James G. Goodwin

Effect of zirconia-modified alumina on the properties of Co/γ-Al2O3 catalysts

Abstract Zr promotion has been shown to improve the activity of Co/SiO 2 and Co/Al 2 O 3 Fischer–Tropsch synthesis (FTS) catalysts. However, little is known about how it works. The focus of this study was to investigate the impact of Zr modification of alumina used to support Co catalysts. Zr was first impregnated into alumina to produce Zr-modified alumina supports containing 2–11xa0wt% of ZrO 2 in the final catalyst. Co catalysts having 20xa0wt% Co were then prepared from these supports by incipient wetness. It was found that Zr modification had a significant impact on the catalyst properties: FTS rate per gram of catalyst increased significantly (>65% at steady state) with Zr modification and Co reducibility, especially during standard reduction, also increased. H 2 chemisorption, however, was found to be essentially constant with Zr modification. The impact of Zr modification is likely due to stabilization of the alumina support, prevention or blockage of Co surface “aluminate” formation, and an increase in Co reducibility to the active catalytic metallic phase. SSITKA results for CO hydrogenation showed an increase in the number of active surface intermediates ( N M ) with Zr modification while the intrinsic activity (1/ τ M ) remained constant, confirming that the major impact of Zr was in increasing the concentration of active Co surface sites. Because of the inconsistency of the turnover frequency calculated based on H 2 chemisorption with the 1/ τ M results, it is suggested that the standard adsorption conditions usually used for Co may not be adequate for modified Co catalysts such as the ones studied here.

Synthesis and characteristics of MCM-41 supported CoRu catalysts

Abstract Supported CoRu catalysts have been prepared with ordered mesoporous silica (MCM-41) as the support using the incipient wetness impregnation method in order to study the effect of ordered mesoporous silica and pore size on cobalt dispersion, reduction behavior, and catalytic properties for the Fischer–Tropsch synthesis (FTS). Metal loadings of 2, 5, 8, and 14xa0wt.% Co with 0.5xa0wt.% Ru were investigated, as well as two different mesoporous silicas having average pore diameters of 3 and 7xa0nm. For comparison purposes, conventional amorphous silica (SiO2) supported CoRu catalysts were also prepared using the same procedure. Due probably to higher concentration of water vapor in the small pores of MCM-41 during metal reduction, CoRu/MCM-41 catalysts showed stronger interaction of Co with the support than CoRu/SiO2 as manifested by lower reducibilities of the catalysts during standard reduction. The Co dispersions were similar on MCM-41 to that on amorphous silica for a given Co loading. MCM-41 supported catalysts exhibited similar selectivities for FTS as to the SiO2-supported ones; however, they had in general greater activities. There was an absence of any obvious pore size effect on product selectivity, probably due to the pores being sufficiently large for the reaction to easily proceed at 1xa0atm and 220xa0°C, the reaction conditions used. Thus, MCM-41 supported CoRu catalysts appear to be suitable for application to FTS. Their unique and tailored properties offer interesting possibilities for catalyst design and application in particular cases.

Co-support compound formation in Co/Al2O3 catalysts: effect of reduction gas containing CO

Abstract The effect of the addition of CO during H2 reduction on Co-support compound formation in a Co/γ-Al2O3 catalyst was investigated. It is known that such compound formation can occur during catalyst reduction, is facilitated by the presence of water vapor, and results in a less active catalyst. In this study, a H2 flow containing CO (1–9xa0vol.%) was used for standard reduction. Water vapor was also added (3xa0vol.%) in one series of experiments in order to examine the impact of CO when relatively high partial pressures of water vapor are also present. After reduction at various conditions, the pretreated catalyst samples were characterized and CO hydrogenation (H2/CO=10/1, 220xa0°C, 1.8xa0atm) also performed. Both initial and steady-state rates during CO hydrogenation went through a maximum for the addition of 3–5xa0vol.% CO during standard reduction. The maximum rate was about four times that in the case where no CO was added. A similar trend was found even where reduction occurred with a high partial pressure of water vapor. It is concluded that the addition of CO during reduction has a significant effect on activity of the catalyst due to increases in both Co reducibility and dispersion.

Characteristics of the active sites on sulfated zirconia for n-butane isomerization

Abstract n -Butane isomerization on sulfated zirconia (SZ) has been studied with the goal of increasing our understanding of the nature of the active sites and the role of the acid (Bronsted and Lewis) sites. Isotopic transient kinetic analysis (ITKA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), butene addition, CO addition, and pretreatment of the catalyst at different temperatures have been used to investigate the nature of the active sites involved in the formation of reaction intermediates and of isobutane. It was found that CO, added from the beginning of the reaction, inhibited the formation of isobutane but it did not prevent formation of carbon from n -butane, resulting in a deactivation of the catalyst proportional with the exposure time to CO. Butene added for 2 min at the beginning of the reaction increased the reaction rate and was involved in the formation of isobutane during multiple turnovers as a result of creation of olefinic modified sites. Addition of CO (for different period of times) along with butene (for 2 min at the beginning of the reaction) did not prevent the formation of the olefinic modified sites when CO was added for a short time. However, such sites participated in the formation of isobutane only after the CO feed was stopped and it desorbed from the surface of the catalyst. These results indicate that Lewis acid sites are not involved in the formation of either deactivating carbon or the olefinic modified sites. Pretreatment of SZ at 500xa0°C improved the activity of SZ due to an increase in the concentration of surface intermediates, N iso-C4 , compared to when it was pretreated at 200 and 315xa0°C and the Bronsted/Lewis acid sites ratio was greater. The results of this study support the hypothesis that the “active centers” for reaction are probably a combination of Bronsted and Lewis acid sites.

Investigation of the initial rapid deactivation of platinum catalysts during the selective oxidation of carbon monoxide

Abstract This paper reports on a study using isotopic transient kinetics analysis (ITKA) to understand the initial rapid deactivation behavior of a 5xa0wt% Pt/ γ -Al 2 O 3 catalyst during selective CO oxidation in hydrogen. The Pt catalyst exhibited the rapid deactivation typically seen for such catalysts during the initial reaction period. Based on ITKA results, the pseudo-first-order intrinsic rate constant was found to be relatively constant with time on stream while the concentration of surface intermediates leading to CO 2 decreased significantly. It can be concluded that the deactivation of this Pt catalyst is mainly the result of a decrease in the concentration of surface intermediates as a result of carbon deposition, not a change in the intrinsic site activity.

Effect of H2 partial pressure on surface reaction parameters during CO hydrogenation on Ru-promoted silica-supported Co catalysts

Abstract Steady-state isotopic transient kinetic analysis (SSITKA), one of the most powerful techniques for the investigation of surface reactions, was used to study the effect of hydrogen partial pressure on the fundamental surface reaction parameters for methanation on ordered mesoporous silica (MCM-41) and amorphous SiO 2 -supported CoRu catalysts. The abundances, coverages, and lifetimes of surface intermediates of the reaction were measured under reaction conditions and their dependence upon hydrogen partial pressure was determined. Although absolute hydrogen coverage under reaction conditions is not measurable due to the hydrogen isotope effect, relative hydrogen surface concentration as a function of P H 2 could be estimated from SSITKA parameters. Increasing the hydrogen partial pressure at a constant reaction temperature of 220xa0°C not only caused the expected increase in the relative surface concentration of hydrogen but also increased the abundance of surface methane intermediates ( N M ), possibly due to increased hydrogenation. The impact of P H 2 on N M for MCM-41-supported CoRu catalysts was similar to that for SiO 2 -supported ones, showing an approximately twofold increase in N M as P H 2 increased from 0.23 to 1.71 bar. The relative concentration of surface hydrogen, however, increased fourfold. The abundance of surface methane intermediates and the surface coverages were significantly higher for the MCM-41-supported CoRu catalysts. The average surface reaction residence time of the methane intermediates ( τ M ) consistently decreased with increasing hydrogen partial pressure due to the fact that the pseudo first order rate constant (1/ τ M ) contains the hydrogen surface concentration term. There was no difference, however, in the intrinsic site activity since the average surface reaction residence times of methane intermediates ( τ M ) for SiO 2 - and MCM-41-supported CoRu catalysts were essentially identical for a given partial pressure of hydrogen, regardless of Co loading. This also indicates that the type of silica support used (amorphous SiO 2 or MCM-41) did not have an impact on surface hydrogen concentration, contrary to the case for H 2 chemisorption at 100xa0°C. The increase in rate with increasing hydrogen partial pressure resulted due to the increase in methane surface intermediates and, more importantly, the increase in hydrogen surface concentration.

Passivation of a Co-Ru/γ-Al2O3 Fischer-Tropsch catalyst

Abstract Passivation of highly dispersed metal catalysts after reduction is necessary prior to exposure to air due to the exothermicity of metal oxidation. This exothermicity can result in a significant increase in temperature of the catalyst resulting in catalyst degradation and a potential fire hazard. This paper reports the results of a study of passivation of Ru-promoted Co/alumina. Passivations using CO and CO+H2 mixtures were compared to the standard method of passivation using small concentrations of O2. Passivation by CO+H2 resulted in a lower temperature rise upon exposure to air than oxygen passivation. Passivation using CO/H2=10 resulted in a catalyst whose catalytic activity for CO hydrogenation was able to be recovered after exposure to air by re-reduction similar to after oxygen passivation. CO passivation yielded a catalyst that was not able to be as well recovered upon re-reduction, probably due to the formation of graphitic carbon. Exposure of the CO/H2 passivated catalyst to air for at least 90xa0min actually made it easier to recover the original activity upon re-reduction. This is probably related to the oxidation of the carbidic passivation layer during air exposure.

Synergy of Components in CuZnO and CuZnO/Al2O3 on Methanol Synthesis: Analysis at the Site Level by SSITKA

In the present study, the effects of the individual components and an Al2O3 support on CuZnO for methanol (MeOH) synthesis were investigated at the site level for the first time using steady-state isotopic transient kinetic analysis and reaction at 250xa0°C and 1.8xa0atm. The presence of ZnO was found to decrease the hydrocarbon (CH4 primarily) formation ability of Cu. By comparing the surface reaction parameters, it could be shown that Cu and ZnO catalysts (supported on Al2O3) exhibit lower MeOH formation rates compared to their combination in either CuZnO or CuZnO/Al2O3 due, especially, to lower intrinsic “site” activities. The synergy between Cu and ZnO was most obvious in the increase in MeOH TOFITK (a measure of site activity for MeOH formation), more than double that for Cu without ZnO. Al2O3 did not seem to impact MeOH synthesis in any way other than to increase dispersion of the CuZnO. However, it did furnish acid sites for the conversion of some MeOH to DME.

Final Technical Report: Effects of Impurities on Fuel Cell Performance and Durability

The main objectives of this project were to investigate the effect of a series of potential impurities on fuel cell operation and on the particular components of the fuel cell MEA, to propose (where possible) mechanism(s) by which these impurities affected fuel cell performance, and to suggest strategies for minimizing these impurity effects. The negative effect on Pt/C was to decrease hydrogen surface coverage and hydrogen activation at fuel cell conditions. The negative effect on Nafion components was to decrease proton conductivity, primarily by replacing/reacting with the protons on the Bronsted acid sites of the Nafion. Even though already well known as fuel cell poisons, the effects of CO and NH3 were studied in great detail early on in the project in order to develop methodology for evaluating poisoning effects in general, to help establish reproducibility of results among a number of laboratories in the U.S. investigating impurity effects, and to help establish lower limit standards for impurities during hydrogen production for fuel cell utilization. New methodologies developed included (1) a means to measure hydrogen surface concentration on the Pt catalyst (HDSAP) before and after exposure to impurities, (2) a way to predict conductivity of a Nafion membranes exposed to impurities using a characteristic acid catalyzed reaction (methanol esterification of acetic acid), and, more importantly, (3) application of the latter technique to predict conductivity on Nafion in the catalyst layer of the MEA. H2-D2 exchange was found to be suitable for predicting hydrogen activation of Pt catalysts. The Nafion (ca. 30 wt%) on the Pt/C catalyst resides primarily on the external surface of the C support where it blocks significant numbers of micropores, but only partially blocks the pore openings of the meso- and macro-pores wherein lie the small Pt particles (crystallites). For this reason, even with 30 wt% Nafion on the Pt/C, few Pt sites are blocked and, hence, are accessible for hydrogen activation. Of the impurities studied, CO, NH3, perchloroethylene (also known as tetrachloroethylene), tetrahydrofuran, diborane, and metal cations had significant negative effects on the components in a fuel cell. While CO has no effect on the Nafion, it significantly poisons the Pt catalyst by adsorbing and blocking hydrogen activation. The effect can be reversed with time once the flow of CO is stopped. NH3 has no effect on the Pt catalyst at fuel cell conditions; it poisons the proton sites on Nafion (by forming NH4+ cations), decreasing drastically the proton conductivity of Nafion. This poisoning can slowly be reversed once the flow of NH3 is stopped. Perchloroethylene has a major effect on fuel cell performance. Since it has little/no effect on Nafion conductivity, its poisoning effect is on the Pt catalyst. However, this effect takes place primarily for the Pt catalyst at the cathode, since the presence of oxygen is very important for this poisoning effect. Tetrahydrofuran was shown not to impact Nafion conductivity; however, it does affect fuel cell performance. Therefore, its primary effect is on the Pt catalyst. The effect of THF on fuel cell performance is reversible. Diborane also can significant affect fuel cell performance. This effect is reversible once diborane is removed from the inlet streams. H2O2 is not an impurity usually present in the hydrogen or oxygen streams to a fuel cell. However, it is generated during fuel cell operation. The presence of Fe cations in the Nafion due to system corrosion and/or arising from MEA production act to catalyze the severe degradation of the Nafion by H2O2. Finally, the presence of metal cation impurities (Na+, Ca 2+, Fe3+) in Nafion from MEA preparation or from corrosion significantly impacts its proton conductivity due to replacement of proton sites. This effect is not reversible. Hydrocarbons, such as ethylene, might be expected to affect Pt or Nafion but do not at a typical fuel cell temperature of 80oC. In the presence of large quantities of hydrogen on the anode side, ethylene is converted to ethane which is very nonreactive. More surprisingly, even more reactive hydrocarbons such as formic acid and acetaldehyde do not appear to react enough with the strong Bronsted acid sites on Nafion at such low temperatures to affect Nafion conductivity properties. These results clearly identify a number of impurities which can have a detrimental impact on fuel cell performance, although some are reversible. Obviously, fuel cells exposed to impurities/poisons which are reversible can recover their original performance capabilities once the impurity flow is stopped. Impurities with irreversible effects should be either minimized in the feed streams, if possible, or new catalytic materials or ion conductors will need to be used to minimize their impact.