(8b) Exploiting Au/TiO2 interface chemistry with water for PrOx of CO in water-gas shift streams

Preferential oxidation (PrOx) of CO is a more attractive and energy efficient solution compared to CO methanation for purifying hydrogen streams from steam reforming processes. Saavedra et al. showed that water enhances CO oxidation activity at low temperatures through activation of O2 to OOH* on Au/TiO2.(1) Under the right conditions, mixtures of H2, O2, and H2O may form reaction intermediates that mimic those derived from H2O2, a potent and green chemical oxidant. This type of chemistry is dominantly reported for Au/TiO2 catalysts at low temperatures, and the interplay between active sites on the metal and the oxide support plays a crucial role in mechanisms for CO oxidation, hydrocarbon oxidation and the water-gas shift reaction. While the role of water on CO oxidation is well studied,(1,2) there has not been much research on the effects of water on H2 oxidation.
Herein, we elucidate the role of water in H2 oxidation on Au/TiO2 employing both experiments and density functional theory(DFT) calculations. The experimentally calculated reaction orders reveal that H2 oxidation is limited by activation of H2 owing to its higher reaction order compared to O2 and H2O. The extensive computational study of several possible elementary steps leads to the same conclusion of H2 activation being the rate determining step. The metal support interface(MSI) sites on Au/TiO2 are found to be the most active towards H2 activation. The higher activity of the MSI sites compared to gold sites is explained by Bader charge analysis, which discloses the heterolytic nature of H2 activation at the interface compared to homolytic activation on gold sites. The experimentally calculated negative reaction orders of water for H2 oxidation are in agreement with an increase of 30 kJ/mol in the barrier for H2 activation in the presence of a layer of water above the active interface sites. Thus, water while enhancing CO oxidation through O2 activation, also poisons the active Au/TiO2 interface sites for H2 oxidation making it a perfect co-catalyst for PrOx of CO in H2/CO mixtures. The findings of this study explain the underlying reasons for the activity of (Au/TiO2 + water) system towards PrOx of CO in water-gas shift streams from both theoretical and experimental standpoints.

References
(1) Saavedra, J.; Doan, H. A.; Pursell, C. J.; Grabow, L. C.; Chandler, B. D.; Science. 345 (2014) 1599-1602
(2) Tran, H. V.; Doan, H. A.; Chandler, B. D.; Grabow, L. C.; Curr. Opin. Chem. Eng. 13 (2016) 100-108