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Reaction Pathways Linking Chemisorption to Desorption of Methylchlorosilanes on Copper(001)

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Reaction Pathways Linking Chemisorption to Desorption of Methylchlorosilanes on Copper(001) REACTION PATHWAYS LINKING CHEMISORPTION TO DESORPTION OF METHYLCHLOROSILANES ON COPPER(001) BY JAMES LALLO A dissertation submitted to the Graduate School|New Brunswick Rutgers, The State University of New Jersey in partial fulfillment of the requirements for the degree of Doctor of Philosophy Graduate Program in Chemistry and Chemical Biology Written under the direction of Professor B.J. Hinch and approved by New Brunswick, New Jersey October, 2012 ABSTRACT OF THE DISSERTATION Reaction pathways linking chemisorption to desorption of methylchlorosilanes on copper(001) by James Lallo Dissertation Director: Professor B.J. Hinch The interactions of silane molecules with metal surfaces are pivotal in many com- mercial processes. Of particular interest is the commercial \Direct Process," in which methyl chloride (CH3Cl) is exposed to silicon, in the presence of a copper catalyst and other promoters, producing dimethyldichlorosilane as the predominant product. Previous UHV studies have investigated the interaction of \pre-dissociated" methyl (CH3) and chlorine (Cl) with copper and copper-silicide surfaces. This thesis investi- gates the reaction mechanisms of \pre-associated" methylchlorosilanes ((CH3)xClySiHz, x+y+z=4) adsorbed on a copper(001) surface. The goal was to develop an understand- ing of the intermediates and transfer processes involved for dissociative adsorption and subsequent desorption of these molecules. Only molecules containing at least one Si- H bond were observed to undergo chemisorption. It was found that dimethylsilane (CH3)2SiH2 and methylsilane (CH3)SiH3 exhibit ligand transfer on the copper sur- face, leading to the desorption of trimethylsilane (CH3)3SiH, in both cases. The sug- gested intermediates present after adsorption of methylsilane were methylsilyl CH3SiH2, methysilylene CH3SiH and methylsilylidyne CH3Si. The proposed mechanisms leading ii to trimethylsilane desorption involve rate liming methyl transfer between silicon cen- ters. Chlorinated silane species also exhibited methyl transfer to form desorbing silane species. While methyl transfer appears facile, chlorine transfer among silicon cen- ters was not observed. Investigations were also conducted on absorbed methyl groups alone on the copper surface. Under certain conditions of azomethane pyrolysis the ad- sorption of both CH3 groups and H atoms on the copper surface was observed. The co-adsorption of methyl and atomic hydrogen leads to the simultaneous desorption of methane and molecular hydrogen at ∼300K. Any remaining methyl groups decompose at 420K, leading to a resumption of the simultaneous methane and H2 desorption. The relative intensities and peak desorption temperatures of the CH4 and H2 desorption were used to study the kinetics of the associative desorption reaction. iii Acknowledgements It has been a long road to this point, and I have had much help along the way. I first need to thank my mother, Madeline. It has just been the two of us for many years now, but she has never wavered from supporting and encouraging me. Even if she never quite understands what it is that I do! My Aunt Helen has always been there for me, with more wisdom then anyone with a doctorate could hope to have. Professionally, I need to thank my advisor Jane. She has provided much support and insight, and endured many long discussions with me. Bob Bartynski was my first advisor at Rutgers. From learning how to tighten bolts to learning how to build a career with a Ph.D., Bob has been there to help. It also must be noted that he is the person who first introduced me to fine Italian espresso. While many students and post- docs have guided my time here at Rutgers, special thanks must be given to Dr. Sylvie Rangan. From providing help and ideas for experiments, to sharing commiseration over coffee, to discussing wine on local public television, the effect of Sylvie on my time here at Rutgers is incalculable. And thanks to Robin, for being with me these past years, and hopefully many years more. iv Table of Contents Abstract :::::::::::::::::::::::::::::::::::::::: ii Acknowledgements ::::::::::::::::::::::::::::::::: iv List of Tables ::::::::::::::::::::::::::::::::::::: viii List of Figures :::::::::::::::::::::::::::::::::::: ix 1. Introduction ::::::::::::::::::::::::::::::::::: 1 1.1. Motivation: The Direct Process production of methylchlorosilanes . 1 1.2. Previous Direct Process Studies . 2 1.3. Structure of thesis . 5 1.4. Acknowledgements . 9 2. Experimental ::::::::::::::::::::::::::::::::::: 12 2.1. UHV Chamber . 12 2.2. Sample preparation and temperature control . 14 2.3. Molecule and radical dosing . 15 2.4. Experimental Methods . 22 3. Azomethane pyrolysis ::::::::::::::::::::::::::::: 33 3.1. Introduction . 33 3.2. Using the azomethane CH3 doser . 35 3.3. Comparing azomethane doser temperatures . 37 3.4. Quantitative comparison . 39 v 3.5. Origins of \325K" feature . 41 3.6. Review of assumed methyl and H surface reactions . 42 3.7. Sources of adsorbed atomic hydrogen . 43 3.8. Mechanisms for generation of vibrationally excited hydrogen . 44 3.9. Other possible sources of adsorbed atomic hydrogen sources . 46 3.10. Conclusion . 47 4. Simultaneous Hydrogen and Methane Desorption ::::::::::: 53 4.1. Introduction . 53 4.2. Previous studies of adsorbed methyl and hydrogen . 53 4.3. Experiment . 55 4.4. Numerical simulation . 56 4.5. Methane and hydrogen desorption . 57 4.6. Methane and hydrogen simulation . 59 4.7. Comparing experiment and simulation . 61 4.8. Implications of ∆THT ≈ 0 ......................... 62 4.9. Other possible pathways . 64 4.10. Conclusion . 67 5. Methylsilanes ::::::::::::::::::::::::::::::::::: 72 5.1. Introduction . 72 5.2. Experimental details . 74 5.3. Product species identification . 75 5.4. Experimental results . 77 5.5. Discussion . 89 5.6. Conclusion . 100 vi 6. Adsorption of Methylchlorosilanes ::::::::::::::::::::: 103 6.1. Introduction . 103 6.2. Experiment . 104 6.3. Results . 105 6.4. Discussion . 116 6.5. Conclusion . 120 7. Conclusion :::::::::::::::::::::::::::::::::::: 123 7.1. Methyl reactions on copper . 123 7.2. Desorption of methylsilanes from Cu(001) . 124 7.3. Desorption of methylchlorosilanes from Cu(001) . 125 7.4. Future directions . 126 Appendix A. Relative hydrogen and methane desorption ::::::::: 127 A.1. Quantification of mass spectrometer signals . 127 A.2. Measurement of the mass spectrometer ion detection function . 128 A.3. Relative calibration of Methane and Hydrogen . 128 Appendix B. Deconvolution and calculation of deuterated mass spectra 130 B.1. Deconvolution . 130 B.2. Deconvolution artifacts . 132 B.3. Deuterated mass spectra . 135 Appendix C. Relative desorption energies :::::::::::::::::: 138 vii List of Tables 4.1. Methane and hydrogen desorption kinetics . 56 B.1. Dimethylsilane cracking pattern . 136 B.2. Singly-deuterated dimethylsilane cracking pattern . 137 viii List of Figures 1.1. Silane molecules . 6 1.2. Silane moieties . 7 1.3. Evidence for TMSD . 8 2.1. Vacuum chamber . 13 2.2. Sample Holder . 14 2.3. Cooling System . 15 2.4. Chlorine uptake . 18 2.5. Azomethane Source . 18 2.6. Azomethane pyrolysis . 20 2.7. Mass Spec . 24 2.8. TPD Setup . 25 2.9. TPD Order . 29 3.1. Azomethane doser temperature comparison . 38 3.2. Comparison of doser temperatures . 39 3.3. Integrated methane desorption . 40 3.4. Methyl and deuterium co-adsorption . 41 3.5. Methane desorption after temp. ramp . 42 4.1. Methane and hydrogen desorption . 58 4.2. Simulated methane and hydrogen desorption . 60 4.3. Comparison of simulation and desorption . 61 4.4. Simulated desorption . 63 ix 4.5. Other desorption products . 64 5.1. Methylsilane pathway tree . 73 5.2. Mass spectra of methylsilanes . 75 5.3. Trimethylsilane exposure . 78 5.4. Trimethylsilane plus deuterium . 79 5.5. Dimethylsilane exposure . 81 5.6. Dimethylsilane plus deuterium . 83 5.7. Dimethylsilane with methyl . 84 5.8. Methylsilane exposure . 85 5.9. Methylsilane with deuterium . 86 5.10. Methylsilane with methyl . 87 5.11. Methylsilane ramp-cool-ramp . 89 5.12. Comparison of methylsilane and dimethylsilane . 94 5.13. Methylsilane reaction pathway . 99 6.1. Mass spectra of methylchlorosilanes . 106 6.2. Methylsilane plus Cl (non-cl) . 108 6.3. Methylsilane plus Cl . 109 6.4. Dimethylsilane plus Cl (non-cl) . 110 6.5. Dimethylsilane plus Cl . 111 6.6. Methylchlorosilane exposure . 112 6.7. Methylchlorosilane plus Cl . 114 6.8. Methyldichlorosilane exposure . 115 6.9. Dimethylchlorosilane exposure . 116 6.10. Trichlorosilane exposure . ..
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