Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO‐Based Catalysts: The Role of Palladium–Zinc Alloy in Selective Methanol Synthesis

Abstract

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Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO‐Based Catalysts: The Role of Palladium–Zinc Alloy in Selective Methanol Synthesis

“…This corresponds to a complete reduction of the palladium present, along with some of the zinc oxide, to yield a highly dispersed palladium-zinc alloy phase (RPd−Zn = 2.59 Å, RPd−Pd = 2.95 Å). [22] Exposure to the reaction mixture, and then carbon dioxide, does not affect the palladium-zinc alloy (Figure 2d). Overall, the fitting of k 3 -weighted Pd K-edge EXAFS of 2Pd-ZnO-np (RPd−Zn = 2.59 Å, RPd−Pd = 2.92 Å) shows that, once reduced in hydrogen, the nanoparticulate palladium-zinc phase is retained under all of the conditions applied, irrespective of the gaseous environment experienced (Tables 1 and S3).…”

“…According to EXAFS analysis (Figure 2d, Table S3), XRD (Figure 2e and Figures S5-S6), and TEM (Figures S7-S8), the resulting phase is a palladium-zinc alloy, in a good agreement with previous studies, where the formation of this phase was observed even at ambient pressure. [17,22,32] The formation of the palladium-zinc alloy phase is facilitated by hydrogen spillover and accumulation of hydrogen inside metallic palladium. [22] Furthermore, and irrespective of the difference in palladium particle size and distribution, the 2Pd-ZnO-i sample shows very similar behavior during operando XAS study (Figure S3).…”

“…[17,22,32] The formation of the palladium-zinc alloy phase is facilitated by hydrogen spillover and accumulation of hydrogen inside metallic palladium. [22] Furthermore, and irrespective of the difference in palladium particle size and distribution, the 2Pd-ZnO-i sample shows very similar behavior during operando XAS study (Figure S3). In contrast to our previous study, [27] where we have observed changes in Cu K-edge XANES that result from an oxidative desegregation of zinc from a copper-zinc alloy phase upon switching to a CO2/H2 mixture, the position of the palladium white line remains constant, not only during carbon dioxide hydrogenation conditions, but even after switching to pure carbon dioxide (Figure 2c).…”

“…Under a hydrogen rich atmosphere and at elevated temperature (≥150 °C), Pd/ZnO is known to form a palladium-zinc alloy phase, which is considered as providing the active sites for various catalytic applications including methanol steam reforming, reverse water gas shift, and hydrogenation reactions. [16][17][18][19][20][21][22] The formation of a palladium-zinc alloy phase is also often associated with high activity and selectivity towards methanol in the hydrogenation of carbon dioxide. [5,[23][24][25] Although the mechanism of methanol synthesis over Pd/ZnO is not fully understood, it is considered that the palladium-zinc alloy is able to generate and stabilize formate intermediate species, which may then be hydrogenated to yield methanol.…”

Recent Progress in Direct DME Synthesis and Potential of Bifunctional Catalysts

“…In Pd-Zn alloys, the SMSI effect between metallic Pd and ZnO at high temperatures ensures the formation of a stable alloy, although the nature of the active sites is the subject of scientific debate [77]. On the one hand, the formation of PdZn alloys often correlates with catalytic activity and methanol selectivity, [78][79][80][81] however, Pd decorated with ZnO x islands are partly discussed as active sites [82].…”