The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure-activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co3O4 with rich oxygen vacancy (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-rich Co3O4 as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO-Co3O4 prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH• first for VO-Co3O4 and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co-OOH•. This work provides insight into the defect mechanism in Co3O4 for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.

Operando Identification of the Dynamic Behavior of Oxygen Vacancy-Rich Co3O4 for Oxygen Evolution Reaction.

The performance of catalysts used for the dry reforming of methane can strongly depend on the selection of active metals, supports and promoters. This work studies their effects on the activity and stability of selected catalysts. Designing an economically viable catalyst that maintains high catalytic activity and stability can be achieved by exploiting the synergic effects of combining noble and/or non-noble metals to form highly active and stable bi- and tri-metallic catalysts. Perovskite type catalysts can also constitute a potent and cost effective substituent. Metal oxide supports with surface Lewis base sites are able to reduce carbon formation and yield a greater stability to the catalyst, while noble metal promoters have proven to increase both catalyst activity and stability. Moreover, a successful metal-support-promoter combination should lead to higher metal-support interacrtion, lower reduction temperature and enhancement of the anti-coking and anti-amalgamation properties of the catalyst. However, the effect of each parameter on the overall performance of the catalyst is usually complex, and the catalyst designer is often faced with a tradeoff between activity, stability and ease of activation. Based on the review carried out on various studies, it is concluded that a catalyst design must take into consideration not only the separate effects of the active metal, support and promoter, but should also include the combined and mutual interactions of these components.

Catalyst design for dry reforming of methane: Analysis review

Dramatically increased CO2 concentration from several point sources is perceived to cause severe greenhouse effect towards the serious ongoing global warming with associated climate destabilization, inducing undesirable natural calamities, melting of glaciers, and extreme weather patterns. CO2 capture and utilization (CCU) has received tremendous attention due to its significant role in intensifying global warming. Considering the lack of a timely review on the state-of-the-art progress of promising CCU techniques, developing an appropriate and prompt summary of such advanced techniques with a comprehensive understanding is necessary. Thus, it is imperative to provide a timely review, given the fast growth of sophisticated CO2 capture and utilization materials and their implementation. In this work, we critically summarized and comprehensively reviewed the characteristics and performance of both liquid and solid CO2 adsorbents with possible schemes for the improvement of their CO2 capture ability and advances in CO2 utilization. Their industrial applications in pre- and post-combustion CO2 capture as well as utilization were systematically discussed and compared. With our great effort, this review would be of significant importance for academic researchers for obtaining an overall understanding of the current developments and future trends of CCU. This work is bound to benefit researchers in fields relating to CCU and facilitate the progress of significant breakthroughs in both fundamental research and commercial applications to deliver perspective views for future scientific and industrial advances in CCU.

Industrial carbon dioxide capture and utilization: state of the art and future challenges

Catalytic reforming of methane (CH4) with carbon dioxide (CO2), known as dry reforming of methane (DRM), produces synthesis gas, which is a mixture of hydrogen (H2) and carbon monoxide (CO). CH4 + CO2 → 2CO + 2H2, ΔH° = 247.3 kJ mol−1, ΔG = 61 770–67.32T. The DRM process has gained much attention recently as it reduces greenhouse gases (GHG), CO2 and CH4, in the atmosphere. In addition to reducing GHG, the DRM process produces valuable chemicals (CO + H2), provides a good approach to utilizing biogas and natural gas with a significant amount of CO2, has good capability as a chemical energy transmission system as compared to steam reforming, and finally yields the desired unity H2/CO ratio for Fischer–Tropsch synthesis. The bimetallic Ni-based catalysts supported on Al2O3/TiO2 and promoted with Ce/ZrO2 show remarkable performances in the DRM process. But, carbonaceous deactivation of the catalysts is the major problem faced during this process. Numerous studies have been cited on various aspects of DRM, and some papers are also devoted to reviewing carbonaceous deposition problems and their remedies. However, some lacunae exist, which are highlighted in the present review paper on strategies to reduce the carbonaceous deactivation of catalysts for improved DRM efficiency by appropriate catalyst development, operating conditions, and flow reactor designs. The disposal of spent catalysts falls under the category of hazardous industrial materials and is also required to comply with stringent environmental regulations. Therefore, regeneration and reclamation techniques for spent catalysts have also been discussed.

An overview on dry reforming of methane: strategies to reduce carbonaceous deactivation of catalysts

Chemical looping offers a versatile platform to convert fuels and oxidizers in a clean and efficient manner. Central to this technology are metal oxide materials that can oxidize fuels, affording a reduced material that can be reoxidized to close the loop. Recent years have seen substantial advances in the design, formulation and manufacture of these oxygen carrier materials and their incorporation into chemical looping reactors for the production of various chemicals. This Review describes the mechanisms by which oxygen carriers undergo redox reactions and how these carriers can be incorporated into robust chemical looping reactors. One promising technology for modern energy and chemical conversions is chemical looping, central to which are redox cycles of metal oxides. This Review describes chemical looping schemes and the mechanisms by which metal oxide particles enable these technologies.

Metal oxide redox chemistry for chemical looping processes

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 demonstra...

Tailoring Lattice Oxygen Binding in Ruthenium Pyrochlores to Enhance Oxygen Evolution Activity

Calcium looping, a CO2 capture technique, may offer a mid-term if not near-term solution to mitigate climate change, triggered by the yet increasing anthropogenic CO2 emissions. A key requirement for the economic operation of calcium looping is the availability of highly effective CaO-based CO2 sorbents. Here we report a facile synthesis route that yields hollow, MgO-stabilized, CaO microspheres featuring highly porous multishelled morphologies. As a thermal stabilizer, MgO minimized the sintering-induced decay of the sorbents’ CO2 capacity and ensured a stable CO2 uptake over multiple operation cycles. Detailed electron microscopy-based analyses confirm a compositional homogeneity which is identified, together with the characteristics of its porous structure, as an essential feature to yield a high-performance sorbent. After 30 cycles of repeated CO2 capture and sorbent regeneration, the best performing material requires as little as 11 wt.% MgO for structural stabilization and exceeds the CO2 uptake of the limestone-derived reference material by ~500%.

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Optimization of the structural characteristics of CaO and its effective stabilization yield high-capacity CO 2 sorbents

CO2 capture and storage is a promising concept to reduce anthropogenic CO2 emissions. The most established technology for capturing CO2 relies on amine scrubbing that is, however, associated with high costs. Technoeconomic studies show that using CaO as a high-temperature CO2 sorbent can significantly reduce the costs of CO2 capture. A serious disadvantage of CaO derived from earth-abundant precursors, e.g., limestone, is the rapid, sintering-induced decay of its cyclic CO2 uptake. Here, a template-assisted hydrothermal approach to develop CaO-based sorbents exhibiting a very high and cyclically stable CO2 uptake is exploited. The morphological characteristics of these sorbents, i.e., a porous shell comprised of CaO nanoparticles coated by a thin layer of Al2O3 (<3 nm) containing a central void, ensure (i) minimal diffusion limitations, (ii) space to accompany the substantial volumetric changes during CO2 capture and release, and (iii) a minimal quantity of Al2O3 for structural stabilization, thus maximizing the fraction of CO2-capture-active CaO.

Muhammad Awais Naeem

Papers

Tailoring Lattice Oxygen Binding in Ruthenium Pyrochlores to Enhance Oxygen Evolution Activity

Optimization of the structural characteristics of CaO and its effective stabilization yield high-capacity CO 2 sorbents

Multishelled CaO Microspheres Stabilized by Atomic Layer Deposition of Al2O3 for Enhanced CO2 Capture Performance

Role of La2O3 as Promoter and Support in Ni/γ-Al2O3 Catalysts for Dry Reforming of Methane

Catalytic performance of CeO2 and ZrO2 supported Co catalysts for hydrogen production via dry reforming of methane