Surface species formed during thermal transformation of ethanol on ZnO powder

Application of modulation excitation-phase sensitive detection-DRIFTS for in situ/operando characterization of heterogeneous catalysts

A new more general method and guidelines for the implementation of modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS).

Effect of ZnO on acid–base properties and catalytic performances of ZnO/ZrO2–SiO2 catalysts in 1,3-butadiene production from ethanol–water mixture

The effect of ZnO and the preparation method of ZnO/ZrO₂–SiO₂ catalysts on their acid–base properties and catalytic performances in the conversion of diluted ethanol mixtures into 1,3-butadiene (BD) is presented.

Comparison of ZnO surface modification with gas-phase propiolic acid at high and medium vacuum conditions

Recent advances in preservation of the morphology of ZnO nanostructures during dye sensitization required the use of a two-step preparation procedure. The first step was the key for preserving ZnO materials morphology. It required exposing clean ZnO nanostructures to a gas-phase prop-2-ynoic acid (propiolic acid) in vacuum. This step resulted in the formation of a robust and stable surface-bound carboxylate with ethynyl groups available for further modification, for example, with click chemistry. This paper utilizes spectroscopic and microscopic investigations to answer several questions about this modification and to determine if the process can be performed under medium vacuum conditions instead of high vacuum procedures reported earlier. Comparing the results of the preparation process at medium vacuum of 0.5 Torr base pressure with the previously reported investigations of the same process in high vacuum of 10^-5Torr suggests that both processes lead to the formation of the same surface species, confirming that the proposed modification scheme can be widely applicable for ZnO sensitization procedures and does not require the use of high vacuum. Additional analysis comparing the computationally predicted surface structures with the results of spectroscopic investigations yields the more complete description of the surface species resulting from this approach.

Universal Calibration of Computationally Predicted N 1s Binding Energies for Interpretation of XPS Experimental Measurements

Computationally predicted N 1s core level energies are commonly used to interpret the experimental measurements obtained with X-ray photoelectron spectroscopy. This work compares the application of Koopmans' theorem to core electrons using the B3LYP functional with two commonly used basis sets, analyzes the factors relevant to the comparison of the computational with experimental data, and presents several correlations that allow an accurate prediction of the N 1s binding energy. The first correlation is obtained with a series of known nitrogen-containing functional groups on well-characterized organic monolayers. This approach can then be reliably extended to a number of nitrogen-containing chemical systems on silicon surfaces in which the nature of the chemical environment of nitrogen atoms had only been proposed based on a number of analytical techniques. In most of those cases, the XPS analysis is consistent with the proposed structures, but is not always sufficient for conclusive assignments. Third, it was attempted to also include N-containing systems on metals. Despite the admittedly oversimplified approach taken in this case (the metal surface is approximated by a single atom), the observed correlations are still experimentally useful, although in this case significant outliers are found. Finally, previously published correlations between experimental and theoretical C 1s data were reexamined, yielding a set of correlations that allow experimentalists to predict C 1s and N 1s XPS spectra with high accuracy.

Chemical Protection of Material Morphology: Robust and Gentle Gas-Phase Surface Functionalization of ZnO with Propiolic Acid

Chemical functionalization of ZnO surface with an alkyne functional group was successfully achieved by exposing ZnO nanopowder to gas-phase propiolic acid in vacuum, which left the alkyne group available for subsequent chemical modification via the azide-alkyne cycloaddition "click" reaction with benzyl azide. The highly selective formation of a bidentate carboxylate linkage and the reaction of benzyl azide were confirmed by solid-state nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Most importantly, scanning electron microscopy revealed that the surface morphology was perfectly preserved by this gas-phase modification, as opposed to the alternative protocols based on liquid phase processing. This simple and precise design can serve as a universal method for the modular functionalization of zinc oxide surface following the initial surface preparation and be further applied to thin films, nanostructures, and powders, where preserving surface morphology during chemical modification is especially important.

Transmetalation Process as a Route for Preparation of Zinc-Oxide-Supported Copper Nanoparticles

Supported nanoparticulate materials have a variety of uses, from energy storage to catalysis. In preparing such materials, precision control can often be achieved by applying chemical deposition methods. However, ligand removal following the initial deposition presents a substantial challenge because of potential surface contamination. Traditional approaches normally include multistep processing and require a substantial thermal budget. Using transmetalation chemistry, it is possible to circumvent both disadvantages and prepare chemically reactive copper nanoparticles supported on a commercially available ZnO powder material by metalorganic vapor copper deposition followed by very mild annealing to 350 K. The self-limiting copper deposition reaction is used to demonstrate the utility of this approach for hexafluoroacetylacetonate-copper-vinyltrimethylsilane, Cu(hfac)VTMS, reacting with ZnO. The low-temperature transmetalation is confirmed by a combination of spectroscopic studies. Model density functional theory calculations are consistent with a thermodynamic driving force for the process.

Selectivity and Mechanism of Thermal Decomposition of β-diketones on ZnO Powder

The thermal chemistry of β-diketones underlies a number of catalytic processes related both to the catalytic reactions yielding commodity chemicals and to the production of supported transition metal catalysts themselves. The mechanisms of decomposition during thermal transformation of three β-diketones, acetylacetone (acacH), 1,1,1-trifluoroacetylacetone (tfacH), and 1,1,1,5,5,5-hexafluoroacetylacetone (hfacH), were studied on ZnO powder surface using Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and Density functional theory (DFT) computational investigation. The initial O-H dissociation leads to the formation of corresponding β-diketonates in all the cases investigated. These diketonates are important surface intermediates that can be generated in a controlled manner in these experiments. The presence on the C-CF₃ entity determines the preferred thermal decomposition pathways, as the C-C bond in this group starts to react with a surface of ZnO around 400 K, followed by immediate decomposition of the resulting CF₃ group. Above 600 K, the presence of the CF₃-substituent leads to the formation of ketene-like structures observed by vibrational spectroscopy. The reaction mechanisms examined with the help of DFT calculations are correlated with vibrational signatures of the species produced and with the F-containing species recorded by XPS.

In situ investigation of organic ligand displacement processes on ZnO powder surface

A variety of surface processes require removal or replacement of surface-bound chemical functional groups to achieve the properties required by a specific application. In the case of ZnO powder, a number of applications require manipulation of surface-bound species including ethoxy, acetoxy, acetylacetate, or 1,1,1,5,5,5-hexafluoroacetylacetate. The displacement of the surface species formed by these compounds on ZnO powder surfaces by a gas-phase reagent is described by a model that takes into account stability of surface species predicted by density functional theory and the strength of binding of the second layer on top of the first, provided by the ΔH of sublimation. This simple model is tested by infrared spectroscopy following the adsorption of one compound and its displacement by the other. A correlation between the enthalpic driving force and the percentage of the displaced species observed experimentally is found. This simple approach can be improved and generalized further to include other surface-bound species and other materials.

Gas-phase oxidation of ethanol over Au/TiO2 catalysts to probe metal–support interactions

Ethanol and oxygen were converted over titania and gold nanoparticles supported on titania to investigate the reactivity of the support, the influence of the metal, and the role of metal–support interactions.