Low-Temperature H2 Reduction of Copper Oxide Subnanoparticles

Small Copper Nanoclusters Synthesized through Solid-State Reduction inside a Ring-Shaped Polyoxometalate Nanoreactor

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu2+ incorporated within the cavity of a ring-shaped POM using H2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H2 or O2). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties.

The discrepancy of NH3 oxidation mechanism between SAPO-34 and Cu/SAPO-34

The difference of NH3 oxidation mechanism over SAPO-34 and Cu-SAPO-34 was studied. XRD (X-ray diffraction), SEM (scanning electron microscopy) and H2-TPR (H2-temperature programmed desorption) were conducted to estimate the Cu species distribution. The quantity of individual Cu2+ ions escalated with the elevation of silicon content in the Cu/SAPO-34 catalysts, leading to an enhancement in the activity of the NH3-SCR (ammonia-selective catalytic reduction) process. This augmentation in activity can be attributed to the increased presence of isolated Cu2+ species, which are pivotal in facilitating the catalytic reaction. In addition, the kinetic test of NH3 oxidation indicated that the CuO species were the active sites for NH3 oxidation. Specifically, the strong structural Brønsted acid sites were the NH3 oxidation active sites over the SAPO-34 support, and the NH3 reacted with the O2 on the Brønsted acid sites to produce the NO mainly. While the NH3 oxidation mechanism over Cu/SAPO-34 consisted of two steps: firstly, NH3 reacted with O2 on CuO sites or residual Brønsted acid sites to form NO as the product; subsequently, the generated NO was reduced by NH3 into N2 on isolated Cu2+ sites. Simultaneously, the isolated Cu2+ sites might demonstrate a significant function in the NH3 oxidation process to form N2. The identification of active sites and corresponding mechanism could deepen the understanding of excellent performance of NH3-SCR over the Cu/SAPO-34 catalyst at high temperature.

A greener approach for synthesizing metal-decorated carbogels from alginate for emerging technologies

In the present work, a series of metal nanoparticle-decorated carbogels (M-DCs) was synthesized starting from beads of parent metal-crosslinked alginate aerogels (M-CAs). M-CAs contained Ca(ii), Ni(ii), Cu(ii), Pd(ii) and Pt(iv) ions and were converted to M-DCs by pyrolysis under a N2 atmosphere up to pyrolysis temperatures of TP = 600 °C. The textural properties of M-CAs are found to depend on the crosslinking ion, yielding fibrous pore networks with a high specific mesoporous volume and specific surface area SV (SV ∼ 480-687 m2 g-1) for M-CAs crosslinked with hard cations, Ca(ii), Ni(ii) and Cu(ii), and comparably loose networks with increased macroporosity and lower specific surface (SV ∼ 240-270 m2 g-1) for Pd(ii) and Pt(iv) crosslinked aerogels. The pyrolysis of M-CAs resulted in two simultaneously occurring processes: changes in the solid backbone and the growth of metal/metal oxide nanoparticles (NPs). The thermogravimetric analysis (TGA) showed a significant influence of the crosslinking cation on the decomposition mechanism and associated change in textural properties. Scanning electron microscopy-backscattered electron imaging (SEM-BSE) and X-ray diffraction revealed that metal ions (molecularly dispersed in the parent aerogels) formed nanoparticles composed of elementary metals and metal oxides in varying ratios over the course of pyrolytic treatment. Increasing the TP led to generally larger nanoparticles. The pyrolysis of the nickel-crosslinked aerogel (Ni-CA) preserved, to a large extent, the mesoporous structure and resulted in the evolution of fine (∼14 nm) homogeneously dispersed Ni/NiO nanoparticles. Overall, this work presents a green approach for synthesizing metal-nanoparticle containing carbon materials, useful in emerging technologies related to heterogeneous catalysis and electrocatalysis, among others.

Ultra-small Mo-Pt subnanoparticles enable CO2 hydrogenation at room temperature and atmospheric pressure

We present a partially-oxidised bimetallic Mo-Pt subnanoparticle (Mo4Pt8Ox) enabling thermally-driven CO2 hydrogenation to CO at room temperature and atmospheric pressure. A mechanistic study explained the full catalytic cycle of the reaction from CO2 activation to catalyst reactivation. DFT calculations revealed that alloying with Mo lowers the activation barrier by weakening the CO adsorption. This finding could be a first step for low-energy CO2 conversion.

Copper-Based Silica Nanotubes as Novel Catalysts for the Total Oxidation of Toluene

Cu (10 wt%) materials on silica nanotubes were prepared via two different synthetic approaches, co-synthesis and wetness impregnation on preformed SiO2 nanotubes, both as dried or calcined materials, with Cu(NO3)2.5H2O as a material precursor. The obtained silica and the Cu samples, after calcination at 550 °C for 5 h, were characterized by several techniques, such as TEM, N2 physisorption, XRD, Raman, H2-TPR and XPS, and tested for toluene oxidation in the 20-450 °C temperature range. A reference sample, Cu(10 wt%) over commercial silica, was also prepared. The copper-based silica nanotubes exhibited the best performances with respect to toluene oxidation. The Cu-based catalyst using dried silica nanotubes has the lowest T50 (306 °C), the temperature required for 50% toluene conversion, compared with a T50 of 345 °C obtained for the reference catalyst. The excellent catalytic properties of this catalyst were ascribed to the presence of easy copper (II) species finely dispersed (crystallite size of 13 nm) on the surface of silica nanotubes. The present data underlined the impact of the synthetic method on the catalyst properties and oxidation activity.

Facile Microwave Synthesis of Hierarchical Porous Copper Oxide and Its Catalytic Activity and Kinetics for Carbon Monoxide Oxidation

The synthesis of copper oxide (CuO)-based nanomaterials has received a tremendous deal of interest in recent years. Particularly, the design and development of novel CuO structures with improved physical and chemical properties have attracted immense attention, especially for catalysis applications. We report on a rational, rapid, and surfactant-free microwave synthesis (MWS) of hierarchical porous copper oxide (HP-CuO) with a three-dimensional (3D) sponge-like topology using an MWS reactor. The activity of the microwave (MW)-synthesized HP-CuO catalysts for carbon monoxide (CO) oxidation was studied and compared to CuO prepared by the conventional heating method (CHM). Results showed that HP-CuO catalysts prepared by MWS for 10 and 30 min surpassed the CuO catalyst prepared by CHM, exhibiting T 80 of 98 and 115 °C, respectively, as compared to 185 °C of CuO prepared by CHM (T80 is the temperature corresponding to 80% CO conversion). In addition, the MW-synthesized HP-CuO catalysts outperformed the CHM-synthesized CuO, achieving a 100% CO conversion at 150 °C compared to 240 °C in the case of CuO prepared by CHM. Interestingly, the HP-CuO catalyst expressed workable CO conversion kinetics with a reaction rate of c.a.35 μmol s-1 g-1 at 150 °C and apparent activation energy (E a) of 82 kJ mol-1. The HP-CuO catalyst showed excellent cycling and long-term stabilities for CO oxidation up to 4 cycles and 72 h on the stream, respectively. The enhanced catalytic activity and stability of the HP-CuO catalyst appear to result from the unique topological and structural features of HP-CuO, which were revealed by SEM, XRD, Raman, BET, TGA, XPS, and TPR techniques.