In Situ and Operando Tools and Methods for Characterization of Heterogeneous Catalysts By Priya Darshini Srinivasan Submitted to the graduate degree program in Chemical and Petroleum Engineering and the Graduate Faculty of the University of Kansas School of Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Chairperson: Juan J. Bravo Suárez Bala Subramaniam Raghunath V. Chaudhari Kevin Leonard Michael Rubin Date Defended: May 3, 2019 The dissertation committee for Priya Darshini Srinivasan certifies that this is the approved version of the following thesis: In Situ and Operando Tools and Methods for Characterization of Heterogeneous Catalysts Chairperson: Juan J. Bravo Suárez Date Approved: ii Abstract In situ and operando spectroscopic characterization of catalysts is a powerful tool to explore the nature of species that may be involved in a heterogeneous catalytic cycle. In combination with methodologies to discriminate passive and active species, it can inform researchers on the presence of spectator and true reaction intermediate species. Towards this goal, this dissertation encompasses novel tools and methods to detect adsorbed species, differentiate spectator species from likely intermediate species, and their application via in situ and operando spectroscopic methods at relevant reaction conditions. First, we introduce a commercial in situ diffuse reflectance (DR) mirror optics cell that was modified for use at high temperatures with fiber optics and with reduced void volume. Such design enabled the development of a technique named oxygen gold plasmon sensing (O2-GPS). The O2-GPS combines in situ/operando DR UV-Vis spectroscopy and the determination of gold surface plasmon resonance (Au SPR) peak shifts as a sensor for adsorption of oxygen on gold catalysts via a simple correlation based on Drude’s free electron model for gold nanoparticles. Such method allowed, for the first time, the observation via in situ/operando UV-Vis spectroscopy of O2 adsorbed at the gold-support interface during O2 flow and under CO oxidation reaction conditions. This work also describes a unique and powerful mathematical framework for the discrimination of spectator species and likely intermediate species. The method is called modulation excitation-phase sensitive detection-diffuse reflectance Fourier infrared spectroscopy (ME-PSD-DRIFTS) as it is applied to in situ/operando DRIFTS data that has been collected under periodic changes of surface species coverages and processed via phase sensitive detection methods that employ Fourier analysis. Such approach allowed the collection of infrared spectra with enhanced signal-to-noise ratio of reacting species while avoiding the presence of spectator species. Here, it is also shown how ME-PSD-DRIFTS can be used to study surface reacting and likely intermediate species on a Co-Al2O3 catalyst with enhanced properties for ethanol dehydration. Along with ex situ characterization and kinetics measurements, ME-PSD-DRIFTS demonstrated the enhanced hydrophobic surface properties of this catalyst which reduced inhibition by water typical of the parent γ-Al2O3. The technique also allowed the sensitive detection of adsorbed ethanol and ethoxide species as well as terminal and bridging hydroxyls bonded to octahedral and tetrahedral Al on Al2O3 (100) and (110) facets as likely reaction intermediates in the conversion of ethanol to diethyl ether and ethylene. iii Acknowledgments I owe my deepest gratitude to all who have contributed towards the successful completion of my research work and had inspired and guided during my doctoral study. First of all, I would like to express my sincere gratitude for the guidance and support from my advisor, Dr. Juan J. Bravo Suárez. His encouragement, inspiring thoughts, and keen eyes on details in the results have been leading us to long lasting and impactful discoveries during my research at the Center for Environmentally Beneficial Catalysis (CEBC). Without his resourceful and continuous untiring supervision, this work would not have been possible. I would like to thank the professors in my Doctoral Committee, Dr. Bala Subramaniam, Dr. Ragunath V. Chaudhari, Dr. Kevin Leonard and Dr. Michael Rubin for their precious time and valuable suggestions at every step of my doctoral study. I would like to thank Dr. Prem Thapa for his help on catalyst characterization with TEM and SEM. I am grateful to our collaborators, Dr. Hongda Zhu from the Center for Environmentally Beneficial Catalysis, Dr. Konstantin Khivantsev and Dr, John Tengco from the University of South Carolina for their valuable help with catalyst preparation. I would also like to thank Ed Atchison for his help in programming and machining of the in situ reaction cells. I have spent almost five years in my lab and it has been a wonderful place where I worked with so many good people. I would like to express my special thanks to my lab mates Thomas Ofosu, Apexa Shah, Maria Ramirez and Bhagyesha Patil. I would like to thank my friends, Dr. Jianfeng Wu, Dr. Kakasaheb Nandiwale and Dr. Andrew Danby for helping me in all possible ways they can and their cheerful attitude. Nothing appropriate can describe the love and support of my parents and my sister, whose constant encouragement and admiration sets new horizons for me to reach, in every facet of my life. Needless to say, it was because of the efforts and constant source of strength of my family today I stand where I am. Their cooperation helped me in pursuing the PhD course and no words are enough to acknowledge them. Finally, I would like to acknowledge National Science foundation (NSF), without its financial assistance this work could not have been completed. iv Table of Contents Abstract…………………………………………………………………………………….. ……iii Acknowledgments…………………………………………………………………………..……iv General Introduction…………………………………………………………………………........1 References………………………………………………………………………………………....5 Chapter 1. Characteristics of In situ and Operando Spectroscopic Diffuse Reflectance Reaction Cells for Heterogeneous Catalysis……………………………………………….…...7 1.1. Introduction……………………………………………………………………………. .…….7 1.2. Ultraviolet-visible spectroscopy……………………………………………………………...8 1.3. Infrared spectroscopy………………………………………………………………………..13 1.4.Diffuse reflectance reaction cells………………………………………………………….…17 1.4.1. Experimental considerations for validity of F(R∞) function……………….…………....18 1.4.2. Integrating spheres…. ……………………………………………………………………20 1.4.3. Mirror optics…………………………………………………….…………….......…......21 1.4.4. Fiber optics……………………………………………………………………………….24 1.5. In situ/operando reaction cells: limitations and opportunities……………………………....27 1.6. Further opportunities………………………………………………………………………...27 1.7. Conclusions………………………………………………………………………………….28 1.8. References…………………………………………………………………………………...29 Chapter 2. Modified Harrick Reaction Cell for in Situ/Operando Fiber Optics Diffuse Reflectance UV-Visible Spectroscopic Characterization of Catalysts………………………35 2.1. Introduction………………………………………………………………………………….35 2.2. Experimental section………………………………………………………………………...37 v 2.2.1. Catalyst preparation…………………………………………………………………...…37 2.2.2. Catalyst characterization…………………………………………………………………38 2.2.3. Modified reaction cell design…………………………………………………………….38 2.2.4. DR UV-Vis spectroscopy technique………………………………………………...…...41 2.2.5. In situ DR UV-Vis reaction cell pulse experiments………………………………...……42 2.2.6. In situ Au surface plasmon resonance following H2/O2 cycle exposures to Au/ZrO2 catalysts…………………………………………………………………………………………..43 2.3. Results and discussion……………………………………………………………………....44 2.3.1. Reaction cell volume vs pulse experiments: residence time distribution (RTD) properties…………………………………………………………………………………………44 2.3.1.1. Reaction cell with large inner volume: RTD characteristics………………………....44 2.3.1.2. Reaction cell with smaller inner void volume: RTD characteristics………………....49 2.3.1.3. Reaction cell volume vs pulse experiments: ideal reactor models…………………...50 2.3.2. Sample vs external cell wall and probe temperatures ………………………………...…54 2.3.3. In situ reaction cell reactivity testing H2/O2 cycles on Au/ZrO2 catalysts…………..…..60 2.4. Conclusion………………………………………………………………………...………...62 2.5. References…………………………………………………………………………………...63 Chapter 3. In Situ Gold Plasmon Sensing of Adsorbed Oxygen (O2-GPS): Activity Trends in CO Oxidation on Supported Gold Catalysts……………………………………………….71 3.1. Introduction………………………………………………………………………………….71 3.2. Experimental section………………………………………………………………………...74 3.2.1. Catalysts preparation……………………………………………………………………..74 3.2.2. Catalysts characterization…………………………………………………………...…...74 vi 3.2.3. In situ gold surface plasmon resonance following O2/He/H2 cycles………………….....75 3.2.4. Operando CO oxidation………………………………………………………………….76 3.3. Results and discussion………………………………………………………………………76 3.3.1. Catalyst synthesis and characterization………………………………………………….76 3.3.2. In situ Au surface plasmon resonance during O2/He/H2 cycle exposures…………….…79 3.3.2.1. Dependence of gold surface plasmon resonance peak position (λm) on support index of refraction (n0) and Au charge transfer (N)……………………………………………………….84 3.3.2.2. Consequences of oxygen/hydrogen adsorption on gold surface plasmon resonance peak position……………………………………………………………………………………..87 3.3.3. Activity trends of CO oxidation on supported gold catalysts……………………………94 3.3.3.1. CO adsorption sites and support effects…………………………………………...…95 3.3.3.2. Oxygen adsorption sites…………………………………………...………………….98 3.4.