Low-temperature Water Gas Shift Research Articles

A noble-metal-free catalyst with MoC nanorod for low-temperature water gas shift reaction

Water gas shift (WGS) reaction is an important process to remove CO contaminant in industrial hydrogen resources. The efficient catalyst that can work at low temperature is keenly desired to achieve high CO conversion from the viewpoint of thermodynamic equilibrium. Here, we develop a noble-metal-free catalyst of MoC nanorod (MoC NR) with the dominant (200) facet for the low-temperature WGS reaction. It exhibits a high performance with ∼ 100 % CO conversion at 175 °C, better than conventional MoC with the dominant (111) facet or the commercial Cu-based catalyst. Moreover, it shows rather better stability due to the presence of the dominant (200) facet. Various characterizations demonstrate that this facet facilitates MoC NR possessing higher removal capability of surface oxygen species to enhance the stability and lower activation energy to endow higher performance compared with conventional MoC. This study provides important implications for the rational design of cost-effective catalysts for the low-temperature WGS reaction.

Understanding the promotion effect of Pt in the reaction system of water gas shift reaction catalyzed by Pt/α-MoC from theoretical perspectives

Pt/α-MoC is an outstanding catalyst for the low-temperature water gas shift (WGS, CO + H2O → H2 + CO2) reaction, exhibiting elevated activity, stability, and selectivity. In this study, three significant active site models (α-MoC-111, Pt-111, and Pt1/α-MoC-111) in Pt/α-MoC catalyst were established to investigate the promotion effect of Pt on the WGS reaction catalyzed by Pt/α-MoC. The results indicate that a synergistic effect at the metal-carbide interface enhances chemical activity compared to pure Pt-111 and α-MoC-111. Firstly, the surface of Pt1/α-MoC-111 exhibits strong metal-support interaction between Pt single atom and α-MoC-111 surface. Compared to the α-MoC-111 surface, the introduction of Pt strengthens the Mo−Mo bond while weakening the Mo−C bond. Secondly, adding a Pt atom significantly increases adsorption strength for CO and H2O, while decreasing it for products (CO2), oxides (O atom), and by-products (C atom). The advantage of suppressing surface carbon deposition and oxidation is observed on the surface of Pt1/α-MoC-111. Furthermore, elementary reactions from the three mainstream reaction pathways of the WGS reaction are systematically calculated to evaluate catalytic performance among these three active sites. On the Pt1/α-MoC-111 surface, activation energy barriers for H2O dissociation and CO oxidation are reduced. This provides theoretical ideas for designing high-efficiency α-MoC based catalysts in the future.

Optimizing Mo-C coordination for enhanced low-temperature water-gas shift activity over α-MoC1-x catalysts: An experimental and theoretical study

Optimizing Mo-C coordination for enhanced low-temperature water-gas shift activity over α-MoC1-x catalysts: An experimental and theoretical study

Reactive Pt-TiN interfacial sites for H2O activation and the low-temperature water gas shift

The reactive interfacial sites upon supported catalysts provide an alternative pathway to enable the reactants activation in an energetically favorable mode, thus decreasing the energy barrier and promoting the catalytic efficiency. By virtue of structural characterization, in-situ spectroscopic analysis, kinetic assessments and density functional theory (DFT) calculations, supported Pt/TiN catalysts are systematically studied to interrogate H2O activation on the Pt-TiN interfacial sites and its catalytic function during the low-temperature water gas shift (WGS). It is found that the partially oxygen-covered TiN surface is highly efficient for H2O activation by oxygen-assisted dissociation with an energy barrier of 0.41 eV, much less than that on metallic Pt surface. The formed hydroxyls binding on Ti sites, observed by in-situ X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD), could readily react with the adjacent CO adsorbed on Pt clusters with the reaction energy barrier of 0.43 eV to attain the individual catalytic cycle. Benefiting from the enhanced H2O activation, the catalytic turnover rate of CO-H2O reaction on Pt/TiN is 3.8 and 20.3 times higher than that on traditional Pt/TiO2 and Pt/SiO2 at 403 K, respectively. The interfacial reactive sites are the origin of low-temperature WGS on Pt/TiN catalysts.

CeO2/Cu2O/Cu Tandem Interfaces for Efficient Water-Gas Shift Reaction Catalysis.

Metal-oxide interfaces on Cu-based catalysts play very important roles in the low-temperature water-gas shift reaction (LT-WGSR). However, developing catalysts with abundant, active, and robust Cu-metal oxide interfaces under LT-WGSR conditions remains challenging. Herein, we report the successful development of an inverse copper-ceria catalyst (Cu@CeO2), which exhibited very high efficiency for the LT-WGSR. At a reaction temperature of 250 °C, the LT-WGSR activity of the Cu@CeO2 catalyst was about three times higher than that of a pristine Cu catalyst without CeO2. Comprehensive quasi-in situ structural characterizations indicated that the Cu@CeO2 catalyst was rich in CeO2/Cu2O/Cu tandem interfaces. Reaction kinetics studies and density functional theory (DFT) calculations revealed that the Cu+/Cu0 interfaces were the active sites for the LT-WGSR, while adjacent CeO2 nanoparticles play a key role in activating H2O and stabilizing the Cu+/Cu0 interfaces. Our study highlights the role of the CeO2/Cu2O/Cu tandem interface in regulating catalyst activity and stability, thus contributing to the development of improved Cu-based catalysts for the LT-WGSR.

Alkali-promoted copper catalyst catalyze low-temperature water-gas shift reaction

The addition of alkali metal promoter is significant to improve the performance of copper catalyst in water–gas shift reaction (WGSR), but the previous theoretical work mainly focuses on the single crystal metal model like Cu(111), rather than the real supported copper clusters. In this paper, Cu4/TiO2(sub-nanometer), Cun/TiO2 (copper nanorod) were chosen to explore the effect of K on WGSR where K is located on TiO2 support, Cu/TiO2 interface, and copper clusters surface. Density functional theory calculation results show that only when K is on Cun/TiO2 and Cu(111) surfaces, can K promote water–gas shift reaction. Microkinetic modeling analysis shows that K mainly promotes the water–gas shift reaction by promoting the decomposition of H2O, and has little influence on the C-O bond formation like carboxyl and CO2. Combined with energetic, electronic and geometric analysis, on the surfaces of Cun/TiO2 and Cu(111), K mainly enhances the adsorption strength of H2O and reduces the barrier of H2O dissociation through direct bonding with oxygenate species, thereby improving the performance of WGSR. The copper clusters with sub-nanometer size cannot reveal the promotion effect of K, and the microkinetic modeling results of Cu(111) surface do not conform to the real situation, but the copper nanorod model can appropriately reflect the promotion effect of K. The structure unit of the active center is [TiO2−Cu−K]. The promotion of other alkali metals was explored, and the microkinetic modeling results show that the order of alkali metal promotion is as follows: Li < Na < K < Rb < Cs. The electronic structure analysis shows that the smaller the electronegativity of alkali metal, the lower the surface work function, the higher the surface dipole moment, and the stronger the promoting effect of alkali metal. This work not only reveals the origin of the K effect on WGSR, but also provides the accurate placement of K promoter.

Low-Temperature Water Gas Shift Reaction over Highly Dispersed Ir on TiO2─Influence of the Ir Dispersed State and the Metal–Support Interface

Oxide-supported iridium (Ir) metal catalysts exhibit superior activity for the low-temperature water gas shift (WGS) reaction. In this study, highly dispersed Ir supported on anatase TiO2 shows a relatively high turnover frequency and a relatively low activation energy for the WGS reaction compared to that reported in the literature. Catalyst characterization reveals the presence of small clusters and single atoms (SAs). Density functional theory (DFT) calculations show that the WGS reaction proceeds preferentially via a carboxyl mechanism with carboxyl formation as the rate-determining step on both TiO2-supported Ir clusters and SAs, which can be supported by experimental observations. In situ diffuse reflectance infrared Fourier transform spectroscopy, corroborated by DFT calculations, indicates that CO co-adsorbed with H2O at the interface between the Ir cluster and TiO2 plays the key role in low-temperature WGS starting from room temperature. DFT calculations show that the energy barrier of carboxyl formation over the Ir cluster is lower than that over Ir SAs, indicating that the Ir clusters may have contributed more to the low-temperature WGS activity. The excellent catalytic activity of the Ir/a-TiO2 catalyst reveals its potential application for low-temperature WGS.

Low-temperature reverse water gas-shift reaction over highly efficient Cu-hydrotalcites: Mechanistic insights on the role of malachite phase

Carbon dioxide (CO2) transformation into valuable fuels and chemicals is in most cases a challenge far from readiness nowadays. One possible route for its conversion is the reverse water gas shift reaction (rWGS), crucial for syngas generation and required for the chemical conversion of CO2 to fuels and platform chemicals. In this paper, well organized Cu/Zn/Al structures were proposed as efficient catalysts for rWGS reaction at low temperatures. The results of in situ XRD revealed the formation of layered structures such malachite and hydrotalcite. The operando DRIFTS-MS studies of those structure suggests a participation of Cu2+/Cu+ pair in the reaction, promoting the redox mechanism and enhancing the activity at lower temperature. This work also provides a new strategy to design Cu-based rWGS catalysts able to prevent the sintering of active phase.

Upgrading the low temperature water gas shift reaction by integrating plasma with a CuOx/CeO2 catalyst

Low temperature WGS reaction is kinetically limited though it is thermodynamically favored. Herein, an outstanding synergistic effect between dielectric barrier discharge (DBD) plasma and non-precious CuOx/CeO2 catalysts was observed to robustly accelerate the water gas shift (WGS) reaction at 150 °C (increase from 6% to 96% after plasma on). The apparent activation energy of the reaction was remarkablly diminished by plasma (69.6 to 15.0 kJ/mol). In-situ characterization experiments (e. g. OES and DRIFTS) revealed that the OH radicals generated by plasma-induced H2O dissociation was found to participate in the WGS reaction. In the furtherly study in DFT calculation and microkinetic modelling investigation, the OH radicals were proved to be one of the reasons for the giant increase in reaction rate. Besides, the vibrational excitation of reactants (e. g. CO) promoted by plasma could also reduce the reaction barriers of some crucial elementary reactions, thus accelerating the reaction rate.

Plasmonic Cu Nanoparticles for the Low-temperature Photo-driven Water-gas Shift Reaction.

The activation of water molecules in thermal catalysis typically requires high temperatures, representing an obstacle to catalyst development for the low-temperature water-gas shift reaction (WGSR). Plasmonic photocatalysis allows activation of water at low temperatures through the generation of light-induced hot electrons. Herein, we report a layered double hydroxide-derived copper catalyst (LD-Cu) with outstanding performance for the low-temperature photo-driven WGSR. LD-Cu offered a lower activation energy for WGSR to H2 under UV/Vis irradiation (1.4 W cm-2 ) compared to under dark conditions. Detailed experimental studies revealed that highly dispersed Cu nanoparticles created an abundance of hot electrons during light absorption, which promoted *H2 O dissociation and *H combination via a carboxyl pathway, leading to the efficient production of H2 . Results demonstrate the benefits of exploiting plasmonic phenomena in the development of photo-driven low-temperature WGSR catalysts.

Conceptual design of a process for hydrogen production from waste biomass and its storage in form of liquid ammonia

In this work we present the simulation of a plant for the exploitation of renewable hydrogen (e.g. from biomass gasification) with production of renewable ammonia as hydrogen vector and energy storage medium. The simulation and sizing of all unit operations were performed with Aspen Plus® as software. Vegetable waste biomass is used as raw material for hydrogen production, more specifically pine sawdust.The hydrogen production process is based on a gasification reactor operating at high temperature (700–800 °C), in the presence of a gasifying agent such as air or steam. At the outlet, a solid residue (ash) and a certain amount of gas, which mainly contains H2, CH4, CO and some impurities (e.g. sulphur or chlorine compounds) are obtained. Subsequently this gas stream is purified and treated in a series of reactors in order to maximize the hydrogen yield. In fact, after the removal of the sulphur compounds through an absorption column with MEA (to avoid poisoning of the catalytic processes), 3 reactors are arranged in series: Methane Steam Reforming (MSR), High temperature Water-Gas Shift (HT-WGS), Low temperature Water-Gas Shift (LT-WGS).In the first MSR reactor, methane reacts at 1000 °C in presence of steam and a nickel-based catalyst, in order to obtain mainly H2, CO and CO2. Subsequently two steps of WGS are present to convert most of the CO into H2 and CO2. Also these reactions are carried out in the presence of a catalyst and with an excess of water.All the oxygenated compounds must be carefully eliminated: the remaining traces of CO are methanated while CO2 is removed by a basic scrubbing with MEA (35 wt%) inside an absorption column. The Haber-Bosch synthesis of ammonia was carried out at 200 bar and in a temperature range between 300 and 400 °C, using two catalysts: Fe (wustite) and Ru/C.As overall balance, from an hourly flow rate of 1000 kg of dry biomass and 600 kg of nitrogen, 550 kg of NH3 at 98.8 wt% were obtained, demonstrating the proof of concept of this newly designed process for the production of hydrogen from renewable waste biomass and its transformation into a liquid hydrogen vector to be easily transported and stored.

Water-gas-shift reaction over Au single-atom catalysts with reversible oxide supports: A density functional theory study

The low-temperature water-gas-shift (LT-WGS) reaction has shown remarkable activity for Au single-atom supported on reducible oxide catalysts. The water dissociation step of the WGS reaction is calculated using density functional theory (DFT) over four single Au atom (Au1)-pristine reversible oxides support systems Au1/MaOb-Ov (Au1/TiO2-x, Au1/ZrO2-x, Au1/CeO2-x and Au1/Co3O4-x) system and four Au1-supports with O vacancy (Ov) systems Au1/MaOb (Au1/TiO2, Au1/ZrO2, Au1/CeO2 and Au1/Co3O4). According to its greatest H2O adsorption energy, lowest water dissociation barrier, and slightest structural distortion, Au1/CeO2-x is chosen as the most beneficial catalyst for the WGS process. Afterwards, for Au1/CeO2-x, three reaction pathways (redox path, formate path and carboxyl path) are calculated. The predominant reaction pathway is the carboxyl pathway, and hydrogen production is the rate-determining step (RDS). For the purpose of designing single metal-support catalysts for the LT-WGS reaction, this paper gives information on the strong metal-support interaction (SMSI).

Gold Atomic Layers and Isolated Atoms on MoC for the Low-Temperature Water Gas Shift Reaction

Au 2–4 atomic layers, monolayers, and isolated atoms were generated by redispersing a Au particle (8 nm) during MoO3 carburization to α-MoC with a CH4/H2 mixture at 600–750 °C. The spherical Au particle initially wetted on the developing molybdenum carbides, reshaped into large thin films, then decomposed to smaller islands, and finally cracked into 2–4 atomic layers at 600 °C. These layered Au species further split into isolated atoms when carburized at 750 °C, which could be reversed into monolayers and 1–2 atomic layers by H2-treatment at 630–650 °C, followed by activation with a CH4/H2 mixture at 590 °C. When tested for the low-temperature water gas shift reaction, the 2–4 atomic layers possessed a much higher activity than the monolayers and isolated atoms. Interestingly, the specific reaction rate showed a volcano-type pattern with respect to the coordination number of Au–Au in the size-specified gold species, where the Au 2–4 atomic layers having a Au–Au coordination number of 7.7 were intrinsically more active. Detailed structural analysis, by STEM and EXAFS, identified that the interfacial gold atom coordinated simultaneously with the Mo atom in MoC and Au atoms in the layers.

Role of interfacial oxygen vacancies in low-loaded Au-based catalysts for the low-temperature reverse water gas shift reaction

Gold-based catalysts have shown high catalytic activity for reverse water gas shift (RWGS) reactions at low temperatures. Despite extensive studies, the RWGS reaction on Au-based catalysts with very low Au content (< 0.1 wt%) has not yet been investigated. In this study, TiO2 and ZrO2 supported gold catalysts with such low gold loading have been synthesized and tested for RWGS. The catalysts were investigated by a series of in/ex-situ characterization techniques, including ICP-OES, XRD, BET, XPS, STEM, STEM-EELS, TAP, in-situ DRIFTS and in-situ EPR. At 250 oC, the Au/TiO2 catalyst showed almost 10 times higher activity than Au/ZrO2. In-situ DRIFTS results suggest that the formate mechanism is the predominant mechanism over Au/ZrO2, while over Au/TiO2 the reaction proceeds via the formation of hydroxycarbonyl intermediates. A combined study including STEM, STEM-EELS, XPS, and in-situ EPR suggests that the interfacial Au–Ov–Ti3+ sites are responsible for the superior activity of Au/TiO2.

Unveiling the mechanism of controllable CO2 hydrogenation by group VIB metal single atom anchored on N-doped graphite: A density functional theory study

CO2 hydrogenation has raised considerable interest due to concerns about climate change. Realizing low-temperature reverse water gas shift (rWGS) reaction remains a significant challenge in the context of coupling it with the C–C growth reactions to convert CO2 to C2+ fuels. We carried out systematic DFT simulations to unveil the underlying low-temperature mechanism for the selective hydrogenation of CO2 to produce CO, over a variety of metal-based single atom catalysts (SACs) supported on the nitrogen-doped graphite. Group VIB metal-based SACs outperformed other 15 metal candidates in terms of versatile capacities in both selective activation of CO2 molecule and facilitating escaping of CO and H2O. Mo1/N3-Gt was especially outstanding by giving rise to spontaneous production of CO and O∗ through an effective electron injection into the CO2 molecule. Water formation has been identified as the potential rate-controlling step in such a catalytic reaction over Mo1/N3-Gt with an energy barrier of 1.10 eV. Herein, the H migration played a pivotal role and had tight affinity to the charge of H∗ on the active site of catalyst. The dynamic coordination environment of Moδ+ was revealed to be the dominant factor affecting the surface H∗ charge, leading to a variety of hydrogenation behaviors. The electron-deficient ligands of CO2 and O∗ on Mo1/N3-Gt, as well as additional adsorbed H2, were effective in adjusting the 4d and 5s electronic structure of central Mo and consequently resulted in nearly electric neutral surface H∗s, thus most benefiting the hydrogenation process. The optimal charge of the coordinated Mo for an outstanding selective hydrogenation performance in this scenario was found to be no less than +1.7e.

Inhibition of water-gas shift reaction activity of oxide-supported Pt catalyst by H2 and CO2

A catalyst composed of platinum-group metals supported on an oxide exhibits high activity in a low-temperature water-gas shift (LT-WGS) reaction; however, the reaction rate is greatly reduced when H2 or CO2, the product gases of the WGS reaction, are included in the reactant stream. In this study, we attempted to understand the origin of this activity inhibition by analyzing the kinetic data with in-situ CO-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The WGS reaction rate decreases more severely by H2 than CO2. The CO-DRIFTS spectra indicate that this can be explained by H2 preoccupying the active sites for the WGS reaction. In addition, by comparing the kinetic data with the literature, it could be inferred that a similar inhibition mechanism is operating in other oxide-supported Pt catalysts. Considering this inhibition mechanism will be important for the development of catalysts with high WGS activity in reformate gas.

Reversible Growth of Gold Nanoparticles in the Low-Temperature Water-Gas Shift Reaction.

Supported gold nanoparticles are widely studied catalysts and are among the most active known for the low-temperature water–gas shift reaction, which is essential in fuel and energy applications, but their practical application has been limited by their poor thermal stability. The catalysts deactivate on-stream via the growth of small Au nanoparticles. Using operando X-ray absorption and in situ scanning transmission electron microscopy, we report direct evidence that this process can be reversed by carrying out a facile oxidative treatment, which redisperses the gold nanoparticles and restores catalytic activity. The use of in situ methods reveals the complex dynamics of supported gold nanoparticles under reaction conditions and demonstrates that gold catalysts can be easily regenerated, expanding their scope for practical application.

Low-temperature Water-gas Shift Reaction Enhanced by Oxygen Vacancies in Pt-loaded Porous Single-crystalline Oxide Monoliths.

Water-gas shift (WGS) reaction at low temperature plays an important role in hydrogen production from fossil fuels and hydrogen purification for proton-exchange membrane fuel cells. However, the activation of H2 O is a critical reaction step that greatly limits the overall performance during WGS reaction. Here we fabricate porous single-crystalline (PSC) MoO3 monoliths at 1 cm scale and deposit atomic-layered Pt clusters at the lattice surface to create the interfacial system toward the low-temperature WGS reaction. The single-crystalline nature stabilizes the oxygen vacancies (VO ) at lattice and facilitates the effective activation of H2 O at the interface. We show the highest Pt-normalized activity of 0.86 molCO molPt -1 s-1 for the ultra-low temperature WGS reaction at 120 °C. The single-crystalline features with enhanced fluxion in porous architectures lead to outstanding performance without visible degradation even after continuous operation for 100 hours.

Low temperature ethanol steam reforming: Selectivity control with lithium doping of Pt/m-ZrO2

Lithium promoted 2%Pt/m-ZrO2 catalysts previously observed to exhibit higher rates for the low temperature water-gas shift (LTS) were tested for the ethanol steam reforming with the aim of exploring the potential tuning of the selectivity. Characterization of catalysts having optimized Li content (0.5–0.75%Li) for LTS exhibited (a) weakened CH bonding of formate, a proposed intermediate in the LTS mechanism, as shown by a shift in the ν(CH) band to lower wavenumbers, (b) a relatively low extent of blocking of Pt, as measured by the ν(CO) band intensity of Pt-CO, (c) increased basicity as measured by CO2 temperature-programmed desorption with mass spectrometry, but not so high as to strongly inhibit CO2 product removal, and finally (d) no evidence of electron transfer from Li to Pt. Here, the same catalysts were tested for ethanol steam reforming (ESR). Results show that Li could likewise weaken the CC bond of the acetate intermediate, the analog of formate in LTS, and facilitate decarboxylation over decarbonylation altering the selectivity in favor of methanation. This trend was confirmed by fixed bed reaction testing, in-situ infrared spectroscopy experiments of transient ESR, and temperature-programmed ESR using MS. The Li-doped catalysts may be used to pre-reform ethanol prior to feeding to a methane steam reformer to increase the overall H2 selectivity of the process. DRIFTS of steady state ESR revealed that deactivation occurs through losses in the Pt-support interface, thereby hindering the turnover of the acetate intermediate.

CFD modelling and simulation of a zeolite catalytic membrane reactor for low temperature water-gas shift reaction

In this work, a catalytic membrane reactor is proposed to couple water-gas shift reaction with selective separation of H2 product to improve the overall conversion. A 3D isothermal CFD model of this catalytic membrane reactor is developed with COMSOL Multiphysics software. The effects of reaction temperature, pressure, sweep gas flow rate and reactor geometry on CO conversion and H2 recovery are analysized in details. Based on the results, the coupling mechanism of reaction, and H2 permeation is proposed. It is found that that the catalyst bed could be divided into active and less-active volume and reducing less-active volume helps improve the utilization efficiency of catalyst. As a conclusion, H2 permeation directly affects H2 mole fraction in near-membrane area, and the effect is spreaded to the rest part to boost the local reaction rate. However, the effect is neutralized by the improved H2 production and permeation affecting volume is mainly less-active volume while active volume is hardly to be affected. Our study indicates that H2 permeation tends to deplete H2 fraction in catalyst volume but the consequent improved reaction rate acts as a resistance and tends to move back to a new steady state.