Realistic Reformate Research Articles

Preferential Methanation of CO using Halogen‐Doped (F, Cl, Br) Ceria‐Supported Nickel Catalysts

AbstractHalogen‐doped Ni(F)/CeO2, Ni(Cl)/CeO2, and Ni(Br)/CeO2 catalysts were prepared by treating halogen‐free Ni/CeO2 with aqueous solutions of NH4F, NH4Cl, and NH4Br, respectively. It was demonstrated that the halogen dopants had distinct effects on the catalyst properties for the CO preferential methanation using realistic reformate gas: fluorine did not change catalytic activity, chlorine inhibited CO2 methanation activity and provided high selectivity towards CO methanation, and bromine completely inhibited both the CO and CO2 methanation activity.

Investigation of Operating Parameters in Conjunction with Catalyst Deactivation of the Water-Gas Shift Reactor in a Fuel Cell System

The operating characteristic of a water-gas shift (WGS) reactor in a fuel cell system was investigated. Deactivation of the catalyst was identified in previous experiments during steady state and after startup/ shutdown, therefore reasons for the deactivation need to be investigated. Experiments on the system level revealed a correlation between deactivation and the fuel used. According to this correlation, a combination of a high amount of aromatic hydrocarbons in the fuel and a high boiling range had a more significant effect than a high amount of aromatics alone, followed by the single effect of a high boiling range. Furthermore, the presence of higher hydrocarbons from incomplete fuel conversion in the autothermal reformer (ATR) was associated with catalyst deactivation of the WGS reactor. To obtain a deeper understanding of the influence of higher hydrocarbons on catalyst activity, experiments were conducted with two commercial noble metal WGS catalysts using ideal and realistic reformate. Results under low-temperature-shift (LTS) conditions showed that both catalysts were only slightly influenced by the amount of higher hydrocarbons added in the steady state, as well as under shutdown conditions.

Robust Au/Ce0.4Zr0.6O2Catalyst for Dynamic Shutdown/Startup of the Water-Gas Shift Reaction in Realistic Reformate with <1 % O2

Development of a water–gas shift (WGS) catalyst durable in dynamic shutdown/startup operation is essential to enable residential applications of H2-powered fuel-cell systems but remains a conundrum for heterogeneous catalysis. We demonstrate herein that by isomorphic substitution of Zr in the lattice of CeO2 to increase the acid density of the catalyst, complemented by dosing a small amount of O2 to realistic reformate to improve the oxidizing potential, the formation of the notorious site-blocking carbonate species on the Au catalyst can be efficiently retarded. This allows the formation of a highly robust Au/Ce0.4Zr0.6O2 WGS catalyst suitable for the challenging shutdown/startup operation in realistic reformate.

A comparative study of the deactivation mechanisms of the Au/CeO2 catalyst for water–gas shift under steady-state and shutdown/start-up conditions in realistic reformate

The deactivation mechanisms of the Au/CeO2 catalyst in steady-state and shutdown/start-up WGS reactions were investigated in realistic reformate at 250°C. Catalyst deactivation due to sintering was excluded. After steady-state operation, the original activity was not recovered by removing the deposited carbonate species. The influences of the component gases of realistic reformate on the activity suggest that loss of the Au–CeO2 interaction caused by the reducing H2 and CO is the main reason for catalyst deactivation. Under the shutdown/start-up condition, the catalyst suffered more drastic deactivation, although it underwent a lesser degree of reduction. A good correlation between the extent of deactivation and the amount of the carbonate species indicates that catalyst deactivation is mainly caused by enhanced formation of the carbonate species, especially through a combined effect of CO2 and H2O, during the low-temperature shutdown and start-up steps. The implications of these findings for improved applications of the Au/CeO2 catalyst in WGS are indicated.

Highly active copper catalyst for low-temperature water-gas shift reaction prepared via a Cu-Mn spinel oxide precursor

The effect of preparation method on structural properties and activity of Cu-Mn spinel oxide catalysts for the low-temperature water-gas shift reaction (LT-WGSR) has been studied. Single-step urea-combustion and coprecipitation procedures were used for synthesis of the catalysts. Catalyst characterization was performed by N2 physisorption, XRD, HRTEM, CO-TPR, CO-TPD and FTIR spectroscopy. The WGS activity was evaluated in a conventional flow reactor in the temperature range of 140–240°C. The influence of reaction gas mixture, including either idealized or realistic reformate, H2O/CO ratio and contact time on activity were investigated. The catalytic activity tests carried out with both idealized and realistic reformate demonstrated the superior performance of the catalyst prepared by the single-step urea-combustion method. Moreover, comparison of the WGS activity of these catalysts with the one of a commercial CuO-ZnO-Al2O3 catalyst points out the potential application of Cu-Mn spinel oxide catalysts in LT-WGSR. The results revealed that the urea-combustion synthesis method is more appropriate than coprecipitation for the preparation of active and stable Cu-Mn spinel oxide catalysts for LT-WGSR.

Effect of Cu loading on Cu/ZnO water–gas shift catalysts for shut-down/start-up operation

Cu/ZnO catalysts with Cu loadings of 44–5 wt% were prepared by coprecipitation and evaluated in temperature-dependant and shut-down/start-up water–gas shift (WGS) reactions using realistic reformate. These catalysts had similar Cu crystallite sizes, and the metallic Cu surface area and surface Cu content increased with the Cu loading. In temperature-dependent reactions, the CO conversion on the 25wt%Cu/ZnO catalyst slightly exceeded that on 44wt%Cu/ZnO. In shut-down/start-up operation, which is imperative for mobile and residential fuel cell applications, the catalysts with Cu loadings higher than 5 wt% suffered slight activity loss. Among them, the 15wt%Cu/ZnO catalyst deactivated the most reluctantly. As a result, after three shut-down/start-up cycles the CO conversion on 15wt%Cu/ZnO, 25wt%Cu/ZnO, and 44wt%Cu/ZnO became comparable. These results demonstrate the feasibility to lower the Cu loading without degrading the WGS performance of the Cu/ZnO catalyst in shut-down/start-up operation, which will guarantee the operation safety when the catalyst will be operated unattended for domestic small-scale fuel cell applications. Unexpectedly, the CO conversion was doubled on 5wt%Cu/ZnO after one shut-down/start-up cycle, which is interpreted as the redispersion of Cu nanoparticles based on transmission electron microscopy (TEM) and temperature-programmed reduction (TPR).

Applying a face-centered central composite design to optimize the preferential CO oxidation over a PtAu/CeO2–ZnO catalyst

The catalytic performance for the preferential oxidation of CO over a 1% (w/w) PtAu/CeO2–ZnO catalyst prepared by co-precipitation was investigated using a full 2k factorial design with three central points and a 95% confidence interval, in order to screen for the importance of the operating temperature (°C) and the H2O and CO2 contents (%) in the simulated reformate gas on the CO conversion and selectivity. The catalyst was characterized by TEM, BET, XRD and FTIR. The temperature and CO2 content had a significant influence on the conversion, whilst the selectivity depended on the temperature only. A face-centered central composite design was then used to evaluate the optimal conditions by simultaneously considering the maximal conversion, selectivity and constraints of the composition of realistic reformate gas. The difference in the estimated response and the experimental one was within ±2% and ±3% for routing simulated and realistic reformate gases, respectively.

Design, scale-out, and operation of a preferential CO methanation reactor with a nickel–ceria catalyst

Preferential CO methanation in a reformate gas was investigated over 10 wt.% Ni/CeO 2 pelleted catalyst in the fixed-bed reactor. It provided the reduction of the CO concentration in the reformate gas to less than 10 ppm over wide temperature interval (250–300 °C), while keeping hydrogen consumption relatively low. The design, scale-out, and operation of a preferential CO methanation reactor with integrated heat-exchanger were reported. The nickel–ceria catalyst was deposited onto metal gauzes, assembled into catalytic blocks. Direct contact of the catalyst with metal support provided high heat conductivity of the assembly and feasible temperature control upon variation of operation regimes. Nickel–ceria catalyst showed high activity and selectivity for the reaction of CO methanation in the presence of CO 2 excess. The preferential CO methanation reactor allowed the decrease of CO concentration to less than 20 ppm in realistic reformate generated by fuel processor via the reaction of methane steam reforming followed by CO water gas shift reaction.

Studies on Au catalysts for water gas shift reaction

Gold-supported catalysts on alumina and ceria were prepared by means of deposition–precipitation method at different pH and molarity of the precursor solution. The screening at the powder level in a fixed bed micro-reactor of the catalytic activity of the 3% Au prepared catalysts, in terms of CO conversion for the WGS reaction, highlighted that the catalysts on alumina were not so active (maximum conversion of 30%), despite a satisfactory gold deposition on the support. On the contrary, ceria-based catalysts displayed better performances. By feeding only CO and H 2O, with H 2O/CO ratio equal to 4, catalysts prepared at different pH and M = 1 × 10 −3 approached satisfactorily the equilibrium WGS conditions, in particular when pH = 8.5 was used. However, catalytic activity tests carried out with a realistic reformate feed (containing also H 2 and CO 2) showed fairly low CO conversions also at high temperature. Then, on this catalyst, tests at different weight space velocities WSV were carried out obtaining better performance by lowering WSV.

Activity, selectivity, and adsorbed reaction intermediates/reaction side products in the selective methanation of CO in reformate gases on supported Ru catalysts

The reaction behavior and mechanistic aspects of the selective methanation of CO over two supported Ru catalysts, a Ru/zeolite catalyst and a Ru/Al 2O 3 catalyst, in CO 2 containing reaction gas mixtures were investigated by temperature-screening measurements, kinetic measurements and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. The influence of other components present in realistic reformate gases, such as H 2O and high amounts of CO 2, on the reaction behavior was evaluated via measurements in increasingly realistic gas mixtures. Temperature screening and kinetic measurements revealed a high activity of both catalysts, with the Ru mass-normalized activity of the Ru/zeolite catalyst exceeding that of the Ru/Al 2O 3 catalyst by about one order of magnitude. Approaching more realistic conditions, the conversion–temperature curve was shifted slightly upwards for the Ru/Al 2O 3 catalyst, whereas for the Ru/zeolite catalyst it remained unaffected. The selectivity was highest for the Ru/zeolite catalyst, where in parallel to full conversion of CO the conversion of CO 2 remained below 10% over a 40 °C temperature window. During selective methanation on the Ru/Al 2O 3 catalyst, CO 2 was converted even though CO was not completely removed from the feed. Transient DRIFTS measurements, following the build-up and decomposition of adsorbed surface species in different reaction atmospheres and in the corresponding CO-free gas mixtures, respectively, provide information on the formation and removal/stability of the respective adsorbed species and, by comparison with the kinetic data, on their role in the reaction mechanism. Consequences on the mechanism and physical reasons underlying the observed selectivity are discussed.

Design, scale-out, and operation of a microchannel reactor with a Cu/CeO 2− x catalytic coating for preferential CO oxidation

A preferential CO oxidation device consisting of an array of 26 parallel microchannel reactors with each a 5 wt% Cu/CeO 2− x catalytic coating was designed for application with a 100 W e fuel cell system. The reactor device is able to reduce the carbon monoxide concentration in a realistic reformate gas from 1.5 vol.% to 10 ppm at an inlet oxygen to carbon monoxide ratio of 1.5, a WHSV of 240 L g −1 h −1, and within the temperature range of 230–240 °C. The device outperformes similar reactor designs as presented in the literature.

Gold-Supported Catalysts for Medium Temperature-Water Gas Shift Reaction

Water gas shift (WGS) reaction can lead to a reduction of the CO content in the H2-rich gas derived from HCs reforming to about 0.5–1%. Au-based catalysts supported on CeO2 and CeO2-based mixed oxides were prepared, characterized and tested for a potential application in a medium temperature WGS stage. In particular, catalytic activity tests performed with a realistic reformate stream (40% H2, 20% H2O, 11% CO2, 5% CO in He) showed how a 3% Au catalyst supported on CeO2 + ZrO2 reached equilibrium conditions at about 400 °C at WSV = 0.333 NL min−1 g cat −1 .

The effect of copper on the selective carbon monoxide oxidation over alumina supported gold catalysts

The selective oxidation of CO in the presence of H 2 was investigated on Au catalysts promoted with different amounts of Cu. Au catalysts were prepared by the deposition–precipitation method and exhibited a satisfactory activity at low temperature with adequate selectivity. A considerable improvement in CO conversion was achieved when the O 2/CO ratio was increased from the value of 0.5–1.0. The addition of Cu to Au/Al 2O 3 catalysts caused an increase in the selectivity to CO oxidation due to an interaction between Au and Cu on the surface of the catalysts. However, this beneficial effect was limited to an optimal content of Cu. The catalysts were characterized by temperature programmed reduction and DRS UV–vis spectroscopy, indicating the formation of small bimetallic Au–Cu particles. The presence of water vapor in the feed stream played a positive effect in the CO conversion and selectivity while the CO 2 presence diminished the CO conversion and selectivity. In the case of a realistic reformate, when both H 2O and CO 2 are present, the positive effect of H 2O was able to compensate the negative effect of CO 2 depending on the temperature of reaction.

PROX reaction over CuO–CeO2 catalyst with reformate gas containing methanol

Abstract In this study, the effect of methanol on the performance of a combustion-synthesized CuO–CeO 2 catalyst in the preferential CO oxidation (PROX) reaction has been investigated. Methanol acts as an inhibitor of CO and H 2 oxidation. This effect is more evident at high methanol concentrations and in the absence of CO 2 and H 2 O in the feed. Complete CO and methanol removal can be achieved at 200 °C for a realistic reformate stream containing 15% CO 2 , 10% H 2 O and 0.1% MeOH at a contact time of 0.30 g s cm −3 .

The Importance of Strongly Bound Pt–CeO x Species for the Water-gas Shift Reaction: Catalyst Activity and Stability Evaluation

We demonstrate ways to prepare active and stable Pt–CeO x catalysts for the water-gas shift reaction (WGSR). Various synthesis protocols are shown to work; the best being the coprecipitation/gelation method, which suppresses the crystal growth of ceria during calcination by the incorporation of some platinum in the bulk oxide. Metallic platinum nanoparticles are not necessary for an active WGSR catalyst. Reaction light-off occurs at ∼120 °C, where Pt2+ species bound to ceria are still present. No activation period, and no hysteresis phenomena were found. During reaction at reducing conditions, some Pt is reduced, but it is reoxidizable. The stability of these low-content (<2 at.%) Pt/CeO x catalysts is high even in realistic reformate gas streams. To avoid cerium(III) hydroxycarbonate formation at room temperature-shutdown with water condensation, a small amount (∼1%) of gaseous oxygen is added to the reaction gas mixture. Cyclic shutdown/startup operation is thus possible without catalyst degradation.

Influence of CO 2 and H 2 on the low-temperature water–gas shift reaction on Au/CeO 2 catalysts in idealized and realistic reformate

The effect of CO 2 and H 2 on the activity and deactivation of Au/CeO 2 catalysts in the low-temperature water–gas shift reaction was investigated by combined kinetic and in situ IR spectroscopic measurements, going stepwise from idealized to realistic reaction atmospheres. The variation of the reaction atmosphere results in considerable variations in the initial activity and deactivation behavior, with significantly lower activity in the presence of H 2 and more pronounced deactivation in CO 2-rich gases. In situ DRIFTS measurements reveal a complex interaction of the different components in the formation of adsorbed reaction intermediates and side products. The bidentate formate species (1588 and 2833 cm −1), which are dominant in H 2-free atmospheres and act as reaction intermediates, are replaced by less reactive bidentate (1560 and 2858 cm −1) and bridge-bonded (2933 and 2961 cm −1) formates in H 2-rich atmospheres. Deactivation is related mainly to the formation of thermally stable monodentate carbonates, which act as reaction-inhibiting spectator species. Their formation is enhanced by CO 2 in the reaction gas, whereas H 2 leads to their reactive removal by reaction to formates. Possible mechanisms for the deactivation process are discussed.

CO removal from realistic methanol reformate via preferential oxidation?performance of a Rh/MgO catalyst and comparison to Ru/$gamma;-Al2O3, and Pt/$gamma;-Al2O3

The performance of a 0.5 wt.% Rh/MgO catalyst for the oxidative removal of CO from H 2 -rich methanol reformate (1% CO, 65% H 2 , 10% H 2 O, rest CO 2 ) was investigated and compared to that of a conventional 0.5 wt.% Pt/γ-Al 2 O 3 catalyst and a 5 wt.% Ru/γ-Al 2 O 3 catalyst. Temperature screening experiments and 10–30 h activity measurements reveal a high activity and selectivity in the temperature region between 175 and 300 °C, in combination with a relatively low deactivation. Water vapor in the feed gas (≤10%) has little effect on the reaction characteristics. The low activity for the reverse water gas shift (RWGS) reaction, whose rate is more than four orders of magnitude lower than the rate for CO oxidation, and for the methanation reaction at temperatures below 300 °C allow to operate Rh/MgO at higher temperatures, around 250 °C, while for Pt/γ-Al 2 O 3 and Ru/γ-Al 2 O 3 the reaction temperatures are limited to 200 and 150 °C, respectively, due to the decrease in selectivity (Pt/γ-Al 2 O 3 ) and the onset of the methanation reaction (Ru/γ-Al 2 O 3 ) at higher temperatures. Model calculations, using kinetic data measured in realistic reformate, predict that the minimum amount of noble metal required for the complete removal of CO from the feed gas (≤10 ppm) is by two orders of magnitude lower for Rh/MgO operated at 250 °C than for Pt/γ-Al 2 O 3 and Ru/γ-Al 2 O 3 catalysts at 200 and 150 °C, respectively, at a comparable O 2 excess. Due to the low RWGS activity the CO exit concentration does not increase significantly above the tolerance limit of state-of-the-art Pt–Ru fuel cell anodes of 100 ppm under dynamic load conditions, down to 1% load. For the other two catalysts the CO exit concentration are significantly higher under these conditions, reaching values above 500 ppm for Ru/γ-Al 2 O 3 at 1% load and the WGS equilibrium value of 2000 ppm for Pt/γ-Al 2 O 3 . The predictions are verified by measurements in a microreactor under similar reaction conditions. The reaction characteristics make Rh/MgO an ideal PROX catalyst for reaction at higher temperatures (250 °C), in particular for applications requiring highly dynamic operation under variable load conditions.

Selective oxidation of CO on Ru/γ-Al2O3 in methanol reformate at low temperatures

The preferential oxidation of CO (PROX) at low temperatures, 80–120°C, over a Ru/γ-Al2O3 supported catalyst pretreated by a low temperature reduction (LTR) process was investigated over a range of CO partial pressures and O2 excess in a realistic methanol reformate reaction environment, in the presence of CO2 and H2O. Based on transmission electron microscopy (TEM) imaging the LTR pretreatment, which involves reduction in pure H2 at 150°C, does not lead to measurable particle growth (d̄Ru=2.5nm), in contrast to the calcination/reduction (CR) pretreatment at 350°C used previously, which results in considerable particle sintering (d̄Ru=11.2nm).Kinetic parameters including the apparent activation energy (E≈48kJ/mol), reaction orders for CO (αCO) and for O2 (αO2) and the selectivity for CO oxidation as well as the influence of the co-reactants CO2 and H2O were evaluated. The results show a significant activity and selectivities of around 55–60% both in idealized reformate, in the absence of CO2 and H2O, and in realistic reformate, already at 80°C. This and the negligible activity for the reverse water gas shift and the CO and CO2 methanation reaction under these reaction conditions make this catalyst suitable for application at low temperatures typical for operation of polymer electrolyte fuel cells (PEFCs), in contrast to the CR pretreated catalyst, where higher process temperatures are required.

CO removal from realistic methanol reformate via preferential oxidation—performance of a Rh/MgO catalyst and comparison to Ru/γ-Al2O3, and Pt/γ-Al2O3

The performance of a 0.5wt.% Rh/MgO catalyst for the oxidative removal of CO from H2-rich methanol reformate (1% CO, 65% H2, 10% H2O, rest CO2) was investigated and compared to that of a conventional 0.5wt.% Pt/γ-Al2O3 catalyst and a 5wt.% Ru/γ-Al2O3 catalyst. Temperature screening experiments and 10–30h activity measurements reveal a high activity and selectivity in the temperature region between 175 and 300°C, in combination with a relatively low deactivation. Water vapor in the feed gas (≤10%) has little effect on the reaction characteristics. The low activity for the reverse water gas shift (RWGS) reaction, whose rate is more than four orders of magnitude lower than the rate for CO oxidation, and for the methanation reaction at temperatures below 300°C allow to operate Rh/MgO at higher temperatures, around 250°C, while for Pt/γ-Al2O3 and Ru/γ-Al2O3 the reaction temperatures are limited to 200 and 150°C, respectively, due to the decrease in selectivity (Pt/γ-Al2O3) and the onset of the methanation reaction (Ru/γ-Al2O3) at higher temperatures.Model calculations, using kinetic data measured in realistic reformate, predict that the minimum amount of noble metal required for the complete removal of CO from the feed gas (≤10ppm) is by two orders of magnitude lower for Rh/MgO operated at 250°C than for Pt/γ-Al2O3 and Ru/γ-Al2O3 catalysts at 200 and 150°C, respectively, at a comparable O2 excess. Due to the low RWGS activity the CO exit concentration does not increase significantly above the tolerance limit of state-of-the-art Pt–Ru fuel cell anodes of 100ppm under dynamic load conditions, down to 1% load. For the other two catalysts the CO exit concentration are significantly higher under these conditions, reaching values above 500ppm for Ru/γ-Al2O3 at 1% load and the WGS equilibrium value of 2000ppm for Pt/γ-Al2O3. The predictions are verified by measurements in a microreactor under similar reaction conditions.The reaction characteristics make Rh/MgO an ideal PROX catalyst for reaction at higher temperatures (250°C), in particular for applications requiring highly dynamic operation under variable load conditions.

Selective oxidation of CO on Ru/$gamma;-Al2O3 in methanol reformate at low temperatures

The preferential oxidation of CO (PROX) at low temperatures, 80–120 °C, over a Ru/γ-Al2O3 supported catalyst pretreated by a low temperature reduction (LTR) process was investigated over a range of CO partial pressures and O2 excess in a realistic methanol reformate reaction environment, in the presence of CO2 and H2O. Based on transmission electron microscopy (TEM) imaging the LTR pretreatment, which involves reduction in pure H2 at 150 °C, does not lead to measurable particle growth (d̄Ru=2.5 nm), in contrast to the calcination/reduction (CR) pretreatment at 350 °C used previously, which results in considerable particle sintering (d̄Ru=11.2 nm). Kinetic parameters including the apparent activation energy (E≈48 kJ/mol), reaction orders for CO (αCO) and for O2 (αO2) and the selectivity for CO oxidation as well as the influence of the co-reactants CO2 and H2O were evaluated. The results show a significant activity and selectivities of around 55–60% both in idealized reformate, in the absence of CO2 and H2O, and in realistic reformate, already at 80 °C. This and the negligible activity for the reverse water gas shift and the CO and CO2 methanation reaction under these reaction conditions make this catalyst suitable for application at low temperatures typical for operation of polymer electrolyte fuel cells (PEFCs), in contrast to the CR pretreated catalyst, where higher process temperatures are required.