F. E. Wagner

Hydrochlorination of acetylene using gold catalysts: A study of catalyst deactivation

A detailed study of the processes occurring during the deactivation of gold catalysts used for the acetylene hydrochlorination reaction is described. Catalyst deactivation is a common feature of supported metal chloride catalysts for this reaction, but in the case of HAuCl4/C catalysts the deactivation is not due to loss of Au from the catalyst during use. The effect of a range of parameters on the deactivation rate is discussed, including method of preparation of the HAuCl4/C catalyst, [Au] and reaction conditions (GHSV and temperature). Deactivated and fresh catalysts were characterised using 197An Mossbauer spectroscopy in combination with X-ray diffractometry, TGA, and BET surface area determinations. On the basis of this study, two deactivation processes have been identified. At low reaction temperatures, 60–100°C, deposition of carbonaceous residues is the predominant deactivation mechanism. However, at higher reaction temperatures, 120–180°C, the main deactivation process is ascribed to the reduction of Au(III), and possibly Au(I), to the less active Au(0) during the catalytic reaction. Consequently, for the experimental conditions investigated in this study, the optimum temperature for operating these catalysts is in the range 100–120°C when both these deactivation processes can be minimised.

Methane oxidation using Au/MgO catalysts

A series of Au/MgO(0.04-15 wt%) catalysts have been investigated for the oxidation of methane. Detailed electron microscopy and Mössbauer spectroscopy studies show that two distinct Au morphologies can be observed: (a) two-dimensional Au rafts and (b) discrete three-dimensional Au particles (5-10 nm in diameter). The two-dimensional rafts are observed as the main form of Au at low loadings and, interestingly, these are observed to poison the methane coupling activity of MgO. As the Au loading is increased the proportion of Au present as discrete particles increases and these are considered to be active for methane oxidation to CO and CO2.

Mössbauer determination of the E2/M1 mixing ratio of the 77 keV transition in197Au and of the sign of the electric field gradient in KAu(CN)2

A Mössbauer measurement of the E2/M1 mixing ratio of the 77 KeV transition in197Au yielded δ=−0.352±0.005. With this result, Mössbauer absorption spectra of KAu(CN)2 single crystals show that the electric field gradient at197Au in this compound is negative. This implies that the field gradient is mainly produced by 6pz electrons. The data also indicate a large vibrational anisotropy of Au in KAu(CN)2.

Mössbauer Spectroscopy in Archaeology: Introduction and Experimental Considerations

An introduction to the Mössbauer method with emphasis on archaeological applications is given and the merits of scattering and transmission experiments in archaeometric work are discussed. A brief outline of the development of Mössbauer spectroscopy in archaeometry is presented and, finally, some of the rarer applications of the method to problems of archaeological nature are described.

Reducing firing of an early pottery making kiln at Batán Grande, Peru: A Mössbauer study

Material from field firing experiments using a 2,700-year old Formative kiln at Batán Grande, Peru, was studied by X-ray diffraction and Mössbauer spectroscopy. The experiments explore the technology involved in producing the gray and black reduced ware for which Cupisnique and other Formative ceramics are justly known. During firing, the iron-bearing compounds in clays undergo characteristic changes which depend on kiln temperature and atmosphere. These changes can be observed in the Mössbauer spectra. By comparing spectra of an appropriate clay fired in field experiments and in the laboratory with the spectra of ancient ceramics, a description of Formative firing techniques in a reducing environment is attempted.

Preparation, magnetic and Mössbauer resonance investigation of some perovskite-type oxides containing iridium +V

Abstract New Ir(+V) perovskites of formula MLaMgIrO 6 (M = Ca, Sr, Ba) have been prepared under high oxygen pressure. The stabilization of Ir(+V) thanks to competition between Ir-O and Mg-O bonds is discussed and compared to that observed for La 2 LiIrO 6 . Magnetic and Mossbauer resonance measurements allow to characterize iridium in the materials, which contain some Ir(+IV).

Clay: An important raw material for prehistoric man

Early techniques of making pottery can be investigated by 57Fe Mössbauer spectroscopy. Iron is generally present in unpurified clays in concentrations of several percent. During firing, the iron undergoes characteristic changes of its chemical and physical state, depending on the kiln atmosphere and on the maximum firing temperature reached. These changes can be followed by Mössbauer spectroscopy. Firing techniques can often be reconstructed when spectra of laboratory and field fired samples are compared with those observed in ancient sherds.

PtRu anodes for methanol electrooxidation: A ruthenium-99 Mössbauer study

Abstract 99Ru Mossbauer spectra have been obtained for a series of PtRu methanol oxidation anodes. For the most catalytically active sample, XPS and Mossbauer data indicate the presence of Ru(IV) species with a small quadrupole splitting. For highly dispersed samples the Mossbauer data indicate that the Ru is present as a mixture of rutile-phase RuO2 and a second Ru(IV) species. Comparison with electrocatalytic results suggests that the second Ru(IV) species is the active catalytic copromoter. The possible identity of this species is discussed.

Neutron spectroscopy of fullerite hydrogenated under high pressure; evidence for interstitial molecular hydrogen

Inelastic neutron scattering spectra of a hydrofullerite quenched after synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and of the same sample after annealing at 300 K for 35 h, which reduced the hydrogen content by molecules per unit, were measured at 85 K. The quenched sample is shown to consist of molecules with and of interstitial molecular hydrogen. The interstitial molecular hydrogen left the sample during annealing at room temperature, whereas the molecules were stable at this temperature. The intramolecular and intermolecular vibrations of and in the fullerite are discussed in view of the measured spectra.

197 Au Mössbauer study of the gold species adsorbed on carbon from cyanide solutions

The gold species present on activated carbon after adsorption from solutions of Au(CN)2− have been studied by197Au Mössbauer spectroscopy as a function of the pH value of the solution, the loading of the carbon, the coadsorption of polyvalent cations, and the treatment of the samples after adsorption. The gold was found to be adsorbed mainly as Au(CN)2−. Coadsorbed polyvalent cations (Ca²+, Gd³+) have no influence on the Mössbauer parameters of the adsorbed gold complex. After adsorption from acidic solutions (pH ≲ 4), one finds a substantial amount of adsorbed gold with Mössbauer parameters similar to those of crystalline AuCN. Presumably, this gold is bound in Aux(CN)x+1 oligomers which form during drying. An additional product with Mössbauer parameters close to those of KAu(CN)2Cl2 was observed on dried samples after adsorption at pH 1. A minor gold species with an uncommonly small electric quadrupole splitting was found on wet carbons but disappeared on drying.