Understanding the complexity in bridging thermal and electrocatalytic methanation of CO2

The selective methanation of CO₂ is an important research area to meet the net-zero emission targets. Furthermore, it is crucial to develop solutions to achieve carbon neutrality, hydrogen utilization, carbon circularity, and chemical-energy storage. This conversion can be realized via the thermocatalytic multistep power-to-X route or by direct electro- (or photoelectro)-catalytic technologies. Herein, we discuss the need to accelerate direct technologies. Improving these technologies requires a better understanding of the catalytic chemistry and the complexity of the aspects to consider in bridging catalytic and electrocatalytic CO₂ methanation. In this tutorial review, initially we analyze the fundamental question of the competitive adsorption of key reactants and regulation strategies to promote the overall reaction. Then, this approach is used to guide the reader in understanding the differences between thermocatalysis and electrocatalysis. Finally, the complexity of the aspects necessary to include in modelling and designing next-generation electrocatalysts for CO₂ methanation is analyzed.

Interfacial Chemistry in the Electrocatalytic Hydrogenation of CO2 over C-Supported Cu-Based Systems

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)-O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe-Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)-O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu-Zn system represents the optimal active ensembles with stabilized Cu(I)-O; DFT simulations rationalize this observation by indicating that Cu-Zn-O neighboring atoms are able to activate CO2, whereas Cu-Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies.

The Role of Substrate Surface Geometry in the Photo-Electrochemical Behaviour of Supported TiO2 Nanotube Arrays: A Study Using Electrochemical Impedance Spectroscopy (EIS)

Highly ordered TiO₂ nanotube (NT) arrays grown on Ti mesh and Ti foil were successfully prepared by a controlled anodic oxidation process and tested for water photo-electrolysis. Electrochemical impedance spectroscopy (EIS), combined with other electrochemical techniques (cyclic voltammetry and chronoamperometry) in tests performed in the dark and under illumination conditions, was used to correlate the photoactivity to the specific charge transfer resistances associated with a 3D (mesh) or 2D (foil) geometry of the support. The peculiar structure of the nanotubes in the mesh (with better light absorption and faster electron transport along the nanotubes) strongly impacts the catalytic performances under illumination. H₂ production and current density in water photo-electrolysis were over three times higher with the TiO₂NTs/Ti mesh, compared to the foil in the same conditions. The results obtained by the EIS technique, used here for the first time to directly compare TiO₂ nanotubes on two different supports (Ti foil and Ti mesh), led to a better understanding of the electronic properties of TiO₂ nanotubes and the effect of a specific support on its photocatalytic properties.

Advanced (photo)electrocatalytic approaches to substitute the use of fossil fuels in chemical production

Electrification of the chemical industry for carbon-neutral production requires innovative (photo)electrocatalysis. This study highlights the contribution and discusses recent research projects in this area, which are relevant case examples to explore new directions but characterised by a little background research effort. It is organised into two main sections, where selected examples of innovative directions for electrocatalysis and photoelectrocatalysis are presented. The areas discussed include (i) new approaches to green energy or H2 vectors, (ii) the production of fertilisers directly from the air, (iii) the decoupling of the anodic and cathodic reactions in electrocatalytic or photoelectrocatalytic devices, (iv) the possibilities given by tandem/paired reactions in electrocatalytic devices, including the possibility to form the same product on both cathodic and anodic sides to "double" the efficiency, and (v) exploiting electrocatalytic cells to produce green H2 from biomass. The examples offer hits to expand current areas in electrocatalysis to accelerate the transformation to fossil-free chemical production.

Assessment of hydrogen production from municipal solid wastes as competitive route to produce low-carbon H2

An economic and CO₂ emission impact assessment of the production of H₂ from municipal solid waste in the two configurations of retrofitting an existing waste to energy plant with an electrolysis unit (WtE + El) and of hydrogen production via waste gasification (WtH₂) is made with respect to reference cases of H₂ production by steam reforming of methane (SMR) or of water electrolysis (El). The results are analyzed with reference to two scenarios depending on whether the fate of waste disposal emissions for SMR and El is accounted. The costs of H₂ production as a function of waste gate fee and CO₂ taxation as well as the CO₂ emissions for both scenarios and the four cases of H₂ production analyzed are reported. The results show that produce H₂ from a WtE plant hybridized with an electrolyzer could be economic only when the plant is free from depreciation costs and no CO₂ taxation exists. Conversely, WtH₂ solution results preferable when CO₂ taxation will be applied to the non-biogenic fraction of waste. Conditions when WtH₂ may results competitive to SMR are defined, in terms of both cost of production and CO₂ emissions. With respect to El case, WtH₂ results more competitive under the assumption made in terms of combined costs and CO₂ emissions.

Catalysis for e-Chemistry: Need and Gaps for a Future De-Fossilized Chemical Production, with Focus on the Role of Complex (Direct) Syntheses by Electrocatalysis

The prospects, needs and limits in current approaches in catalysis to accelerate the transition to e-chemistry, where this term indicates a fossil fuel-free chemical production, are discussed. It is suggested that e-chemistry is a necessary element of the transformation to meet the targets of net zero emissions by year 2050 and that this conversion from the current petrochemistry is feasible. However, the acceleration of the development of catalytic technologies based on the use of renewable energy sources (indicated as reactive catalysis) is necessary, evidencing that these are part of a system of changes and thus should be assessed from this perspective. However, it is perceived that the current studies in the area are not properly addressing the needs to develop the catalytic technologies required for e-chemistry, presenting a series of relevant aspects and directions in which research should be focused to develop the framework system transformation necessary to implement e-chemistry.

Across the Board: Gabriele Centi on Decoupling Electrocatalytic Reactions to Electrify Chemical Production

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. G. Centi, who discusses the decoupling of the electrocatalytic reactions to realize spatiotemporal separation of the anodic and cathodic processes using redox mediators. This solution allows to potentially overcome the limitations due to intermittency of renewable energy production, besides a series of other advantages such as an improved energy efficiency.

Nanocarbon for Energy Material Applications: N2 Reduction Reaction

Nanocarbons are an important class of energy materials and one relevant application is for the nitrogen reduction reaction, i.e., the direct synthesis of NH₃ from N₂ and H₂ O via photo- and electrocatalytic approaches. Ammonia is also a valuable energy or hydrogen vector. This perspective paper analyses developments in the field, limiting discussion to nanocarbon-based electrodes. These aspects are discussed: i) active sites related to charge density differences on C atoms associated to defects/strains, ii) doping with heteroatoms, iii) introduction of isolated metal ions, iv) creation and in situ dynamics of metal oxide(hydroxide)/nanocarbon boundaries, and v) nanocarbon characteristics to control the interface. Discussion is focused on the performances and mechanistic aspects. Aim is not a systematic state-of-the-art report but to highlight the need to use a different perspective in studying this challenging reaction by using selected papers. Notwithstanding the large differences in the proposed nature of the active sites, fall all within a restricted range of performances, far from the targets. A holistic approach is emphasized to make a breakthrough advance.

Reuse of CO2 in energy intensive process industries

Closing the carbon cycle and enabling a carbon circular economy in energy intensive industries (iron and steel, cement, refineries, petrochemistry and fertilizers) are topics of increasing interest to meet the demanding target of defossilizing the production. The focus of this perspective contribution is on CO₂ reuse technologies in this context. While this is a topic with abundant literature, the analysis of applying CO₂ reuse technologies evidences the need to go beyond those receiving most of the attention today, such as conversion of CO₂ to methanol. Depending on the specific context, different scenarios are expected. Some examples illustrating the search for novel solutions are provided, such as those starting from the efficient conversion of CO₂ to CO. Once CO is produced from CO₂ many bio-chemical and catalytic conversion routes open up next to direct uses of CO in the steel and chemical sector.

Tuning the Chemical Properties of Co-Ti3 C2 Tx MXene Materials for Catalytic CO2 Reduction

MXenes, a novel family of 2D materials, are energy materials that have gained considerable attention, particularly for their catalytic applications in emerging areas such as CO₂ and N₂ hydrogenation. Herein, for the first time, it is shown that the surface reducibility of Ti₃ C₂ T_x MXene can be tuned by N doping, which induces a change in the catalytic properties of supported Co nanoparticles. Pristine Co-Ti₃ C₂ T_x MXene favors CO production during CO₂ hydrogenation, whereas CH₄ production is favored when the MXene is subjected to simple N doping. X-ray photoelectron spectroscopy and transmission electron microscopy (TEM) reveal that surface rutile TiO₂ nanoparticles appear on the Ti₃ C₂ T_x support upon N doping, which interact strongly with the supported Co nanoparticles. This interaction alters the reducibility of the supported Co nanoparticles at the interface with the TiO₂ nanoparticles, shifting the product selectivity from CO to CH₄ . This study successfully showcases a practical strategy, based on surface chemistry modulation of 2D MXenes, for regulating product distribution in CO₂ hydrogenation.

Comparing Molecular Mechanisms in Solar NH3 Production and Relations with CO2 Reduction

Molecular mechanisms for N2 fixation (solar NH3) and CO2 conversion to C2+ products in enzymatic conversion (nitrogenase), electrocatalysis, metal complexes and plasma catalysis are analyzed and compared. It is evidenced that differently from what is present in thermal and plasma catalysis, the electrocatalytic path requires not only the direct coordination and hydrogenation of undissociated N2 molecules, but it is necessary to realize features present in the nitrogenase mechanism. There is the need for (i) a multi-electron and -proton simultaneous transfer, not as sequential steps, (ii) forming bridging metal hydride species, (iii) generating intermediates stabilized by bridging multiple metal atoms and (iv) the capability of the same sites to be effective both in N2 fixation and in COx reduction to C2+ products. Only iron oxide/hydroxide stabilized at defective sites of nanocarbons was found to have these features. This comparison of the molecular mechanisms in solar NH3 production and CO2 reduction is proposed to be a source of inspiration to develop the next generation electrocatalysts to address the challenging transition to future sustainable energy and chemistry beyond fossil fuels.

Enhancing N2 Fixation Activity by Converting Ti3 C2 MXenes Nanosheets to Nanoribbons

Metal carbides M₂ C (MXenes) with two-dimensional (2D) structure have been indicated as promising materials for N₂ fixation, with the activity being related to edge planes. Here, it is instead demonstrated that the transformation from a 2D- (nanosheets) to a 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N₂ fixation activity due to the formation of exposed Ti-OH sites. A linear relationship is observed between ammonia formation rate and amount of oxygen on the surface of Ti₃ C₂ MXene.

Elucidating the mechanism of the CO2 methanation reaction over Ni-Fe hydrotalcite-derived catalysts via surface-sensitive in situ XPS and NEXAFS

Hydrotalcite-derived Ni and Fe-promoted hydrotalcite-derived Ni catalysts were found to outperform industrial catalysts in the CO₂ methanation reaction, however the origin of the improved activity and selectivity of these catalysts is not clear. Here, we report a study of these systems by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy elucidating the chemical nature of the catalysts surface under reaction conditions and revealing the mechanism by which Fe promotes activity and selectivity towards methane. We show that the increase of the conversion leads to hydroxylation of the Ni surface following the formation of water during the reaction. This excessive Ni surface hydroxylation has however a detrimental effect as shown by a controlled study. A dominant metallic Ni surface exists in conditions of higher selectivity towards methane whereas if an increase of the Ni surface hydroxylation occurs, a higher selectivity towards carbon monoxide is observed. The electronic structure analysis of the Fe species under reaction conditions reveals the existence of predominantly Fe(iii) species at the surface, whereas a mixture of Fe(ii)/Fe(iii) species is present underneath the surface when selectivity to methane is high. Our results highlight that Fe(ii) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.

Highly Efficient Metal-Free Nitrogen-Doped Nanocarbons with Unexpected Active Sites for Aerobic Catalytic Reactions

Nitrogen (N)-doped nanocarbons (NDN) as metal-free catalysts have elicited considerable attention toward selective oxidation of alcohols with easily oxidizable groups to aldehydes in the past few years. However, finding a new NDN catalytic material that can meet the requirement of the feasibility on the aerobic catalytics for other complicated alcohols is a big challenge. The real active sites and the corresponding mechanisms on NDN are still unambiguous because of inevitable coexistence of diverse edge sites and N species based on recently reported doping methods. Here, four NDN catalysts with enriched pyridinic N species and without any graphitic N species are simply fabricated via a chemical-vapor-deposition-like method. The results of X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra suggest that the dominating N species on NDN are pyridinic N. It is demonstrated that NDN catalysts perform impressive reactivity for aerobic oxidation of complicated alcohols at an atmospheric pressure. Eleven kinds of aromatic molecules with single N species and tunable π conjugation systems are used as model catalysts to experimentally identify the actual role of each N species at a real molecular level. It is suggested that pyridinic N species play an unexpected role in catalytic reactions. Neighboring carbon atoms in pyridinic N species are responsible for facilitating the rate-determining step process clarified by kinetic isotope effects, in situ nuclear magnetic resonance, in situ attenuated total reflectance infrared, and theoretical calculation. Moreover, NDN catalysts exhibit a good catalytic feasibility on the synthesis of important natural products (e.g., intermediates of vitamin E and K3) from phenol oxidation.

CO2 Reduction of Hybrid Cu2 O-Cu/Gas Diffusion Layer Electrodes and their Integration in a Cu-based Photoelectrocatalytic Cell

Cu₂ O/gas diffusion layer (GDL) electrodes prepared by electrodeposition were studied for the electrocatalytic reduction of CO₂ . The designed electrode was also tested in solar-light-induced CO₂ conversion in combination with a CuO/NtTiO₂ photoanode using a compact photoelectrocatalytic (PEC) cell. Both PEC cell electrodes were prepared using non-critical raw materials and low cost, easily scalable procedures. In the PEC experiments, a total carbon faradaic selectivity of about 90 % to formate and about 75 % to acetate was obtained after 24 h of operations without application of potential/current or using sacrificial agents. In electrocatalytic tests of CO₂ reduction at -1.5 V, the same electrode yielded high total faradaic selectivity (>95 %) but formed selectively formate (about 80 % selectivity) rather than acetate. The in situ transformation of the Cu₂ O/GDL electrode leads to the formation of a hybrid Cu₂ O-Cu/GDL system. Cyclic voltammetry data indicate that the potential and the presence of CO₂ (not only of HCO₃ ^- species) are both important elements in this transformation. Data also indicate that the surface concentration of CO₂ (or of its products of transformation) on the electrode is an important factor to determine performance in the conversion of CO₂ .

Needs and Gaps for Catalysis in Addressing Transitions in Chemistry and Energy from a Sustainability Perspective

Catalysis is undergoing a major transition resulting from significant changes in chemical and energy production. To honor the 50th anniversary of establishing the Jerzy Haber Institute of Catalysis and Surface Chemistry, this Essay discusses, from a forward-looking, personal and somewhat provocative perspective, the needs and gaps of catalysis to address the ongoing transition in chemistry and energy from a sustainability perspective. The focus is on a few selected aspects identified as crucial: i) The precise synthesis of catalytic materials, particularly focusing on mesoporous molecular sieves, metal-organic frameworks, and zeolites (particularly two-dimensional type); ii) advanced catalyst characterization methods; iii) new concepts and approaches needed in catalysis to meet the demands of a field of energy and chemistry in transition.

Special Issue to Mark the 50th Anniversary of the Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences

The Special Issue to mark the 50th anniversary of the Jerzy Haber Institute of Catalysis and Surface Chemistry at the Polish Academy of Sciences is introduced by Guest Editors Gabriele Centi and Dorota Rutkowska-Zbik, who look back on the institute's achievements over the past five decades.

Deactivation mechanism of hydrotalcite-derived Ni–AlOx catalysts during low-temperature CO2 methanation via Ni-hydroxide formation and the role of Fe in limiting this effect

Ni–Fe/AlO_x with nanosheet structure, enhance the reducibility and stability of the Ni-hydroxide during the catalytic reaction due to the formation of spinel phase which stabilize smaller Ni nanoparticle with a weaker interaction with the support.

The role of nanosized zeolite Y in the H2-free catalytic deoxygenation of triolein

The reduction in zeolite Y crystal size has improved the triolein conversion, yield of deoxygenated product and diesel range hydrocarbon selectivity.

2D Oxide Nanomaterials to Address the Energy Transition and Catalysis

2D oxide nanomaterials constitute a broad range of materials, with a wide array of current and potential applications, particularly in the fields of energy storage and catalysis for sustainable energy production. Despite the many similarities in structure, composition, and synthetic methods and uses, the current literature on layered oxides is diverse and disconnected. A number of reviews can be found in the literature, but they are mostly focused on one of the particular subclasses of 2D oxides. This review attempts to bridge the knowledge gap between individual layered oxide types by summarizing recent developments in all important 2D oxide systems including supported ultrathin oxide films, layered clays and double hydroxides, layered perovskites, and novel 2D-zeolite-based materials. Particular attention is paid to the underlying similarities and differences between the various materials, and the subsequent challenges faced by each research community. The potential of layered oxides toward future applications is critically evaluated, especially in the areas of electrocatalysis and photocatalysis, biomass conversion, and fine chemical synthesis. Attention is also paid to corresponding novel 3D materials that can be obtained via sophisticated engineering of 2D oxides.

Operando spectroscopy study of the carbon dioxide electro-reduction by iron species on nitrogen-doped carbon

The carbon-carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (-0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.

Hydrotalcite based Ni–Fe/(Mg, Al)Ox catalysts for CO2 methanation – tailoring Fe content for improved CO dissociation, basicity, and particle size

Advantages of hydrotalcite-like precursors and the synergistic effect of bimetallic Ni–Fe alloys are combined and the most appropriate amount of Fe identified with respect to activity, selectivity and stability.

Catalysis by hybrid sp2/sp3nanodiamonds and their role in the design of advanced nanocarbon materials

Hybrid sp²/sp³nanocarbons, in particular sp³-hybridized ultra-dispersed nanodiamonds and derivative materials, such as the sp³/sp²-hybridized bucky nanodiamonds and sp²-hybridized onion-like carbons, represent a rather interesting class of catalysts still under consideration.

New catalytic materials for energy and chemistry in transition

Guest editors Jiří Čejka, Petr Nachtigall and Gabriele Centi introduce theNew catalytic materials for an energy and chemistry transitionthemed issue ofChemical Society Reviews.

Direct versus acetalization routes in the reaction network of catalytic HMF etherification

The etherification of HMF (5-hydroxymethylfurfural) to EMF (5-(ethoxymethyl)furan-2-carbaldehyde) is studied over a series of MFI-type zeolite catalysts containing different heteroatoms (B, Fe, Al), aiming to understand the effect of different isomorph substitutions in the MFI framework on the reaction pathways of HMF conversion.

Beyond Solar Fuels: Renewable Energy-Driven Chemistry

The future feasibility of decarbonized industrial chemical production based on the substitution of fossil feedstocks (FFs) with renewable energy (RE) sources is discussed. Indeed, the use of FFs as an energy source has the greatest impact on the greenhouse gas emissions of chemical production. This future scenario is indicated as "solar-driven" or "RE-driven" chemistry. Its possible implementation requires to go beyond the concept of solar fuels, in particular to address two key aspects: i) the use of RE-driven processes for the production of base raw materials, such as olefins, methanol, and ammonia, and ii) the development of novel RE-driven routes that simultaneously realize process and energy intensification, particularly in the direction of a significant reduction of the number of the process steps.

Enhanced formation of >C1 Products in Electroreduction of CO2 by Adding a CO2 Adsorption Component to a Gas-Diffusion Layer-Type Catalytic Electrode

The addition of a CO₂ -adsorption component (substituted imidazolate-based SIM-1 crystals) to a gas-diffusion layer-type catalytic electrode enhances the activity and especially the selectivity towards >C1 carbon chain products (ethanol, acetone, and isopropanol) of a Pt-based electrocatalyst that is not able to form products of CO₂ reduction involving C-C bond formation under conventional (liquid-phase) conditions. This indicates that the increase of the effective CO₂ concentration at the electrode active surface is the factor controlling the formation of >C1 products rather than only the intrinsic properties of the electrocatalyst.

Waste-to-methanol: Process and economics assessment

The waste-to-methanol (WtM) process and related economics are assessed to evidence that WtM is a valuable solution both from economic, strategic and environmental perspectives. Bio-methanol from Refuse-derived-fuels (RdF) has an estimated cost of production of about 110€/t for a new WtM 300t/d plant. With respect to waste-to-energy (WtE) approach, this solution allows various advantages. In considering the average market cost of methanol and the premium as biofuel, the WtM approach results in a ROI (Return of Investment) of about 29%, e.g. a payback time of about 4years. In a hybrid scheme of integration with an existing methanol plant from natural gas, the cost of production becomes a profit even without considering the cap for bio-methanol production. The WtM process allows to produce methanol with about 40% and 30-35% reduction in greenhouse gas emissions with respect to methanol production from fossil fuels and bio-resources, respectively.