Fred L. Nesbitt

Rate Constant for the Recombination Reaction CH3 + CH3 → C2H6 at T = 298 and 202 K

The recombination of methyl radicals is the major loss process for methyl in the atmospheres of Saturn and Neptune. The serious disagreement between observed and calculated levels of CH3 has led to suggestions that the atmospheric models greatly underestimated the loss of CH3 due to poor knowledge of the rate of the reaction CH3 + CH3 + M → C2H6 + M at the low temperatures and pressures of these atmospheric systems. In an attempt to resolve this problem, the absolute rate constant for the self-reaction of CH3 has been measured using the discharge-flow kinetic technique coupled to mass spectrometric detection at T = 202 and 298 K and P = 0.6-2.0 Torr nominal pressure (He). CH3 was produced by the reaction of F with CH4, with [CH4] in large excess over [F], and detected by low energy (11 eV) electron impact ionization at m/ z = 15. The results were obtained by graphical analysis of plots of the reciprocal of the CH3 signal vs reaction time. At T = 298 K, k 1(0.6 Torr) = (2.15 ± 0.42) × 10-11 cm3 molecule-1 s-1 and k 1(1 Torr) = (2.44 ± 0.52) × 10-11 cm3 molecule-1 s-1. At T = 202 K, the rate constant increased from k 1(0.6 Torr) = (5.04 ± 1.15) × 10-11 cm3 molecule-1 s-1 to k 1(1.0 Torr) = (5.25 ± 1.43) × 10-11 cm3 molecule-1 s-1 to k 1(2.0 Torr) = (6.52 ± 1.54) × 10-11 cm3 molecule-1 s-1, indicating that the reaction is in the falloff region. Klippenstein and Harding had previously calculated rate constant falloff curves for this self-reaction in Ar buffer gas. Transforming these results for a He buffer gas suggest little change in the energy removal per collision, -〈Δ E〉d, with decreasing temperature and also indicate that -〈Δ E〉d for He buffer gas is approximately half of that for Argon. Since the experimental results seem to at least partially affirm the validity of the Klippenstein and Harding calculations, we suggest that, in atmospheric models of the outer planets, use of the theoretical results for k 1 is preferable to extrapolation of laboratory data to pressures and temperatures well beyond the range of the experiments.

Fabrication, Optimization and Characterization of Natural Dye Sensitized Solar Cell

The dyes extracted from pomegranate and berry fruits were successfully used in the fabrication of natural dye sensitized solar cells (NDSSC). The morphology, porosity, surface roughness, thickness, absorption and emission characteristics of the pomegranate dye sensitized photo-anode were studied using various analytical techniques including FESEM, EDS, TEM, AFM, FTIR, Raman, Fluorescence and Absorption Spectroscopy. Pomegranate dye extract has been shown to contain anthocyanin which is an excellent light harvesting pigment needed for the generation of charge carriers for the production of electricity. The solar cell’s photovoltic performance in terms of efficiency, voltage, and current was tested with a standard illumination of air-mass 1.5 global (AM 1.5 G) having an irradiance of 100 mW/cm2. After optimization of the photo-anode and counter electrode, a photoelectric conversion efficiency (η) of 2%, an open-circuit voltage (Voc) of 0.39 mV, and a short-circuit current density (Isc) of 12.2 mA/cm2 were obtained. Impedance determination showed a relatively low charge-transfer resistance (17.44 Ω) and a long lifetime, signifying a reduction in recombination losses. The relatively enhanced efficiency is attributable in part to the use of a highly concentrated pomegranate dye, graphite counter electrode and TiCl4 treatment of the photo-anode.

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

The total rate constant k1 has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction: (1) CH3 + C2H3 Products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C2H4 and CH4, respectively. The kinetic studies were performed by monitoring the decay of C2H3 with methyl in excess, 6 < [CH3]0/ [C2H3]0 < 21. The overall rate coefficient was determined to be k1(298 K) = (1.02 ± 0.53) × 10-10 cm3 molecule-1 s-1 with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C2H3 + H and C2H3 + C2H3. The present result for k1 at T = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH3 + C2H5 and with a value for k1 estimated by the geometric mean rule employing values for k(CH3 + CH3) and k(C2H3 + C2H3). Qualitative product studies at T = 298 K and 200 K indicated formation of C3H6, C2H2, and C3H5 as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH3 + C2H3 reaction. We also observed the secondary C4H8 product of the subsequent reaction of C3H5 with excess CH3; this observation provides convincing evidence for the combination-decomposition channel yielding C3H5 + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted.

The reaction O(³P) + HOBr: Temperature dependence of the rate constant and importance of the reaction as an HOBr stratospheric loss process

The absolute rate constant for the reaction O(³P) + HOBr has been measured between T=233K and 423K using the discharge-flow kinetic technique coupled to mass spectrometric detection. The value of the rate coefficient at room temperature is (2.5±0.6) × 10−11 cm³ molecule−1 s−1 and the derived Arrhenius expression is (1.4±0.5) × 10−10 exp[(−430±260)/T] cm³ molecule−1 s−1. From these rate data the atmospheric lifetime of HOBr with respect to reaction with O(³P) is about 0.6h at z = 25 km which is comparable to the photolysis lifetime based on recent measurements of the UV cross section for HOBr. Implications for HOBr loss in the stratosphere have been tested using a 1D photochemical box model. With the inclusion of the rate parameters and products for the O + HOBr reaction, calculated concentration profiles of BrO increase by up to 33% around z = 35 km. This result indicates that the inclusion of the O + HOBr reaction in global atmospheric chemistry models may have an an impact on bromine partitioning in the middle atmosphere.