The rates of the photocatalytic decolorization of methyl orange solutions in the presence of anatase powders modified by Cr3+ and Sb5+ co-dopants are compared with allowance for data on the actual valence state of antimony in the catalyst, obtained using 121Sb Mössbauer spectroscopy. The reported results point to the catalytic activity of the Cr6+ ions formed to compensate for the Cr3+ charge deficit in antimony-free samples.

Information provided by Sn-119(2+) Mossbauer probe ions, located on surface sites of anatase-type TiO2 microcrystals exposed, at room temperature, to a H2S/H-2 mixture, has permitted to conclude that the interaction of H2S molecules with the substrate surface leads to the dissociation of a fraction of the absorbate molecules. This gives rise to the formation of elemental sulfur which oxidizes the neighboring Sn2+ ions, the produced Sn4+ ions being found coordinated only by S2- anions. Subsequent exposure to ambient air is shown to result in the oxidation of 52- ions, yielding both S and SO42-like species, with concomitant stabilization of Sn4+ ions in coordination polyhedra where they are surrounded by only oxygen anions. (C) 2014 Elsevier B.V. All rights reserved.

The reduction in the squared cosine of angle θ of the Cr-O-Cr bond in the Sm-Ho series in samples containing Sn4+ ions in the Cr3+ position in the crystallite bulk is accompanied by a linear increase in hyperfine field H T→0 K at 119Sn nuclei. Upon annealing in H2, tin attains oxidation state +2 occupying the Cr3+ and R 3+ positions on the surfaces of particles. Mössbauer measurements in situ at 100 K do not reveal the spin polarization of Sn2+, but this polarization does manifest itself for part of the daughter Sn4+ ions formed when samples come into contact with air.

The present study provides the first experimental evidence for the stabilization of tin dopant cations immediately on the surface of an oxide having a tetragonal structure. 119Sn Mössbauer spectra of the dopant, introduced by air annealing into the bulk of anatase microcrystals, showed that it was located, in the tetravalent state, in somewhat distorted octahedral sites of a unique type. On the contrary, the reduced tin species, formed upon subsequent hydrogen annealing the Sn4+-doped samples, are found to occupy different sites being characterized by two sets of the isomer shift δ and quadrupole splitting ΔEQ values (δI = 3.25 mm s−1, ΔEQI = 1.75 mm s−1; and δII = 2.85 mm s−1, ΔEQII = 1.71 mm s−1). Either of them implies both the divalent state of tin atoms and their presence at low-coordination sites that can be assigned to the surface of crystallites. Mössbauer spectra of Sn4+←2+ daughter ions, formed upon contact with air of Sn2+, consist of a symmetrically broadened peak characterized by only slightly different average values of both the isomer shift (<δ> = 0.07 mm s−1) and quadrupole splitting (<ΔEQ> = 0.50 mm s−1), as compared to the δ and ΔEQ values for the bulk-located Sn4+. However, considerable broadening of Sn4+←2+ doublet components (Γ = 0.97 mm s−1) allows one to suggest that these secondary formed ions remain distributed over the non equivalent sites inherited from their Sn2+ precursors. The occurrence of Sn4+←2+ at surface sites is independently proven by XPS measurements that revealed a greater than 10-fold enrichment with tin of 3–5 nm thick surface layers.

The 119 Sn Mössbauer spectra of polycrystalline NiTiO 3 samples impregnated with a solution containing 0.3 at % Sn 4+ are evidence that annealing in H 2 converts tin into the state with the electron density |Ψ(0)| 2 on 119 Sn nuclei corresponding to “Sn 3+ ” ions. The stabilization of tin atoms in such an untypical formal oxidation state occurs at a depth of no more than 2–3 nm from the surface of titanate crystallites. It was revealed that the Sn 3+ ions are not subjected to spin polarization even at temperatures considerably lower than the Néel temperature of NiTiO 3 , which can be explained by their location in the Ni 2+ positions. The formation of Sn3+ prevents the further reduction of tin to the divalent state and, hence, precludes localization of 119 Sn probe cations in positions at the interface.

The results of recent studies involving the application of diamagnetic Mossbauer dopant cations for the investigation of heterogeneous catalytic reactions are described. Possibilities for using the information contained in Mossbauer spectra to clarify the nature of active catalytic centers are discussed. It is shown that, in some cases, the catalytic tests allow the correctness of the chemical interpretation of spectral parameters of the dopant atoms under study to be verified.