Advanced nickel metal catalyst for water-gas shift reaction

A comparative study has been performed to investigate the effectiveness of a Ni metal catalyst before and after impregnation with potassium for the water-gas shift (WGS) reaction. The potassium-modified Ni metal is both more active and more selective for the WGS reaction than the unmodified Ni catalyst. Furthermore, there is no carbon deposition on the modified Ni catalyst. The amount of H₂ produced and the CO conversion via WGS over the potassium-modified Ni catalyst are higher than those for the commercial high-temperature shift (HTS) catalyst under severe experimental conditions (gas-hourly space velocity = 80 000 h⁻¹, CO 60% and H₂ 40%). The suppression of methanation over the modified Ni metal is attributed to the action of the incorporated potassium in increasing the density of the active hydroxyl group that takes part in the WGS reaction to form the intermediate.     

Hydrogen production via water-gas shift reaction over gold supported on Ni-based layered double hydroxides

Co-precipitated NiAl and NiMgAl layered double hydroxides (LDHs) were prepared at M²⁺/Al³⁺ molar ratio of 2.5/1 and subsequently promoted with gold targeting to be studied as catalysts and supports of gold particles in the hydrogen production via water-gas shift (WGS) reaction. Powder X-ray diffraction and N₂ physisorption before and after WGS tests were applied to investigate the impacts of Mg and Au on the structure and catalytic behavior of the systems. Partial replacement of Ni by Mg resulted in moderate activity of NiMgAl and Au/NiMgAl catalysts than NiAl analogues due to: (i) smaller Ni amount that could not supply sufficient number catalytically active sites; (ii) higher thermal stability leading to the creation of the active Ni species at higher temperatures, and (iii) partial regeneration of the layered structure with the assistance of small Au particles, Mg, and the reaction medium as well. Favorable role of gold on Au/NiAl WGS activity was elucidated.     

High pressure palladium membrane reactor for the high temperature water-gas shift reaction

The water-gas shift (WGS) catalytic membrane reactor (CMR) incorporating a composite Pd-membrane and operating at elevated temperatures and pressures can greatly contribute to the efficiency enhancement of several methods of H₂ production and green power generation. To this end, mixed gas permeation experiments and WGS CMR experiments have been conducted with a porous Inconel supported, electroless plated Pd-membrane to better understand the functioning and capabilities of those processes. Binary mixtures of H₂/He, H₂/CO₂, and a ternary mixture of H₂, CO₂ and CO were separated by the composite membrane at 350, 400, and 450 °C, 14.4 bar (P_tube = 1 bar), and space velocities up to 45,000 h⁻¹. H₂ permeation inhibition caused by reversible surface binding was observed due to the presence of both CO and CO₂ in the mixtures and membrane inhibition coefficients were estimated. Furthermore, WGS CMR experiments were conducted with a CO and steam feed at 14.4 bar (P_tube = 1 bar), H₂O/CO ratios of 1.1-2.6, and GHSVs of up to 2900 h⁻¹, considering the effect of the H₂O/CO ratio as well as temperature on the reactor performance. Experiments were also conducted with a simulated syngas feed at 14.0 bar (P_tube  = 1 bar), and 400-450 °C, assessing the effect of the space velocity on the reactor performance. A maximum CO conversion of 98.2% was achieved with a H₂ recovery of 81.2% at 450 °C. An optimal operating temperature for high CO conversion was identified at approximately 450 °C, and high CO conversion and H₂ recovery were achieved at 450 °C with high throughput, made possible by the 14.4 bar reaction pressure.     

Si-modified Pt/CeO2 catalyst for a single-stage water-gas shift reaction

Si-modified Pt/CeO₂ catalysts were prepared for a water-gas shift (WGS) reaction and the effects of this silica addition on the textural and structural characteristics, reducibility and WGS reaction performance of Pt/CeO₂ were investigated. The surface areas of the prepared catalysts increased and both interplanar spacing and average crystalline size of ceria gradually decreased with Si content, resulting in less crystalline and smaller particles. Si addition up to 20 wt. % facilitated the bulk reduction of ceria by inducing significant hydrogen consumption. The oxygen defects in the support, associated with lower valence state cerium, increased with the Si addition. These modifications offer a promising potential to increase the density of hydroxyl groups on the surface of the ceria and consequently increase the concentration of surface intermediate species. The addition of Si to ceria improved the catalytic performance for the WGS reaction, in spite of its irreducible nature. Pt catalysts supported on Si-modified ceria, with a Si content of 5-10 wt.%, exhibited a 2.5-fold increase in reaction rate and turnover frequency (TOF) compared to that of Pt/CeO₂.     

Three-dimensional numerical modeling on high pressure membrane reactors for high temperature water-gas shift reaction

This study presents a three-dimensional numerical model that simulates the H₂ production from coal-derived syngas via a water-gas shift reaction in membrane reactors. The reactor was operated at a temperature of 900 °C, the typical syngas temperature at gasifier exit. The effects of membrane permeance, syngas composition, reactant residence time, sweep gas flow rate and steam-to-carbon (S/C) ratio on reactor performance were examined. Using CO conversion and H₂ recovery to characterize the reactor performance, it was found that the reactor performance can be enhanced using higher sweep gas flow rate, membrane permeance and S/C ratio. However, CO conversion and H₂ recovery limiting values were found when these parameters were further increased. The numerical results also indicated that the reactor performance degraded with increasing CO₂ content in the syngas composition.     

Pt/ZrO2 catalyst for a single-stage water-gas shift reaction: Ti addition effect

Ti modified Pt/ZrO₂ catalysts were prepared to improve the catalytic activity of Pt/ZrO₂ catalyst for a single-stage WGS reaction and the Ti addition effect on ZrO₂ was discussed based on its characterization and WGS reaction test. Ti impregnation into ZrO₂ increased the surface area of the support and the Pt dispersion. The reducibility of the catalyst was enhanced in the controlled Ti impregnation (∼20 wt.%) over Pt/ZrO₂ by the Pt-catalysed reduction of supports, particularly, at the interface between ZrO₂ and TiO₂. The significant CO₂ gas band in the DRIFTS results of Pt/Ti[20]/ZrO₂ indicated that the Ti addition made the formate decomposition rate faster than the Pt/ZrO₂ catalyst, linked with the enhanced Pt dispersion and reducibility of the catalyst. Consequently, Ti impregnation over the ZrO₂ support led to a remarkably enhanced CO conversion and the reaction rate of Pt/Ti[20]/ZrO₂ increased by a factor of about 3 from the bare Pt/ZrO₂ catalyst.     

Effect of precipitation sequence on physicochemical properties of CeO2 support for hydrogen production from low-temperature water-gas shift reaction

A one-step reverse precipitation method has been developed to prepare nano-sized ceria (CeO₂) support with controlled physicochemical properties for low temperature water-gas shift (LT-WGS) reaction. The nano-sized CeO₂ support prepared by reverse precipitation method has a high Brunauer-Emmett-Teller (BET) surface area of 162.8 m²/g. To compare catalytic activity with that of CeO₂ prepared by normal precipitation method, 5 wt% Cu was employed as the active metal, coupled to the CeO₂ support. The catalytic activity of CeO₂ supported Cu catalyst prepared by reverse precipitation method was evaluated for the first time in LT-WGS reaction. Notably, the CeO₂ – R supported Cu catalyst, prepared by reverse precipitation method, showed higher CO conversion and turnover frequency (TOF) values than CeO₂–N supported Cu catalyst prepared by normal precipitation method.     

Hydrogen production from glycerol steam reforming over nickel catalysts supported on alumina and niobia: Deactivation process,effect of reaction conditions and kinetic modeling

Ni catalysts were prepared by wet impregnation of three different supports: alumina, niobia and 10 wt.% niobia/alumina, prepared by (co)precipitation. The catalysts were evaluated on steam reforming of glycerol at 500 °C, for 30 h. The catalyst supported on Nb₂O₅/Al₂O₃ presented the best performance, with higher conversion into gas (80%) during all reaction time and hydrogen yield of 50%. Alumina supported catalyst showed higher deactivation and lower hydrogen yield. All catalysts showed coke formation, but it was formed in larger amount on the catalysts supported on single oxides. A depth study was conducted to evaluate the effect of reaction variables as space velocity, glycerol concentration in feed and temperature on the catalytic performance of the Nb₂O₅/Al₂O₃ catalyst. Kinetic study was also performed for this catalyst using two different approaches, obtaining glycerol and steam orders, as well as the apparent activation energy.     

K-doped LaNiO3 perovskite for high-temperature water-gas shift of reformate gas: Role of potassium on suppressing methanation

LaNiO₃ perovskite has been successfully used as a catalyst precursor for high temperature water-gas shift (HT-WGS) reaction of reformate gas to produce additional hydrogen from the hydrocarbon reforming. The Ni⁰ nanoparticles with the particle size of ca. 21 nm obtained after reduction of LaNiO₃ perovskite can effectively suppress CO methanation during HT-WGS reaction using pure CO/H₂O gas. However, for HT-WGS reaction of reformate gas (including CO, H₂O, CO₂ and H₂), LaNiO₃ perovskite exhibits lower catalytic activity with significant CH₄ formation predominantly via CO₂ methanation. In this work, the CO₂ methanation during HT-WGS reaction of reformate gas was suppressed by the addition of potassium onto LaNiO₃ perovskite. This is due to the adsorption of H₂O on the potassium which is located at the interface between La₂O₃ and Ni⁰ nanoparticle (as deduced from XPS and HRTEM results) that forms stable KOH, blocking the methanation of CO₂ adsorbed on the La₂O₃ with H₂ adsorbed on the Ni⁰ nanoparticles. Moreover, the formation of stable KOH also promotes the formation of formate (HCOO) – a key intermediate for WGS reaction over the reduced LaNiO₃ perovskite – even at high reaction temperature by continuously supplying hydroxyl group to react with CO adsorbed on the Ni⁰ nanoparticle, which helps to maintain the catalytic activity for WGS reaction at high reaction temperature.     

Surface interfaces in low temperature water-gas shift: The metal oxide synergy,the assistance of co-adsorbed water,and alkali doping

With new developments in polymer electrolyte membrane fuel cells, interest is growing in fuel processor technology for converting feedstocks to hydrogen. One critical step in the process to convert CO and purify hydrogen is low temperature water-gas shift (LTS). Control of the LTS rate can be achieved by designing catalysts in a way that produces and rapidly decomposes the surface formate anion intermediate. In this account, examples are provided to demonstrate various interfacial phenomena important for achieving these goals. The interface between a metal and a partially reducible oxide promotes surface reduction of the oxide to the low temperature range, generating sites for the low temperature activation of H₂O on the oxide. Partial reduction of the oxide and surface activation of H₂O at low temperature provide a route for formate production at low temperature. Adjacent co-adsorbed water molecules participate in the transition state of formate decomposition, accelerating the formate turnover rate and altering the selectivity to favor dehydrogenation. The rate-limiting-step involves formate C–H bond scission, with the metal abstracting hydrogen at the interface between metal and oxide, serving as a conduit for hydrogen release. Catalysts may be improved by increasing formate mobility on the oxide; furthermore, the optimization of alkali doping levels can electronically promote formate C–H bond scission.     

Mechanistic aspects of the low temperature steam reforming of ethanol over supported Pt catalysts

The influence of the support of Pt catalysts for the reaction of steam reforming of ethanol at low temperatures has been investigated on Al₂O₃, ZrO₂ and CeO₂. It was found that the conversion of ethanol is significantly higher when Pt is dispersed on Al₂O₃ or ZrO₂, compared to CeO₂. Selectivity toward H₂ is higher over ZrO₂-supported catalyst, which is also able to decrease CO production via the water-gas shift reaction. Depending on catalyst employed, interaction of the reaction mixture with the catalyst surface results in the development of a variety of bands attributed to ethoxy, acetate and formate/carbonate species associated with the support, as well as by bands attributed to carbonyl species adsorbed on platinum sites. The oxidation state of Pt seems to affect catalytic activity, which was found to decrease with increasing the population of adsorbed CO species on partially oxidized (Pt^δ+) sites. Evidence is provided that the main reaction pathway ethanol dehydrogenation, through the formation of surface ethoxy species and subsequently acetaldehyde, which is decomposed toward methane, hydrogen and carbon oxides. The population of adsorbed surface species, as well as product distribution in the gas phase varies significantly depending on catalyst reactivity towards the WGS reaction.     

Hydrogen production by water-gas shift reaction over Co-promoted MoS2/Al2O3 catalyst: The intrinsic activities of Co-promoted and unprompted sites

Co-promoted MoS₂/Al₂O₃ is the industrial-widely used catalyst for hydrogen production by water-gas shift (WGS) reaction under sulfur-containing condition. Despite of the intensive physicochemical characterizations, the intrinsic activities of Co-promoted and unprompted sites on this catalyst are still unreported, mainly owning to the lack of quantification method of catalytic sulfide sites. With low temperature CO adsorption followed by IR spectroscopy, a distinguish technique developed by our group, this short communication reports the temperature-dependent TOFs (turnover frequencies) of these two sites, and reveals that Co-promoted site is intrinsically much more active than unprompted site at low temperatures, while these two sites are catalytically comparable at higher reaction temperatures. The catalytically different performances are related to the different apparent activation energies of WGS reaction on these two sites. This work fills in the long-standing gaps in hydrogen production by WGS reaction over sulfided CoMo/Al₂O₃ catalyst.     

Knowledge extraction for water gas shift reaction over noble metal catalysts from publications in the literature between 2002 and 2012

In this work, a database (containing 4360 experimental data points) on water gas shift reaction (WGS) over Pt and Au based catalysts was constructed using the data obtained from the published papers between the years 2002 and 2012. Then, the database was analyzed using three data mining tools to extract knowledge in three areas: Decision trees to determine the empirical rules and conditions that lead to high catalytic performance (high CO conversion); artificial neural networks (ANNs) to determine the relative importance of various catalyst preparation and operational variables and their effects on CO conversion; support vector machines (SVMs) to predict the outcome of unstudied experimental conditions. It was concluded that, all three models were quite successful and they complement each other to extract knowledge from the past published works and to deduce useful trends, rules and correlations, which are not easily comprehensible by the naked eyes.     

TiO2 and ZrO2 supported Ru catalysts for CO mitigation following the water-gas shift reaction

CO oxidation and methanation over Ru-TiO₂ and Ru-ZrO₂ catalysts were investigated for CO removal for applications in proton exchange membrane fuel cells. The catalysts were synthesised by the deposition precipitation method at a pH of 7–7.5 for better interactions between the support and the active Ru metal. Various characterization experiments such as TPR, XPS, FTIR-CO, CO chemisorption and HRTEM were conducted to better understand the physio-chemical properties of Ru on the supports. Both catalysts showed excellent activity for the total oxidation of CO, however, with the addition of H₂, the catalysts activity to CO oxidation decreased significantly. Higher temperatures for the preferential oxidation reaction indicated that the Ru catalysts not only oxidize CO, but hydrogenate it as well. Furthermore, H₂ oxidation was favoured over the catalysts. Hydrogenation of CO over these catalysts gave high CO conversion and selectivity towards CH₄. Both the catalysts showed similar activity across the temperature range screened and gave maximum CO conversions of 99.9% from 240 °C onwards, with 99.9% selectivity towards CH₄. The catalysts also showed good stability in the reaction and the similarities in the catalytic activity of these were attributed to the well-dispersed Ru metal over the supports. The Ru catalysts effectively reduced CO concentrations in the reformate gas to less than 10 ppm, as is required for practical applications.     

The sulfur tolerance mechanism of the Cu/CeO2 system

The interaction of H₂S with the Cu/CeO₂ system is investigated using the first-principles method. It is found that the formation energy of surface oxygen vacancies is lower than that of interface oxygen vacancies and the spillover of an oxygen ion from ceria to Cu strip is an exothermic process, suggesting that the oxygen ions in the substrate are extremely active. The dissociation of H₂S molecule forms atomic S, which is absorbed preferentially at the Cu strip on both unreduced and reduced Cu/CeO₂(110), instead of interacting with the ceria and diffusing into the ceria bulk, alleviating the deactivation of the ceria. On the other hand, the sulfur atom at the Cu strip could be removed by forming SO₂ at suitable partial pressure of water as suggested by our thermodynamics prediction. Therefore the accumulation of sulfur at the Cu strip and the sulfur poisoning to the Cu/CeO₂ system can be avoided.     

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 H₂O, with H₂O/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 H₂ and CO₂) 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.     

Low-temperature Cu/Zn/SBA-16 integrating carbon membrane reactor for hydrogen production and high CO conversion through water-gas shift reaction

The increasing demand for H₂ energy has led to a great amount of research being conducted in a membrane reactor (MR), in which a membrane is applied during the water-gas shift (WGS) reaction. In this study, Cu/Zn/SBA-16 WGS catalysts and carbon molecular sieve (CMS) membranes were integrated into CMS MRs. To improve the CO conversion and H₂ yield, C MRs were investigated, and different steam/CO (S/C) ratios were used to evaluate the conversion performance. In this study, a tubular CMS membrane was used as the membrane material for a MR. The as-prepared CMS membrane exhibited excellent selectivity of 185.64 for H₂ and CO₂ mixed gas, and an ideal H₂ permeability of 9.7 × 10^?9 mol m^?2 s^?1 Pa^?1 when operated under low temperature/pressure conditions (300 °C/3 bar). The Cu/Zn/SBA-16 catalyst synthesized via coprecipitation was used in the WGS reaction. With a relatively low reaction temperature of 300 °C, 2500 h^?1 gas hourly space velocity, and S/C equal to 1.5, the CO conversion efficiency of MR could reach up to 99%, and the recovery of H₂ was approximately 76%. However, as the S/C increased to 2, the H₂ recovery increased to 99%, whereas the CO conversion decreased to 89% because of the water vapor adsorbed on the active site. The hydrophobic Si/C-modified membrane was further synthesized and showed outstanding performance in CO conversion of over 99% with S/C equal to 2.     

Potential and support-dependent hydrogen evolution reaction activation energies on sulfur vacancies of MoS2 from GC-DFT

We present a detailed mechanistic study of HER at the sulfur vacancy V_S of 2H–MoS₂. We evaluate the Volmer, Tafel, and Heyrovsky transition states for the different possible reaction steps, determining the activation energy as a function of the electrochemical potential via grand-canonical density functional theory. The results show that the Volmer and Heyrovsky steps depend on the electrochemical potential and the activation energies decrease for more negative potentials, while this is not the case for the Tafel step, for which the activation energy is constant. From the activation energies at ?0.2 V vs SHE, it can be concluded that during HER on V_S a first hydrogen atom is adsorbed as a spectator via a Volmer step. Then, the catalytic cycle consists of a Volmer and a Heyrovsky step, with the latter being rate determining. In addition, we investigate for the first time the effect of a conductive support on the HER activity of these sulfur vacancies. Our results show that copper, gold and graphite supports have little effects on the activation energies of all steps. Hence, we conclude that cheap, acid-stable, high-surface area carbon supports are well suited for MoS₂-based HER catalysts.     

Kinetics of the water-gas shift reaction over a La0.7Ce0.2FeO3 perovskite-like catalyst using simulated coal-derived syngas at high temperature

The kinetics of the water-gas shift (WGS) reaction over a novel La_0.7Ce_0.2FeO₃ perovskite-like catalyst is investigated using simulated coal-derived syngas at temperatures of 550 °C and 600 °C which are higher than the maximum operating temperature limit for conventional high temperature WGS catalysts. The influences of CO, CO₂, H₂O and H₂ concentration on WGS reaction rate are determined using selected gas compositions that might be encountered in a coal-based gasification system. An empirical power-law rate model used in this study is found to correlate well with experimental data with good accuracy. Kinetics parameters over La_0.7Ce_0.2FeO₃ obtained in this study are mostly in agreement with those previously measured using Fe-Cr based commercial catalysts in a range of relatively lower temperatures (300-500 °C).