Open Access

Show Abstract ▼

Abstract: Supported bimetallic nanoparticles (BNPs) are promising catalysts, but study on their compositional and structural changes under reaction conditions remains a challenge. In this work, the structure of PtNi BNPs supported on UiO-67 metal-organic framework (MOF) catalyst (i.e., PtNi@UiO-67) was investigated by in situ by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The results showed differences in the reduction behaviour of Ni species in PtNi BNPs and monometallic Ni supported on UiO-67 catalysts (i.e., PtNi@UiO-67 and Ni@UiO-67), suggesting charge transfer between metallic Pt and Ni oxides in PtNi@UiO-67. Under CO oxidation conditions, Ni oxides segregated to the outer surface of the BNPs forming a thin layer of NiOx on top of the metallic Pt (i.e., a NiOx-on-Pt structure). This resulted in a core-shell structure which was confirmed by high-resolution scanning transmission electron microscopy (HR-STEM). Accordingly, the layer of NiOx on PtNi BNPs, which is stabilised by charge transfer from metallic Pt, was proposed as the possible active phase for CO oxidation, being responsible for the enhanced catalytic activity observed in the bimetallic PtNi@UiO-67 catalyst.

Open Access

Show Abstract ▼

Abstract: Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O2) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O2]3− site selectively adsorbs molecular O2, forming a rarely reported electrophilic η2-O2 species at 298 K. Assisted by neighbouring Ce(III) cations, η2-O2 is finally reduced to two O2−, that create two Cu–O–Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s−1 at 373 K and 0.01 bar PCO. The unique electronic structure of [Cu(I)O2]3− site suggests its potential in selective oxidation.

Show Abstract ▼

Abstract: Herein, we report a facile route to synthesize isolated single iron atoms on nitrogen-sulfur-codoped carbon matrix via a direct pyrolysis process in which hemoglobin, a by-product of the meat industry, was utilized as a precursor for iron, nitrogen and sulfur while bamboo-shaped carbon nanotubes served as a support owing to their excellent conductivity and numerous defects. The resulting metal-nitrogen complexed carbon showed outstanding catalytic performance for the oxygen evolution reaction (OER) in alkaline solutions. At an overpotential of 380 mV, the optimal sample yielded a current density of 83.6 mA cm−2, which is 2.5 times that of benchmark IrO2 (32.8 mA cm−2), rendering it as one of the best OER catalysts reported so far. It also showed negligible activity decay in alkaline solutions during long-term durability tests. Control experiments and X-ray absorption fine structure analyses revealed that Fe-Nx species in the samples are the active sites for OER. Further density functional theory calculations indicated that the presence of sulfur in the carbon matrix modified the electronic structures of active species, thereby leading to the superior activity of the sample.

Show Abstract ▼

Abstract: Doped-ZnO nanoparticles, capped with dioctylphosphinate ligands, are synthesised by the controlled hydrolysis of a mixture of organometallic precursors. Substitutional doping of the wurtzite ZnO nanoparticles with 5 mol% Mg(II), Al(III) and Cu(I) is achieved by the addition of sub-stoichiometric amounts of the appropriate dopant [(n-butyl)(sec-butyl)magnesium, triethylaluminium or mesitylcopper] to diethylzinc in the precursor mixture. After hydrolysis, the resulting colloidal nanoparticles (sizes of 2–3 nm) are characterised by powder X-ray crystallography, transmission electron microscopy, inductively-coupled plasma optical emission spectrometry and X-ray photoelectron spectroscopy. A solution of the doped-ZnO nanoparticles and colloidal Cu(0) nanoparticles [M:ZnO : Cu = 1 : 1] are applied as catalysts for the hydrogenation of CO2 to methanol in a liquid-phase continuous flow stirred tank reactor [210 °C, 50 bar, CO2[thin space (1/6-em)]:[thin space (1/6-em)]H2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]3, 150 mL min−1, mesitylene, 20 h]. All the catalyst systems display higher rates of methanol production and better stability than a benchmark heterogeneous catalyst, Cu–ZnO–Al2O3 [480 μmol mmolmetal−1 h−1], with approximately twice the activity for the Al(III)-doped nanocatalyst. Despite outperforming the benchmark catalyst, Mg(II) doping is detrimental towards methanol production in comparison to undoped ZnO. X-Ray photoelectron spectroscopy and transmission electron microscopy analysis of the most active post-catalysis samples implicate the migration of Al(III) to the catalyst surface, and this surface enrichment is proposed to facilitate stabilisation of the catalytic ZnO/Cu interfaces.

Show Abstract ▼

Abstract: Single-site catalysts can demonstrate high activity and selectivity in many catalytic reactions. The synthesis of these materials by impregnation from strongly oxidizing aqueous solutions or pH-controlled deposition often leads to low metal loadings or a range of metal species. Here, we demonstrate that simple impregnation of the metal precursors onto activated carbon from a low-boiling-point, low-polarity solvent, such as acetone, results in catalysts with an atomic dispersion of cationic metal species. We show the generality of this method by producing single-site Au, Pd, Ru and Pt catalysts supported on carbon in a facile manner. Single-site Au/C catalysts have previously been validated commercially to produce vinyl chloride, and here we show that this facile synthesis method can produce effective catalysts for acetylene hydrochlorination in the absence of the highly oxidizing acidic solvents previously used.