Search

The recent findings on the fundamental understanding of the activity of carbon nitride-based materials in the hydrogen evolution reaction (HER) allow to design rationally the photocatalytic systems with improved activities.

Photocatalysis provides a sustainable pathway to produce the consumer chemical H

Here we report a new record of the selective methane photooxidation into methanol under mild conditions promoted by highly dispersed Cu atoms on crystalline carbon nitride, it displayed a remarkable methanol production (2900 μmol g−1) in 4 hours.

Herein, we describe a simple laboratory experiment to address nanomaterial synthesis, plasmon resonance, and its application to detect Cu2+ in ultralow concentrations. The proposed experiment is ve...

The comprehension of the reactivity of the elements is an important goal to be achieved in undergraduate courses in inorganic chemistry. Therefore, at Universidade Federal de Sao Carlos (UFSCar), t...

Photocatalysis provides a sustainable pathway to produce the consumer chemical H2O2 from atmospheric O2 via an oxygen reduction reaction (ORR). Such an alternative is attractive to replace the cumbersome traditional anthraquinone method for H2O2 synthesis on a large scale. Carbon nitrides have shown very interesting results as heterogeneous photocatalysts in ORR because their covalent two-dimensional (2D) structure is believed to increase selectivity toward the two-electron process. However, an efficient and scalable application of carbon nitrides for this reaction is far from being achieved. Poly(heptazine imides) (PHIs) are a more powerful subgroup of carbon nitrides whose structure provides high crystallinity and a scaffold to host transition-metal single atoms. Herein, we show that PHIs functionalized with sodium and the recently reported fully protonated PHI exhibit high activity in two-electron ORR under visible light. The latter converted O2 to up to 1556 mmol L–1 h–1 g–1 H2O2 under 410 nm irradiation using inexpensive but otherwise chemically demanding glycerin as a sacrificial electron donor. We also prove that functionalization with transition metals is not beneficial for H2O2 synthesis, as the metal also catalyzes its decomposition. Transient photoluminescence spectroscopy suggests that H-PHIs exhibit higher activity due to their longer excited-state lifetime. Overall, this work highlights the high photocatalytic activity of the rarely examined fully protonated PHI and represents a step forward in the application of inexpensive covalent materials for photocatalytic H2O2 synthesis.

The hydrogen evolution reaction (HER) is a complex reaction involving many interdependent physicochemical steps. Highly ordered carbon nitride-based materials, such as Na-PHI and K-PHI, display some of the highest activities for H2 evolution among the carbon nitride-based materials, due to their electronic properties, but also the presence of cyanimide terminations, which favors the charge transfer for the Pt cocatalyst nanoparticles (NPs). For such highly optimized semiconductor structures, the necessity to control and improve other steps of the photocatalytic process becomes essential, in particular the poor electron transfer from the Pt NPs to the protons in solution over the Helmholtz or Stern layer. Taking highly ordered Na-PHI as a test material, the influence of water-dissolved alkali cations on the HER is systematically studied and it is experimentally verified that the electron transfer from the Pt NPs to the protons in solution limits the efficiency of heterogeneous carbon nitride-based catalysts. This paper explains how hydrated alkali cations influence electron transfer and are able to boost the H2 evolution rate of the same Na-PHI from 2401 up to 5330 µmol h-1 g-1 with an apparent quantum yield of 13% at 420 nm.

Potassium poly (heptazine imide) (K-PHI), a crystalline two-dimensional carbon-nitride material, is an active photocatalyst for water splitting. The potassium ions in K-PHI can be exchanged with other ions to change the properties of the material and eventually to design the catalysts. We report here the electronic structures of several ion-exchanged salts of K-PHI (K, H, Au, Ru, and Mg) and their feasibility as water splitting photocatalysts, which were determined by density functional theory (DFT) calculations. The DFT results are complemented by experiments where the performances in the photocatalytic hydrogen evolution reaction (HER) were recorded. We show that due to its narrow band gap, Ru-PHI is not a suitable photocatalyst. The water oxidation potentials are straddled between the band edge potentials of H-PHI, Au-PHI, and Mg-PHI; thus, these are active photocatalysts for both the oxygen and hydrogen evolution reactions, whereas K-PHI is active only for the HER. The experimental data show that these are active HER photocatalysts, in agreement with the DFT results. Furthermore, Mg-PHI has shown remarkable performance in the HER, with a rate of 539 μmol/(h·g) and a quantum efficiency of 7.14% at 410 nm light irradiation, which could be due to activation of the water molecule upon adsorption, as predicted by our DFT calculations.

The first page of this article is displayed as the abstract.

Herein we demonstrate that adding single atoms of selected transition metals to graphitic carbon nitrides allows the tailoring of the electronic and chemical properties of these 2D nanomaterials, directly impacting their usage in photocatalysis. These single-atom photocatalysts were successfully prepared with Ni2+, Pt2+ or Ru3+ by cation exchange, using poly(heptazine imides) (PHI) as the 2D layered platform. Differences in photocatalytic performance for these metals were assessed using rhodamine-B (RhB) and methyl orange (MO) as model compounds for degradation. We have demonstrated that single atoms may either improve or impair the degradation of RhB and MO, depending on the proper matching of the net charge of these molecules and the surface potential of the catalyst, which in turn is responsive to the metal incorporated into the PHI nanostructures. Computer simulations demonstrated that even one transition metal cation caused dramatic changes in the electronic structure of PHI, especially regarding light absorption, which was extended all along the visible up to the near IR region. Besides introducing new quantum states, the metal atoms strongly polarized the molecular orbitals across the PHI and electrostatic fields arising from the electronic transitions became at least tenfold stronger. This simple proof of concept demonstrates that these new materials hold promise as tools for many important photocatalytic reactions that are strongly dependent on our ability to control surface charge and its polarization under illumination, such as H2 evolution, CO2 reduction and photooxidation in general.

Ammonia has a large volume energy density and consequently high potential for application as a fuel and energy reservoir in the future. In this study, the dinuclear FeIII metallacycle complex [Fe2(H2opba)2Cl2(dmso)2]·2CH2Cl2·2dmso (1) with excellent capacity to selectively adsorb ammonia (11.3 mmol g−1) was investigated, demonstrating a strong preference for nitrogenous compounds, especially short-chain amines. The adsorption results suggest that the cavity in the structure of 1 has an essential role in the adsorption mechanism i.e. it is the responsible for size selectivity of 1 during the adsorption process. To better understand the adsorption mechanism of ammonia, 1 was fully characterized before and after adsorption. Thermogravimetric-mass spectrometry (TG-MS) and ammonia isotherm experiments revealed that 8 ammonia molecules were physisorbed by the complex cavity (free ammonia) and 4 ammonia molecules were chemisorbed (coordinated ammonia) per dinuclear metallacycle unit. Further characterization also confirmed the proposed adsorption mechanism, corroborating the structural modification of the compound after its interaction with ammonia; in addition, reuse tests showed that despite a small loss of efficiency, the compound was capable of desorbing most of the ammonia and could be used consecutively for at least three cycles. Thus, 1 is a potential ammonia storage material and adsorbent for the removal of ammonia from effluents.

Fe-N-C materials, when prepared as single-atom catalysts (SAC), display excellent activities in oxidation reactions. The systematic investigation of the iron coordination mode revealed that Fe-N4C catalysts are the most active for C-H bond oxidation. However, many of these catalysts are synthesized through pyrolysis, which is characterized by a lack of control and structures with heterogeneous composition, rarely presenting only atomically dispersed Fe−N−C as active sites. Herein, an alternative, reliable and easily reproducible method to obtain highly active Fe SACs (atomically dispersed) with Fe-N4 sites is presented, which is based on ion exchange of sodium from high crystalline sodium poly(heptazine imide) (Na-PHI) by other ions. The obtained catalyst can photocatalytically oxidize C-H bonds selectively toward ketones using only dioxygen. Detailed mechanism investigations indicate that the active species in the C-H bond oxidation are highly valent Fe(IV)/Fe(V)-oxo species, which are further activated by the holes generated at the PHI support under light irradiation.

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.

Red Mud (RM) based gold catalysts were prepared by supporting gold nanoparticles (AuNPs) on the RM residue reduced either with H2 or with ethanol – which promoted partial coating of the RM with carbon. The materials were characterized by X-ray diffraction, Mossbauer spectroscopy, elemental analysis CHN, transmission electron microscopy and EDS microanalysis. Their catalytic performance was evaluated in the liquid-phase oxidation of benzyl alcohol by molecular oxygen, for the first time, as far as we know. The results showed that AuNPs with diameters of around 10 nm were successfully supported and distributed on the surface of red mud based materials. Furthermore, carbon coating allowed better incorporation of AuNPs on the material surface and better dispersion of the material into the reaction solutions. AuNPs supported on RM reduced in ethanol and, therefore, coated with carbon showed better catalytic performance than AuNPs supported on MR reduced in hydrogen.

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.

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.

Herein, the structural defects of metal-free polymeric carbon nitrides were controlled by making use of different precursors in their syntheses, i.e. melamine (CN-M) and thiourea (CN-T), as well as a 1:1 mixture of them (CN-1M:1 T). By controlling the structural defects, the electronic, morphological and chemical properties were modified. Additionally, the activities of synthesized PCNs were evaluated for amoxicillin photodegradation under visible light irradiation (16 mW cm-2). The results of photocatalytic tests showed that CN-T material has better efficiency (100 % removal within 48 h), which is directly related to the greater number of defects present in its structure with consequent improvement of electron-hole pairs separation efficiency. The CN-T material showed excellent stability with only 13 % decrease in its photocatalytic activity after the third cycle. A mechanism for amoxicillin degradation by CN-T was proposed based on the ESI-MS and the in situ EPR allied with spin trapping method investigations.

Tandem reactions consist of a sequence of two or more distinct transformations that can be achieved in a single synthetic step. A new article by Sun and colleagues demonstrates how the catalytic properties of bimetallic nanoparticles can be rationally optimized to enable the one-pot synthesis of polybenzoxazole (PBO).

Ultrathin (1–3 cationic-layers) (CuZn)1–xGax-CO3 layered double hydroxide (LDH) nanosheets were synthesized following the aqueous miscible organic solvent treatment (AMOST) method and applied as catalyst precursors for methanol production from CO2 hydrogenation. It is found that, upon reduction, the aqueous miscible organic solvent treated LDH (AMO-LDH) samples above a critical Ga3+ composition give consistently and significantly higher Cu surface areas and dispersions than the catalysts prepared from conventional hydroxyl-carbonate phases. Owing to the distinctive local steric and electrostatic stabilization of the ultrathin LDH structure, the newly formed active Cu(Zn) metal atoms can be stably embedded in the cationic layers, exerting an enhancement to the catalytic reaction. The best catalyst in this study displayed methanol productivity with a space-time yield of 0.6 gMeOH·gcat–1 h–1 under typical reaction conditions, which, as far as we are aware, is higher than most reported Cu-based catalysts in the literature.

In this work, selective interactions between the constituents of the composite CNT@MnCu (2) prepared using carbon nonotubes (CNTs) (1) and the heterobimetallic chain [MnCu(opba)]n (MnCu), opba = o-phenylenebis(oxamate), were studied mainly by resonance Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). An apparent interaction between CNTs and MnCu complex with the wrapping of the former by the heterobimetallic complex was observed in the microscopy images. The resonance Raman data suggest that the interations between MnCu complex and the CNTs are selective, occurring mainly with metallic CNTs independently of the diameter and excitation energy. However, for semiconducting CNTs, these interactions solely occur with tubes having diameters higher than ca. 1.47 nm.

In this work, the spinel structured iron oxides, MFe2O4 (M = Cu2+ and Co2+), magnetite (Fe3O4) and maghemite (γ-Fe2O3) were investigated as catalyst for the sulfide oxidation in aqueous medium. XRD, Mössbauer, BET, TPR, SEM/EDS and XPS analyses suggest the formation of the spinel phase with surface areas varying from 75 to 105 m2 g-1 and the metal ions Cu2+ and Co2+ located in the octahedral and tetrahedral sites. It was observed that the presence of Co and especially Cu strongly increased the catalytic activity for the oxidation of sulfide to polysulfides, i.e. Sx 2- (x = 1-4) and other S-O species. The strong effect of Cu2+ is discussed in terms of thermodynamically favorable processes comprising a strong surface interaction of S2--Cu2+, an electron transfer from sulfide to produce Cu1+ followed by an electron transfer from Cu1+ to Fe3+ bulk. Fil: Cunha, Igor T.. Universidade Federal de Minas Gerais; Brasil Fil: Teixeira, Ivo F.. University of Oxford; Reino Unido Fil: Albuquerque, Adriana S.. Centro de Desenvolvimento da Tecnologia Nuclear; Brasil Fil: Ardisson, José D.. Centro de Desenvolvimento da Tecnologia Nuclear; Brasil Fil: Macedo, Waldemar A. A.. Centro de Desenvolvimento da Tecnologia Nuclear; Brasil Fil: Oliveira, Henrique S.. Universidade Federal de Minas Gerais; Brasil Fil: Tristão, Juliana C.. Universidade Federal de Viçosa; Brasil Fil: Sapag, Manuel Karim. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina. Universidad Nacional de San Luis; Argentina Fil: Lago, Rochel M.. Universidade Federal de Minas Gerais; Brasil

In this work, blast furnace dust (BFD), a steel industry waste, was used as a demulsifier for highly efficient removal of the organic oil contaminant of metal working wastewater. Batch experiments showed that a simple mixture and stirring of BFD with stable emulsified wastewater containing metalworking oil (TOC ca. 15,000 mg L −1 and 1500 NTU) led to 90–98% TOC removal at pH lower than 7. Scanning electronic microscopy, X-ray diffraction, Mossbauer, thermogravimetric analysis, and Raman analyses suggest that the removal of the stable oil micelles is a combination of two synergic processes: (i) an electrostatic capture of the stable negatively charged oil droplets by the positive BFD particles followed by (ii) a strong interaction with the oil and the hydrophobic carbon surface. Preliminary results showed that BFD can be regenerated and reused by simple extraction or thermal treatment.

Herein, we report a synthetic strategy to convert biomass-derived unsubstituted furan to aromatics at high selectivity, especially to ethylbenzene via alkylation/Diels–Alder cycloaddition using ethanol, while greatly reducing the formation of the main side product, benzofuran, over zeolite catalysts. Using synchrotron X-ray powder diffraction and first-principles calculations, it is shown that the above methodology favors the formation of aromatic products due to ready alkylation of furan by the first ethanol molecule, followed by Diels–Alder cycloaddition with ethylene derived from the second ethanol molecule on a Bronsted acid site in a one-pot synthesis. This gives a double-promoting effect: an alkyl substituent(s) on furan creates steric hindrance to inhibit self-coupling to benzofuran while an alkylated furan (diene) undergoes a Diels–Alder reaction more favorably due to higher HOMO energy.

The use of sunlight to drive chemical reactions via photocatalysis is of paramount importance towards a sustainable future. Among several photocatalysts, earth-abundant polymeric carbon nitride (PCN, often wrongly named g-C3N4) has emerged as an attractive candidate due to its ability to absorb light efficiently in the visible and near-infrared ranges, chemical stability, non-toxicity, straightforward synthesis, and versatility as a platform for constructing hybrid materials. Especially, hybrids with metal nanoparticles offer the unique possibility of combining the catalytic, electronic, and optical properties of metal nanoparticles with PCN. Here, we provide a comprehensive overview of PCN materials and their hybrids, emphasizing heterostructures with metal nanoparticles. We focus on recent advances encompassing synthetic strategies, design principles, photocatalytic applications, and charge-transfer mechanisms. We also discuss how the localized surface plasmon resonance (LSPR) effect of some noble metals NPs (e.g. Au, Ag, and Cu), bimetallic compositions, and even non-noble metals NPs (e.g., Bi) synergistically contribute with PCN in light-driven transformations. Finally, we provide a perspective on the field, in which the understanding of the enhancement mechanisms combined with truly controlled synthesis can act as a powerful tool to the establishment of the design principles needed to take the field of photocatalysis with PCN to a new level, where the desired properties and performances can be planned in advance, and the target material synthesized accordingly.

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.

Gold nanoparticles were supported on the surface of three different matrixes based on red mud waste: (i) pure red mud, (ii) reduced red mud and (iii) partially carbon coated red mud, in order to produce different catalysts for desulfurization reactions. The catalysts were extensively characterized by X-ray diffraction, Mossbauer spectroscopy, thermogravimetric analyses, electron microscopies (SEM and TEM), energy dispersive spectroscopy (EDS) and BET surface area. Results showed that gold was successfully supported and distributed on the surface of red mud based materials as nanoparticles with diameters around 30 nm. The catalyst prepared with carbon coated red mud has shown to be twice more efficient than others for biphasic desulfurization reactions. This result can be associated with its amphiphilicity, which allows the catalyst to be located on the interface of biphasic systems. In this position a synergistic effect occurs between gold nanoparticles that adsorb S containing molecules and red mud Fe sites that promote the formation of OH radicals. The same effect is not observed for catalyst with hydrophilic supports.

By combining synchrotron X-ray powder diffraction, Rietveld refinement and the use of a probe molecule, Brønsted Acid Sites (BAS) of different strengths in steam-treated USY zeolite can be for the first time spatially differentiated: this enables the analysis of different acid-catalysed reactions of dimethylfuran (biomass) by the zeolite using a definitive site blockage strategy.

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.

In this work, the waste red mud (RM) was investigated as catalyst in a new two steps process designed to store, transport and use the associated petroleum gas (APG) lost in petroleum exploration in offshore platforms. In this process the APG is catalytically converted into carbon which can be stored, transported and then converted to syngas by oxidation with H2O. TPCVD and W-TPO experiments combined with Mossabuer, XRD, Raman, SEM, TEM, TG showed that the Fe oxides highly dispersed in the RM waste are reduced and decomposes CH4 decomposition to produce H2 fuel and 27 wt% of carbon deposits. The carbon in the composite can be safely stored, transported and then oxidized to produce H2/CO mixtures leading to the regeneration of the Fe oxides which allows the reuse of the catalyst for a new cycle.

In this proof-of-concept study we demonstrate that co-processing of an acidic bio-oil (pH 12) Red Mud bauxite mining waste originating from the Bayer process can lead to a synergistic upgrading and value addition to both feed materials. The co-processing reaction is carried out in a pressure reactor at 365 °C and yields a stabilized, much less acidic and more energy-dense bio-oil with an oxygen content of 3.5% and heating value of 34 kJ/g compared to 43.1% and 12 kJ/g, respectively, for the crude oil. In the same reaction the Red Mud acts as a sacrificial catalyst and is converted into a partially reduced magnetic neutral gray material with a carbon content of ∼33% (w/w). This neutralized material may have possible use as an iron ore, iron ore pelletization binder or soil additive. The co-processing of the two waste streams – one derived from forestry by-products, the other from bauxite mining and processing operations – may thus offer attractive economic and ecologic synergies. Two principle possible scenarios for an actual realization for such a process on a larger scale are discussed.

Catalytic chemical vapor deposition (CVD) using methane as carbon source was used to produce amphiphilic magnetic nanocomposites from Fe and Mo catalysts supported on alumina nanoparticles. The materials were reduced by hydrogen up to 900 °C and after were partially coated by carbon during a CVD process with CH4 also up to 900 °C. TPR results showed that the Fe and Mo phases present in the composites were reduced mainly to Fe0, Mo0 and iron carbide that consequently confers interesting magnetic properties to the materials. The materials produced with different Fe/Mo contents show saturation of magnetization between 1.1 and 28.1 emu g−1. Thermal analyses, CHN, Raman spectroscopy, SEM, TEM, EDS and BET showed that the higher the metal content, the more carbon was deposited on the materials. Carbon content of the materials varies from 2 to 16% and it is distributed mainly as nanotubes and nanofibers. These composites present excellent interaction on the interface between oil and water and are able to form emulsions as well as to break other stable emulsions. This feature is strategic to these composites as catalysts to act on the interface of biphasic systems. Preliminary studies of the materials as catalysts on biphasic oxidation of lipophilic dye Sudan-IV showed very promising results.

Exfoliated crystalline niobium oxides that contain exposed but interconnected NbO 6 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, HSr 2 Nb 3 O 10 , HCa 2 Nb 3 O 10 , and HNb 3 O 8 , 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 NbO 6 octahedra as proton donors; that of LA is associated with the vacant band position of Nb 5+ to receive electron pairs from substrate; and that of PC is associated with the terminal Nb=O of NbO 6 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 NbO x catalysts for industrially important reactions.

Photoluminescence (PL) and EPR spectroscopy are the two most commonly used techniques to evaluate surface oxygen vacancies (V O 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 31 P-containing probe molecule to obtain the distribution/concentration of V O surface as well as other surface features (hydroxyl groups) of ZnO by using 31 P 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.

Red Mud bauxite mining waste which consists of a highly alkaline mixture of Fe 2 O 3 (typically >60%, w/w), TiO 2 and several complex sodium alumino-silicates is a viable, but non-selective catalyst for the ketonization of carboxylic acids. The active catalyst consists of reduced iron oxide, silicate, aluminate and carbide phases that have to be formed either by pre-reduction of the Red Mud with a blend of formic and acetic acids or in situ by reduction with H 2 (g) at T > 350 °C. Under H 2 (g) this catalyst can convert biomass derived levulinic acid into a blend of C9 alkenes and alkanes in up to 76% (w/w) yield. The reduced Red Mud is a non-alkaline magnetic solid that was analyzed by various techniques (XRD, Mossbauer, EA, Raman, TGA). It can be reused as a catalyst without loss of activity.

The oxidation of aqueous sulfide promoted by activated carbon (AC) was investigated. Raman, infrared, X-ray diffraction, ultraviolet–visible spectroscopy and electron paramagnetic resonance analyses show that AC oxidizes aqueous sulfide to form a complex mixture of polysulfides and sulfur oxide species. This oxidation is strongly promoted by the presence of oxygen surface groups introduced by oxidation of the AC as shown by kinetic measurements. These results are discussed in terms of the similarities the AC have with enzymatic systems, i.e. redox surface groups (e.g. quinone) combined with an efficient electron conduction system.

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.

Graphitic-C3N4 is shown for the first time to catalyse photoacetalization of aldehydes/ketones with alcohols to acetals in high yields using visible light under ambient conditions; transient charge separation over the material is effective to catalyse the reaction in the absence of Lewis or Bronsted acids, giving a new green alternative catalyst.

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.

Biphasic oxidation reactions of organic contaminants with H2O2 were promoted by amphiphilic catalysts prepared from red mud residue and carbon nanostructures. Contaminants oxidation is especially important for petroleum treatment, since S and N compounds are extremely unwanted mainly due to environmental issues. Also, government regulations around the levels of these pollutants in fuels are becoming increasingly strict. The amphiphilic catalysts were tested in oxidation of different model molecules: Sudan IV lipophilic dye, tiophene, dibenzothiophene and quinoline and showed very good removal, reaching 100% of contaminants oxidation. The amphiphilic catalysts act in two steps of biphasic oxidation: (i) favor the formation of a reversible emulsion between the organic contaminated phase and the aqueous oxidized phase and (ii) catalyze Fenton reaction. After reaction, the emulsion can be easily separated by a magnetic process into two phases: decontaminated organic phase and aqueous phase with oxidized contaminants.

International audience; Few layer graphene and natural graphite were chemically modified by oxidation with HNO3 at 80 and 140 °C and used to promote the oxidation of sulfide ions in aqueous solutions. TEM, potentiometric titration, XRD, BET and Raman spectroscopy show that HNO3 treatment even at 140 °C did not modify the graphene and graphite morphologies but produced significant amounts of different oxygen surface groups. The presence of these groups on few layer graphene and graphite strongly increased the Saq2− oxidation, showing activities comparable to a high surface area activated carbon with a similar amount of oxygen surface groups. The obtained results suggest that the sulfide oxidation efficiency requires a balance between two important effects, i.e. the presence of oxygen functionalities to initiate the Saq2− oxidation and the electrical conductivity that is important to further transfer the electrons removed from sulfide.