The photoreduction of CO2 is a growingly interesting research topic due to the intriguing possibility of producing solar fuels from a concerning pollutant. TiO2 photocatalysts were the first materials used for this application, but since then, various strategies have been developed to optimise the catalytic performance and operating conditions to obtain competitive yield. This review presents the findings of the last decade of research on different semiconductors, TiO2 and g-C3N4 and their composites. The main features of the reaction and its key issues are first overviewed, focusing on the effect of different reaction conditions on the performance and recalling the mechanism of the reaction. The strategies developed to overcome the challenges of this demanding reaction are described in the following paragraphs, including the use of dopants or co-catalysts, heterojunctions between different semiconductors and the use of electron transfer mediators. Finally, some unifying concepts are summarised, suggesting the calculation of the stored energy amount and the relative efficiency to allow a safer comparison between literature data collected under widely variable conditions and leading to different products.

The photoreduction of carbon dioxide (CO2) into solar fuels and value-added chemicals is a promising strategy to address climate change and the energy crisis. However, the efficiency of heterogeneous semiconductor-based photocatalytic systems is often hindered by the rapid recombination of charge carriers (electrons and holes). This paper examines the crucial role of the sulfite ion (SO32-) as a hole scavenger during aqueous-phase photoreduction of CO2, achieving the unprecedented productivity of 58 to 67 mol/h kg(cat). A new hybrid homogeneous/heterogeneous process has been developed, activated by UVA and UVC irradiation, never reported previously. Low-energy radiation is sufficient to induce the conventional photogeneration of charge carriers responsible for the well-known heterogeneous photocatalytic pathway of CO2 photoconversion to HCOOH. UVC radiation is able to activate a homogeneous direct photolysis of sodium sulfite, with the generation of solvated electrons as powerful reducing species. The synergistic action of both mechanisms demonstrated the possibility to achieve, for the first time, critically relevant amounts of HCOOH produced in a semipilot-scale reactor (ca. 1.5 L size). The effect of high-pressure operation seems also to be a determinant from a kinetic point of view to increase the concentration of available reacting species.

The continuous-flow technologies in organic synthesis for the production of active pharmaceutical ingredients (APIs) are nowadays more and more applied. In-silico process design is a powerful tool able to support organic synthesis in the field of scale-up and process development. Process design feasibility and reliability depend on the availability of a well-defined chemical reaction kinetic scheme, information which is usually derived from experimental datasets collected on purpose. The latter approach is time-consuming and demanding in terms of resources. Different possibilities are here proposed to valorize widely available experimental data from explorative works with different approaches, depending on the nature, richness, and structure of the datasets. The kinetic parameters (i.e., reaction order, kinetic constant, and activation energy) of some interesting organic reactions have been approximately estimated by applying different computational methodologies, thanks to built-in experimental databases. The numerical algebra approach dealing with linear and non-linear regression analysis for the kinetic parameters has been initially considered and related to the database information for oseltamivir synthesis. The Bayesian statistic was applied to the ibuprofen case through the application of the Markov Chain Monte Carlo (MCMC) method for reaction order estimation. At last, a Machine Learning (ML) approach has been applied to the Rolipram and Pregabalin case study. The in-house developed T-ReX experimental kinetic constant database was exploited, with application of the k-Nearest neighbor algorithm for classification and regular expression pattern recognition. Advantages and limitations of the three approaches are discussed.

Exfoliation of graphitic carbon nitride g-C3N4 by means of ultrasound (US) treatment at varying input power is investigated. Exfoliation of g-C3N4 displays a strong dependence of US input power, with a slightly enhanced bandgap (2.8 eV), but most of all increased lifetime of photogenerated electrons, as observed through diffuse reflectance spectroscopy (DRS) and spectrofluorimetry data. Among all applied power (varied between 30 and 120 W), 120 W sufficiently exfoliated and tuned physicochemical properties of g-C3N4. Compared to bulk as prepared sample, exfoliated g-C3N4 exhibited improvement in photo-induced charge carrier transfer and separation, resulting in higher photocatalytic efficiencies. FE-SEM and TEM images of both bulk and exfoliated g-C3N4 show the effect of the exfoliation power on the nanosheet, pseudo-lamellar structure. A change in US input power correlates well with amonotonous variation of the bandgap, of the charges lifetime of the materials and, most importantly, of the catalytic performance determined through an innovative high-pressure reactor in solid-liquid-gas phase. The catalytic results demonstrate this material as an efficient photocatalyst to obtain high yield of formic acid with net productivities ranging from similar to 5100 to similar to 8200 mmol/kg(cat) h at 80 degrees C in water, which is among the highest reported in the literature.

Several Cu and Ni-based catalysts were synthetized over Ce-based supports, either pure or mixed with different amounts of alumina (1:2 and 1:3 mol/mol). Different metal loadings (10–40 wt%) and preparation methods (wet impregnation, co-precipitation, and flame-spray pyrolysis—FSP) were compared for the oxidative steam reforming of methanol. Characterization of the catalysts has been performed, e.g., through XRD, BET, XPS, TPR, SEM, and EDX analyses. All the catalysts have been tested in a bench-scale continuous setup. The hydrogen yield and methanol conversion obtained have been correlated with the operating conditions, metal content, crystallinity of the catalyst particles, total surface area, and with the interaction of the metal with the support. A Cu loading of 20% wt/wt was optimal, while the presence of alumina was not beneficial, decreasing catalyst activity at low temperatures compared with catalysts supported on pure CeO2. Ni-based catalysts were a possible alternative, but the activity towards the methanation reaction at relatively high temperatures decreased inevitably the hydrogen yield. Durability and deactivation tests showed that the best-performing catalyst, 20% wt. Cu/CeO2 prepared through coprecipitation was stable for a long period of time. Full methanol conversion was achieved at 280 °C, and the highest yield of H2 was ca. 80% at 340 °C, higher than the literature data.

Diclofenac sodium salt was photodegraded by means of advanced oxidation processes (AOPs), such as Fenton, photo-Fenton and heterogeneous photocatalysis. For the latter different photocatalysts were compared, namely commercial titania P25 and titania metallized with gold (0.1 % Au/P25), silver (1 % Ag/P25) and palladium (0.1 % Pd/P25). Homogeneous treatments demonstrated effective in the degradation of the selected pollutant (>80 % conversion @2 h) when the irradiation occurred within the solution. Also, photo-Fenton process assisted by visible light rather than UV was effective but slower and characterized by a toxicity of the residual solution due to unreacted H2O2. The photocatalyzed treatment performed at its best when P25 was used (70 % conversion @2 h), while modified photocatalysts reached the same conversion when H2O2 was added to the solution. Overall, in vitro toxicity tests using Daphnia Magna unveiled that the wastewater treated via M/TiO2 treatment and photo-Fenton under UV in combination with H2O2 showed an acute toxicity comparable with the control group (almost 100 % viability @48 h). Conversely, the other processes failed to degrade completely either the pollutant or the hydrogen peroxide, leading to the mortality of 30-80 % of the individuals. An important outcome of the work is the direct comparison of different treatments to optimise the outcome, i. e. rapidity of degradation and non toxicity of the treated solution for living bodies.

Solar-boosted photo-technology stands out as a powerful strategy for photosynthesis and photocatalytic processes due to its minimal energy requirements, cost-effectiveness and operation under milder, environmentally friendly conditions compared to conventional thermocatalytic options. The design and development of photocatalysts have received a great deal of attention, whereas photoreactor development must be studied deeper to enable the design of efficient devices for practical exploitation. Furthermore, scale-up issues are important for this application, since light distribution through the photoreactor is a concurrent factor. This review represents a comprehensive study on the development of photoreactors to be used mainly for the photoreduction of CO2 to fuels, but with concepts easily transferable to other photosynthetic applications such as ammonia synthesis and water splitting, or wastewater treatment, photovoltaics combined to photoreactors, etc. The primary categories of photoreactors are thoroughly examined. It is also explained which parameters influence the design of a photoreactor and next-generation high-pressure photoreactors are also discussed. Last but not least, current technologies for solar concentrators are recalled, considering their possible integration within the photoreactor. While many reviews deal with photocatalytic materials, in the authors’ view, photoreactors with significant scale and their merged devices with solar concentrators are still unexploited solutions. These are the key to boost the efficiency of these processes towards commercial viability; thus, the aim of this review is to summarise the main findings on solar photoreactors for the photoreduction of CO2 and for related applications.

With the aim to establish a pioneering colony on Mars, the need for refueling the vector to go back to Earth needs to be fulfilled with locally available resources. Based on the advantages of methane as fuel for aerospace applications, the feasibility of a Sabatier reactor providing this fuel on Mars by exploiting the CO2 naturally present in its atmosphere has been evaluated. Alternative options have been considered to provide H2 by using the proven water reserves on the Red planet, through either electrolysis or an electro-thermochemical cycle. Different powering units based on available nuclear reactors have been compared. After preliminary sizing of the plant based on a revised kinetic model for a Ni/Alumina catalyst, different cases have been considered for heating, heat removal, and powering the system. They were characterized by higher energy integration and thus efficiency, or lower efficiency but more compact design and thus lower mass. The overall feasibility has been considered not from a strictly economic point of view, but assessing a reliable overall mass of the equipment to be delivered through the current Starship loading capacity.

Renewable electricity, production/storage and distribution of green hydrogen and carbon dioxide emission reduction are just three of the practices needed for a net zero-emission world. The increasing amount of renewable energy produced requires the development of versatile technologies capable to store the excess renewable electricity produced. Hydrogen production through electrolysis, despite not being still economically viable, can be considered a mature technology. Different strategies to convert H2 into more volumetric dense fuel are under development through Power-to-Chemicals (PtC) process. Among these, CO2 methanation offers the advantage of a wide infrastructure available for the distribution and use of methane as chemical and fuel for both heat and power generation. The latter approach is also called Power-to-Gas (PtG) process. These technologies in the last year have been the focus of research, both public and private. In particular, direct methanation of biogas obtained from anaerobic digestors represents a challenge and an interesting opportunity due to the possibility to avoid the CO2 separation step at the moment needed for biogas purification to biomethane. Since CO2 separation is an additional cost, this route can be the key for economic sustainability of the process. In this review we focus on the main aspects involved in the design of a methanation plant, with a particular focus on the biogas methanation, starting from the reaction thermodynamics and kinetics. Afterwards, light is shed on the most interesting catalysts reported in the literature, with a focus on Ni-based catalysts and considering the support role in the reaction. Different kinetic approaches currently available and promising reactor types, including different simulation models, which are becoming increasingly fundamental in the reactor design and scale-up phase, are reported. Finally, the last chapter contains the most interesting and industrially relevant ongoing projects on direct biogas methanation.

A multi-level program for regressing thermodynamic parameters was tested on a lately quantified salting-out equilibrium involving acetonitrile + water + ammonium bicarbonate (of pseudo-type-II), plus on other two type-I case studies (n-heptane + benzene + methanol and water + ethanol + toluene), both with the UNIQUAC and NRTL models. The presented algorithm is based on already validated ones, and mixes the isoactivity condition of coexisting phases with sequential tests on the Gibb's energy shapes. The thermodynamic parameters are refined with an evolutionary strategy. Evaluating the outcome of 14 regression cases, it was found that: a) the unmodified UNIQUAC equation slightly outperforms the NRTL model for the given systems, b) the consistency check of the excess energy function acts as a bottleneck, and c) the use of different phases optimizer has a great impact on the overall performance.

Solar driven CO2 photoreduction with advancement in heterogeneous catalysis is promising, albeit challenging due to low quantum efficiency and limited solar light absorption. Layered two-dimensional materials like graphitic carbon nitride (g-C3N4) can pave the way considering intriguing properties and practical applications. However, an additional post-synthesis step of exfoliation is needed to enhance surface area and optoelectronic properties of such materials, after their synthesis in bulk form. Herein, we demonstrate exfoliation of graphitic carbon nitride g-C3N4 by means of UltraSound (US) treatment using water as a solvent at varying input power at constant frequency, constant amplitude and time of effective sonication. This can positively contribute to the properties of the final material without critical handling or environmental issues. Exfoliation of g-C3N4 in water displays a strong dependence of US input power, with a slightly enhanced bandgap (2.8 eV), but most of all increased lifetime of photogenerated electrons, as observed through Diffuse Reflectance Spectroscopy (DRS) and Spectrofluorimetry data. Among all applied power (varied between 30W and 120W), 120W sufficiently exfoliated and tuned physicochemical properties of g-C3N4. Compared to bulk as prepared sample, exfoliated g-C3N4 exhibited improvement in photoinduced charge carrier transfer and separation, resulting in higher photocatalytic efficiencies.Accordingly, the bandgap and charges lifetime of the materials correlate well with change in input power and, as well, the catalytic performance determined through an innovative high-pressure reactor in solid-liquid-gas phase. The catalytic results demonstrate this metal free material as an efficient photocatalyst to obtain high yield of formic acid with productivities ranging from ~5100 to ~8200 mmol/kgcath at 80°C in water, which is among the highest reported in the literature.

The photocatalytic reduction of CO2 into solar fuel is considered a promising approach to solving the energy crisis and mitigating the environmental pollution caused by anthropogenic CO2 emission. Some powder photocatalysts have been demonstrated as efficient, but their drifting properties, along with difficult separation (catalyst and product), make continuous mode reaction very challenging, particularly in the liquid phase. In order to make this process commercially viable and economically more efficient, we have developed a simple and scalable method for immobilizing TiO2 P25 over the surface of glass slides using an organic-based surfactant. Improved adhesion properties and the homogeneous dispersion of catalyst nanoparticles were achieved. A holder was designed with 3D printing technology in such a way that it can hold up to six slides that can be dipped simultaneously into the suspension or solution of desired materials for a uniform and homogeneous deposition. The resulting surfaces of the dip-coated materials (e.g., TiO2 P25) were further modified by adding metallic nanoparticles and thoroughly characterized via XRD, DRS UV–Vis, SEM, and SEM–EDX. Photocatalytic tests have been performed for two major applications, viz., hydrogen production via the photoreforming of glucose and the photoreduction of CO2 into different solar fuels. The latter tests were performed in a specially designed, high-pressure reactor with Ag/P25 supported catalysts, which exhibited about three times higher formic acid productivity (ca. 20 mol/kgcat h) compared to the dispersed catalyst, with enhanced stability and recoverability. It is to note that catalysts deposited on the glass slides can easily be recovered and the materials did not show any weight loss. To the best of our knowledge, the obtained formic acid productivity is highest among the published literature.

In this work we present the simulation of a plant for the exploitation of renewable hydrogen (e.g. from biomass gasification) with production of renewable ammonia as hydrogen vector and energy storage medium. The simulation and sizing of all unit operations were performed with Aspen Plus (R) as software. Vegetable waste biomass is used as raw material for hydrogen production, more specifically pine sawdust.The hydrogen production process is based on a gasification reactor operating at high temperature (700-800 degrees C), in the presence of a gasifying agent such as air or steam. At the outlet, a solid residue (ash) and a certain amount of gas, which mainly contains H2, CH4, CO and some impurities (e.g. sulphur or chlorine compounds) are obtained. Subsequently this gas stream is purified and treated in a series of reactors in order to maximize the hydrogen yield. In fact, after the removal of the sulphur compounds through an absorption column with MEA (to avoid poisoning of the catalytic processes), 3 reactors are arranged in series: Methane Steam Reforming (MSR), High temperature Water-Gas Shift (HT-WGS), Low temperature Water-Gas Shift (LT-WGS).In the first MSR reactor, methane reacts at 1000 degrees C in presence of steam and a nickel -based catalyst, in order to obtain mainly H2, CO and CO2. Subsequently two steps of WGS are present to convert most of the CO into H2 and CO2. Also these reactions are carried out in the presence of a catalyst and with an excess of water.All the oxygenated compounds must be carefully eliminated: the remaining traces of CO are methanated while CO2 is removed by a basic scrubbing with MEA (35 wt%) inside an absorption column. The Haber-Bosch synthesis of ammonia was carried out at 200 bar and in a temperature range between 300 and 400 degrees C, using two catalysts: Fe (wustite) and Ru/C.As overall balance, from an hourly flow rate of 1000 kg of dry biomass and 600 kg of nitrogen, 550 kg of NH3 at 98.8 wt% were obtained, demonstrating the proof of concept of this newly designed process for the production of hydrogen from renewable waste biomass and its transformation into a liquid hydrogen vector to be easily transported and stored. (c) 2023 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

In the current research, the productivity of CO2 photoreduction has been boosted by performing the reaction in an innovative photocatalytic reactor, which allows for operation up to 20 bar. A set of photocatalysts were used, including three types of pristine TiO2, i.e., one commercially prepared (Evonik P25), one home-prepared by flame spray pyrolysis (FSP), and one obtained by the hydrolysis of TiCl4 (TiO2exCl), a bare thermo-exfoliated carbon nitride (C3N4-TE), and binary materials composed of TiO2 and C3N4-TE. The photoreduction was carried out in water at pH 14 and in the presence of Na2SO3 as a hole scavenger. Hydrogen and very small amounts of CO were detected in the head space of the photoreactor, while in the liquid phase, the main product was formic acid, along with traces of methanol and formaldehyde. The composites P25/TE and TiO2exCl/TE were found to have a higher productivity if compared to its single constituents used alone, probably due to the heterojunction formed by coupling the two materials. Moreover, the high pressure applied in the photoreactor proved to be very effective in boosting the yield of the organic products.

Bare titania and metal-promoted TiO2 catalysts were employed in the treatment of nitrates, which are ubiquitous pollutants of wastewater. The results show that the process can be carried out under visible light (from a white light LED lamp) and, in the best case, 23.5% conversion of nitrate was obtained over 4 h with full selectivity towards N2 by employing 0.1 mol% Ag/TiO2 prepared by flame spray pyrolysis. Moreover, the performance was worse when testing the same catalysts with tap water (11.3% conversion), due to the more complex composition of the matrix. Finally, it was found that photoreduction of nitrate can be effectively performed in combination with photo-oxidation of ammonium without loss in the activity, opening up the possibility of treating highly polluted wastewater with a single process. The latter treatment employs the two contaminants simultaneously as electron and holes scavengers, with very good selectivity, in a completely new process that we may call Photo-Selective Catalytic Reduction (Photo-SCR).

Photo-catalysts based on titanium dioxide, and modified with highly dispersed metallic nanoparticles of Au, Ag, Pd and Pt, either mono- or bi-metallic, have been analyzed by multiple characterization techniques, including XRD, XPS, SEM, EDX, UV-Vis and N-2 adsorption/desorption. Mono-metallic photo-catalysts were prepared by wet impregnation, while bi-metallic photocatalysts were obtained via deposition-precipitation (DP). The relationship between the physico-chemical properties and the catalyst's behavior for various photo-synthetic processes, such as carbon dioxide photo-reduction to liquid products and glucose photo-reforming to hydrogen have been investigated. Among the tested materials, the catalysts containing platinum alone (i.e., 0.1 mol% Pt/TiO2) or bi-metallic gold-containing materials (e.g., 1 wt% (AuxAgy)/TiO2 and 1 wt% (AuxPtz)/TiO2) showed the highest activity, presenting the best results in terms of productivity and conversion for both applications. The textural, structural and morphological properties of the different samples being very similar, the main parameters to improve performance were function of the metal as electron sink, together with optoelectronic properties. The high activity in both applications was related to the low band gap, that allows harvesting more energy from a polychromatic light source with respect to the bare TiO2. Overall, high selectivity and productivity were achieved with respect to most literature data.

The CO2 photoreduction is a promising way to convert one of the most abundant greenhouse gases to valuable chemicals. The photoreduction in the liquid phase is limited by the low solubility of CO2 in water, but this point is overcome here by using an innovative photoreactor, which allows one to work up to pressures of 20 bar, improving the overall productivity. The photoreduction was performed in the presence of Na2SO3 and using in primis commercial titanium dioxide (P25) and a set of titania catalysts functionalized by surface deposition of either monometallic or bimetallic cocatalysts. The gaseous products were hydrogen and traces of CO, while, in the liquid phase, formic acid/formate, formaldehyde and methanol were quantitatively detected. The pH was observed to shift the products distribution. A neutral environment led mainly to hydrogen and methanol, while, at pH 14, formate was the most abundant compound. The trend for monometallic cocatalysts showed enhanced productivity when using noble metals (i.e., gold and platinum). In order to limit the cost of the catalytic material, bimetallic cocatalysts were explored, adding titania with Au+Ag or Au+Pt. This may open to the possibility of performing the reaction with a smaller amount of the most expensive metals. In the end, we have expressed some conclusions on the cost of the photocatalysts here employed, to support the overall feasibility assessment of the process.

The photoreforming of organic molecules is a growingly interesting technology to achieve faster hydrogen production than with water splitting, while simultaneously mineralising organic pollutants in water. This paper investigates the possibility to produce hydrogen or gaseous fuel mixtures by using simulated wastewaters of the pulping industry. Glucose was first used as model molecule for carbohydrate-containing wastewaters, while tartaric acid was the model for those rich of carboxylic acids. Different titania catalysts were prepared starting form the P25 commercial material and monometallic Pt or Au, or bi-metallic AuxPty formulations were prepared in form of surface-decorated nanomaterials.The effect of pH and co-catalysts addition was explored, achieving after 5 h of irradi-ation the highest glucose conversion (15.2%) and H2 productivity (4.1 mol H2 kgcat-1 hirr-1) with sample 1.0 wt%Au6Pt4/P25. This result competes with the best ones reported in the literature. Testing of tartaric acid showed much faster conversion of the substrate, but limited hydrogen productivity, ethane and CO2 being the main products, anyway leading to a valorisable gaseous fuel mixture.The best performing materials were also tested for the photoreforming of simulated wastewaters representative of pulping industry effluents, according to the two pulp pro-cessing technologies, the Kraft and sulphite ones. Similar results than tartaric acid were obtained when testing the Kraft spent liquor, mainly composed of hydroxycarboxylic acids. By contrast, the Sulphite spent liquor was mainly composed of carbohydrates, acetic and gluconic acids and led to methane as main product, possibly coming from the decarbox-ylation of acetic acid.

Uptake and release capability of a new organoclay material were assessed with the final goal to evaluate its applicability in recovering REs from solutions. Capture and release capability have been tested towards a La ions model solution, and interaction mechanism and reactions, active in both capture and release processes, were related to the nature of the organoclay and the operating process parameters. The studied system, polyamine-based organoclay, is able to capture and release La ions with high efficiency. Capture involves three distinct mechanisms: ion exchange reaction with interlayer cations, surface adsorption, and finally, coordination by the polyamine. 2.5 mmol/g of amino-groups in a linear ethylene-amine based organoclay are enough to remove all the lanthanum ions in solution when contacted with containing 0.48 mmol/gclay, i.e. 2600 ppm/clay. 12 coordinating sites are needed for each lanthanum ion. Finally, the organic part of the sorbent solid is highly stable, being unmodified upon many cycles of use. The release step is greatly pH dependent, and the optimised pH is equal to 1. Under these conditions, release efficiencies are always higher than 80 %. Appling multiple steps and intermediate re-basification, very high and reproducible global efficiencies (up to 90 %) are reached.

The possibility to exploit renewable sources for the production of bulk chemicals is attractive and bio-ethanol has been recently proposed as platform to produce hydrogen via steam reforming and bio-ethylene by dehydration. Another compound with huge industrial applications is ethylene oxide, which may be in principle obtained in two steps following the route bioethanol ( bio-ethylene ( bio-ethylene oxide. Recently, a one-pot synthesis has been proposed. Based on that, the to design from the grass roots a new production plant, including the reactive and purification sections, has been developed, to exploit it industrially after checking its economic sustainability. In this work, the first step for design, i.e. kinetic modelling and reactor design, will be presented and discussed.A reaction pathway with parallel and consecutive reactions has been hypothesized and the kinetic parameters for the five reactions drawn have been retrieved by regression of experimental literature data. The model employed preliminarily is a power law pseudo-homogeneous one, used for a first sizing of the reactor and basic assessment of the technology. Three shell&tube heat exchange reactors were implemented to control the exothermicity of the reaction, with simultaneous steam production. Three catalyst beds were used in such reactors (200, 500 and 2,000 kg) with intercooling. 99.5 % ethanol conversion and 84 % selectivity to ethylene oxide were achieved, with ca. 90 kmol/h productivity, starting from the bioethanol production of a commercial bio-refinery as preliminary criterion for sizing.