Dr. Valdés-Morales, Héctor

Anatase nanoparticles containing surface oxygen vacancies (VO) and Ti3+ are of great importance for applications in photocatalysis, batteries, catalysis, sensors among other uses. The properties of VO and their dependence on the size of nanoparticles are of great research interest and could allow obtaining advanced functional materials. In this work, a complete set of oxygen vacancies in an anatase nanoparticle of size 1.1 nm was investigated and compared to those of a twice larger nanoparticle, having the same shape and surface hydroxylation pattern. It turned out that the decrease in the size of the anatase nanoparticle strongly facilitated creation of surface oxygen vacancies and Ti3+. After their creation, oxygen vacancies undergo three transformation paths — (1) small repulsion of surrounding Ti cations with retention of the vacancy, (2) transfer of oxygen anion, leading to the movement of oxygen vacancy to a more stable position, and (3) collapse of oxygen vacancy accompanied by structure deformation towards Magneli-like phase.

Zeolites are widely used for numerous processes for production of a vast number of chemicals, fuels and commercial goods. Preparation of zeolite catalysts that have improved selectivity for the desired products, operate at lower temperature and possess increased stability is therefore of great interest. The key to such improved zeolite catalysts is in the design of active sites and facilitation of mass transfer via optimization of the porous structure. At the same time, undesirable sites that inhibit desirable properties of the active sites need to be removed or blocked. The strength and structure of either the Brønsted or Lewis acid sites, directly determines their catalytic activity and selectivity for each reaction. In the present study, the structure and acidity of active sites in zeolites are investigated for the example of mordenite using modern semiempirical methods pm7 and scc-dftb (dftb2). Models AlHSi95O192 and Al2H2Si94O192 are used for Brønsted acid sites and Al2Si94O191 for Lewis acid sites. In agreement with previous studies, the stability of T1, T2, T3 and T4 sites is similar. Many different configurations of pair-wise located Al atoms were studied. In the present work it was found that some of the pair-wise located Al atoms possess Brønsted acid sites with strength much higher than that for single Brønsted acid sites. However, since their stability is not the highest among other double sites, special preparation methods need to be developed for selectively obtaining these very active sites. The stability of different Lewis acid sites is also considered.

Anatase (001)nanosheets have recently attracted great attention as very active catalysts and photocatalysts. These graphene analogs have very high surface area and unique surface properties. In the present paper, very thin two-layer anatase nanosheets are investigated computationally in the form of quantum dots of various size. Quantum size effect (QSE)was clearly observed for nanosheets with fully hydroxylated edges and size up to 14 nm and the ultimate band gap is around 3.4 eV. Dehydroxylation of nanosheets obscured QSE, decreased band gap and induced visible light absorption. Therefore, contradictory trends reported in experimental studies for anatase QSE can be ascribed to different degree of hydroxylation of the TiO 2 samples surface. All anatase nanosheet quantum dots retained their flat graphene-like shape. These findings demonstrate that dehydroxylated anatase nanosheet quantum dots are prospective visible-light active photocatalysts even if their inherent band gap is considerably larger than for bulk anatase.

This study aims to evaluate the technical feasibility of applying a low-cost alternative natural bioadsorbent obtained from the cone of the Moroccan cypress Cupressus sempervirens to remove dyes from contaminated waters. Methylene Blue (MB) and Congo Red (CR) dyes are used to represent basic and acid compounds present in wastewater of textile industries. The cone of this medium-sized coniferous evergreen tree was obtained from the Fez area and was characterised by different physical– chemical methods, including nitrogen adsorption–desorption isotherms, Fourier transform infrared spectroscopy, scanning electron microscopy, Boehm titration method and the pH of the point of zero charge (pHpzc). Additionally, the influence of operating conditions such as contact time, initial dye concentration, binary mixture of dye solutions, bioadsorbent dosages and solution pH were evaluated. Experimental results reveal that the adsorption processes take place very rapidly, reaching equilibrium at 30 and 45 min for MB and CR, respectively. Maximum adsorption capacities result to be pH dependents. Hence, MB adsorption is favoured under basic pH conditions, while CR is favoured at acidic pH. A pseudo-second-order kinetic model provides the best fit of the experimental data of MB and CR adsorption onto the biomaterial. Adsorption isotherm data are well represented by Langmuir, Freundlich and Dubinin–Radushkevich models. Langmuir model gives the best fit with a maximum monolayer sorption capacity of 144 and 25.02 mg g–1 for MB and CR, respectively. Experimental results indicate that the cone of Cupressus sempervirens could be used as a potential, low-cost bioadsorbent for the elimination of dyes from contaminated waters.

Gold nanocrystals (AuNCs) were synthesized by economical and green strategy in aqueous medium by using N[3(trimethoxysilyl)propyl]ethylenediamine (TMSPED) as both a reducing and stabilizing mediator to avoid the aggregation of gold nanocrystals. Then, the AuNCs were capped with graphene quantum dots (GQDs) using an ultrasonic method. The resulting nanocomposites of GQD-TMSPED-AuNCs were characterized by X-ray photoelectron, X-ray diffraction, Raman, UV-vis and FTIR spectroscopies. The size and shape of the nanocomposites were confirmed by using transmission electron microscopy and atomic force microscopy. The GQD-TMSPED-AuNCs placed on a glassy carbon electrode enable simultaneous determination of dopamine (DA) and epinephrine (EP) with peak potentials at 0.21 and 0.30 V (vs. Ag/AgCl). The response is linear in the 5 nM – 2.1 μM (DA) and 10 nM – 4.0 μM (EP) concentration ranges, with detection limits of 5 and 10 nM, respectively. The sensor shows good selectivity toward DP and EP in the presence of other molecules, facilitating its rapid detection in practical applications.

A facile combined method, namely sonothermal-hydrothermal, was adopted to assemble titanium dioxide (TiO2) nanoparticles on the surface of reduced graphene oxide (RGO) to form nanocomposites. Characterization techniques confirm that RGO-TiO2 composite is well constituted. Enhanced photocatalytic CO2 reduction to methanol by the composites under UVA and visible irradiation suggests the modification in the band gap of the composite and promotion of the separation of photogenerated carriers, yielding methanol production rate of 2.33 mmol g−1 h−1. Theoretical investigation demonstrated that combining RGO with TiO2 resulted in an upward shift of TiO2 bands by 0.2 V due to the contribution of RGO electrons. Relatively strong adsorption of RGO over the (101) anatase surface with the binding energy of approximately 0.4 kcal mol−1 per carbon atom was observed. Consideration of orbitals of TiO2, RGO and RGO-TiO2 composite led to a conclusion that UVA photoreaction proceeds via the traditional mechanism of photogenerated electron transfer to RGO while visible light CO2 reduction proceeds as a result of charge transfer photoexcitation that directly produces electrons in RGO and holes in TiO2. Superior photocatalytic activity of RGO-TiO2 composite in the present study is attributed to the formation of tight contact between its constituents, which is required for efficient electron and charge transfer.

Cationic doping of TiO2 anatase with sulphur represents a facile method to improve catalytic and photocatalytic activity for hydrogen production and extend the action spectrum of TiO2 into the visible light region. However, there is a lot of misunderstanding when trying to explain the experimental findings and suggest theoretical models. In the present computational research work, novel theoretical models are put forward representing fully hydroxylated small anatase nanoparticles with S(IV) and S(VI) doping in various surface positions and in the bulk. It was found that sulfur in the doped anatase nanoparticles preserves its typical coordination geometries of trigonal pyramid for S(IV) and tetrahedron for S(VI). Doping in the anatase surface is much more energetically favorable compared to doping in the bulk. Doping with S(IV) causes decrease of the band gap from 3.22 to 2.65 eV while S(VI) doping could decrease Eg only to 2.96 eV. Location of photogenerated electrons and holes depends strongly on the position of dopant atoms and their valent state. Contrary to some experimental works, no strong and extended visible light absorption bands could be found with cationic doped hydroxylated anatase nanoparticles. However, improved charges separation is observed indeed and causes improved photocatalytic hydrogen production.

Ethylene is a plant growth regulator that induces accelerated softening and ripening of fruits during transport and storage. Among the most applied methods for ethylene control, adsorption appears as a cheap and efficient technique. In this work, the effect of the incorporation of transition metals into natural Chilean zeolite on ethylene adsorption is investigated. Natural zeolite mainly composed of clinoptilotite and mordenite is modified using copper and zinc nitrate solutions and calcined under oxygen flow at 623 K, generating different transition metal modified zeolites. Parent and modified zeolites were characterised by X-ray diffraction, X-ray fluorescence spectroscopy and nitrogen adsorption. Zeolite surface modifications were assessed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Experimental results reveal the incorporation of Cu2+ and Zn(2+ )as new compensating cations into the zeolite framework. Ethylene adsorption isotherms show an enhancement on the adsorption capacity of Cu-exchanged zeolite. This result is not only associated to the higher micropore surface area of this sample, but also to the higher affinity of ethylene molecules to copper cations incorporated on this zeolite. DRIFTS operando experiments of ethylene adsorption in the absence and in the presence of moisture reveal a competitive mechanism of water and ethylene molecules toward hydroxyl sites. Si-OH-Al and Si-OH sites are rapidly occupied with water, reducing the adsorption of ethylene.

For several decades, it has been known that ozone emissions are harmful to humans, plants, and animals. Heterogeneous catalytic decomposition is an efficient process for removing ozone from air. This study examines the effect of the zeolite's framework and pore width on efficiency for decomposing gaseous ozone. Four highly hydrophobic zeolites are used: a large cavity zeolite (Faujasite/H‐FAU), a medium pore zeolite with parallel channel (Mordenite/H‐MOR), and two medium pore zeolites with interconnected channels (H‐ZSM‐5/H‐MFI and Na‐ZSM‐5/Na‐MFI). Experiments were conducted in fixed‐bed flow reactors loaded with zeolite at ambient conditions (20 °C and 101 kPa). Zeolite surfaces were analyzed during the experiments in order to understand the influence of physical and chemical surface properties on the ozone decomposition mechanism. A higher amount of ozone is eliminated using H‐MOR, compared with the zeolite samples H‐FAU, H‐MFI, and Na‐MFI. Pore width and micropore framework size distribution (channel and cages) appear to be key factors. A narrow channel or cage, slightly larger than the ozone molecule size, seems to promote ozone interactions with Lewis acid sites. Fourier transform infrared spectroscopy shows that Lewis acid sites (LAS), located on the walls of zeolite pores, decompose ozone. This leads to the formation of atomic oxygen species that could react with another ozone molecule to form dioxygen. Hence, LAS are regenerated, ready to decompose another ozone molecule once more.

The influence of seven commercial activated carbons (ACs) to promote hydrogen peroxide decomposition and radical generation is assessed during four operating cycles. The amount of generated hydroxyl radicals is estimated from quenching experiments using methanol as a radical scavenger. The change in chemical surface composition of ACs upon contact with hydrogen peroxide after each operating cycle is measured by Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and by following the change in the value of the pH of the point of zero charge (pHPZC). Results reveal that when ACs are exposed to hydrogen peroxide for extended periods, their chemical surface composition is modified, reducing the capacity of these materials to promote hydrogen peroxide decomposition, and in turn decreasing the generation of hydroxyl radicals. Moreover, DRIFTS analyses show that ACs with an appreciable content of basic surface functionalities, such as chromene-type structures, would guarantee a continuous radical generation, reducing the loss of catalytic activity.