Paweł Nowak

Photocatalytic Activity of Titanium Dioxide Modified by Silver Nanoparticles

Photocatalytic activity of Ag/TiO(2) composites obtained by photoreduction treatment (PRT) was investigated. The composite materials, containing various ratio of silver nanoparticles (0.6-3.7 wt %) were obtained by depositing silver on the Evonic-Degussa P25 titania surface. Selected samples whose color varied between light rose and purple brown were examined by SEM, TEM, XPS, DRS, and BET techniques. Flat band potential was determined using Roy method. TEM analysis showed spherically shaped silver nanoparticles of the diameter 4-12 nm. The XPS measurements revealed that silver particles were obtained mainly in metallic form. DRS spectra and photovoltage measurements showed that silver nanoparticles modified the P25 spectral properties but they changed neither the band gap nor the location of flat band potential. The photocatalytic activity of Ag/P25 composite was compared to the photocatalytic activity of pure P25 in the photooxidation reaction of an important potable water contaminant humic acid (HA) and two model compounds, oxalic acid (OxA) and formic acid (FA). The photodecomposition reaction was investigated in a batch reactor containing aqueous suspension of a photocatalyst illuminated by either UV or artificial sunlight (halogen lamp). The tests proved that a small amount of silver nanoparticles deposited on the titania surface triggers the increase in photocatalytic activity; this increase depends, however, on the decomposed substance.

XPS study of clean metal sulfide surfaces

Abstract XPS spectra of PbS, FeS 2 , CuFeS 2 , Cu 1.87 S and ZnS surfaces prepared by scraping in ultra-high vacuum were measured. The binding energies reported can be used as reference data in more applied research, where these sulfides treated under different conditions are studied. Problems encountered in measuring poorly conducting samples, such as ZnS, with the XPS technique are also shortly discussed.

Electrochemical investigation of the codeposition of SiC and SiO2 particles with nickel

The use of electrochemical impedance spectroscopy (EIS) for the in situ control of the electrolytic codeposition of Ni/SiO2 and Ni/SiC was investigated. An attempt was made to clarify why silica particles hardly codeposit in comparison to silicon carbide particles. It was found that the presence of SiO2 and SiC particles influences the metal deposition process in different ways. SiC particles that are being embedded in the growing metal layer cause an apparent decrease in the electrode surface area, probably due to blocking off a part of the surface by partly engulfed particles. In the case of SiO2 particles, which embed in the metal matrix to a very limited extent, no blocking was observed. It was found that the presence of particles in the solution causes an apparent increase in the electrode surface area, probably due to increased surface roughness.

Influence of the surface properties of silicon carbide on the process of SiC particles codeposition with nickel

The influence of different treatment of SiC particle surface on the rate of codeposition of SiC particles with nickel in the process of SiC/Ni composite preparation was investigated. The hydrophobicity of SiC surface was characterized by contact angle measurements in the system SiC/liquid (water or plating bath)/air. Simultaneously the surface oxidation of SiC was investigated by XPS. The hydrophobic properties of SiC and its susceptibility to codeposition with nickel were compared with those of silica, a strongly hydrophilic substance. It was found that oxidation of the SiC surface, also due to the contact of its surface with plating bath, decreases significantly its hydrophobicity, diminishing at the same time the ratio of codeposition with nickel. However SiC does not oxidize easily and aggressive oxidation (long contact with concentrated H2O2 solution) is required to attain the hydrophilicity of the surface comparable to silica. At moderate oxidizing conditions, intermediate oxidation products (SiOxCy) appear at the SiC surface, which make the surface only partly hydrophilic.

Oxidation of galena surface : an XPS study of the formation of sulfoxy species

There are controversies existing in the literature, concerning the formation of sulfoxy species (sulfates, thiosulfates, sulfites) during the oxidation of galena (lead sulfide) surface. In view of those controversies, the XPS investigations of galena oxidation with the special reference to the oxidation of sulfide sulfur were undertaken. The results of the present work show that the sulfoxy species are formed at the galena surface under favorable conditions, even in the initial stage (first few minutes) of oxidation, and can be investigated by XPS. Those sulfoxy species may be best characterized as a mixture of thiosulfate and adsorbed SOn species. However, they are easily decomposed at the presence of atmospheric carbon dioxide and in the vacuum, even at the room temperature, therefore special precautions have to be undertaken to observe their formation by XPS.

A flotation related X-ray photoelectron spectroscopy study of the oxidation of galena surface

Abstract The initial steps of the oxidation of galena (PbS) surface with special attention to the oxidation in an aqueous environment were investigated by the X-ray photoelectron spectroscopy method (XPS). Galena monocrystals from two different sources were cleaved and exposed to either air or an aqueous environment. The results show that the presence of organic contamination at the galena surface may exert a dramatic effect on the course of its surface oxidation. In the case of aerial oxidation, the presence of impurities changes the reaction path, suppressing the oxidation of sulfide sulfur to sulfate like species selectively. In the case of aqueous oxidation, the presence of organic contamination at the surface (this contaminants are supposed to be mainly the carboxylic acids and their salts) inhibits the oxidation. Very little oxidation products were found on the galena surface contacted with aqueous solutions at the spontaneously established potential, in accordance with the congruent dissolution-oxidation mechanism.

Thermodynamics and kinetics of low pressure methanol synthesis

Abstract The results of our investigations on methanol synthesis, carried out over a number of years, have been presented in the form of an interpretative review. The present article is an attempt to summarize this research against the background of what has already been done in this area in the world. Special attention was focused on the role of CO2 in the process.

On the XPS and IR identification of the products of xanthate sorption at the surface of galena

Abstract XPS investigations of the sorption of methyl, ethyl and amyl xanthates at the surface of lead sulfide and galena, with special attention for the submonolayer region, are described. Lead xanthate in the molecular form as the adsorbing entity is proposed on the basis of the analysis of XPS spectra of the surface after sorption. The uneven distribution of xanthate at the surface, even at low surface coverage, is suggested.

Xanthate adsorption at PbS/aqueous interfaces: Comparison of XPS, infrared and electrochemical results

Abstract In order to elucidate the reasons for the difference in results of measurements of adsorption of ethylxanthate (EtX − ) ion at the surface of lead sulphide and galena, performed using different methods, measurements by X-ray photoelectron spectroscopy and infrared internal reflection spectroscopy were performed on both natural galena and synthetic PbS. The measurements were performed both at spontaneously established potential and under controlled potential conditions. The results obtained using both methods were very similar, showing the formation of submonolayer perpendicularly oriented adsorbed lead ethylxanthate.

Flotation of oxidized lead minerals with derivatives of 2-mercaptobenzothiazole. Part 1: chemical equilibria in the system 6-methyl-2-mercaptobenzothiazole-lead salts

Abstract A new flotation collector 6-methyl-2-mercaptobenzothiazole (MMBT) was investigated. The dissociation constant for the acidic form was measured. Two insoluble lead complexes of MMBT with lead were synthesized and characterized for the first time and their solubility products were measured. On the basis of these data and literature data for the equilibria between lead salts and aqueous solutions, the chemical equilibria for the systems: solution of MMBT-insoluble lead salt were calculated. Lead salts which may form lead minerals or may appear as oxidation products on the surface of galena were taken into account. The domains of existence for different species and equilibrium concentrations for MMBT at different conditions have been presented in the form of distribution diagrams. The results of calculations were compared with the Fourier Transform Infrared Internal Reflection Spectrometric (FT-IR IRS) measurements of the interaction of MMBT solution with the oxidized galena surface.