/ CATALYSIS TODAY ELSEVIER Catalysis Today 24 (1995) 231-236 On the nature of active sites of silica based oxide catalysts in the partial oxidation of methane to formaldehyde A. Parmaliana a, F. Arena a, F. Frusteri b, D. Miceli b, V. Sokolovskii c a Dipartimento di Chimica Industriale, Universita degli Studi di Messina, Salita Sperone c.p. 29, 1-98166 S. Agata (Messina), Italy b lstituto CNR-TAE, Salita S. Lucia 39, 1-98126 S. Lucia (Messina), Italy Department of Chemistry, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa Abstract The partial oxidation of methane to formaldehyde with molecular 02 has been investigated on various commercial bare SiO2 samples and silica supported M003 and V205 catalysts at 550-650°C. Amongst the different SiO2 samples, the highest HCHO productivity (STYr~cHo, g" kg~t~"h- ~) is found with 'precipitated' silica, while 'fumed' SiO2 results in the least reactive silica. Incorporation of molybdena depresses the STYHcH o value for the 'precipitated' silica but enhances the STYr~cHo for bare 'fumed' silica. In contrast, addition of vanadia to either 'precipitated' or 'fumed' silicas leads to higher STYHcHovalues. On the basis of a series of experiments performed by continuous scanning of the reaction mixture with a quadrupole M.S., the participation of lattice oxygen in the main reaction pathway has been ruled out. A straight correlation between the density of reduced sites (p, 1016 st' gc~t~), evaluated in steady-state conditions by 02 chemisorption, and the reaction rate has been disclosed. MoO3 and V205 dopants modify the catalytic properties of SiO2 by affecting the process of oxygen activation on the catalyst surface. I. Introduction In spite of the great research effort dedicated during the last years to the partial oxidation of methane to formaldehyde (MPO) over silica based oxide catalysts, the understanding of the reaction mechanism and the nature of the active sites still remain two challenging issues. Three distinct theoretical models have been proposed to describe the working mechanism of MPO cata- lysts: i) Langmuir-Hinshelwood model or con- certed mechanism; ii) Mars-van Krevelen model or redox mechanism and iii) heterogeneous- homogeneous model or surface assisted gas-phase reaction mechanism [ 1 ]. In order to probe the origin of the oxygen species incorporated into reaction products isotopic labelling techniques 0920-5861/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10920-5861 (95)00030-5 have been used [ 2,3 ]. However, such techniques did not allow to obtain a definitive proof of the participation of lattice or gas-phase oxygen in the product formation since also the reaction products undergo isotopic exchange with labelled oxygen [ 3 ]. On the basis of a comparison of the reaction rates of MPO in the presence and in the absence of 02 in the reaction mixture, we have previously argued that the MPO on SiO2 based oxide cata- lysts, in the range 550-650°C, proceeds via a con- certed mechanism involving the activation of gas-phase 02 on surface reduced sites [4]. In par- ticular, a straight relationship between 'density of reduced sites' under steady-state conditions and the reactivity of SiO2 based oxide catalysts in MPO has been disclosed [ 4,5 ]. A number of stud- ies have been recently addressed to disclose a