Adsorption and decomposition of methanol on gallium oxide polymorphs

SEBASTIÁN E. COLLINS; LAURA E. BRIAND; LUIS A. GAMBARO; MIGUEL A. BALTANÁS; ADRIAN L. BONIVARDI

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Lugar: Washington; Año: 2008 vol. 112 p. 14988 - 15000

1932-7447

The adsorption of methanol was studied on three gallia polymorphs (alfa, beta and gamma), pretreated under oxygenor hydrogen at 723 K. Their Brunauer-Emmett-Teller surface areas were in the range 12-105 m2 g-1.Methanol (or methanol-d3) chemisorbs on the gallium oxides both molecularly, as CH3OHS (or CD3OHS,)and dissociatively, as methoxy (CH3O or CD3O) species, at 373 K. The quantification of the total amount ofchemisorbed methanol at this temperature allowed us to determine the number of available surface activesites per unit area (NS), which is in the range 1-2 micromol m-2 for the oxygen pretreated oxides at 723 K. Thedensity of active sites was moderately smaller (aprox. ­25%) after pretreating the oxides under hydrogen at 723 K.The temperature-programmed surface reaction of adsorbed methanol and methoxy was followed by massspectrometry and infrared spectroscopy under He flow, up to 723 K. It was found that, upon heating above473 K, methoxy oxidized to methylenbisoxi (H2COO) and, then, to formate (HCOO) species, and traces ofdimethyl ether were also detected. Surface formate species further decompose to give CO(g) and CO2(g) attemperatures higher than 573 K, with the concurrent generation of OH and H species over the surface, whichreact toward H2(g). It is suggested that the CO2 production implies the removal of lattice oxygen, generatinga surface oxygen vacancy, which can be restored by water molecules from the gas phase. Thus, gallia can beenvisaged as a promising support for the steam reforming of methanol, as long as a (noble) metal officiates/acts as a rapid H2 releaser from the surface.