Understanding the solar-driven reduction of CO2 on doped ceria

With the appropriate materials, one can construct redox cycles that use CO2 as the oxidant, generating CO as the product. Here, we investigate thermochemical cycles using doped ceria compounds as the oxygen exchange medium. Doped samples are prepared using La, Cr, W, Zr, V, Y, and Ti as dopants. Studying the redox kinetics, we show that doping the pure ceria with zirconium strongly increases overall CO production, albeit at lower reaction rates. This is because the CO2 reduction step is second-order with respect to Ce(III). Doping the fluorite lattice with zirconium cations decreases the number of Ce(III) ions at the surface, and consequently slows down the reaction. This result is counter-intuitive, since normally you would think that the more reduction, the better. But the reactivity towards CO2 is actually determined by the surface Ce(III) ions, and so migration of dopant ions on the surface reduces its reactivity, even though the bulk Ce(III) concentration is higher. Our results demonstrate the importance of understanding surface kinetics when designing oxygen exchange materials for solar reactors.We thank the Dutch National Research School Combination Catalysis (NRSC-C) for funding

Synthesis and Characterization of the Metal–Organic Framework CIM-80 for Organic Compounds Adsorption

Metal–organic frameworks (MOF) are a new type of porous materials that have great potential for adsorption of voltaic organic compounds (VOCs). These types of materials composed of metal ions and organic ligands are easy to synthesize, have high surface areas, their surface chemistry can be adjusted to the desired application, and they can also have good chemical and thermal stability. Therefore, this work focuses on the synthesis of a highly hydrophobic MOF material called CIM-80, a porous material that is made up of the Al3+ cation and the mesaconate linker. This MOF has a B.E.T. of approximately 800 m2/g and has potential applications for the adsorption of hydrophobic organic compounds. However, its synthesis is expensive and very dirty. Therefore, we have studied the synthesis conditions necessary to achieve high synthesis yields (85%) and materials with high crystallinity and accessible porosity. To achieve these results, we have used urea as a mild deprotonation reagent and modulator as an alternative to NaOH, which is traditionally used for the synthesis of this MOF. Once the synthesis of this material was controlled, its adsorption/desorption behavior of water and organic compounds such as toluene, cyclohexane and m-xylene was studied by means of vapor adsorption isotherms. The results show the hydrophobic character of the material and the greater affinity the material has toward aliphatic compounds than toward aromatic ones, with toluene being the most adsorbed compound, followed by cyclohexane and m-xylene.Authors acknowledge financial support by MINECO (Spain) through the project MAT2017-86992-R and “Ministerio de Ciencia e innovación” (PID2020-116998RB-I00). L. Figueroa acknowledges the Generalitat Valenciana (GRISOLIA/2020/123)

Effect of the Synthesis Conditions of MIL-100(Fe) on Its Catalytic Properties and Stability under Reaction Conditions

MIL-100(Fe) is a metal–organic framework (MOF) characterized by the presence of Lewis acid and Fe(II/III) redox sites. In this work, different synthesis methods for the preparation of MIL-100(Fe) are studied. Depending on the source of fluorine, different phases can be obtained: MIL-100(Fe) and an Fe trimesate with unknown structure which we call Fe(BTC). These materials were characterized using numerous techniques and applied in the reaction of CO2 cycloaddition with epichlorohydrin, a reaction catalyzed by Lewis acid sites. It was observed that samples with more Fe(BTC) phase were more active in the reaction. However, all samples, under reaction conditions, transformed into a less active phase.The authors acknowledge financial support by MINECO (Spain) through the project MAT2017-86992-R, “Ministerio de Ciencia e innovación” (PID2020-116998RB-I00), Ministerio de Educación y Formación Profesional (PRX21/00407), and Conselleria de Innovacion, Universidades, Ciencia y Sociedad Digital (CIPROM/2021/022)

Improved mechanical stability of HKUST-1 in confined nanospace

One of the main concerns in the technological application of several metal–organic frameworks (MOFs) relates to their structural instability under pressure (after a conforming step). Here we report for the first time that mechanical instability can be highly improved via nucleation and growth of MOF nanocrystals in the confined nanospace of activated carbons.Financial support from MINECO projects MAT2013-45008-p and CONCERT Project-NASEMS (PCIN-2013-057) is gratefully acknowledged

Manufacture of Carbon Materials with High Nitrogen Content

Nowadays one of the biggest challenges for carbon materials is their use in CO2 capture and their use as electrocatalysts in the oxygen reduction reaction (ORR). In both cases, it is necessary to dope the carbon with nitrogen species. Conventional methods to prepare nitrogen doped carbons such as melamine carbonization or NH3 treatment generate nitrogen doped carbons with insufficient nitrogen content. In the present research, a series of activated carbons derived from MOFs (ZIF-8, ZIF-67) are presented. Activated carbons have been prepared in a single step, by pyrolysis of the MOF in an inert atmosphere, between 600 and 1000 °C. The carbons have a nitrogen content up to 20 at.% and a surface area up to 1000 m2/g. The presence of this nitrogen as pyridine or pyrrolic groups, and as quaternary nitrogen are responsible for the great adsorption capacity of CO2, especially the first two. The presence of Zn and Co generates very different carbonaceous structures. Zn generates a greater porosity development, which makes the doped carbons ideal for CO2 capture. Co generates more graphitized doped carbons, which make them suitable for their use in electrochemistry.Authors acknowledge financial support by MINECO (Spain) through the project MAT2017-86992-R and “Ministerio de Ciencia e Innovación” (PID2020-116998RB-I00)

New Generation of MOF-Monoliths Based on Metal Foams

Herein, it has been developed a method to prepare metallic foams starting from Zamak5 (ZnAlCu alloy) with different pore sizes. The Zamak5 metallic foam is designed to serve as a support and metallic precursor of ZIF-8. In this way, composite materials MOF-metal can be prepared, these composites have a large number of application in energy exchange processe such as: adsorption or chemical reactions. Additionally, this method of sythesizing MOFs is environmentally friendly thanks to absence of solvents. Hanerssing the low melting point of the linker, the linker is infiltrated into the foam where the foam and the linker react to form the ZIF-8. In this way we have managed to transform part of the foam into ZIF-8 crystals that remain adhered to the foam. The foams have been characterized and modeled studying the mechanical and electrical properties, finding that both can be predected by various models. Among these, Ashby and Mortensen models for mechanical properties and Ashby and Percolation model for electrical properties stand.The authors would like to acknowledge the financial support from “Ministerio de Ciencia e innovación” (PID2020-116998RB-I00) and Ministerio de Economía y Empresa (MAT2017-86992-R) and action Mobility of Alicante University

Valorization of CO2 through the Synthesis of Cyclic Carbonates Catalyzed by ZIFs

One way to exploit CO2 is to use it as a feedstock for the production of cyclic carbonates via its reaction with organic epoxides. As far as we know, there is still no heterogeneous catalyst that accelerates the reaction in a selective, efficient and industrially usable way. Cobalt and zinc-based zeolitic imidazole frameworks (ZIFs) have been explored as heterogeneous catalysts for this reaction. In particular, we have prepared ZIF-8 and ZIF-67 catalysts, which have been modified by partial replacement of 2-methylimidazole by 1,2,4-triazole, in order to introduce uncoordinated nitrogen groups with the metal. The catalysts have shown very good catalytic performance, within the best of the heterogeneous catalysts tested in the cycloaddition of CO2 with epichlorohydrin. The catalytic activity is due ultimately to defects on the outer surface of the crystal, and varies in the order of ZIF-67-m > ZIF-67 > ZiF-8-m = ZIF-8. Notably, reactions take place under mild reaction conditions and without the use of co-catalysts.The authors acknowledge financial support by MINECO (Spain) through the projects MAT2017-86992-R and CTQ2017-88171-P, “Ministerio de Ciencia e innovación” (PID2020-116998RB-I00), Ministerio de Educación y Formación Profesional (PRX21/00407), and Conselleria de Innovacion, Universidades, Ciencia y Sociedad Digital (CIPROM/2021/022, MFA/2022/048)

Highly efficient nickel-niobia composite catalysts for hydrogenation of CO2 to methane

We studied the catalytic hydrogenation of CO2 to methane using nickel-niobia composite catalysts. Catalysts containing 10–70 wt% Ni were synthesized by wet impregnation and tested for CO2 hydrogenation in a flow reactor. 40 wt% was found to be the optimum Ni loading, which resulted in CO2 conversion of 81% at 325 °C. We also calcined the Nb2O5 support at different temperatures to study the influence of calcination temperature on the catalytic performance. 40 wt% Ni loaded on Nb2O5, which was calcined at 700 °C gave higher methanation activity (91% conversion of CO2). Time on stream study for 50 h showed a stable activity and selectivity; thus confirming the scope for practical application.E.S.G. and N.R.S acknowledge the financial support from NOW CAPITA project (732.013.002). ASE acknowledges the financial support from the MINECO projects MAT-2013-45008-P and MAT2016-81732-ERC. EVRF gratefully acknowledge support from MINECO for his Ramón y Cajal grant (RyC-2012-11427) and University of Alicante for the project GRE-13-31. Generalitat Valenciana is also acknowledged for financial support (PROMETEOII/ 2014/004)

Enhancing catalytic epoxide ring-opening selectivity using surface-modified Ti3C2Tx MXenes

MXenes are a new family of two-dimensional carbides and/or nitrides. Their 2D surfaces are typically terminated by O, OH and/or F atoms. Here we show that Ti3C2Tx—the most studied compound of the MXene family—is a good acid catalyst, thanks to the surface acid functionalities. We demonstrate this by applying Ti3C2Tx in the epoxide ring-opening reaction of styrene oxide (SO) and its isomerization in the liquid phase. Modifying the MXene surface changes the catalytic activity and selectivity. By oxidizing the surface, we succeeded in controlling the type and number of acid sites and thereby improving the yield of the mono-alkylated product to >80%. Characterisation studies show that a thin oxide layer, which forms directly on the Ti3C2Tx surface, is essential for catalysing the SO ring-opening. We hypothesize that two kinds of acid sites are responsible for this catalysis: In the MXene, strong acid sites (both Lewis and Brønsted) catalyse both the ring-opening and the isomerization reactions, while in the Mxene–TiO2 composite weaker acid sites catalyse only the ring-opening reaction, increasing the selectivity to the mono-alkylated product.TKS was supported by the NWO TOP-PUNT Catalysis in Confined Spaces (Grant 718.015.004). EVRF and ASE acknowledge financial support by MINECO (Spain) through the projects MAT2017-86992-R and MAT2016-80285-P. VN and MWB thank NSF DMR 1740795 for financial support

Highly dispersed Ptδ+ on TixCe(1−x)O2 as an active phase in preferential oxidation of CO

Structure–activity relationships for 1 wt.% Pt catalysts were investigated for a series of TixCe(1−x)O2 (x = 1, 0.98, 0.9, 0.5, 0.2 and 0) supports prepared by the sol–gel method. The catalysts prepared by impregnation were characterized in detail by applying a wide range of techniques as N2-isotherms, XRF, XRD, Raman, XPS, H2-TPR, Drifts, UV–vis, etc. and tested in the preferential oxidation of CO in the presence of H2. Also several reaction conditions were deeply analyzed. A strong correlation between catalyst performance and the electronic properties let us to propose, based in all the experimental results, a plausible reaction mechanism where several redox cycles are involved.Financial support from Generalitat Valenciana and Ministerio de Economía y Competitividad (Spain) through projects PROME-TEOII/2014/004 and MAT2010-21147 is gratefully acknowledged. EOJ also thanks the CNPq – Brazil for her grant. EVRF gratefully acknowledge the Ministerio de Economía y Competitividad (Spain) for his Ramon y Cajal grant (RYC-2012-11427)