Catalysis by Imaging: From Meso- to Nano-scale

Abstract

In-situ imaging of catalytic reactions has provided insights into reaction front propagation, pattern formation and other spatio-temporal effects for decades. Most recently, analysis of the local image intensity opened a way towards evaluation of local reaction kinetics. Herein, our recent studies of catalytic CO oxidation on Pt(hkl) and Rh(hkl) via the kinetics by imaging approach, both on the meso- and nano-scale, are reviewed. Polycrystalline Pt and Rh foils and nanotips were used as µm- and nm-sized surface structure libraries as model systems for reactions in the 10–5–10–6 mbar pressure range. Isobaric light-off and isothermal kinetic transitions were visualized in-situ at µm-resolution by photoemission electron microscopy (PEEM), and at nm-resolution by field emission microscopy (FEM) and field ion microscopy (FIM). The local reaction kinetics of individual Pt(hkl) and Rh(hkl) domains and nanofacets of Pt and Rh nanotips were deduced from the local image intensity analysis. This revealed the structure-sensitivity of CO oxidation, both in the light-off and in the kinetic bistability: for different low-index Pt surfaces, differences of up to 60 K in the critical light-off temperatures and remarkable differences in the bistability ranges of differently oriented stepped Rh surfaces were observed. To prove the spatial coherence of light-off on nanotips, proper orthogonal decomposition (POD) as a spatial correlation analysis was applied to the FIM video-data. The influence of particular configurations of steps and kinks on kinetic transitions were analysed by using the average nearest neighbour number as a common descriptor. Perspectives of nanosized surface structure libraries for future model studies are discussed.

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

“…For reviews, single crystal studies and a recent NAP‐PEEM variant, which enables high resolution (<100 nm) imaging around 1 mbar from 150–1200 K, refer to. [ 95,187,193–198 ]…”

“…For reviews, single crystal studies and a recent NAP-PEEM variant, which enables high resolution (<100 nm) imaging around 1 mbar from 150-1200 K, refer to. [95,187,[193][194][195][196][197][198] Figure 6. a) Illustration of operando photoemission electron microscopy (PEEM) and kinetics by imaging: [97,[185][186][187] a) an ongoing catalytic reaction on a structurally-heterogeneous sample, for example, a polycrystalline Pd foil, is monitored simultaneously by PEEM and MS. Local data obtained from the intensity analysis of PEEM video-sequences for each individual (hkl)-domain (µm-scale) are compared with global (averaged) MS data.…”

“…[185,186] Such kinetic transitions in CO oxidation occur by propagation of reaction fronts (Figure 6c) and have been visualized by PEEM especially on single crystal surfaces [94][95][96][193][194][195] and, more recently, on individual grains of polycrystalline foils. [97,98,179,180,[183][184][185][186][187][188][189][190][191][192] Below the very first PEEM study of CO oxidation on individual oxidesupported µm-sized Pd aggregates is discussed (Figure 13c), which reveals a new phenomenon at the metal/oxide interface. Figures 6 and 13d-l illustrate the operando PEEM experiment.…”

“…reactions can be imaged, sometimes revealing even novel effects such as multifrequential oscillations [179] or how surface structure, defects and metal/oxide interfaces steer kinetic phase transitions. [183][184][185][186][187][188][189][190][191][192] As a downside, PEEM is typically limited to pressures up to ≈10 −4 mbar.…”

Single-Particle Catalysis: Revealing Intraparticle Pacemakers in Catalytic H2 Oxidation on Rh

“… 28 − 30 Since the inactive (oxygen-covered) and active (low oxygen and hydrogen coverage) surfaces significantly differ in their work functions and thus in the FEM image brightness, the switching between the active and inactive state can be monitored in situ by FEM. 31 While the inactive (oxygen-covered) Rh surface exhibits dark contrast (high work function), the active surface with low oxygen and hydrogen coverage appears bright (low work function). By placing regions of interest (ROIs) at chosen positions, as exemplarily shown in Figure 2 a, this process can also be traced locally for crystallographically different nanofacets present on the tip surface.…”