Gary Jacobs

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

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: 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.

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.

CO and CO2 hydrogenation study on supported cobalt Fischer–Tropsch synthesis catalysts

Abstract The conversion of CO/H2, CO2/H2 and (CO+CO2)/H2 mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO2 hydrogenation is slow. For the cases of only CO or only CO2 hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an α of about 0.80; in contrast, the CO2 hydrogenation products contained about 70% or more of methane. Thus, CO2 and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO2 even though the H2O/H2 ratio is at least two times larger for the conversion of CO2. Since the catalyst ages more slowly in the presence of the higher H2O/H2 conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO.

Fischer-Tropsch synthesis: effect of small amounts of boron, ruthenium and rhenium on Co/TiO2 catalysts

The effect of the addition of small amounts of boron, ruthenium and rhenium on the Fischer–Tropsch (F–T) catalyst activity and selectivity of a 10 wt.% Co/TiO2 catalyst has been investigated in a continuously stirred tank reactor (CSTR). A wide range of synthesis gas conversions has been obtained by varying space velocities over the catalysts. The addition of a small amount of boron (0.05 wt.%) onto Co/TiO2 does not change the activity of the catalyst at lower space times and slightly increases synthesis gas conversion at higher space times. The product selectivity is not significantly influenced by boron addition for all space velocities investigated. Ruthenium addition (0.20 wt.%) onto Co/TiO 2 and CoB/TiO2 catalysts improves the catalyst activity and selectivity. At a space time of 0.5 h-g cat./NL, synthesis gas conversion increases from 50–54 to 68–71% range and methane selectivity decreases from 9.5 to 5.5% (molar carbon basis) for the promoted catalyst. Among the five promoted and non-promoted catalysts, the rhenium promoted Co/TiO 2 catalyst (0.34 wt.% Re) exhibited the highest synthesis gas conversion, and at a space time of 0.5 h-g cat./NL, synthesis gas conversion was 73.4%. In comparison with the results obtained in a fixed bed reactor, the catalysts displayed a higher F–T catalytic activity in the CSTR.

Characterization of the morphology of Pt clusters incorporated in a KL zeolite by vapor phase and incipient wetness impregnation. Influence of Pt particle morphology on aromatization activity and deactivation

Abstract Two series of Pt/KL catalysts with varying metal loading were synthesized by the methods of incipient wetness impregnation (IWI) and vapor phase impregnation (VPI) to compare the effects of the different morphologies that result when the metal loading and, in particular, the preparation method are varied. Catalysts were characterized by a variety of techniques. TEM and DRIFTS studies indicated that on the low-loading samples the majority of particles were located inside the channels of the L-zeolite. In agreement with recent studies, the DRIFTS results evidenced the formation of Pt carbonyls, which further support the presence of very small particles. EXAFS and TEM showed that the VPI catalysts resulted in smaller particles than the catalysts prepared by the IWI method. In addition, EXAFS demonstrated for this series a higher degree of interaction with the L-zeolite framework oxygen atoms. Pulse testing of the methylcyclopentane ring opening showed that the very small clusters produced by the VPI preparation did not result in collimation of the MCP molecule, implying that the reactants and products can easily diffuse over the Pt cluster. This is in contrast with the particles produced by the IWI method, which clearly displayed a collimation effect. The characteristic morphology produced by the VPI method was found to improve the performance of the catalyst under clean and sulfur-poisoned conditions, enhancing the catalyst’s resistance to the formation of coke and decreasing the particle agglomeration rate.