Catalysts For Water-gas Shift Reaction Research Articles

Nickel and Iron Biocarbon Catalysts for Water-Gas Shift Reaction

Nickel and Iron Biocarbon Catalysts for Water-Gas Shift 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.

NiCx/Ni-foam discovered as a promising high-temperature WGSR catalyst

Water gas shift reaction (WGSR) is urgently calling for an alternative to the Fe-Cr catalyst. Ni-based catalysts are taken into consideration but remain challenging because of their high methanation activity. Here we report a highly active and selective NiCx/Ni-foam catalyst with clearly detectable Ni3C phase, facilely obtainable by endogenous growth of NiC2O4 onto a Ni-foam and subsequent H2-reduction treatment. The preferred catalyst with high content of NiCx is capable of converting 63.6% or 94.3% CO at 350 °C or 400 °C with trace or < 0.8% CH4 formation for a feed gas with H2O/CO/Ar molar ratio of 4/1/9, and is stable for at least 110 h. It is experimentally and theoretically unveiled that the Ni3C facilitates the dissociative activation of H2O and favors CO to form carboxylate species (favorable for CO2 formation) rather than active Ni(CO)n intermediates for CH4 formation thereby enabling the catalyst with high WGSR activity and selectivity.

Coordination-number-determined activity of copper catalyst in water-gas shift reaction

The water–gas shift reaction (WGSR) is of great significance in industrial hydrogen production. In this paper, Cun/TiO2(n = 1,2,4,8) and Cu (111) crystal planes were studied to explore the effect of size effect on the activity of WGSR. Density functional theory (DFT) calculations showed that the adsorption energy of H2O molecule was significantly enhanced on the surface of Cu1 and Cu2 clusters compared with Cu4 and Cu8 clusters, while CO adsorption strength first increased and then decreased. Among many copper clusters of different sizes, the redox pathway cannot proceed due to the lack of active sites on Cu1 and Cu2 clusters. According to the analysis of the microkinetic model, the order of WGSR activity on the surface of copper clusters of different sizes is: Cu4 > Cu2 ≈ Cu8 > Cu1 > Cu (111). The active center structure of the copper cluster is composed of 3 copper atoms in which the Cu-Cu coordination number is ca. 2 ∼ 3, and the active center is partially oxidized copper. Essentially, the low-coordination copper atoms determined by the geometry structure promote the WGSR activity of the copper clusters. This work not only reveals the copper cluster size effect on WGSR, but also provides the guidance for the size design and control of other supported metal catalysts.

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

AbstractThe 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 *H2O 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.

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.

First‐Principles Density Functional Theory and Machine Learning Technique for the Prediction of Water Adsorption Site on PtPd‐Based High‐Entropy‐Alloy Catalysts

AbstractThe water‐gas shift reaction (WGSR) is employed in industry to obtain high‐purity H2 from syngas, where H2O adsorption is an important step that controls H2O dissociation in WGSR. Therefore, exploring catalysts exhibiting strong H2O adsorption energy (Eads) is crucial. Also, high‐entropy alloys (HEA) are promising materials utilized as catalysts, including in WGSR. The PtPd‐based HEA catalysts are explored via density functional theory (DFT) and Gaussian process regression. The input features are based on the microstructure data and electronic properties: d‐band center (εd) and Bader net atomic charge (δ). The DFT calculation reveals that the εd and δ of each active site of all HEA surfaces are broadly scattered, indicating that the electronic properties of each atom on HEA are non‐uniform and influenced by neighboring atoms. The strong H2O‐active‐site interaction determined by a highly negative Eads is used as a criterion to explore good PtPd‐based WGSR catalyst candidates. As a result, the potential candidates are found to have Co, Ru, and Fe as an H2O adsorption site with Ag as a neighboring atom, that is, PtPdRhAgCo, PtPdRuAgCo, PtPdRhAgFe, and PtPdRuAgFe.

Unravelling the active sites and structure-activity relationship on Cu–ZnO–Al2O3 based catalysts for water-gas shift reaction

Herein, we investigated the main active sites and structure-sensitivity of the water-gas shift (WGS) reaction over ternary Cu–ZnO–Al2O3 (CZA) catalysts. CZA catalysts with various Cu contents were synthesized by the homogeneous one-step coprecipitation method. The Cu content mainly affected the number of active Cu sites and was closely related to the WGS activity. Turnover frequency (TOF) values were independent of Cu dispersion, indicating that the CZA catalyst is structure-insensitive in WGS. The ratio of surface Cu+ species also strongly influenced the activity of the CZA catalyst. TOF based on the total active Cu species showed a constant value, but the CO conversion was linearly increased with the number of surface Cu+ species.

α-MoC Supported Noble Metal Catalysts for Water-Gas Shift Reaction: Single-Atom Promoter or Single-Atom Player.

In this work, we study the water-gas shift (WGS) reaction catalyzed by α-MoC(100) supported typical platinum group metal (PGM) single atoms (Rh1, Pd1, and Pt1) and Au1 via density functional theory calculations. The adsorption energies of key reaction intermediates and the kinetic barriers of the proposed rate-determining step in the WGS were systematically investigated. It is found that Rh1, Pd1, and Pt1 can serve as single-atom promoters (SAPs) to improve the WGS performance of surface Mo atoms on α-MoC(100). The enhanced activity originates from the fact that SAP modifies the electronic structure of Mo active sites. Comparatively, the Au1 species not only acts as an SAP but also directly participates in the catalysis as a single-atom player. The additional experiments with single-atom catalyst performance and kinetic studies confirm the theoretical calculation conclusions. This study can provide a basis to further develop efficient WGS catalysts by tuning the activity of the substrate with intercalation of SAPs.

Structural Evolution of Cu/ZnO Catalysts during Water-Gas Shift Reaction: An In Situ Transmission Electron Microscopy Study.

Supported metal catalysts experience significant structural evolution during the activation process and reaction conditions, which is critical to achieve a desired active surface and interface enabling efficient catalytic processes. However, such dynamic structural information and related mechanistic understandings remain largely elusive owing to the limitation of real-time capturing dynamic information under reaction conditions. Here, using in situ environment transmission electron microscopy, we demonstrate the atomic-scale structural evolution of the model Cu/ZnO catalyst under relevant water-gas shift reaction (WGSR) conditions. Under a CO gas environment, Cu nanoparticles decompose into smaller Cu species and redistribute on ZnO supports with either the crystalline Cu2O or amorphous CuOx phase due to a strong CO-Cu interaction. In addition, we visualize various metal-support interactions between Cu and ZnO under reaction conditions, e.g., ZnO clusters precipitating on Cu nanoparticles, which are critical to understand active sites of Cu/ZnO as catalysts for WGSR. These in situ atomic-scale observations highlight the dynamic interplays between Cu and ZnO that can be extended to other supported metal catalysts.

The role of additives (Ba, Zr, and Nd) on Ce/Cu/Al2O3 catalyst for water-gas shift reaction

Enhanced catalytic performance is required to increase the efficiency of hydrogen production from waste-derived synthesis gas via the high-temperature water-gas shift reaction (HT-WGSR). Herein, the effects of barium, zirconium, and neodymium doping on the physico-chemical properties of a Ce/Cu/Al2O3 catalyst as well as its catalytic performance for HT-WGSR are investigated. Ce/Cu/Al2O3 catalysts with various additives (barium, zirconium, and neodymium) prepared via a sequential impregnation method have been characterized by using N2 adsorption-desorption isotherms, X-ray powder diffraction (XRPD), N2O-titration, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and hydrogen temperature-programmed reduction (H2-TPR). Advantageously, barium and zirconium addition enhance the HT-WGSR activity and stability of the Ce/Cu/Al2O3 catalyst, whereas neodymium doping has a negative effect. Regarding the correlation of catalytic performance with the characterization results, it was found that catalytic activity and stability strongly depended on their oxygen vacancy concentration and strong-metal to support interaction.

Ensemble effect of heterogeneous Cu atoms promoting water-gas shift reaction

It is crucial to explore the best active center for a catalyst in heterogeneous catalysis. Here we probe the performance of B single-atom doped Cu catalyst and the corresponding pure metal catalyst in water-gas shift reaction (WGSR) through density functional theory (DFT). The results show that Cu catalyst modified by B single-atom obtained unprecedented catalytic property compared to the corresponding pure metal catalyst, which is due to the special active site for water dissociation that the embedded B single-atom creates two heterogeneous Cu atoms with different electronic structures. Moreover, due to its charming electronic property, B atom can act as a charge transfer medium, freely switching between anionic and cationic states. More importantly, the synergy of heterogeneous Cu atoms reduces the activation energy barrier of the rate-limiting step (water dissociation) by 0.11 eV on B-doped Cu surface in WGSR. Electronic structure analysis emerges a downshifted d-band center of surface Cu atoms coordinated with B atom and an upshifted d-band center of those by Cu-Cu coordination originated from strain and ligand effects. The findings aim to provide deep insights for people to design new catalysts using earth-abundant elements in WGSR.

Pelletized Cu-Ni/CePr5 catalysts for H2 purification via Water Gas Shift reaction

Water Gas Shift (WGS) reaction recently gained attraction as a purifying method to remove CO in H2 stream produced from bio-alcohols reforming for fuel cells applications. In previous works, we tested Cu-Ni samples supported on Pr promoted ceria as catalysts for WGS reaction. However, powder samples are not recommended for a scale-up process and ceria samples lack of cohesive forces to obtain a pellet. Then, supported catalysts can be an interesting alternative to study. In the present work, alumina spheres were impregnated in order to replicate the same chemical formulation of powder-state samples. It was studied the influence in catalytic performance of several textural and catalytic features such as: pellet size, active phase content and distribution, Cu:Ni ratio in bimetallic samples, and contact time. The Cu samples showed a moderate activity and the Ni ones a better performance, but tendency to generate CH4. For all catalysts, the decrease in the pellet size induced a higher CO conversion, indicating strong mass diffusion effects. Besides, it was observed that small addition of Cu to Ni samples significantly moderated the methanation reaction, but maintaining the activity. The change in active metal content slightly modified the activity and changed the selectivity, in agreement with the characterization techniques results (H2-TPR and OSC). In summary, the sample with Cu:Ni ratio equal to 1:2 with 1.5 wt%, presented a great performance, by removing 8 to 2 %v/v of CO with an acceptable contact time and high selectivity, being the most promissory catalyst for higher scale tests.

Effect of Y-doping on the catalytic properties of CuO/CeO2 catalysts for water-gas shift reaction

CuO/ceria and CuO/Y-doped ceria catalysts were synthesized. The Y-modified supports (1.0, 2.5 and 5.0 wt% Y2O3) were prepared by coprecipitation. CuO (3 wt% Cu) was loaded by deposition-precipitation. Having in mind the known effect of Y3+ modification for the generation of oxygen vacancies in ceria, its positive role on the water-gas shift (WGS) performance was expected. However, the catalytic test showed a trend of decreased WGS activity by increasing the Y-dopant amount, nevertheless that the differences were not very substantial. On the basis of XRD, XPS, EPR, Raman spectroscopy and H2-TPR results the explanation related to the key role of the oxygen mobility influenced by Y-doping could be proposed. The reason of the inferior WGS performance with increasing Y-content would be the higher amount of surface oxygen vacancies around Y3+ ions which disturbed the Cu–Ovac–Ce active sites for WGS reaction. Though, during the long run catalytic tests in WGS reaction a positive effect of Y-doping for improved stability of CuO/ceria catalysts was evidenced.

Experimental and theoretical investigations into the activity and mechanism of the water–gas shift reaction catalyzed by Au nanoparticles supported on Zn–Al/Cr/Fe layered double hydroxides

Water–gas shift reaction (WGSR) is an industrialized reaction with numerous applications concerning CO removal and H2 generation. Since this process is widely used and occurs at elevated temperatures, the development of high efficiency and stable catalysts for WGSR and investigating their catalytic mechanism is a hot topic. In this paper, we demonstrate Au nanoparticles supported on different layered double hydroxides (LDHs) as highly efficient and stable catalysts for WGSR. The incorporation of Au nanoparticles significantly decreases the activation energy and enhances the catalytic activity of LDHs for WGSR, with Au/ZnCr–LDHs exhibiting the best catalytic performance including: 79.4% CO conversion, 102.1 μmol gcat−1 s−1 of reaction rate, 1.01 s−1 TOF values and 41.7 kJ mol−1 of activation energy. TPR experiments suggest that the addition of Au alters the redox cycle on the surface of the catalyst, a key intermediary step involved in the catalytic process. In situ DRIFTS shows that the production of CO2 during WGSR involves the reaction between CO and adsorbed O, which comes from the dissociation of OH species and not the decomposition of formates. DFT calculations indicate that Au–based catalysts can effectively lower the energy barrier of the kinetically relevant step of H2O dissociation, which is the most probable reason for the enhancement of activity. The calculated activation barriers coincide with the experimentally measured values with the order of Au/ZnCr–LDHs<Au/ZnFe–LDHs<Au/ZnAl–LDHs<LDHs. Particularly, redox mechanism B has the lowest activation barriers which is the most potential reaction pathway and perfectly supports the in–situ DRIFTS results.

Effect of precipitation on physico-chemical and catalytic properties of Cu-Zn-Al catalyst for water-gas shift reaction

To investigate the effect of precipitation on physico-chemical and catalytic properties of Cu-Zn-Al catalyst, the pH value and injection rate in the precipitation process were systematically changed, and the water-gas shift reaction was carried out. The Cu-Zn-Al catalyst showed the highest CO conversion when the optimized synthesis parameters (i.e., pH=10-10.5 and injection rate=30 ml/min) were employed. This is mainly due to the enhanced physicochemical properties such as a high Brunauer-Emmett-Teller surface area, small crystallite size, high Cu dispersion, and easier reducibility.

Ammonia-free synthesis of Mo/CoMgAl catalysts with excellent activities in water-gas shift reaction

A series of CoMgAl mixed oxides with homogeneous Co dispersion were prepared using co-precipitation method. The CoMgAl composites were then doped with Mo species, giving Mo/Co(x)MgAl (x=Co/(Co+Mg)) catalysts for water-gas shift reaction (WGSR). In the synthesis of these catalysts, there is limited release of ammonia, which is traditionally common in the generation of Mo-Co catalysts for WGSR. The Mo/Co(x)MgAl catalysts were characterized using techniques such as XRD, N2-physisorption, H2-TPR, H2S-TPS, Raman, HRTEM and XPS. It is noted that the Mo/Co(5%)MgAl catalyst gives 91.2% CO conversion at 350 °C, which is close to the equilibrium CO conversion (91.7%). The H2S-TPS results disclose that there is decrease in activation energy for Mo sulfidation with increase of Co content in Mo/Co(x)MgAl, indicating the presence of Co promotes Mo sulfidation. The outcomes of HRTEM and XPS analyses reveal that the role of Co is to decrease stacking number and slab length of MoS2 moieties. The EDX elemental analysis suggests synergism between Co and MoS2, especially in the case of Mo/Co(5%)MgAl. With the formation of small-size MoS2 slabs and enhanced Co-MoS2 synergistic effect, the Mo/Co(5%)MgAl catalyst has high exposure of active sites, leading to improved catalytic activities in WGSR.

Transition Metal‐Doped Edge‐Terminated MoS2 Superstructures as Efficient Catalysts for H2 Production

AbstractAs the most important layered transition metal dichalcogenide, molybdenum disulfide (MoS2) is applicable for simultaneous H2 production and CO elimination by water‐gas shift reaction (WGSR), while its performance is still not satisfactory. To further promote the catalytic activities of MoS2, herein a series of transition metals (M = Mn, Fe, Co, Ni, Cu, and Zn) into the column‐like superstructures with edge‐terminated MoS2 nanosheet building blocks (termed as M‐ET MoS2 SSs) is doped as the efficient catalysts for WGSR. A precise M doping into ET MoS2 SSs reveals that the M‐ET MoS2 SSs exhibit the volcano‐like activity toward WGSR as a function of specific M with the 5%Ni‐ET MoS2 SSs displaying the best average rate of 50.48 µmolH2 gcat. −1 s−1 in WGSR at 473 K, which is 4 and 13 times higher than those of the ET MoS2 SSs and commercial MoS2, respectively, and is even comparable to many noble‐metal‐based catalysts. The valence band spectra reveal that the boosted WGSR activity of 5%Ni‐ET MoS2 SSs can be ascribed to the downshift of the d‐band center relative to the Fermi level, which facilitates the H desorption to produce H2 in WGSR, showing a highly efficient and earth‐abundant catalysts for WGSR and beyond.

Preparation of CuO/ZnO/Nb2O5 catalyst for the water-gas shift reaction

Abstract CuO/ZnO/Nb2O5 catalyst was prepared by combining the co-precipitation and deposition-precipitation preparation methods in order to develop a novel catalyst for water-gas shift reaction. The effect of alumina substitution for niobia and the influence of zinc oxide presence were evaluated by comparing the catalytic activity of CuO/ZnO/Nb2O5 catalyst with CuO/ZnO/Al2O3 and CuO/Nb2O5 catalytic activities. Nitrogen physisorption, X-ray diffraction, Energy dispersive X-ray spectroscopy, N2O titration and temperature-programmed reduction analysis were performed to evaluate the physicochemical properties of the materials and the catalytic tests were carried out under near-industrial conditions. CuO/Nb2O5 catalyst was inactive during all catalytic tests while CuO/ZnO/Nb2O5 catalyst exhibited significant higher TOF values compared with CuO/ZnO/Al2O3. Catalytic activity measurements revealed that zinc presence was important for catalytic activity under the studied conditions and indicated that the replacement of Al2O3 by Nb2O5 in the ternary catalyst as well as the presence of residual hydroxycarbonates promoted the catalytic activity.

Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen-Etching Technology: An Efficient Support of Gold Catalysts for Water-Gas Shift Reaction.

The surface structure of supports is crucial to fabricate efficient supported catalysts for water-gas shift (WGS). Here, hardly reducible ZrO2 was etched with hydrogen (H), aiming to modify surface structures with sufficient stable oxygen vacancies. After deposition of gold species, the obtained khaki ZrO2-H notably improved WGS catalytic activities and stabilities in comparison to the traditional white ZrO2. The characterization results and quantitative analysis indicate that sufficient surface oxygen vacancies of ZrO2-H support give rise to more metallic Au0 species and higher microstrain, which all boost WGS catalytic activities. Furthermore, optoelectronic properties were successfully used to correlate with their WGS thermocatalytic activities, and then a modified electron flow process was proposed to understand the WGS pathway. For one thing, the introduction of surface oxygen vacancies narrowed the band gap of ZrO2 and decreased the Ohmic barrier, which facilitated the flow of "hot-electron". For another thing, the conduction band electrons can be easily trapped by oxygen vacancies of ZrO2 supports, and then these trapped electrons immediately take part in reduction of H2O to H2. Thus, the electron recombination was suppressed and the WGS catalytic activity was improved. It is worth extending H2-etching technology to improve other thermocatalytic reactions.