ChemInform Abstract: Molybdena‐Alumina Catalysts. Part 12. MoS2 Surface Coverage from CO2 Adsorption.

Abstract

020ChemInform Abstract CO2 adsorption is shown to be a good technique for assaying the surface coverage of the MoS2 in alumina catalysts since it does not adsorb on the MoS2 phase. Corresponding results indicate that in sulfided catalysts up to about 8% Mo on alumina (calcination up to 500 rc C), the MoS2 is dispersed as a monolayer, whereas higher Mo to support surface ratios (calcination at > 500 rc C) can result in multilayer MoS2 phases. A model is proposed in which the MoS2 clusters are oriented flatwise to the support. A catalyst having a complete monolayer coverage in the oxide state results in only 23% coverage in the sulfide state after sulfiding. ESCA results on multilayer oxide catalysts showed qualitative correlation with calculated average numbers of multilayers present.

Estimation of the molybdena coverage in MoO3/alumina catalysts from residual OH groups

“…2 it follows that the adsorption and ISS data are more or less consistent (open points) and suggest that the coverage increases linearly with molybdena loading up to ¾4 Mo atoms nm 2 . The  S values calculated from the OH amounts for samples 1-6 pretreated at 315°C (full points) are well within the range of coverage estimated by other methods up to ¾2 Mo atoms nm 2 and deviate from a straight line at higher molybdena loadings. For sample 5, close values of the coverage were obtained for both pretreatment temperatures of 315 and 500°C.…”

“…However, such a deviation contradicts the results of the XPS measurements performed earlier. 13 The I Mo /I Al intensity ratios corresponding to samples 1-6 increased linearly with amount of deposited molybdena and a small deviation was apparent only at loadings close to 3.7-4.1 Mo atoms nm 2 . This suggests that the surface area occupied by molybdena increased linearly up to relatively high loadings and that deviation in the calculated  E values can be ascribed exclusively to a decreasing OH/Mo stoichiometry.…”

“…The OH density of our alumina support was 11 OH nm 2 at 130°C, which is not far from the value of 12.5 expected for a theoretical, fully packed monolayer of OH on the (100) alumina plane. 15 During thermal treatment of the support at 315°C, the OH density decreased to 8.8 OH nm 2 , representing roughly 75% of the originally occupied area. Then the coverage, related to the whole surface area, was obtained by…”

“…The points in Fig. 1 represent the experimental coverage calculated from the data in Table 1 to deviate from the straight line already at a loading of 2-3 Mo atoms nm 2 . However, such a deviation contradicts the results of the XPS measurements performed earlier.…”

“…12 The amount of removed OH of the support was proportional to the first small portions of MoO 3 and diminished markedly with increasing molybdena content. Two OH groups per Mo atom were removed up to a loading of ¾2 Mo atoms nm 2 ; this number decreased to only 0.8 at the saturation point of 4.1 Mo atoms nm 2 . Later, we attempted to estimate the molybdena coverage of these catalysts and we calculated the  E values from the number of OH groups according to Eqn.…”