Catalysis Today 85 (2003) 279–289 Reaction of methanol on stoichiometric and O-terminated -Cr 2 O 3 (1 0 ¯ 1 2): interconversion of oxygenated C 1 surface intermediates Michael W. Mensch, Chad M. Byrd, David F. Cox ∗ Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA Abstract The reaction of methanol on the nearly stoichiometric -Cr 2 O 3 (1 0 ¯ 1 2) surface gives a spectrum of products includ- ing CH 4 , CH 2 O, CO, CO 2 and H 2 . These products are all generated in a 200 K temperature range (600–800 K) in thermal desorption through a reaction pathway involving methoxide, dioxymethylene and formate surface intermediates. The inter- conversion between oxygenated C 1 surface intermediates involves the reversible reaction of methoxide to dioxymethylene via dehydrogenation and nucleophilic attack of surface lattice oxygen. Formate intermediates are subsequently formed by the dehydrogenation of dioxymethylene. Isotopic labeling studies indicate that the precursor to the formaldehyde reaction product is a dioxymethylene intermediate rather than methoxide. The oxygen-terminated surface is completely unreactive, indicating a structure (or composition) sensitive reaction of methanol on the (1 0 ¯ 1 2) surface. Terminal chromyl oxygen acts as a cation site blocker and eliminates the surface cation–anion site pairs necessary for the dissociation and reaction of methanol. © 2003 Elsevier B.V. All rights reserved. Keywords: Cr 2 O 3 ; Methanol; Methoxide; Dioxymethylene; Formate 1. Introduction Studies of the reaction chemistry of hydrocarbon oxygenates like alcohols, aldehydes and carboxylic acids can enhance our understanding of the surface chemistry of hydrocarbon oxidation processes. These molecules can provide routes to the formation and subsequent study of oxygenated surface intermediates without the associated requirement of oxygen inser- tion and C–O bond formation that is necessary for their formation directly from hydrocarbons. For the adsorption of methanol on oxide surfaces, one typ- ically expects heterolytic dissociative adsorption via O–H bond breaking and loss of the acidic proton from this weak Brønsted acid. The site requirements for ∗ Corresponding author. Tel.: +1-540-231-6829; fax: +1-540-231-5022. E-mail address: dfcox@vt.edu (D.F. Cox). such a process are thought generally to involve a sur- face cation–anion site pair with the proton bound to an oxygen anion and the conjugate base anion, methoxide (CH 3 O - ), bound to a neighboring cation site [1,2]. Cr 2 O 3 is considered an acidic oxide [3,4], but is known to catalyze or promote a variety of reactions including alcohol dehydration and dehydrogenation, methanol synthesis, H 2 –D 2 exchange, double bond migration in olefins, ethylene polymerization, oxida- tive dehydrogenation of alkanes, and hydrocarbon combustion [5–16]. IR investigations of methanol adsorption at room temperature on Cr 2 O 3 powders demonstrate that methoxide is formed by dissociative adsorption of methanol [17–19], while adsorption at elevated temperatures (200–400 ◦ C) leads to the for- mation of formate intermediates [20]. The differences in the IR studies suggest a reaction pathway involving the thermal conversion of methoxide to formate on Cr 2 O 3 as proposed by Tamaru and coworkers [20]. 0920-5861/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0920-5861(03)00394-8