparation method, indicating that the presence of core−shell structures does not adversely or positively affect the catalytic activity for CO oxidation.[26] One possible explanation of this effect is the segregation of Au and Pd alloys that has been noted in Au−Pd alloys under various atmospheres, especially under CO exposure, whereby the strong affinity of CO for Pd causes it to preferentially migrate to the surface.[27]. This structure insensitivity was investigated by Lopez-Sanchez et al using Au−Pd/TiO2 materials prepared by colloidal methods. To account for the variations in metal loadings between catalyst samples, the catalytic activity is expressed throughout this paper as moles of reactant converted per hour per total moles of metal on the catalyst: e.g., MCO converted h−1 Mmetal−1
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