(744e) Catalytic Trends for Nanoscale Catalysts from First-Principles Modeling and Tuning of Catalytic Reactivity for Fuel Production

It is still a rather open area regarding our fundamental understanding for effective manipulation of the activity and selectivity of catalyst for chemicals and fuel production. In this study, a first-principles-based kinetic model has been developed in order to understand the catalytic trends of reactivity and selectivity for hydrogen production on nanoscale single crystal nickel particles. The earth abundant nickel is a widely utilized industrial catalyst for hydrogen production through reforming, which is often accompanied by high temperature water-gas shift reaction (WGSR) that further boosts the hydrogen yield. However, nickel also promotes a number of side reactions, such as methanation and hydrogenolysis, both of which would lower hydrogen selectivity, and thus making the effort to tune the activity and selectivity of nickel catalyst an urgent fundamental research topic.

The mechanistic guidelines were first established using periodic density functional theory (DFT) calculations on the Ni(111), Ni(211) and Ni(100) single crystal facets, followed by the establishment of the most thermodynamically and kinetically relevant hydrogen production pathways. Furthermore, the reaction routes involving CO methanation consuming hydrogen will also be considered. In this investigation, the adsorbate-adsorbate interactions - between the key WGSR and methanation reaction intermediate pairs, such as CO-CO, CO-H₂O, CO-OH, CO-O, and CO-CH – have been extensively considered and shown significant impact on the kinetic trends on nickel single crystal surfaces. It has also been noted that, although most of the lateral interactions are repulsive in nature, catalyst lattice structures and intermolecular hydrogen bonding will create exceptions and complicate the trend analysis. Mean-field microkinetic modelings based on the molecular-level mechanism showed that the catalyst reactivity varies at different temperature regimes. At temperature below 570 K, the H₂ production rate is higher on the open flat surface, such as Ni(100), than on the stepped surface, e.g., Ni(211). Above 570 K, this trend has been reversed. Detailed analysis of the hydrogen production route also revealed that the contribution from the redox and carboxyl pathways also vary at different facets. This study will enrich our understanding of the hydrogen chemistry at the molecular level, and would also provide theoretical insights into tuning the properties of nanoscale transition metal catalysts by manipulating the surface structure and morphology.