An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N2O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.

Heterogeneous catalytic decomposition of nitrous oxide

Copper-based catalysts are active in a wide range reactions of transformation of nitrogen oxides and represent an useful model system to better understand the fundamental aspects of the chemistry and mechanism of reaction of catalytic transformation of these pollutants. After an introduction on the reactivity of copper-based catalysts (supported and unsupported copper oxide, Cu-zeolites, cuprates and other copper compounds) in various reactions of conversion of nitrogen oxides, four main sub-topics are discussed in detail: (i) nature of copper species, (ii) chemisorption and surface transformations of NO, (iii) relationship between copper species and activity in the conversion of nitrogen oxides and (iv) mechanism of reduction of nitrogen oxides to N 2 . Five reactions of transformation of nitrogen oxides are discussed in detail: (i) decomposition of NO, (ii) reduction of NO with ammonia in the presence or not of oxygen, (iii) reduction of NO with hydrocarbons in the presence of oxygen, (iv) reduction of NO with CO and (v) decomposition of N 2 O. The mechanism of reduction of nitrite and N 2 O by copper enzymes is also discussed, with a view to provide some useful insights on the chemistry of transformation. In this review particular attention is directed towards controversial points in the literature, underestimated questions, and hypothesis and theories which do not allow interpretation of all sets of experimental data. Discussion is also focused on the presence of multiple and competitive pathways of transformation, the relative roles of which depend on reaction conditions.

Nature of active species in copper-based catalysts and their chemistry of transformation of nitrogen oxides

The catalytic decomposition of nitrous oxide to nitrogen and oxygen was studied over a number of catalysts. Copper and cobalt exchanged ZSM-5, mordenite, zeolite beta, ZSM-11 and ferrierite are much more active than other cation-exchanged zeolites (Cu-Y, Co-Y, Co-erionite, Cu-L, Co-L, Ni-ZSM-5, Mn-ZSM-5) and metal oxides (CuO/Al2O3, and CoO/Al2O3). For noble metal catalysts, a cation exchanged ZSM-5 has higher activity than the Al2O3 supported form. Rhodium is the most active catalyst, but surprisingly Pt-ZSM-5 is one of the least active catalysts of all. Also, the effects of oxygen and nitric oxide addition, metal loading, and pretreatment upon the catalyst activity were evaluated.

Catalytic decomposition of nitrous oxide on metal exchanged zeolites

Physical properties and mechanisms of formation of nitrous oxide

The kinetics of N 2 O decomposition were investigated over Cu/carbon, Cu/Al 2 O 3 , Cu/SiO 2 , and Cu/ZSM-5 catalysts with the intent of monitoring the requirements for achieving stoichiometric catalytic decomposition over Cu and elucidating the role of the support in the catalytic cycle. The very efficient oxygen scavenging capability of carbon led to higher initial activities and turnover frequencies on Cu/C catalysts compared to Cu/ZSM-5; however, rapid gasification of the carbon support limited the lifetime of these Cu/C catalysts. Very low decomposition activity was exhibited by Cu/Al 2 O 3 and Cu/SiO 2 at temperatures below 823 K; however, addition of CO or H 2 to the reactor feed stream significantly lowered the temperatures required to achieve catalytic N 2 O reduction. With all catalysts, the temperature at which the product ratio in the effluent stream stabilized near the stoichiometric ratio of O 2 /N 2 (or CO 2 /N 2 ) = 1/2 was found to correspond to that at which O 2 either desorbs or is removed by carbon. DRIFT spectra of CO adsorbed at 173 K on these catalysts indicated that both Cu +1 and Cu +2 species coexist during the active phase, as expected for a redox mechanism requiring a balance between these two sites. In contrast, spectra of deactivated catalysts indicated that the Cu sites exist predominantly as Cu +2 species, with only a very small Cu +1 fraction, thus suggesting that the deactivation observed in these catalysts at lower temperatures is primarily due to the inability to reduce Cu +2 cations back to a Cu +1 state. Different kinetic rate expressions, derived from sequences of elementary steps appropriate for each catalyst, fit the data well for N 2 O decomposition on Cu/ZSM-5 and Cu/Al 2 O 3 as well as N 2 O reduction by carbon and CO, and they yielded meaningful values for enthalpies and entropies of adsorption for N 2 O, O 2 and CO.

C.S. Swamy

Papers

Catalytic decomposition of N2O on ternary oxides containing copper in different valence states