Applied Catalysis A: General 226 (2002) 163–174 Effect of Fe-addition on the catalytic activity of silicas in the partial oxidation of methane to formaldehyde A. Parmaliana a,∗ , F. Arena a,b , F. Frusteri b , A. Mart´ ınez-Arias c , M. López Granados c , J.L.G. Fierro c a Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, Casella Postale 29, I-98166 S. Agata, Messina, Italy b Istituto CNR-ITAE, Via Salita S. Lucia Sopra Contesse 5, I-98126 S. Lucia, Messina, Italy c Instituto de Catal´ ısis y Petroleoqu´ ımica, Campus UAM, Cantoblanco s/n, 28049 Madrid, Spain Received 10 July 2001; received in revised form 29 August 2001; accepted 25 September 2001 Abstract The partial oxidation of methane to formaldehyde (MPO) with molecular O 2 has been investigated on various bare and Fe-doped commercial silica catalysts at 650 ◦ C. The influence of the Fe loading (0.01–3.2 wt.%) on the catalytic pattern and steady-state properties of the catalysts have been evaluated. The dispersion and coordination symmetry of surface Fe 3+ species have been investigated by EPR analysis. “Isolated Fe 3+ ions”, “small Fe 2 O 3 clusters” and “Fe 2 O 3 particles” are present on the surface of the FeO x /SiO 2 catalysts, their relative concentration depending upon the loading. Iron species, irrespective of the state of aggregation, enable the reactivity of the silica surface, whilst the highest HCHO productivity is associated with an optimum surface Fe loading (0.05–0.1 Fe at nm -2 ). The different reactivity of the various Fe-doped silica catalysts has been rationalised and normalised in terms of different concentration of “isolated Fe 3+ species” and “aggregated Fe 2 O 3 moieities”. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Fe-doped silica; Methane partial oxidation; Formaldehyde; O 2 chemisorption; EPR; Iron surface species; Reduced sites 1. Introduction The direct partial oxidation of methane to formalde- hyde (MPO) on oxide catalysts is currently one of the most sought goals in the framework of the nat- ural gas conversion to higher added value products. Indeed, this topic has attracted a great research in- terest during the last two decades mainly aimed at discovering very active and/or selective catalysts, while less attention has been addressed to highlight- ∗ Corresponding author. Tel.: +39-090-6765606; fax: +39-090-391518. E-mail address: adolfo.parmaliana@unime.it (A. Parmaliana). ing the nature of the active sites and to defining the surface reaction pathway. Silica based oxide cata- lysts denote a superior functionality in the MPO [1,2] and several clues have been provided for relating the surface structure and the coordination of supported transition metal oxide species with their reactivity [3–6]. However, the peculiar reactivity of the silica surface in MPO resulting in remarkable HCHO pro- ductivity values [7–10] is still an issue not completely rationalised [8,10–12]. We have contributed to this research subject pointing out that the performance of the silica surface in MPO is controlled by the preparation method [7,8], namely for several com- mercial silica catalysts the following reactivity scale 0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0926-860X(01)00897-3