As a promising approach for carbon dioxide capture, chemical looping combustion has been extensively investigated for more than two decades. However, the chemical looping strategy can be and has been ...extended well beyond carbon capture. In fact, significant impacts on emission reduction, energy conservation, and value-creation can be anticipated from chemical looping beyond combustion (CLBC). This article aims to demonstrate the versatility and transformational benefits of CLBC. Specifically, we focus on the use of oxygen carriers or redox catalysts for chemical production – a $4 trillion industry that consumes 40.9 quadrillion BTU of energy. Compared to state-of-the-art chemical production technologies, we illustrate that chemical looping offers significant opportunities for process intensification and exergy loss minimization. In many cases, an order of magnitude reduction in energy consumption and CO 2 emission can be realized without the needs for carbon dioxide capture. In addition to providing various CLBC examples, this article elaborates on generalized design principles for CLBC, potential benefits and pitfalls, as well as redox catalyst selection, design, optimization, and redox reaction mechanism.
Two-dimensional (2D) carbides, nitrides, and carbonitrides known as MXenes are emerging materials with a wealth of useful applications. However, the range of metals capable of forming stable MXenes ...is limited mostly to early transition metals of groups 3–6, making the exploration of properties inherent to mid or late transition metal MXenes very challenging. To circumvent the inaccessibility of MXene phases derived from mid-to-late transition metals, we have developed a synthetic strategy that allows the incorporation of such transition metal sites into a host MXene matrix. Here, we report the structural characterization of a Mo2CT x :Co phase (where T x are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (β-Mo2C:Co) through a two-step synthesis: first an intercalation of gallium yielding Mo2Ga2C:Co followed by removal of Ga via HF treatment. Extended X-ray absorption fine structure (EXAFS) analysis confirms that Co atoms occupy Mo positions in the Mo2CT x lattice, providing isolated Co centers without any detectable formation of other cobalt-containing phases. The beneficial effect of cobalt substitution on the redox properties of Mo2CT x :Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo2CT x catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo2CT x :Co to the favorable binding of hydrogen on the oxygen terminated MXene surface that is strongly influenced by the substitution of Mo by Co in the Mo2CT x lattice. In addition to the remarkable HER activity, Mo2CT x :Co features excellent operational and structural stability, on par with the best performing non-noble metal-based HER catalysts. Overall, our work expands the compositional space of the MXene family by introducing a material with site-isolated cobalt centers embedded in the stable matrix of Mo2CT x . The synthetic approach presented here illustrates that tailoring the properties of MXenes for a specific application can be achieved via substitution of the host metal sites by mid or late transition metals.
Ruthenium pyrochlores, that is, oxides of composition A2Ru2O7−δ, have emerged recently as state-of-the-art catalysts for the oxygen evolution reaction (OER) in acidic conditions. Here, we demonstrate ...that the A-site substituent in yttrium ruthenium pyrochlores Y1.8M0.2Ru2O7−δ (M = Cu, Co, Ni, Fe, Y) controls the concentration of surface oxygen vacancies (VO) in these materials whereby an increased concentration of VO sites correlates with a superior OER activity. DFT calculations rationalize these experimental trends demonstrating that the higher OER activity and VO surface density originate from a weakened strength of the M–O bond, scaling with the formation enthalpy of the respective MO x phases and the coupling between the M d states and O 2p states. Our work introduces a novel catalyst with improved OER performance, Y1.8Cu0.2Ru2O7−δ, and provides general guidelines for the design of active electrocatalysts.
We report an operando examination of a model nanocrystalline In2O3 catalyst for methanol synthesis via CO2 hydrogenation (300 °C, 20 bar) by combining X-ray absorption spectroscopy (XAS), X-ray ...powder diffraction (XRD), and in situ transmission electron microscopy (TEM). Three distinct catalytic regimes are identified during CO2 hydrogenation: activation, stable performance, and deactivation. The structural evolution of In2O3 nanoparticles (NPs) with time on stream (TOS) followed by XANES-EXAFS-XRD associates the activation stage with a minor decrease of the In–O coordination number and a partial reduction of In2O3 due to the formation of oxygen vacancy sites (i.e., In2O3–x ). As the reaction proceeds, a reductive amorphization of In2O3 NPs takes place, characterized by decreasing In–O and In–In coordination numbers and intensities of the In2O3 Bragg peaks. A multivariate analysis of the XANES data confirms the formation of In2O3–x and, with TOS, metallic In. Notably, the appearance of molten In0 coincides with the onset of catalyst deactivation. This phase transition is also visualized by in situ TEM, acquired under reactive conditions at 800 mbar pressure. In situ TEM revealed an electron beam assisted transformation of In2O3 nanoparticles into a dynamic structure in which crystalline and amorphous phases coexist and continuously interconvert. The regeneration of the deactivated In0/In2O3–x catalyst by reoxidation was critically assessed revealing that the spent catalyst can be reoxidized only partially in a CO2 atmosphere or air yielding an average crystallite size of the resultant In2O3 that is approximately an order of magnitude larger than the initial one.
A key step in gene repression by Polycomb is trimethylation of histone H3 K27 by PCR2 to form H3K27me3. H3K27me3 provides a binding surface for PRC1. We show that monoubiquitination of histone H2A by ...PRC1-type complexes to form H2Aub creates a binding site for Jarid2-Aebp2-containing PRC2 and promotes H3K27 trimethylation on H2Aub nucleosomes. Jarid2, Aebp2 and H2Aub thus constitute components of a positive feedback loop establishing H3K27me3 chromatin domains.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, ...in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. {Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2) (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, operando X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition.
Abstract
Early transitional metal carbides are promising catalysts for hydrogenation of CO
2
. Here, a two-dimensional (2D) multilayered 2D-Mo
2
C material is prepared from Mo
2
C
T
x
of the MXene ...family. Surface termination groups
T
x
(O, OH, and F) are reductively de-functionalized in Mo
2
C
T
x
(500 °C, pure H
2
) avoiding the formation of a 3D carbide structure. CO
2
hydrogenation studies show that the activity and product selectivity (CO, CH
4
, C
2
–C
5
alkanes, methanol, and dimethyl ether) of Mo
2
C
T
x
and 2D-Mo
2
C are controlled by the surface coverage of
T
x
groups that are tunable by the H
2
pretreatment conditions. 2D-Mo
2
C contains no
T
x
groups and outperforms Mo
2
C
T
x
, β-Mo
2
C, or the industrial Cu-ZnO-Al
2
O
3
catalyst in CO
2
hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO
2
hydrogenation.
Prosocial behavior at school, such as helping and sharing, contributes to positive individual development, peer relations, and classroom climate. Students with intellectual disabilities (ID) may have ...difficulty to demonstrate prosocial behavior, but little is known about the levels of prosocial behavior and its predictors in this population. This study aims to describe the prosocial behavior of students with ID attending special needs schools and related individual (i.e., age, sex, and general functioning) and classroom level (i.e., classmates' mean prosocial behavior) predictors. School staff members assessed prosocial behavior of 1022 students with ID (69.5% boys; Mage = 11.34 years, SD = 3.73, range: 4-19 years) at the beginning and the end of a schoolyear. We found that students with ID on average demonstrated moderate levels of prosocial behavior, this was lower compared to norms of typically developing students. Correlations within each timepoint proved that prosocial behavior was more present in older students, girls, and students with higher general functioning. Using a longitudinal multilevel model, we found that, prosocial behavior increased more over the schoolyear in older students and in students with higher general functioning. Classmates' mean levels of prosocial behavior did not affect later individual prosocial behavior. We conclude that prosocial behavior in students with ID depends on several individual characteristics, but less on the levels of prosocial behaviors in their special needs classroom peer context.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Transition metal nanoparticles (NPs) are typically supported on oxides to ensure their stability, which may result in modification of the original NP catalyst reactivity. In a number of cases, this ...is related to the formation of NP/support interface sites that play a role in catalysis. The metal/support interface effect verified experimentally is commonly ascribed to stronger reactants adsorption or their facile activation on such sites compared to bare NPs, as indicated by DFT-derived potential energy surfaces (PESs). However, the relevance of specific reaction elementary steps to the overall reaction rate depends on the preferred reaction pathways at reaction conditions, which usually cannot be inferred based solely on PES. Hereby, we use a multiscale (DFT/microkinetic) modeling approach and experiments to investigate the reactivity of the Ni/Al2O3 interface toward water–gas shift (WGS) and dry reforming of methane (DRM), two key industrial reactions with common elementary steps and intermediates, but held at significantly different temperatures: 300 vs 650 °C, respectively. Our model shows that despite the more energetically favorable reaction pathways provided by the Ni/Al2O3 interface, such sites may or may not impact the overall reaction rate depending on reaction conditions: the metal/support interface provides the active site for WGS reaction, acting as a reservoir for oxygenated species, while all Ni surface atoms are active for DRM. This is in contrast to what PESs alone indicate. The different active site requirement for WGS and DRM is confirmed by the experimental evaluation of the activity of a series of Al2O3-supported Ni NP catalysts with different NP sizes (2–16 nm) toward both reactions.
Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO2 emissions. Solid materials capable of reversibly absorbing CO2 have been ...the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO2 absorption both at surfaces and within solid materials.
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