Photochemical Synthesis of Hydrogen Peroxide at Zinc Oxide Surfaces¹

Abstract

Vol. 75 concentration is high. The fates of the heptyl radicals must be diverse, possibly leading to various ketones and aldehydes with seven carbon atoms or less followed by hydrogen abstraction by RO. The presence of unsaturation in the products indicates loss of a second hydrogen by heptyl radicals and also a splitting of these radicals with the formation of short alkyl radicals. This type of reaction does not occur appreciably in the gas phase at room temperature but may be aided in solution by the longer possible interval between formation of a heptyl radical and collision with another radical or with, oxygen.It is not useful to speculate further on the mechanism of these reactions in the liquid phase. The results are exploratory and qualitative, and some doubt exists as to whether this type of experimentation can lead to quantitative interpretations in the reaction kinetic sense. ROCHESTER, SEW YORK [CONTRIBUTION FROM THE MCPHERSON CHEMICAL LABORATORY, THEA quantitative study is made of the heterogeneous reaction leading to hydrogen peroxide formation a t light activated zinc oxide surfaces in contact with oxygen, and pure water or water solutions of various organic additives (e.g., sodium oxalate, sodium formate, etc.). The rates and quantum yields of peroxide formation are determined over a range of oxygen pressures, flow rates, times of gas saturation, amounts of zinc oxide in the suspension, additive types and concentrations, times of exposure, and intensities and wave lengths of the light. The ratios of reactants and products of the zinc oxide-water-oxygensodium oxalate system are determined. From the results the following conclusions are made: (1) The often proposed over-all reaction for the irradiated einc oxide-pure water-oxygen system, 2H20 + 02 4 2H202 (I) occurs only to a very limited extent, if a t all.(2) The presence of an additive increases the peroxide formation, but reaction I is not important. The over-dl reaction in the system containing sodium oxalate additive is shown to be:It is shown that the product hydrogen peroxide is decomposed under certain conditions. In general in the additive-containing system the light-activated zinc oxide catalyzes the occurrence of a normally exothermic reaction.A number of chemical reactions are promoted in systems containing irradiated zinc oxide. For example, zinc oxide in contact with silver oxide gives silver and oxygen among other products.aa-d Also previous investigations have shown that hydrogen peroxide is formed at light-activated zinc oxide surfaces in contact with oxygen, water and miscellaneous ~~m p o u n d s .~~-~ Although no detailed quantitative studies of this heterogeneous reaction have been made, the often proposed over-all reaction4 of peroxide formation is 2H20 f 0 2 + 2He02. Since reaction (1) involves a large increase in free energy ( A E h = 25 kcal./mole H&) the zinc oxide-water-oxygen system is attractive as a possible converter of light energy to chemical energy. The light absorption region of zinc oxide limits the effective wave...

Chemical Reactions Involving Holes at the Zinc Oxide Single Crystal Anode

“…For a primary alcohol, for instance, the two-equivalent oxidation would be the surface reactions :l RCH20-+ p--> RCHO + H. [14] H'--> H + + e [15] In the case of formate, the first oxidation step would be HCOO-+ p--> CO2 + H. It was assumed that the oxidation of R is two-equivalent and that of X one-equivalent. No chemical analyses were made in order to support this hypothesis.…”

The utilization of nitrone spin traps in a study of the photochemistry of pigment dispersions

“…It has long been known that H202 is produced when aqueous ZnO dispersions are irradiated (9) and that the formation of HzOz is enhanced when a suitable donor is added to the system. When DMPO was added to an aqueous ZnO suspension containing no added donors and irradiated (h < 380nm), large quantities of .OH radical adducts were produced.…”

“…This was first suggested by Rubin er al. in 1953 (9) in the photogeneration of H,O, in aqueous ZnO dispersions. The present authors have shown that CdS was capable of reducing methyl viologen at the expense of EDTA.…”

Hydrogen Production Using Highly Active Titanium Oxide-based Photocatalysts

“…Although redox reagents with the highest redox potential were expected to show the highest evolution of O 2 , while those with the lowest redox potential should have shown the highest evolution of H 2 , only a few clear-cut cases were found for the evolution of O 2 . One was H 5 IO 6 , a periodic acid having the highest redox potential while the only other examples found were for IO 3 -, VO 2 + , and BrO 3 -. A series of available redox reagents have been tested as the electron donor in photocatalytic systems for the evolution of H 2 using a non-oxide InP semiconductor.…”

“…Therefore, applying the principle of water decomposition over photoelectrochemical cells to heterogeneous photocatalytic systems using powdered semiconductors is now a field of great interest, encouraging new fundamental investigations into chemical reactions which take place at the electrode surfaces in the electrochemical cells. Significantly, two experimental improvements have supported these investigations, i.e., the accumulation of data on photocatalytic reactions over powdered semiconductors [3][4][5][6] and developments based on the concept of Bard [7][8][9] which has emerged from studies of photochemical cells with semiconductor electrodes in the design of photocatalytic systems using semiconductor particles or powders, as shown in Fig. 5.…”