Photocatalytic hydrogen production using ethanol as sacrificial agent from gas and liquid phases on reduced graphene oxide-TiO2- Pt nanocomposites

ABSTRACT: Various strategies such as heterostructuring, crystal/textural modifications and band gap engineering, have been applied to the improvement of the photocatalytic activity of Titania for hydrogen production from water splitting. In this work deposited Pt on TiO2 is used as electron trap to suppress charge recombination. To reinforce this effect, composites with graphene oxide (GO) have been prepared, exhibiting promising photocatalytic performance for both hydrogen generation and the degradation of ethanol added as hole scavenger. Photocatalytic reactions were conducted in gas and liquid phases.N/

A bifunctional photoaminocatalyst for the alkylation of aldehydes: Design, analysis, and mechanistic studies

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright ©2018 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/abs/10.1021/acscatal.8b01331A bifunctional photoaminocatalyst based on imidazolidinone and thioxanthone is presented. The preparation of these catalysts proceeds in a two-step synthesis that allows an easy tuning of the steric properties. The photophysical and electrochemical data of the imidazolidinone photocatalysts have been determined, indicating that the catalysts can work under visible light conditions. To corroborate the experimental observations, ground state geometry optimization and energy transition studies of thioxanthone and bifunctional catalyst 4c were optimized by time-dependent density functional theory (TD DFT) calculations. The alkylation of aldehydes with this photoaminocatalyst works with high enantioselectivities and yields due to the stereoelectronic properties of the catalyst. A rational mechanistic cycle based on different mechanistic experiments, TD DFT calculations, and laser flash photolysis is presentedThe Spanish Government (CTQ2015-64561-R) and the European Research Council (ERC-CG, Contract 647550) are acknowledge

Influence of surface density on the CO2 photoreduction activity of a DC magnetron sputtered TiO2 catalyst

Advancing in the photocatalyst scale-up is crucial for the development of highly efficient solar fuels production at industrial scale. Here, we report DC-magnetron sputtering as a suitable technique to produce photocatalytic TiO2 coatings for CO2 reduction with a view on process scalability. The crystallinity of the obtained TiO2 coatings varies with surface density, with amorphous or quasi-amorphous coatings obtained with very low densities, while UV light absorption coefficients show the opposite trend, which has been related to the proportionally higher abundace of surface defects and grain boundaries associated to the small crystal size and/or amorphicity of the lightest coatings. The as-prepared samples lead to the reduction of CO2 as demonstrated by 13C isotope tracing. An optimum catalyst area density of 1 g/m2 (by geometric area) is obtained in terms of CO2 photoreduction production, which is ascribed to a compromise situation between crystallinity and absorption coefficient. Selectivity to the different reaction products also varies with the coating characteristics, with amorphous or quasi-amorphous light coatings favouring methanol formation, in contrast with the preferred CO evolution in heavier, crystalline ones. Raman spectroscopy reveals the formation of peroxo and peroxocarbonate species on the photocatalyst surface as oxidation products during the CO2 reduction, the accummulation of which is proposed to be related to the observed catalyst deactivation

Correcting flaws in the assignment of nitrogen chemical environments in N-doped graphene

X-ray Photoelectron Spectroscopy (XPS) applied to N-doped graphene leads to a rather broad N(1s) core level signal that, based on different sources available in the literature, is most often interpreted by fitting the experimental spectra to three peaks. The resulting N(1s) features are assigned to graphitic, pyrrolic, and pyridinic nitrogen, even if these are far from being uniquely defined in the literature. This broadly accepted interpretation has been questioned by recent accurate Hartree-Fock calculations concluding that graphitic and pyrrolic N(1s) core level binding energies are too close to be distinguished. Consideration of models with N in other so far unexplored environments such as N dimers or N at defects show some variations in the calculated core level binding energies. However, these are not large enough to justify a third peak and suggest that the usual three peaks interpretation of the N(1s) XPS in N-doped graphene may be an artefact caused by the fitting procedure. New measurements have been carried out for samples of N-doped graphene and the obtained N(1s) spectra fitted to two or three peaks. It turns out that the spectra can be equally fitted using two or three peaks but only the former is consistent with the results of the unbiased ab initio calculations which calls for a revision of the usual assignment

A molecular approach to the synthesis of platinum-decorated mesoporous graphitic carbon nitride as selective CO2reduction photocatalyst

Altres ajuts: I. A.-P. acknowledges the Universitat Autònoma de Barcelona for his pre-doctoral grant. J.G.-A. acknowledges Serra Húnter Program. The authors thank the Microscopy Service of the Universitat Autònoma de Barcelona for technical assistance with TEM and SEMPlatinum nanoparticles (Pt-NPs) have been directly synthesized through the organometallic approach onto the surface of mesoporous graphitic carbon nitride (mpg-CN) semiconductor with two different metal loadings. Thorough multi-technique characterization reveals a very good dispersion of nanoparticles with a narrow size distribution centered at ca. 2.5 nm, regardless of the metal loading, and composed primarily of platinum metal with a minor contribution of oxidic surface species. Compared to bare mpg-CN, the Pt-NPs decorated materials show improved charge separation properties upon band gap excitation, ascribed to electron extraction by Pt-NPs from the conduction band of mpg-CN, as demonstrated by time-resolved fluorescence measurements. The so-obtained materials show photocatalytic activity for CO2 reduction under both UV and visible light irradiation, with improved selectivity towards highly reduced products such as methanol and methane with respect to the bare semiconductor, which leads to the formation of carbon monoxide as the main product. The obtained results shed light on the pathways that determine selectivity in photocatalytic CO2 conversion, contributing to the development of selective photocatalysts, which is one of the cornerstones in this promising technology for direct solar-to-chemical energy conversio

Iterative dual-metal and energy transfer catalysis enables stereodivergence in alkyne difunctionalization: Carboboration as case study

Stereochemically defined tetrasubstituted olefins are widespread structural elements of organic molecules and key intermediates in organic synthesis. However, flexible methods enabling stereodivergent access to E and Z isomers of fully substituted alkenes from a common precursor represent a significant challenge and are actively sought after in catalysis, especially those amenable to complex multifunctional molecules. Herein, we demonstrate that iterative dual-metal and energy transfer catalysis constitutes a unique platform for achieving stereodivergence in the difunctionalization of internal alkynes. The utility of this approach is showcased by the stereodivergent synthesis of both stereoisomers of tetrasubstituted β-boryl acrylates from internal alkynoates with excellent stereocontrol via sequential carboboration and photoisomerization. The reluctance of electron-deficient internal alkynes to undergo catalytic carboboration has been overcome through cooperative Cu/Pd-catalysis, whereas an Ir complex was identified as a versatile sensitizer that is able to photoisomerize the resulting sterically crowded alkenes. Mechanistic studies by means of quantum-chemical calculations, quenching experiments, and transient absorption spectroscopy have been applied to unveil the mechanism of both stepsPGC2018-098660−B-I00, Horizon2020 (ERC) No.648319, PID2019-106315RB-I00, PID2020-118593RB-C22

Holy Water: Photo-Brightening in Quasi-2D Perovskite Films under Ambient Enables Highly Performing Light-Emitting Diodes

Quasi-2D perovskites provide new opportunities for lighting and display applications due to their high radiative recombination and excellent stability. However, seldom attention has been placed on their self-stability/working operation under ambient storage. Herein, quasi-2D perovskites/Polyethylene oxide (PEO) films are studied, showing an unforeseen photo-brightening effect under ambient storage (i.e., an increase of the photoluminescence quantum yield from 55% to 74% after 100 days). In stark contrast, those stored under a dark/inert atmosphere show a significant decrease down to 38%. This counterintuitive phenomenon responds to the increasing radiative recombination rate caused by the passivation of the surface Br vacancies in the presence of physically adsorbed water molecules, as corroborated by in situ/ex situ X-ray photoelectron spectroscopy and density functional theory calculations. Capitalizing on this surprising effect, stable light-emitting diodes (LEDs) using quasi-2D perovskites/PEO color filters are fabricated, realizing high stabilities of ≈400 h@10 mA under operating ambient conditions, representing a 20-fold enhancement compared to LEDs with 3D counter partners. Hence, this study reveals a unique insight into the impact of water passivation on the optical/structural properties of quasi-2D perovskite films, broadening their applications under operating ambient conditions.Y.D. thanks the financial support from the China Scholarship Council (CSC, no. 201808440326). Financial support has been received from AEI-MINECO/FEDER, UE through the Nympha Project (PID2019-106315RB-I00), the regional government of "Comunidad de Madrid" and the European Structural Funds through FotoArt-CM Project (S2018/NMT-4367). F.O. acknowledges funding from the Marie Skłodowska-Curie grant agreement no 754382. M.U.K. and G.N. thank ELI-ALPS, which is supported by the European Union and co-financed by the European Regional Development Fund (GI-NOP-2.3.6-15-2015-00001). This publication has also received funding from PANOSC, the European Union's Horizon 2020 research and innovation programme under grant agreement no 823852. M.U.K. and G.N. also acknowledge Project no. 2019-2.1.13-TÉT-IN-2020-00059 which has been implemented with the support provided from the National Research, Development and Innovation Fund of Hungary, financed under the 2019-2.1.13-TÉT-IN funding scheme. O.A.R. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 899987. R.D.C. and L.M.C. acknowledge the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 956923

The role of the surface acidic/basic centers and redox sites on TiO2 in the photocatalytic CO2 reduction

The development of sustainable processes for CO reduction to fuels and chemicals is one of the most important challenges to provide clean energy solutions. The use of sunlight as renewable energy source is an interesting alternative to power the electron transfer required for artificial photosynthesis. Even if redox sites are mainly responsible for this process, other reactive acidic/basic centers also contribute to the overall reaction pathway. However, a full understanding of the CO photoreduction mechanism is still a scientific challenge. In fact, the lack of agreement on standardized comparison criteria leads to a wide distribution of reported productions, even using the same catalyst, which hinders a reliable interpretation. An additional difficulty is ascertaining the origin of carbon-containing products and effect of surface carbon residues, as well as the reaction intermediates and products under real dynamic conditions. To determine the elusive reaction mechanism, we report an interconnected strategy combining in-situ spectroscopies, theoretical studies and catalytic experiments. These studies show that CO photoreduction productions are influenced by the presence of carbon deposits (i.e. organic molecules, carbonates and bicarbonates) over the TiO surface. Most importantly, the acid/base character of the surface and the reaction medium play a key role in the selectivity and deactivation pathways. This TiO deactivation is mainly initiated by the formation of carbonates and peroxo- species, while activity can be partially recovered by a mild acid washing treatment. We anticipate that these findings and methodology enlighten the main shadows still covering the CO reduction mechanism, and, most importantly, provide essential clues for the design of emergent materials and reactions for photo(electro)catalytic energy conversion

Photoinduced Charge Transfer and Trapping on Single Gold Metal Nanoparticles on TiO2

We present a study of the effect of gold nanoparticles (Au NPs) on TiO2 on charge generation and trapping during illumination with photons of energy larger than the substrate band gap. We used a novel characterization technique, photoassisted Kelvin probe force microscopy, to study the process at the single Au NP level. We found that the photoinduced electron transfer from TiO2 to the Au NP increases logarithmically with light intensity due to the combined contribution of electron-hole pair generation in the space charge region in the TiO2-air interface and in the metal-semiconductor junction. Our measurements on single particles provide direct evidence for electron trapping that hinders electron-hole recombination, a key factor in the enhancement of photo(electro)catalytic activity.This work was supported by the Office of Basic Energy Sciences (BES) of the U.S. Department of Energy (DOE) under contract DE-AC02-05CH11231 through the Structure and Dynamics of Materials Interfaces Program (FWP KC31SM) and the Molecular Foundry. M.L. acknowledges funds from Comunidad de Madrid (P2018/EMT-4308), a Fulbright grant PRX16/00564, and the MCIU-AEI-FEDERUE (RTI2018-096937-B-C22 and MAT2014-59772-C2-1-P). J.C. acknowledges financial support from Ministerio de Ciencia e Innovación (MICINN) and the European Union through the project PID2019-104272RB-C52. Also, Y.H. acknowledges financial support from MCIU through MAT2014-59772-C2-2- P and L.M. from EC through ERC-2013-SYG-610256. V.A.P.O. and M.B. acknowledge the financial support from EC through ERC CoG HyMAP 648319, MINECO PID2019- 106315RB-I00 and ENE2017-89170-R, ″Comunidad de Madrid″ and European Structural Funds (FotoArt-CM project S2018/NMT-4367) and Fundación Ramón Areces (Art-Leaf project). M.B. also thanks the Juan de la Cierva Incorporación contract (IJC2019042430-I). X.Z. was supported by the NSF-BSF 359 grant number 1906014. The authors thank Prof. Eran Edri, María Ujué González Sagardoy, and Judit Meseguer- Oliver for fruitful discussions and Asylum customer support for help with modifications of the AFM