Water Gas Shift Activity Research Articles

Insights into Mechanisms of Reverse Water Gas Shift Activity Enhancement over Reverse Microemulsion‐Synthesized CuCeO2

AbstractCopper‐doped ceria (CuCeO2) catalysts with 0–26.5 Cu/(Cu+Ce) at% were synthesized via the reverse microemulsion method. X‐ray diffraction analysis of freshly synthesized and spent (post‐reaction) catalysts showed no separate phase of copper or copper oxide, indicating that Cu was incorporated into the CeO2 lattice, replacing Ce. Temperature programmed desorption experiments showed that the activation energy of CO2 desorption increased for higher Cu loadings, indicating stronger CO2 adsorption. This phenomenon was attributed to enhanced formation of oxygen vacancies due to Cu doping. X‐ray photoelectron spectroscopy further confirmed the enhanced generation of oxygen vacancies due to Cu incorporation. Catalytic performance evaluation with the H2/CO2 feed in the 300–600 °C range showed that all catalysts were 100 % selective to CO generation, with higher Cu loadings resulting in CO2 conversion close to equilibrium values at 500–600 °C. The activation energy of the reaction, determined through reaction tests, exhibited inverse relationship with the activation energy of CO2 desorption. The relationship between these two energy barriers is explored, providing valuable insights into the mechanism of RWGS activity enhancement.

Decoding the Promotional Effect of Iron in Bimetallic Pt-Fe-nanoparticles on the Low Temperature Reverse Water-Gas Shift Reaction.

The reverse water-gas shift (RWGS) reaction is a key technology of the chemical industry, central to the emerging circular carbon economy. Pt-based catalysts have previously been shown to effectively promote RWGS, especially when modified by promoter elements. However, their active states are still poorly understood. Here, we show that the intimate incorporation of an iron promoter into metal-oxide-supported Pt-based nanoparticles can increase their activity and selectivity for the low temperature reverse water-gas shift (LT-RWGS) substantially and drastically outperform unpromoted Pt-based materials. Specifically, the study explores the promotional effect of iron in Pt-Fe bimetallic systems supported on silica (PtxFey@SiO2) prepared by surface organometallic chemistry (SOMC). The most active catalyst (Pt1Fe1@SiO2) shows high selectivity (>99% CO) toward CO at a formation rate of 0.192 molCO h-1 gcat-1, which is significantly higher than that of monometallic Pt@SiO2 (96% sel. and 0.022 molCO h-1 gcat-1). In-situ diffuse reflectance FT-IR spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS) indicate a dynamic process at the catalyst surface under the reaction conditions, revealing distinct reaction pathways for the monometallic Pt@SiO2 and bimetallic PtxFey@SiO2 systems.

Frustrated Lewis pairs on Al-doped CeO2 for efficient reverse water–gas shift reaction

The reverse water–gas shift (RWGS) reaction holds promise for producing high value-added CO from CO2, serving as a crucial intermediate to feedstock for producing various hydrocarbons through Fischer-Tropsch synthesis reactions. Metallic catalysts often yield significant methane production as a by-product during operation, whereas non-metallic oxide catalysts offer potential for high CO selectivity. We prepare Al doped CeO2 nanorod by atomic layer deposition (ALD). The incorporation of Al creates more oxygen vacancies on the surface, resulting in increased Frustrated Lewis Pair (FLP) sites. Particularly, the CO yield of ALD-modified CeO2 was enhanced by 134 % compared to that of pristine CeO2 nanorods (CeO2-NR) at 500 °C, with CO selectivity reached 100 %. Density Functional Theory (DFT) calculations reveal a lower energy barrier for H2 dissociation facilitated by the formation of FLP sites, potentially leading to heightened activity in the Reverse Water Gas Shift (RWGS) reaction.

Alkali treatment modulated borate anions in layered double hydroxides as precursors of highly efficient Au-based catalysts for water-gas shift reaction

Layered double hydroxides (LDHs) as an adjustable layered materials were considered as good candidates for the supports of water-gas Shift (WGS) catalysts. However, there is rare report about the exfoliation preparation and WGS application of LDHs containing BO33− as intercalated anions. Herein, we reported a mild alkali treatment modulation of the intercalated borate anions in LDHs for improving the WGS catalytic performance of Au-based catalysts. Compared with inactive Au supported LDHs without alkali treatment, the moderate alkali treatment consumes intercalated BO33− anions in LDHs, arousing the WGS activities of Au-based catalysts. Among the different alkali treatment concentration, Au-LDHs-1M treated with 1 M NaOH present the highest WGS activity and excellent stability with a reduction of only 10 % for 60 h. The moderate alkali treatment promotes the exfoliation of LDHs into nanosheets covered Co3O4 small nanoparticles, which facilitated the high dispersion of Au species and also easily converted into active Fe3O4 and metallic Co phases. While Au-LDHs-5M presents such large Au particles that it exhibits low activity, and Au-LDHs-0M is inactive due to the formation of B2O and Fe23B6 poison phases for the WGS reaction.

Water-assisted oxidative redispersion of Cu particles through formation of Cu hydroxide at room temperature

Sintering of active metal species often happens during catalytic reactions, which requires redispersion in a reactive atmosphere at elevated temperatures to recover the activity. Herein, we report a simple method to redisperse sintered Cu catalysts via O2-H2O treatment at room temperature. In-situ spectroscopic characterizations reveal that H2O induces the formation of hydroxylated Cu species in humid O2, pushing surface diffusion of Cu atoms at room temperature. Further, surface OH groups formed on most hydroxylable support surfaces such as γ-Al2O3, SiO2, and CeO2 in the humid atmosphere help to pull the mobile Cu species and enhance Cu redispersion. Both pushing and pulling effects of gaseous H2O promote the structural transformation of Cu aggregates into highly dispersed Cu species at room temperature, which exhibit enhanced activity in reverse water gas shift and preferential oxidation of carbon monoxide reactions. These findings highlight the important role of H2O in the dynamic structure evolution of supported metal nanocatalysts and lay the foundation for the regeneration of sintered catalysts under mild conditions.

Ex situ and in situ studies on structural features of Pt/Ce0.75Zr0.25O2 catalyst for water gas shift reaction

Pt/Ce1-xZrxO2 catalysts have shown high activity in water gas shift (WGS) reaction, but their structural organization has been studied insufficiently. This work represents a detailed structural study on the supported Pt species and the metal/support interface in a 5 wt% Pt/Ce0.75Zr0.25O2 catalyst in the initial state, after reductive activation, and after WGS reaction in H2-enriched reformate-simulating mixture. A wide range of methods was used: ex situ and in situ X-ray diffraction (XRD) studies, high resolution transmission electron microscopy (HRTEM), X-ray atomic pair distribution function (PDF) method, pseudo in situ X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR). The Ce0.75Zr0.25O2 mixed oxide was shown to provide a highly dispersed state of Pt species. XPS revealed that the initial catalyst contains Pt2+species. PDF analysis allowed us to propose the structure of ultrafine PtO particles and to elucidate the metal-support interaction with fixation of Pt ions on the support surface. In situ XRD, pseudo in situ XPS, and H2-TPR studies revealed the reduction of ultrafine PtO particles in H2 atmosphere at low temperatures (20–90°С) with the formation of metallic Pt0 particles and simultaneous partial reduction of the support surface due to the hydrogen spillover. Catalytic tests of the as-prepared and poisoned with chlorine 5wt% Pt/Ce0.75Zr0.25O2 catalysts in the WGS reaction showed an important role of the oxygen vacancies in the oxide support as active sites. These findings contribute to the understanding catalytic performance of Pt/Ce1-xZrxO2 systems.

Titelbild: Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity (Angew. Chem. 8/2024)

Titelbild: Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity (Angew. Chem. 8/2024)

Cover Picture: Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity (Angew. Chem. Int. Ed. 8/2024)

Cover Picture: Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity (Angew. Chem. Int. Ed. 8/2024)

Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity.

The implementation of supported metal catalysts heavily relies on the synergistic interactions between metal nanoparticles and the material they are dispersed on. It is clear that interfacial perimeter sites have outstanding skills for turning catalytic reactions over, however, high activity and selectivity of the designed interface-induced metal distortion can also obtain catalysts for the most crucial industrial processes as evidenced in this paper. Herein, the beneficial synergy established between designed Pt nanoparticles and MnO in the course of the reverse water gas shift (RWGS) reaction resulted in a Pt/MnO catalyst having ≈10 times higher activity compared to the reference Pt/SBA-15 catalyst with >99 % CO selectivity. Under activation, a crystal assembly through the metallic Pt (110) and MnO evolved, where the plane distance differences caused a mismatched-row structure in softer Pt nanoparticles, which was identified by microscopic and surface-sensitive spectroscopic characterizations combined with density functional theory simulations. The generated edge dislocations caused the Pt lattice expansion which led to the weakening of the Pt-CO bond. Even though MnO also exhibited an adverse effect on Pt by lowering the number of exposed metal sites, rapid desorption of the linearly adsorbed CO species governed the performance of the Pt/MnO in the RWGS.

Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity

AbstractThe implementation of supported metal catalysts heavily relies on the synergistic interactions between metal nanoparticles and the material they are dispersed on. It is clear that interfacial perimeter sites have outstanding skills for turning catalytic reactions over, however, high activity and selectivity of the designed interface‐induced metal distortion can also obtain catalysts for the most crucial industrial processes as evidenced in this paper. Herein, the beneficial synergy established between designed Pt nanoparticles and MnO in the course of the reverse water gas shift (RWGS) reaction resulted in a Pt/MnO catalyst having ≈10 times higher activity compared to the reference Pt/SBA‐15 catalyst with >99 % CO selectivity. Under activation, a crystal assembly through the metallic Pt (110) and MnO evolved, where the plane distance differences caused a mismatched‐row structure in softer Pt nanoparticles, which was identified by microscopic and surface‐sensitive spectroscopic characterizations combined with density functional theory simulations. The generated edge dislocations caused the Pt lattice expansion which led to the weakening of the Pt−CO bond. Even though MnO also exhibited an adverse effect on Pt by lowering the number of exposed metal sites, rapid desorption of the linearly adsorbed CO species governed the performance of the Pt/MnO in the RWGS.

Effect of a Second Promoter on the Performance of a Potassium Doped Silica-Supported Cobalt Catalyst During CO2 Hydrogenation to Hydrocarbons

In this study, the promoting effects of ruthenium, palladium, and copper on the performance of a 15%Co-1%K/SiO2 catalyst were evaluated during CO2 hydrogenation in a fixed-bed reactor. Reactions were carried out at atmospheric pressure and 270 °C with H2/CO2 ratio of 3. All catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and temperature-programmed reduction (TPR). Ruthenium, palladium and copper facilitated the reduction of cobalt oxides and increased cobalt dispersion. In terms of catalyst’s performance, ruthenium addition led to increased CO2 conversion and methane selectivity with a detrimental effect on C5+ hydrocarbons. Palladium also presented a similar pattern at lower loading but a drop in CO2 conversion and increased reverse water–gas shift activity were observed at 3 wt % Pd loading. Promoting with copper resulted in decreased activity, methane selectivity and C5+ hydrocarbons productivity with a much higher CO selectivity.Graphical

Boosted reverse water-gas shift activity via exsolved Cu and Ni in silicalite-1.

The reverse water-gas shift (RWGS) reaction offers a sustainable approach for CO2 utilization, yielding CO for vital catalytic processes. This study compares the catalytic performance of exsolved Cu- and Ni-encapsulated silicalite-1 (S-1) catalysts against those prepared by impregnation methods. Exsolved catalysts, characterized by confined metal nanoparticles and distinct surface chemistry, exhibited higher CO selectivity and lower activation energies of CO formation than their impregnated counterparts. Surface and structural analyses revealed that the exsolution process enhanced RWGS activity, driven by altered metal-support interactions and unique adsorption behaviors, offering insights for improving the efficiency of RWGS catalysis.

Migration and aggregation of Pt atoms on metal oxide-supported ceria nanodomes control reverse water gas shift reaction activity

Single-atom catalysts (SACs) are particularly sensitive to external conditions, complicating the identification of catalytically active species and active sites under in situ or operando conditions. We developed a methodology for tracing the structural evolution of SACs to nanoparticles, identifying the active species and their link to the catalytic activity for the reverse water gas shift (RWGS) reaction. The new method is illustrated by studying structure-activity relationships in two materials containing Pt SACs on ceria nanodomes, supported on either ceria or titania. These materials exhibited distinctly different activities for CO production. Multimodal operando characterization attributed the enhanced activity of the titania-supported catalysts at temperatures below 320 ˚C to the formation of unique Pt sites at the ceria-titania interface capable of forming Pt nanoparticles, the active species for the RWGS reaction. Migration of Pt nanoparticles to titania support was found to be responsible for the deactivation of titania-supported catalysts at elevated temperatures. Tracking the migration of Pt atoms provides a new opportunity to investigate the activation and deactivation of Pt SACs for the RWGS reaction.

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

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.

Effect of Re Addition on the Water–Gas Shift Activity of Ni Catalyst Supported by Mixed Oxide Materials for H2 Production

Water–gas shift (WGS) reaction was performed over 5% Ni/CeO2, 5% Ni/Ce-5% Sm-O, 5% Ni/Ce-5% Gd-O, 1% Re 4% Ni/Ce-5% Sm-O and 1% Re 4% Ni/Ce-5% Gd-O catalysts to reduce CO concentration and produce extra hydrogen. CeO2 and M-doped ceria (M = Sm and Gd) were prepared using a combustion method, and then nickel and rhenium were added onto the mixed oxide supports using an impregnation method. The influence of rhenium, samarium and gadolinium on the structural and redox properties of materials that have an effect on their water–gas shift activities was investigated. It was found that the addition of samarium and gadolinium into Ni/CeO2 enhances the surface area, reduces the crystallite size of CeO2, increases oxygen vacancy concentration and improves Ni dispersion on the CeO2 surface. Moreover, the addition of rhenium leads to an increase in the WGS activity of Ni/CeMO (M = Sm and Gd) catalysts. The results indicate that 1% Re 4% Ni/Ce-5% Sm-O presents the greatest WGS activity, with the maximum of 97% carbon monoxide conversion at 350 °C. An increase in the dispersion and surface area of metallic nickel in this catalyst results in the facilitation of the reactant CO adsorption. The result of X-ray absorption near-edge structure (XANES) analysis suggests that Sm and Re in 1% Re 4% Ni/Ce-5% Sm-O catalyst donate some electrons to CeO2, resulting in a decrease in the oxidation state of cerium. The occurrence of more Ce3+ at the CeO2 surface leads to higher oxygen vacancy, which alerts the redox process at the surface, thereby increasing the efficiency of the WGS reaction.

Facile synthesis of CuFe2O4 catalyst by the electrospinning method to produce hydrogen via the water gas shift of waste-derived syngas

The environmental impact of increasing waste emissions and the demand for clean energy have led to the development of waste-to-energy technologies. Hydrogen, a clean energy carrier, can be produced via the water gas shift (WGS) reaction, accompanied by waste gasification. CuFe2O4 spinel ferrite has been synthesized by various methods, such as co-precipitation, electrospinning, dehydration, sol-gel, and hydrothermal methods, and has been applied to the WGS reaction of waste-derived syngas. Among the different methods, the electrospinning method affords nanofiber-structured CuFe2O4, which showed the highest WGS activity which is attributed to its remarkable reducibility and easier formation of active species. It was also found that the distinct nanofiber structure of this catalyst prevents Cu sintering at high temperatures, resulting in stable activity during the WGS reaction.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water–Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO2-anatase SAC (Pd1/TiO2) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO2 by H2 alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H2 is accompanied by the partial sintering of single Pd atoms (Pd1) into disordered, flat, ∼1 nm diameter clusters (Pdn). The highly active Pd sites in the new coordination environment under H2 are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pdn and facilitates the reduction of TiO2. In contrast, Pd1 sinters into crystalline, ∼5 nm particles (PdNP) during CO treatment, deactivating Pd1/TiO2. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H2 dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H2. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Feed Effects on Water–Gas Shift Activity of M/Co3O4-ZrO2 (M = Pt, Pd, and Ru) and Potassium Role in Methane Suppression

Water–gas shift (WGS) is an industrial process to tackle CO abatement and H2 upgradation. The syngas (CO and H2 mixture) obtained from steam or dry reformers often has unreacted (from dry reforming) or undesired (from steam reforming) CO2, which is subsequently sent to downstream WGS reactor for H2 upgradation. Thus, industrial processes must deal with CO2 and H2 in the reformate feed. Achieving high CO2 or H2 selectivities become challenging due to possible CO and CO2 methanation reactions, which further increases the separation costs to produce pure H2. In this study, M/Co3O4-ZrO2 (M = Ru, Pd and Pt) catalysts were prepared using sonochemical synthesis. The synthesized catalysts were tested for WGS activity under three feed conditions, namely, Feed A (CO and steam), Feed B (CO, H2 and steam) and Feed C (CO, H2, CO2 and steam). All the catalysts gave zero methane selectivity under Feed A conditions, whereas the methane selectivity was significant under Feed B and C conditions. Among all catalysts, PtCZ was found to be the best performing catalyst in terms of CO conversion and CO2 selectivity. However, it still suffered with low but significant methane selectivity. This best performing catalyst was further modified with an alkali component, potassium to suppress undesirable methane selectivity. All the catalysts were well characterized with BET, SEM, TEM to confirm the structural properties and effective doping of the noble metals. Additionally, the apparent activation energies were obtained to showcase the best catalyst.

Performance of Particulate and Structured Pt/TiO2-Based Catalysts for the WGS Reaction under Realistic High- and Low-Temperature Shift Conditions

The water–gas shift (WGS) activity of Pt/TiO2-based powdered and structured catalysts was investigated using realistic feed compositions that are relevant to the high-temperature shift (HTS) and low-temperature shift (LTS) reaction conditions. The promotion of the TiO2 support with small amounts of alkali- or alkaline earth-metals resulted in the enhancement of the WGS activity of 0.5%Pt/TiO2(X) catalysts (X = Na, Cs, Ca, Sr). The use of bimetallic (Pt–M)/TiO2 catalysts (M = Ru, Cr, Fe, Cu) can also shift the CO conversion curve toward lower temperatures, but this is accompanied by the production of relatively large amounts of unwanted CH4 at temperatures above ca. 300 °C. Among the powdered catalysts investigated, Pt/TiO2(Ca) exhibited the best performance under both HTS and LTS conditions. Therefore, this material was selected for the preparation of structured catalysts in the form of pellets as well as ceramic and metallic catalyst monoliths. The 0.5%Pt/TiO2(Ca) pellet catalyst exhibited comparable activity with that of a commercial WGS pellet catalyst, and its performance was further improved when the Pt loading was increased to 1.0 wt.%. Among the structured catalysts investigated, the best results were obtained for the sample coated on the metallic monolith, which exhibited excellent WGS performance in the 300–350 °C temperature range. In conclusion, proper selection of the catalyst structure and reaction parameters can shift the CO conversion curves toward sufficiently low temperatures, rendering the Pt/TiO2(Ca) catalyst suitable for practical applications.