P. Arnoldy

Temperature-programmed reduction of CoOAI2O3 catalysts

It is shown that temperature-programmed reduction (TPR) is a sensitive technique for the characterization of Co- and CoAl-oxidic phases in CoOAl2O3 catalysts. Four different reduction regions can be present for CoOAI2O3catalysts, which are assigned to four Co phases (I, II, III, and IV). Phase I (reduction at ca. 600 K in TPR at 10 K/min) consists of Co34 crystallites. Phase II (reduction at ca. 750 K) consists of Co3+ ions, in crystallites of proposed stoichiometry Co3AlO6 or in well-dispersed surface species. Phase III (reduction at ca. 900 K) consists of surface Co2+ ions. Phase IV (reduction at ca. 1150 K) consists either of surface Co2+ ions (with more Al3+ ions in their surrounding than in phase III) or of subsurface Co2+ ions, occurring in diluted Co2+Al3+ spinel structures or in CoAl2O4. Al3+ ions influence the reducibility of Co ions strongly. This is explained by polarization of CoO bonds by Al3+ ions. Preparation conditions (calcination flow rate and calcination temperature) influence the structure of CoOAl2O3, namely the Co valency, the extent of solid-state diffusion, and the dispersion. Solid-state diffusion of Co2+ and Al3+ ions occurs above ca. 800 K. The implications of this study for CoO-MoO3Al2O3 hydrodesulfurization catalysts are discussed.

Temperature-programmed sulfiding of MoO3/Al2O3 catalysts

The conversion of oxides into sulfides has been studied by means of temperature-programmed sulfiding (TPS). In TPS the H2S, H2O, and H2 concentrations are measured continuously during sulfiding with a H2S/H2/Ar mixture, as a function of temperature. Application of TPS to MoO3Al2O3 hydrodesulfurization catalysts leads to detailed information on the sulfiding rate and mechanism. Sulfiding of MoO3Al2O3 takes place at low temperature in comparison with bulk compounds (MoO3MoO2). The sulfiding mechanism is dominated by O-S exchange reactions. Elemental sulfur is formed by rupture of metal sulfide bonds and is reduced subsequently by H2. In fact, H2 plays only a minor role in sulfiding at low temperatures. Particularly the “H2O content” of the catalysts influences the sulfiding rate to a large extent. “Wet” catalysts, in equilibrium with 3% H2OAr at room temperature, sulfide at very low temperature (typically 400–500 K). “Dry” catalysts, treated in Ar at 775 K, on the other hand, sulfide at relatively high temperature (600–700 K). This H2O effect is explained tentatively by catalysis of OS exchange by Bronsted acid sites. Prereduction hinders sulfiding more than predrying. This suggests a minor importance of reduced intermediates in normal sulfiding procedures. An increase in the Mo content (0.5–4.5 atoms/nm2) leads to sulfiding at lower temperature, but the influence of Mo content is not as pronounced as has been found in TPR reducibility studies. The influence of Mo content on TPR and TPS is explained by detailed consideration of the heterogeneity. Sulfiding of a 4.5 atoms/nm2 catalyst can be completed at ca. 500 K, up to a S/Mo ratio of 1.9, provided that a sufficiently low heating rate is chosen. The fact that such a low temperature is sufficient suggests the initial formation of monolayer-type sulfide species with a S/Mo ratio near 2. These species can exist if steric factors are taken into account.

Temperature-programmed reduction of CoOMoO3Al2O3 catalysts

It is shown that temperature-programmed reduction (TPR) gives new information on the reducibility of CoOMoO3Al2O3 catalysts. The reduction of Mo6+ surface species (monolayer and bilayer species) is not essentially affected by the presence of Co, whereas the reduction of Co2+ ions is strongly influenced by the presence of Mo. There appears to be a strong CoMo interaction at moderate Co contents: the reduction maximum for dispersed Co2+ ions decreases from around 1200 K, found for CoOAl2O3, to 800–850 K for CoOMoO3Al2O3. At high Co contents, Co3O4 crystallites and Co3+ ions in surface positions or in a crystalline Co3+-Al3+-oxide of proposed stoichiometry Co3AlO6 have been found in addition to the Co-Mo interaction phase. Solid-state diffusion of Co2+ ions starts already around 800 K, resulting in destruction of the Co-Mo interaction phase and in formation of subsurface Co2+ ions of low reducibility. By calcination at 1125 K, a significant loss of Mo takes place, while some CoMoO4 microcrystallites are formed. Also some α-Al2O3 is formed, probably initiated by the presence of CoMoO4.

Temperature-programmed reduction of Al2O3-, SiO2-, and carbon-supported Re2O7 catalysts

Temperature-Programmed Reduction (TPR) has been applied to characterize the reducibility of Al2O3-, SiO2-, and carbon-supported Re2O7 catalysts, over a wide range of transition metal content. Dried catalysts are found to contain a so-called monolayer-type Re7+ surface phase as well as crystalline NH4ReO4. Calcination at 575 or 825 K resulted in decomposition of NH4ReO4, formation of the Re7+ surface phase and Re2O7 clusters, and Re loss via sublimation of Re2O7. Differences in reducibility of the various catalyst samples are ascribed to variations in the strength and the heterogeneity of the Re7+-support interaction. The strength of the interaction was found to depend on the support material applied and decreased in the order: Al2O3 > SiO2 > carbon. The heterogeneity was essentially the same for all three supports. The largely varying literature data on the reducibility of Re2O7Al2O3 catalysts is supposedly related with the presence of additives, such as chlorides, which may increase the Re7+-support interaction.

Sulfidability and HDS activity of Co-Mo/Al2O3 catalysts

A series of Co-Mo/Al2O3 catalysts were investigated using Temperature-Programmed Sulfiding and HDS activity measurements. The effect of changing the cobalt content and the temperature of calcination on sulfidability, catalyst structure and thiophene HDS activity was studied in detail. It is found that the effect on the HDS activity of higher temperatures of calcination depends on the Co content: at low Co content the activity drops sharply, for intermediate Co loadings the decline is not as pronounced, while at high Co contents an increase in HDS activity is found when the temperature of calcination is raised above 785 K. The typical “synergistic” maximum observed when HDS activity is plotted versus Co content (or Co/Mo ratio) does not occur when catalysts are calcined above 785 K. Instead HDS activity rises monotonously with an increase in Co content. At least five Co species can be present in sulfided Co-Mo/Al2O3 catalysts but HDS activity can mainly be attributed in all catalysts to one particular phase which contains sulfided Mo and Co. After calcination at 1125 K the activity is increased because the interaction between the active phase and the support has weakened.

Temperature-programmed sulfiding and reduction of CoO/Al2O3 catalysts

Abstract Sulfiding of CoO Al 2 O 3 catalysts has been studied by means of Temperature-Programmed Sulfiding (TPS). TPS patterns were compared with Temperature-Programmed Reduction (TPR) patterns, which give information on the presence of several oxidic Co phases. Sulfiding takes place in a low-temperature region (LT; 295–750 K) and in a high-temperature region (HT; 750–1200 K). Most surface species as well as supported crystallites sulfide in the LT region, whereas in the HT region there is sulfiding of Co2+ ions in subsurface positions and of, probably tetrahedrally coordinated, Co2+ surface ions. For low calcination temperatures (below 800 K) sulfiding in the LT region predominates. The sulfiding pattern shifts from the LT to the HT region with increasing calcination temperature (800–1000 K) due to solid-state diffusion of Co2+ ions. The influence of Co content on the sulfiding rate is relatively small. The sulfiding rate increases slightly with H2O content, which is tentatively explained by polarization of H2S by H2O in the adsorbed state. Generally, sulfiding in H 2 S H 2 (measured by TPS) is much faster than reduction in H2 (measured by TPR) because of different reaction mechanisms. In sulfiding, H2S is the primary reactant in OS exchange reactions, whereas H2 plays only a secondary role, e.g., in reduction of elemental sulfur. H2 can only compete with H2S as primary reactant when H2S diffusion is hindered i.e., in the case of large crystallites. At 1000–1270 K, interconversion of various Co sulfides can be observed. These processes are different for catalysts and bulk compounds, pointing to strong interaction of Co ions with the support.

Thiophene hydrodesulphurization activity of alumina-, silica- and carbon-supported sulphided Re2O7 catalysts

Re2O7 catalysts supported on various carriers (Al2O3, SiO2 and activated carbon), with various rhenium loadings (between 0.01 and 2.5 at. nm−2), and after different heat treatments, at 380, 575 or 825 K, were subjected to a thiophene hydrodesulphurization test at 675 K. The catalysts were very active, viz. 2–20 times more active than molybdenum catalysts with a similar surface coverage. The activity of rhenium sulphides depended slightly on the support used and increased in the order SiO2<Al2O3<carbon. The HDS activity of molybdenum catalysts, however, was much more affected by support choice. Calcination at 575 or 825 K of oxidic catalyst precursors containing NH4ReO4 crystallites led to increased HDS activity, due to increased rhenium dispersion caused by the decomposition of these crystallites. The influence of rhenium content on the HDS activity per mol rhenium was relatively small. Three different regions of rhenium loading could be discriminated. At the lowest loadings (<0.1 at. nm−2) HDS activity was attributed to Al2O3 and SiO2 carrier sites, promoted by low-valent rhenium species. At medium loadings, the activity was fully determined by well-dispersed sulphided rhenium species, and at high loadings, the HDS activity decreased due to the presence of ReS2 crystallites. The capacity for the formation of well-dispersed sulphided rhenium species is much larger on Al2O3 and SiO2 supports (ca. 1.5 at. nm−2) than on carbon (ca. 0.1 at nm−2). The differences between rhenium and molybdenum catalysts are explained in terms of polarization of metal sulphides due to an inductive effect of the supports.

Temperature-programmed reduction of Re2O7/Al2O3 metathesis catalysts; calculation of activation parameters for reduction

Abstract Series of Temperature-Programmed Reduction (TPR) experiments with various heating rates have been used to calculate the activation parameters for the reduction of ReO 3 , NH 3 ReO 4 and Re 2 O 7 /Al 2 O 3 catalysts. The activation energy for the reduction of Re 2 O 7 /Al 2 O 3 catalysts is virtually independent of the Re content (111 ± 8 kJ mol −1 ), but higher than the reference value for the reduction of Al 2 O 3 -supported NH 4 ReO 4 crystallites (95 kJ mol −1 ). In view of the strong interaction between Re 7+ ions and the A1 2 O 3 support, the observed differences in the activation energy values for reduction are surprisingly small; this may be explained by the dominance of autocatalysis in the reduction mechanism. The heterogeneity observed in the reduction of Re 2 O 7 /Al 2 O 3 catalysts is explained for the most part by a 25–40 J mol −1 K −1 increase in the activation entropy with increasing Re content, suggesting an influence of the surface geometry on the freedom of rotation of activated complexes. The sharp increase in metathesis activity with increasing Re content of Re 2 O 7 /Al 2 O 3 catalysts is discussed in the light of the TPR results.

Temperature-programmed sulfiding of Re2O7/Al2O3 catalysts

The conversion of oxides into sulfides has been studied by means of temperature-programmed sulfiding (TPS). In TPS the H2S, H2O, and H2 concentrations are measured continuously during sulfiding with a H2S/H2/Ar mixture, as a function of temperature. Application of TPS to MoO3Al2O3 hydrodesulfurization catalysts leads to detailed information on the sulfiding rate and mechanism. Sulfiding of MoO3Al2O3 takes place at low temperature in comparison with bulk compounds (MoO3MoO2). The sulfiding mechanism is dominated by O-S exchange reactions. Elemental sulfur is formed by rupture of metal sulfide bonds and is reduced subsequently by H2. In fact, H2 plays only a minor role in sulfiding at low temperatures. Particularly the “H2O content” of the catalysts influences the sulfiding rate to a large extent. “Wet” catalysts, in equilibrium with 3% H2OAr at room temperature, sulfide at very low temperature (typically 400–500 K). “Dry” catalysts, treated in Ar at 775 K, on the other hand, sulfide at relatively high temperature (600–700 K). This H2O effect is explained tentatively by catalysis of OS exchange by Bronsted acid sites. Prereduction hinders sulfiding more than predrying. This suggests a minor importance of reduced intermediates in normal sulfiding procedures. An increase in the Mo content (0.5–4.5 atoms/nm2) leads to sulfiding at lower temperature, but the influence of Mo content is not as pronounced as has been found in TPR reducibility studies. The influence of Mo content on TPR and TPS is explained by detailed consideration of the heterogeneity. Sulfiding of a 4.5 atoms/nm2 catalyst can be completed at ca. 500 K, up to a S/Mo ratio of 1.9, provided that a sufficiently low heating rate is chosen. The fact that such a low temperature is sufficient suggests the initial formation of monolayer-type sulfide species with a S/Mo ratio near 2. These species can exist if steric factors are taken into account.