Catalysis Letters 56 (1998) 149–153 149 Selective oxidation of methane to methanol and formaldehyde over V 2 O 5 /SiO 2 catalysts. Role of NO in the gas phase Miguel A. Ba˜ nares a , Jos´ e H. Cardoso a,b , Graham J. Hutchings c , Jose M. Correa Bueno b and Jose L.G. Fierro a a Instituto de Cat´ alisis y Petroleoqu´ ımica, CSIC, Cantoblanco, E-28049 Madrid, Spain b Departamento de Engenharia Quimica, Universidade Federal de S˜ ao Carlos, Brazil c Department of Chemistry, Cardiff University, PO Box 912, Cardiff CF1 3TB, UK Received 4 August 1998; accepted 9 October 1998 The role of nitric oxide incorporation into the reaction feed for the partial oxidation of methane to C 2 -hydrocarbons and C 2 -oxygenates is evaluated. The addition of NO increases the conversion of methane under all the experimental conditions studied and has a strong effect on the product distribution. At low NO concentration the catalysts yield mainly C 2 Hn hydrocarbons, but at higher NO concentrations, carbon oxides dominate. Amongst the C 1 -oxygenates produced, methanol is the major compound observed and its proportion increases with increasing NO concentration. The highest C 1 -oxygenates yield was 7% at atmospheric pressure. Keywords: methane oxidative coupling, nitric oxide effect on methane oxidation, V 2 O 5 catalyst for methane oxidation 1. Introduction The partial oxidation of methane to methanol and formaldehyde and to C 2 H n hydrocarbons has been exten- sively studied, because of the relevance of monomers and C 1 -oxygenates in the petrochemical industry. Apart of this technological importance, most of the efforts to produce them through a single catalytic step did not achieve yields beyond 4% [1,2]. It is known that the heterogeneous– homogeneous nature of methane activation has a great rel- evance on the total activity and product distribution [3,4], and the equilibrium between coupling and oxygenates pro- duction is determined by the nature of the catalyst compo- nents and its BET area. The contribution of the gas-phase reactions, which affords a remarkable effect on product dis- tribution, is affected by the oxygen-to-methane ratio in the feed [6], the presence of void volume [7,8], and the use of radical initiators [9–11]. Thus, the conversion of methane has special requirements since the selectivity can be tuned by choosing the operation parameters. Hutchings et al. showed that the conversion of methane and the selectivity to C 2 H n hydrocarbons decrease during NO feed, whereas that of CO 2 increases [12]. A different trend is reported by Ba˜ nares et al. [13] upon addition of NO, where the conver- sion of methane and the selectivity of C 2 H n hydrocarbons increase. These differences arose mainly from the differ- ent experimental conditions used in these works: while the former feeds ca. 0.4% NO, the later study used a feed of ca. 0.03% NO. It is also interesting to note that on removal of the NO from the reactants the production of C 2 H n hy- drocarbons passes through a maximum in time [12], which is higher than the values afforded under the NO-free feed. This could be associated to a decreasing transient concen- tration of NO upon its removal from the reaction feed, thus getting closer to the reaction conditions reported by Ba˜ nares et al. [13]. This work was undertaken with the aim to evaluate the role of NO concentration in the feed in the conversion of methane on a very low-area silica-supported vanadium ox- ide catalyst. 2. Experimental The low BET area SiO 2 support was prepared from a commercial non-porous silica (Aerosil 200, BET area 174 m 2 /g, particle size ca. 14 nm) by calcination at temper- atures in the range of 1273–1423 K, up to 9 h. No signifi- cant crystallization of the silica is observed despite the high temperatures of the treatment. The low-surface-area silica support (ca. 1 m 2 /g) was impregnated with an aqueous so- lution of ammonium metavanadate (Aldrich) and hydrogen peroxide in a rotary evaporator at 343 K. The vanadium oxide concentration was kept below the monolayer cover- age of the support (0.03% V 2 O 5 ) as confirmed by Raman spectroscopy. The impregnate was dried at 383 K and cal- cined in two steps: 623 K for 2 h and 923 K for 5 h. After calcination they were sieved to particles size range of 0.125–0.250 mm diameter. The area of the catalysts was calculated by the BET method from the N 2 adsorption– desorption isotherms at 77 K using a Micromeritics ASAP 2000 apparatus. The steady-state reaction studies were carried out at at- mospheric pressure in a 10 mm i.d. quartz fixed bed mi- crocatalytic reactor, designed to minimize the dead vol- ume upstream and downstream from the bed of the cat- alyst. The minimum volume upstream decreases the ac- tivation of methane in the gas phase [7] so that all the activity originates from the catalyst alone; in addition, the  J.C. Baltzer AG, Science Publishers