A computational study of catalysis by gold in applications of CO oxidation

Abstract

Au based catalysts have been extensively studied since Masatake Haruta in Japan discovered that small Au nanoparticles supported on transition metal oxides are exceptionally active catalysts for oxidation reactions at low temperature. However, what makes gold, being inert in the bulk form, active is still a big question. Related key challenges to understand are the particle size effect, the role of supports and the nature of the active site. The reactivity of different Au surfaces (given below) and reaction mechanisms for CO oxidation, water gas shift reaction (WGSR) and Preferential oxidation of CO (PrOx) have been studied throughout this thesis. Density Functional Theory has been used to calculate the energetical, geometrical and vibrational properties of the adsorbates as well as minimum energy path for the reactions on the different surfaces. To explore the nature of active sites, decrease in the coordination of gold atoms and co-operative effects between metal and support have been employed. The effect of increasing the degree of coordinative unsaturation of the gold atoms to which the molecules bind has been explored in detail. Adsorption energies, geometrical and vibrational properties, of CO and NO on gold (111), (100), (110), (310) and additional Au atom on (100) which contain coordination numbers in the range of 9 to 4 have been examined. Substantial beneficial effect on the interaction properties of these molecules has been noticed with an increase in unsaturation of coordination. However, dissociation of molecules like O2, H2O and H2 on single low coordinated Au atoms is not possible. Specific sites and structures (bi-atomic rows on (100) in our study) are necessary to adsorb and dissociate O2 on Au. Our bi-atomic rows model predicts O2 dissociation only marginally, because of competition with desorption. It has been concluded that Au alone cannot catalyze reactions like WGSR and selective catalytic oxidation of CO, etc. To understand the role of TiO2 as an active support, the (001) anatase surface has been explored. We found that above mentioned reactions are catalyzed by gold-based catalysts due to co-operative effects between the metal and the active support. For instance, in case of the WGSR, OH and H which are produced as result of water dissociation on the support (TiO2(001)), are proposed to migrate to gold surfaces where H-atoms combine to generate H2 and OHs disproportionate into water and active oxygen, or react with CO to form carboxyl. CO bonded to low coordinated Au atoms consumes O-atoms spontaneously to produce CO2. In addition, the support in the presence of atomic hydrogen, spilled over from Au, serves as capture zone for O2. For purification of reformate gas for fuel cell applications, OH and OOH are key intermediates to oxidize CO selectively. Based on the comparison between activation barriers, Au surfaces are efficient to preferentially oxidize CO as compared with hydrogen in the relevant temperature range of fuel cells.