Andrey Karpov

The Intimate Relationship between Bulk Electronic Conductivity and Selectivity in the Catalytic Oxidation of n-Butane†

The efficient and direct functionalization of alkanes from natural gas or future regenerative carbon-based resources is impeded by the lack of suitable catalysts and the absence of a detailed mechanistic understanding of the few efficient alkane oxidation reactions. The selective oxidation of nbutane to maleic anhydride (MA), an important basic chemical with an annual global production of 1.4 Mt, is one of such scarce commercialized examples. The industrially used vanadium-phosphorous-oxide (VPO) catalyst enabling a maximum MA yield of 65% mainly consists of (VO)2P2O7 (vanadyl pyrophosphate, VPP). Based on experimental evidence it is generally agreed that the reaction proceeds via a two-step mechanism, in which, in Step 1: “lattice” oxygen from the catalyst (or oxygen from an active surface site) is abstracted to oxidize the alkane, and in Step 2: the catalyst is subsequently reoxidized by gas-phase O2. It is however still debated, whether the reaction proceeds on noninteracting single surface sites with the bulk being only an inert support, or if the bulk supplies charge carriers and oxygen. 11–15] The single-site concept would demand spacious active sites to provide the large number of 14 electrons and seven oxygen atoms needed per converted n-butane molecule. In contrast, an unlimited bulk–surface charge and oxygen transfer contradicts the fundamental site-isolation principle of selective oxidation catalysis, which presumes that the (stoichiometric) limitation and spatial isolation of active oxygen prevents the further oxidation of the desired product to COx. Clearly, the investigation of charge-carrier dynamics in selective catalysts is of fundamental importance to disentangle the surface and bulk influence on the catalytic performance. Unfortunately, unstable (Schottky) contact resistances between catalyst particles, electrodes, and at grain boundaries hamper quantitative and sensitive electrical-conductivity investigations by DC or AC contact methods. Although such studies have provided valuable information on the electrical properties of VPO catalysts, the direct participation of bulk charge carriers in the catalytic reaction has not been demonstrated unequivocally yet. Herein, the disadvantages of contact methods could be circumvented by using a noncontact conductivity method based on the microwave cavity perturbation technique (MCPT). MCPT is a highly sensitive technique 21] allowing the non-invasive quantitative measurement of the permittivity and electrical conductivity of polycrystalline samples in a fixed-bed flow-through reactor. The excitation of free charge carriers in the investigated sample by microwaves (at 9.2 GHz) in a calibrated resonant cavity enables the determination of absolute conductivity values. By measuring the change of the resonance frequency and the quality factor of the cavity with and without the sample, its complex permittivity e = e1 + ie2 can be deduced (Figure 1). [20–22] The imaginary part e2 is composed of the dielectric loss, ionic, and electronic conductivity. A major contribution of ionic charge carriers (e.g. O ions) is negligible because their high masses are not able to follow the high-frequency microwave excitation. In addition, no response of the real permittivity e1 of VPO to variations of the gas-phase chemical potential was observed, hence a significant influence of dielectric relaxations (through dipoles) can be excluded. Consequently, the major contribution to e2 in VPO is electronic conductivity. Moreover, pre-investigations showed that the catalyst behaves as a p-type semiconductor with an increasing conductivity in oxidizing and a decreasing conductivity in reducing mixtures (Figure S2 in the Supporting Information). These results are in agreement with contact conductivity investigations. According to X-ray powder diffractometry, the VPO catalyst was phase pure and consisted of vanadyl pyrophosphate (Supporting Information, Figure S1). The catalytic performance and conductivity of VPP were probed simultaneously with the in situ MCPT/GC setup at constant temperature, but in different gas mixtures and at different gas hourly space velocities (GHSV), i.e. at different reaction gas contact times. The catalyst was preheated in air to 400 8C (GHSV: [*] Dr. M. Eichelbaum, Dr. M. H vecker, C. Heine, Dr. A. Trunschke, Prof. Dr. R. Schlçgl Department of Inorganic Chemistry Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) Fax: (49)30-84134401 E-mail: me@fhi-berlin.mpg.de

An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides.

ESCA-measurements on barium platinides provide the first spectroscopic evidence for negative oxidation states of platinum and are in excellent agreement with theoretical predictions based on quantum-chemical calculations.

AgMoVO6: A Promising Catalyst for Selective Gas‐Phase Oxidation of o‐Xylene

With a global consumption of about 4 000 000 t year , phthalic anhydride is among the most important industrial chemical intermediates. It is mainly used for plasticizers, alkyd resins, and unsaturated polyester resins. Currently, the dominant route for the production of phthalic anhydride is the one-step direct gas-phase oxidation of o-xylene with air at 623–673 K over an established heterogeneous catalyst. This catalyst consists mainly of a single layer of promoted vanadium pentoxide deposited onto a TiO2 support, preferentially crystallized in the anatase modification (V/Ti catalyst). The one-step oxidation process typically results in phthalic anhydride yields of 80– 82 mol %. . There are ways to increase this yield by advanced reaction engineering: Higher phthalic anhydride yields could be achieved by a two-step oxidation process, whereby o-xylene is oxidized in the first step either in a liquid phase or in a gas phase to partial C8 oxygenates (o-tolyl aldehyde, o-toluic acid, phthalide, or phthalic anhydride), which are then further oxidized on a conventional V/Ti catalyst to phthalic anhydride with a total phthalic anhydride yield up to 88 mol % (Scheme 1). The highest selectivity to partial C8 oxygenates by oxidation of o-xylene with air in the gas phase was observed by using bronze type catalysts based on d-Ag0.73V2O5-derived crystalline phases (SC8 = 90 % at o-xylene conversion Xo-xyl = 40 %). Since the increase of the phthalic anhydride yield not only saves raw materials but also decreases emission of COx gas, which occurs as the main byproduct in this type of gas phase oxidation reaction, new catalysts for selective oxidation of oxylene with high selectivity to high-value products are not only of great economical importance but also have a noticeable positive environmental impact. To date, general ab initio approaches are known for the design of catalytic materials only for a few chemical reactions, whereas most industrial catalytic processes were developed by empirical methods. Therefore we chose high-throughput experimentation (HTE) for development of new o-xylene oxidation catalysts. Details of the approach chosen for the selection of potential candidate systems are described elsewhere. Testing of the candidate systems were carried out on 1 mL catalyst split samples. In the first stage of screening, the apparent selectivity S [100 SCOx] was used as a basic indicator for the selectivity to high-value products. As a result of testing more than 2000 different catalytic materials, we were able to identify a promising system based on Ag, Mo, V, and O with an apparent selectivity of about 85 % at Xo-xyl 64 % (Table 1). By variation of the Ag/Mo/V ratios it has been found that a local maximum of catalytic performance with respect to apparent selectivity and activity is achieved for a chemical composition of Ag/Mo/V = 1:1:1. This chemical composition corresponds to AgMoVO6, the only crystalline quaternary oxide in the Ag-MoScheme 1. One-step vs. two-steps oxidation of o-xylene to phthalic anhydride.

Formation of SrSnO3 shell-like inclusions in the Bi2Sr2CaCu2O8+x superconductor via chemical reaction

Abstract Superconducting composites Bi-2212–SrSnO 3 have been prepared by reacting between the Bi–Sr–Ca–Cu–O precursor and Sr 2 SnO 4 or Ca 2 SnO 4 at 800–950°C, followed by crystallisation of Bi-2212 from the partial melt at decreasing temperature. The samples have been characterised by powder X-ray diffraction, scanning electron microscopy and magnetic measurements. The materials consist of large Bi-2212 lamellae and complex-shaped fine inclusions of SrSnO 3 . The composite obtained using Sr 2 SnO 4 contains almost all the SrSnO 3 phase in the form of micron-sized spherical shells, which are partly included in Bi-2212 lamellae, partly agglomerated in-between. The shells are perforated, thus allowing the Bi-2212 crystals to grow through them. It has been found that the shell-like grains form at an early stage of the precursor thermal treatment between 800 and 850°C. A mechanism of the SrSnO 3 shell formation is proposed. The composites exhibit T c in the range of 82–87 K and reveal up to five times better magnetic flux pinning at T≥30 K in comparison with the undoped Bi-2212 sample prepared using the same experimental procedure.

A10Tl6O2(A = K, Rb) cluster compounds combining structural features of thallium cluster anions and of alkali metal sub-oxides

New alkali metal thallideoxides, A10Tl6O2 (A = K, Rb), crystallize in a unique structure consisting of hypoelectronic [Tl6]6- clusters in the shape of compressed octahedra, together with oxygen-centred alkali metal octahedra that have been identified as constitutive of alkali metal sub-oxides.

Crystal Structure Elucidation of Anhydrous Rb2[Pt(CN)4] from X-Ray Powder Diffraction Data

The title compound has been synthesized by metathesis of Ba[Pt(CN)4]·4 H2O with Rb2SO4, in aqueous solution. Its crystal structure was solved from X-ray powder diffraction data using the simulated-annealing approach, and refined by Rietveld’s method. The compound crystallizes in space group Imma, a = 11.1432(2), b = 7.4382(1), c = 11.1896(2) Å, V = 927.45(3) Å3, Z = 4, Rp = 0.0402, Rw = 0.0247 (Nhkl = 173). Square-planar tetracyanoplatinate groups stack in an unprecedented eclipsed conformation, forming one-dimensional linear chains of Pt-atoms with Pt-Pt separations of 3.719 Å . Rb2[Pt(CN)4] was characterized by differential thermal analysis, thermogravimetry and infrared spectroscopy.

Pt-dumbbells in Ba3Pt2: Interplay of geometric and relativistic effects on Pt-Pt bonding

Ba3Pt2 has been synthesized by reaction of a 3 : 2 mixture of Ba and Pt at 1223 K in argon, and characterized by single-crystal X-ray structure determination and electrical resistivity measurements. Ba3Pt2 crystallizes in the Er3Ni2 structure type (space group R3̅ with a = 962.40(6), c = 1860.6(1) pm, Z = 9, R(F)N′ = 0.063, N′(hkl) = 777), and is isotypic to Ca3Pt2 and Sr3Pt2. The Pt atoms occur in pairs at a distance of 303 pm. According to the analysis of the Electron Localization Function and the Crystal Orbital Hamilton Population obtained from DFT band structure calculations, covalent bonding can be assumed in the Pt-dumbbells, although it is weaker in Ba3Pt2 than in Ca3Pt2. The peculiarities of the platinum compounds due to relativistic effects are elaborated by a comparison with theoretical results for Ca3Pd2. Ba3Pt2 exhibits metallic conductivity (ρ270 = 0.7 mΩ · cm), which is in accordance with band structure calculations.

Kristallzucht und Strukturaufklärung von K2[Pt(CN)4Cl2], K2[Pt(CN)4Br2], K2[Pt(CN)4I2] und K2[Pt(CN)4Cl2] · 2H2O/ Crystal Growth and Crystal Structure Determination of K2[Pt(CN)4Cl2], K2[Pt(CN)4Br2], K2[Pt(CN)4I2] and K2[Pt(CN)4]Cl2] · 2H2O

Abstract Crystals of K2Pt(CN)4Br2, K2Pt(CN)4I2 and K2Pt(CN)4Cl2 ·2H2O were grown, and their crystal structures have been determined from single crystal data. The structure of K2Pt(CN)4Cl2 has been determined and refined from X-ray powder data. All compounds crystallize monoclinicly (P21/c; Z = 2), and K2Pt(CN)4X2 with X = Cl, Br, I are isostructural. K2Pt(CN)4Cl2: a = 708.48(2); b = 903.28(3); c = 853.13(3) pm; β = 106.370(2)°; Rp = 0.064 (N(hkl) = 423). K2Pt(CN)4Br2: a = 716.0(1); b = 899.1(1); c = 867.9(1) pm; β = 106.85(1)°; R(F)N′ = 0.026 (N’(hkl) = 3757). K2Pt(CN)4I2: a = 724.8(1); b = 914.5(1); c = 892.1(1) pm; β = 107.56(1)°; R(F)N′ = 0.025 (N’(hkl) = 2197). K2Pt(CN)4Cl2 ·2H2O: a = 763.76(4); b = 1143.05(6); c = 789.06(4) pm; β = 105.18(1)°; R(F)N′ = 0.021 (N’(hkl) = 2281). Raman and infrared spectroscopy data are reported.