J.A. Moulijn

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 reduction of NiOWO3/Al2O3 Hydrodesulphurization catalysts

Abstract Temperature-programmed reduction patterns are reported of NiO/Al 2 O 3 , WO 3 /Al 2 O 3 , and NiO WO 3 /Al 2 O 3 catalysts, and of bulk oxides relevant to these catalysts. At loadings below 10% NiO and 19% WO3 nickel and tungsten are present as disperse species and no bulk oxides are found on the support. Tungsten forms a stable monolayer species which is extremely difficult to reduce. At high temperatures of reduction tungsten metal is formed without the intermediate formation of WO 2 . Several nickel species were detected in the supported catalysts, and their characters and amounts depend on the loading and the temperature of calcination. After calcination at low temperatures and at low loadings (2% NiO) nickel is incorporated in the surface layers of the support; at higher loadings a NiO like species is formed which is more difficult to reduce than bulk NiO because of the interaction with the aluminium ions of the support. A high temperature of calcination favours the formation of a diluted spinel phase at the expense of the surface phases. In NiO WO 3 /Al 2 O 3 catalysts a fraction of nickel is present in a mixed NiWOAl phase which is formed by incorporation of nickel in a tungsten containing surface layer. At high temperatures of calcination some disperse NiWO 4 is formed. It is shown that there are two causes for a strong interaction between nickel species and the support: incorporation of nickel ions in the surface layers of the support during impregnation, and solid-state diffusion during calcination of the catalysts. The nickel containing species which are the precursor to the active phases in sulfided HDS catalysts are identified as the nickel species in the surface layers of the support in NiO/Al 2 O 3 samples, and the NiWOAl phase in NiO WO 3 /Al 2 O 3 catalysts.

Alumina supported manganese oxides for the low-temperature selective catalytic reduction of nitric oxide with ammonia

Abstract Alumina supported manganese oxides exhibit a high and selective activity for the catalytic reduction of nitric oxide with ammonia (SCR) between 385 and 575 K. Samples with 3–15 wt.-% manganese were studied at space velocities between 22 000–116 000 h −1 and at standard conditions of 500 ppm NO, 550 ppm NH 3 and 2% O 2 . Manganese acetate results in a better dispersion of the manganese oxide on the support and a higher specific catalyst activity than manganese nitrate as precursor, for which crystalline structures could be detected. Temperature-programmed reduction revealed that acetate yields Mn 2 O 3 and nitrate mainly MnO 2 on the γ-alumina support. The nitric oxide conversion per amount of manganese is fairly independent of the loading for the catalysts prepared from each precursor. The use of 15 NH 3 reveals that it reacts in a 1:1 molar ratio with nitric oxide towards 15 NN and/or 15 NNO. The SCR activity (to nitrogen) is strongly dependent on the oxygen partial pressure, whereas water inhibits reversibly. Lattice oxygen of the catalyst is not able to maintain the SCR reaction in the absence of oxygen. The nitrous oxide formation is independent of the oxygen partial pressure, but increases with increasing manganese loading and with temperature, resulting in lower selectivities for nitrogen formation. The nitrogen and nitrous oxide formation probably occur at different sites. Above 525 K 15 NH 3 oxidation occurs, yielding mainly 15 N 2 O and 15 NO, depending on the temperature. The nitrous oxide is not further reduced by ammonia over this type of catalyst. The addition of tungsten to the catalyst increases the selectivity for nitrogen considerably. The stability of the ex-acetate catalyst is good, for at least 600 h the activity remained constant. The catalysts are sensitive towards sulphur dioxide, the ex-acetate catalysts the least, due to the strong interaction with the alumina support, as is revealed by TPR.

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.

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.

A temperature-programmed sulfiding study of NiO3/Al2O3 catalysts

Abstract The sulfiding of a series of NiO/AI 2 O 3 , WO 3 /Al 2 O 3 , and NiOWO 3 /Al 2 O 3 catalysts has been studied using the temperature-programmed sulfiding technique. For various Ni and W species in the catalysts the temperatures at which sulfiding commences have been determined and the extent of sulfiding after a regular sulfiding treatment has been assessed. It has been found that sulfiding of the supported catalysts starts at room temperature. For NiO/Al 2 O 3 catalysts the sulfiding of different oxidic Ni species can be distinguished. A disperse NiO-like species is sulfidable well below 610 K, whereas Ni ions in the surface layers of the support are sulfided over the whole temperature range up to 1200 K. After pretreatment at a high temperature a dilute spinel phase is formed which is sulfided around 1050 K. WO 3 /Al 2 O 3 catalysts have been found to sulfide at higher temperatures. The lower sulfidability is attributed to the interaction with the support through WOAl links. In NiOWO 3 /Al 2 O 3 catalysts species similar to those in the NiO/Al 2 O 3 and WO 3 /Al 2 O 3 catalysts are found. In addition to these species a mixed phase (“NiWOAI”) is present which is sulfided below 610 K. After calcination at a high temperature, sulfiding of disperse NiWO 4 is found around 920 K.

Mechanism of the potassium catalysed gasification of carbon in CO2

The literature on the potassium catalysed gasification of carbon in CO2 is critically reviewed with respect to the mechanism and the experimental ‘facts’ relevant to the mechanistic considerations. It is concluded that bulk intercalation compounds (C8K, C24K, etc.) are not present under gasification conditions; also other metallic K-species are not the major species during gasification. It is shown that the catalytic activity can be attributed to an oxygen transfer cycle with either reduction of carbon or decomposition of the oxygenated complexes as the rate determining step. In this catalytic cycle only oxidic potassium species are involved.

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.

A temperature-programmed reduction study of sulfided CoMo/Al2O3 hydrodesulfurization catalysts

The reduction of sulfided hydrodesulfurisation catalysts has been studied using temperature-programmed reduction of sulfides (TPR-S). Hydrogenation of stoichiometric sulfur of bulk Co9S8 and MoS2 occurs at much higher temperatures than HDS operating temperatures and is thermodynamically limited under the TPR-S reduction conditions. Al2O3-supported Co and Mo catalysts contain sulfided species which are reduced at lower temperatures than bulk Co9S8 and MoS2. In Co/Al2O3 catalysts with a high Co content Co9S8 is found, while a small amount of a CoS surface species is also present. High temperatures of calcination cause Co to migrate into the support, and this Co species remains oxidic during the sulfiding treatment. On Mo/Al2O3 catalysts it is found that around 600 K a sulfided Mo monolayer species is reduced. At higher temperature, reduction is observed of bulk-like MoS2. In CoMo/Al2O3 catalysts the reduction of Mo occurs at nearly the same temperature as that of Mo/Al2O3. When the Co loading is low, no separate Co sulfide phase is detected, whereas catalysts with a high Co content contain Co9S8. A high temperature of calcination leads to the formation of nonsulfidable Co species. For all supported catalysts the TPR-S patterns indicate that some S is hydrogenated around 400 K. This S is chemisorbed on coordinatively unsaturated sites of Mo and Co sulfides when the H2S pressure is sufficiently high, and it is hydrogenated at a lower H2S/H2 ratio. On Co/Al2O3 catalysts this S species is hydrogenated at a higher temperature than that of the corresponding species in Mo/Al2O3. On CoMo/Al2O3 a similar S species is found; it is hydrogenated at a lower temperature than that found for either Co/Al2O3 or Mo/Al2O3. This is an indication of the formation of a disperse CoMo species. The temperature of hydrogenation of the chemisorbed S species depends on the metal loading and the temperature of calcination. For all CoMo/Al2O3 catalysts, it is found that a low temperature of hydrogenation of S corresponds with a high HDS activity. It is concluded that the capacity to hydrogenate S, which is abstracted from S-containing compounds during the HDS reaction, is a key parameter for the overall HDS activity of CoMO/Al2O3 catalysts.

CO2 gasification of carbon catalysed by alkali metals: Reactivity and mechanism☆

Abstract A systematic study of the catalytic activity of alkali metal carbonates on the CO 2 gasification of activated carbon revealed the following order: Li Na K Rb Cs . Outgassing in an inert gas results in a pronounced activity decrease for Cs, whereas the other alkali metals show a slight increase. The activated carbon itself is unaffected. Apparent activation energies for the CO 2 gasification are also changed by outgassing and decrease from Li to Cs. Upon outgassing of the samples, CO 2 and CO are released in five distinguishable temperature regions, arising from decomposition of surface complexes and carbonate-like species, gasification phenomena and reduction of oxidic species. Outgassing patterns of all alkali metals are quite similar. During alkali-metal-catalysed gasification in CO 2 two types of oxidic species are present: surface bonded -O M species of high stability and oxidic species having less interaction with the carbon surface.