Elucidation of the Reaction Mechanism for High-Temperature Water Gas Shift over an Industrial-Type Copper–Chromium–Iron Oxide Catalyst

The Enigma of Methanol Synthesis by Cu/ZnO/Al2O3-Based Catalysts

The activity and durability of the Cu/ZnO/Al2O3 (CZA) catalyst formulation for methanol synthesis from CO/CO2/H2 feeds far exceed the sum of its individual components. As such, this ternary catalytic system is a prime example of synergy in catalysis, one that has been employed for the large scale commercial production of methanol since its inception in the mid 1960s with precious little alteration to its original formulation. Methanol is a key building block of the chemical industry. It is also an attractive energy storage molecule, which can also be produced from CO2 and H2 alone, making efficient use of sequestered CO2. As such, this somewhat unusual catalyst formulation has an enormous role to play in the modern chemical industry and the world of global economics, to which the correspondingly voluminous and ongoing research, which began in the 1920s, attests. Yet, despite this commercial success, and while research aimed at understanding how this formulation functions has continued throughout the decades, a comprehensive and universally agreed upon understanding of how this material achieves what it does has yet to be realized. After nigh on a century of research into CZA catalysts, the purpose of this Review is to appraise what has been achieved to date, and to show how, and how far, the field has evolved. To do so, this Review evaluates the research regarding this catalyst formulation in a chronological order and critically assesses the validity and novelty of various hypotheses and claims that have been made over the years. Ultimately, the Review attempts to derive a holistic summary of what the current body of literature tells us about the fundamental sources of the synergies at work within the CZA catalyst and, from this, suggest ways in which the field may yet be further advanced.

Neutron Scattering Studies of Heterogeneous Catalysis

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

Theoretical Calculations on Metal Catalysts Toward Water-Gas Shift Reaction: a Review

Water-gas shift (WGS) reaction offers a dominating path to hydrogen generation from fossil fuel, in which heterogeneous metal catalysts play a crucial part in this course. This review highlights and summarizes recent developments on theoretical calculations of metal catalysts developed to date, including surface structure (e. g., monometallic and polymetallic systems) and interface structure (e. g., supported catalysts and metal oxide composites), with special emphasis on the characteristics of crystal-face effect, alloying strategy, and metal-support interaction. A systematic summarization on reaction mechanism was performed, including redox mechanism, associative mechanism as well as hybrid mechanism; the development on chemical kinetics (e. g., molecular dynamics, kinetic Monte Carlo and microkinetic simulation) was then introduced. At the end, challenges associated with theoretical calculations on metal catalysts toward WGS reaction are discussed and some perspectives on the future advance of this field are provided.

Atomically Dispersed Dual Metal Sites Boost the Efficiency of Olefins Epoxidation in Tandem with CO2 Cycloaddition

Tandem catalysis provides an economical and energy-efficient process for the production of fine chemicals. In this work, we demonstrate that a rationally synthesized carbon-based catalyst with atomically dispersed dual Fe-Al sites (ADD-Fe-Al) achieves superior catalytic activity for the one-pot oxidative carboxylation of olefins (conversion ∼97%, selectivity ∼91%), where the yield of target product over ADD-Fe-Al is at least 62% higher than that of monometallic counterparts. The kinetic results reveal that the excellent catalytic performance arises from the synergistic effect between Fe (oxidation site) and Al sites (cycloaddition site), where the efficient CO₂ cycloaddition with epoxides in the presence of Al sites (3.91 wt %) positively shifts the oxidation equilibrium to olefin epoxidation over Fe sites (0.89 wt %). This work not only offers an advanced catalyst for oxidative carboxylation of olefins but also opens up an avenue for the rational design of multifunctional catalysts for tandem catalytic reactions in the future.

A unique Co@CoO catalyst for hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran

The development of precious-metal-free catalysts to promote the sustainable production of fuels and chemicals from biomass remains an important and challenging target. Here, we report the efficient hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran over a unique core-shell structured catalyst, Co@CoO that affords the highest productivity among all catalysts, including noble-metal-based catalysts, reported to date. Surprisingly, we find that the catalytically active sites reside on the shell of CoO with oxygen vacancies rather than the metallic Co. The combination of various spectroscopic experiments and computational modelling reveals that the CoO shell incorporating oxygen vacancies not only drives the heterolytic cleavage, but also the homolytic cleavage of H₂ to yield more active H^δ− species, resulting in the exceptional catalytic activity. Co@CoO also exhibits excellent activity toward the direct hydrodeoxygenation of lignin model compounds. This study unlocks, for the first time, the potential of simple metal-oxide-based catalysts for the hydrodeoxygenation of renewable biomass to chemical feedstocks.

The development of precious-metal-free catalysts to promote the sustainable production of fuels and chemicals from biomass challenging. Here the authors report a unique core-shell structured Co@CoO catalyst which exhibits excellent performance in the hydrogenolysis of biomass-derived compounds.

The Route from Green H2 Production through Bioethanol Reforming to CO2 Catalytic Conversion: A Review

Currently, a progressively different approach to the generation of power and the production of fuels for the automotive sector as well as for domestic applications is being taken. As a result, research on the feasibility of applying renewable energy sources to the present energy scenario has been progressively growing, aiming to reduce greenhouse gas emissions. Following more than one approach, the integration of renewables mainly involves the utilization of biomass-derived raw material and the combination of power generated via clean sources with conventional power generation systems. The aim of this review article is to provide a satisfactory overview of the most recent progress in the catalysis of hydrogen production through sustainable reforming and CO2 utilization. In particular, attention is focused on the route that, starting from bioethanol reforming for H2 production, leads to the use of the produced CO2 for different purposes and by means of different catalytic processes, passing through the water–gas shift stage. The newest approaches reported in the literature are reviewed, showing that it is possible to successfully produce “green” and sustainable hydrogen, which can represent a power storage technology, and its utilization is a strategy for the integration of renewables into the power generation scenario. Moreover, this hydrogen may be used for CO2 catalytic conversion to hydrocarbons, thus giving CO2 added value.

Synthesis of Propylene Carbonate by Urea Alcoholysis—Recent Advances

Organic carbonates are considered the chemicals of the future. In particular, propylene carbonate is widely used as a non-reactive solvent, plasticizer, fuel additive, and reagent, especially in the production of environmentally friendly polymers that are not harmful to human health. This paper reviews recent literature findings regarding the development of propylene carbonate synthetic methods starting from propane-1,2-diol and urea. The ammonia formed during the synthesis is recycled to obtain urea from carbon dioxide.

Manipulating Copper Dispersion on Ceria for Enhanced Catalysis: A Nanocrystal-Based Atom-Trapping Strategy

Due to tunable redox properties and cost-effectiveness, copper-ceria (Cu-CeO2 ) materials have been investigated for a wide scope of catalytic reactions. However, accurately identifying and rationally tuning the local structures in Cu-CeO2 have remained challenging, especially for nanomaterials with inherent structural complexities involving surfaces, interfaces, and defects. Here, a nanocrystal-based atom-trapping strategy to access atomically precise Cu-CeO2 nanostructures for enhanced catalysis is reported. Driven by the interfacial interactions between the presynthesized Cu and CeO2 nanocrystals, Cu atoms migrate and redisperse onto the CeO2 surface via a solid-solid route. This interfacial restructuring behavior facilitates tuning of the copper dispersion and the associated creation of surface oxygen defects on CeO2 , which gives rise to enhanced activities and stabilities catalyzing water-gas shift reaction. Combining soft and solid-state chemistry of colloidal nanocrystals provide a well-defined platform to understand, elucidate, and harness metal-support interactions. The dynamic behavior of the supported metal species can be further exploited to realize exquisite control and rational design of multicomponent nanocatalysts.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.

Effects of oxygen vacancy defect on microwave pyrolysis of biomass to produce high-quality syngas and bio-oil: Microwave absorption and in-situ catalytic

This paper proposed to use ferric oxide (Fe₂O₃) and ferroferric oxide (Fe₃O₄) as catalysts with both microwave absorption and catalytic properties. Carbon dioxide (CO₂) was introduced as the reaction atmosphere to further improve the quality of biofuel produced by microwave pyrolysis of food waste (FW). The results showed the bio-gas yield and the syngas concentration (H₂ + CO) increased to 70.34 wt% and 61.50 mol%, respectively, using Fe₃O₄ as the catalyst. The content of aliphatic hydrocarbons in bio-oil produced with the catalyst Fe₂O₃ increased to 67.48% and the heating value reached 30.45 MJ/kg. Compared with Fe₂O₃ catalyst, Fe₃O₄ exhibited better microwave absorption properties and catalytic properties. Transmission electron microscopy (TEM) and Electron paramagnetic resonance (EPR) characterizations confirmed that the crystal surface of Fe₃O₄ formed more oxygen vacancy defects and unpaired electrons. Additionally, according to the X-ray photoelectron spectroscopy (XPS) analysis, the content of lattice oxygen in Fe₃O₄ was 14.11%, a value that was much lower than Fe₂O₃ (38.54%). The oxygen vacancy defects not only improved the efficient utilization of microwave energy but also provided the reactive sites for the reaction between the volatile organic compounds (VOCs) and CO₂ to generate CO. This paper provides a new perspective for selecting catalysts that have both microwave absorption and catalytic properties during the microwave pyrolysis of biomass.

Elucidation of copper environment in a Cu-Cr-Fe oxide catalyst through in situ high-resolution XANES investigation

Copper containing materials are widely used in a range of catalytic applications. Here, we report the use of Cu K-edge high resolution XANES to determine the local site symmetry of copper ions during the thermal treatment of a Cu-Cr-Fe oxide catalyst. We exploited the Cu K-edge XANES spectral features, in particular the correlation between area under the pre-edge peak and its position to determine the local environment of Cu2+ ions. The information gained from this investigation rules out the presence of Cu2+ ions in a tetrahedral or square planar geometry, a mixture of these sites, or in a reduced oxidation state. Evidence is presented that the Cu2+ ions in the Cu-Cr-Fe oxide system are present in a distorted octahedral environment.

Reversible structural transformation and redox properties of Cr-loaded iron oxide dendrites studied by in situ XANES spectroscopy

Cr-Loaded iron oxide with a dendritic crystalline structure was synthesized and the reversible crystalline phase transition during redox cycling of the iron oxide was investigated. X-ray diffraction and transmission electron microscopy analyses revealed that Cr was well dispersed and loaded in the iron oxide dendrite crystals, whose lattice constant was dependent on the Cr loading. Temperature-programmed oxidation and reduction experiments revealed the reversible redox properties of the Cr-loaded iron oxide dendrites, whose redox temperature was found to be lower than that of Cr-free iron oxide dendrites. In situ Fe K-edge and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated that Cr loading extended the redox reaction window for conversion between Fe3O4 and γ-Fe2O3 owing to compressive lattice strain in the iron oxide spinel structures.

Investigations of Hydrocarbon Species on Solid Catalysts by Inelastic Neutron Scattering

The status of surface species on solid catalysts during heterogeneous catalysis is often mysterious. Investigations of these surface species are crucial to deconvolute the reaction network and design more efficient catalysts. Vibrational spectroscopy is a powerful technique to study the interactions between surface species and the catalysts and infrared (IR) and Raman spectroscopies have been widely applied to study reaction mechanisms in heterogeneous catalysis. However, IR/Raman spectra are difficult to model computationally and important vibrational modes may be IR-, Raman- (or both) inactive due to restrictions by optical selection rules. Inelastic neutron scattering (INS) is another form of vibrational spectroscopy and relies on the scattering of neutrons by the atomic nucleus. A consequence of this is that INS is not subject to any optical selection rules and all vibrations are measurable in principle. INS spectroscopy has been used to investigate surface species on catalysts in a wide range of heterogeneous catalytic reactions. In this mini-review, we focus on applications of INS in two important fields: petrochemical reactions and C1 chemistry. We introduce the basic principles of the INS technique, followed by a discussion of its application in investigating two key catalytic systems: (i) the behaviour of hydrocarbons on metal-oxide and zeolite catalysts and (ii) the formation of hydrocarbonaceous species on methane reforming and Fischer–Tropsch catalysts. The power of INS in studying these important catalytic systems is demonstrated.

High Temperature Water Gas Shift Reactivity of Novel Perovskite Catalysts

High temperature water-gas shift (HT-WGS) is an industrially highly relevant reaction. Moreover, climate change and the resulting necessary search for sustainable energy sources are making WGS and reverse-WGS catalytic key reactions for synthetic fuel production. Hence, extensive research has been done to develop improved or novel catalysts. An extremely promising material class for novel highly active HT-WGS catalysts with superior thermal stability are perovskite-type oxides. With their large compositional flexibility, they enable new options for rational catalyst design. Particularly, both cation sites (A and B in ABO3) can be doped with promoters or catalytically active elements. Additionally, B-site dopants are able to migrate to the surface under reducing conditions (a process called exsolution), forming catalytically active nanoparticles and creating an interface that can strongly boost catalytic performance. In this study, we varied A-site composition and B-site doping (Ni, Co), thus comparing six novel perovskites and testing them for their HT-WGS activity: La0.9Ca0.1FeO3-δ, La0.6Ca0.4FeO3-δ, Nd0.9Ca0.1FeO3-δ, Nd0.6Ca0.4FeO3-δ, Nd0.6Ca0.4Fe0.9Ni0.1O3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ. Cobalt and Nickel doping resulted in the highest activity observed in our study, highlighting that doped perovskites are promising novel HT-WGS catalysts. The effect of the compositional variations is discussed considering the kinetics of the two partial reactions of WGS-CO oxidation and water splitting.

Cr-Free, Cu Promoted Fe Oxide-Based Catalysts for High-Temperature Water-Gas Shift (HT-WGS) Reaction

Ca, Ni, Co, and Ge promoters were examined as potential candidates to substitute for the current toxic Cr in Cu-promoted Fe oxide-based catalysts for the HT-WGS reaction. The Ca and Ni promoters were found to improve catalyst performance relative to promotion with Cr. The HS-LEIS surface analysis data demonstrate that Ca and Ge tend to segregate on the surface, while Ni, Co, and Cr form solid solutions in the Fe3O4 bulk lattice. The corresponding number of catalytic active sites, redox, and WGS activity values of the catalysts were determined with CO-TPR, CO+H2O-TPSR, and SS-WGS studies, respectively. The poorer HT-WGS performances of the Ge and Co promoters are related to the presence of surface Ge and Co that inhibits catalyst redox ability, with the Co also not stabilizing the surface area of the Fe3O4 support. The Ni promoter uniformly disperses the Cu nanoparticles on the catalyst surface and increases the number of FeOx-Cu interfacial redox sites. The Ca promoter on the catalyst surface, however, enhances the activity of the FeOx-Cu interfacial redox sites. The CO+H2O TPSR results reveal that the redox ability of the active sites follows the SS-WGS performance of the catalysts and show the following trend: 3Cu8CaFe > 3Cu8NiFe ≥ 3Cu8CrFe > 3Cu8CoFe >> 3Cu8GeFe. Furthermore, all the catalysts followed a redox-type reaction mechanism for the HT-WGS reaction.

The catalytic mechanism of the Au@TiO2−x/ZnO catalyst towards a low-temperature water-gas shift reaction

A redox mechanism towards the water-gas shift reaction was certified based on in situ/operando experiments and density functional theory calculation studies.