Zakariae Amghouz

Electronic metal-support interactions in single-atom catalysts.

The synthesis of single-atom catalysts and the control of the electronic properties of catalytic sites to arrive at superior catalysts is a major challenge in heterogeneous catalysis. A stable supported single-atom silver catalyst with a controllable electronic state was obtained by anti-Ostwald ripening. An electronic perturbation of the catalytic sites that is induced by a subtle change in the structure of the support has a strong influence on the intrinsic reactivity. The higher depletion of the 4d electronic state of the silver atoms causes stronger electronic metal-support interactions, which leads to easier reducibility and higher catalytic activity. These results may improve our understanding of the nature of electronic metal-support interactions and lead to structure-activity correlations.

Surface-confined atomic silver centers catalyzing formaldehyde oxidation.

Formaldehyde (HCHO) is a prior pollutant in both indoor and outdoor air, and catalytic oxidation proves the most promising technology for HCHO abatement. For this purpose, supported metal catalysts with single silver atoms confined at 4-fold O4-terminated surface hollow sites of a hollandite manganese oxide (HMO) as catalytic centers were synthesized and investigated in the complete oxidation of HCHO. Synchrotron X-ray diffraction patterns, X-ray absorption spectra, and electron diffraction tomography revealed that geometric structures and electronic states of the catalytic centers were tuned by the changes of HMO structures via controllable metal-support interactions. The catalytic tests demonstrated that the catalytically active centers with high electronic density of states and strong redox ability are favorable for enhancement of the catalytic efficiency in the HCHO oxidation. This work provides a strategy for designing efficient oxidation catalysts for controlling air pollution.

Unravelling the onset of the exchange bias effect in Ni(core)@NiO(shell) nanoparticles embedded in a mesoporous carbon matrix

Ni(core)@NiO(shell) nanoparticles (NPs) were synthesized through the pyrolysis of an inorganic precursor taking place within the pores of an active carbon matrix at different temperatures between 673 and 1173 K, and a subsequent oxidation in air. For the lowest temperature (673 K), the smallest average size of the NPs (9 nm) and the largest percentage of NiO (82%) are observed. Upon increasing the temperature up to 1173 K, an average diameter of 23 nm is observed while the NiO percentage decreases below 20%. We found that each NP consists of a Ni core surrounded by a structurally disordered NiO shell with a constant thickness of ∼2 nm, regardless of the core size. The spins inside the NiO shell freeze into a spin glass (SG)-like state below Tf ∼ 40 K. The magnetic exchange coupling between the Ni core and the NiO shell spins gives rise to the occurrence of the exchange bias (EB) effect, whose temperature dependence follows a universal exponential trend in all samples. The SG nature of the shell spins yields a vanishing EB above Tf. This is far below the Neel temperature of bulk antiferromagnetic NiO (TN ∼ 523 K) that usually determines the onset of the EB effect in Ni/NiO interfaces.

Exceptional thermal stability of undoped anatase TiO2 photocatalysts prepared by a solvent-exchange method

A new solvent-exchange technique to prepare anatase nanoparticles with exceptional thermal stability and photocatalytic activity is described here. The process of preparation is accomplished by using organic solvents to precipitate hydrous titania particles from a basic aqueous medium containing a titanium peroxo complex. Undoped titanium dioxide formed via a solvent exchange method has unprecedented thermal stability against transformation to the rutile phase, as opposed to TiO2 prepared by the common method of the gelation of an aqueous titanium peroxo complex. On the basis of X-ray thermodiffraction experiments, it has been established that the thermal treatment at 1000 °C of the titania prepared by ethanol precipitation contains 100% pure anatase phase. The stabilization of anatase is induced by the high defectiveness of the TiO2 nanostructure, which is evidenced from band-gap energy estimation, PXRD and HRTEM studies. The prepared TiO2 nanoparticles show an outstanding photocatalytic activity comparable to the commercial Aeroxide P25 photocatalyst in the UV-assisted decomposition of methylene blue.

Disentangling magnetic core/shell morphologies in Co-based nanoparticles

Co-based nanoparticles (NPs) have been extensively explored due to their prospective applications in areas as diverse as efficient water treatment (Co NPs), hydrogen generation (CoO NPs) and combustion catalysis (Co3O4 NPs). In recent years, the emergence of Co-based entities as bi-magnetic core/shell NPs has opened new avenues for their innovative use in fields ranging from energy storage and magnetic recording to biomedicine. The control and characterization of these nanomaterials thus becomes of paramount importance for targeting their foreseen applications. Here, we show that the intentional oxidation of metallic Co NPs with different sizes (3–50 nm) gives rise to a wide variety of core/shell morphologies including Co, CoO and Co3O4 phases. Bridging the information coming from high-resolution transmission electron microscopy, X-ray absorption spectroscopy and magnetic measurements gives us a self-consistent picture that describes the role played by the morphology and microstructure in the magnetism of Co and its oxides at the nanoscale.

Photocatalytic degradation of 2-(4-methylphenoxy)ethanol over TiO2 spheres

The photocatalytic TiO2-assisted decomposition of 2-(4-methylphenoxy)ethanol (MPET) in aqueous solution has been studied for the first time. The intermediate compounds of MPET photodegradation have been also determined. A toxic p-cresol is formed in significant quantities during the photocatalytic reaction. A solvent-exchange approach for a template-free preparation of spherical TiO2 particles has been described, which is based solely on precipitation of hydrous titania from aqueous titanium peroxo complex by using organic solvents. The proposed method favours the formation of spherical titania particles with a mean size varying from 50 to 260nm depending on the choice of solvent. The procedure for converting nonporous titania spheres into mesoporous material maintaining the same spherical morphology has been developed. The synthesized TiO2 spheres demonstrate a degree of MPET photo-degradation close to that of the commercial titania Aeroxide P25, besides being successfully recovered and reused for four reaction cycles without loss of photocatalytic activity. The effectiveness of the commercial Aeroxide P25 in MPET photodegradation, on the other hand, suffers 10-time drop during the third reaction cycle, which is attributed to its poor recoverability because the photocatalyst is composed of small particles of 20nm size.

Nanosheets of Nonlayered Aluminum Metal-Organic Frameworks through a Surfactant-Assisted Method

During the last decade, the synthesis and application of metal-organic framework (MOF) nanosheets has received growing interest, showing unique performances for different technological applications. Despite the potential of this type of nanolamellar materials, the synthetic routes developed so far are restricted to MOFs possessing layered structures, limiting further development in this field. Here, a bottom-up surfactant-assisted synthetic approach is presented for the fabrication of nanosheets of various nonlayered MOFs, broadening the scope of MOF nanosheets application. Surfactant-assisted preorganization of the metallic precursor prior to MOF synthesis enables the manufacture of nonlayered Al-containing MOF lamellae. These MOF nanosheets are shown to exhibit a superior performance over other crystal morphologies for both chemical sensing and gas separation. As revealed by electron microscopy and diffraction, this superior performance arises from the shorter diffusion pathway in the MOF nanosheets, whose 1D channels are oriented along the shortest particle dimension.

Thermal behavior of layered α-titanium phosphates: from the titanium(IV) bis(hydrogenphosphate) monohydrate to an europium(III)-phase via propylamine intercalation

Polycrystalline layered α-titanium phosphate, α-Ti(HPO4)2·H2O, has been obtained under H3PO4(aq) reflux conditions, and its intercalation compound with propylamine, α-Ti(HPO4)2·2C3H7NH2·H2O, was used for trapping luminescent Eu-centers in two-dimensional confined space. All materials were characterized by X-ray powder diffraction, scanning electron microscopy, solid-state nuclear magnetic resonance (SS-NMR, 31P MAS and 13C CPMAS) and thermogravimetric analysis coupled with mass spectrometry. Moreover, the activation energy of thermal decomposition has been calculated as a function of the extent of conversion, applying both a modified Friedman method developed in our laboratory and the advanced nonlinear method proposed by Vyazovkin.

Sub‐Micron Polymeric Stomatocytes as Promising Templates for Confined Crystallization and Diffraction Experiments

The possibility of using sub-micrometer polymeric stomatocytes is investigated to effectuate confined crystallization of inorganic compounds. These bowl-shaped polymeric compartments facilitate confined crystallization while their glassy surfaces provide their crystalline cargos with convenient shielding from the electron beams harsh effects during transmission electron microscopy experiments. Stomatocytes host the growth of a single nanocrystal per nanocavity, and the electron diffraction experiments reveal that their glassy membranes do not interfere with the diffraction patterns obtained from their crystalline cargos. Therefore, it is expected that the encapsulation and crystallization within these compartments can be considered as a promising template (nanovials) that hold and protect nanocrystals and protein clusters from the direct radiation damage before data acquisition, while they are examined by modern crystallography methodologies such as serial femtosecond crystallography.

Some titanium phosphates as host materials: a crystallographic perspective

Santiago Garcia-Granda1, Jorge García-Glez2, Camino Trobajo2, Zakariae Amghouz3, Sergei A. Khainakov4, Conchi O. Ania5, José B. Parra5, Artem A. Babaryk6, Iván da Silva7, Germán R. Castro8 1Physical And Analytical Chemistry Department, University Oviedo, Oviedo, Spain, 2Departments of Analytical and Physical Chemistry and Organic and Inorganic Chemistry, University of Oviedo-CINN, Oviedo, Spain, 3Department of Materials Science and Metallurgical Engineering, University of Oviedo, University Campus, Gijón, Spain, 4Scientific and Technical Services, University of Oviedo, Oviedo, Spain, 5ADPOR Group, Carbon National Institute (INCAR, CSIC), Oviedo, Spain, 6Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine, 7ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire, United Kingdom, 8SpLine, Spanish CRG Beamline, ESRF, Grenoble, France E-mail: sgg@uniovi.es