Oxygen Vacancies in Cu/TiO2 Boost Strong Metal-Support Interaction and CO2 Hydrogenation to Methanol

[Ce3+-OV-Ce4+] Located Surface-Distributed Sheet Cu-Zn-Ce Catalysts for Methanol Production by CO2 Hydrogenation

The metal-support interaction is crucial for the performance of Cu-based catalysts. However, the distinctive properties of the support metal element itself are often overlooked in catalyst design. In this paper, a sheet Cu-Zn-Ce with [Ce3+-OV-Ce4+] located on the surface was designed by the sol-gel method. Through EPR and X-ray photoelectron spectroscopy (XPS), the relationship between the content of oxygen vacancies and Ce was revealed. Ce itself induces the generation of [Ce3+-OV-Ce4+]. Through ICP-MS, XPS, and SEM-mapping, the Ce-induced formation of [Ce3+-OV-Ce4+] located on the catalyst surface was demonstrated. CO2-TPD and DFT calculations further revealed that [Ce3+-OV-Ce4+] enhanced CO2 adsorption, leading to a 10% increase in methanol selectivity compared to Cu-Zn-Ce synthesized via the coprecipitation method.

CO2 Hydrogenation to CH3OH on Metal-Doped TiO2(110): Mechanisms, Strain Effect and a New Thermodynamic-Kinetic Relation

Surface strain and linear thermodynamic-kinetic relation are interesting topics in catalysis. Development of low temperature methanol catalysts of high activity and selectivity is of particularly importance for conversion of CO2 to methanol. In the present paper CO2 hydrogenation to methanol on Znx@TiO2(110) (x=0-2) was explored using density functional calculations and microkinetic simulations. The reaction mechanisms on the three model systems were determined and it is shown that Zn2@TiO2(110) is the most active. The most favorable pathway on Zn2@TiO2(110) is identified and CO2+H to HCOO is found to be the rate-controlling step. It is demonstrated that there is a linear relation (named AEB relation) between the adsorption energies of the initial states and the barriers for the controlling step on the 18 systems studied. Calculations on strained surfaces show that the AEB relation exists within ±1 % strain. Sr2@TiO2(110) and -1 % strained CaZn and ZnCu doped TiO2(110) are potential good low temperature catalysts and deserve experimental testing.

Tuning Adsorbate-Mediated Strong Metal-Support Interaction by Oxygen Vacancy: A Case Study in Ru/TiO2

The adsorbate-mediated strong metal-support interaction (A-SMSI) offers a reversible means of altering the selectivity of supported metal catalysts, thereby providing a powerful tool for facile modulation of catalytic performance. However, the fundamental understanding of A-SMSI remains inadequate and methods for tuning A-SMSI are still in their nascent stages, impeding its stabilization under reaction conditions. Here, we report that the initial concentration of oxygen vacancy in oxide supports plays a key role in tuning the A-SMSI between Ru nanoparticles and defected titania (TiO2-x). Based on this new understanding, we demonstrate the in situ formation of A-SMSI under reaction conditions, obviating the typically required CO2-rich pretreatment. The as-formed A-SMSI layer exhibits remarkable stability at various temperatures, enabling excellent activity, selectivity and long-term stability in catalyzing the reverse water gas-shift reaction. This study deepens the understanding of the A-SMSI and the ability to stabilize A-SMSI under reaction conditions represents a key step for practical catalytic applications.

Efficient Photoreduction of CO2 to CO with 100% Selectivity by Slowing Down Electron Transport

It remains challenging to obtain a single product in the gas-solid photocatalytic reduction of CO2 because CO and CH4 are usually produced simultaneously. This study presents the design of the I-type nested heterojunction TiO2/BiVO4 with controllable electron transport by modulating the TiO2 component. This study demonstrates that slowing electron transport could enable TiO2/BiVO4-4 to generate CO with 100% selectivity. In addition, modifying TiO2/BiVO4-4 by loading a Cu single atom further increased the CO product yield by 3.83 times (17.33 μmol·gcat-1·h-1), while maintaining 100% selectivity for CO. Characterization and density functional theory (DFT) calculations revealed that the selectivity was mainly determined by the electron transport of the support, whereas CO2 was efficiently adsorbed and activated by the Cu single atom. Such a two-step regulation strategy of combining heterojunction with single atom enhances the possibility of simultaneously obtaining high selectivity and high yield in the photocatalytic reduction of CO2.

Kinetic pathways of sub-bandgap induced electron transfer in Ag/TiO2 and the effect on isopropanol dehydrogenation under gaseous conditions

Electron transfer and its kinetics play a major role in the photocatalysis of metal/semiconductor systems. Using in situ photoconductances, in situ photoabsorption, and photoinduced spectroscopic techniques, the present research aimed to gain a deep insight into electron transfer pathways and their kinetics for Ag/TiO2 systems under sub-bandgap light illumination and gaseous conditions. The results revealed that electrons generated in TiO2 can transfer to Ag nanoparticles at fast rates, and plasmon-generated electrons in Ag nanoparticles can also transfer to TiO2. However, it was found that plasmon-assisted hot electron transfer efficiency is much lower than the electron transition from the valence band to the conduction band of TiO2. Rather than plasmonic active spots, the results showed that Ag nanoparticles acted as co-catalyst sites bridging electron transfer to recombination in a methanol-containing N2 atmosphere. As a result, photocatalytic isopropanol dehydrogenation was decreased. Independent of Ag decorations, it was also indicated that isopropanol dehydrogenation mainly occurred over TiO2 surfaces; therefore, Ag nanoparticles did not increase photocatalytic activities. Our results may provide a different viewpoint on sub-bandgap light-induced Ag/TiO2 photocatalysis under gaseous conditions; this may also facilitate the understanding of the photocatalytic mechanism of metal/semiconductor systems.

Tuning Catalytic Activity of CO2 Hydrogenation to C1 Product via Metal Support Interaction Over Metal/Metal Oxide Supported Catalysts

The metal supported catalysts are emerging catalysts that are receiving a lot of attention in CO2 hydrogenation to C1 products. Numerous experiments have demonstrated that the support (usually an oxide) is crucial for the catalytic performance. The support metal oxides are used to aid in the homogeneous dispersion of metal particles, prevent agglomeration, and control morphology owing to the metal support interaction (MSI). MSI can efficiently optimize the structural and electronic properties of catalysts and tune the conversion of key reaction intermediates involved in CO2 hydrogenation, thereby enhancing the catalytic performance. There is an increasing attention is being paid to the promotion effects in the catalytic CO2 hydrogenation process. However, a systematically understanding about the effects of MSI on CO2 hydrogenation to C1 products catalytic performance has not been fully studied yet due to the diversities in catalysts and reaction conditions. Hence, the characteristics and modes of MSI in CO2 hydrogenation to C1 products are elaborated in detail in our work.

Promotion of Cobalt Oxide Catalysts by Acid-Etching and Ruthenium Incorporation for Chlorinated VOC Oxidation

In this work, Ru-promoted cobalt oxide catalysts with a nanotube morphology were prepared by a synthesis route based on the Kirkendall effect followed by an acid treatment and subsequent optimized Ru impregnation. The resulting samples were thoroughly characterized by means of N2 physisorption, X-ray energy-dispersive spectroscopy, X-ray diffraction, scanning electron microscopy techniques, X-ray photoelectron spectroscopy, and temperature-programmed techniques (O2-temperature-programmed desorption, H2-temperature-programmed reduction, and temperature-programmed oxidation) and evaluated in the gas-phase oxidation of 1,2-dichloroethane. It has been demonstrated that Ru addition improves the oxygen mobility as well as the amount of Co2+ and Oads species at the surface by the formation of the Ru-O-Co bond, which in turn governs the performance of the catalysts in the oxidation reaction. Moreover, the acid-etching favors the dispersion of the Ru species on the surface of the catalysts and strengthens the interaction among the noble metal and the cobalt oxide, thereby improving the thermal stability of the Ru-promoted oxides. Thus, the resulting catalysts are not only active, as the chlorinated pollutant is efficiently converted into deep oxidation products at relatively low temperatures, but also quite stable when operating for 120 h.

Porous In2O3 Hollow Tube Infused with g-C3N4 for CO2 Photocatalytic Reduction

Converting CO2 into energy-rich fuels by using solar energy is a sustainable solution that promotes a carbon-neutral economy and mitigates our reliance on fossil fuels. However, affordable and efficient CO2 conversion remains an ongoing challenge. Here, we introduce polymeric g-C3N4 into the pores of a hollow In2O3 microtube. This architecture results in a compact and staggered arrangement between g-C3N4 and In2O3 components with an increased contact interface for improved charge separation. The hollow interior further contributes to strengthening light absorption. The resulting g-C3N4-In2O3 hollow tubes exhibit superior activity (274 μmol·g-1·h-1) toward CO2 to CO conversion in comparison with those of pure In2O3 and g-C3N4 (5.5 and 93.6 μmol·g-1·h-1, respectively), underlining the role of integrating g-C3N4 and In2O3 in this advanced system. This work offers a strategy for the advanced design and preparation of hollow heterostructures for optimizing CO2 adsorption and conversion by integrating inorganic and organic semiconductors.

dz2 Band Links Frontier Orbitals and Charge Carrier Dynamics of Single-Atom Cocatalyst-Aided Photocatalytic H2 Production

The Cu single-atom catalyst (SAC) supported on TiO2 exhibits outstanding efficacy in photocatalytic hydrogen evolution. The precise operational mechanism remains a subject of ongoing debate. The focus resides with the interplay linking heightened catalytic activity, dynamic valence state alterations of Cu atoms, and their hybridization with H2O orbitals, manifested in catalyst color changes. Taking anatase TiO2 (101) as a prototypical surface, we perform ab initio quantum dynamics simulation to reveal that the high activity of the Cu-SAC is due to the quasi-planar coordination structure of the Cu atom after H2O adsorption, allowing it to trap photoexcited hot electrons and inject them into the hybridized orbital between Cu and H2O. The observed alterations in the valence state and the coloration can be attributed to the H atom released during H2O dissociation and adsorbed onto the lattice O atom neighboring the Cu-SAC. Notably, this adsorption of H atoms puts the Cu-SAC into an inert state, as opposed to an activating effect reported previously. Our work clarifies the relationship between the high photocatalytic activity and the local dynamic atomic coordination structure, providing atomistic insights into the structural changes occurring during photocatalytic reactions on SACs.

Reinforcing the Efficiency of Photothermal Catalytic CO2 Methanation through Integration of Ru Nanoparticles with Photothermal MnCo2O4 Nanosheets

Carbon dioxide (CO2) hydrogenation to methane (CH4) is regarded as a promising approach for CO2 utilization, whereas achieving desirable conversion efficiency under mild conditions remains a significant challenge. Herein, we have identified ultrasmall Ru nanoparticles (∼2.5 nm) anchored on MnCo2O4 nanosheets as prospective photothermal catalysts for CO2 methanation at ambient pressure with light irradiation. Our findings revealed that MnCo2O4 nanosheets exhibit dual functionality as photothermal substrates for localized temperature enhancement and photocatalysts for electron donation. As such, the optimized Ru/MnCo2O4-2 gave a high CH4 production rate of 66.3 mmol gcat-1 h-1 (corresponding to 5.1 mol gRu-1 h-1) with 96% CH4 selectivity at 230 °C under ambient pressure and light irradiation (420-780 nm, 1.25 W cm-2), outperforming most reported plasmonic metal-based catalysts. The mechanisms behind the intriguing photothermal catalytic performance improvement were substantiated through a comprehensive investigation involving experimental characterizations, numerical simulations and density functional theory (DFT) calculations, which unveiled the synergistic effects of enhanced charge separation efficiency, improved reaction kinetics, facilitated reactant adsorption/activation and accelerated intermediate conversion under light irradiation over Ru/MnCo2O4. A comparison study showed that, with identical external input energy during the reaction, Ru/MnCo2O4-2 had a much higher catalytic efficiency compared to Ru/TiO2 and Ru/Al2O3. This study underscores the pivotal role played by photothermal supports and is believed to engender a heightened interest in plasmonic metal nanoparticles anchored on photothermal substrates for CO2 methanation under mild conditions.

Systematic Characterization of Electronic Metal-Support Interactions in Ceria-Supported Pt Particles

Electronic metal-support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal-support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce3+ cations with potentials that include a Ce4f electron in the frozen core, overcoming prevalent convergence and 4f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt8/CeO2(111) model system, revealing the predominant effect of electronic metal-support interactions on Pt atoms located directly at the metal-oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides.

Ultra-Violet-Assisted Scalable Method to Fabricate Oxygen-Vacancy-Rich Titanium-Dioxide Semiconductor Film for Water Decontamination under Natural Sunlight Irradiation

This work reports the fabrication of titanium dioxide (TiO₂) nanoparticle (NPs) films using a scalable drop-casting method followed by ultra-violet (UV) irradiation for creating defective oxygen vacancies on the surface of a fabricated TiO₂ semiconductor film using an UV lamp with a wavelength oof 255 nm for 3 h. The success of the use of the proposed scalable strategy to fabricate oxygen-vacancy-rich TiO₂ films was assessed through UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The Ti 2p XPS spectra acquired from the UV-treated sample showed the presence of additional Ti³⁺ ions compared with the untreated sample, which contained only Ti⁴⁺ ions. The band gap of the untreated TiO₂ film was reduced from 3.2 to 2.95 eV after UV exposure due to the created oxygen vacancies, as evident from the presence of Ti³⁺ ions. Radiation exposure has no significant influence on sample morphology and peak pattern, as revealed by the SEM and XRD analyses, respectively. Furthermore, the photocatalytic activity of the fabricated TiO₂ films for methylene-blue-dye removal was found to be 99% for the UV-treated TiO₂ films and compared with untreated TiO₂ film, which demonstrated only 77% at the same operating conditions under natural-sunlight irradiation. The proposed UV-radiation method of oxygen vacancy has the potential to promote the wider application of photo-catalytic TiO₂ semiconductor films under visible-light irradiation for solving many environmental and energy-crisis challenges for many industrial and technological applications.

Engineering Heterogeneous Catalysis with Strong Metal-Support Interactions: Characterization, Theory and Manipulation

Strong metal-support interactions (SMSI) represent a classic yet fast-growing area in catalysis research. The SMSI phenomenon results in the encapsulation and stabilization of metal nanoparticles (NPs) with the support material that significantly impacts the catalytic performance through regulation of the interfacial interactions. Engineering SMSI provides a promising approach to steer catalytic performance in various chemical processes, which serves as an effective tool to tackle energy and environmental challenges. Our Minireview covers characterization, theory, catalytic activity, dependence on the catalytic structure and inducing environment of SMSI phenomena. By providing an overview and outlook on the cutting-edge techniques in this multidisciplinary research field, we not only want to provide insights into the further exploitation of SMSI in catalysis, but we also hope to inspire rational designs and characterization in the broad field of material science and physical chemistry.

CO2 Activation and Hydrogenation on Cu-ZnO/Al2O3 Nanorod Catalysts: An In Situ FTIR Study

CuZnO/Al2O3 is the industrial catalyst used for methanol synthesis from syngas (CO + H2) and is also promising for the hydrogenation of CO2 to methanol. In this work, we synthesized Al2O3 nanorods (n-Al2O3) and impregnated them with the CuZnO component. The catalysts were evaluated for the hydrogenation of CO2 to methanol in a fixed-bed reactor. The support and the catalysts were characterized, including via in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The study of the CO2 adsorption, activation, and hydrogenation using in situ DRIFT spectroscopy revealed the different roles of the catalyst components. CO2 mainly adsorbed on the n-Al2O3 support, forming carbonate species. Cu was found to facilitate H2 dissociation and further reacted with the adsorbed carbonates on the n-Al2O3 support, transforming them to formate or additional intermediates. Like the n-Al2O3 support, the ZnO component contributed to improving the CO2 adsorption, facilitating the formation of more carbonate species on the catalyst surface and enhancing the efficiency of the CO2 activation and hydrogenation into methanol. The synergistic interaction between Cu and ZnO was found to be essential to increase the space–time yield (STY) of methanol but not to improve the selectivity. The 3% CuZnO/n-Al2O3 displayed improved catalytic performance compared to 3% Cu/n-Al2O3, reaching a CO2 conversion rate of 19.8% and methanol STY rate of 1.31 mmolgcat−1h−1 at 300 °C. This study provides fundamental and new insights into the distinctive roles of the different components of commercial methanol synthesis catalysts.