A Spectroscopic Study of Supported-Phosphate-Catalysts (SPCs): Evidence of Surface-mediated Hydrogen-Transfer

摘要

Supported-phosphate-catalysts (SPCs) are a versatile class of heterogeneous materials used in various industrially relevant processes including Fischer-Tropsch synthesis (FTS), selective catalytic reduction (SCR) of NOx with NH3, and Guerbet reaction of C2H5OH to n-C4H9OH. Herein, model SPCs synthesized by impregnating various supports with phosphoric acid were studied using in-situ chemical probe temperature-programmed-desorption infrared spectroscopy. Hydroxylation of the catalyst surface was observed during the temperature ramp on all SPCs. This behavior was not observed on the bare supports during identical experiments. The results indicate that hydroxylation of the surface occurs via sacrificial hydrogen-transfer reaction, where the chemical probe (NH3 or C2H5OH) can be utilized as the hydrogen-donor. Results herein also show that the extent and the rate of hydrogen-transfer reaction depend on the identity of the support, phosphate loading, and identity of the hydrogen-donor molecule. Surface Bronsted acid sites (P-OH) do not contribute to the reaction and P=O surface sites are proposed as the active sites for the observed hydrogen-transfer. Based on the current work, it is suggested that any catalytic reaction that involves hydrogenation over phosphate-promoted or phosphate-based catalysts needs to account for the surface-mediated hydrogen-transfer capability of the phosphate sites in the overall reaction mechanism.