Liangfeng Luo

Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O2 to propylene oxide and acrolein is of great interest and importance. We report the crystal-plane-controlled selectivity of uniform capping-ligand-free Cu2 O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O2 : Cu2 O octahedra exposing {111} crystal planes are most selective for acrolein; Cu2 O cubes exposing {100} crystal planes are most selective for CO2 ; Cu2 O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One-coordinated Cu on Cu2 O(111), three-coordinated O on Cu2 O(110), and two-coordinated O on Cu2 O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO2 , respectively. These results reveal that crystal-plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.

Catalytically active structures of SiO2-supported Au nanoparticles in low-temperature CO oxidation

Various Au/SiO2 catalysts have been prepared by the deposition–precipitation method followed by calcination in air or reduction in H2. The structures of supported Au nanoparticles were characterized in detail by XRD, TEM, XPS, in situ XANES and operando DRIFTS of CO chemisorption, and their catalytic activity in CO oxidation was evaluated. Calcined in air, the gold precursor decomposes into Au(I) species at low temperatures and further to Au(0) at elevated temperatures, forming supported Au nanoparticles mostly larger than 4.5 nm. Reduced in H2, the gold precursor can be facilely reduced to Au(0) at low temperatures, forming supported Au nanoparticles with different size distributions depending on the reduction temperature. Supported Au nanoparticles around 3–4.5 nm with both abundant low-coordinated Au atoms and bulk Au-like electronic structure effectively chemisorb CO and catalyze CO oxidation at room temperature (RT). Larger supported Au nanoparticles with bulk Au-like electronic structure but few low-coordinated Au atoms do not chemisorb CO and catalyze CO oxidation at RT, and finer supported Au nanoparticles with abundant low-coordinated Au atoms but bulk Au-unlike electronic structure also do not chemisorb CO and catalyze CO oxidation at RT. These results provide solid and comprehensive experimental evidence that supported Au nanoparticles with both abundant low-coordinated Au atoms and bulk Au-like electronic structure are the catalytic active structures for catalyzing CO oxidation at RT without the involvement of oxide supports. The density of low-coordinated Au atoms increases with the decrease of their size, but their electronic structure eventually deviates from bulk Au-like electronic structure; therefore, the catalytic activity of SiO2-supported Au nanoparticles in low-temperature CO oxidation inevitably exhibits a volcano-shaped dependence on their size with the optimum size between 3 and 4.5 nm.

TiO2/Cu2O Core/Ultrathin Shell Nanorods as Efficient and Stable Photocatalysts for Water Reduction

P-type Cu2 O has been long considered as an attractive photocatalyst for photocatalytic water reduction, but few successful examples has been reported. Here, we report the synthesis of TiO2 (core)/Cu2 O (ultrathin film shell) nanorods by a redox reaction between Cu(2+) and in-situ generated Ti(3+) when Cu(2+) -exchanged H-titanate nanotubes are calcined in air. Owing to the strong TiO2 -Cu2 O interfacial interaction, TiO2 (core)/Cu2 O (ultrathin film shell) nanorods are highly active and stable in photocatalytic water reduction. The TiO2 core and Cu2 O ultrathin film shell respectively act as the photosensitizer and cocatalyst, and both the photoexcited electrons in the conduction band and the holes in the valence band of TiO2 respectively transfer to the conduction band and valence band of the Cu2 O ultrathin film shell. Our results unambiguously show that Cu2 O itself can act as the highly active and stable cocatalyst for photocatalytic water reduction.

Methyl Radicals in Oxidative Coupling of Methane Directly Confirmed by Synchrotron VUV Photoionization Mass Spectroscopy

Gas-phase methyl radicals have been long proposed as the key intermediate in catalytic oxidative coupling of methane, but the direct experimental evidence still lacks. Here, employing synchrotron VUV photoionization mass spectroscopy, we have directly observed the formation of gas-phase methyl radicals during oxidative coupling of methane catalyzed by Li/MgO catalysts. The concentration of gas-phase methyl radicals correlates well with the yield of ethylene and ethane products. These results lead to an enhanced fundamental understanding of oxidative coupling of methane that will facilitate the exploration of new catalysts with improved performance.

Utilization of Active Ni to Fabricate Pt–Ni Nanoframe/NiAl Layered Double Hydroxide Multifunctional Catalyst through In Situ Precipitation

Integration of different active sites into metallic catalysts, which may impart new properties and functionalities, is desirable yet challenging. Herein, a novel dealloying strategy is demonstrated to decorate nickel-aluminum layered double hydroxide (NiAl-LDH) onto a Pt-Ni alloy surface. The incorporation of chemical etching of Pt-Ni alloy and in situ precipitation of LDH are studied by joint experimental and theoretical efforts. The initial Ni-rich Pt-Ni octahedra transform by interior erosion into Pt3 Ni nanoframes with enlarged surface areas. Furthermore, owing to the basic active sites of the decorated LDH together with the metallic sites of Pt3 Ni, the resulting Pt-Ni nanoframe/NiAl-LDH composites exhibit excellent catalytic activity and selectivity in the dehydrogenation of benzylamine and hydrogenation of furfural.

The most active Cu facet for low-temperature water gas shift reaction

Identification of the active site is important in developing rational design strategies for solid catalysts but is seriously blocked by their structural complexity. Here, we use uniform Cu nanocrystals synthesized by a morphology-preserved reduction of corresponding uniform Cu2O nanocrystals in order to identify the most active Cu facet for low-temperature water gas shift (WGS) reaction. Cu cubes enclosed with {100} facets are very active in catalyzing the WGS reaction up to 548 K while Cu octahedra enclosed with {111} facets are inactive. The Cu–Cu suboxide (CuxO, x ≥ 10) interface of Cu(100) surface is the active site on which all elementary surface reactions within the catalytic cycle proceed smoothly. However, the formate intermediate was found stable at the Cu–CuxO interface of Cu(111) surface with consequent accumulation and poisoning of the surface at low temperatures. Thereafter, Cu cubes-supported ZnO catalysts are successfully developed with extremely high activity in low-temperature WGS reaction.Nanocrystals display a variety of facets with different catalytic activity. Here the authors identify the most active facet of copper nanocrystals relevant to the low-temperature water gas shift reaction and further design zinc oxide-copper nanocubes with exceptionally high catalytic activity.

Finely Dispersed Au Nanoparticles on SiO2 Achieved by the C60 Additive and Their Catalytic Activity

We have investigated in detail the effect of buckminsterfullerene (C60) additive on the structure and catalytic activity of Au/SiO2 catalysts prepared by a routine deposition–precipitation method employing HAuCl4 as the gold precursor. The structures of various catalysts have been characterized by using N2 adsorption–desorption isotherms, powder X‐ray diffraction, X‐ray photoelectron spectroscopy, transmission electron microscopy, photoluminescence, and Raman spectroscopy. The C60 additive was found to greatly enhance the dispersion of Au nanoparticles supported on SiO2. Supported Au nanoparticles that are about 3–5 nm in size can be synthesized without difficulty on C60/SiO2, whereas those sized about 7–10 nm are usually acquired on bare SiO2. Strong Au–C60 interaction with the charge transfer from Au nanoparticles to C60 has been observed in Au/C60/SiO2 and proven to suppress the agglomeration of supported Au nanoparticles and enhance their dispersion on SiO2. The catalyst Au/C60/SiO2‐10 (Au:C60 molar ratio of 10) exhibits a much better catalytic performance for CO oxidation than Au/SiO2, but is not active for CO oxidation at room temperature, which demonstrates that the intrinsic activity of supported Au nanoparticles increases with decreasing particle size, but supported Au 3–5 nm nanoparticles cannot activate oxygen for CO oxidation at room temperature. We propose that 3 nm is the critical size for Au nanoparticles to exhibit an intrinsic catalytic activity in CO oxidation at room temperature without additional contributions. These results provide novel and important insights into the fundamental understanding of intrinsic structure–activity relation of Au nanoparticles.

A pulse chemisorption/reaction system for in situ and time-resolved DRIFTS studies of catalytic reactions on solid surfaces

A pulse chemisorption/reaction system in combination with Fourier transform infrared spectrometer equipped with a diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) reaction cell and online mass spectrometer is described in detail. Such a system provides an approach to effectively suppress the interference of the gas-phase reactants to the vibrational signals of surface adsorbates during the operando DRIFTS measurements and, thus, allows for in situ and real-time monitor of surface species on catalyst surfaces during chemisorption/reaction processes. Employing this system, we successfully acquired DRIFTS spectra that clearly demonstrate surface species formed by propylene chemisorption and reaction on octahedral Cu2O nanocrystals; we also observed simultaneous chemisorption of CO on top, twofold, and threefold bridged sites of Pd nanoparticles supported on SiO2 upon the collision of CO prior to the saturation of strongly bound sites and the transformation of weakly bound CO(a) into strongly bound CO(a) during the dynamic chemisorption-desorption processes.