(765b) Atomically-Dispersed Pd/ZnO for Selective Non-Aerobic Ethanol Dehydrogenation Reaction

Ethanol dehydrogenation (EDH) is an important first step in selective steam reforming reactions over certain catalysts, but more importantly, it produces valuable chemicals, acetaldehyde and hydrogen. Acetaldehyde is a platform molecule in the synthesis of industrial chemicals and H₂ can be integrated directly with polymer electrolyte fuel cells (PEMFC). Typically, it is conducted under aerobic conditions producing water instead of hydrogen because of catalyst stability issues. Here we consider the non-aerobic EDH reaction. EDH may be accompanied by ethanol decomposition, producing CO in the product. As even a trace amount of CO poisons the PEMFC anode, the development of highly selective EDH catalysts is crucial.

Copper- based catalysts have been widely studied for ethanol dehydrogenation and ethanol steam reforming due to their high selectivity and relatively low cost. Nevertheless, the reaction usually takes place above 200 ^oC, which leads to sintering and deactivation of the copper catalysts. On the other hand, Pd-based catalysts are considered as potential alternatives to copper because of their good long-term stability and resistance to sintering [1]. However, unlike copper, monometallic Pd catalysts bind strongly with carbonyl groups in the form of η²(C, O)-adsorption, resulting in C-C bond breaking so that CO is primarily formed [2]. It has been demonstrated that by introducing another element (Zn, Ga, In) and alloying it with Pd at high temperature (usually 400^oC), the selectivity of Pd for steam reforming of methanol and the dehydrogenation reaction can be dramatically improved [3, 4]. However, a mechanistic understanding of the unique Pd alloying behavior resulting from site isolation of Pd atoms or specific electronic structure due to the neighboring atoms is still lacking. This is true both for the more widely studied methanol as well as for ethanol dehydrogenation reactions.

In the work reported here, we have found that for EDH with high selectivity to acetaldehyde and H₂ isolated Pd sites on solid supports are the catalytically active sites. Atomically dispersed Pd/ZnO is prepared by incipient wetness impregnation, where highly stepped and defective ZnO nanoparticles are synthesized by a solvothermal method to accommodate and stabilize the Pd atoms. Based on catalyst characterization by XPS, CO-DRIFTS, EXAFS and STEM, we have found that Pd is isolated and positively charged, directly binding with oxygen to form Pd^δ+-O species. This atomically dispersed Pd/ZnO is selective to the formation of acetaldehyde and H₂ below 165^oC, while metallic Pd decomposes ethanol to form CO as the main by-product. A different oxide support, Fe₂O₃, was also used to anchor Pd atomically. Both Pd/ZnO and Pd/Fe₂O₃ catalysts show the same apparent activation energy in EDH reaction, suggesting the same active sites in the reaction with no support effect. Our work shows that the geometric structure of Pd matters in the reaction, and isolated Pd sites, unlike the contiguous Pd metal sites catalyze the selective ethanol dehydrogenation to acetaldehyde and H₂. In situ Ethanol-DRIFTS and kinetic isotopic effect studies were conducted to provide a better understanding of the reaction mechanism.

[1 J. Xu, X. Su, X. Liu, X. Pan, G. Pei, Y. Huang, X. Wang, T. Zhang, H. Geng, Methanol synthesis from CO₂ and H₂ over Pd/ZnO/Al₂O₃: Catalyst structure dependence of methanol selectivity, Applied Catalysis A: General, 514 (2016) 51-59.

[2] J. Davis, M. Barteau, Polymerization and decarbonylation reactions of aldehydes on the Pd (111) surface, Journal of the American Chemical Society, 111 (1989) 1782-1792.

[3] M. Armbrüster, M. Behrens, K. Föttinger, M. Friedrich, É. Gaudry, S.K. Matam, H.R. Sharma, The Intermetallic Compound ZnPd and Its Role in Methanol Steam Reforming, Catalysis Reviews, 55 (2013) 289-367.

[4] K. Fottinger, G. Rupprechter, In situ spectroscopy of complex surface reactions on supported Pd-Zn, Pd-Ga, and Pd(Pt)-Cu nanoparticles, Accounts of Chemical Research, 47 (2014) 3071-3079.