Pt–Ni Subsurface Alloy Catalysts: An Improved Performance toward CH₄ Dissociation

Abstract

Methane-dissociative chemisorption is the rate-determining step in the industrially important steam reforming and dry reforming reactions of methane. Widely used industrial catalysts containing Ni as the active metal face the problems of carbon deposition and deactivation, whereas Pt surfaces with lower barrier are expensive to be used in the industrial scale. Using density functional theory calculations, a series of surface and subsurface Ni–Pt bimetallic surfaces were studied to understand the synergistic catalytic activity of alloying elements toward facilitating methane dissociation and in resisting carbon formation. Addition of Ni to Pt(111) decreased activation energy barriers, whereas a linear increase in barrier was found when Pt is added to Ni(111) surface. The observed reactivity trends were explained using surface-based descriptors like work function, surface energy, and d-band center and also using energy-based descriptors, namely, Bronsted–Evans–Polanyi and transition-state scaling relationships. Changes in barrier heights and locations of the barrier with lattice atom motion were calculated to include the effect of surface temperature on dissociation probabilities. Dissociation probabilities thus calculated at different surface temperatures using semiclassical methods showed that reactivity increased with surface temperature on all surface alloys. Overall, two surfaces, viz., Ni9/Pt(111) and sub-Pt9/Ni(111), showed improved behavior toward CH4 dissociation, irrespective of the composition of underlying layers. C2 formation on these two alloys also showed higher barriers compared to pure Ni(111) surface. However, considering all aspects like energy barriers to CH4 dissociation and CH dissociation, carbon adsorption energy, and cost, the subsurface alloy, sub-Pt9/Ni(111), showed an enhanced overall performance as a reforming catalyst.