Simultaneous detection of electronic structure changes from two elements of a bifunctional catalyst using wavelength-dispersive X-ray emission spectroscopy and in situ electrochemistry

Abstract

Multielectron catalytic reactions, such as water oxidation, nitrogen reduction, or hydrogen production in enzymes and inorganic catalysts often involve multimetallic clusters. In these systems, the reaction takes place between metals or metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. In this study, we present a method to detect X-ray emission signals from multiple elements simultaneously, which allows for the study of charge transfer and the sequential chemistry occurring between elements. Kβ X-ray emission spectroscopy (XES) probes charge and spin states of metals as well as their ligand environment. A wavelength-dispersive spectrometer based on the von Hamos geometry was used to disperse Kβ signals of multiple elements onto a position detector, enabling an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the scanning spectrometers, providing data free from temporal and normalization errors and therefore ideal to follow sequential chemistry at multiple sites. We have applied this method to study MnOx-based bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In particular, we investigated the effects of adding a secondary element, Ni, to form MnNiOx and its impact on the chemical states and catalytic activity, by tracking the redox characteristics of each element upon sweeping the electrode potential. The detection scheme we describe here is general and can be applied to time-resolved studies of materials consisting of multiple elements, to follow the dynamics of catalytic and electron transfer reactions.

X-ray Emission Spectroscopy as an in Situ Diagnostic Tool for X-ray Crystallography of Metalloproteins Using an X-ray Free-Electron Laser

“…This is the case also for other measures that account for integrated signal rather than individual feature positions, such as the Integrated Absolute Difference (IAD), which has also been shown to exhibit similar properties as the first moment. 31, 32, 37, 38 Using the first moment, we can track sub-pixel changes in the ‘center-of-mass’ position of the features. The first moment of Mn XES spectrum decreases with increase in the oxidation state, as a result of the red-shift of the Kβ 1,3 peak due to 3p3d exchange interactions, and Kβ 1,3 XES has been shown to be more sensitive to oxidation state than changes in the ligand environment.…”

In situ/Operando studies of electrocatalysts using hard X-ray spectroscopy

“…Probing the changes in elements of interest separately under catalytic conditions can make the comparisons very tricky due to the systematic errors originating from concentration and volume distribution of the sample, data normalization and temporal errors. Based on X-ray emission spectroscopy (XES), we have developed an experimental method to detect changes in the electronic structure of multiple elements simultaneously under experimental conditions [55]. Unlike other X-ray spectroscopies, XES can simultaneously probe multiple metal sites since a single incident X-ray at energy (E 0 ) can excite both of them, when E 0 is higher than the binding energy of the core-level electrons excited in the initial state, as shown in Fig.…”

“…Blue and pink traces represent the pathways of emitted photons from two different elements, Mn and Ni. Figure adapted from reference [55]. …”

“…D) Schematic representation of sequential phase evolution of MnNiO x upon increasing potential. Figure adapted from reference [55]. …”

Metalloprotein structures at ambient conditions and in real-time: biological crystallography and spectroscopy using X-ray free electron lasers

“…One can also envision extending this approach to multiple element systems, by following the emission signal of two elements concurrently. A demonstration experiment of such a system was performed recently at the ALS using a mixed MnNi oxide [55]. …”