Following the Kinetics of Undercover Catalysis with APXPS and the Role of Hydrogen as an Intercalation Promoter

Abstract

While improved catalytic properties of many surfaces covered by two-dimensional materials have been demonstrated, a detailed in situ picture of gas delivery, undercover reaction, and product removal from the confined space is lacking. Here, we demonstrate how a combination of gas pulses with varying compositions and time-resolved ambient pressure photoelectron spectroscopy can be used to obtain such knowledge. This approach allows us to sequentially form and remove undercover reaction products, in contrast to previous work, where co-dosing of reactant gases was used. In more detail, we study CO and H2 oxidation below oxygen-intercalated graphene flakes partially covering an Ir(111) surface. We show that hydrogen rapidly mixes into a p(2 × 1)-O structure below the graphene flakes and converts it into a dense OH–H2O phase. In contrast, CO exposure only leads to oxygen removal from the confined space and little CO intercalation. Finally, our study shows that H2 mixed into CO pulses can be used as a promoter to change the undercover chemistry. Their combined exposure leads to the formation of OH–H2O below the flakes, which, in turn, unbinds the flakes for enough time for CO to intercalate, resulting in a CO structure stable only in coexistence with the OH–H2O phase. Altogether, our study proves that promoter chemistry in the form of adding trace gases to the gas feed is essential to consider for undercover reactions.

Structure and Reactivity of Active Oxygen Species on Silver Surfaces for Ethylene Epoxidation

“…Upon averaging each single O 1s photoemission spectrum from consecutive cycles performed at the same temperature, we plotted and investigated the obtained 200 spectra per gas switch (C 2 H 4 + O 2 → C 2 H 4 + Ar). Such an approach, which is based on previous literature, , allows to improve the signal-to-noise ratio of each spectrum by event-averaging. Subsequently, we developed a batch processing procedure, which selectively deconvolutes each spectrum.…”

Scanning photoelectron spectromicroscopy: From static to operando studies of functional materials

“…Many peculiar properties of this 2D material also require to study of its interactions with gases at near-ambient conditions that has become already possible with classic ambient pressure XPS. Graphene, indeed, is a good playground to follow the kinetics of undercover catalysis where reactions can take place at the interface of graphene and catalyst substrate [53]. Metal-free heteroatom doped-graphene is itself catalytically active for many reactions: oxygen reduction reaction (ORR) [54,55], NO decomposition [56], among others.…”