Ivo F. Teixeira

From Biomass‐Derived Furans to Aromatics with Ethanol over Zeolite

We report a novel catalytic conversion of biomass-derived furans and alcohols to aromatics over zeolite catalysts. Aromatics are formed via Diels-Alder cycloaddition with ethylene, which is produced in situ from ethanol dehydration. The use of liquid ethanol instead of gaseous ethylene, as the source of dienophile in this one-pot synthesis, makes the aromatics production much simpler and renewable, circumventing the use of ethylene at high pressure. More importantly, both our experiments and theoretical studies demonstrate that the use of ethanol instead of ethylene, results in significantly higher rates and higher selectivity to aromatics, due to lower activation barriers over the solid acid sites. Synchrotron-diffraction experiments and proton-affinity calculations clearly suggest that a preferred protonation of ethanol over the furan is a key step facilitating the Diels-Alder and dehydration reactions in the acid sites of the zeolite.

Trimethylphosphine-Assisted Surface Fingerprinting of Metal Oxide Nanoparticle by 31P Solid-State NMR: A Zinc Oxide Case Study

Nano metal oxides are becoming widely used in industrial, commercial and personal products (semiconductors, optics, solar cells, catalysts, paints, cosmetics, sun-cream lotions, etc.). However, the relationship of surface features (exposed planes, defects and chemical functionalities) with physiochemical properties is not well studied primarily due to lack of a simple technique for their characterization. In this study, solid state (31)P MAS NMR is used to map surfaces on various ZnO samples with the assistance of trimethylphosphine (TMP) as a chemical probe. As similar to XRD giving structural information on a crystal, it is demonstrated that this new surface-fingerprint technique not only provides qualitative (chemical shift) but also quantitative (peak intensity) information on the concentration and distribution of cations and anions, oxygen vacancies and hydroxyl groups on various facets from a single deconvoluted (31)P NMR spectrum. On the basis of this technique, a new mechanism for photocatalytic •OH radical generation from direct surface-OH oxidation is revealed, which has important implications regarding the safety of using nano oxides in personal care products.

Hydrazine-Assisted Liquid Exfoliation of MoS2 for Catalytic Hydrodeoxygenation of 4-Methylphenol.

A simple but effective method to exfoliate bulk MoS2 in a range of solvents is presented for the preparation of colloid flakes consisted of one to a few molecular layers by application of ultrasonic treatment in N2 H4 . Their high yield in solution and exposure of more active surface sites allows the synthesis of corresponding solid catalysts with remarkably high activity in hydrodeoxygenation of 4-methylphenol and this method can also be applied to other two dimensional materials.

Cellulose Nanocrystals Assembled on the Fe3O4Surface as Precursor to Prepare Interfaced C/Fe3O4Composites for the Oxidation of Aqueous Sulfide

In this work, composites based on carbon (1, 10 and 20 wt.%) interfaced with Fe3O4 (magnetite) have been studied as catalysts for the oxidation of aqueous sulfide. The composites were prepared by assembling cellulose nanocrystals surrounding Fe3O4 followed by a controlled thermal decomposition at 400, 600 and 800 oC. Mossbauer, X-ray diffractometry (XRD), Raman, thermogravimetry (TG), elemental analysis CHN, scanning electron microscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR) and potentiometric titration indicated that at 400 and 600 oC the cellulose nanocrystals decompose to different carbon forms, i.e., films, filaments and particles attached to the Fe3O4 crystals. At higher temperature, i.e., 800 oC, this carbon on the magnetite surface further reacted to produce Fe0. UV-Vis, Raman and electrospray ionization mass spectrometry (ESI-MS) measurements showed that these composites catalyze the oxidation of aqueous sulfide to convert S2-aq to polysulfides Sn2- (where n = 2-9) and also oxygen containing polysulfides HOSn-. Simple kinetic experiments showed very low sulfide oxidation activities for pure Fe3O4 and pure carbon. On the other hand, the composites, especially with 10% C obtained at 600 oC, were remarkably active. These results are discussed in terms of a possible participation of oxygen based redox groups present on the carbon surface and an electron transfer from the carbon to the Fe3O4 phase.

Mapping surface-modified titania nanoparticles with implications for activity and facet control

The use of surface-directing species and surface additives to alter nanoparticle morphology and physicochemical properties of particular exposed facets has recently been attracting significant attention. However, challenges in their chemical analysis, sometimes at trace levels, and understanding their roles to elucidate surface structure–activity relationships in optical (solar cells) or (photo)catalytic performance and their removal are significant issues that remain to be solved. Here, we show a detailed analysis of TiO2 facets promoted with surface species (OH, O, SO4, F) with and without post-treatments by 31P adsorbate nuclear magnetic resonance, supported by a range of other characterization tools. We demonstrate that quantitative evaluations of the electronic and structural effects imposed by these surface additives and their removal mechanisms can be obtained, which may lead to the rational control of active TiO2 (001) and (101) facets for a range of applications.Metal oxide nanocrystals can be grown with different facets exposed to give variations in reactivity, but the chemical state of these surfaces is not clear. Here, the authors make use of a phosphine probe molecule allowing the differences in surface chemistry to be mapped by NMR spectroscopy.

Probe-Molecule-Assisted NMR Spectroscopy: A Comparison with Photoluminescence and Electron Paramagnetic Resonance Spectroscopy as a Characterization Tool in Facet-Specific Photocatalysis

Photoluminescence (PL) and EPR spectroscopy are the two most commonly used techniques to evaluate surface oxygen vacancies (VO surface) on photoactive transition metal oxides. However, these techniques have shortcomings in the characterization of facet‐dependent catalysis. Here, we used the chemical adsorption of a 31P‐containing probe molecule to obtain the distribution/concentration of VO surface as well as other surface features (hydroxyl groups) of ZnO by using 31P magic‐angle spinning NMR spectroscopy. This facet‐specific technique not only makes cross‐literature comparison feasible but also provides more comprehensive information to lead to a further understanding of the photocatalytic mechanism.

Structure-activity correlations for Brønsted acid, Lewis Acid, and photocatalyzed reactions of exfoliated crystalline niobium oxides

Exfoliated crystalline niobium oxides that contain exposed but interconnected NbO6 octahedra with different degrees of structural distortion and defects are known to catalyze Brønsted acid (BA), Lewis acid (LA), and photocatalytic (PC) reactions efficiently but their structure–activity relationships are far from clear. Here, three exfoliated niobium oxides, namely, HSr2Nb3O10, HCa2Nb3O10, and HNb3O8, are synthesized, characterized extensively, and tested for selected BA, LA, and PC reactions. The structural origin for BA is associated mainly with acidic hydroxyl groups of edge‐shared NbO6 octahedra as proton donors; that of LA is associated with the vacant band position of Nb5+ to receive electron pairs from substrate; and that of PC is associated with the terminal Nb=O of NbO6 octahedra for photon capture and charge transfer to long‐lived surface adsorbed substrate complex through associated oxygen vacancies in close proximity. It is believed that an understanding of the structure–activity relationships could lead to the tailored design of NbOx catalysts for industrially important reactions.

Magnetic amphiphilic nanocomposites based on silica–carbon for sulphur contaminant oxidation

Silica and carbon based magnetic amphiphilic nanocomposites (MANCs) were synthesized and applied for desulfurization in this work. The structure, composition, and magnetic and amphiphilic properties of the resulting MANCs were characterized in detail by physicochemical means such as XRD, elemental analysis, Raman spectroscopy, electron microscopies, thermal analysis, TPR, Mossbauer spectroscopy, magnetization measurements, and contact angle measurements. Different concentrations of Fe and Mo were supported on the surface of silica in order to catalyse the controlled growth of carbon nanotubes and nanofibers by chemical vapor deposition (CVD). The partial coating of hydrophilic silica with hydrophobic carbon nanostructures imparts amphiphilicity, which makes the composites strategic catalysts to promote emulsion formation and to act on the interface. Moreover, during the CVD process magnetic species were produced conferring magnetic properties which can facilitate the emulsion breakage by a simple magnetic process. Studies on desulfurization reactions catalysed by these nanocomposites were promising and showed that Mo plays an important role in the catalyst efficiency.

Photocatalytic Dehydrogenation of Formic Acid on CdS Nanorods through Ni and Co Redox Mediation under Mild Conditions

Selective release of hydrogen from formic acid (FA) is deemed feasible to solve issues associated with the production and storage of hydrogen. Here, we present a new efficient photocatalytic system consisting of CdS nanorods (NRs), Ni, and Co to liberate hydrogen from FA. The optimized noble-metal-free catalytic system employs Ni/Co as a redox mediator to relay electrons and holes from CdS NRs to the Ni and Co, respectively, which also deters the oxidation of CdS NRs. As a result, a high hydrogen production activity of 32.6 mmol h-1  g-1 from the decomposition of FA was noted. Furthermore, the photocatalytic system exhibits sustained H2 production rate for 12 h with sequential turnover numbers surpassing 4×103 , 3×103 , and 2×103 for Co-Ni/CdS NRs, Ni/CdS NRs, and CoCl2 /CdS NRs, respectively.