Catalytic decomposition of nitrous oxide on transition metal exchanged sodium-A zeolites

Abstract

The catalytic decomposition of nitrous oxide (2N20 + 2N2 +02) has been studied on four series of Na-A zeolites exchanged with varying quantities of cobalt, copper, nickel and manganese. Decomposition obeys first-order kinetics without oxygen retention on all catalysts studied. All zeolites except those containing copper show linear Arrhenius plots. The rate-determining step in N,O decomposition is suggested to be electron transfer from the zeolite to adsorbed nitrous oxide. Activation energies are markedly higher for sieves containing an average of only one transition metal ion per unit cell compared with those where the transition metal ion content exceeds 1.5 per unit cell. This is also considered to reflect variation in the electronic properties of the catalysts with transition metal ion content. Transition metal ion exchanged zeolites are very much less active for N,O decomposition than bulk oxides or solid solutions but are of similar activity to transition metal ion exchanged X zeolites.

Low-Temperature Synthesis of MgxCo1-xCo2O4 Spinel Catalysts for N2O Decomposition

“…For example, the conversion activity of catalyst B75 operated at 380−400 °C is in the range of 30.6−38.9 mmol (N 2 O)/g·h (GHSV = 21 200 h -1 , N 2 O = 10 mol % balanced with He), compared to that of 1.6−2.3 mmol (N 2 O)/g·h (GHSV = 30 000 h -1 , N 2 O = 0.1 mol % balanced with He) for an active Co−Mg−Al spinel catalyst (which was derived from the hydrotalcite-like compound Co−Mg−Al HTlc) operated at 350−400 °C. , It has been evidenced that decomposition of nitrous oxide on metal oxide surface involves an electron transferring from a low oxidation state metal cation to an adsorbed nitrous oxide molecule. , Related to the present catalyst system, the following mechanism can be proposed for nitrous oxide decomposition: where Co 2+ is a divalent cobalt surface species and Co 2+ ···ON 2 represents adsorbed nitrous oxide species on an active site (eq 4). For many metal oxide catalysts, charge transfer from a low-valence metal cation to the adsorbed N 2 O is often considered as fast surface reaction, as proposed in eq 5. , Due to the site inversion in the spinel structure, as addressed in the above subsections, divalent Co 2+ and trivalent Co 3+ both exist in tetrahedral sites, in addition to the Mg 2+ . Unlike a proposed mechanism that the charge-transfer process of Co 2+ to Co 3+ causes the CoO phase to change to Co 3 O 4 , the coexistence of Co 2+ and Co 3+ in 4-fold-coordinated sites requires little geometrical change for the two types of cations in the charge-transfer process.…”

“…39,40 It has been evidenced that decomposition of nitrous oxide on metal oxide surface involves an electron transferring from a low oxidation state metal cation to an adsorbed nitrous oxide molecule. 41,42 Related to the present catalyst system, the following mechanism can be proposed for nitrous oxide decomposition:…”

“…For many metal oxide catalysts, charge transfer from a low-valence metal cation to the adsorbed N 2 O is often considered as fast surface reaction, as proposed in eq 5. 41,42 Due to the site inversion in the spinel structure, as addressed in the above subsections, divalent Co 2+ and trivalent Co 3+ both exist in tetrahedral sites, in addition to the Mg 2+ . Unlike a proposed mechanism that the charge-transfer process of Co 2+ to Co…”