SILVER-CLUSTER FORMATION ON AGZR2(PO4)(3) AND CATALYTIC DECOMPOSITIONOF BUTAN-2-OL

A Nasicon-type AgZr2(PO4)(3) material was synthesized and its catalyti c properties were characterized by butan-2-ol conversion in the presen ce and absence of oxygen. The catalytic activity was mainly oriented t oward the production of methyl ethyl ketone and butenes. Activity was shown to depend on the reducibility of the Ag+ ions and on the oxygen concentration in the reactants. Hydrogen, butan-2-ol, and butan-2-ol/O -2 were able to reduce the Ag+ cations located in the three-dimensiona l channels of the phosphate network. Silver clusters appeared on the s urface and protons replaced the Ag+ ions in the structure to form (HPO 4) acidic groups, without any modification in the framework of the pho sphate. Evidence for this phenomenon was provided by (i) XRD peaks cor responding to (111) and (200) metallic silver diffractions, (ii) UV-vi sible spectroscopy which showed conduction bands at 372 and 410 nm att ributed to Ag-0 crystallites and a shoulder in the 250-270 nn region d ue probably to (Ag-n)(delta+) charged clusters, (iii) TEM analysis whi ch revealed that the exposure of the phosphate to the electron beam pr oduced a spontaneous appearance of silver particles, (iv) IR spectrosc opy which demonstrated the protonation of the sample by the appearance of three new hydroxyl bands in the 3000-3500 cm(-1) region, a result supported by H-1 NMR spectroscopy, and (v) TPO experiments performed a fter the reduction of the sample which showed that silver dusters adso rb oxygen in at least two specific ways which are active in the oxidat ive dehydrogenation of the butan-2-ol. It was also shown that the form ation of silver particles was reversible upon reoxidation at 773 K to give the starting material AgZr2(PO4)(3). (C) 1996 Academic Press, Inc .