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Langmuir 1996, 12, 2230-2238
Sorption and Reduction of NO2 on Microporous Ammonium 12-Tungstophosphate R. Be´langer and J. B. Moffat* Department of Chemistry and the Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Ontario, Canada N2L 3G1 Received October 30, 1995X The sorption and reduction of NO2 have been studied on ammonium 12-tungstophosphate, a microporous derivative of 12-tungstophosphoric acid. NO2 penetrates into the bulk structure, where reduction by NH4+ occurs. Nitrogen, oxygen, and nitrous oxide are produced. Nitrogen is desorbed into the gas phase while oxygen apparently participates in a surface oxidation process. Since N2O is produced at higher temperatures, it appears to result from the decomposition of ammonium nitrate formed during the aforementioned processes.
Introduction Interest in the application of heterogenous catalysts for the solution of environmental problems has increased markedly in the last decade.1 Much of this attention has been directed at the oxides of nitrogen, probably because of the increasingly stringent controls placed on these environmentally disadvantageous gases.1,2 While the earliest work on the decomposition of nitric oxide showed that oxygen, one of the products of the decomposition, poisoned the catalyst, more recent work has focused on the reduction process, employing reductants such as ammonia, carbon monoxide, hydrogen, and various hydrocarbons and a variety of heterogeneous catalysts, frequently of a zeolitic nature.3 The preponderance of the work on the reduction of NOx has, understandably, focused on nitric oxide1-3 with considerably less attention being devoted to nitrogen dioxide. Since, at temperatures less than 500 °C, the concentration of NO2 is expected to be greater than that of NO, in the presence of excess oxygen, NO2 must play an important part in the selective catalytic reduction process, in which oxygen is frequently present.4,5 The results of studies of NO2 adsorption on anatase6 and on chromia7 have recently been reported as well as those concerned with the interaction of NO2 and NH3 on anatase.8 The present work is concerned with the application of metal-oxygen cluster compounds (also known as heteropoly oxometalates) and specifically ammonium 12tungstophosphate as catalysts for the reduction of NOx. Although these may be synthesized with a variety of compositions and structures, those most studied for their catalytic properties have structures originally identified by Keggin.9-11 Under ambient conditions these are ionic * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) See, for example, various papers in: Environmental Catalysis; ACS Symposium Series 552; American Chemical Sociey: Washington, DC, 1994; Emerging Technologies. (2) Brandin, J. G. M.; Andersson, L. A. H.; Odenbrand, C. U. I. Catal. Today 1989, 4, 186. (3) See, for example: Adelman, B. J.; Lei, G.-D.; Sachtler, W. M. H. Catal. Lett. 1994, 28, 119 and references therein. (4) Petunchi, J. O.; Hall, W. K. Appl. Catal. 1993, B2, L17. (5) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (6) Kantcheva, M. M.; Bushev, V. P.; Hadjiivanov, K. I. J. Chem. Soc., Faraday Trans. 1992, 88, 3087. (7) Hadjiivanov, K. I.; Klissurski, D. G.; Bushev, V. P. J. Chem. Soc., Faraday Trans. 1995, 91, 149. (8) Kantcheva, M.; Busheva, V.; Klissurski, D. J. Catal. 1994, 145, 96. (9) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: Berlin, 1983.
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solids with discrete cations and anions, the latter being large, approximately 10 Å in diameter, high molecular weight entities. The Keggin anion contains a central atom (in the present work, phosphorus), tetrahedrally bonded to four oxygen atoms, which is enveloped by twelve octahedra with oxygen atoms at each of their vertices and with a peripheral metal atom (in the present work, tungsten) at each of their approximate centers. Oxygen atoms connect peripheral metal atoms to each other and to the central atom. Although a variety of species may be employed to balance the charge of the anion, the synthesis normally yields the acidic form which in the present case is 12tungstophosphoric acid (H3PW12O40‚nH2O). The acid has a cubic (Pn3m) crystallographic structure with up to four approximate planar water molecules hydrogen bonded to the proton while the hydrogen atoms of the former are hydrogen bonded to the terminal oxygen atoms of the anions.12 Although the solid acid has a low surface area (
