The role of the Cu(II) in the catalytic oxidation of CO over Cu/SnO2 with low Cu(II) content was studied by continuous wave EPR, electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectroscopes. Three methods were employed for introducing the copper: (i) by coprecipitation, (ii) impregnation onto SnO2 gel and (iii) impregnation onto calcined SnO2. Two types of Cu(II) species were identified in these calcined Cu/SnO2 materials. Those belonging to the first type, termed B and C, exhibit highly resolved EPR spectra with well defined EPR parameters and are located within the bulk of the oxide. The other group comprises a distribution of surface Cu(II) species with unresolved EPR features and are referred to as S. While the latter were readily reduced by CO the former required long exposures at high temperatures (> 673 K). The specific interactions of the different Cu(II) species with CO were investigated through the determination of the 13C hyperfine coupling of enriched 13CO. The ESEEM spectra of calcined samples, generated either by coprecipitation or impregnation, show after the adsorption of CO signals at the Larmor frequencies of 117,119Sn and 13C and at twice these Larmor frequencies. Although these signals indicate that 117,119Sn and 13C are in the close vicinity of Cu(II), they cannot provide the hyperfine couplings of these nuclei. This problem was overcome by the application of the HYSCORE experiment. The 2D HYSCORE spectra show well resolved cross peaks which provide the hyperfine interaction of these nuclei. Simulations of the HYSCORE spectra yield for 117,119Sn an isotropic hyperfine constant, aiso, of ±4.0 MHz and an anisotropic component, T⊥, of ±2.0 MHz. Pulsed ENDOR spectra also showed 117,119Sn signals which agree with the above values. The 13C cross peaks yield aiso = ±1.0 MHz and T⊥ = ±2.0 MHz. Similar C cross peaks were observed in spectra of calcined Cu/SnO2 after the adsorption of CO2. Based on the same hyperfine couplings in the samples exposed to 13CO and 13CO2 the signals were assigned to surface carbonate species generated by part of the Cu(II) S type species rather then by species B and the role of the Cu(II) in the oxidation process is discussed.
|Number of pages||19|
|Journal||Applied Magnetic Resonance|
|Publication status||Published - 1 Dec 1996|