Low-temperature Water Gas Shift Research Articles

Boosting low-temperature water gas shift reaction over Au/TiO2 nanocatalyst activated by oxygen plasma

The efficient removal of carbon monoxide (CO) from H2-rich stream via the water gas shift (WGS) reaction requires the catalysts that possess high activity at low temperatures to avoid the thermodynamic equilibrium limitation. We report that oxygen (O2) plasma activation enables an Au/TiO2 nanocatalyst to achieve superior activity in the WGS reaction at low temperatures. At 120 °C, O2 plasma activated Au/TiO2 nanocatalyst exhibits CO reaction rate of 0.51 mmol·mg−1·h−1, which is 5 times higher than the conventional activated counterpart. CO conversion over O2 plasma activated Au/TiO2 nanocatalyst only slightly decreases from 73% to 68% during a 4.5 h continuous WGS reaction at 150 °C. The catalyst characterizations indicate that O2 plasma activation could control average size of Au nanoparticles below 3.5 nm, create plenty of coordinatively unsaturated sites (on Au nanoparticles) and sufficient Auδ+-Oδ--Ti sites (at interfaces between Au and TiO2), and modulate the electronic structure of interfaces, which accelerates greatly the WGS reaction. Further, the enhancement mechanism of the WGS reaction over O2 plasma activated Au/TiO2 nanocatalyst was disclosed.

Water-Gas-Shift Reaction on Au/TiO2–x Catalysts with Various TiO2 Crystalline Phases: A Theoretical and Experimental Study

Au supported on TiO2 (Au/TiO2) catalysts have shown excellent activity in low-temperature water-gas-shift (LT-WGS) reaction; however, the effects of different crystalline phases of the support, the oxygen vacancies (Ov) on the support, and the metal-support synergy on the catalytic performance still remain indistinct. In this work, a combination of density functional theory (DFT) calculation, microkinetic modeling, and experimental investigation for the LT-WGS reaction mechanism over the Au8 cluster supported on TiO2 with three crystalline phases (Au/ana-Ov, Au/rut-Ov, and Au/bro-Ov) is systematically performed. Notably, Au/ana-Ov gives the lowest energy barrier at each step of the WGS reaction, indicating a superior catalytic activity, and a redox pathway is confirmed: H2O undergoes the dissociation adsorption on Ov while interface Au serves as an active site for CO activation. The microkinetic modeling results confirm the favorable operating condition (pH2O/pCO ≤ 4) for this reaction. Furthermore, experimental researches verify that the Au/ana-Ov sample exhibits the optimum catalytic activity, which accords well with the prediction conclusions of theoretical calculation. This work provides the detailed information on the metal-support synergistic catalysis as well as the in-depth understanding of the reaction mechanism of LT-WGS over the Au/TiO2–x system, which would pave a way for the development of heterogeneous catalysts toward interface sensitive reactions.

Photocarriers-enhanced photothermocatalysis of water-gas shift reaction under H2-rich and low-temperature condition over CeO2/Cu1.5Mn1.5O4 catalyst

Aiming for cost-efficient hydrogen purification for application of fuel cells, a low-temperature water-gas shift (WGS) reaction over noble-metal-free catalyst to attain completed CO conversion in H2-rich atmosphere is required but still a great challenge. Herein, we present a configuration of semiconductor-bridging active sites in CeO2/Cu1.5Mn1.5O4 catalyst to work for the photothermal WGS reaction under light irradiation; due to the injection of photocarriers from CeO2 into active sites, a 61 % reduction of apparent activation energy enabled the WGS reaction to occur at 225°C. Notably, the catalyst delivered a 96.6 % of CO conversion in 30 min, and subsequently, 0.18 vol.% of CO was left in the system, which matches the industrial standard (< 1 vol.%). This study provides a new design idea on the semiconductor-coupled photothermal catalyst to boost catalytic performance.

Adsorbate-assisted migration of the metal atom in atomically dispersed catalysts: An ab initio molecular dynamics study

We present a phenomenological study of dynamical evolution of the active site in atomically dispersed catalysts in the presence of reaction intermediates associated with CO oxidation and low-temperature water-gas shift reaction. Using picosecond ab initio molecular dynamics, we probe the initiation of adsorbate-induced diffusion of atomically dispersed platinum on rutile TiO2(110). NVT trajectories spanning 5ps at 500K reveal that the dynamical stability of the metal atom is governed by its local coordination to the support and adsorbate. Adsorbates that bind the strongest to Pt typically also lead to the fastest diffusion of the metal atom, and all adsorbates weaken Pt-support interactions, resulting in higher diffusion coefficients compared to bare Pt. We note, however, the absence of quantitative correlations between adsorption characteristics (Pt Bader charge, adsorbate binding energy) and ensemble-averaged quantities (diffusion coefficients). A recurring structural motif identified in several trajectories is a near-linear coordination between support oxygen, Pt, and specific adsorbates. These geometries, on account of enhanced metal support interactions, stabilize Pt and inhibit migration over picosecond timescales. We also identify hydrogen bonding events between the adsorbate and support for OH-containing groups. In the case of OH-bound Pt, for instance, we believe that short-lived H-bonds between OH and support promote Pt migration in the beginning of the NVT trajectory, while the subsequent formation of a near-linear geometry stabilizes the Pt atom despite the continued formation of short-lived hydrogen bonds. These observations are consistent with prior studies that report stabilization of isolated metal atoms in the presence of hydroxyl groups.

Simulation study on the performance of low-temperature water gas shift membrane reactor system

In order to understand the behavior of the low-temperature water gas shift (WGS) reaction membrane reactor system and obtain better performance of the reactor, numerical simulations are performed with a two-dimensional axisymmetric model. Dimensionless analysis is applied to the governing equations, and then a set of dimensionless parameters are established. The effect of the parameters of Damkohler number (Da) and membrane permeability (Pp) over a wide range on the performance of CO conversion and hydrogen recovery is studied, and the discussion of reaction phenomenon is presented. The WGS reaction is simulated under the operating conditions of 2 atm and two temperatures of 200 and 300 °C. The simulation results suggest that Da=10−4 and Pp=10−1 are the optimum operating conditions for the reactor system where the conversion reaches nearly 100% exceeding the thermodynamic equilibrium conversion by 11% at 300 °C and the optimal hydrogen recovery is achieved simultaneously. The distribution of components along the axis of the reactor is checked, and then two crucial indicators are raised to assess the extent of reaction and hydrogen separation respectively, for the purpose of guidance for the length design of the reactor. Finally, the analysis of the effect of sweep gas on reactor performance suggests that the optimum flow rate of sweep gas is 0.2–0.5 L/min, and further enhancement of performance can be achieved by using counter-current sweep gas configuration.

Design of Nanoalloyed Catalysts for Hydrogen Production Processes

The use of supported nanoparticles of metal alloys as the active component makes it possible to modify the activity, selectivity, and stability of conventional monometallic supported heterogeneous catalysts. An approach to directed synthesis of bimetallic powders and supported catalysts based on the decomposition of double complex salts in the pores of the support is described. The efficiency of the proposed strategy was shown earlier by the example of catalysts for hydrogen production and purification processes: Pt0.5Co0.5/SiO2 and Au0.4Cu0.6/CeO2 catalysts for preferential oxidation of CO. Nanopowders of Pt0.5M0.5 (M = Fe, Co, Cu) and Pt0.33Ag0.67 and supported Pt0.5Cu0.5/Ce0.75Zr0.25O2 and Pt–CuOx/Ce0.75Zr0.25O2 catalysts are obtained in this work. It is shown that the nanopowders possess catalytic activity in the reaction of preferential CO oxidation in excess H2, while the Pt–Cu supported catalysts, in the low-temperature water gas shift reaction.

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Summary A new strategy for designing efficient bifunctional heterogeneous catalysts for the water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2) is presented. We avoid conflicting tasks that require ∗OH or ∗O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that is connected by weak hydrogen bonds to the substrate. The task of the substrate is to efficiently channel ∗H on its surface and release them as H2 gas. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates compared with highly reactive substrates that follow either the LH-associative or LH-redox process.

Low Temperature Water-Gas Shift: Enhancing Stability through Optimizing Rb Loading on Pt/ZrO2

Recent studies have shown that appropriate levels of alkali promotion can significantly improve the rate of low-temperature water gas shift (LT-WGS) on a range of catalysts. At sufficient loadings, the alkali metal can weaken the formate C–H bond and promote formate dehydrogenation, which is the proposed rate determining step in the formate associative mechanism. In a continuation of these studies, the effect of Rb promotion on Pt/ZrO2 is examined herein. Pt/ZrO2 catalysts were prepared with several different Rb loadings and characterized using temperature programmed reduction mass spectrometry (TPR-MS), temperature programmed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), an X-ray absorption near edge spectroscopy (XANES) difference procedure, extended X-ray absorption fine structure spectroscopy (EXAFS) fitting, TPR-EXAFS/XANES, and reactor testing. At loadings of 2.79% Rb or higher, a significant shift was seen in the formate ν(CH) band. The results showed that a Rb loading of 4.65%, significantly improves the rate of formate decomposition in the presence of steam via weakening the formate C–H bond. However, excessive rubidium loading led to the increase in stability of a second intermediate, carbonate and inhibited hydrogen transfer reactions on Pt through surface blocking and accelerated agglomeration during catalyst activation. Optimal catalytic performance was achieved with loadings in the range of 0.55–0.93% Rb, where the catalyst maintained high activity and exhibited higher stability in comparison with the unpromoted catalyst.

A stable low-temperature H2-production catalyst by crowding Pt on α-MoC.

The water-gasshift (WGS) reaction is an industrially important source of pure hydrogen (H2) at the expense of carbon monoxide and water1,2. This reaction is of interest for fuel-cell applications, but requires WGS catalysts that are durable and highly active at low temperatures3. Here we demonstrate that the structure (Pt1-Ptn)/α-MoC, where isolated platinum atoms (Pt1) and subnanometre platinum clusters (Ptn) are stabilized on α-molybdenum carbide (α-MoC), catalyses the WGS reaction even at 313kelvin, with a hydrogen-production pathway involvingdirect carbon monoxide dissociation identified. We find that it is critical to crowd the α-MoC surface with Pt1 and Ptn species, which prevents oxidation of the support that would cause catalyst deactivation, as seen with gold/α-MoC (ref. 4), and gives our system high stability and a high metal-normalized turnover number of 4,300,000 moles of hydrogen per mole of platinum.We anticipate thatthe strategy demonstrated here will be pivotal forthe design of highly active and stable catalysts for effective activation of important molecules such aswater and carbon monoxide for energy production.

Insight into the mechanism of the water–gas shift reaction over Au/CeO2 catalysts using combined operando spectroscopies

The mechanism of the low-temperature water-gas shift (LT-WGS) reaction over Au/CeO2 catalysts with different ceria terminations, i.e., (111), (110), and (100) facets, was investigated. Using combined operando Raman and UV-Vis spectroscopy as well as isotope exchange experiments, we are able to draw conclusions about the reducibility behaviour and the exchange of surface oxygen. Additional density functional theory (DFT) calculations facilitate the vibrational bands assignments and enhance the interpretation of the results on a molecular level. A facet-dependent role of gold is observed with respect to the oxygen dynamics, since for the CeO2(111) facet the presence of gold is required to exchange surface oxygen, whereas the CeO2(110) facet requires no gold, as rationalized by the low defect formation energy of this facet. This behaviour suggests that surface properties (termination, stepped surface) may have a strong effect on the reactivity. While the reduction of the support accompanies the reaction, its extent does not directly correlate with activity, highlighting the importance of other properties, such as the dissociative adsorption of water and/or CO2/H2 desorption. The results of our facet-dependent study are consistent with a redox mechanism, as underlined by H218O isotopic exchange experiments demonstrating the ready exchange of surface oxygen.

Experimental and theoretical analysis of a natural gas fuel processor

In this study, a natural gas fuel processor was experimentally and theoretically investigated. The constructed 2.0 kWth fuel processor is suitable for a residential-scale high temperature proton exchange membrane fuel cell. The system consists of an autothermal reformer; gas clean-up units, namely high and low-temperature water-gas shift reactors; and utilities including feeding unit, burner, evaporator and heat exchangers. Commercial monolith catalysts were used in the reactors. The simulation was carried out by using ASPEN HYSYS program. A validated kinetic model and adiabatic equilibrium model were both presented and compared with experimental data. The nominal operating conditions which were determined by the kinetic model were the steam-to-carbon ratio of 3.0, the oxygen-to-carbon ratio of 0.5 and the inlet temperatures of 450 °C for autothermal reformer, 400 °C for high-temperature water-gas shift reactor and 310 °C for low-temperature water-gas shift reactor. Experimental results at the nominal condition showed that the performance criteria of the hydrogen yield, the fuel conversion and the efficiency were 2.53, 93.5% and 82.3% (higher heating value-HHV), respectively. The validated kinetic model was further used for the determination of 2–10kWthermal fuel processor efficiency which was increasing linearly up-to 86.3% (HHV).

Atomically dispersed copper species on ceria for the low-temperature water-gas shift reaction

The structure of copper species, dispersed on nanostructured ceria (particles, rods and cubes), was analyzed by scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). It was interestingly found that the density of surface oxygen vacancies (or defect sites), induced by the shape of ceria, determined the geometrical structure and the chemical state of copper species. Atomically dispersed species and monolayers containing few to tens of atoms were formed on ceria particles and rods owing to the enriched anchoring sites, but copper clusters/particles co-existed, together with the highly dispersed atoms and monolayers, on cubic ceria. The atomically dispersed copper sites and monolayers interacted strongly with ceria, involving a remarkable charge transfer from copper to ceria at their interfaces. The activity for the low-temperature water-gas shift reaction of the Cu/CeO2 catalysts was associated with the fraction of the positively-charged copper atoms, demonstrating that the active sites could be tuned by dispersing Cu species on shape-controlled ceria particles.

Cu/SiO2 derived from copper phyllosilicate for low-temperature water-gas shift reaction: Role of Cu+ sites

Copper phyllosilicate supported over silica was prepared with ammonia evaporation method. Compared with the catalyst synthesized with conventional impregnation method, this novel catalyst showed a much higher activity and stability towards low-temperature water-gas shift reaction. Various characterization methods indicated the Cu nanoparticle in the catalyst had a superb small diameter around 3 nm, co-existent with abundant Cu+ sites. Based on the analysis via CO-adsorption, DRIFTS and in-situ mass spectrum, it was proved that CO could be adsorbed on Cu+ strongly and Cu+ acted as the active sites for CO activation. The reaction mechanism with dual active sites prevented the competitive adsorption of H2O and CO onto Cu0 sites, thereby improving the catalytic activity and stability.

Impact of Incorporation of Active Nanoporous Components or Their Precursors in a CuAlO/CuAl Ceramometal Skeleton on the Properties in the Low-Temperature Water-Gas Shift Reaction

Enhanced activity in low-temperature water-gas shift (LT-WGS) reaction of some ceramometal catalysts compared to conventional Cu–Zn–Al oxide catalyst was demonstrated. Porous ceramometals were synthesized from powdered CuAl alloys prepared by mechanical alloying with the addition of either CuAlexp powders produced by current spark explosion of Cu+Al wires or CuZnAl oxide obtained by coprecipitation. Their structural, microstructural, and textural characteristics were examined by means of X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectrometry, NMR, and adsorption methods, and catalytic properties were studied in the LT-WGS reaction. CuAlO/CuAl ceramometals were found to have mostly the egg-shell microstructure with the metallic cores (AlxCu1–x, Al2Cu, and Al4Cu9) and the oxide shell containing copper oxides and/or mixed oxides of copper and aluminum and, at same time, CuAlO/CuAl ceramometal with incorporated additives was found to create a more complicated microstructure. A large amount of X-ray amorphous oxides of copper and aluminum is typical for all composites. CuAl ceramometal was shown to be more active than the CuZnAl oxide catalyst in spite of a much lower specific surface area. The CuAl+CuZnAl catalyst consisting of prismatic granules showed a higher activity in comparison with CuZnAl oxide consisting of cylindrical granules. The activity of the composite granulated catalyst referred to its unit weight was more than 6-fold higher as compared to the oxide catalyst, while the activity per the surface area was found to be more than an order of magnitude higher due to much higher specific activity of small fraction and additively much lower diffusion limitation of granules.

Use of industrial and sol-gel catalysts in the removal of co from autonomous fuel cells

The low-temperature water-gas shift reaction (WGSR) is already used in several industrial chemical processes. However, interest in this reaction has increased significantly in the last few years because of the advances in fuel cell technology and the need to develop compact reformers for the production of pure hydrogen streams (free from CO). In the present work, the WGSR was carried out on copper-based supported catalysts, one of them already used in the industry and the other one prepared using the sol-gel method with the same composition as the industrial catalyst. The synthesized sol-gel catalyst presented excellent catalytic performance, with a very stable CO conversion of around 60%. The high activity and stability of the sol-gel catalyst were mainly attributed to its large metal surface area and high copper dispersion.

Low temperature water-gas shift: Optimization of K loading on Pt/m-ZrO2 for enhancing CO conversion

Previous work showed that alkali doping of Pt/ZrO2 significantly improved the LT-WGS rate. This improvement was caused by an acceleration in the rate of formate decomposition, the rate limiting step of an associative mechanism, through electronically weakening the CH bond. In this study, the effects that potassium loading on Pt/ZrO2 have on the rate of LT-WGS and the strength of the CH bond in intermediate formate species are explored. Catalysts were investigated using in-situ DRIFTS, TPR-MS, TPD, EXAFS, and catalyst testing. Loadings of 1.7 wt.% potassium or higher shifted the formate ν(CH) band to significantly lower wavenumbers, indicating weakening of the bond. For the optimal loading, 2.6 wt.% K, the formate CH bond was significantly weakened, and the surface of Pt nanoparticles remained largely uncovered. However, loadings of 3.4 wt. % and above blocked the surface of the platinum nanoparticles. This, in addition to an increasing average Pt0 cluster size, hindered platinum from assisting in dehydrogenation of formate during steam promoted formate decomposition occurring in LT-WGS.

Unraveling the Origin of Ceria Activity in Water–Gas Shift by First-Principles Microkinetic Modeling

Water–gas shift (WGS) is a well-known reaction for production and purification of hydrogen from synthesis gas. Transition metals supported on ceria (CeO2) have received much attention as single-step catalysts for the low-temperature WGS reaction. The intrinsic WGS activity of the ceria support indicates that it might play an essential role in the catalytic process. However, there is still no consensus on the governing reaction mechanism at mild conditions. Thus, in this work self-consistent periodic density functional theory and microkinetic calculations were performed to investigate redox, formate-mediated, and carboxyl-mediated pathways on the CeO2(111) surface. Binding energies of reactants (H2O and CO), intermediates (OH, formate, and carboxyl), and products (CO2 and H2) were calculated, together with energy barriers associated with possible elementary reaction steps. These results were used as input in a mean-field microkinetic model. The analysis of degrees of rate control from microkinetic calculations shows the prevalence of a combined redox-associative route for WGS on pure ceria. Overall, these results provide insight into the WGS reaction on ceria and can contribute to experimental research of catalytic systems for low-temperature WGS.

Styrene Hydroformylation with In Situ Hydrogen: Regioselectivity Control by Coupling with the Low-Temperature Water-Gas Shift Reaction.

The hydroformylation of olefins is one of the most important homogeneously catalyzed industrial reactions for aldehyde synthesis. Various ligands can be used to obtain the desired linear aldehydes in the hydroformylation of aliphatic olefins. However, in the hydroformylation of aromatic substrates, branched aldehydes are formed preferentially with common ligands. In this study, a novel approach to selectively obtain linear aldehydes in the hydroformylation of styrene and its derivatives was developed by coupling with a water-gas shift reaction on a Rh single-atom catalyst without the use of ligands. Detailed studies revealed that the hydrogen generated in situ from the water-gas shift is critical for the highly regioselective formation of linear products. The coupling of a traditional homogeneous catalytic process with a heterogeneous catalytic reaction to tune product selectivity may provide a new avenue for the heterogenization of homogenous catalytic processes.

Kinetics of Water Gas Shift Reaction on Au/CeZrO4: A Comparison Between Conventional Heating and Dielectric Barrier Discharge (DBD) Plasma Activation

The kinetics of the low-temperature forward water gas shift (LT-WGS) reaction have been studied over a 2 wt% Au/CeZrO4 (Au/CZO) catalyst using both thermal and dielectric barrier discharge plasma heterogeneous catalyst systems. Using the energy density (e), the apparent activation energy has been calculated under plasma and thermally activated conditions. A substantially lower apparent activation energy is observed in the plasma activated system (9.5 kJ/mol) compared with the thermal-catalysed reaction (132.9 kJ/mol). Different kinetic isotope effect (KIE) values for water were found in thermal (1.43) and plasma (1.89) activated catalytic systems which infer different mechanisms between the two activation processes and also shows the importance of water activation. Furthermore, negative and positive reaction orders with respect to CO and H2O are found for both conditions which are − 1.30, 0.28 under thermal and − 1.53, 0.35 under plasma processes, respectively. The reaction order with respect to H2O and KIE studies demonstrate that the bond cleavage in H2O molecule is a rate determining step in the plasma-assisted LT-WGS, similar to that in the thermal-assisted reaction.

Surface structure–activity relationships of Cu/ZnGaOX catalysts in low temperature water–gas shift (WGS) reaction for production of hydrogen fuel

A new species’ class of Cu-, Ga- and Zn-based rate catalysts was prepared by a systematic co-precipitation technique at the different related pH values (6.5–8.0) along with calcination functional conditions, influencing components’ physical properties, these were characterized, and their application performance for water–gas shift (WGS) reaction was researched. Substances were analysed by various experimental methods, namely chemisorption, temperature-programmed reduction (TPR) characterisation, diffraction, physisorption and microscopy. A homogenous size dispersion of the compounds with smaller granular particles was obtained for catalysis, implemented with high pH-resulting outputs. H2 TPR profiles revealed a tailored stronger effect of Cu–Zn on Ga for process, operated with low pH-conditioned forms. Over Cu/ZnGaOX, WGS was sensitive to Cu, which was primarily active. Catalytic chemical reactivity, activity and selectivity were also found to be critically dependent on material lattice structure, copper surface area and metal–support interaction phenomena. The temperature-programmed surface reaction with mass spectrometry (TPSR–MS) measurements showed that formulations, synthesised at the pH of 8.0, enabled reaching >99% of the equilibrium yield CO conversion at 260 °C. An increase in the converted CO, oxidation and H2 productivity with the integral steam content in gaseous feed flow was achieved. The heterogeneous phase processing at the correlated pH of 7.6 demonstrated the highest formed CO product at the temperature of 200 °C, compared with literature. This is particularly promising for reagent purity hydrogen-fed fuel cells. The kinetics for each co-precipitated solid was evaluated regarding the efficiency for the WGS in a fixed bed reactor.