Preparation of a highly active ternary Cu-Zn-Al oxide methanol synthesis catalyst by supercritical CO2 anti-solvent precipitation

Methanol synthesis using Cu/ZnO/Al2O3 catalysts is a well-established industrial process. Catalyst development is always an important factor and this has resulted in the current fully optimised commercial catalyst that is prepared by co-precipitation via hydroxycarbonate precursors. Recently, the synthesis of a CuZn hydroxycarbonate precursor, analogous to the rare mineral georgeite, was reported to produce a high activity methanol synthesis catalyst. Here we report the addition of Al 3+ , the third component found in industrial catalysts, to the zincian georgeite-derived catalyst prepared using a supercritical CO 2 anti-solvent precipitation methodology. The co-addition of an AlO(OH) sol to the Cu/Zn precursor solution was found to not disrupt the formation of the CuZn georgeite phase, while providing efficient mixing of the Al 3+ within the material. The catalyst derived from the CuZn georgeite precursor phase doped with Al 3+ showed a high level of methanol synthesis productivity, which was comparable to that of the binary CuZn georgeite derived catalyst. This material also exhibited enhanced stability during an accelerated ageing test compared to the non-Al doped zincian georgeite material. Performance was benchmarked against an industrially relevant Cu/ZnO/Al2O3 standard catalyst

Elucidating the role of CO2 in the soft oxidative dehydrogenation of propane over ceria-based catalysts

A mixed oxide support containing Ce, Zr, and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO 2 as the oxidant. The activity of the support was compared with that of fully formulated catalysts containing palladium. The Pd/CeZrAlO x material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h), which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH 3 and CO 2 it was found that the catalyst possessed both acidic and basic sites. In addition, temperature-programmed reduction showed that palladium promoted both the reduction and reoxidation of the support. When the role of CO 2 was investigated in the absence of gas-phase oxidant, using a temporal analysis of products (TAP) reactor, it was found that CO 2 dissociates over the reduced catalyst, leading to formation of CO and selective oxygen species. It is proposed that CO 2 has the dual role of regenerating selective oxygen species and shifting the equilibrium for alkane dehydrogenation by consuming H 2 through the reverse water-gas-shift reaction. These two mechanistic functions have previously been considered to be mutually exclusive

Deactivation of a single-site gold-on-carbon acetylene hydrochlorination catalyst: An X-ray absorption and inelastic neutron scattering study

Single-site Au species supported on carbon have been shown to be the active sites for acetylene hydrochlorination. The evolution of these single-site species has been monitored by Au L3 X-ray Absorption Spectroscopy (XAS). Alternating between a standard reaction mixture of HCl/C2H2 and the single reactants, has provided insights into the reaction mechanism and catalyst deactivation processes. We demonstrate that oxidative addition of HCl across an Au(I) chloride species requires concerted addition with C2H2, in accordance with both the XAS measurements of Au oxidation state and the reaction kinetics being 1st order with respect to each reactant. The addition of excess C2H2 changes the Au speciation and results in the formation of oligomeric acetylene species which were detected by inelastic neutron scattering. Catalyst deactivation at extended reaction times can be correlated with the formation of metallic Au particles. The presence of this Au(0) species generated during the sequential gas experiments or after prolonged reaction times, results in the analysis of the normalised near edge white line intensity of the Au L3 X-ray absorption spectrum alone becoming an unsuitable guide for identifying the active Au species, affecting the strong correlation between normalized white line height and VCM productivity usually observed in the active catalyst. Thus, a combination of scanning transmission electron microscopy and detailed modelling of whole XAS spectrum was required to distinguish active Au(I) and Au(III) species from the spectator Au(0) component

Solvothermal Synthesis of Ultrasmall Tungsten Oxide Nanoparticles

The synthesis of catalytically useful, ultrasmall oxide nanoparticles (NPs) of group 5 and 6 metals is not readily achievable through reported methods. In this work, we introduce a one-pot, two-precursor synthesis route to <2 nm MO_<i>x</i> NPs in which a polyoxometalate salt is decomposed thermally in a high-boiling organic solvent oleylamine. The use of ammonium metatungstate resulted in oleylamine-coated, crystalline WO_<i>x</i> NPs at consistently high yields of 92 ± 5%. The semicrystalline NPs contained 20–36 WO_<i>x</i> structural units per particle, as determined from aberration-corrected high-resolution scanning transmission electron microscopy, and an organic coating of 16–20 oleylamine molecules, as determined by thermogravimetric analysis. The NPs had a mean size of 1.6 ± 0.3 nm, as estimated from atomic force microscopy and small-angle X-ray scattering measurements. Carrying out the synthesis in the presence of organic oxidant trimethylamine <i>N</i>-oxide led to smaller WO_<i>x</i> NPs (1.0 ± 0.4 nm), whereas the reductant 1,12-dodecanediol led to WO_<i>x</i> nanorods (4 ± 1 nm × 20 ± 5 nm). These findings provide a new method to control the size and shape of transition metal oxide NPs, which will be especially useful in catalysis

Interfacial Stabilization of Metastable TiO₂ Films

This work demonstrates a phenomenon that preserves the traditionally metastable anatase crystal structure of thin titania (TiO₂) films along a two-dimensional oxide interface at temperatures well in excess of those that normally trigger a full polymorphic transformation to rutile in higher dimensionality crystalline powders. Whereas atomic surface mobility appears to dominate polymorph transformation processes within bulk TiO₂ powders, a simple reduction in dimensionality to a two-dimensional TiO₂ film (ca. 50–200 nm thick), supported upon a substrate, leads to a remarkable resistance to the calcination-induced anatase-to-rutile transformation. This stabilization does not appear to be specifically reliant on substrate character given its persistence for TiO₂ films prepared on amorphous silica (SiO₂) as well as crystalline TiO₂ substrates. Instead, interface-mediated coordination of the TiO₂ film with the substrate, and the inherent confinement of crystallites in two dimensions, is believed to resist polymorph transformation by mitigation of the atomic surface mobility. Only when temperatures (i.e., >800 °C) that are conducive to bulk atomic mobilization are reached does reconstructive grain growth convert the film into the thermodynamically stable rutile crystal structure

Direct Single-Enzyme Biomineralization of Catalytically Active Ceria and Ceria–Zirconia Nanocrystals

Biomineralization is an intriguing approach to the synthesis of functional inorganic materials for energy applications whereby biological systems are engineered to mineralize inorganic materials and control their structure over multiple length scales under mild reaction conditions. Herein we demonstrate a single-enzyme-mediated biomineralization route to synthesize crystalline, catalytically active, quantum-confined ceria (CeO_2–<i>x</i>) and ceria–zirconia (Ce_1–<i>y</i>Zr_<i>y</i>O_2–<i>x</i>) nanocrystals for application as environmental catalysts. In contrast to typical anthropogenic synthesis routes, the crystalline oxide nanoparticles are formed at room temperature from an otherwise inert aqueous solution without the addition of a precipitant or additional reactant. An engineered form of silicatein, rCeSi, as a single enzyme not only catalyzes the direct biomineralization of the nanocrystalline oxides but also serves as a templating agent to control their morphological structure. The biomineralized nanocrystals of less than 3 nm in diameter are catalytically active toward carbon monoxide oxidation following an oxidative annealing step to remove carbonaceous residue. The introduction of zirconia into the nanocrystals leads to an increase in Ce­(III) concentration, associated catalytic activity, and the thermal stability of the nanocrystals

High Resolution Electron Microscopy Study of Nanocubes and Polyhedral Nanocrystals of Cerium(IV) Oxide

In this research, high resolution transmission electron microscopy (HRTEM) and high angle annular dark field–scanning transmission electron microscopy (HAADF-STEM) studies of ceria­(IV) oxide CeO₂ nanocrystals (NCs) synthesized by a hydrothermal/two phase process were conducted. The synthesis route affords the possibility of controlling the shape of the CeO₂ NCs by changing the oleic acid/cerium ([OLA]/[Ce³⁺]) ratio. At a relatively low [OLA]/[Ce³⁺] ratio of 4, a polyhedral NC morphology was obtained with {111} and {200} termination facets. Increasing the [OLA]/[Ce³⁺] ratio to 8, while maintaining a constant reaction time and temperature during the synthesis, truncated cube-like CeO₂ NCs with {200}, {220}, and {111} termination facets was generated. These morphologies were identified by HRTEM and HAADF-STEM characterization. Fourier transform infrared (FT-IR) analysis and thermogravimetric analysis (TGA) confirm the presence of chemically bonded oleic acid (OLA) on the CeO₂ NC surface. It indicates that there is a relationship between the bonded OLA and the shape of the NC. Additionally, the identification of concave surfaces on {200} facets by HAADF-STEM characterization suggests that the formation of the cube-like CeO₂ morphology is a multiple step mechanism. On the basis of these observations new growth mechanisms for the CeO₂ morphology variants are proposed

The role of defects and growth directions in the formation of periodically twinned and kinked unseeded germanium nanowires

Here we show the impact of preferred growth directions and defects in the formation of complex Ge nanowire (NW) structures grown by a simple organic medium based synthesis. Various types of NWs 15 are examined including: straight defect free NWs; periodically bent NWs with precise angles between the NW segments; NWs with mutually exclusive lateral or longitudinal faults; and more complex ‘wormlike’ structures. We show that choice of solvent and reaction temperature can be used to tune the morphology of the NWs formed. The various types of NWs were probed in depth using transmission electron microscopy (TEM), scanning electron microscopy (SEM), selected area electron diffraction (SAED) and dark field TEM (DFTEM)

Redispersion of Gold Supported on Oxides

Although many gold heterogeneous catalysts have been shown to exhibit significant activity and high selectivity for a wide range of reactions in both the liquid and gas phases, they are prone to irreversible deactivation. This is often associated with sintering or loss of the interaction of the gold with the support. Herein, we report on the use of methyl iodide as a method of dispersing gold nanoparticles supported on silica, titania, and alumina supports. In the case of titania- and alumina-based catalysts, the gold was transformed from nanometer particles into small clusters and some atomically dispersed gold. In contrast, although there was a drop in the gold particle size on the silica support following CH₃I treatment, the size remained in the submicrometer range. The structural changes were correlated with changes in the selectivity and activity for ethanol dehydration and benzyl alcohol oxidation. From these observations, it is clear that this treatment provides a method by which deactivated gold catalysts can be reactivated via redispersion of the gold

Nature of Catalytically Active Sites in the Supported WO₃/ZrO₂ Solid Acid System: A Current Perspective

Tungstated zirconia (WO₃/ZrO₂) is one of the most well-studied solid acid catalyst systems and continues to attract the attention of both academia and industry. Understanding and controlling the properties of WO₃/ZrO₂ catalysts has been a topic of considerable interest over almost the past three decades, with a particular focus on discovering the relationship between catalytic activity and the molecular structure of the surface acid site. Amorphous tungsten oxide (WO_<i>x</i>) species on ZrO₂ surfaces were previously proposed to be very active for different acidic reactions such as alcohol dehydration and alkane isomerization. Recent developments in electron optical characterization and in situ spectroscopy techniques have allowed researchers to isolate the size, structure, and composition of the most active catalytic species, which are shown to be three-dimensional distorted Zr-WO_<i>x</i> clusters (0.8–1.0 nm). Complementary theoretical calculations of the Brønsted acidity of these Zr-WO_<i>x</i> clusters have confirmed that they possess the lowest deprotonation energy values. This new insight provides a foundation for the future characterization and theory of acidic supported metal oxide catalytic materials that will, hopefully, lead to the design of more active and selective catalysts. This perspective presents an up-to-date, comprehensive summary of the leading models of WO₃/ZrO₂ solid acid catalysts