Scaling Relationships for Adsorption Energies on Transition Metal Oxide, Sulfide, and Nitride Surfaces

Abstract

There has been substantial progress in the description of adsorption and chemical reactions of simple molecules on transition-metal surfaces. Adsorption energies and activation energies have been obtained for a number of systems, and complete catalytic reactions have been described in some detail. [1][2][3][4][5][6][7] Considerable progress has also been made in the theoretical description of the interaction of molecules with transition-metal oxides, [8][9][10][11][12][13][14][15][16][17][18][19] sulfides, [20][21][22][23][24][25] and nitrides, [26][27][28][29] but it is considerably more complicated to describe such complex systems theoretically. Complications arise from difficulties in describing the stoichiometry and structure of such surfaces, and from possible shortcomings in the use of ordinary generalized gradient approximation (GGA) type density functional theory (DFT).[30]Herein we introduce a method that may facilitate the description of the bonding of gas molecules to transitionmetal oxides, sulfides, and nitrides. It was recently found that there are a set of scaling relationhips between the adsorption energies of different partially hydrogenated intermediates on transition-metal surfaces.[31] We will show that similar scaling relationships exist for adsorption on transition metal oxide, sulfide, and nitride surfaces. This means that knowing the adsorption energy for one transition-metal complex will make it possible to quite easily generate data for a number of other complexes, and in this way obtain reactivity trends.The results presented herein have been calculated using self-consistent DFT. Exchange and correlation effects are described using the revised Perdew-Burke-Ernzerhof (RPBE) [32] GGA functional. It is known that GGA functionals give adsorption energies with reasonable accuracy for transition metals. [32,33] It is not clear, however, whether a similar accuracy can be expected for the oxides, sulfides, and nitrides, although there are examples of excellent agreement between DFT calculations and experiments, for example, with RuO 2 surfaces.[9] In our study we focused entirely on variations in the adsorption energies from one system to another, and we expected that such results would be less dependent than the absolute adsorption energies on the description of exchange and correlation.For the nitrides, a clean surface and a surface with a nitrogen vacancy were studied. For MX 2 -type oxides or sulfides, an oxygen-or sulfur-covered surface with an oxygen or sulfur vacancy was studied. The structures of the clean surface considered in the present work and their unit cells are shown in Figure 1. The adsorption energies given below are for the adsorbed species in the most stable adsorption site on the surface.By performing calculations for a large number of transition-metal surfaces of different orientations, [31] it was found that the adsorption energy of intermediates of the type AH x is linearly correlated with the adsorption energy of atom A (N, O, S) according to Equation (1):Here the scali...

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