Reactive Water Sorbents for the Sorption-Enhanced Reverse Water–Gas Shift

Catalysis Letters - Sorption-enhanced reverse water–gas shift (SE-RWGS, here designated as ‘COMAX’) was studied with bifunctional reactive sorbents. First proof-of-concept is...     

Theoretical Study of the Water–Gas Shift Reaction on a Au/Hematite Model Catalyst

Topics in Catalysis - Using the density functional theory, the mechanism of the water–gas shift reaction has been investigated employing a model catalyst formed by a Au5 cluster supported on...     

Effect of Molybdenum on the Sulfur-Tolerance of Cerium–Cobalt Mixed Oxide Water–Gas Shift Catalysts

As traditional sources of energy become depleted, significant research interest has gone into conversion of biomass into renewable fuels. Biomass-derived synthesis gas typically contains concentrations ranging from ~30 to 600 ppm H₂S. H₂S is a catalyst poison which adversely impacts downstream processing of hydrogen for gas-to-liquid plants and the deactivation of water–gas shift catalysts by sulfur is typical. Novel catalysts are needed to remain active in the presence of sulfur in order to boost efficiency and mitigate costs. Previous studies have shown molybdenum to be active in concentrations of sulfur >300 ppm. Cobalt has been shown to be active as a spinel in concentrations of sulfur <240 ppm. Ceria has received attention as a catalyst due to its oxygen donating properties. In this study, mixed oxide catalysts were synthesized via Pechini’s method into various blends of metal oxide solutions. Activity testing at low steam-to-carbon ratios (1:1) produced near equilibrium conversions at a GHSV of 6,300 h^?1 and over a temperature range of 350–400 °C for a Ce–Co mixed oxide even after an 800 ppm sulfur treatment. The addition of molybdenum to the Ce–Co base had little effect on sulfur tolerance, but it did lead to a reduction in selectivity for methanation. Specific surface areas generally increased following the sulfur treatments and X-ray diffraction patterns confirmed that bulk sulfiding did not occur.     

Gold Catalysts Supported on Cerium–Gallium Mixed Oxide for the Carbon Monoxide Oxidation and Water Gas Shift Reaction

The synthesis, characterization and catalytic properties of gold supported on ceria, gallia and a cerium–gallium mixed oxide were investigated. The nanostructural characterization of the cerium–gallium support (nominal atomic composition Ce80Ga20) showed that gallium(III) cations are homogenously distributed into the ceria matrix by substituting cerium(IV) cations of the fluorite-type structure of ceria. Au was added to the supports by the deposition–precipitation method using urea. High Au dispersions were achieved for all the fresh materials (D > 60%). The CO oxidation and the water gas shift (WGS) reaction were tested on the whole set of catalysts. All the supported-gold catalysts showed high activity for the CO oxidation reaction. However, those containing gallium in their formulation deactivated due to gold particle sinterization. Au(2%)/CeO₂ was the most active material for the WGS reaction, and the Au(2%)/Ce80Ga20 was as active as a Au(3%)/Ce68Zr32 catalyst for CO oxidation, and even more active than the reference catalyst of the World Gold Council, Au(2%)/TiO₂.     

MgO–Al2O3 Mixed Oxides-Supported Co–Mo-Based Catalysts for High-Temperature Water–Gas Shift Reaction

Silica-supported phosphorus chloride has been proved to be an efficient and recyclable catalyst for Beckmann rearrangement of a variety of ketoximes and dehydration of various aldoximes in anhydrous THF under microwave irradiation. This protocol has advantages of high conversion, high selectivity, short reaction time, no environmental pollution, and simple work-up procedure.     

Low Temperature Water–Gas Shift/Methanol Steam Reforming: Alkali Doping to Facilitate the Scission of Formate and Methoxy C–H Bonds over Pt/ceria Catalyst

Doping Pt/ceria catalysts with the Group 1 alkali metals was found to lead to an important weakening of the C–H bond of formate and methoxy species. This was demonstrated by a shift to lower wavenumbers of the formate and methoxy ν(CH) vibrational modes by DRIFTS spectroscopy. Li and Na-doped Pt/ceria catalysts were tested relative to the undoped catalyst for low temperature water–gas shift and methanol steam reforming using a fixed bed reactor and exhibited higher catalytic activity. Steaming of formate and methoxy species pre-adsorbed on the catalyst surface during in-situ DRIFTS spectroscopy suggested that the species were more reactive for dehydrogenation steps in the catalytic cycle for the Li and Na-doped catalysts relative to undoped Pt/ceria. However, with increasing atomic number over the series of alkali-doped catalysts, the stability of a fraction of the carbonate species was found to increase. This was observed during TPD-MS measurements of the adsorbed CO₂ probe molecule by a systematic increase of a high temperature peak for a fraction of the CO₂ desorbed. This result indicates that alkali-doping is an optimization problem—that is, while improving the dehydrogenation rates of methoxy and formate species, the carbonate intermediate stability increases, making it difficult to liberate the CO₂. Infrared spectroscopy results of CO adsorbed on Pt and ceria suggest that the alkali dopant is located on, and electronically modifies, both the Pt and ceria components. The results not only lend further support to the role that methoxy and formate species play as intermediates in the catalytic mechanisms, but also provide a path forward for improving rates by means other than resorting to higher noble metal loadings.     

Low-Temperature Water–Gas Shift: Doping Ceria Improves Reducibility and Mobility of O-Bound Species and Catalyst Activity

Abstract  

A series of platinum loaded catalysts supported on cation (Me)-doped cerium dioxide (Me = Ba, La, Y, Hf and Zn) was prepared by co-precipitation of the Me-nitrates and impregnation of a Pt precursor. Low temperature water–gas shift activity depends on the nature of dopant employed, varying in the order of Ba > Y > Hf > La > undoped ceria > Zn. TPR-XANES measurements with flowing hydrogen reveal that adding dopants to ceria facilitate ceria reduction and increases the extents of both surface shell and bulk reduction of ceria. Experimental results confirm past theoretical models that dopants enhance both O-mobility and reducibility of ceria. DRIFTS measurements of the transient decomposition of formates in steam suggest that formate half-life follows the trend Zn > undoped ceria > La > Hf > Y > Ba, indicating that the formate decomposition rate is enhanced by the addition of most of the dopants tested. Taken together, the results suggest that dopant addition improves the WGS rate by increasing the O-mobility of O-bound associated intermediates. Therefore, less Pt and Ce, which are expensive, is required to achieve comparable levels of activity.     

Low Temperature Water–Gas Shift: Alkali Doping to Facilitate Formate C–H Bond Cleaving over Pt/Ceria Catalysts—An Optimization Problem

Doping Pt/ceria catalysts with alkali metals was found to lead to an important weakening of the formate C–H bond, as demonstrated by a shift to lower wavenumbers of the ν(CH) vibrational mode. However, with high alkalinity (∼2.5%Na or equimolar amounts of K, Rb, or Cs), a tradeoff was observed such that while the formate became more reactive, the stability of the carbonate species, which arises from the formate decomposition, was found to increase. This was observed by TPD-MS measurements of the adsorbed CO₂ probe molecule. Increasing the amount of alkali to levels that were too high also led to lower catalyst BET surface area, the blocking of the Pt surface sites as observed in infrared measurements, and also a shift to higher temperature of the surface shell reduction step of ceria during TPR. When the alkalinity was too high, the CO conversion rate during water–gas shift decreased in comparison with the undoped Pt/ceria catalyst. However, at lower levels of alkali, the abovementioned inhibiting factors on the water–gas shift rate were alleviated such that the weakening of the formate C–H bond could be utilized to improve the overall turnover efficiency during the water–gas shift cycle. This was demonstrated at 0.5%Na (or equimolar equivalent levels of K) doping levels. Not only was the formate turnover rate found to increase significantly during both transient and steady state DRIFTS tests, but this effect was accompanied by a notable increase in the CO conversion rate during low temperature water–gas shift.     

Comparative study on hydrogen-assisted “one-step” methane conversion over Pd–Co/SiO2 and Pt–Co/NaY catalysts

In our laboratory, methane conversion to higher hydrocarbons in “one-step” process under non-oxidative condition at low temperature was first introduced and investigated over Pd–Co/SiO₂ prepared by sol/gel method [Guczi et al., Catal. Lett. 54 (1998) 33] and over Pt–Co/NaY [Guczi et al., Stud. Surf. Sci. Catal. 119 (1998) 295] bimetallic catalysts. It was found that methane conversion in one-step process is at least 2.5 times higher than that measured in “two-step” process on the same catalysts. In the present work, the two-step and one-step processes are compared. It has been established that in one-step process when methane dissociation occurs in the presence of hydrogen containing helium, not only the production of higher hydrocarbons increases but also the selectivity is shifted towards larger molecules. Palladium–cobalt system proved to be more efficient than the corresponding platinum–cobalt catalysts.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Equilibrium and Kinetic Studies on Lithium Adsorption from Geothermal Water by λ-MnO2

The adsorption equilibria of lithium from geothermal water were investigated by using both powdery and granulated forms of λ-MnO₂ derived from spinel-type lithium manganese dioxide. Optimum amounts of adsorbents were 1.0 g adsorbent/L-geothermal water for powdery λ-MnO₂ and 6.0 g adsorbent/L-geothermal water for granulated λ-MnO₂. The adsorbents exhibited the promising adsorption capacities and their adsorption equilibria of lithium agreed well with the Langmuir adsorption isotherm model. The kinetic data of lithium adsorption have been evaluated using pseudo-first-order, pseudo-second-order kinetics models, as well as Elovich kinetic model. In addition, intra-particle diffusion model has been used for evaluating the kinetic data to evaluate the adsorption mechanism. The adsorption kinetic process was attributed to the gradual adsorption stage where intra-particle diffusion was found as the rate-controlling step.     

H2 and H2/CO oxidation mechanism on Pt/C, Ru/C and Pt–Ru/C electrocatalysts

The oxidation kinetics of H₂ and H₂ + 100 ppm CO were investigated on Pt, Ru and Pt–Ru electrocatalysts supported on a high-surface area carbon powder. The atomic ratios of Pt to Ru were 3, 1 and 0.33. XRD, TEM, EDS and XPS were used to characterize the electrocatalysts. When alloyed with ruthenium, a decrease in mean particle size and a modification of the platinum electronic structure were identified. Impedance measurements in H₂SO₄, at open circuit potential, indicated different mechanisms for hydrogen oxidation on Pt/C (Tafel–Volmer path) and Pt–Ru/C (Heyrowsky–Volmer path). These mechanisms also occur in the presence of CO. Best performances, both in H₂ and H₂ + CO, were achieved by the catalyst with the ratio Pt/Ru = 1. This is due to a compromise between the number of free sites and the presence of adsorbed water on the catalyst. For CO tolerance, an intrinsic mechanism not involving CO electroxidation was proposed. This mechanism derives from changes in the electronic structure of platinum when alloyed with ruthenium.     

Low Temperature Water–Gas Shift Reaction Over Alkali Metal Promoted Cobalt Carbide Catalysts

Low temperature water–gas shift (LT-WGS) was performed over various group I alkali metal (Li, Na, K, Rb, Cs) promoted cobalt carbide (Co₂C) catalysts at temperatures ranging from 453 to 573 K and atmospheric pressure. Cobalt carbide (Co₂C) was found to be active for the WGS reaction. The stability of the catalyst is related to the stability of the cobalt carbide phases under reaction conditions. Potassium promoted cobalt carbide catalysts exhibited higher activity and stability compared to the other alkali promoted catalysts for LT-WGS. X-ray diffraction analyses of fresh and used catalysts suggest that the origin of deactivation of the catalysts is primarily due to the chemical transition of cobalt from carbide to metal during WGS.     

Oxygen-enhanced water gas shift on ceria-supported Pd–Cu and Pt–Cu bimetallic catalysts

Aiming at enhancing H₂ production in water gas shift (WGS) for fuel cell application, a small amount of oxygen was added to WGS reaction toward oxygen-enhanced water gas shift (OWGS) on ceria-supported bimetallic Pd–Cu and Pt–Cu catalysts. Both CO conversion and H₂ yield were found to increase by the oxygen addition. The remarkable enhancement of H₂ production by O₂ addition in short contact time was attributed to the enhanced shift reaction, rather than the oxidation of CO on catalyst surface. The strong dependence of H₂ production rate on CO concentration in OWGS kinetic study suggested O₂ lowers the CO surface coverage. It was proposed that O₂ breaks down the domain structure of chemisorbed CO into smaller domains to increase the chance for coreactant (H₂O) to participate in the reaction and the heat of exothermic surface reaction helping to enhance WGS kinetics. Pt–Cu and Pd–Cu bimetallic catalysts were found to be superior to monometallic catalysts for both CO conversion and H₂ production for OWGS at 300 °C or lower, while the superiority of bimetallic catalysts was not as pronounced in WGS. These catalytic properties were correlated with the structure of the bimetallic catalysts. EXAFS spectra indicated that Cu forms alloys with Pt and with Pd. TPR demonstrated the strong interaction between the two metals causing the reduction temperature of Cu to decrease upon Pd or Pt addition. The transient pulse desorption rate of CO₂ from Pd–Cu supported on CeO₂ is faster than that of Pd, suggesting the presence of Cu in Pd–Cu facilitate CO₂ desorption from Pd catalyst. The oxygen storage capacity (OSC) of CeO₂ in the bimetallic catalysts indicates that Cu is much less pyrophoric in the bimetallic catalysts due to lower O₂ uptake compared to monometallic Cu. These significant changes in structure and electronic properties of the bimetallic catalysts are the result of highly dispersed Pt or Pd in the Cu nanoparticles.     

Fischer–Tropsch Synthesis: Effect of K Loading on the Water–Gas Shift Reaction and Liquid Hydrocarbon Formation Rate over Precipitated Iron Catalysts

The effect of K loading on the water–gas shift (WGS) reaction and hydrocarbon formation rate during Fischer–Tropsch synthesis (FTS) was studied over 100 Fe/5.1 Si/2 Cu/x K (x = 1.25 or 3) precipitated catalysts using a 1-L continuously stirred tank reactor. The catalysts were tested over a wide range of experimental conditions: 260–270 °C, 1.3 MPa, H₂/CO = 0.67 and 20–90 % CO conversions. On the low K loading (1.25 % K) Fe catalyst, the H₂ deficiency required for the FTS reaction was made up by the WGS reaction only at high CO conversion level, i.e. >70 %; however, increasing potassium loading to 3 % dramatically improved the WGS reaction rate which provided enough hydrogen for the FTS reaction even at low CO conversion level, i.e. 30 %. Kinetic analysis suggests that increasing K loading resulted in significant increases in the WGS rate constant relative to that of FTS, which is a major cause of the high WGS activity on the high K loading catalyst. Both the low and high potassium containing iron catalysts have high liquid oil and solid wax formation rates, i.e. 0.78–0.93 g/g-cat/h at 260 °C, 1.3 MPa, H₂/CO = 0.67 and 50 % CO conversion, but increasing potassium loading from 1.25 to 3 % shifted the primary product to wax (70 %) from oil (73.5 %). The wax fraction increased with increasing CO conversion for both iron catalysts. The effect of K loading on initial FTS activity and hydrocarbon distribution/selectivity of the Fe catalysts was also studied. High K loading, i.e. 3 % K, increased the iron carburization rate and significantly shortened the induction period of the FTS reaction. Secondary reactions of olefins were remarkably suppressed and the olefin content was greatly enhanced with increasing K loading from 1.25 to 3 %, consistent with a number of studies in the open literature.     

New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga₂O₃ and In₂O₃ exhibit high catalytic performance for the steam reforming of methanol, CH₃OH+H₂OCO₂+3HH₂, and the dehydrogenation of methanol to HCOOCH₃, 2CH₃OHHCOOCH₃+2HH₂. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.