Mercury-photosensitized Reaction Research Articles

The photochemical and thermal decompositions of hydrogen sulphide

The photochemical decomposition of hydrogen sulphide has been investigated at pressures between 8 and 550 mm of mercury and at temperatures between 27 and 650° C, using the narrow cadmium line ( λ 2288) and the broad mercury band (about λ 2550). At room temperature the quantum yield increases with pressure from 1.09 at 30 mm to 1.26 at 200 mm. Above 200 mm pressure there was no further increase in the quantum yield. Temperature had little effect on the quantum yield at λ 2550, but there was a marked increase in the rate of hydrogen production between 500 and 650° C with 2288 Å radiation. This may have been caused by the decomposition of excited hydrosulphide radicals. The results are consistent with a mechanism involving hydrogen atoms and hydrosulphide radicals. The mercury-photosensitized reaction is less efficient than the photochemical decomposition, the quantum yield being only about 0.45. The efficiency increased with temperature and approached unity at high temperatures and pressures. This agrees with the suggestion that a large fraction of the quenching collisions lead to the formation of Hg ( 3 P 0 ) atoms. The thermal decomposition is heterogeneous at low temperatures and becomes homogeneous and of the second order at 650° C. The experimental evidence suggests the bimolecular mechanism 2H 2 S → 2H 2 + S 2 . The activation energies are 25 kcal/mole (heterogeneous) and 50 kcal/mole (homogeneous).

The Mercury (3P1) Photo-Sensitized Reaction of Ethane at High Temperatures

The mercury photo-sensitized decomposition of ethane has been carried out with particular emphasis on the high temperature reaction. Evidence is given that under these experimental conditions the ethyl radical becomes unstable around 400°C. An activation energy of 39.5 kcal is assigned to the reaction C2H5=C2H4+Hon the basis of the proposed mechanism.

The Mercury Photosensitized Reaction of Dimethyl Ether

The mercury photosensitized reaction of dimethyl ether has been investigated at pressures between 28 and 500 mm and at temperatures between 25°C and 292°C. At 25°C the products consist almost entirely of hydrogen and 1,2-dimethoxyethane, and the initial step has been shown to be the abstraction of a hydrogen atom. At higher temperatures CH2O, CH4, C2H6 and probably CH3OC2H5 become important products. They are derived from the decomposition of the methoxy methyl radical followed by subsequent radical recombination reactions. Carbon monoxide is produced in the later stages from the decomposition of formaldehyde. By varying the light intensity and pressure it has been shown that ethane and dimethoxyethane are produced by second order recombination of methyl and methoxy methyl radicals respectively. The activation energies of the following reactions have been estimated: CH3+CH3OCH3→CH4+CH3OCH2E=9.0 kcal. CH3OCH2→CH3+CH2OE=19 kcal.the only assumption being that methyl and methoxy methyl radicals recombine with zero activation energy.

The Mercury Photo-Sensitized Reaction Between Hydrogen and Oxygen

The mercury sensitized reaction between hydrogen and oxygen in a flowing system was studied. The quantum yield of the reaction was found to be less than unity, and no evidence of a chain reaction at 40° was found. Hydrogen peroxide was the principal product, and yields of peroxide above 90 percent were found for many of the experiments. It was shown that in the gas phase reaction between ozone and hydrogen peroxide, water was rapidly formed. The yield of product was found to increase with increasing oxygen flow rate, with increasing total flow rate for a fixed hydrogen to oxygen ratio, and to increase and then decrease with increasing hydrogen to oxygen ratio. The reaction scheme proposed is Hg+hν→Hg*,Hg*+H2→H+H+Hg,Hg*+O2→O2+Hg,H+O2→HO2,H+M→HM,HM+HM→H2+M,HO2+HO2→H2O2+O2.This leads to the rate equation, d(H2O2)dt=k3/k4′′(O2)k3/k4′′(O2)+1Iak1/k2(H2)k1/k2(H2)+(O2),using the flow rates of hydrogen and oxygen. k1/k2 is the ratio of quenching efficiencies of hydrogen and oxygen for activated mercury or 1.62. k3/k4″ is taken equal to 1.2×10−3.

The Mercury-Sensitized Reaction between Hydrogen and Nitrous Oxide

The mercury photo-sensitized reaction of nitrous oxide and hydrogen to produce nitrogen and water has been studied in the pressure range 10–500 mm. It is shown that hydrogen exerts only a minor influence on the rate. A mechanism has been postulated involving a sensitized decomposition of nitrous oxide into nitrogen and atomic oxygen at a rate roughly one thousand times the rate of reaction of hydrogen atoms with nitrous oxide. The atomic oxygen promotes chains by reaction with hydrogen, the chains are terminated by recombination of hydrogen atoms, on the wall at low pressure and mainly in the gas phase above 100 mm pressure under the experimental conditions.