Showing papers by "Christopher J. Kiely published in 2017"
TL;DR: A catalyst composed of layered gold clusters on molybdenum carbide (MoC) nanoparticles to convert CO through its reaction with water into H2 and CO2 at temperatures as low as 150°C is developed.
Abstract: The water-gas shift (WGS) reaction (where carbon monoxide plus water yields dihydrogen and carbon dioxide) is an essential process for hydrogen generation and carbon monoxide removal in various energy-related chemical operations. This equilibrium-limited reaction is favored at a low working temperature. Potential application in fuel cells also requires a WGS catalyst to be highly active, stable, and energy-efficient and to match the working temperature of on-site hydrogen generation and consumption units. We synthesized layered gold (Au) clusters on a molybdenum carbide (α-MoC) substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction. Water was activated over α-MoC at 303 kelvin, whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity at low temperatures.
484 citations
TL;DR: It is demonstrated that the resulting methanol incorporated a substantial fraction of gas-phase O2, suggesting that the controlled breakdown of H2O2 activates methane, which subsequently incorporates molecular oxygen through a radical process.
Abstract: The selective oxidation of methane, the primary component of natural gas, remains an important challenge in catalysis. We used colloidal gold-palladium nanoparticles, rather than the same nanoparticles supported on titanium oxide, to oxidize methane to methanol with high selectivity (92%) in aqueous solution at mild temperatures. Then, using isotopically labeled oxygen (O2) as an oxidant in the presence of hydrogen peroxide (H2O2), we demonstrated that the resulting methanol incorporated a substantial fraction (70%) of gas-phase O2. More oxygenated products were formed than the amount of H2O2 consumed, suggesting that the controlled breakdown of H2O2 activates methane, which subsequently incorporates molecular oxygen through a radical process. If a source of methyl radicals can be established, then the selective oxidation of methane to methanol using molecular oxygen is possible.
429 citations
TL;DR: An in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions is performed and it is demonstrated that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present.
Abstract: There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination. We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present. We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species.
354 citations
01 Jan 2017
Abstract: Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable—and hence little known and largely ignored—georgeite. The first three of these minerals are widely used as catalyst precursors for the industrially important methanol-synthesis and low-temperature water–gas shift (LTS) reactions, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite; with few exceptions it uses sodium carbonate as the carbonate source, but this also introduces sodium ions—a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.
101 citations
TL;DR: In this paper, the size, structure, and composition of the most active catalytic species were shown to be three-dimensional distorted Zr-WOx clusters (0.8-1.0 nm).
Abstract: 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 (WOx) 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-WOx clusters (0.8–1.0 nm). Complementary theoretical calculations of the Bronsted acidity of these Zr-WOx clusters have confirmed that t...
74 citations
TL;DR: Gold (Au) on ceria-zirconia is one of the most active catalysts for the low-temperature water-gas shift reaction (LTS), but this catalyst rapidly deactivates on-stream and the deactivation mechanism remains unclear.
Abstract: Gold (Au) on ceria–zirconia is one of the most active catalysts for the low-temperature water–gas shift reaction (LTS), a key stage of upgrading H2 reformate streams for fuel cells. However, this catalyst rapidly deactivates on-stream and the deactivation mechanism remains unclear. Using stop–start scanning transmission electron microscopy to follow the exact same area of the sample at different stages of the LTS reaction, as well as complementary X-ray photoelectron spectroscopy, we observed the activation and deactivation of the catalyst at various stages. During the heating of the catalyst to reaction temperature, we observed the formation of small Au nanoparticles (NPs; 1–2 nm) from subnanometer Au species. These NPs were then seen to agglomerate further over 48 h on-stream, and most rapidly in the first 5 h when the highest rate of deactivation was observed. These findings suggest that the primary deactivation process consists of the loss of active sites through the agglomeration and possible dewetting of Au NPs.
45 citations
TL;DR: The addition of Re to Ni on TiO2 yields efficient catalysts for the hydrogenation of acids and esters to alcohols under mild conditions.
39 citations
TL;DR: Interestingly, Ag/MgO was capable of a conversion comparable to current industrial routes of approximately 5 %, and with a high selectivity to cyclohexanol, thus making Ag/ MgO an attractive system for the synthesis of intermediates for the manufacture of nylon fibres.
Abstract: The liquid phase oxidation of cyclohexane to cyclohexanol and cyclohexanone was investigated by synthesizing and testing an array of heterogeneous catalysts comprising: monometallic Ag/MgO, monometallic Pd/MgO and a set of bimetallic AgPd/MgO catalysts. Interestingly, Ag/MgO was capable of a conversion comparable to current industrial routes of ca. 5%, and with a high selectivity (up to 60%) to cyclohexanol, thus making Ag/MgO an attractive system for the synthesis of intermediates for the manufacture of nylon fibres. Furthermore, following the doping of Ag nanoparticles with Pd, the conversion increased up to 10% whilst simultaneously preserving a high selectivity to the alcohol. Scanning transmission electron microscopy and energy dispersive spectroscopy of the catalysts showed a systematic particle size composition variation with the smaller Ag-Pd nanoparticles being statistically richer in Pd. Analysis of the reaction mixture by Electron Paramagnetic Resonance (EPR) spectroscopy coupled with the spin trapping technique showed the presence of large amounts of alkoxy radicals, thus providing insights for a possible reaction mechanism.
36 citations
TL;DR: Zincian georgeite, an amorphous copper–zinc hydroxycarbonate, has been prepared by co-precipitation using acetate salts and ammonium carbonate.
Abstract: Zincian georgeite, an amorphous copper-zinc hydroxycarbonate, has been prepared by co-precipitation using acetate salts and ammonium carbonate. Incorporation of zinc into the georgeite phase and mild ageing conditions inhibits crystallisation into zincian malachite or aurichalcite. This zincian georgeite precursor was used to prepare a Cu/ZnO catalyst, which exhibits a superior performance to a zincian malachite derived catalyst for methanol synthesis and the low temperature water-gas shift (LTS) reaction. Furthermore, the enhanced LTS activity and stability in comparison to that of a commercial Cu/ZnO/Al2O3 catalyst, indicates that the addition of alumina as a stabiliser may not be required for the zincian georgeite derived Cu/ZnO catalyst. The enhanced performance is partly attributed to the exclusion of alkali metals from the synthesis procedure, which are known to act as catalyst poisons. The effect of residual sodium on the microstructural properties of the catalyst precursor was investigated further with preparations using sodium carbonate.
32 citations
TL;DR: This work demonstrates a bioenabled fully aqueous phase and room temperature route to the synthesis of CuInS2/ZnS core/shell quantum confined nanocrystals conjugated to IgG antibodies and used for fluorescent tagging of THP-1 leukemia cells.
Abstract: This work demonstrates a bioenabled fully aqueous phase and room temperature route to the synthesis of CuInS2/ZnS core/shell quantum confined nanocrystals conjugated to IgG antibodies and used for fluorescent tagging of THP-1 leukemia cells. This elegant, straightforward and green approach avoids the use of solvents, high temperatures and the necessity to phase transfer the nanocrystals prior to application. Non-toxic CuInS2, (CuInZn)S2, and CuInS2/ZnS core/shell quantum confined nanocrystals are synthesized via a biomineralization process based on a single recombinant cystathionine γ-lyase (CSE) enzyme. First, soluble In-S complexes are formed from indium acetate and H2S generated by CSE, which are then stabilized by l-cysteine in solution. The subsequent addition of copper, or both copper and zinc, precursors then results in the immediate formation of CuInS2 or (CuInZn)S2 quantum dots. Shell growth is realized through subsequent introduction of Zn acetate to the preformed core nanocrystals. The size and optical properties of the nanocrystals are tuned by adjusting the indium precursor concentration and initial incubation period. CuInS2/ZnS core/shell particles are conjugated to IgG antibodies using EDC/NHS cross-linkers and then applied in the bioimaging of THP-1 cells. Cytotoxicity tests confirm that CuInS2/ZnS core/shell quantum dots do not cause cell death during bioimaging. Thus, this biomineralization enabled approach provides a facile, low temperature route for the fully aqueous synthesis of non-toxic CuInS2/ZnS quantum dots, which are ideal for use in bioimaging applications.
29 citations
TL;DR: A single-enzyme-mediated biomineralization route to synthesize crystalline, catalytically active, quantum-confined ceria and ceria-zirconia nanocrystals for application as environmental catalysts is demonstrated.
Abstract: 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 (CeO2–x) and ceria–zirconia (Ce1–yZryO2–x) 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 i...
TL;DR: In this article, the synthesis of supported gold and gold-palladium nanoparticles without the addition of stabilizing polymers was reported, and the catalysts performed very similarly in the selective oxidation of glycerol and benzyl alcohol suggesting that polymers are not essential to make active catalysts for these reactions.
Abstract: Catalytic properties and stability of Au nanoparticles are sensitive to factors such as the dimensions, shape and composition of the metal nanoparticles. Although colloidal methods provide an easy and quick way to synthesize supported metal catalysts, they typically involve using polymers such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP) as steric stabilizers, which can sometimes be detrimental in subsequent catalytic reactions. Here we report the synthesis of supported gold and gold-palladium nanoparticles without the addition of stabilizing polymers. The catalysts prepared with and without the addition of polymers performed very similarly in the selective oxidation of glycerol and benzyl alcohol suggesting that polymers are not essential to make active catalysts for these reactions. Thus, this new stabilizer free method provides a facile and highly effective way of circumventing the inherent problems of polymer stabilizers when preparing gold and gold palladium catalysts.
TL;DR: An engineered cystathionine γ-lyase enzyme, smCSE that is active for the direct aqueous phase biomineralization of CdSe and Cd selenocystine-CdS core-shell nanocrystals synthesis reveals the powerful potential of biominalization processes.
Abstract: Biomineralization is the process by which biological systems synthesize inorganic materials. Herein, we demonstrate an engineered cystathionine γ-lyase enzyme, smCSE that is active for the direct aqueous phase biomineralization of CdSe and CdSe-CdS core-shell nanocrystals. The nanocrystals are formed in an otherwise unreactive buffered solution of Cd acetate and selenocystine through enzymatic turnover of the selenocystine to form a reactive precursor, likely H2Se. The particle size of the CdSe core nanocrystals can be tuned by varying the incubation time to generated particle sizes between 2.74 ± 0.63 nm and 4.78 ± 1.16 nm formed after 20 min and 24 h of incubation, respectively. Subsequent purification and introduction of l-cysteine as a sulfur source facilitates the biomineralization of a CdS shell onto the CdSe cores. The quantum yield of the resulting CdSe-CdS core-shell particles is up to 12% in the aqueous phase; comparable to that reported for more traditional chemical synthesis routes for core-shell particles of similar size with similar shell coverage. This single-enzyme route to functional nanocrystals synthesis reveals the powerful potential of biomineralization processes.
TL;DR: The result is the identification of a facile yet versatile templating strategy for realizing 3DOm oxides with surface areas that are more than an order of magnitude larger than untemplated control samples, pore diameters and volumes that can be tuned across a continuum of size scales, and selectable polymorphism.
Abstract: Convectively assembled colloidal crystal templates, composed of size-tunable (ca. 15–50 nm) silica (SiO2) nanoparticles, enable versatile sacrificial templating of three-dimensionally ordered mesoporous (3DOm) metal oxides (MOx) at both mesoscopic and microscopic size scales. Specifically, we show for titania (TiO2) and zirconia (ZrO2) how this approach not only enables the engineering of the mesopore size, pore volume, and surface area but can also be leveraged to tune the crystallite polymorphism of the resulting 3DOm metal oxides. Template-mediated volumetric (i.e., interstitial) effects and interfacial factors are shown to preserve the metastable crystalline polymorphs of each corresponding 3DOm oxide (i.e., anatase TiO2 (A-TiO2) and tetragonal ZrO2 (t-ZrO2)) during high-temperature calcination. Mechanistic investigations suggest that this polymorph stabilization is derived from the combined effects of the template–replica (MOx/SiO2) interface and simultaneous interstitial confinement that limit the d...
TL;DR: In this article, a systematic study of the influence of framework topology and exchange properties of the MFI-type Bronsted acidic zeolite ZSM-5 catalysts was carried out and it was shown that surface iron oxide species are spectators in the oxidation of propane with H2O2.
Abstract: Fe- containing ZSM-5 catalysts are reported to be efficient catalysts for the partial oxidation of propane to oxygenated products at reaction temperatures as low as 50 oC in an aqueous phase reaction using the green oxidant H2O2. It was previously proposed that extra framework Fe species at the zeolite's exchange sites are responsible for activation of both the alkane and hydrogen peroxide. Through a systematic study of the influence of framework topology and exchange properties it is now shown that this high catalytic activity is specific to the MFI-type Bronsted acidic zeolite ZSM-5. Furthermore, through a simple aqueous acid washing treatment, leaching ca. 77 % of iron present within a Fe/ZSM-5 catalyst only caused the relative propane conversion to decrease by 17 %; implying that most of the initially loaded Fe does not actually contribute to the catalytic activity. This small change in conversion after 'excess' Fe removal, amounts to a three-fold increase in TOF (Fe) from 66 h-1to 232 h-1 compared with the parent Fe/ZSM-5 catalyst. By comparing these samples, it is shown by NH3-TPD, 27Al MAS NMR, XPS and TEM analysis, that surface iron oxide species are effectively spectators in the oxidation of propane with H2O2. This provides further insight as to the location and true nature of the catalytically active Fe species.
TL;DR: In this article, the authors describe the biomineralization and optical properties of ZnxCd1−xS and Zn xCd 1−xs−ZnS quantum confined nanocrystals.
Abstract: Biomineralization is an intriguing route towards the low temperature, aqueous phase, green synthesis of inorganic functional nanomaterials. Herein we describe the biomineralization and optical properties of ZnxCd1−xS and ZnxCd1−xS–ZnS quantum confined nanocrystals that have potential application in optoelectronics. The reported biomineralization process is reduced to perhaps its simplest form wherein a single recombinant cystathionine γ-lyase (CSE) enzyme is responsible for catalyzing mineralization within an otherwise inert solution and plays a role in controlling the alloy composition. The biomineralized nanocrystals are sphalerite structured with average diameter below 3 nm. Biomineralization of a passivating ZnS shell on Zn0.73Cd0.27S core nanocrystals is achieved through subsequent addition of Zn precursor. This shell growth increases the photoluminescence quantum yield to 7% and increases the radiative decay time to 97.6 ns compared with 40.9 ns for the core materials.
TL;DR: In this article, the authors demonstrate 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.
Abstract: 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 polym...
TL;DR: In this paper, electron microscopy has been used to obtain an unprecedented and complete view of the complex dispersion of metal entities present, including nanoparticles, sub-nm clusters, and atomically dispersed species.
Abstract: Precious metal nanoparticles have important applications in catalysis. In order to maximize the atomic efficiency and therefore reduce the usage of valuable resources, the particle size, morphology and their interaction with the support need to be carefully controlled and optimized. In the case of monometallic Au and Pt catalysts prepared via wet chemical routes, advanced electron microscopy has allowed us to obtain an unprecedented and more complete view of the complex dispersion of metal entities present, including nanoparticles, sub-nm clusters, and atomically dispersed species. With this new information, a better understanding of the catalytically active sites/species can be established. [1] By correlating catalytic performance data with the nanostructures generated by various catalyst preparation methods the most active species can be identified and specifically targeted for synthesis, thus significantly improving the atomic efficiency.