W.F. Huang

Effect of Ag‐promotion on Pd catalysts by XANES

Pd L3 near‐edge absorption measurements (XANES) were performed on four commercial acetylene hydrogenation catalyst samples, with and without the Ag promoter. The Pd L3 edge XANES spectra showed that the Ag‐promoted catalysts have relatively weaker absorption peaks and they follow the same order as the relative commercial performances of the four catalysts studied in terms of selectivity in ethylene purification, which indicates that there are increases in the Pd d‐band electron densities due to the addition of Ag. These results provide a reasonable explanation for the observed improvement in selectivity of the Ag‐promoted acetylene hydrogenation catalysts. The Ag L3 XANES spectra of the supported Pd–Ag catalysts indicate the absence of a white‐line feature which seems to suggest that the charge transferred from Ag to Pd may not be the d‐type.

Perspectives of positron annihilation spectroscopy in heterogeneous catalysis

Abstract The use of polychromatic positron annihilation spectroscopy (PAS) in heterogeneous catalysis is reviewed and its potential applications are discussed. This review covers early and current studies of catalytic oxides and supported metals, refers to the potential use of PAS in the characterization of catalytic sites, and outlines the future of the technique.

Brønsted acidic strength in Y-zeolite by positron spectroscopy

Angular correlation spectra of positron annihilation in Y-zeolite adsorbed with bases (ammonia, water, and alcohol) were studied. Results by the technique successfully reveal that there exist two types of Brønsted acid sites with different acidic strengths which require temperature of desorption of the base at about 200 and 400°C.

Positron spectroscopy of base-adsorbed Y-zeolite

Abstract Angular correlation of gammas from positron annihilation in pyridine-adsorbed Y-zeolite were measured as a function of temperature (and acidity). The desorption of pyridine follows an unusual pattern which could be related to the relatively larger pyridine molecule than other simple base molecules.

Positronium-Brönsted site interaction in silica-alumina catalysis

Abstract The lineshapes of Doppler-broadened positron annihilation radiation were determined at different acidity content of a silica-alumina catalyst. Analysis of the results together with those obtained from temperature programmed desorption and infrared spectra demonstrated that positronium atoms are selectively oxidized by surface protons and do not react with Lewis acid sites. A mechanism for the oxidation reaction is proposed.

Positron annihilation in liquid mercury

Abstract A positron wave function has been calculated for a vacancy and applied to the angular correlation data of liquid mercury. Our results indicate that 19% of all annihilations occur at vacancy sites.

Positron annihilation studies of a conjugated polymer

Abstract Lineshapes of the Doppler broadened annihilation radiation (DBAR) of a conjugated polymer polydiacetylene, 4-butoxycarbonylmethylurethane (P-4BCMU), in its amorphous state, were measured over the temperature range of 30 to 190°C. A differential-scanning-calorimeter (DSC) was used to correlate the transition temperatures. The DBAR lineshapes show a distinct change during the structural transitions of the polymer. A two states trapping model is used to qualitatively describe the temperature dependence of the lineshape. The nature of formation of the trapped states of positrons above T c , through the interplay of electronic structure/conformation change, is discussed.

Positron trap states in amorphous and crystalline conjugated polydiacetylene polymers

Abstract The temperature dependence of the lineshapes S(T) of the Doppler broadened annihilation radiation (DBAR) for polydiacetylenes, amorphous and polycrystalline poly(4BCMU) and crystalline poly(PTS) was investigated experimentally. Away from the transition temperatures dS(T)/T=0 for crystalline polymers, while a significant dS(T)/dT=0.0002/°C was measured for the amorphous polymer. A detailed analysis on the types of trap states that could account for such difference is presented.

Compositional dependence of positron annihilation in HgTl alloys

Abstract Ratios of coincidence counting rates of annihilation gammas at 0 and 10 milliradians for 13 atomic concentrations from 0 to 50% of thallium in liquid mercury were measured. A sharp discontinuity at about 5% of thallium indicates an abrupt change of cluster size and concentration, which may be related to the peaking of electric thermopower of the alloy.