Enhanced water-gas shift reaction performance of MOF-derived Cu/CeO2 catalysts for hydrogen purification

The water-gas shift (WGS) reaction has received renewed interest because it is one of the key reactions for producing hydrogen and renewable energy in contemporary technologies like fuel cells and bio-refineries. Catalysts play an important role in WGS reaction for achieving high CO conversion and hydrogen generation activity. Thus, the performance and stability of catalysts are vital for the WGS reaction. In the present work, the CuCe metal-organic framework (MOF) is used as a template to derive the nanostructured Cu/CeO₂ catalyst. The influence of CuCe-MOF templated approach on the WGS activity of Cu/CeO₂ has been established. Different Cu doping levels had a significant impact on WGS activity. Amongst, the Ce_0.8Cu_0.2O₂ (Cu2Ce) catalyst had a highest CO conversion (96%). The long-term stability tests further prove that the Cu2Ce catalyst had maintained high CO conversion over 100 h reaction time. XRD and TEM results suggest that different loadings of Cu content have a distinct impact on the dispersion of Cu and the catalytic properties. N₂O chemisorption results suggest that 20 wt.% of Cu loading resulted in high Cu dispersion (52%) compared to other loadings. The H₂-temperature programmed reduction (TPR) revealed that the superior catalytic activity of Cu2Ce catalyst could be attributed to the strong reducibility (i.e. lower redox temperature) derived from CuCe-MOF template. It further suggests well-dispersed copper oxide species at low Cu loadings and crystalline copper oxide species at high Cu loadings. This work emphasizes the significance of Cu/CeO₂ catalysts with exceptional catalytic activity and stability for the WGS process with MOF-precursor.     

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.     

The application of MOF-derived CeO2 to synthesize the Cu/CeO2 catalyst for the hydrogen production via water gas shift reaction

The development of catalysts for the water-gas shift (WGS) reaction is attracting attention because of the increased interest in on-site small-scale hydrogen production, which requires highly active and stable catalytic performance under severe conditions. In this study, metal–organic frameworks (MOF), which have been adopted in various fields because of their high surface area, diversity of assemblies, and uniform porosity, were applied to prepare Cu/CeO₂ catalysts for the WGS reaction. MOF-derived CeO₂ (MDC) was obtained from a Ce-BTC-based MOF calcined at different temperatures. Various techniques were used to investigate the physicochemical properties of the Cu/MDC catalysts. Important properties that determine the catalytic performance, such as crystallinity, surface area, Cu dispersion, reducibility, and oxygen storage capacity (OSC), were affected by the treatment temperature of MDC. Among the Cu/MDC catalysts, Cu/MDC prepared with MDC that was treated at 400 °C (Cu/MDC(400)) exhibited the highest CO conversion at reaction temperatures of 200–400 °C. In addition, Cu/MDC(400) maintained 80% of its initial CO conversion after 48 h on stream, even at a very high gas hourly specific velocity of 50,233 mL·g_cat⁻¹·h⁻¹. This result was attributed to the high surface area, Cu dispersion, OSC, and easier reducibility of the Cu/MDC(400) catalyst compared to Cu supported on MDC calcined at other temperatures.     

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.     

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.     

Effects of Pt precursors on Pt/CeO2 to water-gas shift (WGS) reaction activity with Langmuir-Hinshelwood model-based kinetics

The crystallite size effects of Pt nanoparticles on the CeO₂ (Pt/CeO₂) prepared with four different Pt precursors were investigated in terms of their thermal stability and catalytic activity for a water-gas shift (WGS) reaction using the compositions of reformates after a typical steam reforming of propane. The Pt/CeO₂ prepared with a diamine dinitroplatinum (Pt(NO₂)₂(NH₃)₃) precursor, which forms the cationic Pt(NH₃)₂²⁺ species on the negatively-charged CeO₂ surfaces, revealed a superior catalytic activity and thermal stability by forming the partially oxidized smaller Pt nanoparticles decorated with metallic Pt surfaces as well as by forming the strongly interacted PtOx-CeO₂ interfaces. The stable preservation of the pristine smaller Pt nanoparticles with small aggregations even under the hysteresis test from 250 to 400 °C was mainly attributed to the strong metal-support interactions. The optimized Pt/CeO₂ was further studied to obtain kinetic equations derived by Langmuir-Hinshelwood (LH) model, and the optimal operating conditions of WGS reaction were found to be ~280 °C and H₂O/CO molar ratio of 9 with the activation energy of ~78.4 kJ/mol.     

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.     

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.     

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.     

Nanofiber structured oxygen defective CoFe2O4-x catalyst for the water-gas shift reaction in waste-to-hydrogen processes

Waste-to-hydrogen processes are a way to produce hydrogen from waste and reduce the amount of landfill/incineration of wastes simultaneously through the gasification of waste. The water-gas shift (WGS) reaction is a key step in this waste-to-hydrogen process by removing the CO and producing additional H₂. A nanofiber-structured CoFe₂O₄ catalyst was synthesized by the electrospinning method, and the catalytic performance in WGS using waste-derived synthesis gas was compared with that of catalysts prepared by sol-gel, hydrothermal, and co-precipitation methods. The CoFe₂O₄ catalyst synthesized by the electrospinning method showed a clear nanofiber structure and revealed a superior redox property. This superior redox property, which has a large relation with the high oxygen storage capacity of the catalyst, induced the formation of an active phase (Co⁰ and Fe₃O₄) in CoFe₂O₄. As a result, the nanofiber structured oxygen defective CoFe₂O_4-x prepared by the electrospinning method showed the best catalytic activity in this study.     

Investigation of the benefits of diabatic microreactors for process intensification of the water-gas shift reaction within the steam reforming process

Currently, the steam reforming process is the largest industrial source of hydrogen. Improving its efficiency can help to reduce associated carbon emissions and hydrogen production costs. Intensifying the water-gas shift reaction using microreactors with integrated cooling is one way of achieving this. In this study, a 2-D computational model of one of these microreactors is developed, validated with experimental data, and then used to demonstrate how microreactors can enhance the conversion of the water-gas shift reaction beyond what can be achieved using conventional packed bed reactors. These results are then generalized into a full system model of the steam reforming process to demonstrate how microreactors can reduce hydrogen production costs. The results suggest that microreactors can significantly reduce the required reactor volume and catalyst loading for the water-gas shift reaction and can similarly reduce the hydrogen production costs associated with the steam reforming process.     

Preparation of nanocrystalline metal (Cr,Al, Mn,Ce, Ni,Co and Cu) modified ferrite catalysts for the high temperature water gas shift reaction

Water gas shift reaction is an essential process of hydrogen production and carbon monoxide removal from syngas. Fe–Cr–Cu catalysts are typical industrial catalysts for high temperature water gas shift reaction but have environmental and safety concerns related to chromium content. In this work nanocrystalline metal (M)-modified ferrite catalysts (M = Cr, Al, Mn, Ce, Ni, Co and Cu) for replacement of chromium were prepared by coprecipitation method and the effects of promoters on the structural and catalytic properties of the iron based catalysts were studied. Prepared catalysts were characterized using X-ray diffraction (XRD), N₂ adsorption (BET), temperature-programmed reduction (TPR) and transmission electron microscopies (TEM) techniques. Temperature-programmed reduction measurements inferred that copper favors the active phase formation and significantly decreased the reduction temperature of hematite to magnetite. In addition, water gas shift activity results revealed that Fe–Al–Cu catalyst with Fe/Al = 10 and Fe/Cu = 5 weight ratios showed the highest catalytic activity among the prepared catalysts. Moreover, the effect of calcination temperature, GHSV and steam/gas ratio on the catalytic performance of this catalyst was investigated.     

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.