Ind. Eng. Chem. Res. 2007, 46, 7045-7049
7045
KINETICS, CATALYSIS, AND REACTION ENGINEERING Multifunctional Catalyst for de-NOx Processes: The Use of Methanol for the Selective Reduction of NOx Miguel Angel Go´ mez-Garcı´a,*,† Suzanne Libs, Pierre Bernhardt, Veronique Pitchon, and Alain Kiennemann LMSPC, Laboratoire de Mate´ riaux, Surfaces et Proce´ de´ s pour la Catalyse, UMR 7515 du CNRS-ECPM-ULP, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France
The role of methanol in the NOx reduction into N2 has been investigated with the multifunctional catalyst H3PW12O40‚6H2O-Pt (HPW-Pt) supported on CexZr4-xO8. The reduction of NOx is obtained by a two-stage operation: in the first period, during the lean mode (2 min), NOx (NO/NO2 ) 1) are stored in the catalyst; then they are reduced in N2 during the rich period (methanol injection 1 min). With methanol, the NOx storage efficiency of HPW-Pt/CexZr4-xO8 is stable and very high (ca. 100%). It is directly related to the exchange of HPW structural water with methanol. Around 55% of stored NOx are transformed into N2 by alternative lean/rich cycles. CxHyOz intermediate species derived from methanol are proposed as intermediate reductants for NOx. Introduction One of the main challenges for the environmental catalysis is decreasing the NOx (NO and NO2) emissions from both fixed (power and chemical plants) and mobile (automobiles) sources. For stationary sources, even if the selective catalytic reduction (SCR) process is very efficient, critics suggest that transportation and storage of ammonia present unnecessary risks.1 For mobile sources, working at a stoichiometric air/fuel ratio, a well-known three-way catalyst (TWC) is a part of the NOx depollution strategy. However, so far, there has been no mature technology for lean-burn (oxygen rich gas) emissions. In fact, for diesel cars, the NOx trap concept (NSR ) NOx storage and reduction) developed by Toyota,2 even though efficient, needs to be improved specially for the regeneration of the NOx trap of adsorbed sulfur. In the NSR concept, the problem of NOx reduction is undertaken by a two-stage operation: the combination of two different air/fuel ratios instead of a single one. NOx storage takes place during the lean mode. All through this period, NOx are stored as nitrate. After changing to rich conditions, NOx are reduced into N2 by CO, H2, or hydrocarbons remaining (or added) in the exhaust line after the combustion process. However, the presence of CO2 and especially of SO2 in exhaust gases hardly modifies the catalytic NOx storage capacity of sorbent materials by the formation of sulfates and carbonates.3,4 As a consequence, studies need to be undertaken for developing new catalytic compounds able to store NOx in a different way from nitrates.5 Up to now, this possibility is limited to a few compounds: carbonaceous materials (e.g., active carbon fibbers, multiwalled carbon nanotubes),6 zeolite Y,7 and some heteropolyacids (e.g., H3PW12O40‚6H2O ) HPW).8-11 Each of these materials presents also serious drawbacks: oxidative atmospheres for carbonaceous structures, the inhibiting effect * To whom correspondence should be addressed. E-mail:
[email protected]. † On leave from Universidad Nacional de Colombia, Sede Manizales.
of water and sulfur poisoning for zeolites, low-temperature resistance for HPW (
