Characterization of active sites on AgHf2(PO4)3 in butan-2-ol conversion

The silver-hafnium phosphate (AgHf2(PO4)(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 AgHf2(PO4)(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. Tn the second step it increases as the amount of oxidized silver species (AgxOy), which are the active sites for the alcohol dehydrogenation, increases. UV-VIS characterization of AgHf2(PO4)(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 (AgxOy). (C) 2001 Elsevier Science B.V. All rights reserved.