Michele Aresta

Available renewable carbon-sources (biomass, waste-plastics and CO ₂ ) as alternatives to fossil-carbon are categorized. CO ₂ utilization is a strategic technology for the future of human society and for providing feedstock for the chemical industry.

The photochemical and photoelectrochemical reduction of CO₂ is a promising approach for converting carbon dioxide into valuable chemicals (materials) and fuels. A key issue is ensuring the accuracy of experimental results in CO₂ reduction reactions (CO2RRs) because of potential sources of false positives. This paper reports the results of investigations on various factors that may contribute to erroneous attribution of reduced-carbon species, including degradation of carbon species contained in photocatalysts, residual contaminants from synthetic procedures, laboratory glassware, environmental exposure, and the operator. The importance of rigorous experimental protocols, including the use of labeled ¹³CO₂ and blank tests, to identify true CO₂ reduction products (CO2RPs) accurately is highlighted. Our experimental data (eventually complemented with or compared to literature data) underline the possible sources of errors and, whenever possible, quantify the false positives with respect to the effective conversion of CO₂ in clean conditions. This paper clarifies that the incidence of false positives is higher in the preliminary phase of photo-material development when CO2RPs are in the range of a few 10s of μg g_cat⁻¹ h⁻¹, reducing its importance when significant conversions of CO₂ are performed reaching 10s of mol g_cat⁻¹ h⁻¹. This paper suggests procedures for improving the reliability and reproducibility of CO2RR experiments, thus validating such technologies.

Carbon dioxide capture and utilization is a strategic technology for moving away from fossil‐C. The conversion of CO₂ into fuels demands energy and hydrogen that cannot be sourced from fossil‐C. Co‐processing of CO₂ and water under solar irradiation will have a key role in the long‐term for carbon‐recycling and energy products production. This article discusses the synthesis, characterization and application of the two‐phase composite photocatalyst, In₂O₃@g‐C₃N₄, formed by thermal condensation of melamine in the presence of indium(III)nitrate. The composite exhibits a n,n‐heterojunction between two n‐type semiconductors, g‐C₃N₄ and In₂O₃, leading to a more efficient charge separation. The composite has a flat band potential enabling it to effectively catalyze the reduction of CO₂ in the gas phase to produce CO, CH₄ and CH₃OH. While the composite‘s overall photocatalytic efficiency is comparable to that of neat g‐C₃N₄, its ability to promote multielectron‐transfer and Proton Coupled to Electron Transfer (PCET) suggests that there is a potential for further optimization of its properties. The use of labelled ¹³CO₂ has allowed us to clearly exclude that the reduced species are derived from the photocatalyst decomposition or the degradation of contaminants.

The enzymatic effectiveness in the reaction cascade that reduces CO₂ to methanol in water at room‐temperature faces various constraints. One of the major challenges is the short life of costly enzymes: immobilization is used to make them more stable and recyclable. The comparative analysis of the several immobilization techniques reported in the literature is challenging due to the diverse reaction conditions (single enzyme test or pool of enzymes test) and experimental setups, as well as the high variability in the amount of enzymes and cofactor. In the present study, a comparison is presented among three different methods (co‐encapsulation into Ca–alginate beads, co‐absorption onto zirconium(IV) phosphate (ZrP) and covalent binding to dialdehydecellulose [DAC]) of co‐immobilization of the three dehydrogenases F_atedehydrogenase (DH), F_aldDH, and alcohol dehydrogenase, used in equal amount and under the same experimental conditions, so to check at what extension the support and the immobilization method can influence the activity of the enzymatic pool. DAC is used for the first time to support the three DHs and results to be the best method of immobilization with respect to those used here, that also allows longer life on enzymes and repeated recycling of the supported enzymes, increasing the overall methanol production with respect to the free enzymes.

This paper tells the recent history of CO₂ chemistry through the contributions from members of CIRCC to its birth and growth. Starting with the first transition metal complex bearing CO₂ as a ligand synthesized in 1975 by the Aresta Group in Bari, the mode of bonding and the reactivity of coordinated CO₂ are discussed. The early applications in chemo‐thermal, electrolytic, and biotechnological processes, and the use of SC‐CO₂ as solvent and reagent are fields in which CIRCC partners have made pioneering contributions and continue to provide essential inputs. The paper is closed by highlighting key issues and challenges at stake, from capture to large‐scale conversion, investment infrastructure, progressive policies and standardized regulations needed to advocate carbon recycling as a pillar of future innovation in energy and chemical industries. CIRCC has been the nest of modern CO₂ chemistry and the place where CO₂ has always been considered a “resource” rather than a “waste”.

The need to decrease the concentration of CO₂ in the atmosphere has led to the search for strategies to reuse such molecule as a building block for chemicals and materials or a source of carbon for fuels. The enzymatic cascade of reactions that produce the reduction of CO₂ to methanol seems to be a very attractive way of reusing CO₂; however, it is still far away from a potential industrial application. In this review, a summary was made of all the advances that have been made in research on such a process, particularly on two salient points: enzyme immobilization and cofactor regeneration. A brief overview of the process is initially given, with a focus on the enzymes and the cofactor, followed by a discussion of all the advances that have been made in research, on the two salient points reported above. In particular, the enzymatic regeneration of NADH is compared to the chemical, electrochemical, and photochemical conversion of NAD⁺ into NADH. The enzymatic regeneration, while being the most used, has several drawbacks in the cost and life of enzymes that suggest attempting alternative solutions. The reduction in the amount of NADH used (by converting CO₂ electrochemically into formate) or even the substitution of NADH with less expensive mimetic molecules is discussed in the text. Such an approach is part of the attempt made to take stock of the situation and identify the points on which work still needs to be conducted to reach an exploitation level of the entire process.