Mie Resonant Metal Oxide Nanospheres for Broadband Photocatalytic Enhancements

Metal oxides are widely used in heterogeneous catalysis as supports to disperse catalytically active nanoparticles, isolated atomic sites, or even as catalysts themselves. Herein, we present a method to produce optically active metal oxide supports that exhibit size-dependent Mie resonances based on TiO2 nanospheres with tunable size, crystalline phase composition, and optical properties. Mie resonant TiO2 nanospheres were used as supports to disperse Au, Pt, and Pd nanoparticles. We have found up to a 50-fold enhancement of the electric field at the metal oxide/metal interface corresponding to wavelength-dependent multipolar resonances in the TiO2 structure. Using Au/TiO2 as a prototypical photocatalyst, we demonstrate broadband rate enhancements between 400 and 800 nm during CO oxidation, with a noticeable increase below 500 nm. This increased reactivity at higher photon energies is due to improved photon utilization and interband absorption in the gold that results in greater secondary electron generation through electron-electron scattering processes, thus leading to higher rates in conjunction with the Mie scattering TiO2 support. This study not only highlights the potential of Mie resonant TiO2 in broadband photocatalytic enhancements but also for developing various Mie resonant metal oxide supports, such as ZnO or Cu2O, which can improve photocatalytic performance for a number of critical reactions. As the chemical and energy industries move toward conversion technologies driven by renewable energy sources, the strategy of designing optical resonances into oxide supports that are already widely used could enable a straightforward adaptation of photochemical processing based on traditional heterogeneous catalysts.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO₂ catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO₂ support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO₂. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO₂, which can be crucial for supported metal catalysts in biomass conversion.

Subsurface oxygen defects electronically interacting with active sites on In2O3 for enhanced photothermocatalytic CO2 reduction

Oxygen defects play an important role in many catalytic reactions. Increasing surface oxygen defects can be done through reduction treatment. However, excessive reduction blocks electron channels and deactivates the catalyst surface due to electron-trapped effects by subsurface oxygen defects. How to effectively extract electrons from subsurface oxygen defects which cannot directly interact with reactants is challenging and remains elusive. Here, we report a metallic In-embedded In2O3 nanoflake catalyst over which the turnover frequency of CO2 reduction into CO increases by a factor of 866 (7615 h-1) and 376 (2990 h-1) at the same light intensity and reaction temperature, respectively, compared to In2O3. Under electron-delocalization effect of O-In-(O)Vo-In-In structural units at the interface, the electrons in the subsurface oxygen defects are extracted and gather at surface active sites. This improves the electronic coupling with CO2 and stabilizes intermediate. The study opens up new insights for exquisite electronic manipulation of oxygen defects.

Nonoxidative coupling of ethane with gold loaded photocatalysts

Direct and continuous conversion of ethane to yield n-butane and hydrogen at near room temperature (ca. 320 K) was examined with gold loaded gallium oxide and titanium dioxide photocatalysts without the aid of any oxidant in a flow reactor.

Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts

In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO₂ hydrogenation, C–C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

Adsorption of oxygen and CO oxidation on Au/anatase(001) catalysts. A DFT+U study

Active oxygen for CO oxidation over Au/TiO₂ is a highly stable atomic species, which is formed at the perimeter of the Au/oxide interface.

High activity and negative apparent activation energy in low-temperature CO oxidation – present on Au/Mg(OH)2, absent on Au/TiO2

Aiming at a better understanding of the unusual low-temperature CO oxidation reaction behavior on Au/Mg(OH)₂ catalysts, we investigated this reaction mainly by combined kinetic and in situ IR spectroscopy measurements over a wide range of temperatures, from −90 °C to 200 °C.