In this paper, a facile one step synthesis method for the preparation of C-, B-, P-and S doped g-C3N4 by incorporation of small concentration of doping element precursor into urea during thermal polycondensation is reported leading to much lower doping levels than the ones usually reported. The as-obtained doped g-C3N4 photocatalytic materials are deeply characterized in terms of structural, morphological, surface and optical properties. Doping yields beneficial surface morphology modulation along with improved optical, electronic and photocatalytic properties. In particular, C-doped and S-doped g-C3N4 show, after deposition of gold nanoparticles (<1 wt%), enhanced photocatalytic performance (at least twice as high as the undoped photocatalyst, to achieve ca. 610 mmol/h/g) for the production of H2 by water splitting under solar light in the presence of low content (1 vol%) of triethanolamine (TEOA) as sacrificial agent. The most remarkable activity results from the incorporation of traces of S dopant, mainly inserted into interplanar hollow cavities. The enhanced activity is attributed to a combination of high surface area, location of the S dopant and small size of the co-catalyst NPs, which induces enhanced visible light harvesting, enhanced charge carrier separation and enhanced proton recombination. This work highlights the benefits of the optimized low-level doping strategy to overcome the main limitations of g-C3N4-based photocatalysts. <(c)> 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Bimetallic gold-platinum nanoparticles have been widely studied in the fields of nanoalloys, catalysis and plasmonics. Many preparation methods can lead to the formation of these bimetallic nanoparticles (NPs), and the structure and related properties of the nanoalloy often depend on the preparation method used. Here we investigate the ability of thermal dimethylformamide (DMF) reduction to prepare bimetallic gold-platinum sub-nm clusters supported on titania. We find that deposition of Pt preferentially occurs on gold. Formation of sub-nm clusters (vs. NPs) appears to be dependent on the metal concentration used: clusters can be obtained for metal loadings up to 4 wt% but 7-8 nm NPs are formed for metal loadings above 8 wt%, as shown using high resolution transmission electron microscopy (HRTEM). X-ray photoelectron spectroscopy (XPS) shows electron-rich Au and Pt components in a pure metallic form and significant platinum enrichment of the surface, which increases with increasing Pt/Au ratio and suggests the presence of Au@Pt core-shell type structures. By contrast, titania-supported bimetallic particles (typically >7 nm) obtained by sodium borohydride (NaBH4) reduction in DMF, contain Au/Pt Janus-type objects in addition to oxidized forms of Pt as evidenced by HRTEM, which is in agreement with the lower Pt surface enrichment found by XPS. Both types of supported nanostructures contain a gold-platinum interface, as shown by the chemical interface damping, i.e. gold plasmon damping by Pt, found using UV-visible spectroscopy. Evaluation of the materials for plasmon-induced continuous flow CO2 reduction with water, shows that: (1) subnanometer metallic clusters are not suitable for CO2 reduction with water, producing hydrogen from the competing water reduction instead, thereby highlighting the plasmonic nature of the reaction; (2) the highest methane production rates are obtained for the highest Pt enrichments of the surface, i.e. the core-shell-like structures achieved by the thermal DMF reduction method; (3) selectivity towards CO2 reduction vs. the competing water reduction is enhanced by loading of the plasmonic NPs, i.e. coverage of the titania semi-conductor by plasmonic NPs. Full selectivity is achieved for loadings above 6 wt%, regardless of the NPs composition and alloy structure.

Herein, visible light-driven gas-phase photocatalytic CO2 reduction into CH4 is tuned by designing optimized three-component Au/doped C3N4/TiO2 composite photocatalysts. The key point strategy consists in the formation of high-quality C3N4/TiO2 heterojunction by associating low containing doped graphitic carbon nitride to commercially available TiO2 UV-100. Those heterojunctions result in both visible light sensitization and increased charge-carrier separation. Further deposition of small Au nanoparticles (approximate to 3 nm), quite exclusively onto TiO2 surfaces, mainly acts as electron trapping/cocatalytic functions without excluding surface plasmonic effects. The resulting doped g-C3N4 material exhibits enhanced visible light harvesting properties, especially in the case of C-doping. In addition, it is assumed that B- and C-C3N4 doping, leading to a more or less lower conduction band position, is the impacting factor toward total CH4 selectivity achievement. The (0.77 wt%)Au/(0.59 wt%)C-C3N4/TiO2 composite photocatalyst, exhibiting the best compromise between the various impacting factors, leads to a continuous productivity rate of CH4 of 8.5 mu mol h(-1) g(-1) under visible light irradiation over at least 10 h. To the best of knowledge, this level of performance is unprecedented under continuous gas-phase flowing CO2 in the presence of water as reducing agent, without addition of any sacrificial agent.

Here we report on the optimization of NH2-UiO-66/TiO2/Au (ca. 1.5 wt%) composite photocatalysts applied to gas phase CO2 photocatalytic reduction in presence of water as reducing agent by varying NH2-UiO-66/TiO2 ratio and pH during synthesis. It is shown that 10 wt% NH2-UiO-66/TiO2/Au (at pH=7) composite leads to the best cumulated CH4 production rate of 136 mu mol/gcatalyst with 70% electronic selectivity over 5 h of continuous test. This composite exhibits the best compromise between MOF surface area and thus CO2 adsorption sites, visible light photons absorption capacity and a large interface contact area, which ensured preferable metal(Zr)-to-metal(Ti) charge carrier. Au deposition by impregnation/chemical reduction in also required to perform CO2 photoreduction, and may presumably act as electron traps, co-catalyst or surface plasmon resonator.

Au/TiO2 photocatalysts were studied, characterized, and compared for CO2 photocatalytic gas-phase reduction. The impact of the nature of the TiO2 support was studied. It was shown that the surface area/porosity/TiO2 crystal phase/density of specific exposed facets and oxygen vacancies were the key factors determining CH4 productivity under solar-light activation. A 0.84 wt.% Au/TiO2 SG (Sol Gel) calcined at 400 °C exhibited the best performance, leading to a continuous mean CH4 production rate of 50 μmol.h−1.g−1 over 5 h, associated with an electronic selectivity of 85%. This high activity was mainly attributed to the large surface area and accessible microporous volume, high density of exposed TiO2 (101) anatase facets, and oxygen vacancies acting as reactive defects sites for CO2 adsorption/activation/dissociation and charge carrier transport.

Solar energy and photocatalysis will undoubtedly play a key aspect in the energy transition. In this background, hydrogen emerged as the ideal photocatalysis-driven solar fuel due to the simplicity of the water-splitting reaction. But finding a single catalyst with sufficient solar-to-photon conversion efficiency remains an important bottleneck of this technology. A constraint that prompted the advancement of multi-phase composites with broader capabilities. Owing to their unique properties, plasmonic Schottky junctions and Z-scheme heterojunctions arose as promising approaches to enhance hydrogen production. But the selection of suitable components towards multi-phasic composites remains challenging due to the absence of standardization in literature and proper characterization of existing materials. Characterization is especially important since components' plasmon wavelength (visible or NIR) and electronic properties have a significant influence on light-harvesting properties and charge carriers separation of the resulting composites. Plus, the optoelectronic properties of the semiconductors determine the resulting type of heterojunction with potential influence in the charge carrier separation via photosensitization. Recent studies suggest that the combination of TiO2 (broad band gap), gC3N4 (narrow band gap), and gold may achieve efficient plasmonic and Z-scheme heterojunctions. In this review, we cover the synthesis and implementation of these materials (alone or in combination), including key technical aspects in photocatalysis, plasmonics, and hydrogen production subjects. We also address pertinent knowledge, experimental gaps, and point to future perspectives to further improve the development of photocatalytic-driven hydrogen technologies. Globally, this multi-phasic composite Au/TiO2-gC3N4 could be used as a platform to continue reinforcing TiO2 efficiencies via photosensitization, co-catalysis, and surface plasmon resonance.

Preferential oxidation of CO (COPrOx) is a catalytic reaction targeting the removal of trace amounts of CO from hydrogen-rich gas mixtures. Non-noble metal catalysts, such as Cu and Co, can be equally active to Pt for the reaction; however, their commercialization is limited by their poor stability. We have recently shown that CoO is the most active state of cobalt for COPrOx, but under certain reaction conditions, it is readily oxidized to Co3O4 and deactivates. Here, we report a simple method to stabilize the Co2+ state by vanadium addition. The V-promoted cobalt catalyst exhibits considerably higher activity and stability than pure cobalt. The nature of the catalytic active sites during COPrOx was established by operando NAP-XPS and NEXAFS, while the stability of the Co2+ state on the surface was verified by in situ NEXAFS at 1 bar pressure. The active phase consists of an ultra-thin cobalt-vanadate surface layer, containing tetrahedral V5+ and octahedral Co2+ cations, with an electronic and geometric structure that is deviating from the standard mixed bulk oxides. In addition, V addition helps to maintain the population of Co2+ species involved in the reaction, inhibiting carbonate species formation that are responsible for the deactivation. The promoting effect of V is discussed in terms of enhancement of CoO redox stability on the surface induced by electronic and structural modifications. These results demonstrate that V-promoted cobalt is a promising COPrOx catalyst and validate the application of in situ spectroscopy to provide the concept for designing better performing catalysts.

Stefano Fornasaro opened discussion of the paper by Manuela Melucci: Is the production scalable? Can you comment about reproducibility of the results? Manuela Melucci replied: We used commercial graphene oxide powder (Abalonyx) and commercial hollow fiber cartridges (Plasmart 25,

The preferential oxidation of CO in H2-rich mixtures (COPrOx) is a major catalytic reaction utilized for hydrogen purification. In the exploration of alternatives to noble metals, cobalt-based catalysts appear to be a very promising choice. The activity and stability of cobalt in the COPrOx reaction can be improved by the addition of transition metals and manganese is maybe the most prominent among them. Yet, the arrangement of the two components in the catalytically active state is largely unknown, which hinders in-depth understanding of the manganese promotion effect. Here, we compare pure and Mn-modified cobalt catalysts and correlate their structural and chemical characteristics with their COPrOx performance. The Mn-promoted cobalt catalyst is significantly more active than pure cobalt especially at intermediate reaction temperatures (around 200 degrees C). The addition of Mn improves the structural stability of the catalyst and helps to maintain higher specific surface areas. Chemical and microstructural analysis using various operando and in situ techniques revealed that Mn promotes CO conversion by partially stabilizing CoO phase during reaction conditions. It is also suggested that at high temperature, Mn suppress CO methanation reaction but promotes H2 oxidation. Apart of the particular interest in COPrOx reaction, in a general context, this work shows how the spatial distribution of the different catalyst components at nanoscopic level, may affect the surface chemistry and consequently control the reactivity.

Even in ultralow quantities, oxygen vacancies (VO) drastically impact key properties of metal oxide semiconductors, such as charge transport, surface adsorption, and reactivity, playing central roles in functional materials performance. Current methods used to investigate VO often rely on specialized instrumentation under far from ideal reaction conditions. Hence, the influence of VO generated in situ during catalytic processes has yet to be probed. In this work, we assess in situ extrinsic surface VO formation and lifetime under photocatalytic conditions which we compare to photocatalytic performance. We show for the first time that lifetimes of in situ generated atomic VO play more significant roles in catalysis than their concentration, with strong correlations between longer-lived VO and higher photocatalytic activity. Our results indicate that enhanced photocatalytic efficiency correlates with goldilocks VO concentrations, where VO densities must be just right to encourage carrier transport while avoiding charge carrier trapping.

Gold-catalyzed CO oxidation is a reaction of both practical and fundamental interest. In particular, rate-determining oxygen activation pathways have attracted a lot of attention. They have been found to depend on the surface chemistry of the catalyst support, titania providing the most active catalysts and carbon nitride leading to inactive catalysts. Here, we show that C3N4-TiO2 composites with rather similar surface chemistries can be engineered by using titania nanotubes as hard templates and by performing the polycondensation of melamine and dicyandiamide in air and in ammonia. By varying the C3N4 content from 2 to 75 wt %, the mesoporosity can be tuned from 8 to 40 nm. A systematic study of CO oxidation turnover numbers in the absence and in the presence of hydrogen over the composites loaded with well-calibrated 2-4 nm gold nanoparticles clearly shows that (1) the chemical composition of the support surface has much less impact on PROX (preferential oxidation of CO in excess hydrogen) than on dry CO oxidation, (2) NH2-terminated supports are as active as OH-terminated supports in PROX, (3) hydrogen/water-mediated CO oxidation pathways are active on C3N4-based Au catalysts, and (4) PROX activity requires a rather large porosity (40 nm), which suggests the involvement of much larger intermediates than the usually postulated peroxo-type species.

A series of ternary Au/TiO2(P25)-gC(3)N(4) heterostructure photocatalysts with low Au nanoparticle (NP) loading and gC(3)N(4) content were synthesized and evaluated for H-2 production from water with a very low amount of sacrificial agent (1 vol%) under solar and visible-light irradiation. The optimized composite, 0.3 wt% Au/ TiO2(P25)-gC(3)N(4) (95-5), exhibited a remarkable production rate under visible light (419 mu mol(-1) h(-1) gcatalyst of H-2), corresponding to almost twice the cumulated H-2 production over 2.5 h as that of the Au/TiO2 (P25) reference. To best of our knowledge, this high of a yield has never been reported for comparable experimental conditions. The highest performing composite was characterized using UV-vis, oS, UPS, BET, TEM, and TRMC techniques. The performance of the photocatalyst could be correlated to contributions of various phenomena such as effective heterojunction formation of TiO2 with gC(3)N(4) operating in a Z-scheme dynamic allowing TiO2 photosensitization towards visible-light, also leading to new electronic hybrid states, and plasmonic Au/support Schottky junction allowing electronic trapping sites and co-catalytic effects. These contributions resulted in more efficient light harvesting and separation of the charge carriers to enhance visible light driven activity.

Graphite is a widely available natural form of carbon with peculiar chemical and surface properties. It is essentially hydrophobic and consists in very stable stacks of graphene layers held together by highly delocalized π-π interactions. Its use in chemistry and in particular for catalytic applications requires modification of its structure to increase its surface area. This is commonly achieved by harsh oxidation methods which also modifies the chemical composition of graphite and enables subsequent deposition of catalytic phases via common impregnation/reduction methods. Here we show that copper phthalocyanine (CuPc) can be incorporated into unmodified bulk graphite by the straight-forward sonication of a dimethylformamide solution containing CuPc and graphite flakes. Immobilization of the CuPc complex in the graphitic matrix is shown to rely on π-π interactions between the Pc ligand and graphenic surfaces. This strong CuPc-graphene interaction facilitates oxidation of the graphitic matrix upon oxidation of the immobilized complex, as shown by thermogravimetric analysis in air. Nevertheless, a soft oxidation treatment can be designed to produce CuO nanoparticles (NPs) without degrading the dispersing graphitic matrix. These well-dispersed CuO NPs are shown (1) to decrease the degree of stacking of graphite in the solid-state by intercalation in-between graphitic stacks, (2) to be more easily reducible than bulk CuO, and (3) to be catalytically active for the oxidation of carbon monoxide. The higher mass-specific CO oxidation rates observed, as compared with CuO/alumina benchmarks, highlight the beneficial role of the carbon support and the relevance of this new strategy toward the design of copper oxide catalysts from copper phthalocyanine metal complexes.

Reduced Au-Cu/CeO2 catalysts (Cu/Au = 1, 3) are more efficient towards the preferential oxidation of CO (PROX) than their calcined counter-parts. They exhibit lower activation energies and CO partial reaction orders (alpha(CO)), and even more interestingly, significantly higher O-2 partial reaction orders (beta(O2)). For the stoichiometric composition in particular (Au1Cu1/CeO2-R), the partial pressure of oxygen has a higher impact on the reaction rate than the partial pressure of CO (beta(O2) > alpha(CO)), which is unprecedented in low temperature catalysis involving gold and a reducible oxide support. DRIFTS studies further show (1) that Au1Cu1/CeO2-R contains electron-deficient, alloyed Cu atoms (Au-Cu+) and (2) that the formation of CO2 is enhanced by the preadsorption of O-2 rather than the preadsorption of CO. This suggests that CO oxidation proceeds via a Langmuir-Hinshelwood-type, bifunctional mechanism, involving CO adsorbed on Au-0 sites and oxygen adsorbed on electron-deficient, gold-alloyed copper sites. This alloy-mediated oxygen activation could be the key to the superior PROX activity of Au1Cu1/CeO2-R. (C) 2020 Elsevier Inc. All rights reserved.

The use of light as a primary energy source in chemical transformations is one major pillar of the forthcoming energy transition. In this issue of Chem, Han et al. design a quite sophisticated antenna-reactor complex that selectively catalyzes the reductive cleavage of C–O bonds in aryl ethers under mild conditions.

The atmosphere plays a key role in the polycondensation synthesis and the g-C3N4 structure.

2D materials are attracting increasing attention in many strategic applications. In particular, ultra-thin non-layered oxides have been shown to outperform their 3D counter-parts in several health and energy applications, such as the removal of toxic carbon monoxide by low temperature oxidation and the development of high performance supercapacitors. The general reason for that is the increased surface-to-volume ratio, which maximizes exposure of active species and enhances exchange between the (limited) bulk and the surface. The challenge is to synthesize such 2D configurations of 3D oxides, which generally requires quite harsh multi-step, multi-reagent chemical processes. Here we show that natural graphite can be used as a templating matrix to grow non-stoichiometric 2D transition metal oxides. We focus on highly porous, highly reduced cobalt oxides grown from cobalt nitrate and sodium borohydride under sonication. Extensive characterization, including nitrogen physisorption, thermogravimetric analysis (TGA), scanning and transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), temperature programmed oxidation and reduction (TPO/TPR), Fourier transformed infrared (FTIR) and Raman spectroscopies, highlights the specific features of the 2D morphologies (nanosheets and nanofilms) obtained. For comparison, 3D morphologies of Co3O4 spinel nanocrystallites are grown from stacked 2D cobalt phthalocyanine-graphene precursors upon controlled thermal oxidation. Finally, low temperature CO oxidation catalysis evidences the superior performance of the graphene-supported CoO-like cobalt oxide 2D nanosheets.

Sylwester Gawinkowski opened a general discussion of the paper by Jorge Salmon-Gamboa: Why you are using SiO2 nanoparticles? Do they have any function or are they only the substrate to attach other active nanoparticles to? You have shown that only gold nanoparticles attached to silica nanoparti

The use of light as a primary energy source in chemical transformations is one major pillar of the forthcoming energy transition. In this issue of Chem, Han et al. design a quite sophisticated antenna-reactor complex that selectively catalyzes the reductive cleavage of C–O bonds in aryl ethers under mild conditions.