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In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (O v ) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO 2 , the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the O v in the HEO is highlighted for the ammonia production with particular emphasis on the N 2 dissociation step (N 2(ads) → N ads ) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18 O 2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the O v at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce-O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the O v formation in bulk, in medium (Raman), and in short (localized) order (EPR); more O v population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru-O-Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10-50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO 2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the O v population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the O v site adjacent to the adsorption event.

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO 2 is involved. Thus, it is important to understand how to tailor their CO 2 adsorption behavior. In this work, steering the CO 2 activation behavior of Ce-La-Cu-O ternary oxide surfaces through the combined effect of chemical and mechanical strain was thoroughly examined using both experimental and ab initio modeling approaches. Doping with aliovalent metal cations (La 3+ or La 3+ /Cu 2+ ) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO 2 , respectively. Experimentally, microwave-assisted reflux-prepared Ce-La-Cu-O ternary oxides were imposed into mechanical forces to tune the structure, redox ability, defects, and CO 2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (ε C ) and mechanical strain (ε M ) on the modification of the Ce-La-Cu-O surface reactivity toward CO 2 activation. During the ab initio calculations, the stability (energy of formation, E O v f ) of different configurations of oxygen vacant sites (O v ) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO 2 -philicity of the surface was assessed at different levels of the imposed mechanical strain. The E O v f values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest E O v f values for the single subsurface sites, implying that O v may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the O v population was measured using 16 O/ 18 O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of 16 O 18 O formation, R max ( 16 O 18 O), was 13.1 and 8.5 μmol g -1 s -1 for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO 2 activation pathway (redox vs associative) was experimentally probed using in situ diffuse reflectance infrared Fourier transform spectroscopy. It was demonstrated that the mechanical strain increased up to 6 times the CO 2 adsorption sites, though reducing their thermal stability. This result supports the mechanical actuation of the "carbonate"-bound species; the latter was in agreement with the density functional theory (DFT)-calculated C-O bond lengths and O-C-O angles. Ab initio studies shed light on the CO 2 adsorption energy ( E ads ), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge analysis probed the adsorbate/surface charge distribution and illustrated that CO 2 interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. Density of states (DOS) studies revealed a significant E g drop in the presence of double O v and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepared using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal-support interface and Ni catalyst reducibility, thus leading to an increase in the CH 4 and CO 2 conversions. This study opens new possibilities to manipulate the CO 2 activation for a portfolio of heterogeneous reactions.

The optimization of MgO-based adsorbents as advanced CO 2 -capture materials is predominantly focused on their molten-salt modification, for which theoretical and experimental contributions provide great insights for their high CO 2 -capture performance. The underlying mechanism of the promotion effect of the molten salt on CO 2 capture, however, is a topic of controversy. Herein, advanced experimental characterization techniques, including in situ environmental transmission electron microscopy (eTEM) and CO 2 chemisorption by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), transient 18 O-isotopic exchange, and density functional theory (DFT), are employed to elucidate the mechanism of the CO 2 interaction with molten-salt-modified MgO in the 250-400 °C range. Herein, eTEM studies using low (2-3 mbar) and high (700 mbar) CO 2 pressures illustrate the dynamic evolution of the molten NaNO 3 salt promoted and unpromoted MgO carbonation with high magnification (<50 nm). The formation of 18 O-NaNO 3 (use of 18 O 2 ) and C 16 O 18 O following CO 2 interaction, verifies the proposed reaction path: conversion of NO 3 - (NO 3 -  → NO 2 +  + O 2- ), adsorption of NO 2 + on MgO with significant weakening of CO 2 adsorption strength, and formation of [Mg 2+ … O 2- ] ion pairs preventing the development of an impermeable MgCO 3 shell, which largely increases the rate of bulk MgO carbonation., (© 2021 Wiley-VCH GmbH.)

CO elimination through oxidation over highly active and cost-effective catalysts is a way forward for many processes of industrial and environmental importance. In this study, doped CeO 2 with transition metals (TM = Cu, Co, Mn, Fe, Ni, Zr, and Zn) at a level of 20 at. % was tested for CO oxidation. The oxides were prepared using microwave-assisted sol-gel synthesis to improve catalyst's performance for the reaction of interest. The effect of heteroatoms on the physicochemical properties (structure, morphology, porosity, and reducibility) of the binary oxides M-Ce-O was meticulously investigated and correlated to their CO oxidation activity. It was found that the catalytic activity (per gram basis or TOF, s -1 ) follows the order Cu-Ce-O > Ce-Co-O > Ni-Ce-O > Mn-Ce-O > Fe-Ce-O > Ce-Zn-O > CeO 2 . Participation of mobile lattice oxygen species in the CO/O 2 reaction does occur, the extent of which is heteroatom-dependent. For that, state-of-the-art transient isotopic 18 O-labeled experiments involving 16 O/ 18 O exchange followed by step-gas CO/Ar or CO/O 2 /Ar switches were used to quantify the contribution of lattice oxygen to the reaction. SSITKA-DRIFTS studies probed the formation of carbonates while validating the Mars-van Krevelen (MvK) mechanism. Scanning transmission electron microscopy-high-angle annular dark field imaging coupled with energy-dispersive spectroscopy proved that the elemental composition of dopants in the individual nanoparticle of ceria is less than their composition at a larger scale, allowing the assessment of the doping efficacy. Despite the similar structural features of the catalysts, a clear difference in the O lattice mobility was also found as well as its participation (as expressed with the α descriptor) in the reaction, following the order α Cu > α Co > α Mn > α Zn . Kinetic studies showed that it is rather the pre-exponential (entropic) factor and not the lowering of activation energy that justifies the order of activity of the solids. DFT calculations showed that the adsorption of CO on the Cu-doped CeO 2 surface is more favorable (-16.63 eV), followed by Co, Mn, Zn (-14.46, -4.90, and -4.24 eV, respectively), and pure CeO 2 (-0.63 eV). Also, copper compensates almost three times more charge (0.37 e - ) compared to Co and Mn, ca. 0.13 e - and 0.10 e - , respectively, corroborating for its tendency to be reduced. Surface analysis (X-ray photoelectron spectroscopy), apart from the oxidation state of the elements, revealed a heteroatom-ceria surface interaction (O a species) of different extents and of different populations of O a species.

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO 2 associated with Mg 2+ -O 2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO 2 molecular path in MgO-based CO 2 adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO 2 on the surface of MgO exhibited the highest CO 2 uptake (15.7 mmol g -1 ) at 350 °C compared to less than 0.1 mmol g -1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO 2 at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO 3 formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO 2 partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO 2 -capturing performance gained and its considerable significance for future practical designs of precombustion CO 2 capture.

The emerging need for sustainable management of the increasing quantities of urban and industrial organic wastes creates opportunities for the development of alternative strategies for the improvement of degraded soils. The current study was performed to examine the effects of agricultural wastes application on soil bacterial community as well as CO 2 and N 2 O direct gas emissions. Untreated soils were compared with soils, which received the same amount of N (100 μg/g soil) in the form of ammonium nitrate and organic agricultural waste. In particular, soils were incubated with three different organic agricultural wastes, orange (OP), mandarin (MP) and banana peels (BP) and ammonium nitrate (F) after adjusting soil water at 70% of its holding capacity. In the current study, soil chemical characteristics, quantitative PCR of denitrifiers (nirK, nirS, nosZI and nosZII) and16s rRNA amplicon sequencing were assessed to examine the links between the soil microbial communities and short-term soil direct N 2 O emissions when treated with agricultural wastes. The highest soil direct N 2 O emissions were recorded in soils received ammonium nitrate while soils received agricultural wastes exhibited substantially lower soil direct N 2 O emissions. On the contrary, agricultural wastes stimulated CO 2 accumulation as well as the growth of copiotrophic bacterial groups like Proteobacteria and Firmicutes. Interestingly, direct soil N 2 O emissions were decoupled from the density of denitrifier community while agricultural wastes caused a substantial reduction of the relative abundance of bacterial taxa associated with N 2 O emissions in the soil. This study proves evidence that agricultural wastes could be integrated in a waste management strategy, which inter alia includes their direct use in agricultural ecosystems resulting in reduced N 2 O emissions., (Copyright © 2019 Elsevier Ltd. All rights reserved.)