In Situ and Operando X-ray Spectroscopy Studies of Solid/Gas and Solid/Liquid Interfaces

Gas-phase catalytic reactions and electrochemical reactions are two important categories of heterogeneous reactions. In order to determine the reaction mechanisms and potentially improve the technologies based on these reactions, it is essential to understand what happens at relevant solid/gas or solid/liquid interfaces under reaction conditions. X-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) are two element-specific and chemical-state-specific characterization techniques. Recently, researchers have demonstrated that with proper modifications, these vacuum-requiring techniques can also be used to characterize various solid/gas or solid/liquid interface systems under ambient conditions, which opens the way towards in situ and operando characterization of many heterogeneous reaction systems. In this dissertation, the studies on two solid/gas interface systems and two solid/liquid interface systems are presented./nThe solid/gas interface systems were characterized by ambient-pressure XPS (AP-XPS). In the study regarding Co catalyst for Fischer-Tropsch (F-T) synthesis, it was found that CO adsorbs strongly on Co surface; but small amount of sulfur substantially weakens the CO adsorption and consequently poisons the F-T reaction. Under reaction conditions, Co surface remains metallic. At lower temperatures, water molecules in the reaction products can potentially oxidize the Co surface and thereby hinder the catalytic reaction. When the reaction temperature is above 250°C, H2 is responsible for reducing Co to its metallic phase and keeping the catalyst active. In the study of CoxPdy alloy nanoparticles for catalyzing CO oxidation reaction, a clear dependence of catalytic activities on nanoparticle composition was observed. AP-XPS results revealed that Co segregates to the surface and stays in the oxide form after the pretreatment process. Small amount of Co coexisting with Pd on the surface can promote the CO oxidation reaction synergistically and the synergetic effect becomes more prominent with increasing Co content. However, when Co/Pd ratio is too high, after the cleaning processes, the surface of the nanoparticles is fully covered by thin layers of CoOx, preventing CO molecules from binding with Pd, which leads to significant drop in catalytic activity of the alloy nanoparticles./nThe solid/liquid interface systems were characterized by our newly developed in situ and operando XAS technique. Total electrode yield (TEY) signal collected through the working electrode can provide information that is sensitive to the solid/liquid interfaces. At Au/H2O interfaces, the hydrogen-bonding network is greatly disrupted by the interface, leaving more broken hydrogen bonds than in bulk water. But the delocalization of the LUMO orbitals of water molecules into the gold substrate, greatly suppresses the pre-edge feature in O1s XAS spectrum that is typically associated with broken hydrogen bonds. These polar molecules can respond to external electrical field and reorient at the interface, producing potential-dependent O1s TEY XAS spectra. At Pt/H2SO4(aq) or Au/H2SO4(aq) interfaces, similar water reorientation behaviors were observed in the negative potential region. In the positive potential regions, the evolution of O1s XAS spectra is different from that of Au/H2O system. The changes in the spectra are possibly related to the adsorption of sulfate ions at the interface, contradicting many reports that proposed an oxide formation or oxygen/ hydroxyl adsorption processes./nThe studies on four interface systems presented in this dissertation illustrate the capabilities and great potentials of these two in situ x-ray spectroscopy techniques, i.e., AP-XPS and in situ/operando XAS, in the investigation of interfacial phenomena and reaction mechanisms in various heterogeneous reactions. Such mechanistic information can potentially provide insightful guidance for better designs of catalysts and electrode materials.

Advisor: Salmeron, Miquel B
Cuk, Tanja