Characterization of iron catalysts prepared by chemical vapor deposition on nonzeolitic supports.

Chemical vapor deposition (CVD) of FeCl3 has been used to deposit Fe3+ ions on the surface of sulfated zirconia (SZ) and silica-alumina (SA). Upon exposure to FeCl3 vapor most Brønsted acid sites and silanol groups are replaced by Fe, as evidenced by IR. With SZ the concentration of the acid sites and thus the retention of Fe increase with the sulfate loading up to approximately 45% of a monolayer, followed by an abrupt decrease at higher loadings. This indicates condensation of sulfate groups to polysulfates, which is in line with a lower number of Brønsted sites per sulfate. Release of HCl due to the reaction of Brønsted sites with FeCl3 peaks at 85 degrees C for SZ but only at 345 degrees C for SA. After replacing Cl- by OH- and calcining, the materials were tested as De-NOx catalysts and characterized by temperature-programmed reduction (TPR) with H2 or CO. Mononuclear and dinuclear oxo-ions of Fe coexist with Fe oxide particles in calcined Fe/SA, resulting in a low selectivity for NOx reduction. During reduction of Fe/SA up to 800 degrees C, a significant fraction of the Fe forms a chemical compound with SA, possibly an aluminate. In Fe/SZ the Fe dramatically increases the reducibility of the sulfate groups, from 57% partial reduction to SO2 in the absence of Fe, to 90% deep reduction to S2- ions in its presence. Formation of Fe sulfide is indicated by the enhanced sulfur retention upon reduction. Fe/SZ is active for NOx reduction with isobutane. Catalysts with low Fe content that are prepared by controlled sublimation are superior to those prepared by impregnation. At 450 degrees C and GHSV = 30,000 h(-1), 65% of NOx is reduced to N2 in excess O2.

TPD of nitric acid from BaNa-Y: evidence that a nanoscale environment can alter a reaction mechanism.

The mechanism of temperature-programmed desorption (TPD) of nitric acid chemisorbed on BaNa-Y was studied over the temperature range from 200 to 400 degrees C, in the presence and absence of CO. Nitric acid dissociates to form H(+) and NO(3)(-) when chemisorbed on BaNa-Y. The results of these experiments are consistent with H(+) and NO(3)(-) either reacting directly to produce OH and NO(2) or recombining to produce HNO(3), which is desorbed and rapidly decomposes within the zeolite pores to OH and NO(2). The kinetics and stoichiometry suggest that the hydroxyl radicals produced react with CO and NO(2) to form CO(2) + H and NO + HO(2), respectively. The H atoms thus formed react with OH in preference to NO(2), a change in mechanism consistent with literature rate constants and the expectation that the zeolite pore walls act as a third body for the reaction of H with OH. Finally, OH may react with NO(2) to form HO(2), which can undergo further reactions to form O(2), H(2)O, and/or H(2). No reaction between CO and NO(3) or CO and surface-bound NO(3)(-) was observed.