Computational catalysts design for non-transitional metal catalysts

Poul Moses
Haldor Topsoe

We will present two cases where scaling relations; thermochemistry and kinetics have been combined to understand catalytic reactions over non-transitional metal catalysts. Case 1 is trends in hydrodesulfurization catalysis based on realistic surface models. Case 2 is methanol synthesis over ZnO.
In case 1) trends in hydrodesulfurization (HDS) activity are investigated on the basis of surface properties calculated by density functional theory (DFT) for a series of HDS catalysts. It is shown that approximately linear correlations exist between HS group binding energies and activation barriers of key elementary reactions in HDS of thiophene. These linear correlations are used to develop a simple kinetic model, which qualitatively describes experimental trends in activity. The kinetic model identifies the HS-binding energy as a descriptor of HDS activity. This insight contributes to understanding the effect of promotion and structure-activity relationships.
In case 2 we examine the thermochemistry of methanol synthesis intermediates using density functional theory (DFT) and analyze the methanol synthesis reaction network using a steady-state micro-kinetic model. The energetics for methanol synthesis over ZnO(0001) are obtained from DFT calculations using the RPBE and BEEF-vdW functionals. The energies obtained from the two functionals are compared and it is determined that the BEEF-vdW functional is more appropriate for the reaction. The BEEF-vdW energetics are used to construct surface phase diagrams as a function of CO, H2O, and H2 chemical potentials. The computed binding energies along with activation barriers from literature are used as inputs for a mean-?eld micro-kinetic model for methanol synthesis including the CO and CO2 hydrogenation routes and the watergas shift reaction. The kinetic model is used to investigate the methanol synthesis rate as a function of temperature and pressure. The results show qualitative agreement with experiment and yield information on the optimal working conditions of ZnO catalysts.
Case 1 and case 2 illustrates how concepts developed for computational catalysts design over transition metal may be transferred to more complex catalysts systems. The two cases also illustrate where oxides and sulfides differs from metals and where care must be taken in order to correctly model surface chemistry of oxides and sulfides.

Presentation (PDF File)

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