Ignition of CO2 methanation using DBD-plasma catalysis in an adiabatic reactor

In this work, a novel strategy of the use of dielectric barrier discharge (DBD) plasma-catalysis for exothermic reactions is presented. DBD-plasma is used as reaction ignitor, rather than the classical approach of continuous operation, by taking advantage of the synergy between catalytic plasma activation from room temperature and a self-sustained exothermic reaction. CO2 methanation reaction was performed in a thermal insulated reactor using an active nickel-based catalyst loaded in two catalytic bed sections, with electrodes implemented solely in the first section. DBD plasma was employed to activate the reaction from cold conditions with the subsequent increase in reactor temperature and, finally, reaction was self-sustained by thermal-catalysis. The experimental results pointed out the sensitive dependence of the reactor temperature on the gas flow rate during the plasma operation. Low-energy conditions were found in which the reaction could operate in autothermal mode, after plasma shut-off. Power and start-up time were optimized, obtaining a considerable low start-up time from cold conditions (25 ◦C) of only 3 min. Besides, the autothermal operation mode was maintained for 8 h without any energy input. This proof-of-concept work demonstrates that plasma can be implemented as initial power ignition in exothermic reactions using proper reactor design and conditions, and then, the reactor can operate in autothermal mode

Fischer-Tropsch synthesis: Towards a highly-selective catalyst by lanthanide promotion under relevant CO2 syngas mixtures

The role of lanthanides as promoters on cobalt-based catalysts for Fischer-Tropsch synthesis was evaluated under relevant biomass-derived syngas mixtures. Cerium, lanthanum and a combination of them were impregnated on an industrial cobalt-based micro-catalyst. Lanthanide incorporation did not affect significantly the morphology of the catalyst, although it reduced the available surface cobalt. Catalytic tests revealed that both the presence of carbon dioxide in the feed and lanthanides in the catalyst led to similar outcomes; higher selectivity to long-chain hydrocarbons, at the expense of reactivity. Reaction experiments were well aligned with in-situ DRIFTS measurements, which evidenced the modification of the initial reaction mechanism, CO2 conversion and the presence of lower CO-cobalt coverages. This work reports two relevant findings for FTS development. Firstly, the presence of carbon dioxide is beneficial for long-chain hydrocarbon production. Secondly, the incorporation of lanthanides increases the production of gasoline, kerosene and diesel fractions

Bimetallic cobalt catalysts promoted by La2O3 for the production of high-calorie synthetic gas

A new catalytic route for the production of a high-calorie synthetic gas (40-60 MJ/Nm3), composed by C1-C4 hydrocarbons, has industrial interest for gas applications and locations with high heating requirements. In this work, a series of bimetallic Co-X (X = Ni, Pt and Fe) catalysts supported on La2O3 promoted Al2O3 micro-spheres were evaluated using both CO2 and CO carbon sources under mild temperature (T = 200-300 °C), moderate pressure (P = 10 bar·g) and relatively high gas hourly space velocity (40,000 N mL/gcat·h). Experimental results proved that the incorporation of nickel as a second metal is beneficial for high-calorie gas application. Besides, catalytic results showed that the utilization of CO as carbon source is beneficial in both conversion and C1-C4 hydrocarbon selectivities. Co-Ni presented the most interesting results, leading to a heating value of 57.9 MJ/Nm3 (40.01 % CH4 and 50.04 % C2-C4 hydrocarbon) at 250 °C through CO hydrogenation. The enhanced catalytic performance achieved over bimetallic Co-Ni was attributed to CoNi alloy catalytic activity, high reducibility (73.82 %), active metal content (9.65x10-4 mmol/g) and appropriate acid-basic sites for COx activation. In contrast, the conversion of CO2 to high-calorie gas was found to be more challenging and lower gas heating values were achieved (39.73 MJ/Nm3). In this case, an adapted reactor concept using a dual bimetallic catalyst and different reaction conditions is hereby proposed to shift selectivity towards the targeted products. This findings represent a step forwards in catalytic engineering for the development of high-calorie synthetic gas reactors

Passivation of Co/Al2O3 Catalyst by Atomic Layer Deposition to Reduce Deactivation in the Fischer-Tropsch Synthesis

The present work explores the technical feasibility of passivating a Co/γ-Al2O3catalyst byatomic layer deposition (ALD) to reduce deactivation rate during Fischer-Tropsch synthesis (FTS).Three samples of the reference catalyst were passivated using different numbers of ALD cycles (3, 6and 10). Characterization results revealed that a shell of the passivating agent (Al2O3) grew aroundcatalyst particles. This shell did not affect the properties of passivated samples below 10 cycles, inwhich catalyst reduction was hindered. Catalytic tests at 50% CO conversion evidenced that 3 and6 ALD cycles increased catalyst stability without significantly affecting the catalytic performance,whereas 10 cycles caused blockage of the active phase that led to a strong decrease of catalytic activity.Catalyst deactivation modelling and tests at 60% CO conversion served to conclude that 3 to 6 ALDcycles reduced Co/γ-Al2O3deactivation, so that the technical feasibility of this technique was provenin FTS

A MOF-Based Spatial-Separation Layer to Enable a Uniform Favorable Microenvironment for Electrochemical CO2 Reduction

Regulating the local microenvironment of active sites to increase their specific CO2 concentration and pH gradient, is a promising approach to optimize the electrochemical CO2 reduction reaction (eCO2RR). However, currently reported morphological strategies display an uncertainty to the compatibility and distribution between catalytic sites and their microenvironment. Here, a uniform spatial-separation metal-organic framework (MOF) layer between active sites and bulk electrolyte is proposed, which enables each active site to locate in a similarly favorable microenvironment. Zinc oxide (ZnO) nanorods (NR), a representative electrocatalyst for eCO2RR, is covered with a Zeolitic imidazolate framework-8 (ZIF-8) thin layer to serve as a model system. The prepared ZnO NR@ZIF-8 exhibits an enhanced Faradaic efficiency toward CO at a wide range of potentials and reaches a maximum FE of CO (85%) at −1.05 V versus reversible hydrogen electrode, which is one of the best records till date. Moreover, the hydrophobic ZIF-8 layer protects ZnO against self-reduction. Such performance benefits from the porous ZIF-8 shell with high CO2 affinity, realizing efficient CO2 access and retaining an increased local pH near ZnO active sites

Engineering the Interfacial Microenvironment via Surface Hydroxylation to Realize the Global Optimization of Electrochemical CO Reduction

The adsorption and activation of CO on the electrode interface is a prerequisite and key step for electrocatalytic CO reduction reaction (eCO RR). Regulating the interfacial microenvironment to promote the adsorption and activation of CO is thus of great significance to optimize overall conversion efficiency. Herein, a CO-philic hydroxyl coordinated ZnO (ZnO-OH) catalyst is fabricated, for the first time, via a facile MOF-assisted method. In comparison to the commercial ZnO, the as-prepared ZnO-OH exhibits much higher selectivity toward CO at lower applied potential, reaching a Faradaic efficiency of 85% at −0.95 V versus RHE. To the best of our knowledge, such selectivity is one of the best records in ZnO-based catalysts reported till date. Density functional theory calculations reveal that the coordinated surficial −OH groups are not only favorable to interact with CO molecules but also function in synergy to decrease the energy barrier of the rate-determining step and maintain a higher charge density of potential active sites as well as inhibit undesired hydrogen evolution reaction. Our results indicate that engineering the interfacial microenvironment through the introduction of CO-philic groups is a promising way to achieve the global optimization of eCO RR via promoting adsorption and activation of CO

Improvement of carbon dioxide electroreduction by crystal surface modification of ZIF-8

Metal-organic frameworks (MOFs) possess high CO adsorption properties and are considered to be a promising candidate for the electrochemical carbon dioxide reduction reaction (eCORR). However, their insufficient selectivity and current density constrain their further exploration in the eCORR. In this work, by introducing a very small proportion of 2,5-dihydroxyterephthalic acid (DOBDC) into ZIF-8, a surface modified ZIF-8-5% catalyst was synthesized by a post-modification method, exhibiting enhanced selectivity (from 56% to 79%) and current density (from −4 mA cm to −10 mA m) compared to ZIF-8. Density functional theory (DFT) calculations further demonstrate that the boosted eCORR performance on ZIF-8-5% could be attributed to the improved formation of the *COOH intermediate stemming from successful DOBDC surface modification. This work opens a new path for improving the catalytic properties of MOFs via their surface modification

Site-specific axial oxygen coordinated FeN4 active sites for highly selective electroreduction of carbon dioxide

Altres ajuts: ICN2 and IREC were funded by the CERCA Programme/Generalitat de Catalunya.Regulating the coordination environment via heteroatoms to break the symmetrical electronic structure of M-N active sites provides a promising route to engineer metal-nitrogen-carbon catalysts for electrochemical CO reduction reaction. However, it remains challenging to realize a site-specific introduction of heteroatoms at atomic level due to their energetically unstable nature. Here, this paper reports a facile route via using an oxygen- and nitrogen-rich metal-organic framework (MOF) (IRMOF-3) as the precursor to construct the Fe-O and Fe-N chelation, simultaneously, resulting in an atomically dispersed axial O-coordinated FeN active site. Compared to the FeN active sites without O coordination, the formed FeN-O sites exhibit much better catalytic performance toward CO, reaching a maximum FE of 95% at −0.50 V versus reversible hydrogen electrode. To the best of the authors' knowledge, such performance exceeds that of the existing Fe-N-C-based catalysts derived from sole N-rich MOFs. Density functional theory calculations indicate that the axial O-coordination regulates the binding energy of intermediates in the reaction pathways, resulting in a smoother desorption of CO and increased energy for the competitive hydrogen production

Enhanced Photoelectrochemical Water Splitting of Hematite Multilayer Nanowires Photoanode with Tuning Surface State via Bottom-up Interfacial Engineering

The optimization of multiple interfaces in hematite (α-Fe_2O_3) based composites for photoelectrochemical water splitting to facilitate charge transport in the bulk is of paramount importance to obtain enhanced solar-to-fuel efficiency. Herein, we report the fabrication of ITO/Fe_2O_3/Fe_2TiO_5/FeNiOOH multi-layer nanowires and a series of systematic experiments designed to elucidate the mechanism underlying the interfacial coupling effect of the quaternary hematite composite. The hierarchical ITO/Fe_2O_3/Fe_2TiO_5/FeNiOOH nanowires display photocurrents that are more than an order of magnitude greater than those of pristine Fe_2O_3 nanowires (from 0.205 mA cm^(−2) to 2.2 mA cm^(−2) at 1.23 V vs. RHE and 1 Sun), and higher than those of most of the recently reported state-of-the-art hematite composites. Structural, compositional and electrochemical investigations disclose that the surface states (SS) are finely regulated via the atomic addition of an Fe_2TiO_5 layer and FeNiOOH nanodots, while the upgrading of back contact conductivity and charge donor densities originate from the epitaxial relationship and enhanced Sn doping contributed from the ITO underlayer. We attribute the superior water oxidation performance to the interfacial coupling effect of the ITO underlayer (Sn doping and back contact conductivity promoter), the atomic level Fe_2TiO_5 coating (Ti doping, surface state density and energy level modulation) and the FeNiOOH nanodot electrocatalyst (regulating surface state energy level). Our work suggests an effective pathway for rational designing of highly active and cost-effective integrated photoanodes for photoelectrochemical water splitting

Enhanced photoelectrochemical water splitting of hematite multilayer nanowire photoanodes by tuning the surface state via bottom-up interfacial engineering

The optimization of multiple interfaces in hematite (α-Fe₂O₃) based composites for photoelectrochemical water splitting to facilitate charge transport in the bulk is of paramount importance to obtain enhanced solar-to-fuel efficiency. Herein, we report the fabrication of ITO/Fe₂O₃/Fe₂TiO₅/FeNiOOH multi-layer nanowires and a series of systematic experiments designed to elucidate the mechanism underlying the interfacial coupling effect of the quaternary hematite composite. The hierarchical ITO/Fe₂O₃/Fe₂TiO₅/FeNiOOH nanowires display photocurrents that are more than an order of magnitude greater than those of pristine Fe₂O₃ nanowires (from 0.205 mAcm⁻² to 2.2 mAcm⁻² at 1.23 V vs. RHE and 1 Sun), and higher than those of most of the recently reported state-of-the-art hematite composites. Structural, compositional and electrochemical investigations disclose that the surface states (SS) are finely regulated via the atomic addition of an Fe₂TiO₅ layer and FeNiOOH nanodots, while the upgrading of back contact conductivity and charge donor densities originate from the epitaxial relationship and enhanced Sn doping contributed from the ITO underlayer. We attribute the superior water oxidation performance to the interfacial coupling effect of the ITO underlayer (Sn doping and back contact conductivity promoter), the atomic level Fe₂TiO₅ coating (Ti doping, surface state density and energy level modulation) and the FeNiOOH nanodot electrocatalyst (regulating surface state energy level). Our work suggests an effective pathway for rational designing of highly active and cost-effective integrated photoanodes for photoelectrochemical water splitting