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•Cu/CeO2 catalysts were prepared using different impregnation orders for Cu and Ce.•Cu–Ce/CeO2 (prepared by co-impregnation) exhibited the highest catalytic activity.•Cu dispersion ...and Cu particle size are important factors in reaction < 280 °C.•Oxygen vacancy concentration and active Cu species are important factors > 360 °C.
To investigate the effect of Cu/CeO2 catalysts preparation methods for low temperature water−gas shift reaction, Cu/CeO2 catalysts were prepared with various impregnation sequences and applied to the water−gas shift reaction at a high gas hourly space velocity of 36,080 mL/g·h. The catalyst preparation method affected both the Cu−Ce synergistic effect and the physico−chemical properties of the Ce promoted Cu/CeO2 catalysts, such as specific surface area, Cu particle size, Cu dispersion, amount of active Cu species and oxygen vacancy concentration. Among the prepared catalysts, Cu–Ce/CeO2 showed the highest CO conversion because of the high contents of active Cu species and high concentration of oxygen vacancies.
CH
4 steam reforming (SR) and dry reforming (DR) on Rh have been analyzed using a comprehensive, thermodynamically consistent microkinetic model. Our analysis pointed out mechanistic analogies ...between the two processes. In particular, regardless of the co-reactant, methane consumption proceeds via pyrolysis and carbon oxidation by OH* (CH
4 → C
∗ → CO
∗), and the role of the co-reactant (either CO
2 or H
2O) is to provide the main oxidizer, OH
∗. Moreover, in line with isotopic kinetic experiments reported in the literature, methane activation is predicted to be the rate-determining step, and all of the steps involving co-reactant turn out to be quasi-equilibrated. It also was found that under typical experimental conditions, SR and DR always occur with water–gas shift (WGS) reaction close to equilibrium. Adopting a systematic reduction methodology, we propose a hierarchy of models for SR and DR. In particular, first a reduced microkinetic model and then overall rate equations for the SR, DR, and WGS reactions are derived from the microkinetic models. Overall, our kinetic analysis is able to predict correctly the most important features found in experiments, namely that the overall reaction rate exhibits a first-order dependence on CH
4 concentration and is independent of the co-reactant (H
2O or CO
2). Product inhibition, which becomes important at lower temperatures, also is predicted.
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 H
2 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 H
2/He, H
2/CO
2, and a ternary mixture of H
2, CO
2 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. H
2 permeation inhibition caused by reversible surface binding was observed due to the presence of both CO and CO
2 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), H
2O/CO ratios of 1.1–2.6, and GHSVs of up to 2900 h
−1, considering the effect of the H
2O/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 H
2 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 H
2 recovery were achieved at 450 °C with high throughput, made possible by the 14.4 bar reaction pressure.
Iron‐catalyzed reductions: Selective iron‐catalyzed reduction of aldehydes with hydrogen generated in situ by the water–gas shift reaction is presented (see scheme). The generality and selectivity of ...this mild procedure are demonstrated by the efficient reduction of various aromatic, aliphatic and α,β‐unsaturated aldehydes.
Herein, we report a theoretical and experimental study of the water‐gas shift (WGS) reaction on Ir1/FeOx single‐atom catalysts. Water dissociates to OH* on the Ir1 single atom and H* on the ...first‐neighbour O atom bonded with a Fe site. The adsorbed CO on Ir1 reacts with another adjacent O atom to produce CO2, yielding an oxygen vacancy (Ovac). Then, the formation of H2 becomes feasible due to migration of H from adsorbed OH* toward Ir1 and its subsequent reaction with another H*. The interaction of Ir1 and the second‐neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe3+−O⋅⋅⋅Ir2+−Ovac to Fe2+−Ovac⋅⋅⋅Ir3+−O with the involvement of Ovac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir1/FeOx for sequential production of CO2 and H2.
Two sites are better than one: A redox mechanism with dual metal active site was found for the water‐gas shift (WGS) reaction on the Ir1/FeOx single‐atom catalyst by theoretical and experimental studies. The Ir1 and Fe atoms jointly facilitate the creation of an oxygen vacancy at the Fe species neighbouring the Ir1 atom, leading to sequential production of CO2 and H2.
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•Combined operando IR and isotope transient method to count actual CO active sites.•Na modifies local electronic properties of Pt to affect CO and activates H2O.•Transient response ...corrected for CO2 re-adsorption identifies true active species.•Less than 1% surface Pt active on Pt/Al2O3; 100% surface Pt active on 20Na:Pt/Al2O3.•Formate intermediates are spectator species for WGS on Pt/Al2O3 and Na–Pt/Al2O3.
The promotional effect of sodium on Pt/Al2O3 catalysts for the water–gas shift (WGS) reaction was investigated with operando FTIR during steady-state and isotopic transient experiments. The highest turnover frequency (TOF) promotion per surface Pt occurred on a 0.82wt.% Pt/Al2O3 catalyst with a 30:1molar ratio of Na:Pt. This catalyst exhibited a TOF of 0.35s−1 at 250°C and 7% CO, 11% H2O, 9% CO2, and 37% H2 compared to 0.43×10−2s−1 on Pt/Al2O3. Operando IR revealed that Na addition modifies CO adsorption on Pt to create more strongly bound, multiply-bonded CO species at 1768cm−1 and 1697cm−1 compared to predominantly linear CO on Na-free Pt/Al2O3 at 2060cm−1. Transient experiments with 12CO/13CO isotope switches showed that the number of carbon-containing active intermediates increased from less than 1% of the surface Pt for Na-free Pt/Al2O3 to nearly 100% of the surface Pt for 20Na:Pt/Al2O3, which indicates that only a small fraction of the Pt surface on the Na-free samples participates in the WGS reaction. From time-resolved IR spectra during transient WGS with 12CO/13CO and 12CO2/13CO2 isotope switches, we propose that surface formate species are spectators for all Na-promoted Pt/Al2O3 catalysts under these WGS conditions. The results suggest that Na promotes Pt/Al2O3 for WGS by modifying the local electronic properties of Pt and creating new H2O activation sites, which provide greater availability of surface OH/H species to react with nearly all CO on the metallic Pt surface through a non-formate pathway.
Density functional theory is used to investigate the origins of the excellent catalytic activity of the Mo2CTx MXene for the water gas shift reaction. By considering different possibilities for the ...MXene surface termination (Tx = none, O, F, or a mixture of O and F), we conclude that its ideal composition should contain both F and O adatoms, essential for controlling the exothermicity of the reaction and avoiding saturation by oxygenated species. More precisely, while Mo2CO2 and Mo2CF2 are too inert towards water adsorption and dissociation and the bare Mo2C MXene is inactivated upon coverage by oxygenated species, our calculations predict that regions near one or two O adatoms in the midst of F surface terminations should be the active catalytic sites. Indeed, in the vicinity of the O adatoms, water adsorbs with moderate strength, dissociates with a very low energy barrier (0.14–0.20 eV), and the dissociation is moderately exothermic.
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•Mo2CO2 surface is inert towards water adsorption and dissociation.•Water inactivates Mo2C by covering its surface with oxygenated species.•The dissociative adsorption of water on Mo2CF2 leads to surface defluorination.•A Mo2CTx catalyst mixing O and F terminations adsorbs and dissociates water.•Surface terminations control the exothermicity of the reaction.
The APR activities of Pt based catalysts decrease as follow: Pt/MgO
>
Pt/Al
2O
3
>
Pt/CeO
2
>
Pt/TiO
2
>
Pt/SiO
2, which agrees with the degressive sequence of basicity of supports.
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► ...The catalysts with the same Pt loading and dispersion are synthesized. ► The APR of glycerol is more favorable on stronger basicity supported catalyst. ► The catalyst providing a higher ability of WGS shows a better APR activity. ► It is proposed WGS reaction is the key role of the APR of glycerol.
Pt loaded MgO, Al
2O
3, CeO
2, TiO
2 and SiO
2 catalysts were prepared by loading pre-synthesized Pt colloids on support and used for the aqueous-phase reforming (APR) of glycerol to investigate the influence of support properties on catalytic performance. The conversion of glycerol, rate of hydrogen production and composition of gaseous products were measured for the APR process of 5
wt.% glycerol. It was found that the overall catalytic activities for APR of glycerol decreased in the following order for Pt based catalysts: Pt/MgO
>
Pt/Al
2O
3
>
Pt/CeO
2
>
Pt/TiO
2
>
Pt/SiO
2. The WGS reactions were also examined over these five catalysts. The relationship between WGS and APR was discussed and found that WGS played a key role in the process of APR, both are related to the surface properties: the basic sites are prefer for water–gas shift and further enhanced the APR process. Typical solid basic magnesium and alumina mixed oxides supporting Pt catalysts were also tested and exhibited the best APR activity.
The addition of potassium atoms to Cu(111) and Cu/TiO2(110) surfaces substantially enhances the rate for water dissociation and the production of hydrogen through the water–gas shift reaction (WGS, ...CO + H2O → H2 + CO2). In the range of temperatures investigated, 550–625 K, Cu/K/TiO2(110) exhibits a WGS activity substantially higher than those of K/Cu(111), Cu(111), and Cu/ZnO(0001̅) systems used to model an industrial Cu/ZnO catalyst. The apparent activation energy for the WGS drops from 18 Kcal/mol on Cu(111) to 12 Kcal/mol on K/Cu(111) and 6 Kcal/mol on Cu/K/TiO2(110). The results of density functional calculations show that K adatoms favor the thermochemistry for water dissociation on Cu(111) and Cu/TiO2(110) with the cleavage of an O–H bond occurring at room temperature. Furthermore, at the Cu/K/TiO2 interface, there is a synergy, and this system has a unique ability to dissociate the water molecule and catalyze hydrogen production through the WGS process. Therefore, when optimizing a regular catalyst, it is essential to consider mainly the effects of an alkali promoter on the metal–oxide interface.
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