ith an improved EHMO method, three modes (flat-lying, vertical , and in-clined insertion) for adsorption and activation of dioxygen on (100) , (110) and(111) surfaces of Na_2O and of K,O have been exaniinecl, and the ...ith an improved EHMO method, three modes (flat-lying, vertical , and in-clined insertion) for adsorption and activation of dioxygen on (100) , (110) and(111) surfaces of Na_2O and of K,O have been exaniinecl, and the interaction ofthese dioxygen adspecies with CH_4 and with CH_3 · (radical) from gas pliase liasbeen investigated. The results indicate that both oxides tend to forni less cliargedadspecies of dioxygen, with the flat-lying adsorption on (110) surface most favor-able energetically. All these cliemisorbed dioxygen species are capable of interactingeffectively with CH_4 and Abstract ing one hydrogen atom from the CH_4 molecule andtheir tendencies to reassociate with CH_3 · , which would easily lead to deep oxida-tion of the fragments of hydrocarbons, are enhanced with inereasing negativecliarges on them. In comparison with Na_2O, K_2O has a little stronger tendency tostabilize less charged dioxygen adspecies; this has a close relation with the knownexperimental fact that K￣+ showed better promoting effect than Na￣+ in improvingC_2-selectivity in methane oxidative coupling (MOC ).展开更多
The adsorption of O2 over defective La2O3-based OCM catalysts with anionic vacancies , and the reaction of surface oxygen species with CH4 were studied by means of in situ confocal microprobe Raman spectra. The partia...The adsorption of O2 over defective La2O3-based OCM catalysts with anionic vacancies , and the reaction of surface oxygen species with CH4 were studied by means of in situ confocal microprobe Raman spectra. The partially reduced oxygen species O2, O , O (O<&<1 ) and even the lattice oxygen ion O2-can be detected on the surface of O2-pretreated La2O3-based catalysts. At lower temperatures (< 573 K) it is peroxide species O- or O- that is more essential for the coupling of methane,while at higher temperature. the superoxide O2 and lattice oxygen species O2-are rather important. It is easier for fluoride-containing La2O3-based catalysts to induce the oxygen species with fewer negative charges and the basicity of the cat- alyst surface is weakened , so that the C2 selectivity for OCM reaction is improved.展开更多
The influence of steam on catalytic performance for the oxidative coupling of methane(OCM) over Ba 2+ promoted La 2 O 3 catalysts was studied. It was shown that the presence of a suitable amount of stea...The influence of steam on catalytic performance for the oxidative coupling of methane(OCM) over Ba 2+ promoted La 2 O 3 catalysts was studied. It was shown that the presence of a suitable amount of steam led to a significant increase of the yield and selectivity to C 2 hydrocarbons, a decrease of selectivity to carbon monoxide and some increase of selectivity to carbon dioxide with increasing the amount of water in the feed was also observed. The activity and selectivity of the catalysts are more sensitive to the effect of steam with the variation of Ba 2+ content in the catalysts at a relatively low temperature(650℃). The comparative experiments for OCM with methane oxygen mixture diluted with nitrogen or steam were carried out in order to investigate the role of steam.展开更多
文摘ith an improved EHMO method, three modes (flat-lying, vertical , and in-clined insertion) for adsorption and activation of dioxygen on (100) , (110) and(111) surfaces of Na_2O and of K,O have been exaniinecl, and the interaction ofthese dioxygen adspecies with CH_4 and with CH_3 · (radical) from gas pliase liasbeen investigated. The results indicate that both oxides tend to forni less cliargedadspecies of dioxygen, with the flat-lying adsorption on (110) surface most favor-able energetically. All these cliemisorbed dioxygen species are capable of interactingeffectively with CH_4 and Abstract ing one hydrogen atom from the CH_4 molecule andtheir tendencies to reassociate with CH_3 · , which would easily lead to deep oxida-tion of the fragments of hydrocarbons, are enhanced with inereasing negativecliarges on them. In comparison with Na_2O, K_2O has a little stronger tendency tostabilize less charged dioxygen adspecies; this has a close relation with the knownexperimental fact that K￣+ showed better promoting effect than Na￣+ in improvingC_2-selectivity in methane oxidative coupling (MOC ).
文摘The adsorption of O2 over defective La2O3-based OCM catalysts with anionic vacancies , and the reaction of surface oxygen species with CH4 were studied by means of in situ confocal microprobe Raman spectra. The partially reduced oxygen species O2, O , O (O<&<1 ) and even the lattice oxygen ion O2-can be detected on the surface of O2-pretreated La2O3-based catalysts. At lower temperatures (< 573 K) it is peroxide species O- or O- that is more essential for the coupling of methane,while at higher temperature. the superoxide O2 and lattice oxygen species O2-are rather important. It is easier for fluoride-containing La2O3-based catalysts to induce the oxygen species with fewer negative charges and the basicity of the cat- alyst surface is weakened , so that the C2 selectivity for OCM reaction is improved.
文摘The influence of steam on catalytic performance for the oxidative coupling of methane(OCM) over Ba 2+ promoted La 2 O 3 catalysts was studied. It was shown that the presence of a suitable amount of steam led to a significant increase of the yield and selectivity to C 2 hydrocarbons, a decrease of selectivity to carbon monoxide and some increase of selectivity to carbon dioxide with increasing the amount of water in the feed was also observed. The activity and selectivity of the catalysts are more sensitive to the effect of steam with the variation of Ba 2+ content in the catalysts at a relatively low temperature(650℃). The comparative experiments for OCM with methane oxygen mixture diluted with nitrogen or steam were carried out in order to investigate the role of steam.
