Burtron H. Davis

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

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.

Production of hydrogen from ethanol: review of reaction mechanism and catalyst deactivation.

Mechanism and Catalyst Deactivation Lisiane V. Mattos,† Gary Jacobs,‡ Burtron H. Davis,‡ and Fab́io B. Noronha* †Departamento de Engenharia Química e de Petroĺeo, Universidade Federal Fluminense (UFF), Rua Passo da Pat́ria, 156-CEP 24210-240, Niteroí, RJ, Brazil ‡Center for Applied Energy Research, The University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States Instituto Nacional de Tecnologia−INT, Av. Venezuela 82, CEP 20081-312, Rio de Janeiro, Brazil

Sulfated zirconia as a hydrocarbon conversion catalyst

Abstract Platinum containing sulfated zirconia is a more active catalyst for hydrocarbon isomerization and cracking than most, if not all, zeolite cracking catalysts. A survey of the literature data suggests that the preparation technique, initial calcination and final catalyst treatment play important roles in determining catalyst activity. The final treatment probably is the most important and least controlled step in most studies to date. Whether the catalytic site involves Lewis or Bronsted acidity is debated; our data indicate Bronsted acidity as more likely. The role of a supported metal, e.g., Pt, is to provide a hydrogenation role but many attribute a second role for it in generating acidity through hydrogen spillover. For hexadecane conversions, increasing the hydrogen pressure increases the cracking to lower molecular weight hydrocarbons. The presence of small amounts of hydrogen donors are claimed to improve isomerization selectivity.

Low temperature water–gas shift: in situ DRIFTS-reaction study of ceria surface area on the evolution of formates on Pt/CeO2 fuel processing catalysts for fuel cell applications

Abstract Steady state infrared (IR) measurements for adsorption of only CO and under water–gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, and that their concentrations vary with surface area of partially reduced ceria. Under steady state WGS, the concentrations of surface formates are strongly limited at high CO conversions. However, at low temperatures and conversions, the formates are close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. Comparisons at constant temperature indicate that formate bands from IR may provide an indication of the number of active sites present on the catalyst surface, as the rates varied accordingly. The IR results favor a formate intermediate mechanism to explain WGS. However, more kinetic studies are required, and over a broad range of temperatures, to verify this conclusion. Previous low temperature kinetic studies at a relatively high CO/H2O ratios have produced a zero-order dependency for CO and the authors related this to a mechanistic scheme involving reaction of Pt-CO with CeO2 to yield CO2, followed by reoxidation of Ce2O3 by H2O, with liberation of H2. The zero-order was suggested to be due to saturation of noble metal surface with CO during WGS. Saturation of ceria with carbonates was also reported. In this study, a high H2O/CO ratio was used where the CO rate dependency was first-order. This criteria requires that the surface coverage of the adsorbed CO intermediate should be reaction rate limited. Therefore, the formates are suggested to be the intermediates.

FISCHER-TROPSCH SYNTHESIS: DEACTIVATION OF NOBLE METAL-PROMOTED CO/AL2O3 CATALYSTS

Abstract 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 : current mechanism and futuristic needs

Mechanisms for the generation of hydrocarbon and oxygenate products from synthesis gas using the Fischer–Tropsch synthesis are presented. The data generated to date indicate that, while there are similarities between iron and cobalt catalytic synthesis mechanisms, the details differ. For the iron catalyst, it is concluded that an oxygenate mechanism is more appropriate in light of todays data. While less mechanistic data for the cobalt catalyst are available, it appears that a surface carbide mechanism is a better choice.

Fischer–Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts☆ ☆

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.

Fischer–Tropsch synthesis XAFS: XAFS studies of the effect of water on a Pt-promoted Co/Al2O3 catalyst

Abstract The impact of water on the deactivation of a 0.5% Pt-promoted 15% Co/Al 2 O 3 catalyst was studied by XAFS. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under an inert gas blanket. Synthesis operating conditions were maintained constant while differing amounts of argon were replaced by added water. Below 25% added water (H 2 O/CO=1.2; H 2 O/H 2 =0.6), the slight negative effect on activity was reversible, and no changes were observed in the EXAFS or XANES spectra. This indicates that the effect of water in this range is most likely kinetic. However, XAFS results strongly suggest that, above 25% water addition, the sudden irreversible loss in activity is due to reaction of the cobalt clusters with the support, forming cobalt aluminate-like species. The XAFS and previously reported activity data indicate that there are two regions for the water effect: at lower H 2 O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H 2 O/CO ratios irreversible oxidation of cobalt occurs.

Zirconium oxide crystal phase: The role of the pH and time to attain the final pH for precipitation of the hydrous oxide

Precipitated hydrous zirconium oxide can be calcined to produce either a monoclinic or tetragonal product. It has been observed that the time taken to attain the final pH of the solution in contact with the precipitate plays a dominant role in determining the crystal structure of the zirconium oxide after calcination at 500 /sup 0/C. The dependence of crystal structure on the rate of precipitation is observed only in the pH range 7--11. Rapid precipitation in this pH range yields predominately monoclinic zirconia, whereas slow (8 h) precipitation produces the tetragonal phase. At pH of approximately 13.0, only the tetragonal phase is formed from both slowly and rapidly precipitated hydrous oxide. The present results, together with earlier results, show that both the pH of the supernatant liquid and the time taken to attain this pH play dominant roles in determining the crystal structure of zirconia that is formed after calcination of the hydrous oxide. The factors that determine the crystal phase are therefore imparted in a mechanism of precipitation that depends upon the pH, and it is inferred that it is the hydroxyl concentration that is the dominant factor.

Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust

Cleaning Diesel Exhaust One strategy for removing pollutants from diesel engine exhaust is to trap the unburned carbon soot and then to combust the soot with the NO2 that is generated from NO; the two pollutants are then converted to N2 and CO2. Diesel exhaust is relatively cold, compared to gasoline engine exhaust, and conversion of NO to NO2 has required the use of platinum catalysts. W. Wang et al. (p. 832) now report that a more earth-abundant catalyst, based on Mn-mullite (Sm, Gd)Mn2O5 metal oxides was able to oxidize NO in simulated diesel exhaust at temperatures as low as 75°C. Spectroscopic studies and quantum chemical modeling suggested that Mn-nitrates formed on Mn-Mn dimer sites were the key intermediates responsible for NO2 formation. Costly platinum catalysts for removing nitrogen oxide pollutants could potentially be replaced with metal oxide catalysts. Oxidation of nitric oxide (NO) for subsequent efficient reduction in selective catalytic reduction or lean NOx trap devices continues to be a challenge in diesel engines because of the low efficiency and high cost of the currently used platinum (Pt)–based catalysts. We show that mixed-phase oxide materials based on Mn-mullite (Sm, Gd)Mn2O5 are an efficient substitute for the current commercial Pt-based catalysts. Under laboratory-simulated diesel exhaust conditions, this mixed-phase oxide material was superior to Pt in terms of cost, thermal durability, and catalytic activity for NO oxidation. This oxide material is active at temperatures as low as 120°C with conversion maxima of ~45% higher than that achieved with Pt. Density functional theory and diffuse reflectance infrared Fourier transform spectroscopy provide insights into the NO-to-NO2 reaction mechanism on catalytically active Mn-Mn sites via the intermediate nitrate species.