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In Situ Raman evidence for a barium solid state phase that is active in nitric oxide decomposition: influence of preparation parameters

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Mestl,  Gerhard
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;
Department of Chemistry, Texas A&M University, College Station, TX, USA;

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Citation

Mestl, G., Xie, S., Rosynek, M. P., & Lunsford, J. H. (1998). In Situ Raman evidence for a barium solid state phase that is active in nitric oxide decomposition: influence of preparation parameters. In Preparation of Catalysts VII: Proceedings ofthe 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4, 1998 (pp. 459-467). Amsterdam: Elsevier.


Cite as: https://hdl.handle.net/21.11116/0000-0007-75DB-F
Abstract
Barium on MgO and CaO and, to a lesser extent, pure barium are effective catalyst for NO decomposition. A very distinct catalytic activity is observed on MgO and CaO, depending on the barium loading. Identical Ba loadings on amphoteric Al2O3 or basic La2O3 with comparable specific surface areas give inactive materials. Barium loadings on MgO and CaO, exceeding 12 mol%, and pure BaO, result in an unusual steady-state behavior. The N2 formation rate goes through a sharp maximum. This behavior is not observed for comparable Ba loadings on Al2O3 or La2O3 or barium loadings below 12 mol% on MgO and CaO. The exact temperature, at which the maximum conversion occurs, depends on the NO concentration: it increases with increasing NO pressure from 630 °C in 1% NO/He to 700°C in 4% NO/He over 14 mol% Ba/MgO. The rate of reaction also undergoes an unusual transient effect with respect to changes in the NO partial pressure. In situ Raman spectroscopic characterization of 14 mol% Ba/MgO, Ba/CaO and pure BaO shows the presence of a coordinatively bound nitro species (Ba-NO2) on the catalyst. Its Raman intensity varies on the same time scale as the rate of N2 formation. This strong temporal correlation of the Raman intensity variation and the N2 formation rate suggests that the Ba- nitro species plays an active role as an intermediate in the catalytic reaction. The Raman intensity variations of ionic Ba(NO3)2 and nitrito species (Ba-ONO) do not correlate with the changes in NO conversion. These species, therefore, are considered to play no active role in the catalytic reaction. At the critical temperature, at which the sharp decrease in catalytic activity is observed, the Raman bands of all three species exhibit a sharp decrease in intensity and the spectrum of BaO is detected. Therefore, the sharp reduction in catalytic activity is related to a type of phase transition, which only can occur within Ba particles of considerable size.

This active Ba-nitro species and the related phase transitions could not be detected in Ba/ Al2O3 and Ba/ La2O3 materials, despite comparable specific surface areas. The importance of support specific surface area and support basicity/acidity, thus, can be excluded for catalytic activity. The support structure, therefore, is suggested to play an important role in the development of active NO decomposition catalysts.