Atomic‐Scale Observations of the Manganese Porphyrin/Au Catalyst Interface Under the Electrocatalytic Process Revealed with Electrochemical Scanning Tunneling Microscopy

Abstract

O 2 + 2H 2 O + 4e − ) due to the large overpotential necessary, which remains highly interesting as a research topic. [8] Building upon studies on the rare noble metal catalysts (Pt, Ir, Ru), [9] mononuclear molecular catalysts, like metalloporphyrin, have many attractive features, including low cost, intuitive information on the reaction mechanism, and simple assembly in various energy devices, [10] prompting the improvement of the low-efficient OER catalyst. [11] However, it is still debated whether the true active catalyst on electrode potential is the robust molecular complex itself or the metal oxide compounds produced by the decomposition of the molecular complex as a precatalyst. [12] For this reason, untangling this argument paves the way toward the ultimate goal of a cost-effective, robust, and efficient electrocatalyst. [13] A simple model study is a prerequisite for a fundamental understanding of the catalytic processes at the solid-liquid interfaces, [14] due to the lack of inclusive conception and the complexity of molecular catalysts under reaction conditions. [15] Therefore, we focus on 2D well-defined molecular catalysts, particularly porphine, which is the simplest porphyrin group, [16] via in situ electrochemical scanning tunneling microscopy (EC-STM) at the molecular level for OER in alkaline solution. Our system is a good model for an oxygen-evolving complex containing an active manganese cluster, [17] and for the monolayer structure of the simple porphine molecules still not fully investigated; [16] we report the topographic characterization of Au-supported 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine manganese (II) (MnOEP), whose OER activity is drastically enhanced due to their synergistic effects. [18] Since the octaethylporphyrin shows the highest stability, based on its geometry with maximized π-π interaction to the substrate, [19] it facilitates the observation of well-ordered MnOEP molecular arrangements in alkaline solution using EC-STM. As electrode potential increases, starting with the reversible adsorption of hydroxyl anions, the MnOEP/ Au interface undergoes a progressive oxidation reaction leading to structural changes. Moreover, the catalytic activity for OER is increased after the first moderate potential sweep with the emergence of Au features, such as OH − ion adsorption and Au oxidation peaks in cyclic voltammogram (CV). A detailed explanation of this phenomenon is not yet clear, [20] but changes in the MnOEP adlayer through an initial oxidation-reduction As a promising molecular catalyst for oxygen evolution reaction (OER), metalloporphyrin is a good model system that is extensively studied. The catalytic efficiency of metalloporphyrin can be improved with deeper insight into its complex issues, such as structural stability and catalytic activity. Using in situ electrochemical scanning tunneling microscopy (EC-STM) and X-ray photoelectron spectroscopy, the morphological evolution of the manganese porphyrin/Au(111) interface affected by the electrocatalytic reaction is...

Metal Porphyrins as Single Site Catalyst Models Explored by Electrochemical Scanning Tunneling Microscopy: A New Perspective in Electrocatalysis

“…EC-STM provides atomic-or molecular-level surface structures of electrocatalysts and chemical processes at the solid-liquid interface, monitoring, in operando condition, the electrocatalyst surface structures and electrode processes including , d) E app = OCP, e) E app = 0.95 V, f) E app = 1.05 V, g) E app = 1.10 V, h) E app = 1.25 V, i) E app = 0.85 V. Adapted with permission. [33] Copyright 2020, Wiley.…”

“…Kim et al [33] very recently reported the atomic-scale observation of a manganese porphyrin toward oxygen evolution reaction (OER). OER retains a relevant role in the development and spreading of clean energy conversion since, together with hydrogen evolution reaction (HER), it takes part to electrochemical water splitting.…”

“…Among these applications, catalysis retains a major role, and a number of works are devoted to examine heterogenous catalysis by means of UHV-STM. [33][34][35] The enthusiastic approach to catalysis unveiling with STM technique should be downsized, because in all these cases, a common point was the employment of single crystals as metal substrates for the catalysis characterization. Here arises one major issue of STM, namely the use of metallic single crystals as substrates.…”

“…Figure 18. EC-STM images of MnOEP on Au(111) in 0.1 m NaOH at a) E app = 0.65 V, b) E app = 0.85 V. c) topographic profiles corresponding to white dashed line of figure b), d) E app = OCP, e) E app = 0.95 V, f) E app = 1.05 V, g) E app = 1.10 V, h) E app = 1.25 V, i) E app = 0.85 V. Adapted with permission [33]. Copyright 2020, Wiley.…”

Synthesis of extended covalently bound porphyrins on the Au(111) surface

“…Kim et al discovered that the interaction between metalloporphyrins and the underlying Au(111) substrate results in synergistic effects on the catalytic activity for the oxygen evolution reaction. 34 Rana et al conducted research on a double decker molecule that contains phthalocyanine and observed that, on the Au(111) surface, the Au atoms bond to the double decker molecules, resulting in a significant decrease in the molecules' mobility when compared to studies conducted on a highly oriented pyrolytic graphite surface. 35 Moreover, recent studies have demonstrated that the Au(111) herringbone affects the electronic states of a TFPP porphyrin, leading to the formation of two equivalent conformations of the molecule with shifted electronic states.…”