Early transition metal (Group IV-VI) carbides and nitrides are a fascinating class of materials with a broad range of potential applications including use as catalysts for energy and environmentally sustainable processes. These relatively low cost materials can be produced in nanoscale form with surface areas as high as 200 m2/gr, have been demonstrated to be active for a number of industrially significant reactions including water gas shift and alkane isomerization, and are thermally and chemically stable. The design of nanoscale early transition metal carbide- and nitride-based catalysts would benefit significantly from a better understanding of relationships between their structural, compositional and functional properties.
The goal of this research is to elucidate key structural and compositional features that govern the catalytic properties of early transition metal carbides and nitrides using two commercially relevant test reactions, the water gas shift (WGS) and Fischer-Tropsch Synthesis (FTS). The principal techniques that will be employed to accomplish the project goal are ultrahigh resolution transmission electron microscopy (TEM) and in situ x-ray absorption spectroscopy (XAS). The TEM and XAS results will be complemented by results from infrared spectroscopy and x-ray photoelectron spectroscopy.
This effort is responsive to the Catalysis and Biocatalysis Program goal of supporting multidisciplinary research on the synthesis and characterization of catalysts that function at the nanoscale. Potential applications for nanoscale carbide- and nitride-based catalysts include the production of hydrogen and "green gasoline" via FTS, and consequently the project supports efforts related to energy diversity and reductions in greenhouse gas emissions.
The intellectual merit of the proposed research lies in a significant expansion of the knowledge base for early transition metal carbide and nitride catalysts. While the database of reactions catalyzed by these materials has grown, there is little fundamental information about relationships between their structure, composition and function. The ultrahigh resolution TEM will allow, perhaps for the first time, high resolution imaging of the non-metal atoms in the metal atom matrix of the catalyst. In addition, XAS, one of only a few in situ methods, will provide the type of compositional, structural and electronic information needed to develop unambiguous correlations. Relationships derived during the proposed research will significantly enhance our fundamental understanding of the character of carbides and nitrides, and could facilitate their design and development as catalysts and catalyst supports for the sustainable production of a variety of chemicals and fuels.
With regard to the broader impacts, the proposed research will engage under-represented minority high school, undergraduate and graduate students in socially relevant research. The proposed project will leverage on-going education and outreach activities including the Michigan-Louis Stokes Alliance for Minority Participation and Undergraduate Research Opportunity Program. Through an expansion of the University of Michigan Chemical Sciences at the Interface of Education (CSIE) program, undergraduate and post-doctoral chemical engineering and materials science students will be able to add education to their professional studies. In addition, through a collaboration with the IDEA Institute, teachers from the Detroit and Ypsilanti School Districts, both of which have large populations of underserved students, will be engaged in summer camp programs focused on microscopy and surface science. Researchers engaged in the proposed project have long-standing commitments to increasing the participation of underrepresented groups in authentic research and integrating research into educational activities.
Early transition metal (Group IV–VI) carbides and nitrides are a fascinating class of materials with potential for use as catalysts in energy and environmental sustainable applications. Recently we discovered that the nitrides and carbides are active for reactions of importance in hydrogen production including the water gas shift and hydrocarbon steam reforming. A better understanding of relationships between their structural, compositional and catalytic properties would facilitate the design of new carbide and nitride based catalysts. The goal of research carried out in this project was to define key structural and compositional features that govern the catalytic properties of early transition metal carbide and nitride based catalysts using two commercially relevant test reactions, the water gas shift (WGS) and Fischer-Tropsch Synthesis (FTS). Our approach for the project involved detailed characterization of the compositional and structural properties of the materials using techniques including in situ x-ray absorption spectroscopy and ultrahigh resolution transmission electron microscopy (TEM). X-ray absorption spectroscopy (XAS) is one of only a few truly in situ methods that can provide the type of compositional, structural and electronic information we seek. The TEM and XAS results were complemented by results from x-ray photoelectron spectroscopy and thermal desorption spectroscopy. The principal technical objectives for this project were to: prepare bulk, high surface area early transition metal carbide and nitride, prepare carbide supported metal (e.g. Pt, Ni and Cu) catalysts, evaluate catalytic properties of selected materials for the WGS and FTS reactions, characterize key structural and compositional properties of the materials using ex situ and in situ techniques, correlate the characterization and reaction results to determine key properties of the active sites for WGS and FTS over the carbide and nitride based catalysts. We successfully met the objectives of the project. During the project we demonstrated methods to prepare carbide and nitride based catalysts with high surface areas and high WGS and FTS activities. In fact, some of these materials were as or more active than Cu-Zn-Al oxide and supported Fe catalysts that are used commercially for the WGS and FTS reactions, respectively. With regard to the WGS, our work focused on Mo2C supported metals including Pt, Ni, Cu and Fe. Our results are consistent with the active sites for these catalysts being at the interface of the metal particle and carbide support. For example for Pt/Mo2C, we believe that the CO adsorbs to Pt sites then reacts, at the interface between Pt and the Mo2C, with oxygen released during the reduction of H2O on Mo2C sites (this appeared to be the rate determining step). This information will be useful in designing better performing materials. In particular one would seek architectures that maximize the density of sites at the metal-carbide/nitride interface. With regard to the FTS reaction, we evaluated the performance of bulk carbides and nitrides of Mo, W, V, and Nb. The Mo2C, W2C, VN, and NbN catalysts exhibited similar rates while rates for the Mo2N and W2N catalysts were more than an order of magnitude lower. Primary products for the carbide and nitride catalysts were hydrocarbons and CO2 with CH4 and light hydrocarbons (C2-C4) being the prominent. Commercial FTS catalysts typically produce higher hydrocarbons suggesting that product distributions for the carbide/nitride catalysts will have to be improved despite the high activities. By varying the H2 and CO feed concentrations, the reaction orders with respect to H2 and CO were for the Mo2C catalyst. The reaction order with respect to H2 was ~1. The reaction order with respect to CO was ~ −0.5. This information will help in determine strategies to improve the carbide and nitride catalysts. This effort is responsive to the Catalysis and Biocatalysis Program goal of supporting multidisciplinary research on the synthesis and characterization of catalysts that function at the nanoscale. The research also enabled the engagement of under-represented minority undergraduate students in socially relevant research through the Research Experience for Undergraduates program. Their training to think critically, and judiciously use state-of-the-art surface and bulk materials characterization tools directly supports the National Science Foundation’s strategic goals. The project also leveraged on-going education and outreach activities including the Michigan-Louis Stokes Alliance for Minority Participation (MI-LSAMP). The MI-LSAMP partners the University of Michigan, Western Michigan University, Michigan State University and Wayne State University in an effort to double the number of underrepresented minority students earning science, technology, engineering and mathematics (STEM) baccalaureate degrees from our institutions, and increase the number of URM students interested in, academically qualified for and matriculation into graduate study programs. Two under-represented minority high school students were mentored through collaboration with the MI-LSAMP. Finally, the PI was able to include results from the project in course related materials including home problems and in-class examples.
