Alexander Samokhvalov

Review of Experimental Characterization of Active Sites and Determination of Molecular Mechanisms of Adsorption, Desorption and Regeneration of the Deep and Ultradeep Desulfurization Sorbents for Liquid Fuels

This review analyzes the recent literature on the direct experimental determination of molecular and atomic-level nature of adsorption sites, mechanisms of adsorption under the mild (close to ambient) conditions, desorption of sulfur-aromatic compounds, and surface chemical reactions upon regeneration of the “spent” sorbents for desulfurization of the liquid fuels. Supported noble, transition metals and metal oxides, binary, and ternary metal oxides, activated carbons, zeolites, supported polymers, pervaporation membranes and other advanced adsorptive desulfurization materials are discussed. The recent trends in developing the deep and ultradeep desulfurization sorbents are discussed, and challenges of the direct determination of molecular mechanisms are described.

Oxidation Potentials of Human Eumelanosomes and Pheomelanosomes

Abstract Eumelanosomes and pheomelanosomes isolated from black and red human hair, respectively, were studied by photoelectron emission microscopy (PEEM). PEEM images were collected at various wavelengths between 207 and 344 nm, using the spontaneous emission output of the Duke OK-4 free electron laser (FEL). Analysis of the FEL-PEEM data revealed ionization thresholds of 4.6 and 3.9 eV corresponding to oxidation potentials of −0.2 and +0.5 V vs normal hydrogen electrode for eumelanosomes and pheomelanosomes, respectively. The difference in oxidation potential is attributed to the pigment content of the melanosome, namely whether it contains primarily eumelanin and pheomelanin. The effect of added melanosomes on the reduction of Fe(III)-cytochrome showed pheomelanosomes are stronger reducing agents than eumelanosomes, consistent with the measured oxidation potentials. The FEL-PEEM experiment offers to be an important new approach for quantifying the effects of age, oxidation and metal accumulation on the oxidation potentials of intact melanosomes.

Characterization of the Fe(III)-binding Site in Sepia Eumelanin by Resonance Raman Confocal Microspectroscopy¶

Abstract The resonance Raman spectrum of Sepia eumelanin is discussed by analogy to model compounds containing catechol (CAT)–like structural units. These data are then compared with the analogous data on Fe(III)-enriched Sepia eumelanin. In contrast to the natural eumelanin, the Fe(III)-enriched samples exhibit absorption features in the visible and near-IR spectral regions, which are attributed to ligand-to-metal charge-transfer (LMCT) bands. Resonance Raman spectra collected by exciting these LMCT bands reveal bands at 580 and 1470 cm−1; the intensity of these features increases with increasing Fe(III) content. The 580 and 1470 cm−1 bands are assigned to Fe–OR stretching and ring deformation modes, respectively. These data further substantiate that the Fe(III)-melanin–binding site in melanin is composed of CAT-like structural units.

Desulfurization of Real and Model Liquid Fuels Using Light: Photocatalysis and Photochemistry

Ultra-deep desulfurization of liquid fuels is crucial for the environment, longer lifetime of combustion engines, and emerging “green,” sustainable, carbon-neutral fuels for fuel cell applications. Current interest is towards photocatalysis and photochemistry for production of clean fuels and valuable chemicals. This critical Review provides systematization and analysis of studies on photocatalytic, photosensitized, and photochemical desulfurization of liquid fuels in the recent approximately 20 years. Zeolites, single-site photocatalysts, TiO2, and photosensitizers are discussed with their characterization and photocatalytic, photosensitized, and photochemical reaction networks. Pilot-scale photoreactors, kinetics of fuel desulfurization using sunlight, and uses of by-products of photodesulfurization of liquid fuels are covered (181 references).

Production of Hydrogen by Glycerol Photoreforming Using Binary Nitrogen–Metal‐Promoted N‐M‐TiO2 Photocatalysts

The need for renewable energy focuses attention on hydrogen obtained by using sustainable and green methods. The sustainable compound glycerol can be used for hydrogen production by heterogeneous photocatalysis. A novel approach involves the promotion of the TiO2 photocatalyst with a binary combination of nitrogen and transition metal. We report the synthesis and spectroscopic characterization of the new N-M-TiO2 photocatalysts (M=none, Cr, Co, Ni, Cu), and the photocatalytic reforming of glycerol to hydrogen under ambient conditions and near-UV or visible light versus benchmark P25 TiO2 . In units of activity μmol m(-2)  h(-1) , N-Ni-TiO2 is five-fold more active than P25, and N-Cu-TiO2 is 44-fold more active. The photocatalytic activity of N-M-TiO2 increases from Cr to Co and Ni, whereas the photoluminescence decreases; the change in activity is due to the modulation of charge recombination.

Porous calcium titanate and sorption and desorption of water under ambient conditions: a study by conventional and synchronous luminescence spectroscopy

We report chemical synthesis and characterization of micro/mesoporous CaTiO3 by X-ray diffraction (XRD), nitrogen sorption/desorption, small angle XRD, diffuse reflectance optical spectroscopy, and luminescence spectroscopy under ambient conditions. We determined the energies of absorption and emission transitions through electronic midgap states in micro/mesoporous calcium titanate by optical spectroscopy, “conventional” photoluminescence (PL) spectroscopy and, for the first time, synchronous luminescence spectroscopy at 25 °C. Micro/mesoporous CaTiO3 reversibly sorbs and desorbs water vapor in ambient air, with water sorption capacity being close to the total pore volume. The luminescence of micro/mesoporous CaTiO3 in visible range at room temperature is repeatedly significantly increased upon desorption of water and decreased (quenched) upon sorption of water vapor in air, due to the interactions of the adsorbate with electronic surface midgap states.

Selective Activation of C=C Bond in Sustainable Phenolic Compounds from Lignin via Photooxidation: Experiment and Density Functional Theory Calculations.

Lignocellulosic biomass can be converted to high‐value phenolic compounds, such as food additives, antioxidants, fragrances and fine chemicals. We investigated photochemical and heterogeneous photocatalytic oxidation of two isomeric phenolic compounds from lignin, isoeugenol and eugenol, in several nonprotic solvents, for the first time by experiment and the density functional theory (DFT) calculations. Photooxidation was conducted under ambient conditions using air, near‐UV light and commercial P25 TiO2 photocatalyst, and the products were determined by TLC, UV–Vis absorption spectroscopy, HPLC‐UV and HPLC‐MS. Photochemical and photocatalytic oxidation of isoeugenol proceeds via the mild oxidative “dimerization” to produce the lignan dehydrodiisoeugenol (DHDIE), while photooxidation of eugenol does not proceed. The DFT calculations suggest a radical stepwise mechanism for the oxidative “dimerization” of isoeugenol to DHDIE as was calculated for the first time.

Adsorption of aromatic sulfur compounds on metal-organic frameworks (MOFs)

Large-ring aromatic sulfur compounds, e.g., benzothiophene (BT), dibenzothiophene (DBT), present in petroleum and liquid fuels are harmful to human health and the environment. These compounds cause acid rain due to SO2 formation upon combustion of liquid fossil fuels. We studied the use of Metal Organic Frameworks (MOFs), e.g., Cu-MOF Basolite C300, to remove aromatic sulfur compounds from model liquid fossil fuels in a selective way and in a non-destructive fashion.

Note: Heated sample platform for in situ temperature-programmed XPS

We present the design, fabrication, and performance of the multi-specimen heated platform for linear in situ heating during the Temperature-Programmed XPS (TPXPS). The platform is versatile, compatible with high vacuum (HV) and bakeout. The heater platform is tested under in situ linear heating of typical high surface area sorbent∕catalyst support--nanoporous TiO(2). The platform allows the TPXPS of multiple samples located on specimen disk that can be transferred in and out of the TPXPS chamber. Electric characteristics, temperature and pressure curves are provided. Heating power supply, PID temperature controller, data-logging hardware and software are described.

Characterization of the Fe(III) Binding Site in Sepia Eumelanin

The resonance Raman spectrum of Sepia eumelanin is discussed by analogy to model compounds containing catechol (CAT)-like structural units. These data are then compared with the analogous data on Fe(III)-enriched Sepia eumelanin. In contrast to the natural eumelanin, the Fe(III)-enriched samples exhibit absorption features in the visible and near-IR spectral regions, which are attributed to ligand-to-metal charge-transfer (LMCT) bands. Resonance Raman spectra collected by exciting these LMCT bands reveal bands at 580 and 1470 cm(-1); the intensity of these features increases with increasing Fe(III) content. The 580 and 1470 cm(-1) bands are assigned to Fe-OR stretching and ring deformation modes, respectively. These data further substantiate that the Fe(III)-melanin-binding site in melanin is composed of CAT-like structural units.