Gastón O. Larrazábal

Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes

This review highlights recent developments and future perspectives in carbon dioxide usage for the sustainable production of energy and chemicals and to reduce global warming. We discuss the heterogeneously catalysed hydrogenation, as well as the photocatalytic and electrocatalytic conversion of CO2 to hydrocarbons or oxygenates. Various sources of hydrogen are also reviewed in terms of their CO2 neutrality. Technologies have been developed for large-scale CO2 hydrogenation to methanol or methane. Their industrial application is, however, limited by the high price of renewable hydrogen and the availability of large-volume sources of pure CO2. With regard to the direct electrocatalytic reduction of CO2 to value-added chemicals, substantial advances in electrodes, electrolyte, and reactor design are still required to permit the development of commercial processes. Therefore, in this review particular attention is paid to (i) the design of metal electrodes to improve their performance and (ii) recent developments of alternative approaches such as the application of ionic liquids as electrolytes and of microorganisms as co-catalysts. The most significant improvements both in catalyst and reactor design are needed for the photocatalytic functionalisation of CO2 to become a viable technology that can help in the usage of CO2 as a feedstock for the production of energy and chemicals. Apart from technological aspects and catalytic performance, we also discuss fundamental strategies for the rational design of materials for effective transformations of CO2 to value-added chemicals with the help of H2, electricity and/or light.

Towards sustainable fuels and chemicals through the electrochemical reduction of CO2: lessons from water electrolysis

The storage of renewable energy through the electrochemical reduction of CO2 (eCO2RR) is an attractive strategy to transform the current linear utilisation of carbon fuels (extraction–combustion–CO2 release) into an increasingly cyclic one. An electrochemical alternative for energy storage is the production of H2 from water splitting, studied for decades and commercially available up to the megawatt range. By comparing the technological similarities between these two processes, it is possible to extract both a global perspective and research directions for the eCO2RR. Herein, the main limiting phenomena affecting CO2 electrolysers and their causes are outlined first. This is followed by the derivation, for several eCO2RR products, of targets in terms of durability, current efficiency, energy efficiency, faradaic efficiency, and overvoltage, which must be achieved for the eCO2RR to reach a similar energy storage capability as the electrochemical production of H2. By comparing these figures of merit with the eCO2RR literature, we conclude that the conversion of carbon dioxide to carbon monoxide and formic acid lead the race towards practical use, although both still exhibit major performance gaps. The present energy efficiencies are low, mainly due to high overvoltages, and durability is not yet a developed research area. Besides the development of more efficient electrocatalysts, research advances on the fronts of electrode and electrolyser design, coupled with the optimisation of methods for the preparation of electrodes, are expected to push forward the electrochemical reduction of CO2 on its way to viability.

Lignin repolymerisation in spruce autohydrolysis pretreatment increases cellulase deactivation

This study presents a modified autohydrolysis pretreatment which helps to overcome the recalcitrance of softwood for enzymatic hydrolysis of its cellulose. Autohydrolysis pretreatments of spruce wood were performed with 2-naphthol, which prevents lignin repolymerisation reactions, thereby increasing the enzymatic digestibility of cellulose by up to 64%. The negative influence of repolymerised lignin structures on enzymatic hydrolysis was confirmed by the addition of resorcinol in autohydrolysis, which is known to promote repolymerisation reactions and decreased the biomass digestibility. Several analyses were performed to study the underlying mechanism of this effect on hydrolysis, indicating that cellulolytic enzymes are adsorbed and deactivated especially by repolymerised lignin structures, which accounts for the high differences in biomass digestibility. It was shown that lignin repolymerisation significantly increases its specific surface area through modification of the lignin nanostructure, which is supposed to increase the unproductive binding of enzymes.

CuCrO2 Delafossite: A Stable Copper Catalyst for Chlorine Production

CuCrO2 Delafossite: A Stable Copper Catalyst for Chlorine Production With time comes wisdom : Since the implementation of CuCl2 for HCl oxidation by Deacon in 1868, the search for stable copper catalysts has been futile. Cuprous delafossite, CuCrO2 (see picture), is shown to have unprecedented stability against chlorination, allowing for durable Cl2 production with no metal loss. Based on this, a highly active CuCrO2-CeO2 composite was developed, a cost-effective Cl2 recovery method. Angewandte Chemie

Building Blocks for High Performance in Electrocatalytic CO2 Reduction: Materials, Optimization Strategies, and Device Engineering

In recent years, screening of materials has yielded large gains in catalytic performance for the electroreduction of CO2. However, the diversity of approaches and a still immature mechanistic understanding make it challenging to assess the real potential of each concept. In addition, achieving high performance in CO2 (photo)electrolyzers requires not only favorable electrokinetics but also precise device engineering. In this Perspective, we analyze a broad set of literature reports to construct a set of design-performance maps that suggest patterns between performance figures and different classes of materials and optimization strategies. These maps facilitate the screening of different approaches to electrocatalyst design and the identification of promising avenues for future developments. At the device level, analysis of the network of limiting phenomena in (photo)electrochemical cells leads us to propose a straightforward performance metric based on the concepts of maximum energy efficiency and maximum product formation rate, enabling the comparison of different technologies.

Solvothermally-Prepared Cu2O Electrocatalysts for CO2 Reduction with Tunable Selectivity by the Introduction of p-Block Elements.

The electroreduction of CO2 to fuels and chemicals is an attractive strategy for the valorization of CO2 emissions. In this study, a Cu2 O electrocatalyst prepared by a simple and potentially scalable solvothermal route effectively targeted CO evolution at low-to-moderate overpotentials [with a current efficiency for CO (CECO ) of ca. 60 % after 12 h at -0.6 V vs. reversible hydrogen electrode, RHE], and its selectivity was tuned by the introduction of p-block elements (In, Sn, Ga, Al) into the catalyst. SEM, HRTEM, and voltammetric analyses revealed that the Cu2 O catalyst undergoes extensive surface restructuring (favorable for CO evolution) under the reaction conditions. The modification of Cu2 O with Sn and In further enhanced the current efficiency (CE) for CO (ca. 75 % after 12 h at -0.6 V). In contrast, the introduction of Al altered the selectivity profile of the catalyst significantly, decreasing the selectivity toward CO but promoting the reduction of CO2 to ethylene (CE≈7 %), n-propanol, and ethanol (CE≈2 % each) at -0.8 V vs. RHE. This result is related to a decreased reducibility of Al-doped Cu2 O that might preserve Cu+ species (favorable for C2 H4 production) under the reaction conditions, which is supported by XRD, X-ray photoelectron spectroscopy, and H2 temperature-programmed reduction observations.

Autohydrolysis pretreatment of softwood – enhancement by phenolic additives and the effects of other compounds

The effects of different additives on lignin repolymerisation in the autohydrolysis pretreatment of softwood and the consequences for enzymatic cellulose digestibility have been studied. The study comprised 35 substances including alcohols, amines, heterocyclic and other compounds. It has been shown that lignin repolymerisation does not only hinder hydrolysis by the deactivation of cellulases, but can also obstruct their access to cellulose. A new class of phenolic additives has been discovered that can block lignin repolymerisation and thus increase glucose yields in hydrolysis by more than 40%. Dimethylphloroglucinol was found to be even more effective than 2-naphthol, the most effective lignin repolymerisation blocker reported to date. The study reveals that effective additives have to be highly nucleophilic and must not act as a crossing agent for lignin fragments, which can dramatically worsen glucose yields. Phenolic compounds activated by several hydroxy groups with only a single reactive aromatic site are however very beneficial in enhancing pretreatment. The order of effectiveness of the tested compounds is consistent with the hypothesis that they compete with the aromatic rings present in lignin for lignin carbocations. These ions have been proposed earlier to be intermediates in the formation of repolymerised lignin structures. While compounds activated towards electrophilic substitution generally had a high impact, compounds that can stop radical repolymerisation had no effect. The gained insights open up the possibility to identify numerous further additives that can enhance autohydrolysis, steam and acidic pretreatments of lignocellulose.

Microfabricated electrodes unravel the role of interfaces in multicomponent copper-based CO 2 reduction catalysts

The emergence of synergistic effects in multicomponent catalysts can result in breakthrough advances in the electrochemical reduction of carbon dioxide. Copper-indium catalysts show high performance toward carbon monoxide production but also extensive structural and compositional changes under operation. The origin of the synergistic effect and the nature of the active phase are not well understood, thus hindering optimization efforts. Here we develop a platform that sheds light into these aspects, based on microfabricated model electrodes that are evaluated under conventional experimental conditions. The relationship among the electrode performance, geometry and composition associates the high carbon monoxide evolution activity of copper-indium catalysts to indium-poor bimetallic phases, which are formed upon exposure to reaction conditions in the vicinity of the interfaces between copper oxide and an indium source. The exploratory extension of this approach to the copper-tin system demonstrates its versatility and potential for the study of complex multicomponent electrocatalysts.The development of efficient catalysts for electrochemical carbon dioxide conversion is hindered by a lack of rationalization. Here, authors use microfabricated electrodes to study the birth of active sites around interfaces in multicomponent copper-based catalysts during carbon dioxide reduction.