The catalytic valorization of carbon dioxide (CO2) is a current challenge with implications in the mitigation of global warming, as well as to set up circular industrial processes. In this work, we present a novel photothermal approach to selectively convert it into carbon monoxide. To this end, the co-doping of nanostructured, high surface-area indium oxide is achieved using cobalt and iron. The joint use of microscopy and spectroscopic X-ray techniques was able to show that the two cations occupy cationic network positions of the oxide. Nevertheless, while cobalt is preferentially located at near-surface positions, iron occupies bulk ones. The catalytic response of the co-doped sample outperforms the single-doped materials and makes fruitful use of the combination of heat and light through a synergistic catalytic effect. The co-doped material would thus open a way to intensify the classical thermal process, rendering a highly active and selective system for the valorization of carbon dioxide.

Co modulation of In–Ti photothermal valorization of CO 2 .

A series of cobalt-promoted indium-titanium composite oxides was synthesized using a microemulsion method. Their functional properties were investigated for the photothermal reduction of carbon dioxide. In this series, the indium content varied between 2.5 and 20%, while the cobalt percentage was kept constant at 4% throughout the series. In all cases, carbon monoxide formation occurred selectively through the reverse water gas shift reaction. The sample with 2.5% In maximized the synergistic use of the two energy sources, while the sample with 10% In showed the highest catalytic activity under both thermal and dual photo-thermo conditions. A physicochemical characterization was performed for all samples. The use of microscopy and X-ray absorption spectroscopy demonstrated that sub-nanometric indium entities, in combination with atomically dispersed cobalt oxide entities, achieved a balance between high thermal activity and significant synergy between light and heat in a catalytic process. The system with 10% indium is thus able to improve the thermal catalytic process through the use of light, providing an intensification procedure for the classic process.

The co-doping of the indium oxide structure with cobalt and niobium is attempted using a microemulsion method to produce high-surface-area, mesoporous, functional materials. The co-doping leads to a major nanometric bixbyite oxide phase, with both cations occupying cation positions and affecting the size and shape of the oxide entities. For specific cobalt to niobium atomic ratios, the presence of a second surface phase is encountered. Such a surface phase corresponds to an ill-defined indium hydroxide-type phase. The coexistence of the two phases is a unique phenomenon associated with the presence of both cations, which play a decisive role in promoting the CO2 adsorption capability of the solids, as well as the valorization of the molecule under dual heat-light excitation. The co-doping of indium oxide thus appears to be a unique way to intensify the analogous thermal (classical) catalytic process for the elimination/valorization of carbon dioxide.

This work investigates Ru-CeO2-TiO2 catalysts for the CO2 methanation reaction and compares their performance with that of previously studied Ru-CeO2 systems. Despite the lower Ru loading, the TiO2-containing catalysts exhibit a significantly higher activity. To understand this behavior, in situ X-ray absorption spectroscopy (XAS) was carried out at the Ru K-edge and Ce L-3-edge. Unlike Ru-CeO2, which displays the reversible redox behavior of Ru, the Ru-CeO2-TiO2 catalysts show irreversible Ru reduction and a substantially higher fraction of Ce3+ species under all tested conditions (H-2, CO2, H-2/CO2). The stabilization of metallic Ru during methanation, together with the enhanced formation of Ce3+ promoted by TiO2 through interfacial electronic transfer, accounts for the catalyst's high activity. Complementary in situ DRIFTS measurements reveal the formation and rapid consumption of bidentate carbonates and formates. These species act as a key intermediate in methane formation. Overall, these findings highlight the crucial role of the mixed CeO2-TiO2 oxide in tuning the surface chemistry of the catalysts by stabilizing metallic Ru, enhancing ceria reducibility, and promoting efficient reaction pathways for CO2 methanation. The manipulation of metal <-> oxide-oxide interactions can be a very useful tool when dealing with the valorization of CO2.

Enhancing the performance of titania-based systems has emerged as a central goal to allow the light-triggered continuous production of hydrogen using renewable energy and bio-based sacrificial molecules. A cheap and widely available promoter based on magnesium is here exploited to provide a long-term stable catalytic response and maximize the formation of hydrogen. Using the International Union of Pure and Applied Chemistry (IUPAC)-recommended quantum efficiency as the parameter to measure activity, this study demonstrates an 8.2-fold increase in photocatalytic activity compared with the Pt-TiO2 reference catalyst, achieving a quantum efficiency of 3.0% under sunlight. The physicochemical study provides evidence that the increase in activity is achieved through a precise synthesis procedure, resulting in the formation of atom-dispersed Mg(ii) entities. The origin of the promotion effect appears to be related to the reaction mechanism, with a limited effect from charge carrier generation or recombination. An in situ infrared study showed that the surface effects connected to the adsorption of alcohol, as well as the generation and (subsequent) evolution of bridge carbonate species, appear at the core of the enhancement in activity. The magnesium promoter, spread as single-atom entities over titania, is thus shown as a way to obtain highly active and stable photocatalytic systems for green hydrogen production.Keywords: Single-atom promoter; Sunlight; Quantum efficiency; Mechanism; Hydrogen.

The performance of Ru/CeO2 catalysts under CO2 hydrogenation conditions was studied using in situ X-ray absorption fine structure (XAFS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to understand the structural evolution and chemical nature of each component under reaction conditions. The catalysts were prepared using a reverse microemulsion method that maximized the dispersion of RuOx particles on ceria. A RuOx -> Ru transformation was observed upon exposure of the RuOx/CeO2 systems to H-2 at 250 degrees C. For a sample with 5 % molar Ru, two-dimensional clusters (2-4 atoms) of Ru formed on top of the ceria support. An increase in the loading of ruthenium to 20 % molar led to the formation of nanoplates of the metal 2-3 at. layers thick. Both Ru structures were highly dispersed on ceria. They were oxidized after exposure to CO2 at 250 degrees C. However, under CO2/H-2 mixture, they remained in a metallic state as two-dimensional clusters and nanoplates. In situ DRIFTS studies, the 5 % and 20 % Ru/CeO2 catalysts showed distinct reaction intermediates when exposed to CO2 or CO2/H-2 (1/4) reaction mixtures. Normalized to the Ru molar percentage, the 5 % Ru/CeO2 system was the most active catalyst exhibiting a selectivity of similar to 60 % CH4 and 40 % CO at 250 degrees C. On the other hand, under the same reaction conditions, the 20 % Ru/CeO2 system was less active but had a CH4 selectivity close to 80 %. These results highlight the importance of the structure of the metallic Ru particles as a factor that determines the catalytic performance.

Hydrogen generation through a photocatalytic process appears to be a promising technology to produce this energy vector through a novel, efficient, green, and sustainable process. The fruitful use of sunlight as an excitation source and renewable bio-derived reactants as well as the development of highly efficient catalysts are required to achieve this goal. In this perspective article, we focus on describing how to braid energy and sustainability sides of hydrogen photo-generation into a single parameter, allowing quantitative measurement and trustful comparison of different catalytic systems. Starting from the energy-related efficiency parameters defined by the IUPAC, we present novel approaches leading to parameters enclosing energy and sustainability information. The study is completed with the analysis of other, non-IUPAC, parameters of broad use such as the solar-to-hydrogen observable. The set of results available in the literature for the water splitting reaction and the use of bio-derived sacrificial molecules are reviewed to assess the potential of such reactions in the energy-efficient and sustainable production of hydrogen.

The valorization of CO2 is an important challenge within the current panorama, since this molecule is probably the main contributor to climate change. In this study, the synthesis of materials based on a nanostructured batonnet-type indium oxide is carried out. In them, different amounts of Co are introduced, varying between 2 and 8% mol. It is verified that the most active sample in the transformation of carbon dioxide to carbon monoxide contains 6 mol %. of Co. This sample's activity under dual excitation exceeds the thermal counterpart by more than 30%. After carrying out a complete physical and chemical characterization with the help of X-ray absorption spectroscopy and other techniques, it is shown that catalysts with amounts of cobalt equal to or below 4 mol % contain isolated single-atom species, while those with higher amounts of metal display a Co-Co interaction which triggers the evolution of the samples under reaction conditions. The optimum control of this Co-Co interaction and the nature of the final cobalt-containing species determine dual photothermal catalytic properties. This work establishes a structure-activity relationship to interpret the catalytic behavior of highly dispersed subnanometric cobalt species, and thus an avenue to optimize the photothermal valorization of carbon dioxide.

The synthesis and properties of stoichiometric, reduced, and Co-doped In2O3 are described in the light of several experimental techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet (UV)-visible spectroscopy, porosimetry, and density functional theory (DFT) methods on appropriate models. DFT-based calculations provide an accurate prediction of the atomic and electronic structure of these systems. The computed lattice parameter is linearly correlated with the experimental result in the Co concentration ranging from 1.0 to 5.0%. For higher Co concentrations, the theoretical-experimental analysis of the results indicates that the dopant is likely to be preferentially present at surface sites. The analysis of the electronic structure supports the experimental assignment of Co2+ for the doped material. Experiments and theory find that the presence of Co has a limited effect on the material band gap.

The synthesis, physicochemical, and functional properties of composite solids resulting from the surface spread of oxidized indium species onto nanoplatelets of anatase were investigated. Both the size and the interaction between the indium- and titanium-containing components control the functional properties. In the reduction of CO2 to CO, the best samples have an indium content between ca. 2 and 5 mol % and showed an excess rate over the photo and thermo-alone processes above 33% and an energy efficiency of 1.3%. Subnanometric (monomeric and dimeric) indium species present relatively weak thermal catalytic response but strong thermo-photo promotion of the activity. A gradual change in functional properties was observed with the growth of the indium content of the solids, leading to a progressive increase of thermal activity but lower thermo-photo promotion. The study provides a well-defined structure-activity relationship rationalizing the dual thermo-photo properties of the catalysts and establishes a guide for the development of highly active and stable composite solids for the elimination and valorization of CO2.

In this work, ruthenium-promoted nitrogen-doped carbon nanotubes (Ru/NCN) composite materials were synthesized and utilized for the dual thermo-photo catalytic production of hydrogen using biomass-derived reactants. A spectroscopic and microscopy-based characterization showed that the composites contain rather small metallic ruthenium entities uniformly distributed and stabilized through interaction with the carbon-containing component. A highly active and (under reaction conditions) stable Ru/NCN material maximized hydrogen output under continuous operation, showing outstanding reaction rates up to ca. 30 mmol g(-1) h(-1) and quantum efficiencies of ca. 7.5 %. The synergy reached by the combined use of two energy sources was studied quantitatively and analyzed in the temperature range going from room temperature to 300 degrees C. The combination of heat and light triggers a mechanism by which the bio-molecule, methanol, is activated by light-triggered hydroxyl-type radical species and promotes the reaction up to temperatures close to 300 degrees C. The new composite materials based on nitrogen-doped carbon nanotubes show thus high potential to improve classic, thermal-based processes for hydrogen production.

Carbon nitride MXene exhibits good metal conductivity, high photothermal conversion, carrier mobility, and high exposure of active sites, which makes it a promising co-catalyst for photothermal synergistic transformation of CO2. In this paper, Ti3CN/TiO2 heterojunction was constructed in situ using Ti3CN as TiO2 precursor to investigate the performance of Ti3CN MXene in photothermal synergistic transformation of CO2, and then the monolayer structure was utilized to enhance the interfacial charge transfer and improve the photothermal catalytic activity of Ti3CN. The catalysts were characterized by SEM, XRD, XPS, and UV-Vis DRS, and it was found the heterojunction constructed by monolayer MXene had a narrower bandgap and a higher carrier generation mobility, which, combined with the catalytic activity test, proved the single monolayer Ti3CN MXene had better photothermal synergistic conversion efficiency of CO2, and the heterojunction yield was 11.36 μmol·g−1·h−1 after layering, compared with that before layering (9.41%), which was 1.2 times higher than that before layering (9.41 μmol·g−1·h−1).

The development of the photo-catalysis field is limited by a deficient quantitative assessment of photo-activity. The interplay between mass and momentum transport together with radiative transfer phenomena taking place at any photo-catalytic reaction or process makes complex such quantitative assessment. To reach this goal, the review studies the measurement, meaning, and analysis of three types of observables. The first family of observables has the reaction rate and closely connected observables as the turnover frequency as central pieces. The second family owns the so-called efficiency observables, starting from the photonic yield and quantum efficiency of the reaction and ending in the global efficiency of the process. Finally, the review studies kinetic constant observables. The contribution focusses on most rencet contribution analyzing these observables in terms of their (adequate) measurement conditions and physico-chemical interpretation, in order to unveil their full potential in the context of the photo-catalysis field.

Morphologically controlled indium oxide nanocubes were synthesized, contacted with pure anatase nanoparticles, and used in the dual photothermocatalytic valorization of carbon dioxide. The series of samples with an indium oxide loading up to 20 wt % were subjected to a detailed physicochemical analysis. The catalysts synthesized are high surface area mesoporous materials. Interface effects coming from the contact between the two oxide components were analyzed and found to control the charge carrier recombination taking place under illumination. The catalytic activity under dual photothermo conditions showed optimum profit of the combination of the two energy sources with a sample having a 10 wt % of indium oxide. This sample shows a synergistic use of the dual excitation with an excess of ca. 50% as measured by the reaction rate. This excess appears at the top of the literature. The synergy observed would be linked with the optoelectronic effects controlled by the oxide-oxide interface.

In this research, composite catalysts based on mixtures of titanium and cobalt oxides were synthesized and tested in the thermo-photo reduction of carbon dioxide. The cobalt loading varied between 2% and 8 M %, providing an outstanding activity in the selective production of carbon monoxide through the reverse water gas reaction. The sample doped with 4% Co optimized catalytic performance, with an excess over the photo and thermo alone processes above 27 % and an energy efficiency of 1.1 %. These values indicate that the sample displays the top performance reported so far. A physico-chemical characterization of the catalysts was also carried out showing that nearly amorphous cobalt (II) oxide entities are embedded within nanocrystalline anatase nanoplatelets within an inverse-type configuration. The use of in-situ infrared provides conclusive evidence that the promotion of the catalytic performance under dual excitation is intimately linked with the formation and evolution of formate-type species at the catalyst surface. A novel dual-excitation catalytic process using novel composite CoOx-TiO2 catalysts would open new ways for the energy-efficient elimination and valorization of carbon dioxide.

Statistical procedures to analyse sets of X-ray absorption near-edge structure (XANES) spectra are briefly described. Correlation and factor-analysis procedures are among the most widely used. Representative examples of their application to XANES analysis in a broad range of scientific disciplines are discussed.

The role of a Pd-based co-catalyst supported on a high surface area anatase powder is analyzed for the photo-production of hydrogen from methanol:water mixtures. The co-catalyst chemical composition was modified by replacing palladium atoms with copper. The preparation method renders alloyed particles with a primary particle size in the 5-6 nm interval. A Pd:Cu 3:1 ratio promotes strongly the activity of the noble metal. According to the quantum efficiency parameter, the Pd:Cu 3:1 alloy increases by ca. 2 the activity of the monometallic Pd counterpart. The analysis of the photoactivity points out that alloying does not affect positively charge recombination and handling

In this research study, the impact of adding zirconium oxide to titanium oxide-supported gold systems on photocatalytic hydrogen production was analyzed. The activity was evaluated as a function of the zirconium content, with up to 5 mol.% tested under UV, visible light, and sunlight illumination. It was found that the highest activity was achieved with a 1.75 mol.% zirconium oxide, as measured using reaction rate and quantum efficiency parameters under various illumination conditions. The results showed a quantum efficiency close to 1.9% under sunlight, which represents a substantial improvement of over two orders of magnitude compared to the original titanium system. The addition of zirconium influenced the recombination of charge carriers and boosted photoactivity through the interaction between gold and zirconium.