Operando Surface Spectroscopy and Microscopy during Catalytic Reactions: From Clusters via Nanoparticles to Meso‐Scale Aggregates

Abstract

described as metal dispersion (number of metal surface atoms relative to the total number of metal atoms in the catalyst), whereas the support is characterized by the specific surface area (m 2 g −1). For hemispherical metal particles of ≈1.5, 2, 5, and 10 nm diameter, the dispersion is about 0.8, 0.6, 0.3, and 0.1, respectively, illustrating why the nanoscale is required not to waste precious metal inside the particles. The ultimate dispersion is, of course, provided by single metal atoms and this concept is followed in "single atom catalysis" or "single site catalysis". [10-17] However, the adsorption and reaction steps of a catalytic reaction do typically not occur on isolated single atoms, but also involve neighboring sites, for example, of the oxide support [18] or of a metal matrix (for bimetallic nanoparticles, active metal atoms may be embedded in an inert or less-active metal matrix [11,19-23]). The so-called site isolation of active atoms may prevent CC bond cleavage (coking), which requires at least two neighboring active metals. In any case, the stability of the single atoms is the key challenge, as atoms may diffuse and sinter. Furthermore, for example, Pt, Pd, Cu, or Au atoms are mobilized by CO, [24-28] forming CO-M 1 units that may eventually sinter into larger clusters. Herein, we define metal clusters as entities with less than ≈100 atoms, characterizing the so-called "non-scalable" regime (Figure 1a). [29] Their atomic and electronic structure is significantly different from that of bulk metals, for example, with respect to atom positions (icosahedral vs face centered cubic fcc), distances (lattice contraction or expansion), and band structure (semiconductor vs metal). Clearly, these properties are size-dependent ("each atom counts" [30,31]), with pronounced impact on adsorption and reaction properties. [5,32] Accordingly, nanoparticles with more than ≈100 atoms are in the "scalableregime" and start to have or exhibit bulk atomic and electronic structure. [6,32-34] Nevertheless, the relative contributions of corner, edge, step, terrace and phase boundary sites on the surface of a nanoparticle (Figure 1b,c) are still size-dependent. [35,36] Meso-scale ("black") powders of metals are clearly bulk-like and have very low dispersion (Figure 1d), but the µm-sized aggregates still consist of adjoining nanocrystals with structured surfaces, comparable to that of true nanoparticles. Operando characterization of working catalysts, requiring per definitionem the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure, composition and adsorbed species. Frequently applied operando techniques are discussed, including X-ray absorption spectroscopy, near ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy. In contrast to these area-averaging spectroscopies, operando surface microscopy by photoemission electron microscopy delivers spatially-resolved data, directly visualizing catalyst heterogeneity. For thorough interpretation, the exper...

Confinement Effects in Individual Carbon Encapsulated Nonprecious Metal‐Based Electrocatalysts

“…The in situ Raman and infrared spectroscopy could also provide the information on chemical state and phase transition, and surface adsorbates/reaction intermediates. [ 205,206 ] Hence, it is a big requirement to introduce or coalesce more effective in situ characterization techniques to understand the electrocatalytic nature and process in this NPM‐based core@carbon shell electrocatalysts.…”

Dynamic Evolution of Active Sites in Electrocatalytic CO2 Reduction Reaction: Fundamental Understanding and Recent Progress

“…[37] In terms of CO 2 RR, recently developed operando techniques have shown the great significance in combining advanced characterizations with catalytic performance evaluation under operating conditions to uncover the real-time structure-performance sensitivity of electrocatalysts. [36,39,104] In this part, state-of-the-art operando techniques for monitoring dynamic evolution of active sites and reaction intermediates in CO 2 RR, are summarized. An overview of schematic of several involved operando techniques is shown as Figure 5.…”

“…[47,119] On the other hand, the traditional XPS measurement of catalysts is generally conducted in an ultrahigh vacuum circumstance, making the operando investigations under electrochemical conditions difficult in which the reactor for in situ XPS has to be exquisitely and specifically designed. [36,120] The quasi and in situ XPS techniques are emerging in recent years, which allow the realtime chemical analysis of heterogeneous catalysts at or near an ambient circumstances. [118] As an example in CO 2 RR, it was revealed by in situ XPS that N atoms on InN nanosheets were partly removed by cathodic potential during electrolysis, leaving the In-rich surface on InN catalyst contributing to enhanced adsorption of HCOO* intermediates and promoted formate production.…”

Direct CO2 capture and conversion to fuels on magnesium nanoparticles under ambient conditions simply using water

“…In situ ATR-FTIR studies were carried out to examine the mechanistic pathways. 68 Before in situ ATR-FTIR studies, the temperature-programmed desorption (TPD) experiment was carried out under a He ow using 20 mg catalysts with a heating rate of 10 C min À1 up to 350 C. Analysis of desorbed species was carried out by using a differentially-pumped quadrupole mass spectrometer (Prisma Plus QMG 220, Pfeiffer Vacuum).…”