Dennis E. Sparks

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.

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.

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.

XPS investigation of an iron/manganese/sulfated zirconia catalyst

Abstract A sample of Fe–Mn–SO 4 2− –ZrO 2 has been heated at 500°C in air for 98 h. At intervals, the sample was evacuated and transferred without atmospheric exposure to an XPS chamber. As noted with Pt–SO 4 2− –ZrO 2 , the O 1s peak resolved to a doublet; one of these peaks is interpreted to result by dehydration of the sulfate group. Following the 98 h air treatment, the sample was treated at 150°C at 1 atm in flowing hydrogen for a total of 78 h. The XPS spectra, obtained at intervals during the heating in hydrogen, showed that both Fe and Mn remained in an oxidized state.

Paraffin dehydrocyclization. Part 8. Conversion of n-octane with mono and bifunctional Pt-Al2O3 catalysts at 100 psig

Abstract Pt monofunctional and bifunctional catalysts produce a common set of products apart from the C 8 -aromatic and i-C 8 -alkane distributions at low conversions. The difference between the two types of catalysts therefore is essentially that of the rate and selectivity of producing aromatics vs. C 1 C 7 products. Overall, the bifunctional pathway provides both the most rapid and the most selective pathway for the formation of aromatics from n -alkanes. The bifunctional pathway provides a nearly equilibrium distribution of methyl heptanes whereas the monofunctional platinum pathway produces predominantly the isomers allowed from hydrogenolysis of alkylcyclopentanes formed by the five-carbon ring cyclization of the n -alkane. The non-acidic Pt catalyst produces aromatics by direct six-carbon ring formation. The acidic catalyst produces essentially a mixture of C 8 -aromatics that is near the thermodynamic equilibrium mixture, presumably because of isomerization reactions that proceed by metal catalyzed cyclization and by acid catalysed cyclization.

Paraffin dehydrocyclization. Part 10. Conversion of n-octane with supported Pt—Sn catalysts at 100 psig

Abstract The n -octane conversion data at 100 psig conditions show that the presence of tin in a Pt-alumina catalyst increases conversion when a non-acidic support is used but decreases the activity of a catalyst with an acidic alumina support. The above observation applies for catalysts prepared by coimpregnation of Sn and Pt onto the support, but not for a catalyst prepared by coprecipitation of a tin-aluminum oxide support which is then impregnated with Pt. For the catalysts prepared by coimpregnation, tin decreases the activity by poisoning acid sites that are needed for the bifunctional cyclization pathway but this is not the case for the coprecipitated catalyst for a similar Sn/Pt ratio. The role of tin in altering the activity and selectivity of Pt catalysts is correlated to catalyst characterization data.

The regeneration of Pt promoted SO42−–ZrO2 catalysts

Different regeneration strategies for a Pt-SO42−–ZrO2 catalyst were investigated. Two of the regeneration methods tested on the same deactivated catalyst included sulfur addition via a SO2 step followed by a reactivation in situ step. These methods produced regenerated catalysts, which were active for the conversion of n-hexadecane. However, their high activity was not adequately sustained when compared to a fresh catalyst. Two oxidation/regeneration methods were also investigated. One oxidation method was successful in producing a catalyst that could undergo multiple regenerations with the restoration of the activity to the level of the fresh catalyst and the retention of that activity over a period of time that was comparable to, or exceeded, that of the fresh catalyst.

Carbon-14 tracer study of the conversion of labeled n-propylcyclopentane during n-octane aromatization with a Pt-zeolite L catalyst

n-Propylene cyclopentane or n-propylcyclopentane labeled in the ring with {sup 14}C was converted together with n-octane using a Pt-KL zeolite catalyst operating at 482 C and ca. 14 bar. The products indicate that hydrogenolysis to produce isooctanes, not ring expansion to produce aromatics, is the major reaction pathway for the alkyl cyclopentane compound. Dilution of the {sup 14}C activity in n-propylcyclopentane during the conversion shows that C{sub 5} as well as C{sub 6} cyclization occurs during the conversion of n-octane. The current data were obtained with a catalyst that has a Pt crystal size range that is similar to those reported earlier. Furthermore, the conversion data for n-octane and n-propylcyclopentane using the Pt-KL zeolite catalyst are very similar to data obtained with catalysts based on other nonacidic supports where the Pt crystals cannot be located in a zeolite type channel. Thus, for n-octane conversion, it appears that the Pt in L zeolite catalysts has selectivities that are similar to Pt on other nonacidic supports.

A Comparison of Fischer-Tropsch Synthesis in a Slurry Bubble Column Reactor and a Continuous Stirred Tank Reactor

A Slurry Bubble Column Reactor (SBCR) is a gas-liquid-solid reactor in which the finely divided solid catalyst is suspended in the liquid by the rising gas bubbles. SBCR offers many advantages over fixed-bed type reactors such as: 1) improved heat transfer and mass transfer; 2) isothermal temperature profile is maintained; and 3) relatively low capital and operating cost. Fischer-Tropsch Synthesis (FTS) takes place in a SBCR where the synthesis gas is converted on catalysts suspended as fine particles in a liquid. The synthesis gas flows in a bubble phase through the catalyst/wax suspension. The volatile products are removed with unconverted gases, and the liquid products are separated from the suspension. A gas distributor located in the bottom of the reactor produces the bubbles in the reactor. A considerable interest has been expressed in using the SBCR to carry out FTS particularly for the conversion of stranded natural gas into liquids. Currently, the Center for Applied Energy Research (CAER) is utilizing a Prototype Integrated Process Unit (PIPU) system for scale-up research of the FTS. The purpose of this study was to compare the performance and activity decline of a precipitated Fe/K Fischer Tropsch Synthesis (FTS) catalyst in a revamped slurry bubble column reactor (SBCR) to that of previous CSTR and SBCR runs using the same catalyst and operating conditions. The activity decline measured in the revamped SBCR system was shown to be similar to that of the CSTR experiments. The apparent activity decline in a previous SBCR run was due a transient startup effect from the slurry filtration system.

Deactivation of Pt/ZrO2/SO4 Catalyst

Publisher Summary Interest in solid superacid catalysts has resulted from hydrocarbon conversion data, which suggest they have the requirements for an ideal hydrocracking catalyst. The addition of Pt to a sulfated ZrO2 catalyst has been shown to produce a catalyst that will isomerize n-butane and n-pentane at low temperatures. The production of isoalkanes, which are favored thermodynamically at low temperatures, have higher octane numbers and are suitable for the production of reformulated gasolines. There have been many studies on the preparation and activity measurements of these catalysts; however, few studies have been done to determine the deactivation characteristics of Pt/ZrO2SO4 catalysts other than the comparison of the activities of these catalysts with and without Pt in runs lasting over 100 hours. The presence of Pt reduced the coke precursors by its hydrogenation function. The on-going research at the Center for Applied Energy Research in the preparation, characterization, and activity testing of these catalysts has produced a number of continuous plug-flow and trickle bed reactor runs of 20 to over 600 hours duration using different hydrocarbon feedstocks. At the completion of experiments in which the activity, defined by conversion of the feedstock, has declined, experiments to determine if the acid and/or metal function was responsible for the observed deactivation were performed. The results of these experiments are presented in the chapter.