Samples containing chromium (both 53Cr-enriched and non-enriched) have been prepared by equilibrium adsorption or impregnation methods at low loadings (<0.5%) using ZrO, , y-Al,O, , SnO,, TiO, (anatase) and SiO, as supports. Heating in O,, generally at 773 K, yielded mononuclear CrV species in a square-pyramidal configuration, Crgc (A), on all supports with the exception of SiO, where CrV is in a tetrahedral configuration, Crxc (A). H, O or NH,, both a t room temperature (RT), yielded the Crxv (A) species from CrEc (A), that is, the complex changes its coordination from five to six. After H, O adsorption and evacuation at RT, Crgc (A) is reversibly restored; however, N H, adsorption and evacuation at increasing temperature gives a new CrV species at 473 K. In the EPR signal of this species, designated Cr& (B), the perpendicular component is split into three lines with 14NH3 (A,4N = 4.0 G) and t w o lines with 15NH3 (A,5N = 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial 0,ligand replaced by a nitrogen-containing NH:-3 species, possibly the NH; anion. With H,O or NH, at RT, the Crgc (B) species is transformed into the corresponding hexacoordinated species, Cr& (B).Upon adsorption of small H, O doses on the CrO,/SiO, sample, the Crgc (A) species is transformed into Crgc (A) and Crxc (A). In the presence of excess water, the chromyl species on SiO, becomes unstable, undergoing disproportionation t o Cr"' and Cr"' On adsorption of NH,, Cr& (6) is formed from Cr& (A) at RT. With 5N H,, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us t o assign the Cr& (B) species o n SiO, t o a slightly distorted chromyl complex with slightly nonplanar equatorial ligands.

CuO x /ZrO2 samples prepared by adsorption from copper solutions or by impregnation were characterized by means of FTIR, XPS, ESR, DRS, volumetric CO adsorption, and redox cycles with H2 and O2. In samples prepared by adsorption, the maximum copper uptake corresponded to an extended plateau at 2.5 atoms nm-2. In as-prepared samples, isolated CuII species were in a distorted octahedral configuration, and in samples heated in dry O2 at 773 K, in a square-pyramidal configuration. Water vapor adsorption transformed the latter species into distorted octahedral complexes. In all samples, heating in O2 at 773 K anchored copper to the zirconia surface. All copper was present as CuII. Evacuation of these samples at 773 K caused no copper reduction, whereas heating with H2 above 450 K reversibly reduced CuII to metal copper. Evacuation of as-prepared samples differed according to how samples were prepared. In particular, evacuation of samples prepared from Cu−acetylacetonate or Cu−acetate reduced CuII to CuI at 473 K and to copper metal at higher temperature. Evacuation reduced copper because acetylacetonates and acetates underwent oxidation during desorption. Evacuation up to 773 K of samples prepared from Cu−nitrate caused no copper reduction. In samples heated in O2 at 773 K, CO adsorption at RT yielded CuI−CO and carbonates. Volumetric CO adsorption combined with FTIR showed that copper was highly dispersed on the ZrO2 surface up to 2.5 atoms nm-2.

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