A Computational Study on the Adsorption Configurations and Reactions of Phosphorous Acid on TiO2 Anatase (101) and Rutile (110) Surfaces

Abstract

We report the result of a density functional theory study on the adsorption and decomposition pathways of phosphorous acid (H3PO3) on TiO2 anatase (101) and rutile (110) surfaces. The most stable adsorption structure for H3PO3 and its isomer, HP(O)(OH)(2), is a monodentate adsorption mode for the anatase surface with calculated adsorption energies 23.5 and 38.5 kcal/mol and a bidentate adsorption mode for rutile surface with 26.7 and 36.6 kcal/mol. The mechanisms for the surface reactions of these species have been explicitly elucidated with the computed potential energy surfaces. The barriers for the stepwise H3PO3 H-migration to two nearby bridged O atoms reactions on anatase leads to Ti-OP(OH)O-Ti(a) + 2H-O-b(a) with 7.9 and 6.8 kcal/mol barriers. Even lower activation barriers (1.3 and 2.9 kcal/mol) have been obtained on the rutile (110) surface for the same bond breaking modes. The intermediate Ti-OP(OH)O-Ti(a) thus formed on both surfaces can further decompose via two distinct pathways through H-migration to the P atom and H2O elimination to produce Ti-OP(H)(O)O-Ti(a) and Ti-OPO-Ti(a), respectively. In addition, we have calculated the adsorption and reactions of the dimer of H3PO3 on both surfaces. The most noticeable difference occurs in the energy levels of the H3PO3 reactions on the anatase and rutile surfaces, with the rutile being more reactive than the anatase surface. The predicted adsorption energies show that Ti-OP(OH)O-Ti(a) with two hydrogen atoms on bridged surface oxygen atoms is 47.1 kcal/mol for anatase and 42.4 kcal/mol for rutile; both are low when compared with the Ti-OB(OH)O-Ti(a) on the same surfaces, 140.1 and 134.6 kcal/mol, respectively. Our density of states analysis shows that OB(OH)O has a larger overlap with the TiO2 surface than OP(OH)O has, favoring the former's charge transfer efficiency.