Mechanistic Studies of Reducible Metal Oxides as Hydrodeoxygenation Catalysts

Hydrodeoxygenation of phenol to benzene using ruthenium supported titania catalysts strongly varies depending on the support crystal structure and preparation conditions. Here, we performed spectroscopic characterization of titania supports to identify the surface impurities common to commercial and synthesized titania samples using a variety of spectroscopic methods. Sulfate impurities were detected for the commercial anatase samples and a procedure for their elimination was proposed so that inactive catalysts gained reactivity. Surface hydroxyls of different TiO2 samples (anatase, rutile, and pyrogenic) were identified using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments performed on vigorously cleaned surfaces and a facet-specific assignment was proposed using DFT calculations performed by our collaborators. In addition, the electronic structure of TiO2 samples were studied using the reaction of vigorously cleaned TiO2 samples with H2/D2. Our results revealed that sulfate impurities of the commercial anatase samples change their electronic structure consistent with creation of deep electronic trap states within the band gap. Our results are used to derive structure-activity relationships for the Ru/titania catalyzed hydrodeoxygenation reactions of phenol

Fabrication of Multifunctional Electronic Textiles Using Oxidative Restructuring of Copper into a Cu-Based Metal–Organic Framework

This paper describes a novel synthetic approach for the conversion of zero-valent copper metal into a conductive two-dimensional layered metal–organic framework (MOF) based on 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to form Cu3(HHTP)2. This process enables patterning of Cu3(HHTP)2 onto a variety of flexible and porous woven (cotton, silk, nylon, nylon/cotton blend, and polyester) and non-woven (weighing paper and filter paper) substrates with microscale spatial resolution. The method produces conductive textiles with sheet resistances of 0.1–10.1 MΩ/cm2, depending on the substrate, and uniform conformal coatings of MOFs on textile swatches with strong interfacial contact capable of withstanding chemical and physical stresses, such as detergent washes and abrasion. These conductive textiles enable simultaneous detection and detoxification of nitric oxide and hydrogen sulfide, achieving part per million limits of detection in dry and humid conditions. The Cu3(HHTP)2 MOF also demonstrated filtration capabilities of H2S, with uptake capacity up to 4.6 mol/kgMOF. X-ray photoelectron spectroscopy and diffuse reflectance infrared spectroscopy show that the detection of NO and H2S with Cu3(HHTP)2 is accompanied by the transformation of these species to less toxic forms, such as nitrite and/or nitrate and copper sulfide and Sx species, respectively. These results pave the way for using conductive MOFs to construct extremely robust electronic textiles with multifunctional performance characteristics

Host–Guest Interactions and Redox Activity in Layered Conductive Metal–Organic Frameworks

This paper describes the identification of specific host–guest interactions between basic gases (NH3, CD3CN, and pyridine) and four topologically similar 2-dimensional (2D) metal–organic frameworks (MOFs) comprising copper and nickel bis(diimine) and bis(dioxolene) linkages of triphenylene-based ligands using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy (EPR), and powder X-ray diffraction (PXRD). This contribution demonstrates that synthetic bottom-up control over surface chemistry of layered MOFs can be used to impart Lewis acidity or a mixture of Brønsted and Lewis acidities, through the choice of organic ligand and metal cation. This work also distinguishes differences in redox activity within this class of MOFs that contribute to their ability to promote electronic transduction of intermolecular interactions. Future design of structure–function relationships within multifunctional 2D MOFs will benefit from the insights this work provides

Combining Benzyl Alcohol Oxidation Saturation Kinetics and Hammett Studies as Mechanistic Tools for Examining Supported Metal Catalysts

Understanding and quantifying how the active sites in supported metal catalysts can be modified are critical for rationally designing catalysts. This problem is particularly complex for reactions that occur at the metal-support interface (MSI) because of the multiple chemistries associated with the metal and the support. In this study, we used the oxidation of substituted benzyl alcohol over Au/TiO2 and Au/Al2O3 to probe MSI chemistry. Substituents impacted substrate binding, deprotonation, and the rate-limiting transfer of a hydride from benzyl alcohol to Au, as shown by a combination of Michaelis-Menten (M-M) saturation kinetics and kinetic isotope effects. Hammett studies performed with a single substrate versus those done with two substrates together in competition experiments showed significant differences, which were attributable to stronger competitive adsorption on the support by more electron-rich alcohols. The M-M analysis showed that alcohol substitution impacts substrate binding and deprotonation equilibria, which in turn affect the number of active alkoxides adsorbed at the MSI. Hammett slopes should therefore be measured under saturating conditions using one substrate at a time. The Hammett slopes measured for heterogeneous systems in this manner agree well with the KIE-Hammett slope relationship determined in homogeneous systems, which provide information on the early or late nature of the transition state. Our results show that the combination of Michaelis-Menten and Hammett techniques for benzyl alcohol oxidation provides mechanistic information associated with the MSI chemistry of supported Au catalysts as well as information on active site electronics

Kinetics of H\u3csub\u3e2\u3c/sub\u3e Adsorption at the Metal–Support Interface of Au/TiO\u3csub\u3e2\u3c/sub\u3e Catalysts Probed by Broad Background IR Absorbance

H2 adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H2 adsorption on Au/TiO2 catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H2 adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H2 adsorption kinetics measured with the BBA show a lower Eapp value (23 kJ mol−1) for H2 adsorption than previously reported from proxy H/D exchange (33 kJ mol−1). We also identify a previously unreported H-O-H bending vibration associated with proton adsorption on electronically distinct Ti-OH metal-support interface sites, providing new insight into the nature and dynamics of H2 adsorption at the Au/TiO2 interface

Direct Evidence for Sulfur-Induced Deep Electron and Hole Traps in Titania and Implications for Photochemistry

Numerous studies show that sulfating titania narrows its band gap, facilitating longer-wavelength photochemistry, but there are contradictory reports on whether this improves or degrades UV photocatalysis and whether reactions proceed via electron- or hole-mediated pathways. The widely proposed role of sulfur is that it induces deep electron traps, which increase hole lifetimes. There is, however, no direct evidence for unoccupied states deep in the band gap. By contrast, transient absorption spectroscopy indicates that sulfur induces hole traps. We present experiments on sulfur-free and sulfated titania in which dissociation of hydrogen generates electrons that fill the lowest unoccupied states. The energy of these electrons relative to the conduction band minimum (CBM) was measured with diffuse reflectance spectroscopy in the infrared and UV–vis ranges. For all commercial sulfur-containing anatase materials, conversion of tridentate sulfate species into sulfur substituted on lattice sites occurred under highly oxidizing conditions above 400 °C and led to partially unoccupied states ∼2.8 eV below the CBM. We assign this deep trap state to sulfur atoms substituted on a titanium lattice site with a formal charge of S5+ in non-stoichiometric TiO2+x, based on agreement between the experiment and the predicted UV–vis spectrum of Harb, Sautet, and Raybaud, using HSE06 density functional perturbation theory. Our band structure calculations demonstrate that titanium vacancies (or excess oxygen) are necessary to create partially unoccupied states, and X-ray diffraction Rietveld analysis confirms the existence of these vacancies. The partial occupancy of these states, along with sulfur’s ability to switch oxidation states, explains their role as both electron (S5+ + e– → S4+) and hole (S5+ + h+ → S6+) traps, reconciling previous work. We discuss how relative rates of electron vs hole trapping can enhance or degrade activity depending on the pathway and the TiO2+x non-stoichiometry. We consider how increasing the dopant concentration can induce band bending or pin the Fermi level and shift the redox reactions that are thermodynamically accessible