ABSTRACT
A relation among activity, composition, and structure was determined for a working catalyst by means of a stainless-steel reactor cell of novel design that permitted operation at temperatures and pressures similar to those in industrial reactors. Molybdenum K-edge x-ray absorption spectra were used to probe the structural environment of molybdenum in CoMoS/[unknown]-alumina catalysts while hydro-desulfurization of benzothiophene was proceeding at high temperature and pressure. For catalyst samples with different contents of cobalt, radial structure functions obtained from extended x-ray absorption fine structure data presented the same features as those obtained from the spectra of MoS(2)/[unknown]-alumina reference samples. Moreover, Mo-S and Mo-Mo coordination numbers were maximum for the sample with an atomic ratio of Co to (Co + Mo) of 0.33; this sample was also the most active catalyst tested.
ABSTRACT
The superparamagnetic behavior of very small particles of metallic iron (ca 1.5 nm), with about half of their atoms at the surface, is changed reversibly by adsorption and desorption of hydrogen below the superparamagnetic transition temperature. The change after adsorption implies a lowering of the anisotropy energy barrier for the magnetic relaxation of iron and is ascribed to a change in crystalline shape. No such changes are observed for larger particles of iron (ca 8 nm) with about 10% of their atoms at the surface.
ABSTRACT
Tungsten carbide catalyzes the formation of water from hydrogen and oxygen at room temperature, the reduction of tungsten trioxide by hydrogen in the presence of water, and the isomerization of 2,2-dimethylpropane to 2-methylbutane. This catalytic behavior, which is typical of platinum, is not exhibited at all by tungsten. The surface electronic properties of the latter are therefore modified by carbon in such a way that they resemble those of platinum.
ABSTRACT
The self-diffusion coefficient D of water occluded in samples of near-faujasite has been determined by pulsed field-gradient spin-echo nuclear magnetic resonance. The value of D in square centimeters per second x 10(5) at 30 degrees C is 1.34, 1.65, and 1.88 in the following zeolites, respectively: Na X, Ca X, and Ca Y (X and Y being an indication of ratio of silicon to aluminum in the zeolites). By comparison, the value of D in pure water at 30 degrees C is 2.5 x 10(-5) cm(2)/sec. Arrhenius activation energies for D are 6.9, 6.8, and 5.6 kilocalories per mole, respectively, for the three faujasites and 5.0 kcal/mole for pure water. Thus, there appears to be little difference in diffusion behavior between free water and water occluded in faujasite.
