Ethylene Epoxidation at the Phase Transition of Copper Oxides

In Situ Formation of Platinum-Carbon Catalysts in Propane Dehydrogenation

The catalytic production of propylene via propane dehydrogenation (PDH) is a key reaction in the chemical industry. By combining operando transmission electron microscopy with density functional theory analysis, we show that the intercalation and ordering of carbon on Pt interstitials to form Pt-C solid solutions is relevant for increasing propylene production. More specifically, we found that at the point of enhanced propylene formation, the structure of platinum nanoparticles is transformed into a transient caesium chloride-type Pt-C polymorph. At more elevated temperatures, the zincblende and rock salt polymorphs seemingly coexist. When propylene production was highest, multiple crystal structures consisting of Pt and carbon were occasionally found to coexist in one individual nanoparticle, distorting the Pt lattice. Catalyst coking was detected at all stages of the reaction, but did initially not affect all particles. These findings could lead to the development of novel synthesis strategies towards tailoring highly efficient PDH catalysts.

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation

Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.

Electrical-Driven Directed-Evolution of Copper Nanowires Catalysts for Efficient Nitrate Reduction to Ammonia

The electrocatalytic conversion of nitrate (NO3 - ) to NH3 (NO3 RR) at ambient conditions offers a promising alternative to the Haber-Bosch process. The pivotal factors in optimizing the proficient conversion of NO3 - into NH3 include enhancing the adsorption capabilities of the intermediates on the catalyst surface and expediting the hydrogenation steps. Herein, the Cu/Cu2 O/Pi NWs catalyst is designed based on the directed-evolution strategy to achieve an efficient reduction of NO3 ‾. Benefiting from the synergistic effect of the OV -enriched Cu2 O phase developed during the directed-evolution process and the pristine Cu phase, the catalyst exhibits improved adsorption performance for diverse NO3 RR intermediates. Additionally, the phosphate group anchored on the catalyst's surface during the directed-evolution process facilitates water electrolysis, thereby generating Hads on the catalyst surface and promoting the hydrogenation step of NO3 RR. As a result, the Cu/Cu2 O/Pi NWs catalyst shows an excellent FE for NH3 (96.6%) and super-high NH3 yield rate of 1.2 mol h-1  gcat. -1 in 1 m KOH and 0.1 m KNO3 solution at -0.5 V versus RHE. Moreover, the catalyst's stability is enhanced by the stabilizing influence of the phosphate group on the Cu2 O phase. This work highlights the promise of a directed-evolution approach in designing catalysts for NO3 RR.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Understanding Copper(I)-Ethylene Bonding Using Cu K-Edge X-ray Absorption Spectroscopy

Copper plays many important roles in ethylene chemistry, thus generating significant interest in understanding the structures, bonding, and properties of copper(I)-ethylene complexes. In this work, the ethylene binding characteristics of a series of isolable Cu(I)-ethylene compounds supported by a systematic set of fluorinated and nonfluorinated bis- and tris(pyrazolyl)borate and the related bis(pyrazolyl)methane ligands have been investigated. Through a combination of X-ray absorption spectroscopy and quantum chemical calculations, we characterize their geometric and electronic structures and the role that fluorinated ligands play in lowering the electron density at Cu sites. Such ligands increase the ethylene-to-Cu σ-donor interaction and, correspondingly, decrease the Cu-to-ethylene π back-bonding. This latter interaction leads to a partial vacancy in the Cu 3d level, which manifests experimentally as a low-energy feature in the Cu K pre-edge, allowing for its direct observation and comparison within a series of Cu(I) compounds. The pre-edge feature is reproduced by TD-DFT calculations, and its energy position and total intensity are used to quantitatively probe Cu-ethylene bonding. The variations in the Cu electronic structure influence the stability and overall ethylene bonding strength of these compounds, ultimately showing how substituents on the supporting ligands have a notable effect on their physical and chemical properties.

Quo Vadis Dry Reforming of Methane?—A Review on Its Chemical, Environmental, and Industrial Prospects

In recent years, the catalytic dry reforming of methane (DRM) has increasingly come into academic focus. The interesting aspect of this reaction is seemingly the conversion of CO2 and methane, two greenhouse gases, into a valuable synthesis gas (syngas) mixture with an otherwise unachievable but industrially relevant H2/CO ratio of one. In a possible scenario, the chemical conversion of CO2 and CH4 to syngas could be used in consecutive reactions to produce synthetic fuels, with combustion to harness the stored energy. Although the educts of DRM suggest a superior impact of this reaction to mitigate global warming, its potential as a chemical energy converter and greenhouse gas absorber has still to be elucidated. In this review article, we will provide insights into the industrial maturity of this reaction and critically discuss its applicability as a cornerstone in the energy transition. We derive these insights from assessing the current state of research and knowledge on DRM. We conclude that the entire industrial process of syngas production from two greenhouse gases, including heating with current technologies, releases at least 1.23 moles of CO2 per mol of CO2 converted in the catalytic reaction. Furthermore, we show that synthetic fuels derived from this reaction exhibit a negative carbon dioxide capturing efficiency which is similar to burning methane directly in the air. We also outline potential applications and introduce prospective technologies toward a net-zero CO2 strategy based on DRM.

Phase Coexistence and Structural Dynamics of Redox Metal Catalysts Revealed by Operando TEM

Metal catalysts play an important role in industrial redox reactions. Although extensively studied, the state of these catalysts under operating conditions is largely unknown, and assignments of active sites remain speculative. Herein, an operando transmission electron microscopy study is presented, which interrelates the structural dynamics of redox metal catalysts to their activity. Using hydrogen oxidation on copper as an elementary redox reaction, it is revealed how the interaction between metal and the surrounding gas phase induces complex structural transformations and drives the system from a thermodynamic equilibrium toward a state controlled by the chemical dynamics. Direct imaging combined with the simultaneous detection of catalytic activity provides unparalleled structure-activity insights that identify distinct mechanisms for water formation and reveal the means by which the system self-adjusts to changes of the gas-phase chemical potential. Density functional theory calculations show that surface phase transitions are driven by chemical dynamics even when the system is far from a thermodynamic phase boundary. In a bottom-up approach, the dynamic behavior observed here for an elementary reaction is finally extended to more relevant redox reactions and other metal catalysts, which underlines the importance of chemical dynamics for the formation and constant re-generation of transient active sites during catalysis.

The Lord of the Chemical Rings: Catalytic Synthesis of Important Industrial Epoxide Compounds

The epoxidized group, also known as the oxirane group, can be considered as one of the most crucial rings in chemistry. Due to the high ring strain and the polarization of the C–O bond in this three-membered ring, several reactions can be carried out. One can see such a functional group as a crucial intermediate in fuels, polymers, materials, fine chemistry, etc. Literature covering the topic of epoxidation, including the catalytic aspect, is vast. No review articles have been written on the catalytic synthesis of short size, intermediate and macro-molecules to the best of our knowledge. To fill this gap, this manuscript reviews the main catalytic findings for the production of ethylene and propylene oxides, epichlorohydrin and epoxidized vegetable oil. We have selected these three epoxidized molecules because they are the most studied and produced. The following catalytic systems will be considered: homogeneous, heterogeneous and enzymatic catalysis.

Synthesis of a Gold-Metal Oxide Core-Satellite Nanostructure for In Situ SERS Study of CuO-Catalyzed Photooxidation

This work reports on an assembling-calcining method for preparing gold-metal oxide core-satellite nanostructures, which enable surface-enhanced Raman spectroscopic detection of chemical reactions on metal oxide nanoparticles. By using the nanostructure, we study the photooxidation of Si-H catalyzed by CuO nanoparticles. As evidenced by the in situ spectroscopic results, oxygen vacancies of CuO are found to be very active sites for oxygen activation, and hydroxide radicals (*OH) adsorbed at the catalytic sites are likely to be the reactive intermediates that trigger the conversion from silanes into the corresponding silanols. According to our finding, oxygen vacancy-rich CuO catalysts are confirmed to be of both high activity and selectivity in photooxidation of various silanes.

In situ tuning of electronic structure of catalysts using controllable hydrogen spillover for enhanced selectivity

In situ tuning of the electronic structure of active sites is a long-standing challenge. Herein, we propose a strategy by controlling the hydrogen spillover distance to in situ tune the electronic structure. The strategy is demonstrated to be feasible with the assistance of CoO_x/Al₂O₃/Pt catalysts prepared by atomic layer deposition in which CoO_x and Pt nanoparticles are separated by hollow Al₂O₃ nanotubes. The strength of hydrogen spillover from Pt to CoO_x can be precisely tailored by varying the Al₂O₃ thickness. Using CoO_x/Al₂O₃ catalyzed styrene epoxidation as an example, the CoO_x/Al₂O₃/Pt with 7 nm Al₂O₃ layer exhibits greatly enhanced selectivity (from 74.3% to 94.8%) when H₂ is added. The enhanced selectivity is attributed to the introduction of controllable hydrogen spillover, resulting in the reduction of CoO_x during the reaction. Our method is also effective for the epoxidation of styrene derivatives. We anticipate this method is a general strategy for other reactions.

In situ tuning of the electronic structure of active sites is a long-standing challenge. Here, the authors report an approach to tune the electronic structure of cobalt species during the styrene epoxidation reaction by the introduction of controllable hydrogen spillover for enhanced selectivity.

Chemical Batteries with CO2

Efforts to obtain raw materials from CO₂ by catalytic reduction as a means of combating greenhouse gas emissions are pushing the boundaries of the chemical industry. The dimensions of modern energy regimes, on the one hand, and the necessary transport and trade of globally produced renewable energy, on the other, will require the use of chemical batteries in conjunction with the local production of renewable electricity. The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary, robust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO₂ does not yet exist.

In situ observation of oscillatory redox dynamics of copper

How a catalyst behaves microscopically under reaction conditions, and what kinds of active sites transiently exist on its surface, is still very much a mystery to the scientific community. Here we present an in situ study on the red-ox behaviour of copper in the model reaction of hydrogen oxidation. Direct imaging combined with on-line mass spectroscopy shows that activity emerges near a phase boundary, where complex spatio-temporal dynamics are induced by the competing action of simultaneously present oxidizing and reducing agents. Using a combination of in situ imaging with in situ X-ray absorption spectroscopy and scanning photoemission microscopy, we reveal the relation between chemical and morphological dynamics and demonstrate that a static picture of active sites is insufficient to describe catalytic function of redox-active metal catalysts. The observed oscillatory redox dynamics provide a unique insight on phase-cooperation and a convenient and general mechanism for constant re-generation of transient active sites.

How a catalyst behaves microscopically under reaction conditions, and what kinds of active sites transiently exist on its surface, is still very much a mystery to the scientific community. Here the authors report on in situ observation of a redox active copper catalyst by a combination of in situ imaging and spectroscopy tools.

A DFT and microkinetic study of propylene oxide selectivity over copper-based catalysts: effects of copper valence states

The development of high-performance copper-based catalysts is critical for the selective oxidation of propylene in both technology and scientific fields.

Ca-Ag compounds in ethylene epoxidation reaction

The ethylene epoxidation is a challenging catalytic process, and development of active and selective catalyst requires profound understanding of its chemical behaviour under reaction conditions. The systematic study on intermetallic compounds in the Ca-Ag system under ethylene epoxidation conditions clearly shows that the character of the oxidation processes on the surface originates from the atomic interactions in the pristine compound. The Ag-rich compounds Ca₂Ag₇ and CaAg₂ undergo oxidation towards fcc Ag and a complex Ca-based support, whereas equiatomic CaAg and the Ca-rich compounds Ca₅Ag₃ and Ca₃Ag in bulk remain stable under harsh ethylene epoxidation conditions. For the latter presence of water vapour in the gas stream leads to noticeable corrosion. Combining the experimental results with the chemical bonding analysis and first-principles calculations, the relationships among the chemical nature of the compounds, their reactivity and catalytic performance towards epoxidation of ethylene are investigated.

A mini review of in situ near-ambient pressure XPS studies on non-noble, late transition metal catalysts

The rich surface chemistry of Fe, Co, Ni and Cu during heterogeneous catalytic reactions from the perspective of NAP-XPS studies.

Anisotropic Reactivity of CaAg under Ethylene Epoxidation Conditions

The chemical behavior of CaAg as catalyst for ethylene epoxidation was studied using a combination of experimental (X-ray powder diffraction, scanning electron microscopy, thermal analysis and infrared spectroscopy), and quantum chemical techniques as well as real-space chemical bonding analysis. Under oxidative ethylene epoxidation conditions, the CaAg (010) surface possesses an outstanding stability during long-term experiments. It is caused by the formation of an ordered, stable and dense CaO passivation layer with a small amount of embedded Ag atoms. On this way, the (010) surface constitutes a kinetic barrier for further incorporation of oxygen into the subsurface region and thereby prevents  further oxidative decomposition of CaAg. The calculated adsorption energies of the reaction species show strong adsorption of the reaction products that may explain the observed low conversion of ethylene toward ethylene oxide using CaAg as catalyst.

Molecular and dissociative O2 adsorption on the Cu2O(111) surface

Dissociative O₂ is preferred on the reconstructed Cu₂O(111) surface thermodynamically at high coverage.

The Electro-Deposition/Dissolution of CuSO4 Aqueous Electrolyte Investigated by In Situ Soft X-ray Absorption Spectroscopy

The electrodeposition nature of copper on a gold electrode in a 4.8 pH CuSO₄ solution was inquired using X-ray absorption spectroscopy, electrochemical quartz crystal microbalance, and thermal desorption spectroscopy techniques. Our results point out that the electrodeposition of copper prompts the formation of stable oxi-hydroxide species with a formal oxidation state Cu⁺ without the evidence of metallic copper formation (Cu⁰). Moreover, the subsequent anodic polarization of Cu₂O_aq yields the formation of CuO, in the formal oxidation state Cu²⁺, which is dissolved at higher anodic potential. It was found that the dissolution process needs less charge than that required for the electrodeposition indicating a nonreversible process most likely due to concomitant water splitting and formation of protons during the electrodeposition.