Electrochemical studies of hot corrosion of type 347H stainless steel

Abstract

Electrochemical studies of the hot corrosion of AISI SA 213 TP 347H stainless steel have been carried out in a mixture of 80 wt-% V₂O₅ + 20 wt-% Na₂SO₄. The range of temperatures was 540–680°C at intervals of 20 K and the techniques employed included corrosion potential, Tafel polarisation, and electrochemical noise measurements. At 620°C the corrosion potential, measured against a platinum reference electrode (PRE), decreases from ?350 mV to ?480 mV and remains at this level during the first 8 h. Using Tafel polarisation, it was found that, with change in the temperature from 540 to 680°C, the corrosion potential decreased on initial heating to 600°C and then increased again at higher temperatures, the corrosion rate increasing continuously with increasing temperature. However, at constant temperature (620°C) the corrosion rate increased with time during the first 8 h, after which it decreased and reached a steady state after 27 h, probably owing to the formation of a surface film. Electrochemical noise measurements, of both voltage and current noise, indicated a combination of general corrosion, probably owing to the formation of a surface layer, and localised corrosion in the grain boundaries.     

Co–Mg–Al Hydrotalcite Precursors for Catalytic Total Oxidation of Volatile Organic Compounds

Co–Mg–Al hydrotalcite type solids were synthesized as precursors of catalysts for the total oxidation of toluene. After calcination at 500 °C, different mesoporous mixed oxides were obtained with high specific surfaces. The comparison of the catalytic activities of the calcined hydrotalcites with those of calcined hydroxides evidenced the superiority of the first oxides explained meanly by higher specific surfaces and more easily reducible particles. DRIFT “operando” allowed to follow the oxidation reaction and the formation of light coke and carbonate species.     

Comparison of the first stage of the thermal decomposition of Polish coals by diffuse reflectance infrared spectroscopy

Diffuse reflectance Fourier transform infrared spectroscopy was applied to study the decomposition of hard and brown coal during mild pyrolysis. The pyrolysis of investigated samples was conducted with a heating rate of 10 K/min up to 673 K under ambient pressure and a nitrogen flow of 4 L/h. Curve-fitting analysis was employed to characterise the coal structures evolving in temperatures ranging from 323 to 673 K. This study gives the information about the degradation of hydrogen bonds and oxygen structure. The comparison of the calculated structural parameters shows that bonds assigned to C_ar in hard coal are more stable at the investigated temperature range than the bonds in brown coal. The same phenomenon was observed for the CH bonds in aromatic formations. It was noted that during the pyrolysis the first changes in the structural parameters of lignite are visible at temperatures above 423 K and in case of hard coal above 473 K.     

Synthesis and characterization of mesoporous silica thin films as a catalyst support on a titanium substrate

Mesoporous silica films with a thickness of 500-900 nm were synthesized on a titanium substrate by the evaporation-induced self-assembly method (with 900-1200 rpm for 90 s) using cetyltrimethylammonium bromide (CTAB) as structure-directing agent and tetraethyl orthosilicate as the silica source. Prior to coating deposition, the titanium substrate was oxidized to increase the surface roughness up to 500 nm and to produce a thin titania layer. Just before the synthesis, the titania layer was made super hydrophilic by an UV treatment for 2 h to provide a better adhesion of the silica film to the substrate. Films with hexagonal and cubic mesostructures with a uniform pore size of 2.8 nm and a surface area of 1080 m²/g were obtained and characterized by different methods. An alternative approach for surfactant removal by gradual heating up to 250 °C in vacuum was applied. Complete removal of CTAB from the as-synthesized silica films was confirmed by infrared spectroscopy.     

DRIFT study of CO chemisorption on organometallics-derived Pd/MgO catalysts: the effect of chlorine

Magnesia-supported palladium catalysts were prepared from chemical vapour deposition (CVD) of [Pd(C₃H₅)(C₅H₅)] and incipient wetness impregnation of [Pd(C₃H₅)Cl]₂ and [Pd(acac)₂]. DRIFT spectroscopy of adsorbed CO on prereduced catalysts indicates that the electronic state of metal particles depends on the preparation methodology and markedly on the organometallic precursor. Inn-heptane reforming at 500°C, the highest activity and selectivity were shown by the CVD-based system. Chloride ions deriving from the impregnation solvent exchange with surface hydroxyls. Acidic Mg-Cl sites are thus formed, which induce a beneficial effect on the catalytic properties. The reforming activity collapsed when a chlorine-containing precursor was used, due to a partial coverage of the palladium surface with chemisorbed chlorine atoms.     

Non-stationary catalytic cracking of methane over ceria-based catalysts: Mechanistic approach and catalyst optimization

The non-stationary cracking of methane over various noble metal/CeO₂-doped catalysts at 400 and 600 °C was followed by DRIFT spectroscopy and on the basis of the identified elementary steps a simplified kinetic modeling is proposed. The production of H₂ by direct decomposition of CH₄ on the noble metal is improved by the capacity of ceria to store carbonaceous surface species thanks to: (i) the spillover of carbonyls from noble metal particles towards basic hydroxyls formed on partially reduced Ce sites and (ii) the reverse spillover of ceria oxygen towards metal to oxidize the carbon issued from methane cracking. The resulting formate adspecies are in turn oxidized into carbon dioxide during the regeneration step. Doping the ceria with basic lanthanide oxides and replacing Pt by more efficient and eventually better dispersed metals for methane decomposition like Rh and Ir lead to significant improvements in the hydrogen productivity.     

The existence of dual Cu site involved in the selective catalytic reduction of NO with propene on Cu/ZSM-5

In order to establish the role of surface species in the selective catalytic reduction (SCR), in situ IR studies were carried out using a DRIFT (diffuse reflectance infrared Fourier transform) cell in gas mixtures of various C₃H₆/NO ratios containing excess oxygen. The location and mobility of Cu ions were investigated by recording the relevant bands of CO adsorbed on Cu/ZSM-5. The nitro species coordinated on Cu²⁺ and the -NCO surface complex as possible intermediates were observed in the reduction of NO with propene on Cu/ZSM-5 between 350 and 400°C. The reactivities of these species toward NO, O₂ and propene were examined. The nitro species can react with propene very rapidly to form N₂ without the formation of NCO species. NCO also reacts with NO₂ and/ or NO at 350°C. IR spectra of CO adsorbed on cuprous ions show that two kinds of Cu ions, which are responsible for the activation of NO and propene respectively, exist on Cu/ZSM-5. From these results, a dual site mechanism involving nitro species and -NCO species as intermediates is suggested.     

Siting of Co2+ Ions in Cobalt-Modified High-Silica Zeolites Probed by Low-Temperature Molecular Hydrogen Adsorption

Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy was used to characterize the effects of introducing cobalt into the zeolites ZSM-5 and mordenite. Aqueous impregnation of the hydrogen form of the zeolite and subsequent in vacuo treatment at temperatures up to 920 K results in partial exchange of protons in bridging hydroxyl groups by Co²⁺ cations. These changes are evidenced by a decrease in intensity of the bands at 3606-3609 cm^-1 characteristic of Brønsted acid sites. DRIFT spectra of hydrogen adsorbed at 77 K also confirm the exchange of protons for Co²⁺ cations, as evidenced by a decrease in the intensity of the band at 4106 cm^-1 for H₂ adsorbed on protons. By contrast, the bands at 3905, 3965, and 4010 cm^-1 for H₂ adsorbed on the Co²⁺ cations increase in intensity. With increasing Si/Al ratio at a constant Co loading of 1 wt% the intensity of the band at 3905 cm^-1 for H-ZSM-5 strongly increases in intensity relative to the other bands. This feature is attributed to Co²⁺ cations interacting with two adjacent cation-exchange sites located in a 10-membered ring. It is hypothesized that the Lewis acidity of Co²⁺ cations in such environments is higher than that of Co²⁺ cations associated with oxygen atoms in individual five- or six-membered rings containing two Al atoms, because the Co²⁺ cation can interact with only two of the four basic oxygen anions located in the ring. It is proposed that Co²⁺ cations in the latter type of sites are identified by the bands at 3965 cm^-1 and 4010 cm^-1 for adsorbed H₂.     

Recent Research and Progress in Food,Feed and Nutrition with Advanced Synchrotron-based SR-IMS and DRIFT Molecular Spectroscopy

Ultraspatially resolved synchrotron radiation based infrared microspectroscopy is able to detect the structure features of a food or feed tissue at cellular and molecular levels. However, to date, this advanced synchrotron-based technique is almost unknown to food and feed scientists. The objective of this article was to introduce this novel analytical technology, ultra-spatially resolved synchrotron radiation based infrared microspectroscopy (SR-IMS) to food, feed, conventional nutrition, and molecular nutrition scientists. The emphasis of this review focused on the following areas: (1) Principles of molecular spectroscopy for food and feed structure research, such as protein molecular structure, carbohydrate conformation, heating induced protein structure changes, and effect of gene-transformation on food and feed structure; (2) Molecular spectral analysis methodology; (3) Biological applications of synchrotron SR-IMS and DRIFT spectroscopy; and (4) Recent progress in food, feed and nutrition research program. The information described in this article gives better insight in food structure research progress and update.