Published as part of The Journal of Physical Chemistry virtual special issue “Michael R. Hoffmann Festschrift”.
Introduction
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Summarizing one’s achievements, especially for a distinguished scholar like Michael (Mike) R. Hoffmann, poses an absolute challenge. It is particularly poignant that due to his current health situation, he is unable to personally write a biography. After extensive discussions among the coauthors (D.B., W.C., and K.C.), we have decided to make our best efforts in collecting, editing, and reorganizing Mike’s previously written articles to create this “quasi-autobiography.” The biography encompasses various elements, including (i) a message from Mike (reproduced from the preface of abstract book for “3rd International Workshop on Frontiers in Environmental Chemical Research in honor of Prof. Michael Hoffmann’s 70th birthday” held in Pohang University of Science and Technology (POSTECH)), (ii) an academic career overview, (iii) his research trajectory, and (iv) his contributions to environmental sustainability. These sections aim to provide a partial portrayal of Mike’s academic journey, primarily as a professor first at University of Minnesota (1975–1980) and subsequently (and mostly) at the California Institute of Technology (Caltech, 1980–present). We sincerely invite the readers, including his academic descendants as well as the past, present, and hopefully “future” colleagues, to gain a deeper understanding of how his accomplishments have significantly influenced the fields of environmental and energy science and engineering in “practical” ways.
Message from Mike
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(Reproduced from “3rd International Workshop on Frontiers in Environmental Chemical Research in honor of Prof. Michael Hoffmann’s 70th birthday”, 2016, Pohang University of Science and Technology (POSTECH), Pohang, Korea)
It has been hard for me to fully understand how fast the years of the calendar have been peeled away since November 1946. During my early years, I never imagined that I would pursue an academic career at the interface of science and engineering. In retrospect, the path toward a professional life in the “ivory tower” of an American University began most likely with the brain-stretching toys such as the Gilbert Chemistry and Microscope Sets, Erector Sets, and Tinker Toys that my father brought home early during my elementary school days. Those intellectual playthings coupled with professional chemistry and chemical engineering magazines at home led to building a home chemistry lab in my parent’s basement. I am truly lucky that I survived that period of time without destroying our house or losing a few body parts. Somehow, I continued on that early path through high school, college, graduate school, and beyond in spite of spending much of my time involved in athletics during high school and college. My interests in chemistry with a special focus on environmental chemistry grew during graduate school and certainly intensified during my postdoctoral years at Caltech and my first professorship at the University of Minnesota. It has been [now 43] years since I returned to Caltech. Sometimes I feel as if I am the luckiest person in the world to have been able to be a professor at Caltech that, in spite of its small size, is well-known and influential globally.
I am truly grateful to the many students, postdocs, Caltech faculty colleagues, and professional colleagues around the world, who have made significant intellectual contributions to my professional career. Without the many invaluable contributions to my knowledge base, I would not be able to survive in this increasingly more competitive global academic world. I am truly honored by [this Festschrift] and I appreciate your support, advice, and friendship over the years. I feel lucky to be able to “stand on the shoulders of so many giants”.
Academic Career Overview
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Prof. Hoffmann received a BA from Northwestern University (1968) and a Ph.D. degree from Brown University (1974). He was appointed as a postdoctoral fellow at Caltech from 1973 to 1976. From 1975 to 1980, he served as a professor of Civil/Chemical Engineering at the University of Minnesota. He joined the Environmental Science & Engineering (ESE) faculty at Caltech in 1980. His academic positions at Caltech include Professor of Environmental Chemistry (1986–1995), James Irvine Professor of Environmental Science (1996–2015), Dean of Graduate Studies (2002–2009), and Theodore Y. Wu Professor (2016–present).
Prof. Hoffmann has more than 78,200 citations on Google Scholar with H-Index of 124 (as of June 2023). Prof. Hoffmann has also been recognized by Web of Science as one of the most highly cited researchers in engineering in the world. He was awarded the Alexander von Humboldt Prize in 1991, the American Chemical Society Award for Creative Advances in 2001, and the Jack E McKee Medal by the Water Environment Federation in 2003. He was honored as a Distinguished Chair Professor of Atmospheric Sciences, Chemistry, and Environmental Engineering at the National Taiwan University, Taipei in 2010. Prof. Hoffmann is a Member of the National Academy of Engineering and the De Tao International Master’s Academy based in Beijing and Shanghai. In 2012, he received a prize from the Bill and Melinda Gates Foundation for his group’s work on solar-powered electrochemical treatment of human waste as applied to sanitation in the developing world. In 2012, he was recognized as a Distinguished Visiting Fellow of the British Royal Academy of Engineering and as a Global Vision Scholar by Tsinghua University. Prof. Hoffmann has been named as an Honorary Professor at the Beijing University of Chemical Technology, the Beijing University of Science and Technology, and the Beijing University of Aeronautics and Astronautics (i.e., Beihang University). The full list of awards and society memberships can be found in his CV (Supporting Information).
Research Trajectory
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During his early professional years, Mike’s primary focus was exploring the potential of hydrogen peroxide and related peroxides (peroxymonosulfate, peroxydisulfate, and peroxyacetic acid) for pollution control applications. His pioneering work laid the foundation for the extensive use of hydrogen peroxide in various pollution control applications, including hazardous waste treatment using Fenton’s reagent and peroxone (a combination of hydrogen peroxide and ozone), as well as the remediation of methyl tert-butyl ether (MTBE) contaminated groundwater. Hydrogen peroxide is now widely used to mitigate hydrogen sulfide generation and emissions in sanitary sewer systems, wastewater pumping basins, and wastewater treatment plants. In recognition of his collaborative work on groundwater remediation at JFK International Airport in New York City, in partnership with Brown and Caldwell and American Airlines, Mike was honored with the Jack G. McKee Medal by the Water Environment Federation in 2003.
Building upon his early interest in sulfur pollution control, Mike initiated a new area of research in 1980 focused on semiconductor photocatalysis for the control of reduced sulfur compounds. The research conducted in his laboratory centered around synthesizing and characterizing metal oxide semiconductors that could be activated using solar light. This involved exploring the use of readily available metal oxides such as Fe2O3, TiO2, and ZnO. One of the major challenges during this period was developing stable colloid suspensions of nanometer-sized semiconductors for practical applications. The Hoffmann lab developed several innovative synthetic methods for producing semiconductor quantum dots in the particle size range of 1 to 4 nm. Notably, they were the first to develop sol–gel synthesis techniques for TiO2 and ZnO quantum dots (also known as quantum-sized semiconductors) without the need for organic surfactants as stabilizers. Leveraging their in-depth knowledge of the surface chemistry of metal oxide and metal sulfide materials, they successfully synthesized stable suspensions of quantum dots in water and other polar solvents.
In 1995, Mike and his colleagues (including W.C. and D.B.) published a review article introducing the environmental applications of semiconductor photocatalysis. This seminal publication has since been cited over 23,600 times (as of June 2023). Furthermore, Mike’s earlier publication with W.C. on the metal-ion doping of quantum-sized TiO2 has garnered more than 5,000 citations in the primary literature (as of June 2023). These citation metrics solidify Mike’s status as an influential figure in the field of semiconductor photocatalysis, which still remains a prominent research area for his group. For example, their recent research focuses on utilizing earth-abundant semiconducting materials for solar fuels production through artificial photosynthesis and water treatment applications.
Expanding their work on semiconductor metal oxides, the Hoffmann group (including K.C.) has applied them to photovoltaic-powered electrochemical wastewater treatment coupled with solar energy conversion, primarily with major funding from the Gates Foundation since 2011. They have extensively investigated the engineering aspects of “wastewater electrolysis cells” for on-site treatment of human waste alongside decentralized molecular hydrogen (H2) production. Key electrocatalysts include bismuth-doped TiO2 functionalized mixed metal oxide anodes, which exhibit reliable electrocatalytic activity for oxidizing Cl– to reactive chlorine species. This oxidation process helps degrade environmental pollutants such as chemical oxygen demand, proteins, NH4+, urea, and total coliforms. On the cathodes, the reduction of water, protons, and CO2 can produce molecular hydrogen, among other energy carriers, with moderate levels of current and energy efficiency. The significant implications of these works are shown with more details in the following section.
Contributions to Environmental Sustainability
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Mike and his research group have collaborated with several prominent companies to translate their laboratory-scale research into commercially viable processes. Prof. Hoffmann has cultivated strong industry connections worldwide. Notable companies that have provided support for Mike’s research include DuPont, Chevron, the Eaton Corporation, Northrop-Grumman and Bell Helmets, 3M, Falcon Water-Free Technologies, and Kohler, among others. The various research projects have aimed to achieve practical engineering objectives, such as utilizing quantum dots in photoresist polymerization, mitigating wax deposition in oil production wells, controlling antioxidant wastes in metals manufacturing, developing self-cleaning surfaces for military aircraft and vehicles, creating advanced self-cleaning gas masks and protective coatings for soldiers and first responders, and designing water-free urinals and self-contained electrochemical toilet wastewater treatment systems.
Atmospheric Chemistry, Air Pollution, Clouds, Fogs, and Haze Aerosols
The Hoffmann group has made significant contributions to our understanding of the role of water in the atmosphere, particularly in controlling the chemical transformation pathways of acidic gas precursors such as sulfur dioxide and nitrogen oxides, and the subsequent acidification of the atmosphere. They have also elucidated the role of ammonia as an atmospheric base in neutralizing the formation of inorganic acids (such as sulfuric and nitric acids), leading to the formation of hygroscopic aerosols like ammonium sulfate and ammonium nitrate, which contribute to haze aerosol formation in polluted urban environments like New Delhi, Beijing, and Mexico City. Mike’s discovery of hyper acidic clouds and fogs in areas such as Los Angeles, San Francisco, and the San Joaquin Valley resulted in major California state laws. These laws mandated the switch from high-sulfur fuels to natural gas (CH4) as the only acceptable fossil fuel source for power generation statewide. Furthermore, these laws and subsequent regulations in California and the United States imposed stricter controls on the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx) to mitigate atmospheric acidification on urban, rural, and continental scales. Additionally, Mike’s group has made pioneering contributions in understanding the kinetics and detailed mechanisms of SO2 oxidation by ozone, hydrogen peroxide, and oxygen in water, with a particular focus on the pH of clouds and atmospheric water droplets. Their work has revealed that a significant portion of total sulfur dioxide oxidation to sulfuric acid occurs in clouds, fogs, and haze aerosols, challenging the previous belief that sulfur dioxide was primarily controlled by gas-phase reactions with hydroxyl radicals. Overall, the Hoffmann group’s research has greatly advanced our knowledge of atmospheric chemistry, shedding light on the role of water in chemical transformations and the resulting impacts on acidification and aerosol formation, leading to significant regulatory measures and a more comprehensive understanding of global sulfur dioxide oxidation processes.
Advanced Oxidation Technologies
In recognition of his significant contributions to the exploration and implementation of advanced oxidation and reduction technologies for the remediation and control of recalcitrant environmental pollutants, Mike was elected to the US National Academy of Engineering in 2011. His fundamental laboratory studies paved the way for advocating the use of various oxidants such as hydrogen peroxide, ozone, peroxydisulfate, peroxydiphosphate, peroxone, peroxymonosulfate, and semiconductor photocatalysis as major options for pollution control. Prior to 1980, potassium permanganate and chlorine in the form of hypochlorous acid/hypochlorite were the primary oxidants used for water pollution control in the United States. Today, hydrogen peroxide, ozone, peroxone, and persulfate are routinely employed for groundwater remediation, tertiary water treatment, and direct potable water reuse in locations such as Orange County/Los Angeles, CA, and Singapore. Many of the highly cited publications from the Hoffmann group focus on the use of nanoparticulate semiconductors for photocatalytic oxidation and reduction of water and atmospheric contaminants. In the early 1980s, Professor Hoffmann was among a small group of researchers advocating for the use of titanium dioxide photocatalysis. Since then, there has been a surge of university and industrial research exploring the application of inexpensive metal oxide semiconductors for pollution control. Mike’s election to the US National Academy of Engineering reflects the impact and recognition of his groundbreaking work in the field, which has revolutionized the use of advanced oxidation and reduction technologies in environmental remediation and pollution control.
Integrated Engineering Systems for Decentralized Wastewater Treatment
Mike has made significant advancements in developing small-scale human wastewater treatment systems that combine biochemical and electrochemical treatment methods. These systems are designed to provide decentralized pollution control and can be utilized in urban, rural, peri-urban, and remote areas worldwide. Since 2011, the Gates Foundation has provided substantial funding to the Hoffmann group to further enhance photovoltaic-powered treatment systems for use in developing countries. The aim is to address the issue of open defecation, which contributes to water-borne diseases and contamination of surface and groundwater, resulting in numerous premature deaths, particularly in Africa and India. The progress made by the Hoffmann group in employing integrated engineering approaches for small-scale human wastewater treatment led to their recognition and receipt of the Gates Foundation Prize for Reinventing the Toilet in 2012. This achievement propelled the formation of a joint-venture company, Eco-San, in Yixing, China, in collaboration with colleagues from Tsinghua University. Eco-San now manufactures integrated toilet and waste treatment systems that are used in India, tourist sites in southern China (Jiangsu, Yunnan, Sichuan, and Hainan provinces) under the Chinese National Tourism Authority (CNTA), and remote schools in the East London district of South Africa. The joint-venture company also produces semiconductor electrodes developed in the Hoffmann Lab, which are now manufactured in Yixing and distributed worldwide. Eco-San’s integrated biological and electrochemical treatment strategy allows for on-site treatment of human waste, achieving near-total elimination of chemical oxidation demand, complete denitrification, and disinfection of microorganisms. The treated water can be recycled internally as flushing water for toilets. Projections indicate that production levels in the near future could reach 2,000 to 5,000 units per year. Furthermore, Eco-San manufactures electrochemical reactor systems for various environmental and industrial applications beyond toilet wastewater treatment. In May 2016, Eco-San was awarded the first prize by the CNTA. These contributions have led to the other commercialization of projects such as Seva (Self-contained Toilet Wastewater Treatment) and ULTRON (Ultraportable Three-stage Oxidative Nanofiltration).
International Collaborations
In recognition of his contributions to decentralized wastewater treatment and other areas, Mike served as a Global Vision Scholar associated with the School of the Environment at Tsinghua University from March 2013 to June 2017. During this time, he engaged in research and teaching activities, delivering lectures on environmental chemistry and technology to students. He also traveled to various universities in Asia to give research seminars, sharing his expertise with a wider audience. In addition to his role as a Global Vision Scholar, Mike has been lauded as an honorary professor at the Beijing University of Chemical Technology. He also serves as a member of the external advisory board for the Department of Environmental Engineering at the South University of Science and Technology of China in Shenzhen.
Throughout his career, Mike has hosted numerous academic scholars from around the world, particularly from Korea and China, who came to the Hoffmann group at Caltech for sabbatical leaves to collaborate on research topics. He has also hosted many Ph.D. students who received grants for studying abroad as part of their doctoral research. Furthermore, Mike has welcomed postdoctoral scholars to his laboratories at Caltech (including the Keck and Linde+Robinson laboratories). These academic exchanges and collaborations have resulted in joint journal papers, showcasing the fruitful outcomes of these research collaborations. Mike actively engages with the visiting students and postdoctoral scholars, meeting with them regularly to discuss their research projects, provide updates, and offer feedback to facilitate their progress. He emphasizes the importance of an open exchange of ideas and the benefits of dynamic collaboration within a laboratory setting and during weekly group meetings. By fostering an environment of intellectual exchange, Mike helps researchers make rapid progress while ensuring they address fundamental aspects of their work.
Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.
Detlef Bahnemann - Leibniz
University Hannover, Callinstraße 5, 30167 Hannover, Germany;
Saint-Petersburg
State University, Ulyanovskaya
str. 1, Saint-Petersburg, 198504 Peterhof Russia;
https://orcid.org/0000-0001-6064-6653;
Email: [email protected]
Notes
Views expressed in this preface are those of the authors and not necessarily the views of the ACS.
Acknowledgments
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The authors acknowledge Nora Oshima (Administrative officer in Caltech division of geological and planetary sciences) and Jiseon Kim (visiting student researcher in the Hoffmann lab) for their assistance with the Supporting Information.
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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system:
http://pubs.acs.org/page/copyright/permissions.html.
2024-11-02T23:23:27.814-07:00
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