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

Photo-Induced Self-Cleaning and Wettability in TiO2 Nanocolumn Arrays Obtained by Glancing-Angle Deposition with Sputtering

In this work, the preparation of regular nanosized columnar structures of titanium dioxide by means of glancing angle deposition with magnetron sputtering (MS-GLAD) followed by thermal annealing is reported. MS-GLAD gives rise to metallic titanium columnar structures with regular width and length that after thermal treatment are fully oxidized to form TiO2 nanocolumns that maintain the morphological features of the original metallic ones. Further functionalization with gold by means of multiple ion cluster source results in well-dispersed Au nanoparticles across the nanocolumns’ surface with a narrow size distribution centered at ca. 8.5 nm. The obtained nanostructures show photocatalytic self-cleaning activity as shown by the elimination of an organic layer deposited on their surface and the detection of hydroxyl radicals. Photoelectrochemical measurements show a better charge separation at the Au/TiO2 interface. In addition, wettability studies show that the degree of hydrophobicity of the surface is increased by the presence of nanocolumns, both in the dark and under UV illumination. This behavior is not modified by the presence of Au nanoparticles on the surface. The obtained results open up interesting implications in the tunability of the properties of nanostructured thin films for this kind of photo-activated application.Financial support from the Spanish Ministry of Science, Innovation, and Universities (MICINN) through the projects SOLPAC (ENE2017-89170-R, MCIU/AEI/FEDER, EU), MAT2014-59772-C2-1-P, and MAT2014-59772-C2-2-P is gratefully acknowledged. The authors also acknowledge the service from the MiNa Laboratory at IMN funded by Comunidad de Madrid (S2018/NMT-4291 TEC2SPACE), MICINN (CSIC13-4E-1794), and the EU (FEDER, FSE). Also, this work has been funded by the regional government of Madrid and European Structural Funds through their financial support to FotoArt-CM program (S2018/NMT-4367), and from Fundación Ramon Areces though the ArtLeaf project. M.B. thanks MICINN for a Juan de la Cierva Incorporación (IJC2019-042430-I) grant.Peer reviewe

Improved Methane Production by Photocatalytic CO2 Conversion over Ag/In2O3/TiO2 Heterojunctions

In this work, the role of In2O3 in a heterojunction with TiO2 is studied as a way of increasing the photocatalytic activity for gas-phase CO2 reduction using water as the electron donor and UV irradiation. Depending on the nature of the employed In2O3, different behaviors appear. Thus, with the high crystallite sizes of commercial In2O3, the activity is improved with respect to TiO2, with modest improvements in the selectivity to methane. On the other hand, when In2O3 obtained in the laboratory, with low crystallite size, is employed, there is a further change in selectivity toward CH4, even if the total conversion is lower than that obtained with TiO2. The selectivity improvement in the heterojunctions is attributed to an enhancement in the charge transfer and separation with the presence of In2O3, more pronounced when smaller particles are used as in the case of laboratory-made In2O3, as confirmed by time-resolved fluorescence measurements. Ternary systems formed by these heterojunctions with silver nanoparticles reflect a drastic change in selectivity toward methane, confirming the role of silver as an electron collector that favors the charge transfer to the reaction medium

Improved Methane Production by Photocatalytic CO2 Conversion over Ag/In2O3/TiO2 Heterojunctions

In this work, the role of In2O3 in a heterojunction with TiO2 is studied as a way of increasing the photocatalytic activity for gas-phase CO2 reduction using water as the electron donor and UV irradiation. Depending on the nature of the employed In2O3, different behaviors appear. Thus, with the high crystallite sizes of commercial In2O3, the activity is improved with respect to TiO2, with modest improvements in the selectivity to methane. On the other hand, when In2O3 obtained in the laboratory, with low crystallite size, is employed, there is a further change in selectivity toward CH4, even if the total conversion is lower than that obtained with TiO2. The selectivity improvement in the heterojunctions is attributed to an enhancement in the charge transfer and separation with the presence of In2O3, more pronounced when smaller particles are used as in the case of laboratory-made In2O3, as confirmed by time-resolved fluorescence measurements. Ternary systems formed by these heterojunctions with silver nanoparticles reflect a drastic change in selectivity toward methane, confirming the role of silver as an electron collector that favors the charge transfer to the reaction medium.This research was funded by the European Union’s Horizon 2020 research and innovation program under the European Research Council (ERC) through the HyMAP project, grant agreement No. 648319. Additional funding by the Spanish MCIN/AEI/10.13039/501100011033/FEDER through the Nympha Project (PID2019-106315RB-I00), the regional government of “Comunidad de Madrid” and the European Structural Funds through FotoArt-CM program (S2018/NMT-4367), and Fundación Ramón Areces through the ArtLeaf project is gratefully acknowledged.Peer reviewe

Influence of nickel loading on the hydroisomerization of n-dodecane with nickel-tungsten oxide-alumina supported catalysts

[EN] Heterogeneous catalysts based on alumina-supported tungsten oxides (15 wt% W) with different loadings of nickel (0.5, 1, 2, 4, 6, 8 and 10 wt% Ni) were selected to study the influence of Ni loading on the hydroisomerization of n-dodecane. The catalysts were prepared by applying the wetness impregnation method on the supports to introduce W and Ni. The characterization techniques applied for determining physicochemical properties of the catalysts were N adsorption-desorption at 77 K (textural properties), X-ray diffraction (structure and crystalline phases), H-TPR (redox properties), FTIR, NH-TPD (acid sites analyses) and XPS (chemical surface analysis). The catalytic properties of such catalysts were found to be crucial in the n-dodecane conversion. The NH-TPD profiles indicate that the medium acid sites are the main sites responsible for the reaction performance. The formation of bulky crystal structures of nickel species in the high nickel loading catalyst (10 wt% Ni) was confirmed by XRD and XPS results, resulting in the largest cracking activity. The conversion of n-dodecane and selectivity to i-C+branched C tend to increase with Ni loading until the catalyst contains 6 wt% Ni (28% n-C conversion and 94% of branched C selectivity). The lower selectivity at high nickel loading is due to metal-based cracking reaction. An optimum balance between metal and acid centers is needed to achieve a compromise between conversion and selectivity, avoiding or minimizing cracking reactions.The support of MICIN/AEI (Spain) through project ENE2016-74889-C4-3-R is acknowledged. DGP acknowledges MICIN/AEI for her contract (BES-2017-079679) (Spain). This research is part of the CSIC program for the Spanish Recovery, Transformation, and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by Regulation (EU) 2020/2094 (TRE2021-03-012). We acknowledge the support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI

Nickel ferrite supported on calcium-stabilized zirconia for solar hydrogen production by two-step thermochemical water splitting

[EN] Solar hydrogen production by thermochemical cycles from water using oxide redox pairs represents a promising technology at medium-long term to storage intermittent solar irradiation. Redox oxides based on nickel ferrite supported on modified zirconia have been prepared and studied for renewable H production from water by two-step thermochemical cycles. We report herewith a new material based on nickel ferrite supported on Ca-doped zirconia substrate that presents high efficiency for hydrogen production, which is ascribed to a high active phase dispersion promoted by the inclusion of part of Fe cations in the support lattice and by the formation of calcium zirconate, acting as a physical barrier that hinders sintering.The present work was performed within the research programs supported by the Secretaría de Estado de Investigación, Desarrollo e Innovación and CAM (Spain) under projects CTQ2013-48669-P and S2013/MAE-2882, respectively. María Retuerto acknowledges the Juan de la Cierva program of the Spanish Ministry of Economy and Competitiveness for a grant (FPDI-2013-17582)

Biodiversity and distributional patterns in deep-sea vulnerable marine ecosystems and associated fish communities of the Seco de los Olivos Bank

Seamounts are ubiquitous in the global ocean enhancing locally the biological biodiversity in the deep-sea and hosting multiple Vulnerable Marine Ecosystems (VMEs), like cold-water corals (CWCs) and sponge grounds. In the western Mediterranean basin (Alborán Sea), the Seco de Los Olivos Seamount (Chella Bank) is considered a hotspot of deep-sea biodiversity, being part of the protected marine network in Europe (Natura 2000). While previous studies provided extensive qualitative information about what species and habitats occur in the seamount, there is still a challenge to have accurate quantitative information to better understand the spatial distribution patterns of the megabenthic species and its functional role. Using underwater video transects recorded by a Remotely Operated Vehicle (ROV) during the MEDWAVES survey (EU project ATLAS H2020), we quantitatively characterize the megabenthic communities in the Seco de Los Olivos Seamount. A total of 62 operational taxonomic units (OTUs) grouped into five major assemblages were identified, which were mainly associated with the substrate type. The main groups typifying these assemblages included massive sponges, black corals, sea pens, and ray-finned fishes. In addition, taxonomic diversity was evaluated and the patterns observed were also associated with the substrate type and to the flank orientation in a lesser extent. Besides our ecological findings, we found a variety of human impact evidence (e.g., pieces of fishing lines, glass bottles, and trawl marks). Nonetheless, there is a management plan in progress aiming to regulate different human activities (e.g., fisheries) carried out in this area declared a site of ecological importance. Abundances and densities obtained through this study for the most representatives OTUs represent an important contribution to improve our understanding of the biodiversity, ecological associations and potential threats of deep-sea communities and thus, will help to enhance the spatial management and conservation plans for these vulnerable marine ecosystems

Direct Evidence of Photoinduced Electron Diffusion and Trapping in Single Metal Nanoparticles on TiO2 by High Resolution Surface Photovoltage Imaging

Trabajo presentado en la Global Conference on Nanotechnology - NanoSeries, celebrada de forma virtual, del 21 al 24 de junio de 2022We studied the effect of gold nanoparticles (Au NPs) deposited 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 (PA-KPFM), 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 performance [1]

Description and quantification of the deep-sea megabenthic communities in Ormonde seamount (Gorringe Bank, NE Atlantic) using video analysis

The Gorringe Bank is a seamounts complex located at the eastern tip of the Azores-Gibraltar plate boundary separating Eurasia and Africa. Despite its early discovery at the end of the XIX century, the knowledge of the benthic communities and other associated fauna is still very scarce in this region, particularly in the deep areas below 200 m. The Ormonde seamount is one of the two main summits of Gorringe Bank, which rises from 5000 m to 33 m depth. In this study, we characterised the deep-sea communities in the Ormonde seamount from 900 to 2000 m depth by analysing underwater videos recorded by remotely operated vehicle (ROV) during the MEDWAVES survey (EU H2020 project ATLAS). We provided for the first time, detailed and quantitative information on species composition, densities, spatial distribution and state of conservation compared to other well studied seamounts. The surveyed areas were strongly dominated by sponges, but the presence of patches of cold-water corals were also notable and some other vulnerable marine ecosystems (VMEs) were identified. Spatial patterns in the deep-sea communities were also analysed as a result of the different water masses that bathed the area, exhibiting higher biodiversity in the presence of the Mediterranean Outflow Water (MOW). Studies of deep-sea ecosystems not only improve our knowledge of the deep ocean but also are essential defining an appropriate management plan and conservation measures