The electronic state and local surrounding of both iron ions and tin dopant ions in Lu 1 – x Ca x Fe 0.997 Sn 0.003 O 3 ( x = 0.003; 0.1; 0.2) compounds were studied by 57 Fe and 119 Sn Mössbauer spectroscopy. The analysis of 57 Fe spectra showed that in the Lu 0.8 Ca 0.2 Fe 0.997 Sn 0.003 O 3 sample, obtained by air annealing, the charge deficit created by the substitution of Lu 3+ by Ca 2+ was compensated by the partial transition of Fe 3+ to the +5 oxidation state. With a further increase in the value of x (in the Lu 0.8 Ca 0.2 Fe 0.997 Sn 0.003 O 3 compound) a mixed compensation mechanism appeared, including the formation of oxygen vacancies VO in addition to Fe 5+ ions. Annealing the sample with x = 0.1 in H 2 at 400°C led to the reduction of Fe 5+ to Fe 3+ and, accordingly, to the pure vacancy mechanism of the Ca 2+ charge deficit compensation. The analysis of 119 Sn spectra showed that the substitution of Lu 3+ by Са 2+ in the Lu 0.8 Ca 0.2 Fe 0.997 Sn 0.003 O 3 structure led to the increase in the value of the magnetic field H , probed by some Sn 4+ ions, as compared to the value of H > observed in the case of Lu 0.997 Ca 0.003 Fe 0.997 Sn 0.003 O 3 . This change can be explained by a local increase in the angle of the indirect exchange interaction in the chain Fe 3+ –О 2 ––Sn 4+ , reflecting the presence of Ca 2+ cation, larger than that of Lu 3+ , in the vicinity of Sn 4+ ion.

A synthesis procedure that allows us to obtain ZnO:0.3 at % 119Sn samples with Sn2+ ions on the surfaces of oxide particles containing structure-forming cations on tetrahedral sites is developed for the first time. Analysis of the 119Sn Mössbauer spectra of obtained samples does not confirm the existence of the localized magnetic moments at ZnO grain boundaries recently suggested in the literature.

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.

The irradiation with UV light of anatase powders containing additives of Sb3+ on the surfaces of crystallites, submerged in water, changes the parameters of 121Sb Mössbauer spectrum indicating the transition of antimony to the pentavalent state. It is shown that this transformation is due to the emergence of holes in the valence band of TiO2.

When operating the reforming units with continuous catalyst regeneration there is the problem of optimizing the multiplicity of the catalyst circulation in the reactor-regenerator. This problem can be solved with a combination of natural and computer simulation through a study of the formation of coke on the catalyst surface. Based on the results of TGA of the industrial catalyst Pt-Sn/γ-Al2O3 concluded that amorphous coke is formed on the catalyst surface in reforming process, the whose number of coke at the outlet of the reactor block is 4–6 % depending on the composition of need materials and process parameters. The specific surface area of samples (m2/g): for the original – 152, after regeneration – 140, at the outlet of the reactor – 118, which correlates with the amount of coke on the surface of the samples. Mathematical analysis of processes of coke formation in the reforming reactor, a moving granular bed showed that the multiplicity of the catalyst circulation should be maintained in the range 0,008–0,010 m3/m3 to improve the efficiency of industrial plant. Maintaining the optimum conditions in the reactor and regenerator unit will allow to control the coke formation and to maintain the coke concentration on the minimum possible, and specific surface area of the catalyst at the highest possible level.

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.

Catalytic reforming is one of the most important processes for high octane gasoline manufacture and aromatic hydrocarbons production. There are several ways of this process modernization. It is effective to use mathematical modeling method to examine all possible variants because it requires low time and financial investments. In this work we analyzed the set of possible reactions, proposed kinetic model for catalytic reforming process, calculated thermodynamic and kinetic parameters, investigated catalyst layer hydrodynamics. A mathematical model for a moving-bed catalytic reforming reactor taking into account activity and circulating factor of the catalyst was presented. The model allows analyzing various variants of process modernization and can help to find optimal operation regimes.

During the operation of continuous catalyst regeneration reformers, the problem of optimizing the catalyst circulation ratio in the reactor-regenerator system arises. This problem is solved by a combination of real and computational experiments to investigate the regularities of coking on a catalyst’s surface. Based on TGA results for industrial Pt-Sn/γ-Al 2 O 3 catalyst, it is concluded that amorphous coke is formed on the catalyst’s surface during reforming, its quantity at the reactor block outlet being 4–6%, depending on the feed composition and technological parameters of the process. The specific surface of samples is 152 m 2 /g for the fresh catalyst, 140 m 2 /g after regeneration, and 118 m 2 /g at the reactor outlet, which correlates with the quantity of coke on the surface of samples. Mathematical analysis of the coking processes in a reformer with a moving bed show that the catalyst circulation ratio must be maintained in the range of 0.008–0.010 m 3 /m 3 to increase the operating efficiency of an industrial unit. Maintaining optimal conditions enables us to control the coking process, keeping coke concentration as low as possible and the catalyst specific surface as high as possible.

A mathematical model of the catalytic reforming of gasolines in a reactor with continuous catalyst regeneration is proposed. The model takes into account the motion of the catalyst, changes in its activity along the bed height, and the dependence of its activity on the circulation ratio. The kinetic parameters of the Pt-Sn catalyst are determined under operational conditions by solving the inverse kinetic problem. The composition of the reformate component as calculated by the model coincides with the experimental data within the accuracy of chromatographic analysis. The proposed model is invariant to the composition of raw materials and can be used for predictive calculations.

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.

Annealing of a hydroxide precursor containing equimolar amounts of Mg 2+ and Ti 4+ and small additions of Sn 4+ (0.1 at %) in air at 900°C leads to titanate MgTiO 3 with an ilmenite structure. The 119 Sn Mössbauer spectrum of the sample (unresolved doublet with the isomer shift δ = 0.10 ± 0.01 mm/s and the quadrupole splitting Δ = 0.49 ± 0.02 mm/s) is evidence that the tin atoms are still in the oxidation state +4. Annealing of the precursor at the same temperature in a hydrogen atmosphere yields MgTiO 3 containing Sn 2+ ions (a doublet at δ = 2.82 ± 0.01 mm/s and Δ = 1.66 ± 0.03 mm/s) (the Sn 2+ /MgTiO 3 sample). According to the spectral parameters, the 119 Sn 2+ ions have a low coordination number (CN ≪ 6) and are abnormally resistant to reduction to the metal. Analogous features of the crystal-chemical behavior of 119 Sn 2+ were previously observed during the Mössbauer study of the samples containing tin on the surface of Cr 2 O 3 , α-Al 2 O 3 , and MgO crystallites. The conclusion drawn from analysis of the 119 Sn 2+ Mössbauer parameters that tin in the Sn 2+ /MgTiO 3 sample has surface localization was supported by X-ray photoelectron spectroscopy. Mössbauer measurements show that the tin of Sn 2+ /MgTiO 3 when in contact with air is oxidized much more slowly than on the surface of Cr 2 O 3 , α-Al 2 O 3 , or MgO crystallites. The inhibition of the oxidation reaction is explained to be due to passivation of adsorbed O 2 molecules caused by their interaction with mobile t 2 g electrons of Ti 3+ forming in titanate during high-temperature annealing in H 2 . In addition to the Sn 2+ doublet, the 119 Sn spectrum shows a spectral component with parameters (δ ∼ 1.6 mm/s, Δ ≤ 0.2 mm/s) not fitting the known tin species that can form in MgTiO 3 . This component is explained by persistence in titanate of some Sn 4+ ions immobilizing the mobile t 2 g electron at one of their neighboring Ti 4+ cations.

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.

Analysis of the Mössbauer spectra of dopant 119Sn in cubic MgO has demonstrated that the Sn2+ ions can be stabilized on the surface of crystallites of an oxide with a structure differing from the corundum structure. The Mössbauer parameters (at 100 K, the isomer shift is δ = 2.50 ± 0.01 mm/s and the quadrupole splitting is Δ = 2.30 ± 0.02 mm/s) point to the stereochemical activity of the lone pair of Sn2+. Being in contact with oxygen at 295 K, tin is rapidly converted to the tetravalent state (δ = 0.08 ± 0.01 mms, Δ = 0.58 ± 0.01 mm/s). The lack of formation of Sn2+ on the surface of another cubic oxide (MnO) can be explained by rapid segregation of tin from the bulk of crystallites as stannate clusters.