CO2 adsorption on the pristine and reduced CeO2 (111) surface: Geometries and vibrational spectra by first principles simulations

Creating CoRu Dual Active Sites Codecorated Stable Porous Ceria Support for Enhanced Li-CO2 Batteries Cathodes

Lithium-carbon dioxide (Li-CO2) battery represents a high-energy density energy storage with excellent real-time CO2 enrichment and conversion, but its practical utilization is hampered by the development of an excellent catalytic cathode. Here, the synergistic catalytic strategy of designing CoRu bimetallic active sites achieves the electrocatalytic conversion of CO2 and the efficient decomposition of the discharge products, which in turn realizes the smooth operation of the Li-CO2 battery. Moreover, obtained support based on metal-organic frameworks precursors facilitates the convenient diffusion and adsorption of CO2, resulting in higher reaction concentration and lower mass transfer resistance. Meanwhile, the optimization of the interfacial electronic structure and the effective transfer of electrons are achieved by virtue of the strong interaction of CoRu at the support interface. As a result, the Li-CO2 cell assembled based on bimetallic CoRu active sites achieved a discharge capacity of 19,111 mA h g-1 and a steady-state discharge voltage of 2.58 V as well as a cycle life of >175 cycles at a rate of 100 mA g-1. Further experiments combined with density-functional theory calculations achieve a deeply view of the connection between cathode and electrochemical performance and pave a way for the subsequent development of advanced Li-CO2 catalytic cathodes.

Surface Chemistry of Biologically Active Reducible Oxide Nanozymes

Reducible metal oxide nanozymes (rNZs) are a subject of intense recent interest due to their catalytic nature, ease of synthesis, and complex surface character. Such materials contain surface sites which facilitate enzyme-mimetic reactions via substrate coordination and redox cycling. Further, these surface reactive sites are shown to be highly sensitive to stresses within the nanomaterial lattice, the physicochemical environment, and to processing conditions occurring as part of their syntheses. When administered in vivo, a complex protein corona binds to the surface, redefining its biological identity and subsequent interactions within the biological system. Catalytic activities of rNZs each deliver a differing impact on protein corona formation, its composition, and in turn, their recognition, and internalization by host cells. Improving the understanding of the precise principles that dominate rNZ surface-biomolecule adsorption raises the question of whether designer rNZs can be engineered to prevent corona formation, or indeed to produce "custom" protein coronas applied either in vitro, and preadministration, or formed immediately upon their exposure to body fluids. Here, fundamental surface chemistry processes and their implications in rNZ material performance are considered. In particular, material structures which inform component adsorption from the application environment, including substrates for enzyme-mimetic reactions are discussed.

Decoupling the Chemical and Mechanical Strain Effect on Steering the CO2 Activation over CeO2-Based Oxides: An Experimental and DFT Approach

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO2 is involved. Thus, it is important to understand how to tailor their CO2 adsorption behavior. In this work, steering the CO2 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 (La3+ or La3+/Cu2+) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO2, 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 CO2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (εC) and mechanical strain (εM) on the modification of the Ce-La-Cu-O surface reactivity toward CO2 activation. During the ab initio calculations, the stability (energy of formation, EOvf) of different configurations of oxygen vacant sites (Ov) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO2-philicity of the surface was assessed at different levels of the imposed mechanical strain. The EOvf values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest EOvf values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov population was measured using 16O/18O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of 16O18O formation, Rmax(16O18O), 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 CO2 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 CO2 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 CO2 adsorption energy (Eads), 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 CO2 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 Eg drop in the presence of double Ov 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 CH4 and CO2 conversions. This study opens new possibilities to manipulate the CO2 activation for a portfolio of heterogeneous reactions.

Can oxygen vacancies in ceria surfaces be measured by O1s photoemission spectroscopy?

X-ray photoemission spectroscopy is a standard technique for materials characterization and the O 1s binding energy is commonly measured for oxides. Here we use density functional theory calculations to investigate how the O 1s binding energy in CeO₂(111) is influenced by the presence of oxygen vacancies. The case with point vacancies in CeO₂(111) is compared to complete reduction to Ce₂O₃. Reduction of CeO₂by oxygen vacancies is found to have a minor effect on the O 1s binding energy. The O 1s binding energy is instead clearly changed when the character of the chemical bond for the considered oxygen atom is modified by, for example, the formation of OH-groups or carbonates.

A theoretical study of the electrochemical reduction of CO2 on cerium dioxide supported palladium single atoms and nanoparticles

Pd/CeO₂ catalysts show superior catalytic performance owing to their optimal cycling activity and stability. In this study, single-atom Pd and eight-atom Pd nanoparticle clusters were supported on the surface of CeO₂(110) to investigate the effect of loaded-metal size on the catalytic performance of the Pd-CeO₂ system for CO₂ reduction. We investigated the CO₂ reduction reaction (CRR) that produces C₁ products (CO, HCOOH, CH₃OH, and CH₄) on Pd₈/CeO₂ and Pd/CeO₂ by density functional theory. The structures, CO₂ adsorption configurations, and CO₂ reduction mechanisms of these two electrocatalysts were systematically studied. Subsequently, different reduction pathways on Pd₈/CeO₂ and Pd/CeO₂ were investigated to identify the optimal reaction pathway for further assessment. The results showed that both of these catalysts are more selective towards the production of CH₃OH than CH₄. Moreover, compared to Pd/CeO₂ and Pd₄/CeO₂ (from a previously reported study) the production of CH₃OH via the CRR on Pd₈/CeO₂ exhibited the lowest limiting potential. These results demonstrate the superiority of Pd₈/CeO₂ as an electrocatalyst for the electrochemical reduction of CO₂ to CH₃OH.