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THE MERCURY PHOTOSENSITIZED DECOMPOSITION OF WATER VAPOR PDF

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THE MERCURY PHOTOSENSITIZED DECOMPOSITION OF WATER VAPOR Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By HAROLD BASSECHES, B.S. « V The Ohio State University 1951 Approved by: TABLE OF CONTENTS Jagi Introduction . . . . . ........................................... 1 Historical Review .................................................... k Methods and Techniques Used in This Research 11 A. Use of Flow Method 11 B. Choice of Radiation Source 12 C. Analytical Techniques 111 1. Hydrogen Peroxide iu 2. Mercuric Oxide and Mercuric Ion 15 3. Oxygen 16 U. f^rdrogen 17 Description of Apparatus ....................................... 19 Calibration Techniques ........................................... 50 A. Lamp Calibration 50 B. Flow Rate Calibration 53 C. Volume and Pressure Measurements 55 D. Temperature Measurement 56 E. Other Operating Conditions 57 Run Procedure............................................................ 59 Experimental Results .............................. . . . . 78 A. A uxiliary Data 78 B. hydrogen Peroxide C. Mercuric Oxide and Mercuric Ion D. Gaseous Products E. The A ctivation Energy of the Process for the Decomposition of ?/ater Vapor i S82477 page F. Dissociation Energy of Water and of OH 107 G. Quantum Yields 111 H. Effect of Change of Flow Rate 111 VIII. Discussion of Experimental Results ....................... 116 A. Limiting Values for the Dissociation Energy 116 B. Discussion of Number of Degrees of Freedom of the System 119 C. hydrogen Peroxide 133 D. Quantum Yields 13U E. Effect of Flow Rate - Difference Between Static and Flow Methods 135 F. Discussion of Errors 137 IX. Formulation of Mechanism ...................... ll|i| A. Discussion of the Primary Reaction iLjli B. Elementary Secondary Reactions Involving Atoms and Radicals 15U C. Proposed Mechanism 17U X. Summary...................................................................................................193 XI. Bibliography.................................................................... 195 XII. Acknowledgments....................................................................................199 XIII. Autobiography.....................................................................................201 ii THE MERCURY PHOTO SENSITIZED DECOMPOSITION OF WATER VAPOR I. INTRODUCTION The present research is concerned with a re-examination of the mercury photosensitized decomposition of water vapor by the use of a flow method. The reaction involves the irradiation of a mixture of mercury vapor and water vapor by the resonance line, ^ 2537A, from a low pres­ sure mercury-neon gas discharge. The resonance radiation excites the mercury atoms from the ground state to the excited state (^P]_). By inelastic collisions of the second kind the excitation energy of the 3p]_ mercury atom is transferred to the water molecule. This excitation energy (112.651 kcal/mole) is not quite sufficient to bring about the dissociation of water according to the reaction H20 - H + OH (1) A small amount of energy is needed to supply the deficiency. This energy is usually furnished by the thermal motion of the particles. Only one other flow investigation of the reaction has been made. This investigation was very incomplete and the effect of the change of such variables as pressure, temperature and flow rate was not studied. Both hydrogen and oxygen were found as gaseous products. Although several static investigations were made, only one is fairly complete. No oxygen was found in any of the static studies. Although the dis­ sociation energy of water into hydrogen atoms and hydroxyl radicals and 1 the dissociation energy of hydroxyl radicals into tydrogen atoms and oxygen atoms can be derived by the study of the reaction, fairly reli­ able values by this method were reported in only one of all of these investigations (See Part II). One of the purposes of this research was to see if it was possi­ ble to confirm the observation that oxygen is found when a flow system is used. It was also desired to determine why oxygen was found when a flow method was used and why it was absent when a static method was used. Since the process of the mercury photosensitized decomposition of water vapor is a kinetic phenomenon the interpretation of the results in regard to the dissociation energies depends on the mechanism assumed for the reaction. No detailed mechanism has been suggested by any previous investigation and the dissociation energy results reported are based on mechanisms with insufficient basis. By more complete qualitative and quantitative techniques for the analysis of the reaction products it is the purpose of this research to make a judicious choice of possible elementary reactions that are in­ volved and to formulate a mechanism which describes the reaction. With improved techniques, the present research attempts to deter­ mine more precise values for the dissociation energy of water according to reaction (1) and for the dissociation energy of the hydroxyl radical into oxygen and hydrogen atoms. The problem of whether the present method leads to limiting values for these dissociation energies is in­ vestigated. (See Part VIII A). The present method is examined to see whether the measurement of the yield of gaseous products with tempera- 2 ture can be used to determine the number of degrees of freedom for the system Hg* + I^O. The number of degrees of freedom has a bearing on one method of calculation of the dissociation energy of water. (See Part VIII B.) The reaction was studied over a range of pressures to check a proposal recently advanced that the activation energy for the decompo­ sition of the water vapor varies with pressure. (See Part VIII B.) 3 II. HISTORICAL REVIEW The first study of the mercury photosensitized decomposition of water vapor seems to have been made by Senftleben and Rehren^ in 1926. They performed the test under static conditions. In most of their tests the water vapor pressure was kept at U.6 mm. The water they used was vacuum distilled before use. The tests were conducted at room temperature. They used a water-cooled quartz mercury lamp as a light source but did not report aqy intensity. The analysis of the gaseous products was based on the measurement of the heat conductivity of the gas. From their analysis they concluded that hydrogen was the only gaseous product formed. In searching for an explanation of the absence of oxygen they proposed the reaction 2H2O = Hg + H202 (1) which implies the recombination of OH radicals to form the tydrogen peroxide. They were not able to detect any H2O2 by any chemical means3 however the methods used were not reported. They did not believe that oxygen was adsorbed on the walls of the vessel or that atomic oxygen was involved. They did not believe that the mercury could have combined with the oxygen. They established that the reaction depended on collisions of the second kind with excited mercury since in the absence of the mercury, no reaction was observed when the water vapor was irradiated. They concluded that the ^ l8i;9A line was not effective in the reaction, by k interposing fluor-spar between the lamp and the vessel. The fluor-spar would have absorbed the line. Since no differences were observed when the fluor-spar was removed the absence of ary effect by the line was demonstrated. They concluded that of the following processes for de­ composition of the water vapor H20 - H + H + 0 (2) H20 = H2 + 0 (3) H20 = H + OH (U) only (k) was probable. They used a thermochemical cycle to eliminate (2) and (3)» Using more modern data for the dissociation energies in­ volved their method for the elimination of (2) and (3) remains valid. They placed an upper limit of 112 kcal/Wle (U-9 ev) for the dis­ sociation energy of water into H and OH, although this does not seem to be a value corrected to 0° K and probably applies to room temperature. p Gaviola and Wood in 1928 studied the sensitized band fluorescence of a number of molecules, among which OH and water were included. Their findings in regard to the primary reaction are discussed in detail in Part IX A. They confirmed that dissociation of water into H and OH does take place from the observation of HgH and OH bands. From their spectro­ scopic studies, they deduced that the main effect of water molecules on mercury atoms was to bring these atoms down to the metastable ^PQ state. From estimates of the number of effective collisions they deduced that l|.9ev was not quite sufficient energy for the dissociation of the $ ■water molecule into H and OH and that dissociation takes place only in the very few cases when the difference of energy can be obtained from the kinetic energy of high velocity molecules. From a thermochemical cycle they deduced that the dissociation energy of water should be about 5.2 ev and that for OH should also be about 5.2 ev. However, they utilized a value of 7.02 ev for the dis­ sociation energy of oxygen which is considerably different from the mare modern value of 5.080 ev as listed by Herzberg.^ Therefore, we cannot attach too much importance to their quantitative calculations. They also made estimates on the rate of formation of OH molecules and concluded that this estimate would lead to best agreement with a dis­ sociation energy of water of 5.1 ev. However, since they did not feel they knew the output of their lamp with sufficient accuracy, they did not stress the calculation. In addition it may be noted that their argument assumed that OH molecules disappeared by the reaction OH + OH * HgOg which is not justified. They claimed that Bates and Taylor^ and Bon- hoeffer and Loeb detected and measured the rate of H2O2 formation but this cannot be verified in these references. Riechemeir, Senftleben, and Pastorff^ (referred to hereafter as RSP) in 193U, undertook a reinvestigation of the system studied earlier *L by Senftleben and Rehren from a more quantitative viewpoint. They took cognizance of the revised estimate of "the dissociation energy of water by Gaviola and Wood2 and the value given ty Bonhoeffer and n Reichardt of 5*0 + 0.1 ev (115 +2.5 kcal/mole) on the basis of spec­ troscopic measurements. Assuming the validity of the proposal of 6 o Gaviola and Wood that the difference between the excitation energy of the excited mercury atom and the dissociation energy of -water is made up from the thermal energy of the colliding species, they reasoned that the yield of dissociation products should increase as the tempera­ ture increased. From such a rise a means -was provided for determining the energy to be added to the excitation energy to bring about dissoci­ ation. They measured the yield of hydrogen as a function of tempera­ ture and compared their results with the theoretical expression based on the Boltzmann formula. See Part VIII B for the mathematical detail. The experiments were conducted by a static method. The water vapor pressure was varied over the range 0.77 to 10.5 mm of Hg. The temperature was varied over the range from 300 to 500° K. The concen- 6 tration of water was kept constant. ESP did not report the intensity of the lamp they used. The authors apparently assumed from the earlier work of Senftleben and Rehren^ that the only gaseous product was hydro­ gen. They did not seem to make any tests for hydrogen peroxide al­ though they stated that it was one of the products in their freezing trap. As a result of their measurements, RSP found that 0.21 + O.Ol* ev of thermal energy had to be supplied to the water molecule in addition to the h.9 ev of excitation energy of the mercury atom (^P-^) to dis­ sociate the water molecule into H and OH. From this value they calcu­ late that the dissociation energy of water is 5*11 + O.Oi* ev (117.9 + 0.9 kcal/mole). Utilizing a value of the dissociation energy of water into its atoms as 10.U + 0.1 ev they calculate a dissociation energy of OH into its atoms equal to 5.29 + O.llt ev. The temperature to which 7

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