Density functional theory (DFT) calculations are often used to model surface reactions and the effects of coverage therein, typically using flat periodic single-crystal surface models. These flat periodic surface models, however, lack curvature and thus cannot allow for lateral relaxation of strained adlayers created by repulsive co-adsorbate interactions or by transition states larger than their relevant precursors (reactions with positive activation areas). The inability of the adlayer to laterally relax leads to inaccurate coverage estimates and coverage effects. Here, we will examine Ir-catalyzed alkane hydrogenolysis on H*-covered surfaces and Ru-catalyzed CO-H2 reactions on CO*-covered surfaces and demonstrate the necessity for curved catalyst models in estimating the saturation coverage of H* and CO* species, the site-requirements for surface reactions, and the influence of activation area and co-adsorbate interactions on the rates of high-coverage reactions. Site requirements for a well-studied probe reaction, ethane hydrogenolysis reactions on H*-covered Ir surfaces, were determined for both Ir(111) and Ir119 hemispherical particles (1.6 nm). These results demonstrate that the Ir(111) surface model incorrectly predicts ethane hydrogenolysis rates and site requirements (and therefore H2-pressure dependencies) because the H*-adlayer cannot relax during formation of the kinetically relevant transition state from the H*-covered surface (a positive activation area) while the Ir119 model allows for H* adlayer relaxation provides quantitative agreement with kinetic data. CO-H2 reactions create space on crowded Ru surfaces as the CO*-activation transition state occupies a smaller area than the CO* which it replaces (a negative activation area). Adlayer strain is relieved upon formation of the CO*-activation transition states, therefore decreasing activation energies as CO* coverage increases and ultimately mitigating CO inhibition. Without these repulsive stresses which favor CO* activation, Fischer-Tropsch synthesis would be impossible. Furthermore, hydrocarbon chains on Ru surfaces prevent CO* readsorption and thus facilitate vicinal CO* activation reactions which enable rapid chain growth via CHx*-insertion reactions (known as ‘carbene’ mechanisms).

Impacts of activation area during high-coverage alkane hydrogenolysis and CO hydrogenation

2019 Spring National Meeting

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