Characterization of active sites on AgHf 2(PO 4) 3 in butan-2-ol conversion

Abstract

The silver–hafnium phosphate (AgHf 2(PO 4) 3), belonging to the Nasicon-type structure, was synthesized by a sol–gel method and characterized by several techniques. The Ag + cations were found to be, as in zeolite frameworks, easy to reduce to metallic silver. This reduction was investigated by STEM/EDX which showed that, under an intense electron beam, the Ag + ions diffuse toward the surface of the sample and form metallic particles of sizes varying from 3 to 15 nm. The X-ray diffraction patterns confirmed that the reduction does not damage the phosphate structure. The Ag + ions were replaced in the structure by protons giving rise to (PO–H) acid groups. The catalytic behavior of AgHf 2(PO 4) 3 was studied in butan-2-ol conversion. In the absence of O 2, the reaction leads to an abrupt decrease of the dehydrogenation activity and a dehydration reaction that reaches a pseudo-stationary state. Concomittantly, the Ag + cations are reduced to Ag 0. In the presence of O 2, the dehydrogenation reaction undergoes a complex transitory state which can be decomposed into two steps. During the first one, the activity decreases in concert with the reduction of Ag + ions. In the second step it increases as the amount of oxidized silver species (Ag x O y ), which are the active sites for the alcohol dehydrogenation, increases. UV–VIS characterization of AgHf 2(PO 4) 3 in dynamic conditions similar to those used in the catalytic tests confirmed that butan-2-ol reduces Ag + ions to metallic silver which, in the presence of O 2 in the reaction mixture, are oxidized to (Ag x O y ).