Role of ZnO and CeOx in Cu-Based Model Catalysts in Activation of H2O and CO2 Dynamics Studied by in Situ Ultraviolet–Visible and X-ray Photoelectron Spectroscopy

High-area alumina supported Cu-Ce atomic species for water-gas shift reaction

Atomically dispersed cerium species, anchored to high-area alumina by unsaturated penta-coordinated aluminum, strongly interact with atomically dispersed Cu species to provide active centers for water-gas shift reaction (WGSR). The alumina-anchored Ce3+ species stabilize atomically dispersed Cu+ to form Cu+-Ce3+ active complexes and they work synergistically to enhance low-temperature WGSR activity. This work offers alternative approaches to developing low-cost catalysts for the WGSR process.

The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum-ceria catalyst. At low temperatures (25-100 °C and 1 mbar H2) surface O-H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O-H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring.

Unveiling the Atomic Structure and Growth Dynamics of One-Dimensional Water on ZnO(10-10)

The adsorption and organization state of water on the metal oxide surface is of critical importance for wide fields where interface chemistry dominates. On the technically important ZnO(10-10) surface, we found water assembles into an one-dimensional (1D) chain structure at submonolayer coverage instead of the well-known half-dissociated two-dimensional (2D) island. With a combination of high resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we clearly distinguished the single and double water chains, which are composed of dissociated monomers and half-dissociated dimers, respectively. Moreover, we unambiguously determined that single water molecules dissociate spontaneously before agglomerating into ordered phase, which is contrary to the proposition of previous studies. These results have deepened our understandings of the adsorbed water species on the ZnO surface, which may bring new insights into the mechanisms of water-stimulated surface reactions.

Metal–support interaction induced ZnO overlayer in Cu@ZnO/Al2O3 catalysts toward low-temperature water–gas shift reaction

The water–gas shift reaction (WGSR) plays a pivotal role in many important industrial processes as well as in the elimination of residual CO in feed gas for fuel cells. The development of a high-efficiency low-temperature WGSR (LT-WGSR) catalyst has attracted considerable attention. Herein, we report a ZnO-modified Cu-based nanocatalyst (denoted as Cu@ZnO/Al₂O₃) obtained via an in situ topological transformation from a Cu₂Zn₁Al-layered double hydroxide (Cu₂Zn₁Al-LDH) precursor at different reduction temperatures. The optimal Cu@ZnO/Al₂O₃-300R catalyst with appropriately abundant Cu@ZnO interface structure shows superior catalytic performance toward the LT-WGSR with a reaction rate of up to 19.47 μmol_CO g_cat⁻¹ s⁻¹ at 175 °C, which is ∼5 times larger than the commercial Cu/ZnO/Al₂O₃ catalyst. High-resolution transmission electron microscopy (HRTEM) proves that the reduction treatment results in the coverage of Cu nanoparticles by ZnO overlayers induced by a strong metal–support interaction (SMSI). Furthermore, the generation of the coating layers of ZnO structure is conducive to stabilize Cu nanoparticles, accounting for long-term stability under the reaction conditions and excellent start/stop cycle of the Cu@ZnO/Al₂O₃-300R catalyst. This study provides a high-efficiency and low-cost Cu-based catalyst for the LT-WGSR and gives a concrete example to help understand the role of Cu@ZnO interface structure in dominating the catalytic activity and stability toward WGSR.

The water–gas shift reaction (WGSR) plays a pivotal role in many important industrial processes as well as in the elimination of residual CO in feed gas for fuel cells.

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

The active sites of Cu-ZnO catalysts for water gas shift and CO hydrogenation reactions

Cu–ZnO–Al₂O₃ catalysts are used as the industrial catalysts for water gas shift (WGS) and CO hydrogenation to methanol reactions. Herein, via a comprehensive experimental and theoretical calculation study of a series of ZnO/Cu nanocrystals inverse catalysts with well-defined Cu structures, we report that the ZnO–Cu catalysts undergo Cu structure-dependent and reaction-sensitive in situ restructuring during WGS and CO hydrogenation reactions under typical reaction conditions, forming the active sites of Cu_Cu(100)-hydroxylated ZnO ensemble and Cu_Cu(611)Zn alloy, respectively. These results provide insights into the active sites of Cu–ZnO catalysts for the WGS and CO hydrogenation reactions and reveal the Cu structural effects, and offer the feasible guideline for optimizing the structures of Cu–ZnO–Al₂O₃ catalysts.

Identification of active sites of a catalyst is the Holy Grail in heterogeneous catalysis. Here, the authors successfully identify the Cu_Cu(100)- hydroxylated ZnO ensemble and Cu_Cu(611)Zn alloy as the active sites of Cu-ZnO catalysts for water gas shift and CO hydrogenation reactions, respectively.

Construction of Dual-Active-Site Copper Catalyst Containing both CuN3 and CuN4 Sites

Clear recognition and rational construction of suitable active center for specific reaction is always of great significance in designing highly efficient catalysts. Herein, a dual-active-site copper catalyst (DAS-Cu) containing both CuN₃ and CuN₄ sites is reported. Such catalysts show extremely high catalytic performance (yield: up to 97%) toward oxyphosphorylation of alkenes, while catalysts with single active site (CuN₃ or CuN₄ ) are chemically inert in this reaction. Combined with theoretical and experimental results, the different roles of two different Cu active sites in this reaction are further identified. CuN₃ site captures the oxygen and trigger further oxidizing process, while CuN₄ site provides moderate adsorption sites for the protection of phosphonyl radicals. This work deeply discloses the significant cooperated role with two single-atomic sites in one catalytic active center and brings up a valuable clue for the rational design of better-performing heterogeneous catalyst.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Cu-Based mixed metal oxides for an efficient photothermal catalysis of the water-gas shift reaction

Cu–ZnO catalyst with a well-designed nanojunction structure was fabricated for the photothermal catalysis of the water-gas shift (WGS) reaction.

The most active Cu facet for low-temperature water gas shift reaction

Identification of the active site is important in developing rational design strategies for solid catalysts but is seriously blocked by their structural complexity. Here, we use uniform Cu nanocrystals synthesized by a morphology-preserved reduction of corresponding uniform Cu₂O nanocrystals in order to identify the most active Cu facet for low-temperature water gas shift (WGS) reaction. Cu cubes enclosed with {100} facets are very active in catalyzing the WGS reaction up to 548 K while Cu octahedra enclosed with {111} facets are inactive. The Cu–Cu suboxide (Cu_xO, x ≥ 10) interface of Cu(100) surface is the active site on which all elementary surface reactions within the catalytic cycle proceed smoothly. However, the formate intermediate was found stable at the Cu–Cu_xO interface of Cu(111) surface with consequent accumulation and poisoning of the surface at low temperatures. Thereafter, Cu cubes-supported ZnO catalysts are successfully developed with extremely high activity in low-temperature WGS reaction.

Nanocrystals display a variety of facets with different catalytic activity. Here the authors identify the most active facet of copper nanocrystals relevant to the low-temperature water gas shift reaction and further design zinc oxide-copper nanocubes with exceptionally high catalytic activity.

Artificial Photosynthesis of Alcohols by Multi-Functionalized Semiconductor Photocathodes

Novel artificial photosynthesis systems are devised as cells of dye/Pd/NR-MO_x (M=Ti, Zn)∥CoPi/W:BiVO₄ that convert efficiently CO₂ to alcohols. The photocathodes are aminofunctionalized, palladium-deposited, and in situ sensitized nano-TiO₂ or ZnO/FTO (FTO: fluorine-doped tin oxide) electrodes that are characterized by X-ray photoelectron spectroscopy (XPS), TEM, XRD, UV/Vis spectra, and evaluated by electrochemical techniques. The cell of dye/Pd/S-TiO₂ ∥CoPi/W:BiVO₄ uniquely generates ethanol under irradiation of 200 mW cm^-2 , reaching 0.56 % quantum efficiency (QE) at -0.56 V and 0.13 % QE without external electron supply. The cell of dye/Pd/ N-ZnO∥CoPi/W:BiVO₄ produces solely methanol at a rate of 42.8 μm h^-1  cm^-2 at -0.56 V of a Si solar cell, which is far less than the electrochemical voltage of water splitting (1.23 V). Its QE reaches to 0.38 %, which is equal to plants. The isotopic labeling experiments confirm the carbon source and oxygen releasing. The selectivity for alcohols of multi-functionalized semiconductors is discussed.

Enhancing CO2 Electroreduction with the Metal-Oxide Interface

The electrochemical CO₂ reduction reaction (CO₂RR) typically uses transition metals as the catalysts. To improve the efficiency, tremendous efforts have been dedicated to tuning the morphology, size, and structure of metal catalysts and employing electrolytes that enhance the adsorption of CO₂. We report here a strategy to enhance CO₂RR by constructing the metal-oxide interface. We demonstrate that Au-CeO_x shows much higher activity and Faradaic efficiency than Au or CeO_x alone for CO₂RR. In situ scanning tunneling microscopy and synchrotron-radiation photoemission spectroscopy show that the Au-CeO_x interface is dominant in enhancing CO₂ adsorption and activation, which can be further promoted by the presence of hydroxyl groups. Density functional theory calculations indicate that the Au-CeO_x interface is the active site for CO₂ activation and the reduction to CO, where the synergy between Au and CeO_x promotes the stability of key carboxyl intermediate (*COOH) and thus facilitates CO₂RR. Similar interface-enhanced CO₂RR is further observed on Ag-CeO_x, demonstrating the generality of the strategy for enhancing CO₂RR.

Pd(0)-CMC@Ce(OH)(4) organic/inorganic hybrid as highly active catalyst for the Suzuki-Miyaura reaction

A very easy sequential metathesis for the synthesis of Pd(II)-CMC@Ce(OH)₄ organic/inorganic hybrid and its application as effective pre-catalyst for the Suzuki-Miyaura reaction have been reported. It was found that the Pd nanoparticles (Pd NPs) were formed in situ in the course of the Suzuki-Miyaura couplings when Pd(II)-CMC@Ce(OH)₄ was used as a pre-catalyst. The activity of the Pd NPs in the reaction was enhanced synergistically by the unique redox properties (Ce³⁺/Ce⁴⁺) of Ce(OH)₄ and coordination with carboxyl groups as well as free hydroxyl groups of the hybrid of CMC@Ce(OH)₄. The results exhibit the Pd(0)-CMC@Ce(OH)₄ is super over Pd(II)@CMC, Pd(II)@CeO₂, and Pd(II)@Ce(OH)₄ catalysts in the Suzuki-Miyaura reaction. Moreover, the catalyst could be easily separated by simple filtration and reused at least seven runs without losing its activity.