Elucidating the Nature of the Cu(I) Active Site in CuO/TiO2 for Excellent Low-Temperature CO Oxidation

Unraveling the Mechanistic Origin of High N2 Selectivity in Ammonia Selective Catalytic Oxidation on CuO-Based Catalyst

NH3 emissions from industrial sources and possibly future energy production constitute a threat to human health because of their toxicity and participation in PM2.5 formation. Ammonia selective catalytic oxidation to N2 (NH3-SCO) is a promising route for NH3 emission control, but the mechanistic origin of achieving high N2 selectivity remains elusive. Here we constructed a highly N2-selective CuO/TiO2 catalyst and proposed a CuOx dimer active site based on the observation of a quadratic dependence of NH3-SCO reaction rate on CuOx loading, ac-STEM, and ab initio thermodynamic analysis. Combining this with the identification of a critical N2H4 intermediate by in situ DRIFTS characterization, a comprehensive N2H4-mediated reaction pathway was proposed by DFT calculations. The high N2 selectivity originated from the preference for NH2 coupling to generate N2H4 over NH2 dehydrogenation on the CuOx dimer active site. This work could pave the way for the rational design of efficient NH3-SCO catalysts.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

FeOx-Modified Ultrafine Platinum Particles Supported on MgFe2O4 with High Catalytic Activity and Promising Stability toward Low-Temperature Oxidation of CO

Catalytic oxidation is widely recognized as a highly effective approach for eliminating highly toxic CO. The current challenge lies in designing catalysts that possess exceptional low-temperature activity and stability. In this work, we have prepared ultrafine platinum particles of ~1 nm diameter dispersed on a MgFe2O4 support and found that the addition of 3 wt.% FeOx into the 3Pt/MgFe2O4 significantly improves its activity and stability. At an ultra-low temperature of 30 °C, the CO can be totally converted to CO2 over 3FeOx-3Pt/MgFe2O4. High and stable performances of CO-catalytic oxidation can be obtained at 60 °C on 3FeOx-3Pt/MgFe2O4 over 35 min on-stream at WHSV = 30,000 mL/(g·h). Based on a series of characterizations including BET, XRD, ICP, STEM, H2-TPR, XPS, CO-DRIFT, O2-TPD and CO-TPD, it was disclosed that the relatively high activity and stability of 3FeOx-3Pt/MgFe2O4 is due to the fact that the addition of FeOx could facilitate the antioxidant capacity of Pt and oxygen mobility and increase the proportion of adsorbed oxygen species and the amounts of adsorbed CO. These results are helpful in designing Pt-based catalysts exhibiting higher activity and stability at low temperatures for the catalytic oxidation of CO.

Hydroxyls on CeO2 Support Promoting CuO/CeO2 Catalyst for Efficient CO Oxidation and NO Reduction by CO

Transition metal catalysts, such as copper oxide, are more attractive alternatives to noble metal catalysts for emission control due to their higher abundance, lower cost, and excellent catalytic activity. In this study, we report the preparation and application of a novel CuO/CeO2 catalyst using a hydroxyl-rich Ce(OH)x support for CO oxidation and NO reduction by CO. Compared to the catalyst prepared from a regular CeO2 support, the new CuO/CeO2 catalyst prepared from the OH-rich Ce(OH)x (CuO/CeO2-OH) showed significantly higher catalytic activity under different testing conditions. The effect of OH species in the CeO2 support on the catalytic performance and physicochemical properties of the CuO/CeO2 catalyst was characterized in detail. It is demonstrated that the abundant OH species enhanced the CuOx dispersion on CeO2, increased the CuOx-CeO2 interfaces and surface defects, promoted the oxygen activation and mobility, and boosted the NO adsorption and dissociation on CuO/CeO2-OH, thus contributing to its superior catalytic activity for both CO oxidation and NO reduction by CO. These results suggest that the OH-rich Ce(OH)x is a superior support for the preparation of highly efficient metal catalysts for different applications.

Tailoring Copper Chemical Status and Hydrophobicity of Biomimetic Photocatalytic Films for Carbon Dioxide Conversion

Naturally hierarchical nanostructures of leaves were successfully replicated on thermally stable polyimide (PI) films to obtain biomimetic substrates for the grafting of p-type semiconductor, cuprous oxide (Cu₂O). The chemical states of Cu₂O and the hydrophobicity on the photocatalytic films were tunable by altering the process time of ion-exchange or chemical reduction. The obtained photocatalytic films showed activity to photocatalytically convert carbon dioxide (CO₂) into carbon monoxide (CO) under visible light illumination. The yield of CO was initially improved with the increasing hydrophobicity on the film but then leveled off. The photocatalytic activity could be further improved by tailoring the amount or composition of copper oxides. An optimum ratio of Cu₂O and moderate basicity on the surface, as well as more metallic Cu from the bulk, will achieve more efficient interfacial charge transfer, resulting in a higher CO production rate.

Insight into the Effect of Oxygen Vacancy Prepared by Different Methods on CuO/Anatase Catalyst for CO Catalytic Oxidation

In this study, CuO loaded on anatase TiO2 catalysts (CuO/anatase) with oxygen vacancies was synthesized via reduction treatments by NaHB4 and H2 (CuO/anatase-B, CuO/anatase-H), respectively. The characterizations suggest that different reduction treatments bring different concentration of oxygen vacancies in the CuO/anatase catalysts, which finally affect the CO catalytic performance. The CuO/anatase-B and CuO/anatase-H exhibit CO conversion of 90% at 182 and 198 °C, respectively, which is lower than what occurred for CuO/anatase (300 °C). The XRD, Raman, and EPR results show that the amount of the oxygen vacancies of the CuO/anatase-H is the largest, indicating a stronger reduction effect of H2 than NaHB4 on the anatase surface. The in situ DRIFTS results exhibit that the Cu sites are the adsorption sites of CO, and the oxygen vacancies on the anatase can active the O2 molecules into reactive oxygen species. According to the in situ DRIFTS results, it can be concluded that in the CO oxidation reaction, only the CuO/anatase-H catalyst can be carried out by the Mvk mechanism, which greatly improves its catalytic efficiency. This study explained the reaction mechanisms of CO oxidation on various anatase surfaces, which offers detailed insights into how to prepare suitable catalysts for low-temperature oxidation reactions.

Hierarchical Porous Carbon Nitride-Crumpled Nanosheet-Embedded Copper Single Atoms: An Efficient Catalyst for Carbon Monoxide Oxidation

Rational design of metal single-site embedded porous graphitic carbon nitride (P-g-C₃N₄) nanostructures exploiting maximum atom utilization is warranted to enhance the thermal CO oxidation (CO_Ox) reaction. Herein, a facile, green, one-pot, and template-free approach is developed to fabricate the hierarchical porous P-g-C₃N₄-crumpled ultrathin nanosheets atomically doped with copper single atoms (Cu-P-g-C₃N₄). Mechanistically, the quick protonation of melamine and pyridine under acidic conditions induces deamination to form melem, which is polycondensed under heating. The interconnected pores, high surface area (240 m²g^-1), and maximized exposed isolated Cu atomic active sites (1.8 wt %) coordinated with nitrogen atom P-g-C₃N₄ are the salient features of Cu- P-g-C₃N₄ that endowed complete conversion to CO₂ at 184 °C. In contrast, P-g-C₃N₄ only converted 3.8% of CO even at 350 °C, implying the electronic effect of Cu single atoms. The abundant Cu-nitrogen moieties can drastically weaken the binding affinity of the CO-oxidation (CO_Ox) intermediates and products, thus accelerating the reaction kinetics at a low temperature. This study may promote the fabrication of P-g-C₃N₄ doped with various single atoms for the oxidation of CO.

Cu-VWT Catalysts for Synergistic Elimination of NOx and Volatile Organic Compounds from Coal-Fired Flue Gas

A dual-function catalyst, designated as Cu5-VWT, has been constructed for the synergistic removal of NO_x and volatile organic compounds under complex coal-fired flue gas conditions. The removal of toluene, propylene, dichloromethane, and naphthalene all exceeded 99% (350 °C), and the catalyst could effectively block the generation of polycyclic aromatic hydrocarbons. Mechanistic studies have shown that Cu sites on the Cu5-VWT catalyst facilitate catalytic oxidation, while V sites facilitate NO_x reduction. Thus, toluene oxidation and NO_x reduction can proceed simultaneously. The removal of total hydrocarbons and nonmethane total hydrocarbons from 1200 m³·h^-1 real coal-fired flue gas by a monolithic catalyst were determined as 92 and 96%, respectively, much higher than those of 54 and 72% over a commercial VWT catalyst, indicating great promise for industrial application.

Solid-State Construction of CuOx/Cu1.5Mn1.5O4 Nanocomposite with Abundant Surface CuOx Species and Oxygen Vacancies to Promote CO Oxidation Activity

Carbon monoxide (CO) oxidation performance heavily depends on the surface-active species and the oxygen vacancies of nanocomposites. Herein, the CuO_x/Cu_1.5Mn_1.5O₄ were fabricated via solid-state strategy. It is manifested that the construction of CuO_x/Cu_1.5Mn_1.5O₄ nanocomposite can produce abundant surface CuO_x species and a number of oxygen vacancies, resulting in substantially enhanced CO oxidation activity. The CO is completely converted to carbon dioxide (CO₂) at 75 °C when CuO_x/Cu_1.5Mn_1.5O₄ nanocomposites were involved, which is higher than individual CuO_x, MnO_x, and Cu_1.5Mn_1.5O₄. Density function theory (DFT) calculations suggest that CO and O₂ are adsorbed on CuO_x/Cu_1.5Mn_1.5O₄ surface with relatively optimal adsorption energy, which is more beneficial for CO oxidation activity. This work presents an effective way to prepare heterogeneous metal oxides with promising application in catalysis.

Modeling surface spin polarization on ceria-supported Pt nanoparticles

In this work, we employ density functional theory simulations to investigate possible spin polarization of CeO₂-(111) surface and its impact on the interactions between a ceria support and Pt nanoparticles. With a Gaussian type orbital basis, our simulations suggest that the CeO₂-(111) surface exhibits a robust surface spin polarization due to the internal charge transfer between atomic Ce and O layers. In turn, it can lower the surface oxygen vacancy formation energy and enhance the oxide reducibility. We show that the inclusion of spin polarization can significantly reduce the major activation barrier in the proposed reaction pathway of CO oxidation on ceria-supported Pt nanoparticles. For metal-support interactions, surface spin polarization enhances the bonding between Pt nanoparticles and ceria surface oxygen, while CO adsorption on Pt nanoparticles weakens the interfacial interaction regardless of spin polarization. However, the stable surface spin polarization can only be found in the simulations based on the Gaussian type orbital basis. Given the potential importance in the design of future high-performance catalysts, our present study suggests a pressing need to examine the surface ferromagnetism of transition metal oxides in both experiment and theory.

Neighboring sp-Hybridized Carbon Participated Molecular Oxygen Activation on the Interface of Sub-nanocluster CuO/Graphdiyne

Activation of O₂ is a crucial step in oxidation processes. Here, the concept of sp-hybridized C≡C triple bonds as an electron donor is adopted to develop highly active and stable catalysts for molecular oxygen activation. We demonstrate that the neighboring sp-hybridized C and Cu sites on the interface of the sub-nanocluster CuO/graphdiyne are the key structures to effectively modulate the O₂ activation process in the bridging adsorption mode. The as-prepared sub-nanocluster CuO/graphdiyne catalyst exhibited the highest CO oxidation activity and readily converted 50% CO at around 133 °C, which is 34 and 94 °C lower than that for CuO/graphene and CuO/active carbon catalysts, respectively. In situ diffused reflectance infrared Fourier transform spectroscopy and density functional theory calculation results proved that the neighboring sp-hybridized C is more favorable to promote the rapid dissociation of carbonate than sp²-hybridized C without overcoming any energy barrier. The gaseous CO directly reacts with the active molecular oxygen and tends to proceed through the E-R mechanism with a relatively low energy barrier (0.20 eV). This work revealed that sp-hybridized C of graphdiyne-based materials could effectively improve the O₂ activation efficiency, which could facilitate the low-temperature oxidation processes.

Facile co-removal of As(V) and Sb(V) from aqueous solution using Fe-Cu binary oxides: Structural modification and self-driven force field of copper oxides

Currently, the environmental and ecological damage caused by As(V) and Sb(V) co-contamination has attracted widespread attention worldwide. Due to the similar intrinsic structure configuration and electrostatic repulsion of As(V) and Sb(V), the long-standing issue of their low co-removal capacity remains unresolved. In this study, novel Fe-Cu (FC) binary materials with varied Fe/Cu proportions were synthesized via a simple co-precipitation method to co-eliminate aquatic As(V) and Sb(V). A 2/1 ratio of Fe/Cu was determined to be a suitable proportion with a higher co-adsorption capacity, specifically 70.9 mg·g^-1 for As(V) and 94.3 mg·g^-1 for Sb(V). Detailed morphological and structural analyses indicated that the FC material gradually changed from microscale aggregates to nanoscale spheres with increasing Cu content, accompanied by an increasing crystalline degree and higher surface area. Additionally, the transformation of amorphous ferrihydrite (FO) into FeO(OH) was suppressed by Fe-Cu complexion during the co-adsorption process, in which ferrihydrite (FO) had more adsorption sites than FeO(OH). In addition, the addition of Cu promoted the pH_pzc of FC materials from the acidic range into the neutral or alkaline range. The increased potential difference of FC materials accelerated the As(V) and Sb(V) diffusion rate and effectively offset native electrostatic repulsion, which exhibited a considerable effect than the adsorption sites. Through detailed kinetic data analysis, it was determined that the proportion of the diffusion layer thickness around Sb(V) was suppressed to the As(V) level, and the adsorption kinetics of the two species were both promoted by the self-driven force field. All the results indicated that the co-adsorption capacity depended on the coupling contribution of Fe and Cu, where Fe oxide acted as the major adsorption potential and Cu provided a self-driven force for As(V) and Sb(V) diffusion. This study may provide a novel prospective for homogeneous metal ion co-removal.

New insights into the influence mechanism of H2O and SO2 on Pt-W/Ti catalysts for CO oxidation

A series of anatase TiO2 loaded with 0.1 wt.% Pt and n% WO3 (0.1Pt-nW/Ti-A, n=0, 1, 2, 5, 10) were prepared using the step-impregnation method. Among the catalysts, 0.1Pt-5W/Ti-A showed...

Formulating Multiphase Medium Anti-wetting States in an Air-Water-Oil System: Engineering Defects for Interface Chemical Evolutions

Studies which regulate macroscopic wetting states on determined surfaces in multiphase media are of far-reaching significance but are still in the preliminary stage. Herein, inspired by the wettability subassembly of fish scales, Namib desert beetle shell, and lotus leaf upper side, interfaces in the air-water-oil system are programmed by defect engineering to tailor the anti-wetting evolution from double to triple liquid repellency states. By controlling the visible light irradiation and plasma treatment, surface oxygen vacancies on Cu_xO@TiO₂ nanowires (NWs) can be healed or reconstructed. The original membrane or the membrane after plasma treatment possesses abundant surface oxygen vacancies, and the homogeneous hydrophilic membrane shows only double anti-wetting states in the water-oil system. By the unsaturated visible light irradiation time, the surface oxygen vacancy partially healed, the heterogeneous hydrophilic-hydrophobic components occupied the membrane surface, and the anti-wetting state finally changed from double to triple in the air-water-oil system. After the illumination time reaches saturation, it promotes the healing of all surface oxygen vacancies, and the membrane surface only contains uniform hydrophobic components and only maintains double anti-wetting state in the air-oil system. The mechanism of the triple anti-wetting state on a heterogeneous surface is expounded by establishing a wetting model. The wetting state and the adhesion state of the Cu_xO@TiO₂ NW membrane show regional specificity by controlling the illumination time and region. The underwater oil droplets exhibit the "non-adhesive" and "adhesive" state in a region with unsaturated irradiation time or in an unirradiated region, respectively. Underwater oil droplet manipulation can be accomplished easily based on switchable wettability and adhesion. Current studies reveal that defect engineering can be extended to anti-wetting evolution in the air-water-oil system. Constructing an anti-wetting interface by heterogeneous components provides reference for designing the novel anti-wetting interface.

Engineering the Nucleophilic Active Oxygen Species in CuTiOx for Efficient Low-Temperature Propene Combustion

Industrialization has resulted in the rapid increase of volatile organic compound (VOC) emissions, which have caused serious issues to human health and the environment. In this study, an extensive Cu incorporating TiO₂ induced nucleophilic oxygen structure was constructed in the CuTiO_x catalyst, which exhibited superior low-temperature catalytic activity for C₃H₆ combustion. Thorough structural, surface characterization and density functional theory (DFT) calculations revealed that the Cu-O-Ti hybridization induced nucleophilic oxygen initiates C₃H₆ combustion by abstracting the C-H bond. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that incorporated copper species acted as the major adsorbent site for the propene molecule. In combination of the DRIFTS and DFT results, the promotion effect of the nucleophilic O on the C-H bond abstraction and CO₂ formation pathway was proposed. The surface doping induced nucleophilic oxygen as strong Brønsted basic sites for low-temperature propene combustion exemplified an efficient strategy for rational design of next-generation environmental catalysts.

The solid-state in situ construction of Cu2O/CuO heterostructures with adjustable phase compositions to promote CO oxidation activity

Cu₂O/CuO heterojunctions were fabricated via in situ solid-state technology. Tuning the ratio of reactants enables optimization of the components of the Cu₂O/CuO heterostructures and their catalytic activities for CO oxidation.