Understanding and improving catalytic methane combustion using uniform palladium-based nanostructured catalysts [electronic resource]
Keywords: Chemical Engineering.

Increased availability of cheap natural gas from advances in the extraction of natural gas from shale has led to renewed interest in the use of methane, its main constituent, in more efficient engines, lower-temperature fuel cells, and the direct production of olefins and aromatics. In all these applications, methane combustion at low temperatures (< 300˚C) is crucial for these technologies to perform while limiting harmful emissions of methane, the second most prevalent greenhouse gas. Despite advances in recent years, supported heterogeneous catalysts still require rates to be improved, especially in the presence of steam. Also, the alternative homogeneous combustion systems are far from practical due to the production of toxic gases, such as CO and NOx. Palladium is widely recognized as one of the promising materials for this reaction and has received continuous attention for the past few decades. Unfortunately, there are still questions on the active site of the reaction, the role of the support, the effect of steam on structural properties, and the effect of promoters on activity. To gain a fundamental understanding of this system and provide insights to further improve the activity and stability of Pd-based catalysts, systematic studies using size-, shape-, and composition-controlled Pd-based catalysts are essential. The first portion of this dissertation focuses on the development of libraries of uniform Pd-based heterogeneous catalysts. This will begin by presenting an improved synthesis for nanometer-sized controlled Pd nanocrystals (NCs), in which design rules are demonstrated. Building on this knowledge of monometallic Pd NCs, we present a novel and general synthesis for a wide library of multimetallic PdM (M = V, Mn, Fe, Co, Ni, Zn, and Sn) NCs. With this synthesis, control over size and composition, as well as, a proposed mechanism is demonstrated. Overall, we show that, with an understanding of the synthesis mechanism and design rules, nanocrystals can be synthesized with control over size, shape, and composition. We then use our uniform Pd NCs as catalyst precursors to prepare a library of well-defined materials to systematically describe structure-property relationships for methane complete combustion, with respect to effects of particle size, support, and steam. With the help from theory, in-situ X-ray absorption spectroscopy, physical models, and detailed kinetic measurements, we confirm that PdO is the most active phase in steam free-conditions, provide an analysis of causes for observed mild structure sensitivities, demonstrate the support exerts limited influence on the PdO activity, and show that the introduction of steam causes severe deactivation due to coverage effects. This study clarifies contrasting reports in literature about active phase and stability of Pd-based materials. Finally, we present a systematic study of promoters for Pd-catalyzed methane combustion using supported Pd-based multimetallic catalysts. This study demonstrates the requirement for control over the co-localization of both promoter and active phases to truly identify promoters and understand their effects. We then show, with control over both active and promoter phases, that some metals (Fe, Co, and Sn) inhibit sintering of the active Pd phase, and others (Ni and Zn) increase its intrinsic activity compared to monometallic Pd catalyst. Broadly, this study provides a generalized procedure for investigating promotional effects of secondary metal and metal oxide phases for a variety of active metal-promoter combinations and catalytic reactions. In summary, this thesis provides design rules for more active and stable Pd-based methane combustion catalysts. We demonstrate that the ideal Pd-based methane combustion catalysts is composed of highly dispersed PdO nanocrystals supported on a thermal stable support with the addition of secondary metal/metal oxides, such as Fe, Co, Ni, Zn, or Sn, to inhibit sintering and stabilize the PdO phase. Studies of water inhibition demonstrate that further development of Pd-based materials which alter the binding energies of inhibitory intermediates is required. Finally, these studies lay the foundation for future systematic studies of Pd-catalyzed reactions.