Ragnar Larsson

Why do homogeneous analogs of phillips (CrO3/SiO2) and union carbide (Chromocene/SiO2) polyethylene catalysts fail? Some answers from ir investigations

Both title catalysts show strong i.r. bands after adsorption of CO, which are assigned to bridging CO molecules. These CO i.r. bands are at 2120, 2100 and 2035 cm−1 at low temperatures (−145 °C) for the CO-reduced Phillips catalyst (Zecchina , 1975) or at 1920, 1825 and 1620 cm−1 (Union Carbide catalyst).Single-bonded CO is observed showing i.r. bands at 2190, 2186 and 2179 cm−1—shifted to higher wavenumbers than CO gas (2143 cm−1), in contrast to bridging CO — or at 2046 and 1974 cm− (and 2008 cm−1), respectively. The single-bonded CO is replaced at low temperatures (Phillips catalyst) or after some days at room temperature (Union Carbide catalyst) by bridging CO molecules. These findings support not only the presence of dinuclear chromium surface complexes, but also further indicate that such surface complexes represent the majority ofthe chromium. The CO i.r. studies make it possible to propose detailed models of the dinuclear chromium surface complexes. The initiation reaction for the polymerization of ethylene could be the addition of one ethylene molecule to one dinuclear surface complex by forming a bridge. Homogeneous analogs for the title catalysts should therefore consist of coordinatively unsaturated dinuclear complexes, which was not the case in previous studies.

ESCA and electrochemical studies on pyrolysed iron and cobalt tetraphenylporphyrins

Catalysts for the reduction of O2 have been prepared by pyrolysis of CoTPP and FeClTPP in their crystalline state as well as in different mixtures with carbon black. ESCA measurements on these pyrolysed porphyrins have been correlated to their catalytic properties. When porphyrins are pyrolysed with carbon black, sublimed chelates re-adsorb on the surface of carbon black forming a stable chelate—carbon support bond, which is believed to increase the activity and stability of the catalyst. Porphyrins pyrolysed without carbon black become electrically conducting after pyrolysis as a result of a polymerization reaction with the chelates intact. For porphyrins pyrolysed without carbon black the main active site is believed to be a metal chelate but for the Co-catalyst there is evidence that other surface components, formed as a result of decomposition of the chelate, influence the activity.

X-ray photoelectron and mössbauer spectroscopy on a variety of iron compounds

Abstract A combination of X-ray photoelectron spectroscopy (ESCA) and Mossbauer spectroscopy has been used to study the chemical shifts of a series of iron complexes. Obtained experimental data will constitute a basis for comparison with theoretical studies.

A model of selective energy transfer at the active site of the catalyst

Abstract A model is described of catalyst-reactant interaction. It implies that one role of the catalyst is to supply energy into that vibrational mode of the reactant that most effectively takes the system to the activated state. In this way the dissipation of energy from the excited molecule is effectively counteracted. By treating the system as the coupled, damped oscillator of classical mechanics, an expression is derived for the isocatalytic temperature (describing the ‘compensation effect’) in terms of the vibrational frequencies of the reactant and the catalyst system. Examples are given of the application of this model to studies on heterogeneous as well as homogeneous systems.

On the core electron binding energy of carbon and the effective charge of the carbon atom

From Mulliken population data based on ab initio calculations of a series of substituted aromatic compounds and from well-calibrated XPS data (core electron binding energies, Eb) an empirical relation between Eb(C1s) and the charge on the carbon atoms, qc, has been found: Eb(C1s = 6.42qc + 4.52q2c + 285.8 (eV). It is also found that XPS data for other carbon-containing species, notably the cyanide ion, can be described by this relation.

On the catalytic decomposition of nitrous oxide over metal oxides

Abstract In literature a lot of data has accumulated on the activation energy of of N2O decomposition over a large number of different catalysts. If one constructs a histogram of the partition of the data one observes that the values of activation energies seem to gather themselves into a series of rather sharply defined regions. These regions are spaced regularly with an approximate interval of 3.5 kcal/mol. This empirical observation fits the suggestion made previously that the change of activation energy for a series of related catalysts for one and the same reaction occurs in a stepwise manner. This increment should correspond to an energy quantum of the vibration mode that distorts the molecule to reaction. A quantitative treatment is attempted to determine the anharmonicity constant of that vibration mode from all data presented. One than gets about 7 cm−1 compared to 3.2 cm−1 for the gaseous molecule. Introducing the expression Ea = E + RT one obtains the anharmonicity constant 3.4 cm−1, in close agreement with the above value from molecular spectroscopy. The findings support the theories of resonance in catalysis, indicating a selective transfer of energy from the catalyst to the vibration mode of the reactant that has to be activated to reach the ‘activated state’. Using this model a quantitative description of the ‘compensation effect’ has been made. The isocatalytic temperature, characteristic of a series of catalysts obeying the compensation rule, can be expressed as a function of the vibration frequency of the molecule, ν, and that of the energy pool of the catalyst, ω. The results of this treatment are applied to the material of N2O. It follows that the experimentally determined isocatalytic temperature, Θ, can be calculated if one uses the value of the N-O stretching frequency that is obtained from the treatment of the activation energy data. The dependence of Ea on lattice parameters follows from the presented model. The model presumes a special relation between adsorption enthalpy and activation energy, somewhat different from the one emerging from the equilibrium model that is usually applied.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over silica-supported vanadium oxides

Vanadium oxides on silica gel (Davison 952) with varying vanadium concentrations have been investigated for the activities and selectivities in the oxidation of toluene. These results were correlated with the features shown by X.r.d., ESCA, u.v.–visible and i.r. studies of adsorbed CO. The catalysts show a low activity in comparison with V/alumina. The support alone shows 60% selectivity for benzaldehyde formation. From i.r. data it is suggested that this benzaldehyde is formed on sites in association with sodium impurities. Introduction of vanadium increases the rate of reaction but the selectivity is shifted towards carbon oxides. The increased activity at low vanadium content is due to isolated four-coordinated vanadium species. U.v.–visible data show that the isolated species agglomerate at further loading to polyvanadium chains. These polymers possess an increased activity as a further route of reduction of VV to VIV is present. Five-coordinated vanadium of the same activity as the chains and producing selective oxidation products other than benzaldehyde is found with a loading of 2% V. Larger agglomerates and V2O5 crystallites are present on the 10% V catalyst. These species only give a somewhat increased activity, but with a product pattern different from crystalline V2O5 and 10% V/alumina.

On the catalytic decomposition of formic acid. I. The activation energies for oxide catalysis

Abstract Literature data of the activation energies for the dehydration and the dehydrogenation reactions of formic acid over a series of oxide catalysts are treated to exemplify the idea of a stepwise variation of activation energies. By applying the formalism for the energy of an anharmonic vibrator, one can derive that very vibrational mode of the reacting intermediate (formate ion) that must be excited for the two reactions to occur. For the dehydration this is mainly the anharmonic O–C–O stretch and for the dehydrogenation reaction it is mainly the O–C–O in plane bending mode. An algorithm SETOS is presented that facilitates the analysis of the data.

The kinetics and mechanism of the reaction between carbon dioxide and a series of amines: Observation and interpretation of an isokinetic effect

Abstract Kinetics investigations have been made on the reaction of some organic amines with dimethylcarbonate (DMC) in the presence of carbon dioxide. N -alkyl-carbamates were formed and measurements were made for a series of substituents in the amine. Second-order kinetics was used in the analysis. The rate constants have been analyzed and the resulting Arrhenius parameters indicate an isokinetic effect. The stepwise increase of the activation energies and the value of the isokinetic temperature are such that the selective energy transfer (SET) model suggests a δ (COC) vibration mode of the dimethylcarbonate to be activated in the rate determining reaction step. The DMC molecule is hydrogen bonded to the quartenary ammonium cation. The energy source is suggested to be a closely related vibration mode of the dimethylcarbonate serving as solvent.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over vanadium oxides supported on titania

Catalysts containing 0.1–10 wt% V have been prepared by impregnating TiO2(Degussa P25) by aqueous NH4VO3 and calcining at 773 K. The catalytic performance in the oxidation of toluene was investigated and correlated with the features shown by X.r.d., ESCA, u.v.–visible spectroscopy and i.r. spectroscopy of adsorbed CO. Both the activity in the oxidation of toluene and the initial selectivity for benzaldehyde increase rapidly with increasing vanadium loading up to 2 wt% V. The changes between 2 and 10 wt% V are small. Surface areas, pore structure, primary particle size and anatase/rutile ratio change with the vanadium content. Up to 2 wt% V, a small increase in the anatase/rutile ratio is obtained. In these catalysts, surface-bonded vanadium species are present, and it is suggested that they induce a reconstruction in the titania surface. Larger and opposite effects are observed for 10 wt% V, where crystalline V2O5 facilitates the anatase to rutile phase transition. Sintering of the primary particles and a large reduction in the specific surface area are also obtained. U.v.–visible spectroscopy of reduced catalysts indicated the predominance of VIV species in tetrahedral coordination. VIII species could not be observed by i.r. spectroscopy of adsorbed CO. I.r. spectra indicate the presence of V—OH groups. Breakpoints in the spectral data were obtained at ca. 1.1 wt% V corresponding to the consumption of two support OH groups per vanadium species. It is suggested that the isolated vanadium species are oxohydroxyvanadium bonded to titania through two oxygen bridges. These constitute the active site at low loadings. Two adjacent vanadium species are thought to form the reduced tetrahedral structure. New active sites present at higher loadings may be formed by the reaction between V—OH groups and vanadium precursors. The new sites possibly represent a VV/VIV redox couple: It is suggested that its presence is an important factor governing the catalytic activity.