THE DECO~WOSITION OF AMMONIA ON TUNGSTEN SURFACES

THE DECO~WOSITION OF AMMONIA ON TUNGSTEN SURFACES

THE DECOMPOSITION OF AMMONIA ON TUNGSTEN SURFACES by YU.KWANG PENG, DIPL. CHEM. A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment of the Requirements for the Degree Doctor of Philosophy McMaster University (February) 1973 @ Yu Kwang Pang 1973

OOCTOR OF PHILOSOPHY (Chemistry) McMASTER UNIVERSITY Hamilton, Ontario. TITLE: The Decomposition of Ammonia on Tungsten Surfaces AUTHOR: Yu Kwang Peng, B.Sc. Dipl. Chem. (Cheng Kung'University) (Technical University Hannover, Germany) SUPERVISOR: Professor P. T. Dawson NUMBER OF PAGES: XV, 20S SCOPE AND CONTENTS: This thesis presents an experimental study of the decomposition of ammonia on tungsten surfaces. The investigations were made at ammonia pressures between 10-7 and 10-2 torr, and at temperatures between 200 and SOOoK by employing thermal desorption mass spectrometry and field emission microscopy. The surface species formed during the interaction were first isolated and the rate parameters for the formation and desorption of these species were determined. The properties of the adlayer of these species were also studied. Based on the rate parameters for the formation and desorption of these species a satisfactory mechanism for the decomposition of ammonia on tungsten surfaces has been deduced. ii

ACKNOWLEDGEMENTS The author wishes to express his most sincere gratitude to his supervisor, Professor P. T. Dawson, for his guidance and encouragement throughout the course of this work, and for his valuable criticisms and suggestions during the preparation of this thesis. The author wishes to thank Professors R. B. Anderson and C. Calvo for their helpful suggestions and reading of the manuscript. The author also wishes to thank his wife, Anita, for her patience and constant encouragement. Finally, the author wishes to acknowledge the receipt of financial support from the Chemistry Department for all the time spent in this university. iii

TABLE OF CONTENTS CHAPTER 1 1.1 1.2 1.3 CHAPTER 2 2.1. 2.2 2.2.1 ~NTRODUCTION General Current status of studies on the interaction of ammonia with tungsten surfaces Scope of the present work FLASH DESORPTION SPECTROMETRY AND FIELD EMISSION MICROSCOPY Ultra-high vacuum Flash desorption spectrometry Single binding state with constant activation energy PAGE 1 1 2 7 9 9 10 10 2.2.2 Single binding state with activation 13 2.23 2.3 2.3.1 2.3.2 2.3.3 2.3.4 energy depen~ent on coverage Multiple binding states Field electron emission microscopy Field electron emission from metals Work function of the clean surface Field electron emission microscope Work function change and its me"asurement iv 16 16 17 20 20 23

CHAPTER 3 EXPERIMENTAL METHODS 29 3.1 Materials 29 3.2 Flash desorption experiments 30 3.2.1 Decompos±tion of ammonia 30 3.2.2 Activated nitrogen adsorption 39 3.3 Field emission experiments 41.CHAPTER 4 RESULTS 49 4.1 Interaction of ammonia with tungsten 49 4.1.1 Interaction of ammonia with tungsten 49 at different temperatures 4.1.2 State of hydrogen desorbing from 57 the n-species 4.1.3 Quantitative estimates of surface 68 coverage 4.1.4 Pressure dependence of the n- 75 formation reaction 4.1.5 Hydrogen and nitrogen ratios in the 78 formation and desorption of the n-species 4.1.6 Interaction of ammonia with 6-nitro- 82 gen at different temperatures 4.1.7 Kinetics of formation and desorption 84 of n-species 4.1.8 Attempts to form the n-species from 88. nitrogen and hydrogen v

4.2 ~-state of nitrogen on tungsten 90 surfaces 4.2.1 Thermal desorption spectra obtained 91 after adsorption of activated nitrogen 4.2.2 Desorption kinetics 93 (a) Desorption in a closed system 94 with fast heating (b) Kinetic analysis for a pumped 106 system with slow heating rate 4.2.3 Isotope exchange experiments 108 4.2.4 The adsorption of non-activated 111 nitrogen 4.2.5 Desorption in a closed system with 113 varying heating rate 4.3 Field emission results 117 4.3.1 Formation of o-nitrogen 117 (a) Repeated dosing at 300 0 K 118 (b) Ammonia interaction under low 125 pressure with tungsten at 500 0 K 4.3.2 Ammonia interaction under higher 130 pressure with tungsten at 500 0 K 4.3.3 Formation and desorption of n-species 135 4.3.4 Ammonia interaction with tungsten 144 at 700 and BOOoK under high exposure vi

CHAPTER 6 CHAPTER 7 APPENDICES A B REFERENCES CHAPTER 5 DISCUSSION 156 5.1 Mechanism and kinetics of ammonia 156 decomposition 5.2 Desorpt~on model for the X-state of 164 nitrogen 5.2.1 The immobile limit 166 5.2.2.Isotopic mixing 173 5.3 Surface configurations of different 174 ad1ayers 5.3.1 5-nitrogen adlayer 174 5.3.2 n-species 180 5.4 Interpretation of earlier experi- 186 mental results based on present understanding of ammonia decomposition SUMMARY SUGGESTIONS FOR FUTURE RESEARCH The fraction of n-hydrogen detected as mass 1 in the line of sight experiment Kinetic analysis of the n-formation 194 198 200 200 201 204 vii

LIST OF FIGURES FIGURE 1 2 3 4 5 Potential energy diagrams for electrons at a metal surface in the presence of an applied field Schematic diagram of field emission microscope Schematic diagram of electron trajectories in a field emission tube Field emission pattern of clean tungsten Schematic diagram indicating lens effect of PAGE 18 22 22 24 24 conducting humps on a tip surface 6 7 Ultra-high vacuum system used for thermal desorption studies of ammonia decomposition Hydrogen and nitrogen desorption spectra obtained with the reaction vessel at room temperature and 77 0 K 31 35 8 Ultra-high vacuum system with a line of sight 38 9 10 11 between the sample filament and mass spectrometer Ultra-high vacuum system used for the studies of activated nitrogen adsorption Ultra-high vacuum system used for field emission studies Electronic arrangement used in the field 40 43 45 emission studies viii

_---r 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Hydrogen desorption spectra, obtained using Method I, from adsorption of ammonia at temperatures between 200 and 700 0 K Nitrogen desorption spectra, obtained using Method II, from adsorption of ammonia at temperatures between 200 and 700 0 K The formation of a-nitrogen by repeated. adsorption of ammonia at room temperature and heating to 800 0 K Hydrogen and nitrogen desorption spectra, obtained using Method II, from the adsorption of ammonia at different prsssures at 700 0 K Hydrogen and nitrogen desorption spectra, obtained using Method III, from the adsorption of ammonia at different pressures at 700 0 K Hydrogen and nitrogen desorption spectra, obtained using s1.ow heating rate, from the adsorption of ammonia Atomic and molecular hydrogen desorption spectra obtained from the adsorption of ammonia Probability of occurrence for certain mass 2 50 52 54 56 58 61 63 65 to mass 1 ratio ix

20 Desorption spectra qf mass 2, 3 and 4 obtained from ammonia adsorption in a reaction vessel where the walls were saturated with deuterium atoms 67 21 Relative intensities of mass 2, 3 and 4 when 69 hydrogen and deuterium gases were allowed to flow through the system 22 Variation in hydrogen, B-nitrogen and 0 71 nitrogen coverage with increasing ammonia dosing pressure 23 Hydrogen desorption spectra from adsorption of 76 ammonia at 700 0 K for different doses 24 Nitrogen desorption spectra from adsorption 77 of ammonia at 700 0 K for different doses 25 Variation in total surface coverage of hy- 79 drogen and nitrogen with increasing ammonia exposure at 700 0 K 26 Nitrogen desorption spectra, obtained using 83 Method III, from adsorption of ammonia on a clean tungsten surface and a. o-nitrogen adlayer 27 Variation in total surface coverage of nitro- 86 gen with increasing ammonia exposure at different temperatures x

28 29 Eva~uation of the activation energy for the formation of the n-species Eva~uation of the activation energy for the desorption of the n-species 87 89 30 Thermal desorption spectra obtained from acti- 92 vated nitrogen adsorption at 300 0 K for different exposures 95 3~ Thermal desorption traces obtained from activated nitrogen adsorption at 300 0 K emp~oying 32 different heating rates -1 Computer fitting of the 886 deg sec trace of 99 Figure 31 33 Evaluation of the activation energy for desorp-~07 tion of A-nitrogen 34 Mass 28, 29 and 30 desorption spectra ob- 1~0 tained from isotope exchange experiment 35 Therma~ desorption spectra obtained from ad ~12 36 sorption of non-activated nitrogen gas Thermal desorption traces for B-nitrogen and 115 (A+B)-nitrogen in a closed system 37 Variation in work function with temperature 119 of a tungsten emitter which had been repeatedly dosed with ammonia at 300 0 K 121 38 Field emission patterns obtained for the repeated adsorption of ammonia as described in Figure 37 xi

39 Variation in work function with temperature 126 of a tungsten emit.ter which had been dosed at 500 o K for 10-5 torr-sec 40 Field emission patterns obtained for the ad- 128 sorption of ammonia as described in Figure 39 41 Field emission patterns obtained for the ad- 131 sorption of ammonia at 500 0 K for 0.1 torr-sec and its subsequent decomposition 42 Field emission patterns obtained for the ad- 133 sorption of ammonia at 500 0 K for 1 torr-sec and its subsequent decomposition 43 Field emission patterns obtained for ammonia 136 interaction for 0.05 torr-sec at 700 0 K and its 44 45 46 subsequent decomposition Field emission patterns obtained for ammonia interaction for 0.2 torr-sec at 700 0 K and its subsequent decomposition Field emission patterns obtained for ammonia interaction for 1 torr-sec at 700 0 K and its subsequent decomposition Variation in work function and A log A with temperature of a tungsten emitter which had been dosed with anunonia for 0.05, 0.2 and 1 torr-sec at 700 0 K 138 140 142 xii

-, 47 "Variation in work function with temperature 145 of a tungsten emitter which had been dosed 48 49 with ammonia at 500 0 K for 10 torr-sec Fie1d emission patterns obtained for ammonia interaction for 10 torr-sec at 500 0 K and its subsequent decomposition Variation in work function and A log A with 146 149 temperature of a tungsten emitter which had 50 51 been dosed with ammonia for 10 torr-sec at 700 and 800 0 K Fie1d emission patterns obtained for ammonia interaction for 10 torr-sec at 700 0 K and its subsequent decomposition Fie1d emission patterns obtained for ammonia interaction for 10 torr-sec at 800 0 K and" its subsequent decomposition 150 153 52 Tungsten atom arrangement for a {100} p1ane 157 53 169 Variation in the number of adjacent atompairs with fractiona1 coverage for a random 54 55 immobi1e desorption process. Variation in work function with surface coverage upon popu1ating the o-nitrogen on the W{100} p1ane Schematic diagram for possib1e o-nitrogen surface configuration xiii 176 178

56 57 58 A possib1e nitrogen arrangement for the n-species on a W{10~} p1ane Comparison of work function versus temperature p10t obtained for W2t~H2 with that obtained for the o-nitrogen Effect of different heating periods on the work function versus temperature p10t 183 189 191 A-1 Position of the samp1e fi1ament with respect 200 to the ionization chamber of mass spectrometer