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 ∼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 °C for SZ but only at 345 °C for SA. After replacing Cl- by OH- and calcining, the materials were tested as De-NOx catalysts and characterized by temperature-programed 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 NO x reduction. During reduction of Fe/SA up to 800 °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 NO x reduction with isobutane. Catalysts with low Fe content that are prepared by controlled sublimation are superior to those prepared by impregnation. At 450 °C and GHSV = 30 000 h-1, 65% of NO x is reduced to N2 in excess O2.