Aerosol Assisted Chemical Vapor Deposition of Transparent Conductive Zinc Oxide Films

Highly conductive and transparent ZnO films were synthesized by the reaction of diethyl zinc (in toluene) with methanol by dual source aerosol assisted chemical vapor deposition. These films displayed a stable sheet resistance of 7.2 Ω/□ and high transmission across the visible region comparable to commercial transparent conducting oxides (TCOs) based on oxides of tin or indium. Doping the zinc oxide structure with fluorine (trifluorotoluene) resulted in dense compact films with improved electrical properties than ZnO films with a sheet resistance of 4.5 Ω/□. These films also displayed idealized surface texturing for photovoltaic applications. Fluorine concentration was 2 at.% determined by wavelength dispersive X-ray analysis (WDX). Aluminum doped zinc oxide films were also synthesized by introducing dopant amounts of trimethylaluminium solution (in toluene) into the system. These films exhibited low sheet resistances of 14 Ω/□. The aluminum concentration in the films was 4 at.% determined by WDX

Controlling the Cross-Sensitivity of Carbon Nanotube-Based Gas Sensors to Water Using Zeolites

Carbon nanotube-based gas sensors can be used to detect harmful environmental pollutants such as NO₂ at room temperature. Although they show promise as low-powered, sensitive, and affordable monitoring devices, cross-sensitivity of functionalized carbon nanotubes to water vapor often obscures the detection of target molecules. This is a barrier to adoption for monitoring of airborne pollutants because of the varying humidity levels found in real world environments. Zeolites, also known as molecular sieves because of their selective adsorption properties, are used in this work to control the cross-sensitivity of single-walled carbon nanotube (SWCNT)-based sensors to water vapor. Zeolites incorporated into the sensing layer are found to reduce interference effects that would otherwise obscure the identification of NO₂ gas, permitting repeatable detection over a range of relative humidities. This significant improvement is found to depend on the arrangement of the SWCNT-zeolite layers in the sensing device, as well as the hydrophilicity of the chosen zeolite

Zn–Co Double Metal Cyanides as Heterogeneous Catalysts for Hydroamination: A Structure–Activity Relationship

Zn–Co double metal cyanide (DMC) materials are effective heterogeneous catalysts for intermolecular hydroaminations. Using the reaction of 4-isopropylaniline with phenylacetylene as a test, the effect of different catalyst synthesis procedures on the catalytic performance is examined. The best activities are observed for double metal cyanides with a cubic structure and prepared with a Zn²⁺ excess, and for nanosized particles prepared via a reverse emulsion synthesis. Detailed study of the active Zn²⁺ sites in the cubic material by EXAFS gives evidence for coordinative vacancies around the Zn, with four cyanide ligands in close proximity of the Zn. The substrate scope of the hydroaminations was successfully expanded to both aromatic and aliphatic alkynes and other aromatic and aliphatic amines. Even with styrenes the reaction proceeded with aromatic amines. The DMC catalysts are truly heterogeneous, possess a high thermal stability and are perfectly reusable

Following the Creation of Active Gold Nanocatalysts from Phosphine-Stabilized Molecular Clusters

The phosphine-stabilized gold cluster [Au₆(Ph₂P-<i>o</i>-tolyl)₆]­(NO₃)₂ is converted into an active nanocatalyst for the oxidation of benzyl alcohol through low-temperature peroxide-assisted removal of the phosphines, avoiding the high-temperature calcination process. The process was monitored using in situ X-ray absorption spectroscopy, which revealed that after a certain period of the reaction with tertiary butyl hydrogen peroxide, the phosphine ligands are removed to form nanoparticles of gold which matches with the induction period seen in the catalytic reaction. Density functional theory calculations show that the energies required to remove the ligands from the [Au₆L_<i>n</i>]²⁺ increase significantly with successive removal steps, suggesting that the process does not occur at once but sequentially. The calculations also reveal that ligand removal is accompanied by dramatic rearrangements in the topology of the cluster core

Understanding Structure–Function Relationships in Zeolite-Supported Pd Catalysts for Oxidation of Ventilation Air Methane

Catalytic combustion of ventilation air methane (VAM) is a potential solution for abatement of this greenhouse gas. In this study, we evaluate the combustion of VAM (with methane concentrations below 1%) spanning over 100 h time on stream (TOS) during reaction over a Pd/HZSM-5 catalyst. The aim is to understand the structural changes that lead to catalyst deactivation. We observe the formation of carbonaceous deposits even under oxygen-rich conditions, which are an important contributor to deactivation. X-ray absorption spectroscopic (XAS) investigation shows that, in addition to carbon deposits, the growth of Pd oxide clusters leads to a reduced number of accessible sites and in turn intrinsic activity. STEM-EDS analysis disclosed the presence of the carbonaceous deposit on the surface of the used catalyst, and TGA confirmed the presence of different carbon species on the used catalyst under very lean conditions. Structural changes show that Pd–O/acid–base interactions have a significant influence on the structure of the active site. This assertion is consistent with findings from acid–base characterization experiments. Although the catalyst displayed a high level of stability over the first 10 h of VAM combustion, long-term reaction, in the presence of water vapor, is associated with partial rearrangement of the zeolite, accompanied by a gradual deactivation of the catalyst. This rearrangement is associated with a decrease in surface area and pore volume, which is consistent with the significant changes observed in the Al-X-ray absorption near-edge spectroscopic (XANES) analysis. A comparison of the NH₃-TPD of fresh and used Pd/HZSM-5 catalysts shows that the strengths of the acid sites are significantly reduced. This is a consequence of the changing nature of transition metal interaction with the zeolite, which is accompanied by the dealumination of the zeolite support, thereby enhancing Pd agglomeration and the emergence of two low index surface orientation facet planes identified as PdO(101) and PdO(100). A higher turnover frequency (TOF) (0.031 s^–1) for reactivated Pd/HZSM-5 after removing all carbonaceous material compared to the TOF (0.024 s^–1) for used Pd/HZSM-5 was observed. The catalyst regained 75% of its initial catalytic activity after removing carbonaceous compound from the used catalyst. We propose the formation of a palladium carbonaceous complex manifesting itself in carbonate and a carbonyl group observed in used Pd/HZSM-5. These species act as an important contributor to catalyst deactivation and cause partial reversible deactivation during long-term VAM combustion

Reduction of Propionic Acid over a Pd-Promoted ReO_<i>x</i>/SiO₂ Catalyst Probed by X‑ray Absorption Spectroscopy and Transient Kinetic Analysis

A Pd-promoted Re/SiO₂ catalyst was prepared by sequential impregnation and compared to monometallic Pd/SiO₂ and Re/SiO₂. All samples were characterized by electron microscopy, H₂ and CO chemisorption, H₂ temperature-programmed reduction, and <i>in situ</i> X-ray absorption spectroscopy at the Re L_III and Pd K-edges. The samples were also tested in the reduction of propionic acid to 1-propanol and propionaldehyde at 433 K in 0.1–0.2 MPa H₂. Whereas monometallic Pd was inactive for carboxylic acid reduction, monometallic Re catalyzed aldehyde formation but only after high-temperature prereduction that produced metallic Re. When Pd was present with Re in a bimetallic catalyst, Pd facilitated the reduction of Re in H₂ to ∼+4 oxidation state at modest temperatures, producing an active catalyst for the conversion of propionic acid to 1-propanol. Under the conditions of this study, the orders of reaction in propionic acid and H₂ were approximately zero and one, respectively. Transient kinetic analysis of the carboxylic acid reduction to alcohols revealed that at least 50% of the Re in the bimetallic catalyst participated in the catalytic reaction. The Pd is proposed to enhance the catalytic activity of the bimetallic catalyst by spilling over hydrogen that can partially reduce Re and react with surface intermediates

Optimized Atmospheric-Pressure Chemical Vapor Deposition Thermochromic VO₂ Thin Films for <i>Intelligent</i> Window Applications

Monoclinic vanadium­(IV) oxide (VO₂) has been widely studied for energy-efficient glazing applications because of its thermochromic properties, displaying a large change in transmission of near-IR wavelengths between the hot and cold states. The optimization of the reaction between VCl₄ and ethyl acetate via atmospheric-pressure chemical vapor deposition (APCVD) was shown to produce thin films of monoclinic VO₂ with excellent thermochromic properties (Δ<i>T</i>_sol = 12%). The tailoring of the thermochromic and visible light transmission was shown to be possible by altering the density and morphology of the deposited films. The films were characterized by X-ray diffraction, atomic-force microscopy, scanning electron microscopy, ellipsometry, and UV–vis spectrometry. This article provides useful design rules for the synthesis of high-quality VO₂ thin films by APCVD

A Comprehensive Scenario of the Crystal Growth of γ‑Bi₂MoO₆ Catalyst during Hydrothermal Synthesis

In a previous study, we confirmed by in situ combined high-resolution powder diffraction/X-ray absorption spectroscopy and Raman scattering experiments that the crystal formation of the well-known γ-Bi₂MoO₆ catalyst occurred in two steps through the formation of an intermediate fluorite structure (Kongmark, C. Chem. Commun. 2009, 4850−4852; Catal. Today 2010, 157, 257–262). Here, for the first time, by combining these results with complementary ex situ studies (HTXRD, SEM/EDX, Raman scattering), it was possible to elucidate the nature of the intermediate phase and to propose a complete scenario of the growth of γ-Bi₂MoO₆ crystals. In addition, we performed a kinetic analysis from the three different in situ characterization techniques and confirmed a 2D-diffusion limited growth process

Active Nature of Primary Amines during Thermal Decomposition of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles

Although [Ni­(S₂CNBuⁱ₂)₂] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 °C in oleylamine to give pure NiS nanoparticles, while in <i>n</i>-hexylamine at 120 °C a mixture of Ni₃S₄ (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral <i>trans-</i>[Ni­(S₂CNBuⁱ₂)₂(RNH₂)₂] result as shown by <i>in situ</i> XANES and EXAFS and confirmed by DFT calculations. Heating to 90–100 °C leads to changes consistent with the formation of amide-exchange products, [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­R}] and/or [Ni­{S₂CN­(H)­R}₂]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni­{S₂CN­(H)­Hex}₂] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 °C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni­{S₂CN­(H)­Hex}₂]. DFT modeling of [Ni­{S₂CN­(H)­R}₂] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S₂CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni­{S₂CN­(H)­R}­(μ-SH)]₂. These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amide-exchange that account primarily for the formation of the observed nanoparticulate nickel sulfides

Isolated Fe Sites in Metal Organic Frameworks Catalyze the Direct Conversion of Methane to Methanol

Hybrid materials bearing organic and inorganic motifs have been extensively discussed as playgrounds for the implementation of atomically resolved inorganic sites within a confined environment, with an exciting similarity to enzymes. Here, we present the successful design of a site-isolated mixed-metal metal organic framework (MOF) that mimics the reactivity of soluble methane monooxygenase enzyme and demonstrates the potential of this strategy to overcome current challenges in selective methane oxidation. We describe the synthesis and characterization of an Fe-containing MOF that comprises the desired antiferromagnetically coupled high-spin species in a coordination environment closely resembling that of the enzyme. An electrochemical synthesis method is used to build the microporous MOF matrix while integrating the atomically dispersed Fe active sites in the crystalline scaffold. The model mimics the catalytic C–H activation behavior of the enzyme to produce methanol and shows that the key to this reactivity is the formation of isolated oxo-bridged Fe units