STUDIES OF THE HYDROGEN HELD BY SOLIDS .27. HXMOS2

The sorption of hydrogen measured as HxMoS2 has been studied at 0.72 less-than-or-equal-to P(H2) less-than-or-equal-to 65 atm on both single-crystalline MoS2 and microcrystalline material prepared by decomposition of (NH4)2MoS4 in 14% H2S/H2. The surface area was varied from about 60 to 7 m2 g-1 by hydrogen treatments at various temperatures. Studies were also made with both sulfided and reduced molybdena-alumina catalysts. The (550-Torr) isobars obtained with all these materials were remarkably similar. They revealed a molecular adsorption at low temperatures in amounts smaller than, but commensurate with V(m), the nitrogen monolayer capacity, and an activated sorption to an equilibrium state above 400 K. Uptakes on the supported catalysts were about 10-fold larger than on the microcrystalline material. For MoS2, uptakes in the range 0.016 less-than-or-equal-to x less-than-or-equal-to 0.045 were recorded at 1 atm and these data were cross-checked by several methods. They were measured at P(H2) < 1 atm by the standard volumetric method in a BET system and at 1 less-than-or-equal-to P(H2) less-than-or-equal-to 35 atm on a high-pressure Cahn microbalance. These were compared with amounts desorbed in TPD measurements from samples equilibrated in H-2 at elevated temperatures and pressures before quenching to room temperature to stabilize the sorptions. Similar preparations were measured by the isotope dilution method by exchange with D2 to ensure that total sorption was being obtained. All of these measurements were in agreement within an estimated experimental error of +/- 10%. Uptakes increased with P(H2) over the entire range examined and with the N2 surface areas at constant pressure. Since the latter plots extrapolated to finite intercepts at zero surface area, a portion of x is evidently held at sites unavailable to N2 molecules. This was confirmed by measurements on single crystals where the sorptions exceeded monolayer capacity. H-1 NMR measurements were made, and calibrated integrated intensities of the signal showed that approximately 75% of the sorbed H was being detected by this method. The signal was nearly symmetric and Gaussian in shape. A chemical shift of about 2 ppm was determined; this is consistent with the assignment of Hx to SH groups.