Photoinduced reactivity of titanium dioxide

Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.

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Cobalt particle size effects in the fischer- : Tropsch reaction studied with carbon nanofiber supported catalysts

Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism.

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Heterogeneous Catalyst Deactivation and Regeneration: A Review

National Natural Science Foundation of China [20625310, 20923004]; National Basic Research Program of China [2005CB221408, 2010CB732303]; Research Fund for the Doctoral Program of Higher Education [20090121110007]; Key Scientific Project of Fujian Province [2009HZ0002-1]

Development of Novel Catalysts for Fischer–Tropsch Synthesis: Tuning the Product Selectivity

Temperature programmed reduction (TPR) and hydrogen chemisorption combined with reoxidation measurements were used to define the reducibility of supported cobalt catalysts. Different supports (e.g. Al2O3, TiO2, SiO2, and ZrO2 modified SiO2 or Al2O3) and a variety of promoters, including noble metals and metal cations, were examined. Significant support interactions on the reduction of cobalt oxide species were observed in the order Al2O3>TiO2>SiO2. Addition of Ru and Pt exhibited a similar catalytic effect by decreasing the reduction temperature of cobalt oxide species, and for Co species where a significant surface interaction with the support was present, while Re impacted mainly the reduction of Co species interacting with the support. For catalysts reduced at the same temperature, a slight decrease in cluster size was observed in H2 chemisorption/pulse reoxidation with noble metal promotion, indicating that the promoter aided in reducing smaller Co species that interacted with the support. On the other hand, addition of non-reducible metal oxides such as B, La, Zr, and K was found to cause the reduction temperature of Co species to shift to higher temperatures, resulting in a decrease in the percentage reduction. For both Al2O3 and SiO2, modifying the support with Zr was found to enhance the dispersion. Increasing the cobalt loading, and therefore the average Co cluster size, resulted in improvements to the percentage reduction. Finally, a slurry phase impregnation method led to improvements in the reduction profile of Co/Al2O3.

Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts

Fresh and used, unpromoted and noble metal-promoted 15% Co/Al 2 O 3 catalysts were analyzed by XANES and EXAFS to provide insight into catalyst deactivation. XANES analysis of the catalysts gave evidence of oxidation of a fraction of the cobalt clusters by water produced during the reaction. Comparison of XANES derivative spectra to those of reference materials, as well as linear combination fitting with the reference data, suggest that some form of cobalt aluminate species was formed. Because bulk oxidation of cobalt by water is not permitted thermodynamically under normal Fischer–Tropsch synthesis (FTS) conditions, it is concluded that the smaller clusters interacting with the support deviate from bulk-like cobalt metal behavior and these may undergo oxidation in the presence of water. However, in addition to the evidence for reoxidation, EXAFS indicated that significant cobalt cluster growth took place during the initial deactivation period. Promotion with Ru or Pt allowed for the reduction of cobalt species interacting with the support, yielding a greater number of active sites and, therefore, a higher initial catalyst activity on a per gram catalyst basis. However, these additional smaller cobalt clusters that were reduced in the presence of the noble metal promoter, deviated more from bulk-like cobalt, and were therefore, more unstable and susceptible to both sintering and reoxidation processes. The latter process was likely in part due to the higher water partial pressures produced from the enhanced activity. The rate of deactivation was therefore faster for these promoted catalysts.

Fischer-tropsch synthesis: deactivation of noble metal promoted co/al2o3 catalysts

Fischer-tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts

A direct relationship between the Fischer–Tropsch synthesis (FTS) rate and the number of surface cobalt atoms available for reaction is usually obtained. For Co/Al2O3 catalysts in particular, the site density depends on two primary factors: (1) the average size of the cobalt clusters on the support; and (2) the fraction of cobalt reduced to the metallic state.

The addition of small amounts of noble metal promoters, such as Pt and Ru to cobalt alumina, may catalyze the reduction of cobalt oxide shifting the temperature of reduction of both steps (Co3O4→CoO and CoO→Co0) to lower temperatures. However, Re affects only the second step. Re is reduced at a higher temperature than Pt or Ru, and at approximately the same temperature as the first step of cobalt reduction (Co3O4→CoO). Thus, Re metal is present to catalyze only the second step.

In situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Re has been used to show that there is direct contact of Re with cobalt atoms, while evidence for ReRe bonds is not observed. Even though direct atom-to-atom contact is found, temperature-programmed reduction (TPR) suggests that hydrogen spillover from the promoter to cobalt oxide clusters is important for the catalysis of cobalt oxide reduction.

In situ EXAFS at the K edge of Co shows that the average cobalt cluster size decreases with increasing Re loading. Re promotes reduction of smaller species which interact with the support, and therefore, for a given reduction temperature, the average cobalt metal cluster size decreases as a function of increasing Re content.

After reduction at a temperature slightly above the first peak in the TPR (Co3O4→CoO), the species remaining on the surface displayed XANES spectra identical to that of CoO. After reduction at a temperature above the second broad TPR peak, XPS showed that a residual oxide species was present, with a binding energy equivalent to cobalt aluminate.

Fischer–Tropsch synthesis: study of the promotion of Re on the reduction property of Co/Al2O3 catalysts by in situ EXAFS/XANES of Co K and Re LIII edges and XPS

Abstract The unpromoted and promoted Fischer–Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer–Emmett–Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al2O3 catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.

Tapan K. Das

Papers

Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts

Fischer-tropsch synthesis: deactivation of noble metal promoted co/al2o3 catalysts

Fischer-tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts

Fischer–Tropsch synthesis: study of the promotion of Re on the reduction property of Co/Al2O3 catalysts by in situ EXAFS/XANES of Co K and Re LIII edges and XPS

Fischer-tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts