B. Mile

The location of nickel oxide and nickel in silica-supported catalysts: two forms of NiO and the assignment of temperature-programmed reduction profiles

Abstract The preparation of nickel catalysts supported on a range of silicas results in the formation of two distinct types of “NiO” which reduce at very different temperatures under temperature-programmed reduction (TPR) conditions. Examination of the effects of pore structure and experiments designed to concentrate the oxides in either the smaller (~9 nm) or the larger pores (15–30 nm) show that the more reducible oxide is located mainly in the small pores and the less reducible oxide in the large pores. The more reducible oxide resembles bulk NiO and has negligible interaction with the silica. The less reducible oxide is either in the form of crystallites so small as to make nucleation of the reduction to metal difficult or as surface nickel silicates or hydroxysilicates. Reoxidation of the reduced catalyst followed by TPR shows that the nickel oxide and nickel crystallites are immobilized in the pores of the silica at temperatures up to 600 °C.

ESR spectrum of Cu5, a trigonal bipyramidal copper cluster☆

Abstract The neutral copper cluster 63 Cu 5 has been isolated in C 6 D 12 and its ESR spectrum recorded. The spectrum can be analyzed in terms of a 63 (2)_= 608 G, a 63 (2) = 15 G, a 63 (1) = 30 G and g = 2.055. The structure of this species is best described as a distorted trigonal bipyramid with ≈60% unpaired 4s spin population on two of the equatorial copper atoms and very little unpaired 4s spin population on the remaining equatorial and two axial copper atoms.

The electron spin resonance spectrum of the lithium trimer in hydrocarbon matrices

Abstract The lithium metal trimer, Li 3 , has been prepared in adamantane using a rotating cryostat and its electron spin resonance spectrum recorded from 4 to 298 K. It has three magnetically equivalent Li nuclei with parameters a 7 (3) = 33.1 G and g = 2.001. Consequently a change of matrix from argon to adamantane has no effect on the spin distribution and structure of Li 3 which appears to be a pseudorotating dynamic Jahn-Teller molecule at all temperatures. In adamantane it is stable for several hours even at room temperature.

An EPR study of diffusion of iron into rutile

In situ high temperature EPR measurements of the growth of the signal of substitutional Fe(III) ions have been used to study the diffusion of Fe in the rutile form of titanium dioxide. Two preparations, characteristic of the two main processes employed for the production of titanium dioxide have been studied. The first preparation, designated TiO2(SO4), was made from precipitated TiO2. It was calcined at ca. 850°C and cooled slowly to room temperature. The second preparation, designated TiO2(Cl), was from the gas phase oxidation of TiCl4 at above 1200°C. The resulting TiO2 was rapidly quenched to room temperature. The surfaces of both samples were impregnated with 0.030% Fe and the development of an EPR signal at g = 8.11, characteristic of Fe(III) substituting for titanium ions in the rutile lattice was monitored in situ at temperatures up to 730°C by using a high temperature EPR cavity. For both TiO2(SO4) and TiO2(Cl) the g = 8.11 signal showed a parabolic dependence of intensity with time typical of many diffusion processes. The temperature dependence of the slope of the intensity (I) s. time0.5 plots allows estimates of the activation energies for the diffusion to be made. Values of 110 ± 30 kJ mol−1 for TiO2(SO4) and 50 ± 20 kJ mol−1 for TiO2(Cl) are determined. The much lower value for the TiO2(Cl) is attributed to the presence of metastable defects which, because of the rapid cooling, persist in this rutile. This interpretation is supported by an observed increase in activation energies on heating the rapidly quenched TiO2(Cl) prior to the diffusion experiment. Pre-annealing at 700°C to reduce the concentration of defects, increased the activation energy for diffusion in TiO2(Cl) to 90 ± 30 kJ mol−1. The activation energy for diffusion of Fe is significantly lower than that for Cr (150 kJ mol−1). Reasons for this are discussed.

Applications of EPR to a study of the hydrogenation of ethene and benzene over a supported PD catalyst - detection of free radicals on a catalyst surface

EPR has not been used extensively in the field of catalysis despite it being the most sensitive technique available for detecting free radicals and paramagnetic metal ions, which are intermediates or participants in many catalytic processes. In this paper, we describe its application to the detection of paramagnetic intermediates on the surface of a palladium catalyst during heterolytic hydrogenation reactions. EPR, in conjunction with spin trapping, is shown to provide a convenient, simple method for detecting hydrogen adatoms generated by the dissociative chemisorption of hydrogen on alumina-supported palladium catalysts (<0.04% Pd) at room temperature. By using D2 we have also been able to demonstrate directly the occurrence of hydrogen/deuterium spillover onto the alumina surface by H/D atom transfer to surface hydroxy groups. Alkyl and aromatic free redical intermediates formed by reaction of the H˙ adatoms with alkenes and benzene have also been observed by EPR at a catalyst surface for the first time. The hydrogen atoms reacting with the spin traps, ethene and benzene, are not strongly chemisorbed hydrogen (Pd—H) but those weakly adsorbed H-adatoms in equilibrium with H2(g).

Electron paramagnetic resonance study of the reaction of ground-state aluminium atoms with a series of ethers in a rotating cryostat

Ground-state Al atoms react with symmetric cyclic and acyclic ethers at 77 K in a rotating cryostat to give a host of mononuclear organoaluminium compounds. Magnetic parameters determined from EPR spectra given by Al atoms and acyclic ethers are consistent with the formation of novel C—C and C—H aluminium atom insertion products. A third species has been tentatively assigned to the product given by Al atom insertion into the C—O bond. In addition a carbon-centred radical is formed by loss of a hydrogen atom from carbon adjacent to the alkoxyl group. In contrast the cyclic ethers are resistant to alumino hydride formation and in most cases a carbon radical resulting from ring opening at the C—O bond is formed. Mono- and di-ligand complexes, Al[ether] and Al[ether]2, are tentatively identified and may be the primary intermediates for all the reactions that occur.

Reaction of ground-state aluminium atoms with diethyl ether. Electron spin resonance evidence for insertion into the CH and CC bonds

Ground-state Al atoms react with diethyl ether on solid hydrocarbon surfaces at 77 K in a rotating cryostat to give several mononuclear aluminium(o) compounds. The magnetic parameters of two of these compounds are consistent with the formation of CH3CHAlH(OCH2CH3) and/or CH3CH2OCH2CH2AlH and CH3AlCH2OCH2CH3 produced by insertion of the Al atom into the CH and CC bonds of the ether. A third species could be CH3CH2OAlCH2CH3, given by insertion into the CO bond, but this assignment is somewhat tentative.

The electron spin resonance spectrum of Al[C2H4] in hydrocarbon matrices

Al[C2H4] is formed from Al atoms and C2H4 on solid hydrocarbons at 77 K in a rotating cryostat. It has the magnetic parameters a‖(Al)= 123 MHz, a‖(Al)=–14 MHz, aAl= 31 MHz, aH(4)= 7.5 MHz, g‖= 2.0026, g⊥= 2.0047 and g= 2.004. This complex is remarkably stable and persists in adamantane up to 297 K. The structure and bonding of Al[C2H4] is still in some doubt, but the magnetic parameters and stability suggest a strong σ as well as π interaction between the Al atom and the ethylene group.

Exact analytical solution for the powder pattern of orthorhombic-g systems. Predicted EPR powder spectrum and application to powder TiO2 : Cr3+ in the rutile phase

An exact analytical solution for the powder pattern of an ion with orthorhombic effective g factor is found by use of the isotimic method. Theoretical first-derivative powder EPR spectra are obtained by convoluting the product of the analytic powder pattern and the transition probability, at the sites of the main spectral features, with first-derivative Lorentzian lineshape functions of constant-in-field and constant-in-frequency widths. The calculation is made in a PC capable of handling the MATHEMATICA program. The EPR spectrum of powder TiO2 : Cr3+ in the rutile phase, which is assumed to be composed of two orthorhombic g spectra, is interpreted through these results. Unambiguous assignments of five of the six anisotropic g parameters are made from the five features composing the observed orthorhomic g powder spectrum. These are gz= 5.04, gy= 2.63, gx= 1.64 and g′z= 5.82, g′y= 1.33. The value of g′x= 1.14 which corresponds to a spectral feature not yet observed in the spectrum is deduced from the others by means of the existing theory for g values arising from Kramers doublets. Boltzmann factors of the populations giving rise to the two spectra are deduced from line-intensity measurements. Theoretical powder EPR spectra of the two orthorhombic-g systems calculated with the above six g values and Boltzmann factors are compared with experiment for the different kinds of Lorentzian lineshape functions employed. This suggests that a broadening of the elementary lineshape function is responsible for the dissapearance of the feature at H(g′x)= 597 mT.

The reactions of silicon monoxide and aluminium monofluoride with 2,2′-bipyridyl

Abstract The high temperature species, SiO and AlF, have each been treated with 2,2′-bipyridyl in tetrahydrofuran solution at − 105 °C and in an adamatane matrix at − 196 °C to give products which have spectroscopic properties closely resembling the known compound [SiCl 2 (bipy) 2 ]. When initially formed the products are highly coloured and paramagnetic in solution or in the adamantane matrix and their EPR spectra show the presence of singlet and triplet species. In the triplet species, two electrons are transferred from the silicon or aluminium centre to two bipyridyl ligands which must lie orthogonal to each other across the metal centres but it is uncertain whether the species are monomeric or oligomeric. Evaporation of the solutions causes irreversible precipitation of pyrophoric solids of composition SiO(bipy) or AlF(bipy).