(664c) Design of Multi-Functional Catalytic Interfaces from First Principles: Modelling Water Gas Shift on Pt/MgO

Pushkar G. Ghanekar¹ and Jeffrey P. Greeley^1*
[1] Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA.

Metal nanoparticles dispersed on oxide supports have found applications in a vast number of chemical processes related to catalysis, energy generation, and environmental remediation. In some cases, the interface between the metal and the supporting oxide is theorized to exhibit unique reactivity compared to just the metal or the oxide in isolation[1-3]. The Water-Gas Shift reaction (WGSR), in particular, is known to show sensitivity to such metal/oxide interfaces [4-5], and our previous results on Au/MgO have shown the importance of carefully modelling the interfacial region for this catalyst[6]. Expanding on those results, we now look at Pt nanoparticles supported MgO. Platinum is known to be active for WGSR but is quite electronically distinct from Au[7], permitting us to establish broader reactivity trends for supported metal catalysts. Using periodic DFT calculations, a highly optimized model of Pt nanowire on MgO substrate is developed which avoids effects of unphysical interfacial strain or quantum size effects usually anticipated in small cluster models[8]. DFT-derived reaction energies, coupled with rigorous dual-site microkinetic modelling, are used to elucidate the activity of the catalyst. The interfacial region is shown to greatly accelerate the water dissociation step which has an exceedingly high barrier on just the metal(Pt) or oxide(MgO) in isolation. The results for low adsorbate coverages suggest that two competing pathways contribute to the reaction (carboxyl and redox pathways). However, refined entropic analyses and explicit optimization of energetics at high coverages of CO, which partially poisons the platinum nanoparticle, demonstrates that the major contribution stems from the carboxyl mechanism. Finally, building upon this DFT/microkinetic model, we briefly discuss efforts to tune and optimize WGSR rates by perturbing the electronic properties of the catalysts via substitutional doping of the MgO matrix, and we present a descriptor that captures some of the observed trends from these perturbations.

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