The surface and materials science of tin oxide

Cu(II)/CeO 2 catalyst materials have been prepared by two routes, coprecipitation from aqueous solutions containing Cu 2+ and Ce 4+ ions and sorption of Cu 2+ ions on to ceria gel, and their constitutions after thermal treatment in the temperature range 333-1273 K investigated by nitrogen adsorption, powder X-ray diffraction, EPR, and EXAFS. EXAFS data show that the initial Cu(II) species are polymeric Cu(OH) 2 and hexaaqua {Cu(H 2 O) 6 } 2+ ions sorbed onto the surface of the ceria particles from the coprecipitation or impregnation routes, respectively. The materials are mesoporous except after calcination at 1273 K when they become nonporous and crystallites of CuO are apparent. Promotion of ceria with copper(II) enhances the activity toward the oxidation of carbon monoxide dramatically, 100% conversion occurring at 343 K even for high CO concentrations and stoichiometric CO/O 2 compositions. The most active catalyst material is formed by thermal processing of materials obtained by either route at ca. 673 K producing a material comprising small particles of ceria on the surface of which copper(II) is dispersed amorphously. EPR shows that, after calcination at 573 K, the copper(II) is present in both types of material as a mixture of isolated Cu 2+ ions and amorphous clusters or aggregates of Cu 2+ ions. Additionally Cu 2+ dimers are formed at 873 K. In situ redox studies show that the amorphous copper(II) aggregates are reduced most easily by exposure to CO at 473 K, followed by the dimer species at 573 K and finally the isolated Cu 2+ monomers at 673 K when signals due to Ce 3+ appear (i.e. reduction of the support). Reoxidation of all types of Cu + can be achieved by exposure of the reduced catalyst material to either O 2 or NO, although under the same pressure (200 Torr) oxidation by NO is achieved at a lower temperature than with O 2 . The mode of action of these catalyst materials appears to be synergistic in nature with the principal role of Cu(II) being mainly in electron transfer, abstracting the negative charge remaining when oxygen vacancies are formed following desorption of CO 2 . The significant enhancement of catalytic activity is due in large part to the efficiency of the Cu(II)/Cu(I) couple in this process. Catalyst deactivation at low temperatures under reducing conditions is due to depletion of the catalyst surface of active oxygen, but activity is restored by treatment in air at moderate temperatures. Deactivation at high temperatures is irreversible and is due to the phase separation of copper(II) oxide coupled with a dramatic increase in particle size.

Nature and surface redox properties of copper(II)-promoted cerium(IV) oxide co-oxidation catalysts

Nanocrystals of pure, Cu2+ and Fe3+ doped SnO2 have been prepared by a sol−gel route and were investigated by a combination of EXAFS and XRPD measurements. Surface doped samples were also studied, and these were prepared by immersing pure nanocrystals in aqueous solutions of the dopant cations. The XRPD results showed the average size of the crystallites when freshly prepared was 2−3 nm and was not affected by the dopant. The Sn K-edge EXAFS of the pure SnO2 nanocrystals was consistent with their size and suggested the level of disorder in the crystallites was comparable to that in bulk SnO2. The K-edge EXAFS of the dopants showed that up to at least 10 mol % nominal doping of both Cu2+ and Fe3+ ions in sol−gel prepared samples were situated on Sn4+ sites in the SnO2 cassiterite lattice. In the case of the surface doped samples the EXAFS showed no penetration of the dopant into the SnO2 lattice. The growth of the crystallites on heating was monitored by XRPD and was found to become clearly evident at abou...

A combined exafs and diffraction study of pure and doped nanocrystalline tin oxide

Tin oxide nanocrystals, both pure and Cu2+ and Fe3+ doped, have been prepared by a sol–gel process The response of these materials to carbon monoxide in dry air has been investigated as a function of annealing temperature The growth of the crystallites was monitored by XRPD from room temperature to about 900 °C and the response of the material to CO was studied for materials annealed over that temperature range All of the materials were shown to respond to low concentrations of CO with a narrow peak in sensitivity at an operating temperature of about 200 °C A good response to CO was also observed at an operating temperature of about 400 °C No improvements in selectivity to CO were observed by the addition of either of the cation dopants The sensitivity to CO was shown to decrease as crystallite size increased The addition of the metal cation dopants impeded crystallite growth Our previously reported Cu K-edge and Fe K-edge EXAFS measurements, on the Cu2+ and Fe3+ doped materials respectively, showed the dopant cations to move from ordered Sn4+ substitutional lattice sites in the as-prepared materials to more disordered regions, most likely the surface regions, as the materials were annealed This dopant migration begins at about 400 °C and is accompanied by a corresponding large decrease in response to CO at an operating temperature of about 200 °C (the peak in sensitivity) This is attributed to the migration of the dopants to the surface of the crystallites and speculative explanations are given The reduction in response of the same materials at an operating temperature of about 400 °C is not so large, indicating the response mechanisms at 200 and 400 °C to be different

The effects of crystallite growth and dopant migration on the carbon monoxide sensing characteristics of nanocrystalline tin oxide based sensor materials

CATALYTIC oxidation of CO, both by O2 and by nitrogen oxides (NOx), is of considerable relevance to the removal of toxic CO and/or NOx from exhaust gases. Certain applications such as automobile exhaust control in cold-start conditions, and the removal of CO from tobacco smoke by use of a catalytic filter, demand not only catalytic activity at moderately low temperatures, but also effectiveness in the presence of water vapour. The low-temperature activities of most catalysts are at best (for example precious metals) partially destroyed, or at worst (for example base metal oxides) totally destroyed by poisoning due to water vapour. The results presented here demonstrate that, not only is the low-temperature activity of the Pd–SnO2 system not poisoned by water vapour, but rather its presence causes a marked enhancement in activity for the oxidation of CO both by O2 and by NO.

M.J. Fuller

Papers

The catalytic reduction of nitric oxide by carbon monoxide over SnO2CuO gels