Characterization of tungsten sulfide catalysts

Abstract

Unsupported tungsten sulfide catalysts of high surface area (60 m 2/g) were prepared by in situ decomposition of ammonium tetrathiotungstate (ATT) in flowing helium at 450 °C. This catalyst (stoichiometry approx WS 2.2) was subjected to various pretreatments in both hydrogen and a mixture of 15% hydrogen sulfide in hydrogen at elevated temperatures (up to 450 °C). Temperature-programmed reduction (TPR) in hydrogen showed two peaks, indicating the presence of two species of sulfur with different binding energies. Surface areas and pore-size distributions of the catalysts were found to be relatively unaffected by treatment in hydrogen at 450 °C for 1 h, but treatment for 12 h caused the surface area to decrease from 60 to 35 m 2/g. Examination of catalyst samples under a scanning electron microscope showed that the large number of microcracks developed on decomposition of ATT were considerably reduced by a 12-h treatment in hydrogen at 450 °C. Low-temperature oxygen chemisorption (LTOC) was measured on tungsten sulfide subjected to pretreatment in hydrogen for different times and temperatures. Samples with the greatest sulfur loss showed the highest LTOC, suggesting that oxygen chemisorbs at surface anion vacancies (coordinatively unsaturated tungsten ions). Appreciable LTOC was generated only upon removal of sulfur corresponding to the second peak. The ratio of surface area to LTOC for a sample treated for 12 h in hydrogen led to a value of 170 A ̊ 2 O 2 molecule ( 85 A ̊ 2 O atom ). Catalyst activity for propylene hydrogenation at 100 and 150 °C was measured using a pulsed-flow microcatalytic reactor. Activity was found to increase with increasing LTOC.