Patricia M. Patterson

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 Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation.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.

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

The addition of platinum metal to cobalt/alumina-based Fischer-Tropsch synthesis (FTS) catalysts increases both the reduction rate and, consequently, the density of active cobalt sites. Platinum also lowers the temperature of the two-step conversion of cobalt oxide to cobalt metal observed in temperature programmed reduction (TPR) as Co3O4 to CoO and CoO to Co0. The interaction of the alumina support with cobalt oxide ultimately determines the active site density of the catalyst surface. This interaction can be controlled by varying the cobalt loading and dispersion, selecting supports with differing surface areas or pore sizes, or changing the noble metal promoter. However, the active site density is observed to depend primarily on the cluster size and extent of reduction, and there is a direct relationship between site density and FTS rate. In this work, in situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Pt was used to show that isolated Pt atoms interact with supported cobalt clusters without forming observable Pt--Pt bonds. K-edge EXAFS was also used to verify that the cobalt cluster size increases slightly for those systems with Pt promotion. X-ray absorption near-edge spectroscopy (XANES) was used to examine the remaining cobalt clusters after the first stage of TPR, and it revealed that the species were almost entirely cobalt (II) oxide. After the second stage of TPR to form cobalt metal, a residual oxide persists in the sample, and this oxide has been identified as cobalt (II) aluminate using X-ray photoelectron spectroscopy (XPS). Sequential in situ reduction of promoted and unpromoted systems was also monitored through XPS, and Pt was seen to increase the extent of cobalt reduction by a factor of two.

Compensation behaviour in the hydrogenation of benzene, toluene and o-xylene over Ni/SiO2. Determination of true activation energies

The vapour phase hydrogenation of benzene, toluene and o-xylene has been studied over an Ni/SiO2 catalyst prepared by homogeneous precipitation/deposition. The hydrogenations were carried out in the absence of diffusion limitations, catalyst deactivation or secondary conversions. Turnover frequencies and reaction probabilities decrease in the order benzene > toluene > o-xylene. The appearance of well defined reversible maxima (Tmax) in the rate vs. temperature plots is reported and discussed. The reaction orders with respect to the aromatic and hydrogen partial pressures are plotted as a function of temperature. The effect of temperature on the effective surface collisions is also considered. The temperature dependence of the rate constants was fitted to Arrhenius equations and generated positive (T⩽Tmax) and negative (TTmax) activation energies. The derivation of true activation energies and heats of adsorption from the kinetic data is presented. The turnover frequencies and apparent activation energies obtained in this study are compared to similar and different systems reported in the literature. A compensation effect is established for the experimentally determined or apparent kinetic parameters. The effect is attributed to variations in the temperature dependences of the surface concentration of the reactive aromatic species.

Low Temperature Water–Gas Shift: Role of Pretreatment on Formation of Surface Carbonates and Formates

Recently, the role of ceria vacancies on water–gas shift activity has been explained in terms of a redox mechanism, whereby CO adsorbed on a metal reduces the ceria surface to generate CO2, and water reoxidizes the ceria surface to CeO2, liberating hydrogen in the process. In this study, we examine the possibility of a ceria-mediated redox mechanism by examining more closely the evolution of carbonate and formate bands under different controlled treatment environments, and utilizing different reduction procedures. Earlier it was claimed that the decomposition of carbonates by water was consistent with a redox process, whereby the CO2 product could spillover to the support. We found that the observation of carbonate formation and decomposition by water was a result of the treatment procedure used in the earlier work, and that, once bridging OH groups are produced in the presence of water, the reaction more likely proceeds via a formate intermediate, which is produced by reaction of CO with the active bridging OH groups. However, the vacancies appear to play an important role in generating these active sites. Possible pathways to active site generation are discussed.

Fischer-Tropsch synthesis : influence of support on the impact of co-fed water for cobalt-based catalysts

Co catalysts were prepared with variable cobalt oxide-support interactions through judicious selection of the cobalt loading, the type of support utilized, and the promoter employed, if any, along with its loading. For a comparable Co loading range, while a positive effect of water was found for catalysts identified to have supports that only weakly interacted with the cobalt clusters, an adverse impact of water was recorded when cobalt was supported on more strongly interacting supports, such as TiO 2 and especially, Al 2 O 3 . However, alumina supported cobalt catalysts were found to have much higher active site densities in the cobalt loading range explored, due to a smaller average crystallite size. More robust Co/Al 2 O 3 catalysts, less sensitive to the negative effect of water, were obtained at higher Co loadings, where the average cluster size was > 10 nm.