CO bonding and hydrogenation activity of promoted noble metal catalysts during restructuring in high pressure CO hydrogenation: FeIr/SiO2

Bimetallic FeIr/SiO2 catalysts that are active for methanol prodn. from synthesis gas (CO + H2) bind CO less strongly, and exhibit higher activity for the hydrogenation of ethylene in the presence of CO, than catalysts that produce mainly methane. It is argued that promotion of the noble metal serves to weaken the adsorption of CO, thereby lowering its tendency to dissoc., and, most importantly, enhancing the surface coverage of hydrogen. Both factors are favorable for methanol formatio

An EXAFS study on the so-called "Co-Mo-S" phase in Co/C and CoMo/c, compared with a Mössbauer emission spectroscopy study

EXAFS measurements were performed on four sulfided catalysts with the same compn. as their 57Co-contg. counterparts which were previously characterized by Moessbauer emission spectroscopy. The Co species in Co/C catalysts with a Co-Mo-S-like Moessbauer emission spectrum after sulfidation at 373 K is similar to the species with the Co-Mo-S spectrum in CoMo/C catalysts. In both cases the Co atoms are present in a very highly dispersed Co species. In the catalyst without Mo this sulfidic Co species strongly sinters during sulfidation at 673 K, resulting in a Co9S8-type phase. In the CoMo/C catalyst, the Mo hinders sintering of the Co species. [on SciFinder (R)

An EXAFS study on the so-called "Co-Mo-S" phase in Co/C and CoMo/c, compared with a Mössbauer emission spectroscopy study

EXAFS measurements were performed on four sulfided catalysts with the same compn. as their 57Co-contg. counterparts which were previously characterized by Moessbauer emission spectroscopy. The Co species in Co/C catalysts with a Co-Mo-S-like Moessbauer emission spectrum after sulfidation at 373 K is similar to the species with the Co-Mo-S spectrum in CoMo/C catalysts. In both cases the Co atoms are present in a very highly dispersed Co species. In the catalyst without Mo this sulfidic Co species strongly sinters during sulfidation at 673 K, resulting in a Co9S8-type phase. In the CoMo/C catalyst, the Mo hinders sintering of the Co species. [on SciFinder (R)

CO bonding and hydrogenation activity of promoted noble metal catalysts during restructuring in high pressure CO hydrogenation: FeIr/SiO2

Bimetallic FeIr/SiO2 catalysts that are active for methanol prodn. from synthesis gas (CO + H2) bind CO less strongly, and exhibit higher activity for the hydrogenation of ethylene in the presence of CO, than catalysts that produce mainly methane. It is argued that promotion of the noble metal serves to weaken the adsorption of CO, thereby lowering its tendency to dissoc., and, most importantly, enhancing the surface coverage of hydrogen. Both factors are favorable for methanol formatio

EXAFS study of the local structure of Ni in Ni-MoS2/C hydrodesulfurization catalysts

To study the local structure of the Ni promoter atom, the Ni and Mo K edge EXAFS spectra of Ni-MoS2/C hydrodesulfurization catalyst were measured in an in-situ EXAFS cell at 77 K. The Ni atom is situated in a square pyramid of five S atoms at a distance of 2.21 Å from the S atoms. In addition an EXAFS contribution due to a Mo atom at 2.82 Å from the Ni atom could be identified. This local structure indicates that the Ni atoms are situated on top of the S4 squares at the MoS2 edges in millerite-type Ni sites. The Ni atoms are situated in the planes of the Mo atoms and not in the intercalation plane midway between successive MoS2 sandwich layers

The effect of passivation on the activity and structure of sulfided hydrotreating catalysts

Air exposure (passivation) and subsequent resulfidation caused a substantial increase in the thiophene hydrodesulfurization activity of sulfided Co-Mo/Al2O3 catalysts. Since no effect was observed for Mo/Al2O3 and Co/Al2O3 catalysts, the passivation effect must be related to the Co---Mo---S structure. EXAFS, Mssbauer emission spectroscopy, and transmission electron microscopy measurements showed that the passivation effect is neither related to a change in the MoS2 dispersion, nor to a change in number and nature of the Co sites. A Co-Mo/TiO2, a Co-Mo/C, and a Co-Mo/Al3O3 catalyst, which was prepared with the aid of nitrilotriacetic acid, hardly showed an increase in the catalytic activity after passivation. Because of this, and since the Mo-S coordination number for a Co-Mo/Al2O3 catalyst is increased by passivation, it is concluded that the activity increase originates from a decrease in Mo-support interactions. These interactions are broken when Co atoms migrate to Mo---O---Al bonds during a passivation and resulfidation treatment and assist in the sulfidation of these bonds. Removal of steric hindrance is held responsible for the activity increase. The effect of passivation on the catalytic activity decreased with increasing Co/Mo ratio, probably because at high Co loadings most Mo---O---Al bonds are already sulfided in the first sulfidation step. A maximum activity increase was obtained at a passivation temperature of 340 K. At lower temperatures the oxidation and migration of Co is too slow, whereas at higher temperatures new Mo---O---Al bonds are formed in the passivation treatment. Ni-Mo/Al2O3 catalysts showed the same behavior as Co-Mo/Al2O3 catalysts