A review of the effect of the addition of hydrogen in the selective catalytic reduction of NOx with hydrocarbons on silver catalysts

Research is progressing fast in the field of the hydrogen assisted hydrocarbon selective catalytic reduction (HC-SCR) over Ag-based catalysts: this paper is a review of the work to date in this area. The addition of hydrogen to the HC-SCR reaction feed over Ag/Al2O3 results in a remarkable improvement in NO (x) conversion using a variety of different hydrocarbon feeds. There is some debate concerning the role that hydrogen has to play in the reaction mechanism and its effect on the form of Ag present during the reaction. Many of the studies use in situ UV-Vis spectroscopy to monitor the form of Ag in the catalyst and appear to indicate that the addition of hydrogen promotes the formation of small Ag clusters which are highly reactive for NO (x) conversion. However, some authors have expressed concern about the use of this technique for these materials and further work is required to address these issues before this technique can be used to give an accurate assessment of the state of Ag during the SCR reaction. A study using in situ EXAFS to probe the H-2 assisted octane-SCR reaction has shown that small Ag particles (containing on average 3 silver atoms) are formed during the SCR reaction but that the addition of H-2 to the feed does not result in any further change in the Ag particle size. This points to the direct involvement of H-2 in the reaction mechanism. Clearly the addition of hydrogen results in a large increase in the number and variety of adsorbed species on the surface of the catalyst during the reaction. Some authors have suggested that conversion of cyanide to isocyanate is the rate-determining step and that hydrogen promotes this conversion. Others have suggested that hydrogen reduces nitrates to more reactive nitrite species which can then activate the hydrocarbon; activation of the hydrocarbon to form acetates has been proposed as the key step. It is probable that all these promotional effects can take place and that it very much depends on the reaction temperature and feed conditions as to which one is most important.

Electrochemical kinetics of Ag vertical bar Ag+ and TMPD vertical bar TMPD+center dot in the room-temperature ionic liquid [C(4)mpyrr][NTf2]; toward optimizing reference electrodes for voltammetry in RTILs

The voltammetry and kinetics of the Ag vertical bar Ag+ system (commonly used as a reference electrode material in both protic/aprotic and RTIL solvents) was studied in the room-temperature ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C(4)mpyrr][NTf2] on a 10 mu m diameter Pt electrode. For the three silver salts investigated (AgOTf, AgNTf2, and AgNO3, where OTf- = trifluoromethanesulfonate, NTf2- = bis(trifluoromethylsulfonyl)imide, and NO3- = nitrate), the voltammetry gave rise to a redox couple characteristic of a

Reduced-size polarized basis sets for calculations of molecular electric properties. III. Second-row atoms

Reduced-size polarized (ZmPolX) basis sets are developed for the second-row atoms X = Si, P, S, and Cl. The generation of these basis sets follows from a simple physical model of the polarization effect of the external electric field which leads to highly compact polarization functions to be added to the chosen initial basis set. The performance of the ZmPolX sets has been investigated in calculations of molecular dipole moments and polarizabilities. Only a small deterioration of the quality of the calculated molecular electric properties has been found. Simultaneously the size of the present reduced-size ZmPolX basis sets is about one-third smaller than that of the usual polarized (PolX) sets. This reduction considerably widens the range of applications of the ZmPolX sets in calculations of molecular dipole moments, dipole polarizabilities, and related properties.

Rapid, quantitative analysis of ppm/ppb nicotine using surface-enhanced Raman scattering from polymer-encapsulated Ag nanoparticles (gel-colls)

Rapid, quantitative SERS analysis of nicotine at ppm/ppb levels has been carried out using stable and inexpensive polymer-encapsulated Ag nanoparticles (gel-colls). The strongest nicotine band (1030 cm(-1)) was measured against d(5)-pyridine internal standard (974 cm(-1)) which was introduced during preparation of the stock gel-colls. Calibration plots of I-nic/I-pyr against the concentration of nicotine were non-linear but plotting I-nic/I-pyr against [nicotine](x) (x = 0.6-0.75, depending on the exact experimental conditions) gave linear calibrations over the range (0.1-10 ppm) with R-2 typically ca. 0.998. The RMS prediction error was found to be 0.10 ppm when the gel-colls were used for quantitative determination of unknown nicotine samples in 1-5 ppm level. The main advantages of the method are that the gel-colls constitute a highly stable and reproducible SERS medium that allows high throughput (50 sample h(-1)) measurements.

On the nature of the silver phases of Ag/Al2O3 catalysts for reactions involving nitric oxide

The nature of the silver phases of Ag/Al2O3 catalysts (prepared by silver nitrate impregnation followed by calcination) was investigated by X-ray diffractograms (XRD), transmission electron microscopy (TEM) and UV-VIS analyses and related to the activity of the corresponding materials for the oxidation of NO to NO2. The UV-VIS spectrum of the 1.2 wt.% Ag/Al2O3 exhibited essentially one band associated with Ag+ species and the NO2 yields measured over this material were negligible. A 10 wt.% Ag/Al2O3 material showed the presence of oxidic species of silver (as isolated Ag+ cations and silver aluminate), but the UV-VIS data also revealed the presence of some metallic silver. The activity for the NO oxidation to NO2 of this sample was moderate. The same 10% sample either reduced in H-2 or used for the C3H6-selective catalytic reduction (SCR) of NO showed a significantly larger proportion of silver metallic phases and these samples displayed a high activity for the formation of NO2. These data show that the structure and nature of the silver phases of Ag/Al2O3 catalysts can markedly change under reaction feed containing only a fraction of reducing agent (i.e. 500 ppm of propene) in net oxidizing conditions (2.5% O-2). The low activity for N-2 formation during the C3H6-SCR of NO (reported in an earlier study) over the high loading sample can. therefore, he related to the presence of metallic silver. which is yet a good catalyst for NO oxidation to NO2. The reverse observations apply for the oxide species observed over the low loading sample, which is a good SCR catalyst but do not oxidize NO to NO2. (C) 2002 Elsevier Science B.V. All rights reserved.

Fragmentation and plasmid strand breaks in pure and gold-doped DNA irradiated by beams of fast hydrogen atoms

The results of an investigation into the damage caused to dry plasmid DNA after irradiation by fast (keV) hydrogen atoms are presented. Agarose gel electrophoresis was used to assess single and double strand break yields as a function of dose in dry DNA samples deposited on a mica substrate. Damage levels were observed to increase with beam energy. Strand break yields demonstrated a considerable dependence on sample structure and the method of sample preparation. Additionally, the effect of high-Z nanoparticles on damage levels was investigated by irradiating DNA samples containing controlled amounts of gold nanoparticles. In contrast to previous (photonic) studies, no enhancement of strand break yields was observed with the particles showing a slight radioprotective effect. A model of DNA damage as a function of dose has been constructed in terms of the probability for the creation of single and double strand breaks, per unit ion flux. This model provides quantitative conclusions about the effects of both gold nanoparticles and the different buffers used in performing the assays and, in addition, infers the proportion of multiply damaged fragments.

Theory of laser-driven double-ionization of atoms at Ti : sapphire laser wavelengths

Progress in the theoretical understanding of non-sequential double-ionization of atoms is reviewed from its beginnings with Kuchiev's work in the late 1980s and Corkum's work in the early 1990s to the present day. The crucial role of laboratory experiment as a persistent stimulus to theoretical endeavour is underlined but the predictive roles of simple, yet fundamental, theory and also of a full quantum mechanical description are not forgotten. A theoretical forward look is provided.