Gas Shift Research Articles

In Situ Characterization of CuFe2O4 and Cu/Fe3O4 Water−Gas Shift Catalysts

Mixtures of copper and iron oxides are used as industrial catalysts for the water−gas shift (WGS, CO + H2O → H2 + CO2). In-situ time-resolved X-ray diffraction, X-ray absorption fine structure, and atomic pair distribution function analysis were used to study the reduction of CuFe2O4 with CO and the behavior of CuFe2O4 and Cu/Fe2O3 catalysts under WGS reaction conditions. Metal↔oxygen↔metal interactions enhance the stability of Cu2+ and Fe3+ in the CuFe2O4 lattice, and the mixed-metal oxide is much more difficult to reduce than CuO or Fe2O3. Furthermore, after heating mixtures of CuFe2O4/CuO in the presence of CO or CO/H2O, the cations of CuO migrate into octahedral sites of the CuFe2O4 lattice at temperatures (200−250 °C) in which CuO is not stable. Above 250 °C, copper leaves the oxide, the occupancy of the octahedral sites in CuFe2O4 decreases, and diffraction lines for metallic Cu appear. From 350 to 450 °C, there is a massive reduction of CuFe2O4 with the formation of metallic Cu and Fe3O4. At this point, the sample becomes catalytically active for the production of H2 from the reaction of H2O with CO. Neutral Cu0 (i.e., no Cu1+ or Cu2+ cations) is the active species in the catalysts, but interactions with the oxide support cannot be neglected. These studies illustrate the importance of in situ characterization when dealing with mixed-metal oxide WGS catalysts.

Oxygen-Vacancy-Stabilized Positively Charged Au Nanoparticles on CeO2(111) Studied by Reflection−Absorption Infrared Spectroscopy

To examine the active form of Au on Au/CeO2 catalysts in the low-temperature water−gas shift (WGS) reaction, the effect of surface oxygen vacancies on CeO2(111) on the charged state of Au nanoparticles (or clusters) deposited at 85 K was studied by combination of structural investigation by scanning tunneling microscopy (STM) and charged state estimation by reflection−absorption infrared spectroscopy (RAIRS) using CO molecule as a probe. On a stoichiometric CeO2(111) without oxygen vacancies, only neutral Au species, which gave a ν(CO) peak at ∼2100 cm−1 for bound CO, were observed independent of preannealing temperatures up to 400 K. Introduction of surface oxygen vacancies by the electron stimulated desorption (ESD) process showed the majority of Au deposited at 85 K was positively charged, giving ν(CO) peaks at approximately 2130 and 2145 cm−1 bound to positively charged Au at different degree. Thermal stability measurements for the species in vacuum revealed that less positively charged Au species rem...

Noble Metal Core−Ceria Shell Catalysts For Water−Gas Shift Reaction

The metal−support interface is known to play a key role in many catalysis reactions. Using the microemulsion technique we report the preparation of a nanosized noble metal particle with an intimate contact with cerium oxide. The materials display better catalytic performance in the water−gas shift (WGS) reaction over the hydrogen-rich reformate−steam mixture without methane (side product) formation as compared with catalysts prepared by traditional methods. It is evident that the catalysts prepared by the microemulsion method show an unusual morphology and unique catalytic properties with the noble metal nanoparticle being embedded in a thin layer of cerium oxide, giving a maximum metal−support interface of three-dimensional interactions. In light of results obtained from material characterization as well as reaction rate orders and switching experiment studies over the catalysts, a discussion on possible reaction mechanism(s) taking place at the interface of this new type of catalyst is included in this paper.

A potential ionic liquid for CO2-separating gas membranes: selection and gas solubility studies

The production of hydrogen from fossil fuels by steam reforming/water gas shift can be enhanced by separating the reaction byproduct, CO2, within the reactor as it is produced. Such a separation-enhanced reaction not only has higher conversion efficiency, but can also be considered a greener process which produces high-purity hydrogen with little CO2 contamination. Supported ionic liquid membranes may be able to achieve this separation task since they are known to have high CO2 and low H2 solubilities. In this study, the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide family of ionic liquids has been selected for this purpose, based on limited literature data. The solubilities of major reaction gases, namely CO2, H2, CO, and CH4, in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared to one another. In addition, the solubilities of CO2 and H2 in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared. The results indicate, from a thermodynamic point of view, the possibility of using this family of ionic liquids as separation membranes with practical CO2/H2 selectivities.

Effect of Reaction Temperature on the Performance of Thermal Swing Sorption-Enhanced Reaction Process for Simultaneous Production of Fuel-Cell-Grade H2 and Compressed CO2 from Synthesis Gas

A novel cyclic thermal swing sorption-enhanced reaction (TSSER) process concept was recently proposed for the simultaneous production of fuel-cell-grade H2 and compressed CO2 from synthesis gas containing CO and H2O. The process carried out the catalytic water−gas shift (WGS) reaction (CO + H2O ↔ CO2 + H2) with simultaneous removal of CO2 from the reaction zone by a reversible, water-tolerant, CO2-selective chemisorbent in order to circumvent the thermodynamic limitation of the WGS reaction and enhance the rate of the forward reaction. The chemisorbent was periodically regenerated using the principles of thermal swing adsorption by purging the sorber−reactor with superheated steam at different pressures and temperatures. Several intermediate process steps were employed to produce a pure and compressed CO2 byproduct during the thermal desorption process. The present work reports (a) new experimental data demonstrating the concept of the sorption-enhanced WGS reaction at different temperatures using a commercial WGS catalyst and Na2O-promoted alumina as the CO2 chemisorbent and (b) the effect of the sorption−reaction temperature on the TSSER process performance estimated by model simulation. Relatively slower kinetics of the sorption-enhanced WGS reaction imposes a lower bound (∼200 °C), whereas the thermal stability of the chemisorbent and the use of carbon steel sorber−reactors set the upper bound (∼550 °C) of temperatures for practical operation of the TSSER process. Simulated process performances (sorption−reaction at 200 and 400 °C and regeneration at 550 °C) show that the operation of the sorption−reaction step at 200 °C increases the H2 and CO2 productivities of the process by ∼38% and 35%, respectively, without changing (a) the number of moles of H2 produced per mole of CO in the feed gas or (b) the net CO2 recovery as a compressed byproduct gas. The total steam duty for the sorbent regeneration increases by ∼14% for operation at the lower sorption−reaction temperature. Another major benefit of operation at the lower reaction temperature is a very large increase in the pressure of the CO2 byproduct (e.g., 40 and 21 atm at 200 and 400 °C, respectively) when the reactor feed gas contained 20% CO + 80% H2O at a total pressure of 15 atm.

Catalytic Carbon Dioxide Reforming of Methane to Synthesis Gas over Activated Carbon Catalyst

The catalytic activity and kinetic behavior of catalytic reforming of methane with carbon dioxide over activated carbon were investigated as a function of reaction temperature, gas hourly space velocity (GHSV), and partial pressures of CH4 and CO2. The CH4 and CO2 conversions were greatly influenced by the reaction temperature in the range of 850−1050 °C. The apparent activation energies for CH4 and CO2 consumption and CO and H2 production were 32.63 ± 1.06, 25.54 ± 1.79, 24.81 ± 3.06, and 32.99 ± 2.58 kcal/mol, respectively. The curves of reaction rates versus GHSV showed various trends at different temperatures and indicated 7500 mL/h·g-cat was sufficient for operation in the kinetic regime. The reaction rate of methane and carbon dioxide over activated carbon was affected significantly by the partial pressures. Under a higher CO2 pressure, the excess CO2 reacted with H2 through the reverse water−gas shift (RWGS) reaction. The predictions of the CH4 and CO2 reaction rates based on a semiexperimental for...

Modeling of Multiple Cycles for Sorption-Enhanced Steam Methane Reforming and Sorbent Regeneration in Fixed Bed Reactor

Mathematical models of multiple cycles for sorption-enhanced steam methane reforming and Ca-based sorbent regeneration in a fixed bed reactor are developed. Empirical correlations are used to describe steam methane reforming, the water−gas shift, and CO2 capture kinetics. The processes of multiple carbonation/calcination cycles of a Ca-based sorbent are taken into account to describe the sorption-enhanced hydrogen production process with the simultaneous removal of carbon dioxide by a Ca-based sorbent. The mathematical models are validated through comparing simulated results with experimental data. The model results qualitatively agree with experimental data. The effect of reactivity decay of dolomite, CaO/Ca12Al14O33, and limestone sorbents on sorption-enhanced hydrogen production and sorbent regeneration processes was studied through numerical simulation. The simulated results indicated that the operation time of producing high-purity hydrogen (that is defined as the time for the start of breakthrough, ...

Simulation and Modeling Study of Anode-Supported Planar SOFC Operating with Synthesis Gas

A two-dimensional model of an anode-supported planar SOFC operating with synthesis gas was developed based on co-flow and counter-flow configurations. Mass, energy and charge transport equations coupled with electrochemical reactions (H2, CO oxidation and O2 reduction) as well as chemical (water- gas shift) reaction were considered in the model. The performance comparison obtained from the model reveals that the cell performance for counter-flow configuration is higher than that of co-flow configuration. Unfortunately, due to the high temperature gradient at the fuel inlet region, the counter-flow configuration may not be the best mode of operation.

Continuous Production of Hydrogen from Sorption-Enhanced Steam Methane Reforming in Two Parallel Fixed-Bed Reactors Operated in a Cyclic Manner

Hydrogen with purity higher than 90% was produced continuously by sorption-enhanced steam methane reforming reactions in two parallel fixed-bed reactors operated in a cyclic manner. The process involved sorption-enhanced steam methane reforming reactions and a sorbent regeneration reaction. First, through addition of a CO2 sorbent into a reforming reactor, the reactions of reforming, water−gas shift, and CO2 sorption were combined, and more CH4 was expected to convert H2 in one reactor. Second, regeneration of the sorbent was carried out in the other reactor. The hydrogen production and sorbent regeneration processes were carried out simultaneously in the two fixed-bed reactors, operated in a cyclic manner by switching a methane/steam feed and an Ar-containing feed between the two reactors at a fixed feed switchover time. The critical value of the feed switchover time for achieving the production of higher concentrations of hydrogen was investigated and analyzed.

Electrical efficiencies of methane fired, high- and low- temperature fuel cell power plants

The important system difference between power plants based on low temperature and high temperature fuel cells is that gas reforming and shift conversion is thermally decoupled from the cell in low temperature cell power plants whereas the gas process steps are performed at close to the elevated fuel cell temperatures in high temperature fuel cell power plants. This article elucidates the consequences: assuming equal electrical efficiencies for the respective cells (50%) it is shown that thermal decoupling leads to energy and exergy losses and sizably lower electrical system efficiencies because heat for the generation of the process steam necessitates the combustion of methane. Also hydrogen losses in the step for preferential oxidation of carbon monoxide (Selox process) and several heat transfer steps add to the lower efficiency of low temperature systems. Low temperature fuel cell power plants need 15–17% more fuel than high temperature fuel cell power plants for the same amount of electric energy. The theoretical comparison of an adiabatic LT and HT fuel cell process reveals that, with postulated electrical cell efficiencies of 50%, the theoretical electrical efficiency of the LT process is 6–7% points lower than that for the HT-process (35 vs. 41%). For exergy efficiencies also taking into account rejected heats, the numbers read 43 and 58%.

Weighting Variation of Water−Gas Shift in Steam Reforming of Methane over Supported Ni and Ni−Cu Catalysts

In this work, catalysts of 2 wt % Ni supported on samaria-doped ceria (SDC), gadolinia-doped ceria, and α-Al2O3, as well as SDC-supported Ni−Cu catalysts with 0.5 wt % Ni or Cu and addition of 0.01−0.1 wt % Cu or Ni were prepared. Activity tests for steam reforming of methane over these catalysts were carried out at temperatures of 673−823 K. A method for analyzing the weighting of the water−gas shift (WGS) activity in steam reforming is proposed. The effects of temperature, support, and Ni−Cu ratio on the WGS weighting variations were studied. The results indicate that the weighting of the WGS activity decreases with increasing temperature. The extent of the variation of the WGS weighting is larger with doped ceria as the support than with α-Al2O3. The addition of Cu into a Ni catalyst enhances the WGS activity, and this enhancement effect can be quantitatively related to the amount of the bimetallic Cu−Ni species. The simple analysis of WGS weighting is able to give some ideas on the operation and catal...

Methane Reforming with Carbon Dioxide. The Behavior of Pd/α-Al2O3 and Pd−CeOx/α-Al2O3 Catalysts

The catalytic reforming of methane with carbon dioxide on Pd (∼1%)/α-Al2O3 and Pd (∼1%)−CeOx/α-Al2O3 catalysts was studied at 627 °C using a stoichiometric feed mixture. Characterization of fresh and used samples was carried out by TEM and TG analyses. Preliminary experiments in the 600−730 °C temperature range demonstrated that, on Pd/α-Al2O3, the conversion of CO2 was larger than that of CH4 because of the reverse water−gas shift (RWGS) reaction, although the CO/H2 ratio was close to 1. This behavior is attributed to the predominant CH4 decomposition reaction that leads to carbon deposition and catalyst deactivation. The decline in activity was also due to palladium sintering, a process favored by the reducing atmosphere generated by the presence of CO and H2. Kinetic information obtained in experiments carried out under differential conditions was used to evaluate the importance of mass- and heat-transport limitations. It is shown that intraparticle mass transfer and interparticle heat resistances can ...

Low-Pressure Sorption-Enhanced Hydrogen Production

The sorption-enhanced production of hydrogen involves the simultaneous steam−methane reforming, water−gas shift, and CO2 removal reactions in the presence of a reforming catalyst and CO2 sorbent. High concentrations of H2 with low concentrations of CO are produced in a single reaction step. Pure CO2 suitable for use or sequestration may be produced during sorbent regeneration. This study examined the sorption-enhanced H2 production reactions at low pressure, which also requires a low reaction temperature so that the sorbent will retain its effectiveness for CO2 capture. H2 concentrations in excess of 96 mol % (dry basis) and CO concentrations as low as 7 ppmv (dry basis) were produced in tests between 1 and 5 bar and 400 and 460 °C using a H2O-to-CH4 feed ratio of 3. Calcined Arctic SHB dolomite proved to be an effective CO2 sorbent that did not require pretreatment for sulfur removal.

Microkinetic Modeling for Water-Promoted CO Oxidation, Water−Gas Shift, and Preferential Oxidation of CO on Pt

A comprehensive surface reaction mechanism on Pt is presented that is capable of describing CO oxidation, H2 oxidation, water−gas shift (WGS), preferential oxidation (PROX) of CO, and the promoting role of H2O on CO oxidation reasonably well. This mechanism consists of a literature CO oxidation model, a surface reaction mechanism for H2 oxidation on Pt developed here, and coupling reactions between the CO and H2 chemistries included for the first time. Thermodynamic consistency, which is shown to be essential for WGS, is ensured in all steps of the entire mechanism. The CO−H2 coupling via the CO + OH reaction, which may involve direct CO2 formation, CO* + OH* ↔ CO2* + H*, as well as an indirect pathway via the carboxyl intermediate, is explored. It is shown that this coupling plays a significant role in capturing the promoting effect of H2O on the CO oxidation-temperature-programmed reaction experiments at low temperatures as well as the overall speed of the WGS and PROX reactions. With the parameters use...

Kinetics of Aqueous-Phase Reforming of Oxygenated Hydrocarbons: Pt/Al2O3 and Sn-Modified Ni Catalysts

Reaction kinetics studies were conducted on the aqueous-phase reforming of ethylene glycol to produce hydrogen at temperatures near 500 K over Pt/Al2O3 and Raney NiSn catalysts. Ethylene glycol reforming proceeds through similar mechanisms over Pt and NiSn catalysts, involving initial dehydrogenation of ethylene glycol, followed by C−C bond cleavage and water−gas shift. The initial dehydrogenation of ethylene glycol appears to be kinetically significant over Pt/Al2O3, whereas the subsequent rate of C−C cleavage appears to be kinetically significant over R−Ni14Sn. The reforming reaction is fractional order in the ethylene glycol concentration, because of the strong adsorption of the oxygenated reactant, and negative order in the system pressure, through product inhibition by adsorption of H2 and/or CO at high pressures. High selectivity for hydrogen production is achieved for gas-phase products over Pt/Al2O3, whereas the addition of Sn is necessary to avoid alkane formation by methanation over Ni-based catalysts. The rate of methane formation increases at high system pressures, suggesting that the R−Ni14Sn catalyst is still vulnerable to methanation at high H2 and CO2 partial pressures. The reforming of ethylene glycol is accompanied by significant production of acetic acid through bifunctional dehydrogenation/isomerization and dehydration/hydrogenation routes over the metal and support. These bifunctional routes can be used to produce long-chain alkanes or partially reduced chemical intermediates by appropriate use of catalyst−support combinations, catalyst modifiers, and process conditions.

Assessing High-Temperature Water−Gas Shift Membrane Reactors

A simple two-step microkinetic model for the high-temperature water−gas shift was developed using existing experimental data. This two-step redox model uses only three adjustable parameters, and it is capable of predicting the inhibitory effect of CO2 on the kinetics of the reaction. It was used to simulate the performance of an adiabatic membrane reactor for the water−gas shift where the membrane is based on Pd. The simulations show that excess steam in the feed is desirable to control the adiabatic temperature rise; a 3:1 ratio of steam to carbon monoxide was found to be near optimum. The simulations further suggest that the rate of reaction is the limiting process in the membrane reactor, not the permeation of hydrogen through the membrane. At 90% hydrogen yield, finding a perfect membrane would only reduce the reactor size by 12%, whereas eliminating the inhibitory effect of CO2 would reduce the reactor size by 76%.

Surface Structure Sensitivity of the Water−Gas Shift Reaction on Cu(hkl) Surfaces: A Theoretical Study

The surface structure sensitivity of the water−gas shift (WGS) reaction (CO + H2O → CO2 + H2) over the Cu(111), Cu(100), and Cu(110) surfaces has been studied by first-principles density functional calculations together with the UBI-QEP approach. The Cu(hkl) surfaces are simulated by the Cu10 and Cu14 cluster models. The selectivity of the WGS reaction on the well-defined single-crystal surfaces is closely associated with the differences in the dissociation energies of H2O on the metal surfaces. The trend in the calculated dissociation energies and activation barriers follows the order Cu(110) < Cu(100) < Cu(111), suggesting that the most efficient crystal surface for catalyzing the WGS reaction is Cu(110), closely followed by the Cu(100) surface, and that the more densely packed Cu(111) surface is the least active among the Cu(hkl) surfaces studied here. The present calculations are in good agreement with experimental observations.

Hydrogen Production Using Sorption-Enhanced Reaction

This study examined the sorption-enhanced production of H2 via the steam−methane reforming process using a mixture of Ni-based commercial reforming catalyst and Ca-based sorbent obtained from commercial dolomite. The rates of the reforming, water−gas shift, and CO2 removal reactions are sufficiently fast that combined reaction equilibrium was closely approached, allowing for >95 mol % H2 (dry basis) to be produced in a single step. A dolomite pretreatment procedure was developed to remove sulfur, which was necessary to avoid poisoning of the reforming catalyst. The multicycle durability of the catalyst−sorbent mixture was studied as a function of regeneration temperature and gas composition using a laboratory-scale fixed-bed reactor. Twenty-five-cycle tests showed only moderate activity loss under most of the regeneration conditions studied. The primary loss in activity was associated with the inexpensive sorbent instead of the more expensive catalyst.

Evaluation of Reaction Order and Activation Energy of Char Combustion by Shift Technique

Combustion of chars obtained from the pyrolysis of lignocellulosic materials at 1200 K has been studied using a TGA at different temperatures (583-670 K) and oxygen partial pressures (10-101 kPa). The kinetic parameters vary significantly depending on burn-off when calculated on the basis of multiple and independent combustion runs. The apparent reaction order increases from 0.64 to 0.95 and apparent activation energy increases from 116 to 125 kJmol−1, Experiments carried out with gas and temperature shifts suggest that the dependency of the kinetic parameters on bum-off are spurious consequences of changes in the internal area. The shift technique yields an oxygen reaction order of 0.5, which decreases slightly with bum-off, and an activation energy of 120 KJmol−1, which increases slightly with burn-off. These values are found for the combustion of nine chars irrespective of their panicle sizes, ash content or parent material. The data obtained are attributed to a strong pore diffusion control on a common intrinsic kinetics characterised by zero reaction order and the activation energy of 240 Umol−1.

Effect of Adding Co to MoS2/Al2O3 upon the Kinetics of the Water−Gas Shift

A microkinetic model for the kinetics of the water−gas shift over sulfided CoMo/Al2O3 catalysts was developed starting from a similar model for unpromoted sulfided Mo/Al2O3 catalysts. Co was found to promote the catalyst's activity only at low CO/H2O ratios; at high ratios the Mo catalyst was marginally more active than the CoMo catalyst. The most important difference between the two models was the strength of interactions between the surface and hydroxyl groups. The addition of Co increased the stability of hydroxyl groups relative to sulfhydryl groups, and at higher H2O concentrations this allowed oxidized surface sites to more readily participate in both steam adsorption and hydrogen desorption steps. The results are most easily reconciled in terms of a promotional model where the Co and Mo are in close proximity and the active sites are similar to sites on unpromoted Mo catalysts.