CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives

Developing efficient technologies to decrease CO2 emissions and dealing with climate change issues are among the most critical challenges in worldwide research. This review discusses the most recent advances on the electrochemical transformation of CO2 to alcohols, mainly methanol, ethanol and n-propanol, as a promising way to produce renewable liquid fuels. The main focus is given to copperbased electrocatalyst with different structures (Cu nanoparticles, oxide-derived Cu, and Cu composites) because Cu is up to now the heterogeneous catalyst with the most relevant activity for producing valuable C1+ hydrocarbons and alcohols via CO2 co-electrolysis. Several factors that impact the reaction activity and selectivities, such as the catalyst morphology, composition, surface structure, electrolyte effects and the electrocatalytic cell design (including liquid-phase and catholyte-free systems) are considered and analysed. This review reports an overview of the state-of-the-art with the most recent investigation highlights. It aims to provide guidance on the best experimental practices, new research directions, and strategies to develop efficient electrocatalysts. An outlook about the main challenges to be still resolved for a future practical application of this technology is also provided, toward a future based on sustainability and independence from fossil fuels

A review on the catalytic combustion of soot in Diesel particulate filters for automotive applications: From powder catalysts to structured reactors

Abstract The current soot oxidation catalyst scenario has been reviewed, the main factors that affect the activity of powder catalysts have been highlighted and kinetic soot oxidation models have been examined. A critical review of recent advances in modelling approaches has also been presented in this work. The multiscale nature of DPFs lends itself to a hierarchical organization of models, over various orders of magnitude. Different observation scales (e.g., wall, channel, entire filter) have often been addressed with separate modelling approaches that are rarely connected to one another, mainly because of computational difficulties. Nevertheless, DPFs exhibit an intrinsic multi-scale complexity that is reflected by a trade-off between fine and large-scale phenomena. Consequently, the catalytic behavior of DPFs usually results in a non-linear combination of multi-scale phenomena

Strategies for improving GDE performance by a uniform dispersion of catalyst nanoparticles and an optimal Nafion content

In the context of the strategies needed to mitigate CO2 emissions and combat climate change, the electrochemical CO2 reduction represents a promising alternative. Among the different reactors, GDE-based ones are widely studied systems: here, the limitations shown by configurations with CO2 dissolved in electrolyte solutions can be overcome by feeding CO2 directly in gaseous form. In this work, the manufacturing process of the Cu-based gas diffusion electrode, namely the catalytic ink deposition on a porous carbon paper support, was carried out both by airbrushing (manual) and by spray-coating (automated) techniques. The characterization of the electrodes was performed by using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) techniques. To assess electrodes behavior, cyclic and linear sweep voltammetry techniques were conducted. When comparing the achieved current densities at the highest applied potential, the electrode obtained with the spray coater displayed a better electrocatalytic activity (~10 mA/cm2 higher at about -2.25 V vs Ag/AgCl), with respect to that fabricated with the airbrush. A thorough study of the GDEs performance was accomplished, testing the so obtained electrodes and thereby evaluating the effect of a variation of Nafion content in the productivity and selectivity results toward the desired products. The catalyst layer dispersion is a critical aspect of electrochemical CO2 reduction and, confirming previous studies on the different deposition methods, a more uniform distribution of the catalyst particles enabled the spray coated GDEs to outperform the hand-made ones. Furthermore, the variation of Nafion content on the GDE structure had a relevant effect on the electrode performance, allowing to considerably reduce the side-production of hydrogen and increasing at the same time the CO generation

Influence of sonication on co-precipitation synthesis of copper oxide catalyst for CO2 electroreduction

The need to reduce greenhouse gas emissions and increase our energy supply makes the electrochemical reduction of CO2 (CO2R) a very attractive alternative to produce non-fossil-based fuels or chemicals. Copper-based catalysts is one of the catalyst that most efficiently promote the formation of species with one or more carbon-carbon bonds from the electrochemical reduction of CO2 [1]. Because the catalyst preparation method has an influence on the physicochemical properties and on the electrocatalytic performance[2], in this work, it was decided to evaluate the effect of the ultrasound application (US) on the shape and size of the particles obtained, its electrocatalytic activity and its selectivity to products of interest. For this purpose, sonication was carried out at different percentage amplitudes of ultrasonic power (23, 30 and 37%) during the aging time of the synthesis. Physical characterization was carried out by using different techniques including X-ray diffraction, BET and filed-emission scanning electron microscopy (FESEM). Electrochemical tests for CO2 reduction were done under ambient conditions. Regarding the physical characteristics, we found that pore size distribution is narrower by increasing the US amplitude. On the other hand, there is no significant difference in morphology and dimension of particles. However, the surface area increased with the use of ultrasound, this is attributed to a better dispersion created by acoustic cavitation. Ultrasound has also an effect on Copper-based catalysts performance; in this case, the selectivity towards H2 and C1 products (CO and formate) was enhanced. In addition, an increase in productivity of CO2R products was obtained with respect to the synthesized catalysts that were not assisted by ultrasound (> 3-fold). These results motivate us to further explore in what other ways acoustic cavitation phenomenon can influence the physical characteristics of the catalysts and, in turns, their performance for the electrochemical reduction of CO2

Investigation of Gas Diffusion Electrode systems for the electrochemical CO2 conversion

In the context of climate change and carbon management, electrochemical CO2 reduction represents a promising solution. In this study, the electrochemical conversion of CO2 under atmospheric conditions has been performed in a continuous flow gas diffusion electrode (GDE)-based cell configuration. A porous and conductive support has been employed to this end, where a Cu-based catalyst has been manually deposited in a GDE by means of an airbrusher. With the aim to increase the production of CO2 reduction liquid products, several variables of the studied system have been assessed. The most promising conditions have been explored among the applied potential, catalyst loading, Nafion content, KHCO3 electrolyte concentration and the presence of metal oxides, like ZnO or/and Al2O3. In particular, it has been found that the binder content has affected the production of CO, leading to syngas with a H2/CO ratio of ⁓1 at the lowest Nafion content (15%). In contrast, the highest Nafion content of 45% has led to an increase of C2+ products formation and a decrease of CO selectivity by 80%. The obtained results revealed that liquid crossover affects the GDE performance by severely compromising the CO2 transport to the active sites of the catalyst, thus reducing the CO2 conversion efficiency. A mathematical model confirmed the role of a high local pH, combined with electro-wetting, in promoting the formation of bi-carbonate species: salts formation may cause the catalyst deactivation and hinder the mechanisms for C2+ liquid products. The ultimate intent of this work is to direct the attention of the scientific community to other involved factors of the CO2 reduction process rather than the catalytic activity of the materials, which can impact on both kinetics and mass transport and in turns on the final efficiency of this kind of devices

Catalysis in Diesel engine NOx aftertreatment: a review

AbstractThe catalytic reduction of nitrogen oxides (NOx) under lean-burn conditions represents an important target in catalysis research. The most relevant catalytic NOx abatement systems for Diesel engine vehicles are summarized in this short review, with focus on the main catalytic aspects and materials. Five aftertreatment technologies for Diesel NOx are reviewed: (i) direct catalytic decomposition; (ii) catalytic reduction; (iii) NOx traps; (iv) plasma-assisted abatement; and (v) NOx reduction combined with soot combustion. The different factors that can affect catalytic activity are addressed for each approach (e.g. promoting or poisoning elements, operating conditions, etc.). In the field of catalytic strategies, the simultaneous removal of soot and NOx using multifunctional catalysts, is at present one of the most interesting challenges for the automotive industry

Rheological Properties Comparing Hot and Cold Bituminous Mastics Containing Jet Grouting Waste

The use of reclaimed asphalt pavement is a practice that is adding significant environmental value to road technologies, not only due to the reduction of materials sent to landfill but also because of the mechanical properties of the reclaimed asphalt (RA) that can be reused. This research focuses on the rheological properties of hot and cold bituminous mastics made up as follows: (1) hot mastics mixed with limestone filler (LF) and bitumen, (2) hot mastics, made from bitumen mixed with jet grouting waste (JW), a mixture of water, cement, and soil derived from land consolidation work in underground tunnels, and (3) hot mastics mixed with LF and JW as filler and bitumen. Three different ratios (0.3, 0.4, and 0.5) of filler per unit of neat bitumen (B50/70) were studied. The same number was used for mixing cold mastics, by using an appropriate laboratory protocol designed since the adoption of a cationic bituminous emulsion. A total of 18 mastics were prepared and investigated. The comparison was carried out using the frequency sweep (FS) test, analysing shear modulus G∗, applying the multistress creep and recovery (MSCR) test (40°C and 60°C) as well as the delta ring and ball (ΔR&B) test, focusing on two main issues: (1) the stiffening effect caused by the filler type used for mixing each mastic, and (2) a comparison, in terms of stiffening effects and nonrecoverable creep compliance (Jnr) of hot and cold mastic performance to highlight JW reuse in mastics. The results showed that the best G∗ performance at test temperatures higher than 30°C is given by cold mastic after 28 days of curing time when JW is added to LF and bitumen. The lowest Jnr value was 40°C and 60°C for the same mastic

Development of Cu-based hybrid catalysts for the electrocatalytic CO2 reduction to added value products

The simultaneous need to reduce greenhouse gas emissions and increase our energy supply makes the electrochemical reduction of CO2 a very attractive alternative [1]. In this context, science seeks effective methods to transform CO2 into chemicals of economic value. Among the possible products to obtain, we are especially interested in species with one or more carbon-carbon bonds, these types of compounds are favoured using copper as catalyst. Six catalysts were synthesized with different ratios of Cu, Zn Al and subsequently exposed to a thermal treatment to obtain the correspondent oxidized compounds. These kinds of catalyst are traditionally used in thermocatalysis for the efficient production of methanol at high temperature and pressure conditions [2]. Noting the good performance of this catalyst in thermocatalysis, it was chosen to carry out the experiment in the electrochemical reduction of CO2 at ambient conditions. Electrochemical tests were carried out in the rotating disk electrode (RDE) in order to reduce the mass transfer limitations that may exist due to the low solubility of CO2 in an aqueous medium. The chemical-physical properties of the catalyst were studied by several characterization techniques (e.g. XRD, XPS, BET, among others) to understand the role of the modification of the catalyst components during operation in the final selectivity and activity. Among the liquid products obtained are acetone, ethanol, isopropanol, formic acid and in some cases, methanol was also found. Moreover, gaseous products obtained were hydrogen, carbon monoxide and methane, being these last - gaseous products - those that present the highest faradaic efficiencies. These results were compared with the performance of the catalysts in a Gas Diffusion Electrode (GDE) cell, to obtain commercially-relevant current densities