The goal of this research is to explore novel catalytic material and systems for effective conversion of C1 feed. Catalysis of C1 chemistry is of critical importance for the clean production of fuels and chemicals and future energy sustainability. Herein, two processes were studied: In the first section, a comprehensive study of oxidative coupling of methane (OCM) using novel nanofiber catalysts of mixed metal oxides was undertaken and in the second section, direct catalytic conversion of carbon dioxide (CO2) to methanol was studied, which resulted in discovery of a superior catalytic system for CO2 hydrogenation to methanol.

Section 1: Utilization of natural gas as an alternate chemical feedstock to petroleum has been a highly desirable but difficult goal in industrial catalysis. Accordingly, there has been a substantial interest in the oxidative coupling of methane (OCM), which allows for the direct catalytic conversion of methane into economically valuable C2+ hydrocarbons. OCM is a complex reaction process involving heterogeneous catalysis intricately coupled with gas phase reactions; hence, despite decades’ worth of research, it has yet to be commercialized. The lack of progress in OCM is primarily due to the following reasons: 1. The absence of a highly active and robust catalyst that can operate at lower temperatures; and 2. Our inadequate understanding of the underlying detailed chemical kinetics mechanism (DCKM) of the OCM process, which impedes the undertaking of quantitative simulations of novel reactor configurations and/or operating strategies. To address these issues, we undertook the following program of studies: 1. Further improved the synthesis of novel nanofiber catalysts by electrospinning, building on the early discovery that La2O3-CeO2 nanofibers were highly active and robust OCM catalysts; 2. Applied our novel microprobe sampling system to OCM reactors for the acquisition of spatially resolved species concentration and temperatures profiles within the catalytic zone. Our novel sampling approach led to the important discovery that H2 is produced very early in the OCM catalytic zone, an observation that was completely missed in all prior studies. The application of our novel microprobe system to a dual-bed OCM reactor also demonstrated the feasibility to significantly improve C2+ product yields to 21% (from 16% for single bed) which we plan to further improve by considering more sequential beds; 3. Outlined development and validation of new generation of DCKM for the OCM process using the high-information content of spatial concentration profiles obtained in part 2. Most importantly, to improve the current DCKM literature by considering surface reactions that result in early H2 formation. Validated DCKM represent highly valuable numerical tools that allow for the prediction of the OCM performance of different reactor configurations operating under a broad range of conditions, e.g. high pressures, porous wall reactors etc. Consequently, this new generation of comprehensive DCKM based on the sampling profiles, detailed in this report, will be of considerable use in improving the yields of useful products in the OCM process; 4. Explore novel conditions that include oxygen-feed distributed packed bed OCM reactors and coupled catalytic and non-thermal plasma OCM reactors, again to further push the yields for useful C2+ products. The details of the proposed approach for implementing such reactor configurations and development of a new generation of DCKM for the OCM process is outlined in the future work, Chapter 4, of section 1 of the report.

Section 2: Direct catalytic conversion of carbon dioxide to liquid fuels and basic chemicals, such as methanol, using solar-derived hydrogen at or near ambient pressure is a highly desirable goal in heterogeneous catalysis. When realized, this technology will pave the way for a sustainable society together with decentralized power generation. Here we report a novel class of holmium (Ho) containing multi-metal oxide Cu catalysts discovered through the application of high-throughput methods. In particular, ternary systems of Cu-GaOx-HoOy > Cu-CeOx-HoOy ~ Cu-LaOx-HoOy supported on γ-Al2O3 exhibited superior methanol production (10x) with less CO formation than previously reported catalysts at atmospheric pressure. Holmium was shown to be highly dispersed as few-atom clusters, suggesting that the formation of tri-metallic sites could be the key for the promotion of methanol synthesis by Ho.

The emission of greenhouse gases into the atmosphere has been linked to global warming. Carbon dioxide's (CO2) one of the most abundant greenhouse gases. Natural gas, mainly methane, is the cleanest fossil fuel for electricity production helping meet the United States ever growing energy needs. The methanation of CO2 has the potential to address both of these problems if a catalyst can be developed that meets the activity, economic and environmental requirements to industrialize the process. Producing methane using carbon dioxide as a reactant would have the dual effect of keeping CO2 from entering earth's atmosphere by consuming it to produce natural gas which in turn can produce electricity to meet growing power needs. This thesis aims to introduce the current methanation of carbon dioxide research, provide guidance into what needs to be considered before entering this field and how one might go about it.

The synthesis of solids with finely turned nanostructures that offer superior catalytic performance is a major challenge in heterogeneous catalysis for gas phase reactions. Industrial catalysts are almost universally composed of quasi-spherical nanoparticles, or powders plagued with particle agglomeration, migration and sintering problems that lead to deactivation. In this work, quasi-cylindrical nanofibers are electrospun and extensively utilized for the oxidative coupling of methane (OCM), as well as for propylene epoxidation and the catalytic partial oxidation (CPO) of methane.

Electrospun nanofibers of metal oxides may be tuned to have high surface areas but typically possess no internal porosity, reducing diffusion limitations that would lengthen the exposure of target intermediate oxidation products to unselective catalysis. Additionally, experiments and density functional theory (DFT) studies have previously shown that pentagonal Ag nanowires exhibit higher selectivity than conventional particles in ethylene epoxidation since their surfaces are terminated mainly by the (100) surface facet rather than the lowest energy (111) facet that dominates particles. Hence, nanofibers may elevate catalytic performance in broad range of partial oxidation reaction schemes.

Research into the oxidative coupling of methane, or, the catalytic conversion of methane to ethane and ethylene by molecular oxygen, almost exclusively utilized powders and failed to result in viable catalyst despite four decades of intense, global efforts. Accordingly, the use of catalytic nanofibers provides a potentially fruitful path towards a solution. Here, nanofiber fabrics of La2O3-CeO2 were electrospun and used in fixed bed OCM reactors to achieve 70% selectivity and 16% yield for C2+ hydrocarbons at a CH4/O2 feed ratio of 7 and remarkably low feed temperature of 470 ?C. Powders of La2O3-CeO2 documented in the literature exhibit similar selectivity and yield, but with the feed at 715 ?C. The electrospun fabrics used in this research were found to have dense nanofibers of diameters typically within the 20 – 200 nm range and, accordingly, surface areas of 10 – 20 m2/g as well as thinner fibers tending towards both higher C2+ selectivity and CH4 conversion.

While performing reaction engineering studies using the aforementioned fabrics, it was found that designing reactors comprising dual catalytic La2O3-CeO2 fabric beds with inter-stage O2 injection and cooling pushes yields to 21%. Moreover, a novel in-situ microprobe sampling technique for acquiring spatial temperature and concentration profiles within these OCM reactors was developed, providing a means to formulate and validate detailed chemical kinetic mechanisms. This has led to the discovery of prompt H2 formation in OCM, a feature previously unidentified that may break ground in mechanism refinement. Additionally, spatial concentration and temperature profiles were acquired in fixed bed reactors comprising La2O3-CeO2 fabrics doped with varying levels of Ir and fed CH4/O2 mixtures to gain insight into the transition from OCM to the catalytic partial oxidation of methane. It was found that, in general, OCM and CPO appear to occur both in parallel and sequentially in a fixed bed, evidenced by the temporary rise and subsequent destruction of C2+ hydrocarbons when the catalyst is doped with 0.05 wt% Ir. Clearly, this sampling technique has broad applicability in catalysis research over a limitless number of reactions for the acquisition of comprehensive data sets potentially useful for formulating and refining detailed chemical kinetic mechanisms (DCKM), thus furthering a fundamental understanding of the catalysis and advancing faster towards the development of higher performing materials.