ZnO Supports Research Articles

Redispersing Ir Nanoparticles via a Carbon-Assisted Pyrolysis Process to Break the Activity-Stability Trade-Off of H2 Sensors.

Oxide semiconductor-supported metal nanoparticles often suffer from a high-temperature gas sensing process, resulting in agglomeration and coalescence, which significantly decrease their surface activity and stability. Here, we develop an in situ pyrolysis strategy to redisperse commercial Ir particles (∼15.6 nm) into monodisperse Ir species (∼5.4 nm) on ZnO supports, exhibiting excellent sintering-resistant properties and H2 sensing. We find that large-size Ir nanoparticles can undergo an unexpected splitting decomposition process and spontaneously migrate along the encapsulated carbon layer surface during high-temperature pyrolysis of ZIF-8. This resultant monodisperse status can be integrally reserved, accompanying further oxidation sintering. The final Irred/ZnO-450-based sensor exhibits outstanding stability, H2 response (10-2000 ppm), fast response/recovery capability (7/9.7 s@100 ppm), and good moisture resistance. In situ Raman and ex situ XPS further experimentally verify that highly dispersive Ir species can promote the electron transfer process during the gas sensing process. Our strategy thus provides important insights into the design of agglomeration-resistant gas sensing materials for highly effective H2 detection.

Stability and potential degradation of the α',β'-epoxyketone pharmacophore on ZnO nanocarriers: insights from reactive molecular dynamics and density functional theory calculations.

We investigate the structure and dynamics of a zinc oxide nanocarrier loaded with Carfilzomib, an epoxyketone proteasome inhibitor developed for treating multiple myeloma. We demonstrate that, even though both bare and functionalized zinc oxide supports have been used for drug delivery, their interactions with the reactive functional groups of the ligands could be detrimental. This is because pharmacophores like α',β'-epoxyketones should preserve the groups required for the drug activity and be capable of leaving the vehicle at the target site. Earlier studies showed that even when ZnO is functionalized with oleic acid surfactants, the drug could reach parts of the surface and remain stably adsorbed. Herein, we have used reactive molecular dynamics simulations and quantum chemistry calculations to explore the potential interactions of the Carfilzomib functional groups with the typical surfaces of ZnO supports. We have found that Carfilzomib can adsorb on the (0001)Zn-terminated polar surface through the carbonyl oxygens and the epoxyketone moiety. These strong connections could prevent the drug release and induce the epoxy ring opening with its consequential inactivation. Therefore, regulating the dosage to maintain the desired level of drug bioavailability is paramount. These findings emphasize the need for appropriate carrier functionalizations to efficiently entrap, transport, and release the cargo at the target sites and the crucial role played by predictive/descriptive computational techniques to complement and drive experiments to the most appropriate selections of the materials to optimize drug delivery.

CO2 Hydrogenation over Copper/ZnO Single-Atom Catalysts: Water-Promoted Transient Synthesis of Methanol.

The hydrogenation of CO2 by renewable power-generated hydrogen offers a promising approach to a sustainable carbon cycle. However, the role of water during CO2 hydrogenation is still under debate. Herein, we demonstrated that either too low or too high contents of water hampered the methanol synthesis over Cu/ZnO based catalysts. For Cu single atoms on ZnO supports, the optimal content of water was 0.11 vol. % under 30 bar (CO2 : H2 =1 : 3) at 170 °C. Upon the introduction of optimal-content water, the methanol selectivity immediately became 99.1 %, meanwhile the conversion of CO2 underwent a volcano-type trend with the maximum of 4.9 %. According to mechanistic studies, water acted as a bridge between H atoms and CO2 /intermediates, facilitating the transformation of COOH* and CH2 O*. The enhanced activity induced the generation of more water to react with CO via water-gas shift reaction, resulting in the increase in methanol selectivity.

CO2Hydrogenation over Copper/ZnO Single‐Atom Catalysts: Water‐Promoted Transient Synthesis of Methanol

AbstractThe hydrogenation of CO2by renewable power‐generated hydrogen offers a promising approach to a sustainable carbon cycle. However, the role of water during CO2hydrogenation is still under debate. Herein, we demonstrated that either too low or too high contents of water hampered the methanol synthesis over Cu/ZnO based catalysts. For Cu single atoms on ZnO supports, the optimal content of water was 0.11 vol. % under 30 bar (CO2 : H2=1 : 3) at 170 °C. Upon the introduction of optimal‐content water, the methanol selectivity immediately became 99.1 %, meanwhile the conversion of CO2underwent a volcano‐type trend with the maximum of 4.9 %. According to mechanistic studies, water acted as a bridge between H atoms and CO2/intermediates, facilitating the transformation of COOH* and CH2O*. The enhanced activity induced the generation of more water to react with CO via water‐gas shift reaction, resulting in the increase in methanol selectivity.

ZIF-67 membranes supported on porous ZnO hollow fibers for hydrogen separation from gas mixtures

Co-based zeolitic imidazolate framework-67 (ZIF) membranes present impressive potential to be applied for efficient gas separation. However, the synthesis of ZIF-67 membrane is challenging because ZIF-67 crystals tend to nucleate and grow in the reaction solution rather than on the substrate surface. In this work, ZIF-67 membranes were developed on porous ZnO hollow fibers via in situ solvothermal growth method. The effects of different ZnO supports, surface activation, growth duration, and growing temperatures on the resultant membrane performance were carefully investigated. The optimized ZIF-67 composite hollow fiber membrane displayed desirable permeation performance, ideal selectivities for single gas, and separation factors for gas mixtures. The achieved H2 permeance was 8.98 × 10−8 mol m−2 s−1·Pa−1and ideal selectivities for H2 gas pairs with CO2, N2, and CH4 were 5.14, 11.75, and 15.89, respectively, which compared favorably with other reports in the literature. The developed ZIF-67 membrane exhibited good stability in terms of gas permeances between 30 and 200 °C. Furthermore, the membrane showed almost constant H2 permeance and H2/CH4 separation factor during the 50-h run at 150 °C, demonstrating the excellent thermal stability of membrane. The fabrication of the excellent ZIF-67 membrane can be achieved just in one cycle of growth with high reproducibility.

Effects of ZnO crystal facet on the ethanol detection by the Au/ZnO sensors

In this paper, Au nanoparticles loaded ZnO nanocones with the dominant {101¯1} facet, nanoflakes with the dominant {0001} facet, and nanorods with the dominant {101¯0} facet were synthesized and the crystal facet effects of the ZnO on the ethanol-sensing property of the Au/ZnO sensors were investigated. The size of gold nanoparticles and the ethanol-sensing response strongly depended on the facet structures of the ZnO supports. The high surface energy of {101¯1} facet led to the strong interaction between the Au nanoparticles and the ZnO nanocones, which was beneficial to the stability of Au nanoparticles and the increase of the catalytic activity of the Au/ZnO nanocones. Temperature-programmed surface reaction results indicated that the high activity of the Au/ZnO nanocones accelerated the ethanol-sensing reaction. The acceleration resulted in the remarkable change of depletion layer and high ethanol-sensing response.

Synthesis of cubic and hexagonal ZnTiO3 as catalyst support in steam reforming of methanol: Study of physical and chemical properties of copper catalysts on the H2 and CO selectivity and coke formation

Performance of cubic and hexagonal phases of ZnTiO3 as catalyst supports were studied in steam reforming of methanol and the obtained data were compared with TiO2 and ZnO supports. The ZnTiO3 phases were synthesized with the sol-gel method in low temperature and copper (as the active metal) was deposited on the surface of the ZnTiO3 nanoparticles with incipient wetness impregnation method. The Cu/cubic-ZnTiO3 catalyst exhibits high activity, selectivity and low coke formation in this reaction. This high activity is associated with the development of low-moderate acid sites in the cubic sample. Also, inappropriate byproducts such as carbon monoxide were not detected in the Cu/cubic-ZnTiO3 catalyst, which are related to the oxygen vacancy defects and acid sites on the cubic-ZnTiO3 surface. Consequently, our findings prove that the Cu/cubic-ZnTiO3 catalyst has commercial (inexpensive raw materials) and industrial (high conversion and selectivity) advantages.

Defective Mesocrystal ZnO-Supported Gold Catalysts: Facilitating CO Oxidation via Vacancy Defects in ZnO

We present a strategy to prepare a highly active Au/ZnO catalyst for CO oxidation by introducing abundant Zn- and O-vacancy defects into a ZnO support of mesocrystal form. Two different ZnO supports were chosen for comparison; almost defect-free ZnO nanorods (NR-ZnO) and twin-brush-like ZnO mesocrystals (TB-ZnO) with rich Zn/O-vacancy defects gave Au/NR-ZnO and Au/TB-ZnO upon deposition of gold nanoparticles. The catalytic test of CO oxidation over Au/TB-ZnO catalyst showed an enhanced catalytic activity that was 153 times greater than the activity of Au/NR-ZnO. The dramatic enhancement in CO oxidation is attributed to a room-temperature Mars–van Krevelen (MvK) mechanism on the surface of the Au/TB-ZnO catalyst, which was promoted by extensive vacancy defects in TB-ZnO. To elucidate the increase in activity, the vacancy ratio (i.e., [VO•]/[VZn•]) of TB-ZnO was systematically modulated by adjusting calcination conditions. The defective ZnO support altered the tendency in the variation of size, valence stat...

Fabrication of MAF-4 membrane on porous ZnO support for H<sub>2</sub>/CO<sub>2</sub> separation

Membrane separation is a cost-effective and energy-saving technology compared with physical adsorption, chemical absorption, distillation, and other conventional methods. With high surface area, uniform and adjustable pore aperture size/shape, etc., porous coordination polymers or metal-organic frameworks (MOFs) have demonstrated outstanding potentials for storage, separation, catalysis, sensing and many other applications. MOFs are also emerging as promising membrane materials in recent years. Nevertheless, most MOF-based membranes still suffer from complicated synthesis, low stability, low permeability, and/or low selectivity. SOD-[Zn(mim)2] (MAF-4 or ZIF-8, Hmim=2-methylimidazole) with a very small aperture size (0.34 nm) is a zeolitic MOF material having high chemical stability and thermal stability. Besides adsorption, catalysis, and many other functions, MAF-4 has also been extensively studied as a membrane material, and many membrane synthesis methods and membrane supports have been reported. As a Zn(II)-based MOF, MAF-4 crystals/membranes can grow on ZnO surface by partial conversion of the oxide to the MOF component. Therefore, porous ZnO may be a suitable support for a MAF-4 membrane. However, for known MAF-4 membrane synthesis methods, the ZnO layers need to be coated on the surface of conventional supports such as porous α-Al2O3, silica, and titania, by complicated procedures. To utilize the advantage of ZnO and simplify the membrane preparation process, porous ZnO supports were used in this work, and MAF-4 membranes were successfully grown by the facile secondary growth method. Powder X-ray diffraction, energy dispersive X-ray spectroscopy and scanning electron microscope showed that the obtained membranes were compact and continuous with thickness of 4 μm. Single-component gas permeation experiments gave an H2/CO2 ideal selectivity of 7.5 and an H2 permeance of 2.1×10-7 mol/(m2 Pa s) under a pressure drop of 100 kPa at 25°C. When the temperature increased to 100°C, the H2/CO2 ideal selectivity increased to 25, while the H2 permeance remained 2.0×10-7 mol/(m2 Pa s). For equimolar H2/CO2 mixture, the obtained MAF-4 membrane showed a selectivity of 5.7 for H2/CO2 and a H2 permeance of 1.2×10-7 mol/(m2 Pa s) under a pressure drop of 100 kPa at 25°C. This work indicates porous ZnO as a suitable support for fabrication of other Zn-based MOF membranes.

Support effect of Ag/ZnO catalysts for partial oxidation of methanol

Three ZnO supports with different morphologies (fusiform, nanoparticle and rod-shaped sphere) were synthesized successfully by facile hydrothermal method, over which Ag were loaded to evaluate for the hydrogen production in partial oxidation of methanol in the temperature range of 150–400°C. The activity data showed that the catalytic activities of Ag/ZnO catalysts were markedly affected by the morphologies of the supporting materials, among which their activities ranked as fusiform>nanoparticle>rod-shaped sphere. The best performing AgZn-F catalyst shows the highly efficient catalytic performance, achieving almost 100% conversion and very low CO selectivity at 360°C. These results illustrate that the morphology of support plays a crucial role on the Ag dispersion and the interaction of Ag and ZnO support, and thus determines the catalytic activity in partial oxidation of methanol.

Interaction of bimetallic PtCo layers with bare and graphene-covered ZnO(0001) supports

PtCo bimetallic overlayers supported on bare and graphene covered ZnO(0001) substrates have been successfully prepared and used to investigate the effect of graphene interlayer on the arrangement and the redox behaviour of PtCo. We found that Co is readily oxidized at the PtCo/ZnO interface during annealing in ultra-high vacuum (UHV) and low pressure O2 atmosphere, while after inserting a layer of graphene in-between, the oxidation of Co is restricted. In addition, the reduction of Co oxides by H2 is more pronounced when PtCo is supported on graphene covered ZnO. Apart from the cobalt oxidation state, graphene insertion at the interface also influences the PtCo arrangement by favouring their intermixing. Raman spectra show that low intensity defects are introduced into graphene layer after the deposition of PtCo and are enhanced by high temperature annealing. This study highlights the prospect of using graphene to tune the interaction between alloys and oxide supports which finds potential applications in catalysis.

Reactive Adsorption Desulfurization on Cu/ZnO Adsorbent: Effect of ZnO Polarity Ratio on Selective Hydrogenation

The desulfurization activity and selective hydrogenation of Cu/ZnO adsorbents on the different polarity ratios of ZnO as supports was investigated in reactive adsorption desulfurization. The ZnO particles were synthesized by the hydrothermal process, and CuO/ZnO adsorbents were synthesized by incipient impregnation method. The structure and morphology of the ZnO and CuO/ZnO were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, X-ray photoelectron spectra (XPS), scanning electron microscope/selected area electron diffraction (SEM/SAED), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR). The surface area and polarity ratio of ZnO supports were controlled by the calcination temperature and concentration of P123, respectively. More reactive activity sites were provided by the high surface area of ZnO supports, thus improving the desulfurization activity. The polarity ratio of ZnO may strongly influence the hydrogenation reactions of olefins. The selective hy...

Influence of Support Structure on the Ultraviolet Photoluminescence Enhancement from Graphene/ZnO Hybrid Structures

Novel application of graphene combined with light emitting materials has been proposed recently due to the plasmonic effects of graphene. Here, we report our investigations on the structural and optical properties of two graphene/ZnO hybrid structures that fabricated based on different ZnO supports. Plasmon-enhanced ultraviolet photoluminescence has been observed from both samples. The combined Raman and photoluminescence studies suggest a strong interaction between ZnO and graphene, which is affected by the surface structures of ZnO. Our results develop insights about the influence of ZnO supports on the PL enhancement and interfacial coupling in graphene/ZnO hybrid structures, which provides a reference for the design and fabrication of optoelectronic devices with high efficiency.

Pd3Sn nanoparticles on TiO2 and ZnO supports as catalysts for semi-hydrogenation: Synthesis and catalytic performance

The two catalysts Pd3Sn/TiO2 and Pd3Sn/ZnO were prepared via a one-pot procedure based on the “polyol method” with the addition of a capping agent (polyvinylpyrrolidone) to control the particle size distribution. The same procedure was used to prepare Pd/TiO2 and Pd/ZnO for comparison. All four catalysts showed high activity and selectivity for the selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) in the liquid phase under identical conditions. However, Pd3Sn/TiO2 and Pd3Sn/ZnO show selectivities to alkene significantly higher than that of the Pd catalysts. Specifically, the selectivity increases from 96.4% to 97.4% on TiO2 support, and 96.2% to 97.6% on ZnO support, at 90% conversion. Transition electron microscopy shows nanoparticles evenly dispersed on the support, with mean particle sizes as low as 4.1 (±0.8) nm when Sn is incorporated into the catalyst. Unsupported Pd3Sn was prepared using the same method and characterised by powder X-Ray diffraction followed by the Rietveld refinement. Pd3Sn was found to be single-phase and isostructural to Pd metal with a face centred cubic unit cell.

Synergy of Contact between ZnO Surface Planes and PdZn Nanostructures: Morphology and Chemical Property Effects in the Intermetallic Sites for Selective 1,3-Butadiene Hydrogenation

In this work, differently shaped ZnO supports were employed to study their effect on PdZn nanostructures. Our goal was to understand the role of ZnO morphology and consequently the major exposed faces (polar (0001), nonpolar (1120), or nonpolar (1010) facets) on the structural and electronic properties of the formed PdZn alloys and how it affects their chemical properties in selective 1,3-butadiene hydrogenation. A multitechnique approach that included in situ synchrotron X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) measurements combined with in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS) has been accomplished to understand the formation of the PdZn alloy and the structural and electronic properties. Complementary characterization was obtained by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy analysis of adsorbed CO (CO-FTIR), and X-ray photoelectron spectroscopy (XPS)...

A Comparative Ambient Pressure X-ray Photoelectron and Absorption Spectroscopy Study of Various Cobalt-Based Catalysts in Reactive Atmospheres

The oxidation state of cobalt in various forms (single crystal, nanopowder, deposited on CeO2 and ZnO supports) was investigated in situ under O2, H2 and ethanol steam reforming (ESR) conditions (C2H6O:H2O = 1:3) by using ambient pressure X-ray photoelectron and absorption spectroscopies. In O2 atmosphere single crystalline, nanopowder and CeO2-supported cobalt is readily oxidized into a spinel Co3O4 phase, while over the ZnO a mixed Zn1−xCoxO oxide is formed. In H2 and ESR atmospheres, Co3O4 oxide reduces to the metallic state, with an evident effect of the gas atmosphere and the sample type in this process. On the other hand the Zn1−xCoxO oxide proved to be extremely stable in reductive conditions. Finally, under the ESR atmosphere the amount of carbon deposition and the type of the adsorbed oxygen species considerably varies between supported and single crystalline/nanopowder cobalt catalysts.

Synthesis and photocatalytic activity of ZnO-Au25 nanocomposites

ZnO-Au25 nanocomposites were synthesized by doping Au25 nanoclusters into the porous ZnO nanospheres. It was notable that the ultrasmall Au25 nanoclusters possessed uniform sizes and fine dispersibility on the porous ZnO supports. A considerable correlation between the loading of Au25 nanoclusters and the photocatalytic activity was found. Compared with the pure ZnO nanospheres, the ZnO-Au25 nanocomposites exhibited more efficient photocatalytic activity in terms of degradation of Rhodamine B (RhB) in an aqueous solution. In addition, the possible photocatalytic mechanisms are discussed in this work. This strategy may be helpful for preparing other novel hybrid nanocomposites with well-defined structures and superior performances.

Promoting Strong Metal Support Interaction: Doping ZnO for Enhanced Activity of Cu/ZnO:M (M = Al, Ga, Mg) Catalysts

The promoting effect of Al, Ga, and Mg on the support in Cu/ZnO catalysts for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports were synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, and calcined and activated catalysts were characterized by various methods, including contactless (microwave) conductivity measurements under different gas atmospheres. Small amounts of promoters were found to exhibit a significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water-gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H2 are decreased upon promotion of the ZnO support with the trivalent promoters Al3+ and Ga3+, indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al3+ and Ga3+. A detrimental effect of Mg2+ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system.

Oxidation of palladium on Au(111) and ZnO(0001) supports.

The oxidation behavior of supported Pd-deposits on Au(111) and ZnO(0001) single crystals has been studied by x-ray photoemission spectroscopy (XPS). Oxidation has been carried out ex situ in a high-pressure cell and subsequent vacuum-transfer and characterization by ultra-high vacuum XPS, as well as in situ characterization by synchrotron based near ambient pressure XPS. On Au(111) alloying of Pd with the substrate competes with oxidation and only for sufficiently thick Pd films oxidation is obtained. For Pd deposits on ZnO the oxidation condition depends on the amount of deposited Pd. Thicker Pd-deposits behave similar to bulk Pd-samples, while for thinner films the oxidation temperatures may be lowered. Interestingly, for very small amounts of Pd, in situ XPS shows full oxidation at room temperature and at less than 0.6 mbar O2 pressure. This indicates lowering of the kinetic barriers for oxidation of very small supported Pd-clusters. The formed oxide is, however, not stable in ultra high vacuum and a slow reduction is observed. The instability of this oxide in UHV indicates that the formed Pd-oxide at the interface to ZnO may have different chemical properties compared to bulk PdO or surface oxides on Pd.

Influence of ZnO Facets on Pd/ZnO Catalysts for Methanol Steam Reforming

A series of Pd/ZnO catalysts with different Pd loadings were prepared using needlelike ZnO crystallites (ZnO-N) with predominant (10–10) nonpolar facets exposed and commercial ZnO (ZnO-P) without any dominant facets. The Pd/ZnO catalysts were characterized using complementary techniques, such as nitrogen physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Fourier transform infrared spectroscopy analysis of adsorbed CO (CO-FTIR). The effect of ZnO crystallite faceting on the activity and selectivity of the Pd/ZnO catalysts was studied in methanol steam reforming (MSR). It was found that the Pd-rich phases (PdxZny, x > y) are predominantly formed at low Pd loadings on both ZnO supports (ZnO-N and ZnO-P), resulting in high CO selectivity. As Pd loading increases, the x/y ratio in the PdxZny phases decreases, leading to decreased CO selectivity. At similar Pd loadings, Pd/ZnO-P catalysts are more selective in MSR than Pd/ZnO-N, which is due to more facile formation of the stable PdZnβ phase on polar ZnO (0001) facets than on nonpolar ZnO (10–10) facets. The current study provides insight into the structure–performance relationships in Pd/ZnO catalysts for MSR, helping shed light on the rational design of selective MSR catalysts to minimize CO formation.