R Roel Prins

Carbon-supported sulfide catalysts

The activities of sulfided MoC, WC, CoC, NiC, Co-MoC, and Ni-WC catalysts for thiophene hydrodesulfurization and butene hydrogenation were studied using a flow microreactor operating at atmospheric pressure. The following parameters were varied: type of carbon support, carbon pretreatment, catalyst preparation method, and content of active material. The results are compared with those obtained for series of sulfided Mo/γ-Al2O3, W/γ-Al2O3, MoSiO2, and WSiO2. Some samples, viz., MoC, CoC, and Co-MoC, were also studied by X-ray photoelectron spectroscopy (XPS). The carbon-supported catalysts demonstrated outstanding performance for thiophene hydrodesulfurization. XPS analysis showed the presence of low-valence-state sulfur, e.g., S− or (S-S)2−, in MoC and CoC catalysts with low molybdenum or cobalt content. These sulfurspecies are supposedly connected with the catalytic activity for hydrodesulfurization. CoC and NiC were found to have a hydrodesulfurization activity which was higher (Co) or the same (Ni) as that measured for MoC or WC. Therefore Co (Ni) ions in Co (Ni)-Mo (W)C catalysts are considered as promoters for the MoS2 (WS2) phase (low CoMo or NiW ratios) or as additional active species.

Determination of metal particle size of highly dispersed Rh, Ir, and Pt catalysts by hydrogen chemisorption and EXAFS

Abstract Hydrogen-to-metal ( H M ) ratios exceeding unity for Pt and Rh and exceeding 2 for Ir were measured for highly dispersed Pt, Rh, and Ir catalysts supported on Al 2 O 3 and SiO 2 . Since the coordination of hydrogen to metal atoms is unknown for such highly dispersed catalysts, the metal surface area of these catalysts cannot be calculated from the hydrogen chemisorption values. Therefore EXAFS (extended X-ray absorption fine structure) measurements were performed to determine the metal particle size and thereby to calibrate hydrogen chemisorption results. The H M ratio determined by hydrogen chemisorption is a linear function of the average metal coordination number determined by EXAFS. This linear relationship is independent of support but varies with the metal with the H M ratio increasing in the order Pt H M values are discussed. Spillover and subsurface hydrogen are excluded as explanations and only multiple adsorption of hydrogen on metal surface atoms is shown to be capable of explaining all experimental observations. The H M surface stoichiometry differs among Pt, Rh, and Ir in the order H Pt H Rh H Ir , analogous to the order of stability of corresponding metal polyhydride complexes and of theoretical expectation.

The effect of phosphate on the hydrodenitrogenation activity and selectivity of alumina-supported sulfided Mo, Ni, and Ni-Mo catalysts

Abstract Al 2 O 3 -supported Mo, Ni, NiMo, and Rh catalysts, prepared by sequential aqueous impregnation and in situ sulfidation, were investigated in the hydrodenitrogenation (HDN) of quinoline at 643 K and 3 MPa and in the hydrodesulfurisation (HDS) of thiophene at 673 K and 0.1 MPa. The Ni and Mo catalysts had a very low conversion of quinoline to hydrocarbons which improved only slightly in the presence of phosphate. The Rh catalysts had a high conversion and a high selectivity for propylcyclohexane and showed no deactivation with time. The addition of Ni to MO/Al 2 O 3 and of phosphate to NiMo/Al 2 O 3 and Rh/Al 2 O 3 catalysts increased the HDN conversion significantly. The selectivity for propylbenzene and the apparent HDN activation energy increased with increasing P-loading. Ni increased the thiophene conversion of Mo/Al 2 O 3 , but phosphate had almost no influence on the HDS activity of NiMo/Al 2 O 3 and Rh/Al 2 O 3 . The effect of phosphate is due to a combination of structural and catalytic factors. Phosphate improves the activity by inducing the formation of the type II NiMoS structure, but also, especially at high Ni loading, lowers the activity by inducing a decrease in the dispersion of the NiMoS phases and a segregation of Ni 3 S 2 . Phosphate also promotes the S- and N-elimination reactions, but this only has an influence on the overall catalyst activity if the preceding hydrogenation reactions are not rate determining.

Effect of the support on the structure of Mo-based hydrodesulfurization catalysts: Activated carbon versus alumina☆

The structure of oxidic and sulhded MO catalysts supported on activated carbon was studied by means of X-ray photoelectron spectroscopy (XPS), temperature programmed sulfiding (TPS), and suIfur analysis measurements. In the oxidic state the MO phase was highly dispersed as isolated or polymerized monolayer species at MO loadings helow 3 wt% and as very tiny three-dimensional particles at higher loadings. Upon sulfiding particle growth took place, although the size of the sulfide particles remained below 4.6 nm even in the sample with the highest MO loading (14.1 wt%). TPS patterns showed that sulfiding proceeded via a mechanism of 0-S substitution reactions and was completed at temperatures below 560 K. In the suhided catalysts only MO(W) was detected by XPS and S/MO stoichiometries determined by XPS, TPS, and chemical sulfur analysis varied between 1.5 and 2.0, demonstrating that MO& was the major phase present after sulfidation. The higher catalytic activity for MO/C compared to Mo/A120, is explained by differences in the structure of the sulfide phases present and in the interaction between these phases and the respective supports.

The role of cobalt and nickel in hydrodesulfurization: Promoters or catalysts?

Theories of hydrodesulfurization have assumed that W and Mo are catalysts for the reaction and that Co and Ni are promoters which enhance or preserve the activity of the catalysts. To test this, samples of sulfided C-supported Co-, Ni-, W-, and Mo-containing samples were tested as catalysts for thiophene hydrodesulfurization. The results do not clearly define the role of Co and Ni; however, at low concentrations, the promoter function is probably an adequate explanation, but at higher concentrations, more study is indicated to clearly define the role of Co and Ni. Catalytic effects of other C-supported sulfided metals are now being studied. (BLM)

Periodic trends in the hydrodenitrogenation activity of carbon-supported transition metal sulfide catalysts

Periodic trends of transition metals for the catalysis of reactions such as hydrogenation, hydrogenolysis, isomerization and hydrogen oxidation have been well studied. When activity versus position of the transition metal in the periodic table is plotted, quite often these trends are manifested in the form of so-called volcano-type curves. In the present study, the authors have chosen the HDN of quinoline at moderately high pressure as a model reaction, and they have used the same carbon-supported transition metal sulfide catalysts studied by Vissers et al. Results are shown for the following transition metals: V, Cr, Mn, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, and Pt. 9 references.

MAS NMR, TPR, and TEM studies of the interaction of NiMo with alumina and silica–alumina supports

Abstract 1 H MAS NMR, 1 H spin-echo MAS NMR with Al irradiation, 29 Si MAS NMR, and 1 H→ 29 Si CP MAS NMR were used to investigate the deposition of Mo and Ni species on the surface of alumina and silica–alumina. Mo and Ni species first occupy the alumina sites and then the silica sites. The results of temperature-programmed reduction show that the weaker interaction between the Mo and Ni species and the silica–alumina support leads to better reducibility of the metal oxides on silica–alumina than on Al 2 O 3 . Mo and Ni species also interact with each other. Transmission electron microscopy proved that, after sulfidation, higher stacks of MoS 2 are formed on the silica–alumina support than on the Al 2 O 3 support. The higher stacking is responsible for the higher hydrodenitrogenation activity of the NiMoS catalysts supported on silica–alumina.

Characterization of supported cobalt and cobalt-rhodium catalysts. I. Temperature-programmed reduction (TPR) and oxidation (TPO) of Co---Rh/Al2O3

Temperature-programmed reduction and oxidation (TPR and TPO) have been used to study the state of cobalt and rhodium in a series of Co---Rh/?-Al2O3 catalysts. The results show that rhodium enhances the reducibility of part of the cobalt, but that it does not prevent the formation of cobalt aluminate, which is irreducible below 773 K. TPR of the coimpregnated Co---Rh/?-Al2O3 catalyst shows a reduction peak at a much lower temperature than that of Co/Al2O3. This and the slight shift relative to the peak of Rh/Al2O3 indicates that cobalt and rhodium ions are not far apart after coimpregnation, which explains the easy formation of bimetallic particles during reduction. Passivation (oxidation at room temperature) of the reduced bimetallic catalyst leaves the structure of the bimetallic particles largely intact, but cobalt is oxidized to a great extent while rhodium remains metallic. Passivated Co-Rh particles thus consist of a rhodium kernel covered by cobalt oxide. TPR of passivated catalysts also suggests that already in the reduced state the bimetallic particles are surface-enriched in cobalt. A thorough oxidation of the bimetallic catalysts, on the other hand, leads to a restructuring i.e., the formation of metal oxide particles which are in close proximity.

Characterization of supported cobalt and cobalt-rhodium catalysts : III. Temperature-Programmed Reduction (TPR), Oxidation (TPO), and EXAFS of Co---Rh/SiO2

Temperature-Programmed Reduction, Oxidation, and Extended X-Ray Absorption Fine Structure (TPR, TPO, and EXAFS) experiments supply clear evidence for the formation of bimetallic particles in CoRhSiO2 catalysts. After coimpregnation and drying, as well as after oxidation, the reduction of CoRhSiO2 catalysts proceeds at lower temperatures than the reduction of comparable CoSiO2 catalysts, indicating that rhodium catalyzes the reduction of the cobalt metal salt and cobalt oxide. EXAFS of the Rh K-edge of the CoRhSiO2 catalyst shows that after reduction the rhodium atoms in the catalyst have less cobalt neighbors than those in the CoRh alloy. The RhCo and RhRh peak intensities in the Fourier transform of the Rh EXAFS were only slightly influenced by adsorption of oxygen at room temperature, whereas the EXAFS spectrum of the cobalt K-edge changed completely to that of cobalt oxide. From these results it is concluded that the reduced catalyst contains bimetallic CoRh particles, the interiors of which are enriched in rhodium, while the outer layers contain more cobalt.

A temperature programmed reduction study of Pt on Al2O3 and TiO2

The reducibility of platinum on γ-Al2O3- and TiO2 was studied with the aid of temperature programmed reduction. The reduction peak temperature was found to be dependent on the temperature of the primary oxidation after impregnation and drying. The higher the oxidation temperature the lower the TPR peak temperature and the higher the H2 consumption. During the oxidation small PtO2 particles were formed which were more easily reduced than the original isolated Pt2+ ions. For Pt/Al2O3 no decomposition of PtO2 was observed up to 750 K, while bulk PtO2 decomposed around 600 K. This demonstrated that there is a substantial interaction between PtO2 and Al2O3. For PtO2 supported on TiO2 and SiO2 this interaction is much weaker and on these supports PtO2 decomposed at lower temperatures than on Al2O3. Reduction of passivated catalysts, with H/Pt < 0.8 in the metallic state, took place even at 223 K. After passivation these catalysts consist of a metal core surrounded by a metal oxide skin. Due to the presence of the metallic core, H2 can be dissociatively chemisorbed at low temperatures and induce reduction of the oxide layer. The implications of this for the O2-H2 titration method are discussed. Reduction of Pt/TiO2 led to metal assisted reduction of the support. Below 500 K only a small part of the support is (reversibly) reduced in the near vicinity of the metal particles. Above 500 K further metal assisted reduction of the TiO2 support takes place, probably promoted by increased ion mobility.