Date of Award

Spring 2020

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Jochen Lauterbach

Abstract

In this work, we set out to establish strong structure/activity relationships for various catalytic compositions and reactions. Through in situ spectroscopic approaches, specifically DRIFTS, Raman XPS, and XAFS, we were able to discern the reactive species in CO2 hydrogenation over highly active cobalt nanostructures, the relevant ensemble size and composition of single site catalysts for CO2 hydrogenation, and active vibrational modes of mixed oxide catalysts for ethane partial oxidation (EPO).

First, we illustrate how tailoring surface orientations of Co3O4 catalysts on the nanoscale results in control over catalytic performance via the preferential formation of active surface species during CO2 hydrogenation. This resulted in over an order of magnitude increase in the methane turnover frequency on Co3O4 nanorods with the exposed {110}/{001} family of surface facets, as opposed to conventional Co3O4 nanoparticles with the exposed {111}/{001} family of surface facets. We found via in situ DRIFTS studies that this difference in catalytic performance for the Co3O4 nanorods was due to the inhibition of the formate spectator species. Furthermore, by studying the second hydrogenation step in CO2 hydrogenation, which is CO hydrogenation, we were able to discern that the formation of bridged CO was the key difference between the two catalyst. Second, cobalt and ruthenium single site catalyst were explored due to their highly uniform active sites; allowing for definitive claims as to which surface species are responsible for the reaction mechanisms. To characterize the structure and dispersion of the single-site catalysts, techniques such as UV-vis, XAFS, XPS, TPR, and Raman were vi utilized under ambient conditions as well as under reductive environments to simulate reaction conditions. For the case of cobalt single sites, the surface moieties under ambient and reductive environments coupled with their corresponding catalytic performance during CO2 hydrogenation allowed us to discern how the transition between isolated atoms to small nanoparticles affects the reaction mechanism. For ruthenium single site catalysts supported on boronnitride, we found atomic and/or subnanometer clusters to be over an order of magnitude more active than their analogous nanoparticles

First, we illustrate how tailoring surface orientations of Co3O4 catalysts on the nanoscale results in control over catalytic performance via the preferential formation of active surface species during CO2 hydrogenation. This resulted in over an order of magnitude increase in the methane turnover frequency on Co3O4 nanorods with the exposed {110}/{001} family of surface facets, as opposed to conventional Co3O4 nanoparticles with the exposed {111}/{001} family of surface facets. We found via in situ DRIFTS studies that this difference in catalytic performance for the Co3O4 nanorods was due to the inhibition of the formate spectator species. Furthermore, by studying the second hydrogenation step in CO2 hydrogenation, which is CO hydrogenation, we were able to discern that the formation of bridged CO was the key difference between the two catalyst. Second, cobalt and ruthenium single site catalyst were explored due to their highly uniform active sites; allowing for definitive claims as to which surface species are responsible for the reaction mechanisms. To characterize the structure and dispersion of the single-site catalysts, techniques such as UV-vis, XAFS, XPS, TPR, and Raman were vi utilized under ambient conditions as well as under reductive environments to simulate reaction conditions. For the case of cobalt single sites, the surface moieties under ambient and reductive environments coupled with their corresponding catalytic performance during CO2 hydrogenation allowed us to discern how the transition between isolated atoms to small nanoparticles affects the reaction mechanism. For ruthenium single site catalysts supported on boronnitride, we found atomic and/or subnanometer clusters to be over an order of magnitude more active than their analogous nanoparticle reaction.

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