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

Titania-catalysed oxidative dehydrogenation of ethyl lactate: effective yet selective free-radical oxidation

We research here the catalytic oxidative dehydrogenation of ethyl lactate, as an alternative route to ethyl pyruvate. Testing various solid catalysts (Fe2O3, TiO2, V2O5/MgO–Al2O3, ZrO2, CeO2 and ZnO), we find that simple and inexpensive TiO2 efficiently catalyses this reaction under mild conditions. Furthermore, molecular oxygen was used as the terminal oxidant. Importantly, this reaction runs well also using inexpensive commercial solvent mixtures. Both the desired reaction and the by-products formation follow a free-radical mechanism. Remarkably, adding activated carbon, a solid radical scavenger, hardly affects the catalytic activity, but enhances the product selectivity. This is because this solid radical scavenger hampers the formation of undesired products in solution, without suppressing the oxidation at the catalyst surface

Silica-supported sulfonic acids as recyclable catalyst for esterification of levulinic acid with stoichiometric amounts of alcohols

Converting biomass into value-added chemicals holds the key to sustainable long-term carbon resource management. In this context, levulinic acid, which is easily obtained from cellulose, is valuable since it can be transformed into a variety of industrially relevant fine chemicals. Here we present a simple protocol for the selective esterification of levulinic acid using solid acid catalysts. Silica supported sulfonic acid catalysts operate under mild conditions and give good conversion and selectivity with stoichiometric amounts of alcohols. The sulfonic acid groups are tethered to the support using organic tethers. These tethers may help in preventing the deactivation of the active sites in the presence of water

Conversion of CO2 by non- thermal inductively-coupled plasma catalysis

CO2 decomposition is a very strongly endothermic reaction where very high temperatures are required to thermally dissociate CO2. Radio frequency inductively-coupled plasma enables to selectively activate and dissociate CO2 at room temperature. Tuning the flow rate and the frequency of the radio frequency inductively-coupled plasma gives high yields of CO under mild conditions. Finally the discovery of a plasma catalytic effect has been demonstrated for CO2 dissociation that shows a significant increase of the CO yield by metallic meshes. The metallic meshes become catalysts under exposure to plasma to activate the recombination reaction of atomic O to yield O2, thereby reducing the reaction to convert CO back to CO2. Inductively-coupled hybrid plasma catalysis allows access to study and to utilize high CO2 conversion in a non-thermal plasma regime. This advance offers opportunities to investigate the possibility to use radio frequency inductively-coupled plasma to store superfluous renewable electricity into high-valuable CO in time where the price of renewable electricity is plunging.</p

Dry Reforming of Methane under Mild Conditions Using Radio Frequency Plasma

Dry reforming of methane (DRM) is a challenging process wherein methane reacts with CO2 to give syngas. This reaction is strongly endothermic, typically requiring temperatures higher than 500 °C. Catalysts can be used, but the high temperatures (which are a thermodynamic requirement) often lead to catalyst deactivation. Herein, the reaction from another conceptual direction is approached, using low‐power radio frequency inductively coupled plasma (RF‐ICP). It is demonstrated that this system can give high conversions of methane and CO2 at near‐ambient temperatures. Importantly, the energy costs in this system are considerably lower compared with other plasma‐driven DRM processes. Furthermore, it is shown that the yield of hydrogen can be increased by minimizing the C2 compound formation. The factors that govern the DRM process and discuss Hα emission and its influence on H atom recycling in the process are examined.</p

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

CO2 Hydrogenation at Atmospheric Pressure and Low Temperature Using Plasma-Enhanced Catalysis over Supported Cobalt Oxide Catalysts

CO2 is a promising renewable, cheap, and abundant C1 feedstock for producing valuable chemicals, such as CO and methanol. In conventional reactors, because of thermodynamic constraints, converting CO2 to methanol requires high temperature and pressure, typically 250 °C and 20 bar. Nonthermal plasma is a better option, as it can convert CO2 at near-ambient temperature and pressure. Adding a catalyst to such plasma setups can enhance conversion and selectivity. However, we know little about the effects of catalysts in such systems. Here, we study CO2 hydrogenation in a dielectric barrier discharge plasma-catalysis setup under ambient conditions using MgO, γ-Al2O3, and a series of CoxOy/MgO catalysts. While all three catalyst types enhanced CO2 conversion, CoxOy/MgO gave the best results, converting up to 35% of CO2 and reaching the highest methanol yield (10%). Control experiments showed that the basic MgO support is more active than the acidic γ-Al2O3, and that MgO-supported cobalt oxide catalysts improve the selectivity toward methanol. The methanol yield can be tuned by changing the metal loading. Overall, our study shows the utility of plasma catalysis for CO2 conversion under mild conditions, with the potential to reduce the energy footprint of CO2-recycling processes

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)