Jacek Ziółkowski

ESCA studies of copper oxides and copper molybdates

Abstract Valence band, Cu 2p 3 2 , O 1s, Mo 3d , and Cu L 3 M 45 M 45 photoelectron and X-ray-induced Auger spectra were recorded for metallic copper, Cu 2 O, CuO, Cu 2 Mo 3 O 10 , Cu 6 Mo 4 O 15 , CuMoO 4 , Cu 3 Mo 2 O 9 , and Cu 3.85 Mo 3 O 12 . Cu 2p 3 2 binding energy is 0.9 eV lower for Cu + -containing molybdates than for Cu 2 O and 0.7 eV higher for Cu 2+ -containing molybdates with respect to that of CuO. Calculation of net chemical shift demonstrates the influence of Madelung potential on the binding energy of core electrons. On the basis of differences in binding energy it was possible to distinguish between various Cu-containing phases and to follow the surface redox processes of copper molybdates which, as it was seen, follow the same reactions as in the bulk processes. Auger spectra suggest the presence of a very thin layer of “surface phase” common for all five studied molybdates and independent of bulk structure and composition.

Oxidation of butane and butene on the (100) face of (VO)2P2O7: A dynamic view in terms of the crystallochemical model of active sites

The structure of the (100) face of (VO)2P2O7 and its performance in the oxidation of n-butane and butenes to maleic anhydride have been analyzed in terms of the crystallochemical model of active sites (CMAS). Analysis involves the heats of adsorption of oxygen, hydrogen (as a component of OH), and water as well as the heats of their movement along the surface, which allows determination of the energetically easiest pathways of elementary steps and gives insight into the reaction dynamics. The catalyst (I00) (VO)2P2O7 is found to work in a surface-oxidized state, all cations being covered with oxygen. The active site for the direct oxidation of n-butane to maleic anhydride is found to be situated between four protruding, undersaturated oxygens (2 × VO, 2 x PO). The reaction is thought to be initiated by H bonding at both terminal carbons. After removal of two terminal hydrogens, strong CterminalOsurf bonds anchor the molecule sufficiently long enough for the reaction to be completed. The assemblage of unsaturated oxygens around the indicated site is geometrically and energetically convenient both for the abstraction and removal of eight hydrogens and for the insertion of three oxygens necessary for the formation of maleic anhydride. The desorption of water and migration of surface oxygen (which produces the pairs of adjacent vacancies to be filled by O2 molecules) that constitute a substep of the concerted reoxidation seem to be rate determining. Oxidation of butenes on (100) (VO)2P2O7 is thought to be initiated by adsorption of C=C over unsaturated oxygens. In view of the surface structure, this adsorption limits the number of active oxygens with which the hydrocarbon may interact and favors a mild and nonselective oxidation to epoxybutanes, crotonaldehyde, hydrofuran, furan, and acetaldehyde. Minor yields are expected due to difficult reoxidation and competitive adsorption. Theoretical predictions are shown to agree with experimental data.

Catalytic anisotropy of MoO3 in oxidation reactions in the light of bond-strength model of active sites

Abstract Catalytic anisotropy of molybdic oxide in oxidation of propylene has been observed by Volta et al. (7th Int. Congr. Catal. Commun. C4, Tokyo, 1980; Faraday Disc. 72/13, Selectivity in Heterogeneous Catalysis, Nottingham, 1981). High selectivity to acrolein in the indicated reaction was ascribed to the (100) plane while the (010) plane was found to yield CO 2 . The critical discussion of Voltas experimental data, performed in this paper, has shown that they may be interpreted in three alternative ways, differing in the ascription of the reaction products to the crystallographic planes. Different crystallographic planes exposed by the grains of MoO 3 have been analyzed in terms of the bond-strength model of active sites (Ziolkowski, J. Catal. , submitted) developed under the following main assumptions: (i) the reaction path depends on the number and configuration of the active oxygen atoms in the vicinity of the adsorption site, and (ii) the individual catalytic activity of a given surface oxygen atom is proportional to the reciprocal sum of the strength of the bonds to it from the adjacent cations. The analysis provided the arguments to indicate the most probable reaction pattern. According to it the main products expected to be formed in oxidation of propylene on different planes of MoO 3 are (100), CO, CO 2 ; (001), acrolein (acrylic acid, C 2 O, CO, CO 2  mainly at longer contact time); (101) and (101), acrolein; (010), inactive (possible minor yield of hexadiene and benzene).

Polymorphism of the bivalent metal vanadates MeV2O6 (Me = Mg, Ca, Mn, Co, Ni, Cu, Zn, Cd)

Abstract Based on the literature data, our former findings and additional DTA and high-temperature X-ray studies performed for CdV 2 O 6 , MgV 2 O 6 , and MnV 2 O 6 , a consistent scheme of the phase transformations of the Me V 2 O 6 ( Me = Mg, Ca, Mn, Co, Ni, Cu, Zn, Cd) metavanadates is constructed at normal pressure between room temperature and melting points. Three types of structures exist for the considered compounds: brannerite type (B), pseudobrannerite type (P), and NiV 2 O 6 type (N). The following phase transformations have been observed: Me = Mg, B → P at 535°C; Me = Mn, B → P at 540°C; Me = Co, N → B at 660°C; Me = Cu, B (with triclinic distortion) → B at 625°C (secondary order); and Me = Cd, B → P at 170°. CaV 2 O 6 P, NiV 2 O 6 N, and ZnV 2 O 6 B exist in unique form in the entire temperature range. P-form seems to be favored by Me of larger ionic radii. N-form seems to appear at a peculiar d -shell structure and small Me size. Preliminary explanation of the dependence of the structure type on Me size is offered. New X-ray data are given for CdV 2 O 6 B, CdV 2 O 6 P, MgV 2 O 6 B, MgV 2 O 6 P, and MnV 2 O 6 P.

Advanced bond-strength model of active sites on oxide catalysts

Recent studies provide several examples of catalytic anisotropy of oxides in simple or oxidative dehydrogenation, dehydration, and oxidation reactions. Among others, these data include conversion of methanol, ethanol, and propylene on MoO3 and conversion of propylene and o-xylene on Mn1 − xφxV2 − 2xMo2xO6. The univocal ascription of the formation of a given product to the defined crystallographic face of catalyst makes it possible a more profound discussion of the possible structure of active sites. Catalytic reaction can develop if geometric and energetic fit is achieved between adsorbed molecule and the neighborhood of adsorption site. In the proposed model structural considerations concerning various planes of catalyst are based on crystallographic data and on known shapes and dimensions of organic molecules. Adsorption of reactants is assumed to take place on coordinatively unsaturated surface metal and oxygen atoms. Bond strength, calculated according to the bond-length-bond-strength concept, is taken as a measure of binding energy, and consequently (beside geometry) as a factor determining the catalytic reaction pathway. Elementary steps consisting in the exchange of hydrogen or oxygen atom between organic species and catalyst surface are thought to proceed on the condition that the indicated atoms are more firmly bonded to the presumed step product than to the substrate. On this basis the sites are indicated on which dehydrogenation, oxidation (oxygenation), deoxygenation, and water evolution steps may take place. Combination of dehydrogenation and deoxygenation is thought to result in dehydration. Possible mechanisms of catalyst reoxidation are also discussed qualitatively in terms of the proposed model.

New method of calculation of the surface enthalpy of solids

Abstract A new method of calculation of the surface enthalpy of solids Ec in the so-called rigid lattice approximation is proposed. The formerly found [J. Solid State Chem. 57 (1985) 269, 291] empirical relationship between bond length and bond energy in oxide crystals is used for this purpose. It is sufficient to add the energies of all bonds cut in the process of surface formation over the surface unit cell and to divide the sum by double the unit cell area. The advantage of this widely applicable method consists in the fact that it is sufficient to know the structure of the considered crystal; for oxides all coefficients of the proposed equation have already been determined and listed in the quoted paper. Generalization of the method on the non-oxide (X) crystals is dependent on the determination of the respective coefficients for the element-X bonds. But at present the method is also applicable to non-oxide crystals of high symmetry (e.g. NaCl-type, CsCl-type, ZnS-type, fluorite-type). It is argued that Ec may be thought of as active surface energy responsible for various aspects of the surface reactivity and cleavage, while the passive surface energy of the relaxed surface governs static phenomena. As the passive surface energy (or free energy) is not easy to be obtained either experimentally or theoretically, an attempt is made to predict the equilibrium form of crystals with Curie-Wulff plots using Ec instead of the free energy. In most cases the predictions agree well with accessible literature data. Data involving cleavage planes and direct experimental determinations of the surface energy are also used in the evaluation of the proposed method. The paper is illustrated with numerical data of Ehklc calculated for a number of oxides and halides of the NaCl-type structure as well as for oxides of the rutile-type, TiO2-anatase, V2O5 and MoO3 including the reduced faces of the two latter oxides.

Crystallochemical model of active sites on oxide catalysts

Abstract Recently the bond strength model of active sites (BSMAS) on oxide catalysts has been formulated (Ziolkowski, J., J. Catal. 81 , 311, 1983; 80 , 265, 1983; 84 , 317, 1983), based on the conviction that the pathway of catalytic reaction depends on the geometric and energetic fit between adsorbed molecule and the neighborhood of adsorption site. In terms of BSMAS structural considerations concerning various morphological planes of catalyst are found on crystallographic data and on known shapes and dimensions of molecules; bond strength is taken as a measure of binding energy. In this paper the bond-length-bond-strength-bond-energy concept (Ziolkowski J., J. Solid State Chem. 57 , 269, 291, 1985) is used to develop BSMAS into the crystallochemical model of active sites (CMAS). The geometric ground of CMAS is essentially the same as that of BSMAS, while the energetic factor is expressed in the actual energy units. Applications of CMAS are illustrated by considering a number of catalytic reactions (propylene → acrolein; methanol → dimethyl ether, methylal; acetone → propylene) going on various morphological planes of MoO 3 . Detailed, molecular mechanisms of these reactions are proposed as well as their energetic pathways. Assuming that the higher enthalpy of the elementary step, the higher the activation energy, the rate-determining steps are indicated.

Catalytic properties of defective brannerite-type vanadates: I. Reactive specificity of (202̄) and (201) crystallographic planes of Mn1 − xφxV2 − 2xMo2xO6 in oxidation of propylene

Oxidation of propylene in the presence of Mn1 − xφxV2 − 2xMo2xO6 solid solutions (0 ≤ x ≤ 0.3, φ denotes a cation vacancy at the Mn2+ site) has been studied in an integral-flow system at 360–440 °C over a wide range of contact time. Acrolein, acrylic acid, acetic acid, acetaldehyde, CO, and CO2 were identified as the reaction products. Reactive specificity of the (202) and (201) crystallographic planes has been observed. The (202) plane is much more active and characterized by at least two types of active centers (yielding C3 and C2C1 products) able to incorporate, without desorption, more than one oxygen atom into the molecule of propylene or into the products of its degradation. On the contrary, on the (201) plane predominate centers capable of the successive, one by one, incorporation of oxygen into the organic molecule, separated by desorption-adsorption processes. A common feature of both planes is that the total combustion markedly diminishes with the increase of the composition parameter x. Detailed schemes of the reaction on both crystallographic planes are proposed and discussed.

Catalytic properties of defective brannerite-type vanadates: II. A model of sites active in oxidation of propylene on the (201) and (202̄) planes of Mn1 − xφxV2 − 2xMo2xO6

A model of sites active in oxidation of propylene on (201) and (202) planes of Mn1 − xφxV2 − 2xMo2xO6 catalyst has been developed. Propylene is considered to be adsorbed on the coordinatively unsaturated surface metal atoms as a π-complex. The mechanism of its transformation is thought to depend on the number and configuration of active oxygen atoms around the adsorption center. Structural considerations are based on crystallographic data and the individual activities of surface oxygen atoms are assumed to be proportional to the reciprocal sum of the strength of bonds to them from the adjacent cations. The latter are calculated according to the bond-length-bond-strength concept. The conclusions from the model are very consistent with the experimental facts, published previously (Ziolkowski and Janas, J. Catal.81, 298 1983), which reveal important differences between the mechanism of propylene oxidation on the (201) and (202) crystallographic planes. The method applied to determine the structure of the active sites seems to have more general significance and may be applied to other oxide catalysts.

Defect structures in the brannerite-type vanadates. I. Preparation and study of MN1−xфxV2−2xMo2xO6 (0 ≤ x ≤ 0.45)

Abstract Phases of the formula Mn1−xфxV2−2xMo2xO6 with the brannerite-type (α) structure, where ф represents a vacancy at the Mn2+ site, have been sythesized and characterized by X-ray diffraction and DTA. The X-ray data are listed for MnV2O6 and solid solution with x = 0.40. They indicate the random distribution of V and Mo over the original V sites and the random distribution of Mn and vacancies over the original Mn sites. The monoclinic cell dilates with increasing x, primarily in the direction of the b-axis. The phase diagram of the pseudobinary MnV2O6MoO3 system has been determined. The extent of the stability region for the investigated brannerite solid solution has been established (xmax = 0.45 at 583°C). Other features determined in this system were: (a) little solubility of MoO3 in the high-temperature (β) modification of MnV2O6, (b) a two-phase area of α- and β-type solid solution coexistence, (c) a eutectic point between α-type solid solution and MoO3 at 583°C and 75 mole% of MoO3 and (d) phase relationships at the liquidus.