Carbon Nitride-Based Catalysts for High Pressure CO2 Photoreduction

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.

Electrochemical Reduction of Carbon Dioxide at TiO2/Au Nanocomposites

Herein, we report on the facile synthesis of nanocomposite consisting of TiO₂ and Au nanoparticles (NPs) via a tailored galvanic replacement reaction (GRR). The electrocatalytic activity of the synthesized TiO₂/Au nanocomposites for CO₂ reduction was investigated in an aqueous solution using various electrochemical methods. Our results demonstrated that the TiO₂/Au nanocomposites formed through the GRR process exhibited improved catalytic activities for CO₂ reduction, while generating more hydrocarbon molecules than the typical formation of CO in contrast to polycrystalline Au. GC analysis and NMR spectroscopy revealed that CO and CH₄ were the gas products, whereas HCOO^-, CH₃COO^-, CH₃OH, and CH₃CH₂OH were the liquid products from the CO₂ reduction at different cathodic potentials. This remarkable change was further studied using the density functional theory (DFT) calculations, showing that the TiO₂/Au nanocomposites may increase the binding energy of the formed ^·CO intermediate and reduce the free energy compared to Au, thus favoring the downstream generation of multicarbon products. The TiO₂/Au nanocomposites have high catalytic activity and excellent stability and are easy to fabricate, indicating that the developed catalyst has potential application in the electrochemical reduction of CO₂ to value-added products.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

Formaldehyde (HCHO) is a crucial C₁ building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO₂ ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO₂ -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO₂ to HCHO are discussed.

Universal Kinetic Mechanism Describing CO2 Photoreductive Yield and Selectivity for Semiconducting Nanoparticle Photocatalysts

Photocatalytic conversion of CO₂ to generate high-value and renewable chemical fuels and feedstock presents a sustainable and renewable alternative to fossil fuels and petrochemicals. Currently, there is a dearth of kinetic understanding to inform better catalyst design, especially at uniform reaction conditions across diverse catalytic species. In this work, we investigate 12 active, stable, and unique but common nanoparticle photocatalysts for CO₂ reduction at room temperature and low partial pressure in aqueous phase: TiO₂, SnO₂, and SiC deposited with silver, gold, and platinum. Our analysis reveals a single consistent chemical kinetic mechanism, which accurately describes the yield and selectivity of all single-carbon containing (C1) products obtained in spite of the diverse catalysts employed. Formaldehyde is predicted as the first product in the reaction network and we report, to the best of our knowledge, the highest selectivity to date toward formaldehyde during CO₂ photoreduction when compared against all other C1 products (∼80%) albeit at low CO₂ conversion (<0.5 μmol g_cat^-1 h^-1, <16.8 nmol m^-2 h^-1). Further, we observe a volcano-like relationship between the electron-transfer rate of a given photocatalyst for CO₂ reduction and the net rate at which reduced products are produced in the reaction mixture taking into account unfavorable product oxidation. We establish an empirical upper limit for the maximum rate of production of CO₂ reduction products for any nanoparticle photocatalyst in the absence of a hole-scavenging agent. These results form the basis for the design and optimization of the next generation of highly efficiency and active photocatalysts for CO₂ reduction.

Low Metal Loading (Au, Ag, Pt, Pd) Photo-Catalysts Supported on TiO2 for Renewable Processes

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 N₂ 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% (Au_xAg_y)/TiO₂ and 1 wt% (Au_xPt_z)/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.

Photocatalytic Reduction of Nitrates and Combined Photodegradation with Ammonium

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).

Effect of Metal Cocatalysts and Operating Conditions on the Product Distribution and the Productivity of the CO2 Photoreduction

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.

Recent Advances of Photocatalytic Hydrogenation of CO2 to Methanol

Constantly increasing hydrocarbon fuel combustion along with high levels of carbon dioxide emissions has given rise to a global energy crisis and environmental alterations. Photocatalysis is an effective technique for addressing this energy and environmental crisis. Clean and renewable solar energy is a very favourable path for photocatalytic CO2 reduction to value-added products to tackle problems of energy and the environment. The synthesis of various products such as CH4, CH3OH, CO, EtOH, etc., has been expanded through the photocatalytic reduction of CO2. Among these products, methanol is one of the most important and highly versatile chemicals widely used in industry and in day-to-day life. This review emphasizes the recent progress of photocatalytic CO2 hydrogenation to CH3OH. In particular, Metal organic frameworks (MOFs), mixed-metal oxide, carbon, TiO2 and plasmonic-based nanomaterials are discussed for the photocatalytic reduction of CO2 to methanol. Finally, a summary and perspectives on this emerging field are provided.

The Influence of High-Energy Faceted TiO2 Supports on Co and Co-Ru Catalysts for Dry Methane Reforming

The reforming of methane from biogas has been proposed as a promising method of CO2 utilization.  Co-based catalysts are promising candidates for dry methane reforming. However, the main constraints limiting the large-scale use of Co-based catalysts are deactivation through carbon deposition (coking) and sintering due to weak metal-support interaction. We studied the structure-function properties and catalytic behavior of Co/TiO2 and Co-Ru/TiO2 catalysts using two different types of TiO2 supports, commercial TiO2 and faceted non-stoichiometric rutile TiO2 crystals (TiO2*). The Co and Ru metal particles were deposited on TiO2 supports using a wet-impregnation method with the percentage weight loading of Co and Ru of 5% and 0.5%, respectively. The materials were characterized using SEM, STEM-HAADF, XRD, XPS and BET. The catalytic performance was studied using the CH4:CO2 ratio of 3:2 to mimic the methane-rich biogas composition. Our results indicate that the addition of Ru to Co catalysts supported on TiO2* reduces carbon deposition and influences oxygen mobility. Co and Co-Ru catalysts supported on TiO2* has superior activity with the highest conversion of CO2 and CH4 of 34.7% and 23.5%, respectively. Despite the improved performance, the Co-Ru/TiO2* catalyst has limited stability due to the proliferation of nanoparticle growth and TiOx layers on the surface of the nanoparticles indicating the prevalence of the strong-metal support interaction.

Photo-Oxidation of Ammonia to Molecular Nitrogen in Water under UV, Vis and Sunlight Irradiation

Titanium dioxide-based photocatalysts have been used to perform the photo-oxidation of ammonium/ammonia to molecular nitrogen. Different light sources were employed, i.e., UV, LED visible light and natural sunlight, and their performance was compared in order to understand which setup was the most efficient. It was found that under selected conditions, the LED lamp, in combination with silver-promoted TiO2, was able to push the conversion of ammonium toward 48% after 4 h of reaction time. On the other hand, with a more powerful UV lamp, lower conversion was achieved, ca. 40%. Natural sunlight under the same conditions attained more than 38% conversion, but the fluctuation of the reaction conditions remain a very critical issue for the real exploitation of sunlight in water treatment.

Improvement of CO2 Photoreduction Efficiency by Process Intensification

This paper addresses an innovative approach to improve CO2 photoreduction via process intensification. The principle of CO2 photoreduction using process intensification is presented and reviewed. Process intensification via concentrating solar light technology is developed and demonstrated. The concept consists in rising the incident light intensity as well as the reaction temperature and pressure during CO2 photoreduction using concentrating solar light. A solar reactor system using concentrated sunlight was accordingly designed and set up. The distribution of light intensity and temperature in the reactor was modeled and simulated. CO2 photoreduction performance in the reactor system was assessed, and the reaction temperature and pressure evolution were recorded. The results showed that the light intensity, temperature, and pressure could be effectively increased and irradiation on the catalyst surface followed a Gaussian distribution. The CO2 photoreduction reaction rates were enhanced to hundreds of times.

Photoreforming of Glucose over CuO/TiO2

Hydrogen production has been investigated through the photoreforming of glucose, as model molecule representative for biomass hydrolysis. Different copper- or nickel-loaded titania photocatalysts have been compared. The samples were prepared starting from three titania samples, prepared by precipitation and characterized by pure Anatase with high surface area, or prepared through flame synthesis, i.e., flame pyrolysis and the commercial P25, leading to mixed Rutile and Anatase phases with lower surface area. The metal was added in different loading up to 1 wt % following three procedures that induced different dispersion and reducibility to the catalyst. The highest activity among the bare semiconductors was exhibited by the commercial P25 titania, while the addition of 1 wt % CuO through precipitation with complexes led to the best hydrogen productivity, i.e., 9.7 mol H2/h kgcat. Finally, a basic economic analysis considering only the costs of the catalyst and testing was performed, suggesting CuO promoted samples as promising and almost feasible for this application.

Catalytic, Photocatalytic, and Electrocatalytic Processes for the Valorization of CO2

Worldwide yearly CO2 emissions reached 36 Gt in 2014, whereas they amounted to ca [...]