Selective phase transformation of layered double hydroxides into mixed metal oxides for catalytic CO oxidation

Phase transformation from layered double hydroxides (LDHs) into mixed metal oxides (MMOs) has been widely used in various catalytic applications owing to its numerous advantages over conventional synthesis methods. Herein we report the results of selective phase transformation of LDHs into spinels and delafossites for the preparation of phase-pure MMO catalysts. Pure cuprous delafossites and cupric spinels were selectively obtained through heat treatment of Cu-based LDHs followed by post-treatments. This enabled the study of the crystalline-phase-dependent CO oxidation activity of the MMO catalysts and their physicochemical properties. The spinel catalysts exhibited higher CO oxidation activities, in comparison with those of the delafossites, with greater redox properties and improved active sites for CO adsorption. Although the crystalline phases were derived from the same LDH precursors, the catalytic properties of the end product were greatly influenced by their crystal structures

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School of Energy and Chemical Engineering (Chemical Engineering)ope

Boosting Support Reducibility and Metal Dispersion By Exposed Surface Atom Control for Highly Active Supported Metal Catalysts.

The synthetic strategy of facet control combined with metal doping in a well-defined structure is introduced for the first time to control the catalytic activity. This research will contribute to improving the catalytic activity through understanding of the relationship between the shape of oxide and the doped and loaded metals

Catalytic CO Oxidation over Au Nanoparticles Supported on CeO2 Nanocrystals: Effect of the Au-CeO2 Interface

Gold nanoparticles (NPs) have attracted attention due to their superior catalytic performance in CO oxidation at low temperatures. Along with the size and shape of Au NPs, the catalytic function of Au-catalyzed CO oxidation can be further optimized by controlling the physicochemical properties of oxide-supporting materials. We applied a combinatorial approach of experimental analyses and theoretical interpretations to study the effect of a surface structure of supporting oxides and the corresponding CO oxidation activity of supported Au NPs. We synthesized Au NPs (average d ≈ 3 nm) supported on shape-controlled CeO2 nanocrystals, Au/CeO2 cubes, and Au/CeO2 octahedra for experimental analyses. The catalysts were modeled as Au/CeO2(100) and Au/CeO2(111) via density functional theory (DFT) calculations. The DFT calculations showed that the O-C-O type reaction intermediate could be spontaneously formed at the Au-CeO2(100) interface upon sequential multi-CO adsorption, accelerating CO oxidation via the Mars-van Krevelen mechanism. The additional kinetic process required for O-C-O formation at the Au-CeO2(111) interface slowed down the reaction. The experimental turnover frequency (TOF) of the Au/CeO2 cubes was 4 times greater than that of the Au/CeO2 octahedra (under 0.05 bar CO and 0.13 bar O2). The increasing TOF as a function of CO partial pressure and the positive correlation between the reducibility of CeO2 and the catalytic activity of Au/CeO2 catalysts confirmed the theoretical prediction that CO molecules occupy the surface of Au NPs and that the oxidation of Au-bound CO occurs at the Au-CeO2 interface. Through a comparative study of DFT calculations and in-depth experimental analyses, we provide insights into the catalytic function of CeO2-supported Au NPs toward CO oxidation depending on the shape of CeO2 and ratio of CO/O2

Versatile Layered Hydroxide Precursors for Generic Synthesis of Cu-Based Materials

The ability to mix multiple elements in a structure is crucial for obtaining Cu-based nanostructures and microstructures with desirable physicochemical properties. Precursors containing multiple metal cations, such as layered double hydroxides, have been used for the synthesis of multielement materials. However, these precursors experience difficulty containing large cations, which limits the functionalities of the derived materials. Herein, the development of Cu-based hydroxy double salts (HDSs), versatile precursors that accommodate a broad range of metal cations with homogeneous distributions, is reported. Up to 25 different metal cations are mixed with Cu in an HDS single phase, individually and simultaneously. The HDSs further exhibit useful properties with respect to anion exchange, exfoliation, and phase transformation to metal oxides. During the metal oxide transformation process, the formed crystal structures are mainly dependent on the ionic radius of the secondary metal cations. To prove their utility as precursors, the metal oxides derived from the HDSs are tested and found that they exhibited high catalytic activities for CO oxidation. This study significantly expands the compositional and structural freedom of Cu-based multi-elemental materials

Specific Metal-Support Interactions between Nanoparticle Layers for Catalysts with Enhanced Methanol Oxidation Activity

Oxide supports often play a critical role in metal-supported catalysts owing to their charge transfer phenomena that can alter catalytic performance. Herein, in place of conventional bulk oxide supports, monodisperse oxide nanoparticles (NPs) were exploited as supports for Pt catalysts. Depending on the type of oxide NP, Pt/oxide layered catalysts exhibited dramatic changes in the catalytic activity and selectivity for methanol oxidation. While Pt NPs deposited on MnO, Fe3O4, Co3O4, Cu2O, and ZnO NPs had comparable turnover frequencies to that of the pure Pt NP catalyst, Pt deposited TiO2 NPs changed the reaction rate significantly, with preferential selectivity observed toward partial oxidation products. Facet-specific interactions between Pt and TiO2 NPs were demonstrated by density functional theory calculations and catalytic reactions using shape-controlled TiO2 NPs. When Pt NPs were attached to spherical and rhombic TiO2 NPs with abundant (001) surfaces, methanol conversion was enhanced 10-fold owing to strong charge transfer from TiO2 to Pt

Enhanced hot electron generation by inverse metal???oxide interfaces on catalytic nanodiode

Identifying the electronic behavior of metal???oxide interfaces is essential for understanding the origin of catalytic properties and for engineering catalyst structures with the desired reactivity. For a mechanistic understanding of hot electron dynamics at inverse oxide/metal interfaces, we employed a new catalytic nanodiode by combining Co3O4 nanocubes (NCs) with a Pt/TiO2 nanodiode that exhibits nanoscale metal???oxide interfaces. We show that the chemicurrent, which is well correlated with the catalytic activity, is enhanced at the inverse oxide/metal (CoO/Pt) interfaces during H2 oxidation. Based on quantitative visualization of the electronic transfer efficiency with chemicurrent yield, we show that electronic perturbation of oxide/metal interfacial sites not only promotes the generation of hot electrons, but improves catalytic activity

Structural evolution of ZIF-67-derived catalysts for furfural hydrogenation

Zeolitic imidazolate framework-67 (ZIF-67) can be converted to metallic Co nanoparticles supported on N-doped carbon (Co/NC) through reduction. However, its unique properties, including extremely high surface area, isoreticular pore structure, and regular metal???organic network, disappear after high-temperature (>500 ??C) reduction. Aggregated CoOx particles reduce the number of surface-active sites, resulting in poor catalytic activity. If the original ZIF-67 structure is maintained after the high-temperature reduction, promoting the uniform distribution of active sites in the porous carbon, the catalytic performance can be further improved. Herein, the correlation between the catalytic furfural hydrogenation performance, Co/NC morphology, and oxidation state of Co was investigated as a function of the H2 reduction temperature and time. The reduction of ZIF-67 at 400 ??C for 6 h yields a highly dispersed Co/NC catalyst, while preserving the overall morphology. The resulting Co/NC-400-6 catalyst exhibits the highest activity, promoting high selectivity toward 2-methylfuran. The product selectivity can be further altered by incorporating Cu into ZIF-67 to produce furfuryl alcohol. With proper H2 treatment to minimize the damage to the intrinsic surface area and pore structure, metal???organic frameworks can be utilized as high-performance heterogeneous catalysts by maximizing the distribution of active sites

Influence of the Pt size and CeO2 morphology at the Pt-CeO2 interface in CO oxidation

Understanding the inherent catalytic nature of the interface between metal nanoparticles (NPs) and oxide supports enables the rational design of metal-support interactions for high catalytic performance. Electronic interactions at the metal-oxide interface create active interfacial sites that produce distinctive catalytic functions. However, because the overall catalytic properties of the interface are influenced by several complex structural factors, it is difficult to express the catalytic activity induced by the interfacial site through a simple descriptor. Based on a combinatorial study of density functional theory calculations and catalytic experiments, we focus on two structural design factors of metal NP-supported oxide catalysts: the size of Pt NPs and the morphology of the CeO2 support. Pt NPs with sizes of 1, 2, and 3 nm were supported on the surface of CeO2-cubic ({100} facet) and -octahedral ({111} facet) nanocrystals. During catalytic CO oxidation, the activity of the Pt/CeO2-cube was higher than that of the Pt/CeO2-octahedron, regardless of the size of the NPs. Although 1 nm Pt NPs donate a similar number of electrons per Pt atom to CeO2-cubes and CeO2-octahedra, the inherently low oxygen vacancy formation energy of the CeO2(100) surface leads to the higher catalytic activity of the Pt-CeO2-cube interface. However, the intrinsic catalytic activity of the interface between Pt NPs and two CeO2 nanocrystals converges as the size of Pt NPs increases. Because large Pt NPs interact more strongly with CeO2(100) than CeO2(111), the positive effect of the low vacancy formation energy of CeO2(100) is compensated by the strengthened Pt-O interaction. This study elucidates how the interfaces formed between the shape-controlled CeO2 and the size-controlled Pt NPs affect the resultant catalytic activity