Oxygen species adsorbed on CeO2/SiO2 supported catalysts

Abstract

The nature of the oxygen species adsorbed on slightly reduced Ce02/Si02 supported catalysts has been investigated using oxygen enriched with I7O2. The observed hyperfine splitting was found to be associated with the central component of the g tensor ; it indicated that the corresponding orbital contained the unpaired electron and therefore thecentral component was defined as gxx. With this assumption, the e.s.r. spectra are consistent with the adsorption of oxygen as 0; at 77 K with gyy = 2.0109,gxx = 2.0158 and gzz = 2.028 with a hyperfinesplitting(A,,) of 75 G about gxx. The 0; is adsorbed with the internuclear axis along the surface of the catalyst at a site corresponding to a cerium ion. There is some indication that interaction with the metal ion orbitals is perturbing the energy levels of the adsorbed 0; ion.Recently, catalysts based on pure or supported CeO, have been used in oxidation reaction^."^ In order to understand the reaction mechanisms involved it is useful to identify the nature of the oxidizing species present since this may greatly influence the activity of the ~a t a l y s t . ~A number of different oxygen species such as 0-, 0, and 0 ; have been observed recently on MgO 5-7 and their reactivity ~haracterized.~. The most reactive species appears to be 0and similar conclusions were reached by Kazansky for 0formed on V 2 0 5 supported by silica.The various possible species of oxygen can be distinguished unambiguously by the use of isotopically labelled oxygen in e.s.r. studies.'O As a continuation of the studies of adsorbed oxygen species,' we used the hyperfine structure resulting from the nuclear spin of 5/2 for 1 7 0 to identify the form of adsorbed oxygen on reduced CeO, /Si02 catalysts. EXPERIMENTAL Supported Ce02 was prepared by impregnation of a Si02 carrier (specific surface area 640m2/g) with a Ce(N03)3 aqueous solution of known concentration. The catalyst was further washed with doubly distilled water, dried at 380 K and calcined in an oxygen atmosphere at 800 K ; the catalyst was found to contain 5.4 % by weight of cerium. Before oxygen adsorption the samples were outgassed in vaczio at 800 K for 12 h in silica tubes and oxygen was adsorbed at pressure of 10-2-10-1 Torr at 77 K using 1 6 0 2 and isotopically enriched O2 (58 atom % of 1 7 0 ) on various samples. E.s.r. measurements were carried out on a Varian V 4502 spectrometer at 9.2 GHz. Diphenylpicrylhydrazyl was used as a standard for g value calibrations. The values of the g tensor components were measured at the turning points as described in ref. (6) and the absolute accuracy is believed to be within JrO.0010. The values are quoted to 4 decimal t On leave of absence from Institut de Recherches sur la Catalyse.

Physicochemical characterization of Au/CeO2 solid. Part 1: The deposition–precipitation preparation method

“…Therefore, the total spin density must be in the 2p orbital of the oxygen atom. Furthermore, taking only the A xx constant into consideration (since A yy ¼ 0 and A zz ¼ 0), the total spin density located between the two oxygen atoms of the O À 2 ion has been calculated when this latter was adsorbed on different matrices [85,86]. It has been found that the total spin density values are very close to unity when the a iso value is about 25 G. This value is similar to the one obtained in this study.…”

“…In some specific situations, the values for A zz have been determined [87] but rarely for A yy . Furthermore, the two oxygen nuclei are usually equivalent and this is due to adsorbed ionic molecular oxygen having its inter-nuclear axis (z direction) parallel to the surface [85,86]. Theoretical calculations lead to A xx values ranging between 74 G and 80 G, indicating that the localization of the unpaired electron on the O À 2 ion is largely independent of the support.…”

Ceria‐Based Solid Catalysts for Organic Chemistry

“…[4] Since the pioneering works of Yao and Yu Yao [2] and of Su et al, [5,6] many studies have been devoted to improving knowledge of the kinetics of OSC [7][8][9] and of the mechanisms implicated in surface and bulk oxygen mobility in ceria and related compounds, [10][11][12][13][14] with a special insight into CeZrO x oxides. [15][16][17][18][19] Oxygen diffusivity in these materials can be measured by 18 O/ 16 O isotopic exchange, [20][21][22] while oxygen species (e.g., superoxides, peroxides) that might be involved in the diffusion process can be investigated by electron spin resonance (ESR) [23][24][25][26][27] or FTIR. [18,[28][29][30][31] ESR studies have shown that the presence of a metal (Pt, Rh) drastically changes the nature of the oxygen species; the metal favoring the formation of O À ions instead of superoxide species.…”

Do observations on surface coverage-reactivity correlations always describe the true catalytic process? A case study on ceria

“…Oxygen vacancies formed on the surface of the catalyst can be quantitatively investigated by means of EPR spectroscopy. 17,18,19,20 O 2 can be used as probe molecule for this purpose, because it binds to defects forming paramagnetic superoxide…”