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 μmol/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.
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•H2 production by water splitting on traces non-metal doped g-C3N4 with 1 vol% TEOA.•Enhanced H2 production rate induced by traces level (<0.1 wt%) of S-dopant.•Benefits of low-level doping strategy to overcome the main limitations of g-C3N4.•One-pot easy synthesis of non-metal doping of g-C3N4 thermal polycondensation.•Those photocatalyst exhibit enhanced visible light harvesting properties.
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.
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 μmol/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.
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•NH2-UiO-66/TiO2/Au composites synthetized with different MOF/TiO2 ratios and various pH.•Rate of H2 and CH4 production obtained after solar light irradiation during 5 h.•10%wt. NH2-UIO-66/TiO2/Au composite prepared at pH 7 presents the best production of CH4 of 136 μmol/gcatalyst.•Suggested mechanism with ligand-to-metal charge transfer and metal(Zr)-to-metal(Ti) charge carrier at MOF/TiO2 interface.
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.
Here for the first time the design and optimization are presented of a three‐component Au/TiO2–gC3N4 nanocomposite photocatalyst able to efficiently produce H2 from water using very low amounts of ...sacrificial agents and under visible light irradiation. This enhanced photocatalytic behavior compared to Au/TiO2 and Au/gC3N4 materials is the result of synergetic effects due to high quality assembly and interface between the three components. This optimized nanoscale assembly characterized by simultaneous favorable nanoheterojunction formation between g‐C3N4 and TiO2 semiconductors, as well as AuNPs/gC3N4 and AuNPs/TiO2 junctions, leads to enhanced visible light harvesting, charge separation, and H2 production. This composite photocatalyst yields a high H2 production (350 µmol−1 h−1 gcatalyst−1) under visible light irradiation with minimal amounts of sacrificial agent (≤1 vol%), corresponding to activities much higher than reported so far under comparable conditions.
A optimized combination of TiO2, g‐C3N4, and Au nanoparticles within a composite material enables highly efficient photocatalytic H2 production from water under visible‐light with low amounts of sacrificial agents. The high quality assembly and contact between the three constituents result in a synergetic effect leading to better visible light absorption and better use of the photogenerated charge carriers.
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•Reduced AuCu/CeO2 show lower CO reaction orders than calcined catalysts.•Reduced catalysts exhibit higher O2 reaction orders than calcined AuCu/CeO2.•For reduced Au1Cu1/CeO2, the ...limiting step is the activation of O2 rather than CO.•Oxygen adsorption occurs on gold-alloyed copper atoms in reduced Au1Cu1/CeO2.•Atomic scale mixing of O2 and CO activation sites results in higher PROX efficiency.
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 (αCO), and even more interestingly, significantly higher O2 partial reaction orders (βO2). 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 (βO2 > αCO), 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 O2 rather than the preadsorption of CO. This suggests that CO oxidation proceeds via a Langmuir-Hinshelwood-type, bifunctional mechanism, involving CO adsorbed on Au0 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.
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