CO_(2) conversion to solar fuels and chemicals:Opening the new paths

This future article discusses the new prospects and directions of CO_(2)conversion via the photo-electrocatalytic(PEC)route.The second(2nd)generation solar fuels and chemicals(SFs)are generated directly in PEC systems via electrons/protons reactions without forming molecular H_(2)as an intermediate,overcoming the thermodynamics limitations and practical issues encountered for electro-fuels produced by multistep thermocatalytic processes(i.e.CO_(2)conversion with H_(2)coming from water electrolysis).A distributed and decentralized production of SFs requires very compact,highly integrated,and intensified technologies.Among the existing reactors of advanced design(based on artificial leaves or photosynthesis),the integrated photovoltaic plus electrocatalytic(PV-EC)device is the only system(demonstrated at large scale)to produce SFs with high solar-to-fuel(STF)efficiency.However,while the literature indicates STF efficiency as the main(and only)measure of process performance,we remark here the need to refer to productivity(in terms of current density)and make tests with reliable flow PEC systems(with electrodes of at least 5–10 cm^(2))to accelerate the scaling-up process.Using approaches that minimize downstream separation costs is also mandatory.Many limitations exist in PEC systems,but most can be overcome by proper electrode and cell engineering,thus going beyond the properties of the electrocatalysts.As examples of current developments,we present the progress of(i)artificial leaf/tree devices for green H_(2)distributed production and(ii)a PEC device producing the same chemicals at both cathode and anode parts without downstream operations for green solvent distributed production.Based on these developments,future directions,such as producing fertilizers and food components from the air,are outlined.The aim is to provide new ideas and research directions from a personal perspective.

碳基催化剂:为开发下一代纳米工程催化材料开辟新方法(英文)

This essay analyses some of the recent development in nanocarbons (carbon materials having a defined and controlled nano-scale dimension and functional properties which strongly depend on their nano-scale features and architecture), with reference to their use as advanced catalytic materials. It is remarked how their features open new possibilities for catalysis and that they represent a new class of catalytic materials. Although carbon is used from long time in catalysis as support and electrocatalytic applications, nanocarbons offer unconventional ways for their utilization and to address some of the new challenges deriving from moving to a more sustainable future. This essay comments how nanocarbons are a key element to develop next-generation catalytic materials, but remarking that this goal requires overcoming some of the actual limits in current research. Some aspects are discussed to give a glimpse on new directions and needs for R&D to progress in this direction.

Enhanced performance in the direct electrocatalytic synthesis of ammonia from N2 and H2O by an in-situ electrochemical activation of CNT-supported iron oxide nanoparticles

The direct electrocatalytic synthesis of ammonia from N2 and H2O by using renewable energy sources and ambient pressure/temperature operations is a breakthrough technology,which can reduce by over 90%the greenhouse gas emissions of this chemical and energy storage process.We report here an in-situ electrochemical activation method to prepare Fe2O3-CNT(iron oxide on carbon nanotubes)electrocatalysts for the direct ammonia synthesis from N2 and H2O.The in-situ electrochemical activation leads to a large increase of the ammonia formation rate and Faradaic efficiency which reach the surprising high values of 41.6μg mgcat^−1 h^−1 and 17%,respectively,for an in-situ activation of 3 h,among the highest values reported so far for non-precious metal catalysts that use a continuous-flow polymer-electrolytemembrane cell and gas-phase operations for the ammonia synthesis hemicell.The electrocatalyst was stable at least 12 h at the working conditions.Tests by switching N2 to Ar evidence that ammonia was formed from the gas-phase nitrogen.The analysis of the changes of reactivity and of the electrocatalyst characteristics as a function of the time of activation indicates a linear relationship between the ammonia formation rate and a specific XPS(X-ray-photoelectron spectroscopy)oxygen signal related to O2−in iron-oxide species.This results together with characterization data by TEM and XRD suggest that the iron species active in the direct and selective synthesis of ammonia is a maghemite-type iron oxide,and this transformation from the initial hematite is responsible for the in-situ enhancement of 3-4 times of the TOF(turnover frequency)and NH3 Faradaic efficiency.This transformation is likely related to the stabilization of the maghemite species at CNT defect sites,although for longer times of preactivation a sintering occurs with a loss of performances.

Etherification of HMF to biodiesel additives: The role of NH4+ confinement in Beta zeolites

The role of NH4^+ ion confinement in the catalytic etherification of HMF(5-hydroxymethylfurfural) with ethanol to biodiesel additives was evidenced by studying the catalytic behavior of NH4^+-Beta zeolites with SiO2/Al2O3 ratios of 25 and 75.In order to affect the strength and distribution of the acidic sites, as well as the mobility of NH4^+ ions in the zeolites cages, a secondary level of porosity was introduced in the NH4^+-Beta, presenting a different stability versus alkaline treatment, by using a thermal or an ultrasound assisted method.By analyzing the catalytic behavior in these two series of samples with respect to the changes in porosity by nonlocal density functional theory, structure by XRD, amount of acid sites by FT-IR and mobility of NH4^+ cations by measurements of reversible NH4^+ exchange capacity, was evidenced a decrease in catalytic performances both in terms of rate of HMF depletion and productivity to the main products, when confinement of the ammonium ions is lost due to the introduction of mesoporosity.The high capability of ammonium ions release, associated to the mono-dentate configuration,and the minor confinement effect inside the zeolite pore system, due to the more opened pores structure of mesoporous zeolites, hinders both the direct etherification of HMF to EMF [5-(ethoxymethyl)furan-2-carbaldehyde] and the parallel reaction pathway via acetalization, favoring the rapid desorption of the HMFDEA [5-(hydroxymethyl)furfural diethyl acetal] product out of the crystal and the consequent inhibition of the consecutive reactions to EMFDEA [5-(ethoxymethyl)furfural diethyl acetal] and EMF.

Transforming catalysis to produce e‐fuels:Prospects and gaps

After short introducing the crucial role of e‐fuels to meet net‐zero emissions targets,this perspective paper discusses the differences between reactive catalysis(electro‐,photo‐and plasma‐catalysis,with focus on the first for conciseness)and thermal catalysis used at most.The main point is to evidence that to progress in producing e‐fuels,the gap is not in terms of scaling‐up and pilot testing,but rather in the fundamental needs to turn the current approach and methodologies to develop reactive catalysis,including from a mechanistic perspective,to go beyond the current methods largely derived from thermal catalysis.Developing thus new fundamental bases to understand reactive catalysis is the challenge to accelerate the progress in this area to enable the potential role towards a sustainable net‐zero emissions future.Some novel aspects are highlighted,but the general aim is rather to stimulate discussion in rethinking catalysis from an alternative perspective.

Semiconductor,molecular and hybrid systems for photoelectrochemical solar fuel production

The paper shortly reviews the basic direct approaches applied in searching for viable solutions to solar fuel production. These are generally distinguished in molecular and semiconductor(non-molecular)systems, however, hybrid strategies, proposed recently, have also been included. The most promising efforts are considered, highlighting key aspects and emerging critical issues. Special attention is paid to aspects such as electrode architecture, device design, and main differences in the scientific vision and challenges to directly produce solar fuels. This overview could be useful to orientate the readers in the wide panorama of research activities concerning water splitting, natural and artificial photosynthesis, and solar fuel production through the identification of common aspects, specialties and potentialities of the many initiatives and approaches that are developing worldwide in this field with the final aim to meet world energy demand.

Role of size and pretreatment of Pd particles on their behaviour in the direct synthesis of H_2O_2

Two families of catalysts, based on Pd nanoparticles supported on ceramic asymmetric tubular alumina membranes, are studies in the direct synthesis of H2O2. They are prepared by depositing Pd in two ways：（i） reduction with N2H4 in an ultrasonic bath and（ii） by impregnation-deposition. The first preparation leads to larger particles, with average size of around 11 nm, while the second preparation leads to smaller particles, with average size around 4 nm. The catalytic membranes were tested as prepared, after thermal treatment in air and after further pre-reduction with H2 in mild（100 ℃） conditions. Samples were characterized by TEM, CO-chemisorption monitored by DRIFTS method and TPR, while catalytic tests have been performed in a semi-batch recirculation membrane reactor. Experimental catalytic results were analysed using two kinetics models to derive the reaction constants for the parallel and consecutive reactions of the kinetic network. Smaller particles of Pd show lower selectivity due to the higher rate of parallel combustion, even if the better dispersion of Pd and thus higher metal surface area in the sample lead to a productivity in H2O2 similar or even higher than the sample with the larger Pd particles. Independently on the presence of smaller or larger Pd nanoparticles, an oxidation treatment leads to a significant enhancement in the productivity, although the catalyst progressively reduces during the catalytic process. The inhibition of the parallel combustion reaction（to water） induced from the calcination treatment remains after the in-situ reduction of the oxidized Pd species formed during the pre-treatment.This is likely due to the elimination of defect sites which dissociatively activate oxygen, and tentatively attributed to Pd sites able to give three- and four-fold coordination of CO.

The energy-chemistry nexus: A vision of the future from sustainability perspective

The changing energy-chemistry nexus is discussed in this perspective paper about the future of sustainable energy and chemical production to identify the priorities and open issues on which focus research and development. Topics discussed regard （i） the new sustainable energy scenario, （ii） the role of energy storage （from smart grids to chemical storage of energy）, （iii） the outlooks and role of solar （bio）refineries and solar fuels, （iv） how to integrate hio- and solar-refineries to move to new economy, （v） the role of methanol at the crossover of new energy-chemistry nexus, （vi） the role of chemistry in this new scenario, （vii） the role of nanomaterials for a sustainable energy, （viii） the use of nanocarbons to design advanced energy conversion and storage devices, and （ix） possibilities and routes to exploit solar energy and methane （shale gas）. The contribution provides a glimpse of the emerging directions and routes with some elements about their possible role in the future scenario, but does not orovide a detailed analysis of the state of the art in these directions

Photoactive materials based on semiconducting nanocarbons——A challenge opening new possibilities for photocatalysis

This perspective paper introduces the concept that nanocarbons and related materials such as carbon dots are an interesting intrinsic photocatalytic semiconducting material, and not only a modifier of the existing (semiconducting) materials to prepare hybrid materials. The semiconducting properties of the nanocarbons, and the possibility to have the band gap within the visible-light region through defect band engineering, introduction of light heteroatoms and control/manipulation of the curvature or surface functionalization are discussed. These materials are conceptually different from the 'classical' semiconducting photocatalysts, because semiconductor domains with tuneable characteristics are embedded in a conductive carbon matrix, with the presence of various functional groups (as C=0 groups) enhancing charge separation by trapping electrons. These nanocarbons open a range of new possibilities for photocatalysis both for energetic and environmental applications. The use of nanocarbons as quantum dots and photo luminescent materials was also analysed. (C) 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Controlled location of S vacancies in MoS_(2) nanosheets for high-performance hydrogenation of CO_(2) to methanol

Introduction In a joint contribution among many Chinese research Institutes,a recent paper published on Nature Catalysis reported the use of sulfur vacancy-rich MoS_(2)as a novel catalyst for the hydrogenation of CO_(2)to methanol[1].A commentary on this paper was also published on the same journal[2]reporting as a summary:"Highly active,selective and stable catalysts for the hydrogenation ofC0_(2)to methanol are immensely sought after.Now,using a broad range of spectroscopic methods,in-plane double sulfur vacancies of MoS_(2)sheets have been suggested to catalyse this reaction using an unusual mechanism".

Analysis of the factors controlling performances of Au-modified TiO2 nanotube array based photoanode in photo-electrocatalytic(PECa)cells

The efficiency of photo-electrocatalytic（PECa） devices for the production of solar fuels depends on several limiting factors such as light harvesting, charge recombination and mass transport diffusion. We analyse here how they influence the performances in PECa cells having a photo-anode based on Au-modified TiOnanotube（TNT） arrays, with the aim of developing design criteria to optimize the photo-anode and the PECa cell configuration for water photo-electrolysis（splitting） and ethanol photo-reforming processes.The TNT samples were prepared by controlled anodic oxidation of Ti foils and then decorated with gold nanoparticles using different techniques to enhance the visible light response through heterojunction and plasmonic effects. The activity tests were made in a gas-phase reactor, as well as in a PECa cell without applied bias. Results were analysed in terms of photo-generated current, Hproduction rate and photoconversion efficiency. Particularly, a solar-to-hydrogen efficiency of 0.83% and a Faradaic efficiency of 91%were obtained without adding sacrificial reagents.

Electrocatalytic production of glycolic acid via oxalic acid reduction on titania debris supported on a TiO_(2)nanotube array

Electrodes prepared by anodic oxidation of Ti foils are robust and not toxic materials for the electrocatalytic reduction of oxalic acid to glycolic acid, allowing the development of a renewable energy-driven process for producing an alcoholic compound from an organic acid at low potential and room temperature. Coupled with the electrochemical synthesis of the oxalic acid from CO_(2),this process represents a new green and low-carbon path to produce added value chemicals from CO_(2). Various electrodes prepared by anodic oxidation of Ti foils were investigated. They were characterized by the presence of a TiO_(2) nanotube array together with the presence of small patches, debris, or TiO_(2) nanoparticles. The concentration of oxygen vacancies, the amount of Ti^(3+) measured by X-ray photoelectron spectroscopy(XPS) and the intensity of the anodic peak measured by cyclic voltammetry, were positively correlated with the achieved oxalic acid conversion and glycolic acid yield. The analysis of the results indicates the presence of small amorphous TiO_(2) nanoparticles(or surface patches or debris) interacting with TiO_(2) nanotubes, the sites responsible for the conversion of oxalic acid and glycolic acid yield. By varying this structural characteristic of the electrodes, it is possible to tune the glycolic acid to glyoxylic acid relative ratio. A best cumulative Faradaic efficiency(FE) of about 84% with FE to glycolic acid around 60% and oxalic conversion about 30% was observed.

Smart catalytic materials for energy transition

Energy transition,and the related chemistry transition due to their strong nexus,is creating a major worldwide change in the current production system,driven initially by social and environmental pressures(cleaner production,reduced greenhouse gas emissions),but today instead is pushed by economic(renewable energy sources are becoming progressively the more economic energy form)and geopolitical(energy security)motivations.Oil and natural gases are the building blocks of the current refinery and(petro)chemistry,but going beyond fossil fuels is the challenge associated with this transition.This has also major implications on the technologies and processes actually in use,further pushed from another emerging direction associated with the progressive change from centralized to delocalized productions,for a better link with the territory and the local resources.The combined effect of these two emerging directions determines a radical change in the energy and chemical production systems,with major technological implications.Current process technologies in the area of chemical and fuel production cannot just be adapted,they need to be fully redesigned(also in terms of concepts,materials,engineering)to address the new challenges of using renewable energy sources in delocalized productions(small‐scale production at the regional level using local resources and in strong symbiosis to other local productions).