G. Munuera

Study of CeO2 XPS spectra by factor analysis: reduction of CeO2

Abstract The evaluation of the reduction state of CeO2 by XPS is still controversial because of the presence of more than 10 peaks in the Ce(3d) photoemission spectra due to Ce3+ and Ce4+, some of them at similar binding energies. This makes difficult conventional fitting using nonlinear least-square (NLLSF) methods with Gaussian–Lorentzian curves that require the knowledge of the number of bands. In the present work, we have tried to overcome this problem using factor analysis (FA), a method that has been applied with success to other spectroscopic problems. In order to obtain confident results for the individual shapes of the components, (the basis for a correct quantification) we have combined target testing (TT) and iterative target transformation factor analysis (ITTFA). We have shown that the combined use of these two methods results in a more accurate evaluation (ca. ±2%) of the reduction state of partially reduced CeO2. The election of the “real factors” corresponding to Ce4+ and Ce3+ species is discussed in detail.

XPS study of oxidation processes of CeOx defective layers

Abstract A CeO 2 thin film has been subjected to Ar + bombardment at 298 K to induce the reduction of its outmost layers by preferential removal of oxygen. An XPS study of the altered layer at normal and grazing angle has been carried out. The Factor Analysis (FA) of the XPS spectra of this Ar + reduced film shows that it has a stoichiometry close to Ce 2 O 3 , being Ce 3+ the dominant species at both collection angles. Simultaneously, the O1s spectra depict a lateral peak whose relative intensity is higher for those spectra recorded at grazing angle. Exposure to successive doses of O 2 at 298 K of the reduced layers produces the increase of the O/Ce ratio and a progressive reoxidation of Ce 3+ into Ce 4+ as determined by FA of the Ce3d spectra. Simultaneously, the lateral component at the O1s peak also decreases, thus discarding that it is due to surface contamination by –OH or similar species, as previously suggested in the literature. After exposure to a high pressure of oxygen (ca. 1 Torr), the XPS spectrum obtained at a normal collection angle shows an almost complete oxidation of the film to CeO 2 . However, in the spectrum at grazing angle, Ce 3+ species and the lateral component of oxygen are still detected. The lateral O1s component is tentatively attributed to oxygen ions with unusual coordinations in a defective CeO x ( x 3+ ions might be due to fully coordinated species. Enrichment of the surface of the defective cerium oxide with these oxygen species seems to be a result of the same structural rearrangements that favour the observed stabilization of Ce 3+ species at the surface.

Spectroscopic characterization of Tio2/SiO2 catalysts

Abstract TiO2/SiO2 (7 wt% TiO2) has been prepared via modification of the SiO2 surface by reaction with Ti[OCH(CH3)2]4. Physical characterization of the material was performed by means of XRD, diffuse reflectance spectroscopy (DRS), XPS, and low-temperature IR spectroscopy of adsorbed CO. TiO2 exists in highly dispersed form on the SiO2 surface. Interestingly, a quantum size effect leads to a shift of the absorption edge of TiO2 toward higher energy. XPS difference spectra indicate the formation of Ti3+ during reduction. This is confirmed by CO adsorption in the temperature range 80

Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 1.—Role of hydroxyl groups in photo-adsorption

The photo-activity of TiO2(anatase P. 25) samples with a “fully hydroxylated surface” was studied in the presence of oxygen. Photo-adsorption of oxygen was observed in the early stages of u.v. illumination reducing the pressure to < 0.1 N m–2. After this fast adsorption, slow photo-desorption of oxygen was detected continuing for several hours. Thermal removal of water and hydroxyl groups reduced photo-adsorption and the process became diffusion controlled. Rehydration restored fast photo-adsorption and first order kinetics. Photo-desorption of water accompanied photo-adsorption of oxygen on these “fully hydrated surfaces”. A mechanism involving the formation of OH· radicals and their reaction to give H2O2, which decomposes leading to oxygen photo-desorption, is proposed to explain these facts.

Compositional changes induced by 3.5 keV Ar+ ion bombardment in Ni-Ti oxide systems: A comparative study

Abstract Compositional changes induced by 3.5 keV Ar + sputtering in TiO 2 , NiO, NiTiO 3 and a (TiO 2 + NiO) mixture have been quantitatively studied by XPS. Although all the samples show important changes in their stoichiometry, the extent of the decomposition depends on the compound. The stability of Ti 4+ appears to be enhanced by the presence of Nisu2+ cations which, on the other hand, are more easily reduced to Ni 0 than in pure NiO. To explain these results a redox solid state reaction between the intermediate phases formed during sputtering is proposed, which tends to preserve the most stable phases.

Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 2.—Study of radical intermediates by electron paramagnetic resonance

The effect of pretreatment temperature on the u.v. photo-adsorption of O2 on TiO2 has been studied by e.p.r. Different oxygen species are found if the sample is completely hydroxylated or strongly dehydroxylated, this being related to two different photo-adsorption mechanisms. O2H· and V-centres are found for the hydrated samples and O–2, O–3 O3–3 for the dehydroxylated samples. The origin of all these species is discussed.

XPS study of the surface carbonation/ hydroxylation state of metal oxides

Abstract The carbonation and hydroxylation state of a series of basic oxides (La 2 O 3 , RhLaO 3 and YBa 2 Cu 3 O 7 ) has been examined by XPS. While hydroxyl groups are characterized by an O(1s) peak at ∼531.4 eV, when carbonate species are present C(1s) and O(1s) peaks at 289.4 and 531.4 eV are recorded, giving an atomic O/C ratio near 3. Heating of samples in UHV up to 973 K leads to removal of most of the carbonate and hydroxyl species, carbonate being much more stable than hydroxyls remaining partially even at the highest temperatures.

Characterisation of iron/titanium oxide photocatalysts. Part 2.—Surface studies

A study of the surfaces of particulate solids, formed by the dispersion of iron(III) ions in various titanium dioxide preparations, has revealed that significant changes occur during the calcination of these materials in air at progressively higher temperatures (T/K ⩽ 1273). In the more dilute systems, containing nominal iron(III) concentrations of 0.5–1.0 atom%, solid solutions in the anatase phase exhibit good degrees of dispersion which are maintained as the phase transformation into rutile progresses at more elevated temperatures (T/K 923). With the larger nominal iron concentrations (2 and 5 atom%) X-ray photoelectron (XPS) and diffuse-reflectance (DRS) spectroscopic measurements show that the formation of iron-rich surfaces has commenced already at 773 K through there being a lower limit of solubility of iron in the rutile phase and through the anatase to rutile phase transition becoming detectable at lower temperatures in these solids. The Fe : Ti atomic ratios at the surface with the larger nominal concentrations of iron are consistent with the formation of iron-rich surface phases, α-Fe2O3 and Fe2TiO5. Inhomogeneities in the distribution of the surface ion species have also been demonstrated by EDX measurements.The structural character of these solids has been discussed in relation to their photocatalytic activities for the reductive fixation of dinitrogen by water, in which only single-phase systems were found to be active. The inactivity of the multi-phasic solids has been considered in terms of the masking of the active phase by surface layers which create electronic heterojunctions that encourage hole–electron recombination.

Titania-supported bimetallic catalyst synthesis by photocatalytic codeposition at ambient temperature : preparation and characterization of Pt-Rh, Ag-Rh, and Pt-Pd couples

Abstract We have examined the possibilities of preparing some bimetallic catalysts supported on TiO2 (Degussa P-25) by UV illumination (λ > 300 run, radiant flux, 60 MW cm−2), at room temperature for 24 h, of aqueous suspensions containing 500 mg of this oxide, and cations of the desired metals in amounts corresponding to metal loadings between 0.5 and 2.5 wt% if complete reduction were achieved. In the case of the H2PtC16RhCl3 couple, Rh deposition almost did not take place. For the mixture AgNO3RhC13 at pH 2.6, Rh deposition was substantial but still incomplete. In contrast, Rh deposition was complete in aqueous ammonia (pH 11). TEM and STEM-EDX analysis of the solid recovered in this latter case showed that Rh formed 1- to 3-nn particles and was also found in 8- to 30-nm particles containing ca. 80 wt% Ag (with respect to Rh) presumably as a result of its deposition on more rapidly crystallized Ag. Solids prepared from H2PtC16 and Pd(NO3)2 were studied in more detail because both metals were completely deposited, at least at levels ≤1 wt%. They were heterogeneous, since Pd and Pt formed ca. 1- or 3-nm crystallites, respectively; however, most of the crystallites were found to contain both metals with a high and varied proportion of Pd. An XPS Ar+-sputtering depth profile of the I wt% Pt-1 wt% Pd/TiO2 sample showed a similar behavior for Pt and Pd, which might indicate the formation of a solid solution in some particles. As this formation is attributed to similar reduction rates for both metallic salts, it is inferred that alloys are obtained only in cases where the salts and conditions can be chosen to fulfill this requirement. This limits the capabilities of the photoassisted codeposition method for preparing effective bimetallic supported catalysts.

An XPS study of the Ar+-induced reduction of Ni2+ in NiO and Ni-Si oxide systems

The reduction effect on the Ni2+ species induced by 3.5 keV Ar+ bombardment of NiO particles dispersed on SiO2 and on NiSiO3 as well as on two NiO powder samples with different grain sizes has been quantitatively studied by XPS. In all the cases reduction of the Ni2+ species to metallic Ni° has been observed. However, it is also shown that Ar+-bombardment-induced reduction of Ni2+ is strongly enhanced by the presence of Si4+ cations, which on the other hand remain stable.