Rh single atom catalysis on the magnetite (001) surface

Marcus Andrew Sharp

doi:10.7273/000007090

Surface science has long played an important role in the characterization of metal oxide surfaces and in heterogeneous catalysis. With the emergence of single-atom heterogeneous catalysts, atomic-level characterization is essential to understand the nature of and mechanisms behind catalytic reactions occurring on these sites. Further, the stability and characteristics of single-atom sites are dependent upon the supporting material. Thus, the characterization of the adsorbate-support interaction is also critical in understanding single-atom catalysts. By studying the interactions of adsorbates with the magnetite Fe3 O4 (001) single crystal surfaces and Rh single atoms supported on Fe3 O4 (001), this work aims to add to the limited body of atomic level characterization of these surface sites using a combination of molecular beam dosing, temperature programmed desorption, X-ray photoelectron spectroscopy, scanning tunneling microscopy, low energy electron diffraction, infrared reflection absorption spectroscopy, and density functional theory. Firstly, the adsorption of the C2 hydrocarbons (ethane, ethene, and ethyne) and formic acid are studied on bare Fe3 O4 (001). The C2 hydrocarbons are found to physisorb weakly on the Fe3 O4 (001) surface. Inversion analysis is used to extract coverage-dependent kinetic data for each molecule. The desorption energies of the C2 hydrocarbons increase with increasing bond order due to enhanced interaction of the π system to coordinatively unsaturated octahedral Fe sites. In contrast, formic acid chemisorbs strongly on the Fe3 O4 (001) surface and dissociates already at 80 K, producing formate bidentate and hydroxyl species. Formate decomposes along the decarbonylation pathway yielding CO and Mars-van Krevelen mediated H2 O between 450-600 K. Additionally, CO2 and recombinative HCOOH are produced as minor products. The desorption of CO follows the production of water indicating the reaction of formate with oxygen vacancies is a key intermediate step in CO formation. Further, CO exhibits two desorption pathways which are dependent upon surface hydroxyl coverage. To study the model single atom catalysts, the preparation conditions and thermal stability of Rh single atom sites was investigated on Fe3 O4 (001). Our studies demonstrated the formation of model catalysts containing pure or nearly pure Rh species bound as adatoms, substitutional octahedral atoms within the Fe3 O4 (001) surface, as well as nanoparticles. CO and CO2 adsorption was used to probe the properties of different Rh sites. Adatoms and nanoparticles exhibit high-temperature CO desorption (250–600 K) with some CO oxidation to CO2 via the Mars–van Krevelen mechanism. In contrast, CO2 was found to interact weakly with all Rh sites, but a small quantity of CO2 reduced to CO on Rh adatoms and small nanoparticles. Adsorption of formic acid on these Rh moieties demonstrated a switching of the reaction pathway from decarbonylation to CO to decarboxylation and CO2 production. Single substituted Rh octahedral sites, which is inactive for CO oxidation and CO2 reduction, are found to activate in the presence of the chemisorbed surface intermediates. Through hydroxyl adsorption experiments, these structurally stable Rh octahedral sites become destabilized through hydroxyl recombination to water which reduces Rh coordination triggering its conversion to active Rh adatoms and small clusters. The newly formed adatoms and small clusters can revert back to the stable but inactive Rh octahedral after the reaction and reaching 700 K. These results demonstrate the complex interactions between substrate and intermediate species and their effect in the activation and destabilization of single atom sites. Ultimately, they highlight the continuing need for atomic level investigations of the catalyst active site throughout the course of the reaction process.

Rh single atom catalysis on the magnetite (001) surface

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