A DFT study of associative and dissociative chemical adsorption of DMMP onto SnO2(110) surface nano-cluster

Density functional theory has been applied to a computational study of associative and dissociative adsorption of dimethyl methylphosphonate (DMMP) on SnO2 (cassiterite). Rutile SnO2(110) surface, free of defects and of OH, has been modeled using free-standing nano-cluster with unsaturated edge atoms terminated with hydrogen. Associative (molecular) adsorption occurs via the formation of a Sn5c center dot center dot center dot O=P dative bond along with a very strong interaction between the adjacent Sn-5c site and the methoxy O atom of the adsorbate. C-H center dot center dot center dot O hydrogen bonds between CH3 groups and surface O atoms also contribute. Associative adsorption of DMMP on SnO2(110) has been compared to TiO2(110). Two hypothetically possible products have been considered for the first step in dissociative adsorption of DMMP on the adsorbent. Dissociative adsorption of DMMP on the SnO2(110) surface shows formation of two dative bonds via O atoms of the O-P=O (bridge) and formation of a bidentate structure. The key requirement for the dissociative adsorption is the availability of pairs of under-coordinated metal cations and surface base sites (oxygen). Thus, the coordination environment of the surface cations and anions is the critical surface characteristics for describing surface reactions of Lewis and Bronsted acids.