Brian G. Wicke

Dynamics of the chemiluminescent oxidation of zinc atoms by nitrous oxide

The dynamics of the chemiluminescent oxidation of zinc atoms by nitrous oxide to form ZnO* have been studied utilizing a laser vaporization pulsed beam source to produce the zinc reactant. The kinetic energy dependence of the chemiluminescent reaction cross section has a threshold at 1.6×10−19 J/molecule (1.0 eV) in the center of mass; this threshold value has been used to estimate the dissociation energy of ZnO as D≥4.48×10−19±0.32×10−19 J [2.8(±0.2) eV]. The reaction cross section increases with increasing kinetic energy in accord with the Arrhenius model from threshold to 3.2×10−19 J (2.0 eV), but then increases more rapidly. Above threshold, small amounts of thermal energy added to the N2O reactant increase the reaction cross section 70 times more than the equivalent energy supplied as relative kinetic energy. This energy selectivity is interpreted in terms of an electron jump reaction mechanism which is significantly enhanced by the ν2 bending vibrational mode of N2O.

Reaction of Nitrogen Oxides with Black Carbon: An FT-IR Study

Qualitative and quantitative studies of the reaction of black carbon with the oxides of nitrogen, including NO, NO2/N2O4, N2O, and N2O3, have been carried out with the use of Fourier transform infrared spectroscopy (FT-IR). The active reactant is shown to be NO2, whether it acts as a disproportionation product or as an impurity in the gas under study. FT-IR spectra of the surface species identify them as resulting from reaction of carbon with NO2. For paraffin candle soot which was exposed simultaneously to oxygen atoms, and nitric oxide at 298 K, the surface species also are due to NO2, formed by oxidative adsorption of NO on the soot surface.

Room temperature oxidation of soot by oxygen atoms

Abstract Soot from a paraffin candle diffusion flame was collected on a quartz plate and exposed to oxygen atoms in a flow tube apparatus under carefully controlled conditions. The gaseous combustion products, CO 2 and CO, were measured as a function of time during the combustion using a mass spectrometer, while the weight change of the soot was monitored by a microbalance. The striking feature of this oxidation chemistry is the substantial net adsorption of oxygen atoms by the soot. Under our experimental conditions, the soot adsorbed up to 25 % by weight oxygen, compared to an initial soot composition of less than 5 % by weight oxygen. This is a chemical adsorption, in which the oxygen atoms are strongly bonded to carbon atoms in the soot. Statistically this 25 % oxygen corresponds to one oxygen atom for every four carbon atoms in the soot. At 23 °C, oxygen atom adsorption on fresh soot is much faster than the production of gas-phase oxidation products. Furthermore, the oxygen laden soot does not produce gaseous CO 2 and CO at room temperature in the absence of O atoms; additional energy and/or chemistry is required to produce these gaseous oxidation products.

Cyanuric acid + nitric oxide reaction at 700°C and the effects of oxygen

The reaction of cyanuric acid, (HNCO)/sub 3/, with nitric oxide has been examined in a flow tube under conditions similar to those initially reported for RAPRENO/sub chi/. Surface interactions are shown to play an important role in the observed chemistry. In a quartz flow tube at 700{sup 0}C, (HNCO)/sub 3/ decomposes slowly; addition of nitric oxide does not affect the (HNCO)/sub 3/ decomposition, and no NO reduction occurs. In an otherwise equivalent stainless-steel flow system, (HNCO)/sub 3/ decomposes rapidly to H/sub 2/, CO, and N/sub 2/ at 700{sup 0}C. In this stainless-steel flow tube, NO is efficiently reduced to N/sub 2/ by (HNCO)/sub 3/. At 700{sup 0}C, the stoichiometry of this fast chemistry is 2(HNCO)/sub 3/ + 9 NO{yields}3 H/sub 2/O + 7.5 N/sub 2/ + 6 CO/sub 2/. O/sub 2/ also reacts rapidly with (HNCO)/sub 3/ vapor at 700{sup 0}C in stainless steel. The dominant nitrogen-containing product of this reaction is NO. This reaction of (HNCO)/sub 3/ vapor with O/sub 2/ is faster than the corresponding reaction with NO. Under conditions examined here in stainless steel, reduction of NO by (HNCO)/sub 3/ in the presence of O/sub 2/ occurs only after the O/sub 2/ is consumed.

Nitric oxide inhibition of soot oxidation by oxygen atoms at 298K

Abstract Nitric oxide is observed to inhibit the rate of soot oxidation by oxygen atoms at 298K. Small amounts of added NO reduce the rates of production of CO 2 and CO by up to 35%. We show experimentally that NO is not reducing the gas phase O atom concentration. Thermal desorption mass spectrometry shows a small adsorption of NO on the soot; this NO adsorption corresponds to 1.5% of the carbon atoms on the surface of the individual soot spheres. This inhibition is interpreted in terms of a relatively small number of reactive sites on the soot at which soot gasification occurs and which are effectively blocked by NO. When considered together with our previously reported work on oxidation of soot by oxygen atoms at 298K, these results allow a partial mechanism to be formulated for this soot oxidation process.

Porosity changes in soot resulting from oxygen atom adsorption at 298 K

Abstract Previously reported experiments from our laboratory showed that soot oxidation by oxygen atoms at 298 K was dominated by large net oxygen atom adsorption on this soot. The adsorbed oxygen was stable at 298 K; at elevated temperatures, this oxygen desorbed entirely as carbon monoxide and carbon dioxide. Here, we describe experiments using electron microscopy and nitrogen adsorption measurements to characterize the as-collected soot, soot loaded with varying amounts of adsorbed oxygen, and this oxygen-loaded soot that has been thermally degassed. The electron micrographs show small changes in the soot except at very long oxygen-atom exposure times, for which significant carbon gasification has occurred. The nitrogen adsorption measurements show large increases in the micropore structure of soot on thermal desorption of the adsorbed oxygen. These results confirm our earlier conclusion that oxygen atom adsorption occurs throughout the soot particles, not just on the surface. These data further show that thermal desorption (of CO and CO 2 ) occurs from within the soot particles and does not result from migration of the chemisorbed oxygen to the soot surface.

Fast, two‐channel analog‐to‐digital converter for DEC microcomputers

A circuit is described which provides fast acquisition of two analog signals by the Digital Equipment Corporation family of LSI 11, 11/2, and 11/23 microcomputers. This circuit digitizes each signal with 12 bits of resolution and stores the results directly in microcomputer memory via direct memory access. Real‐time data‐acquisition speeds up to three microseconds per pair of signals may be obtained.

The roles of innate oxygen and adsorbed oxygen in the chemistry of soot oxidation by oxygen atoms at 298 K

We have previously reported work examining the oxidation of soot by oxygen atoms at 298 K. We found that this oxidation produced CO 2 and CO gasification products, but that the chemistry was dominated by large net oxygen atom adsorption on the soot. In the present work we have used O 16 and O 18 in various sequential oxygen atom exposure experiments to study the roles of oxygen which is part of the as-prepared soot (hereafter referred to as innate oxygen) and adsorbed oxygen in the chemistry of this soot oxidation. We find that oxygen innate in the soot appears exclusively in the CO 2 gasification products. Adsorbed oxygen appears in both CO 2 and CO gasification products. A significant fraction of gasification product molecules contain exclusively oxygen atoms which had been adsorbed previously on the soot. That is, the oxygen atom responsible for the evolution of a gasification product molecule may not appear in this product molecule. We interpret these results to mean that the kinetically limiting reaction step in this soot gasification is product desorption; heat liberated by adsorption of an oxygen atom on the soot produces sufficient local surface energy to allow the activated desorption of CO 2 and CO.

Chemiluminescent oxidation of gas phase carbon species by molecular oxygen

Abstract A new technique—pulsed laser vaporization of thin carbon films—has been developed for the production of gas phase carbon species under well controlled experimental conditions. Using this technique, we have observed chemiluminescence from the oxidation of these species by molecular oxygen. The carbon beam is shown by mass spectrometry to contain the following species (in order of their relative abundances): C3 >C >C2 >C4 ∼- C5. Spectroscopic analysis of the chemiluminescence produced in the reaction of these gas phase species with molecular oxygen is used to identify the reaction product as chemiexcited diatomic carbon, C2∗. The pressure dependence of the chemiluminescence and Spectroscopic analysis are consistent with the following reaction: C 3 + O 2 → C 2 ∗ + CO 2 . Preliminary experiments under single collision molecular beam conditions confirm that this single reaction step does occur.