NO reduction over La2O3 using methanol

Nitric oxide (NO) reduction by methanol was studied over La2O3 in the prese nce and absence of oxygen. In the absence of O-2, CH3OH reduced NO to both N2O and N-2, with selectivity to dinitrogen formation decreasing from aroun d 85% at 623 K to 50-70% at 723 K. With 1% O-2 in the feed, rates were 4-8 times higher, but the selectivity to N-2 dropped from 50% at 623 K to 10% a t 723 K. The specific activities with La2O3 for this reaction were higher t han those for other reductants; for example, at 773 K with hydrogen a speci fic activity of 35 mu mol NO/s m(2) was obtained whereas that for methanol was 600 mu mol NO/s m(2). The Arrhenius plots were linear under differentia l reaction conditions, and the apparent activation energy was consistently near 14 kcal/mol with CH3OH. Linear partial pressure dependencies based on a power rate law were obtained and showed a near-zero order in CH3OH and a near-first order in H-2. In the absence of O-2, a Langmuir-Hinshelwood type model assuming a surface reaction between adsorbed CH3OH and adsorbed NO a s the slow step satisfactorily fitted the data, and the model invoking two types of sites provided the best fit and gave thermodynamically consistent rate constants. In the presence of O-2 a homogeneous gas-phase reaction bet ween O-2, NO, and CH3OH occurred to yield methyl nitrite. This reaction con verted more than 30% of the methanol at 300 K and continued to occur up to temperatures where methanol was fully oxidized. Quantitative kinetic studie s of the heterogeneous reaction with O-2 present were significantly complic ated by this homogeneous reaction.