Local reaction kinetics by imaging

Abstract

In the present contribution we present an overview of our recent studies using the "kinetics by imaging" approach for CO oxidation on heterogeneous model systems. The method is based on the correlation of the PEEM image intensity with catalytic activity: scaled down to the μm-sized surface regions, such correlation allows simultaneous local kinetic measurements on differently oriented individual domains of a polycrystalline metal-foil, including the construction of local kinetic phase diagrams. This allows spatially- and component-resolved kinetic studies and, e.g., a direct comparison of inherent catalytic properties of Pt(hkl)- and Pd(hkl)-domains or supported μm-sized Pd-powder agglomerates, studies of the local catalytic ignition and the role of defects and grain boundaries in the local reaction kinetics.

Keywords: CO oxidation; Catalysis; Catalytic ignition; Palladium; Photoemission electron microscopy; Platinum.

Graphical abstract

Fig. 1

An example of a polycrystalline Pt foil: a) as imaged in PEEM; and b) identification of individual surface domains by EBSD. Note the inverse pole figure assigning the corresponding directions. From .

Fig. 2

Principle of the experiment. The ongoing catalytic reaction is simultaneously monitored by PEEM and MS. Global (averaged) MS data (lower inset) can be correlated with the spatially resolved data resulting from the intensity analysis of the video-PEEM images (upper inset). From .

Fig. 3

Global mass spectroscopy (MS) studies of CO oxidation on polycrystalline Pt foil: (a) hysteresis of the CO₂ production rate at cyclic variation of the CO pressure at an O₂ pressure of 1.3 × 10^− 5 mbar and a temperature of 417 K (red line) and 441 K (black line). The τ_A and τ_B values obtained from the hysteresis curves in (a) are used for the phase diagrams in Fig. 3b and c as indicated by the arrows. (b) Global kinetic phase diagram at constant oxygen pressure of 1.3 × 10^− 5 mbar for CO oxidation on polycrystalline Pt foil, as obtained by in situ MS. (c) Corresponding kinetic phase diagram at constant temperature of 441 K for CO oxidation on polycrystalline Pt foil as obtained by in situ MS. From .

Fig. 4

Global (MS) versus local (spatially-resolved, PEEM) monitoring of catalytic ignition: a) global MS measurements of ignition (red squares) and extinction curves (black triangles) on polycrystalline Pd foil. CO₂ production rate measured by MS during cyclic variation of the sample temperature (rate: 0.5 K s^− 1) at constant p_CO = 5.8 × 10^− 6 mbar and p_O2 = 1.3 × 10^− 5 mbar is plotted versus sample temperature. Simultaneously recorded PEEM video-sequences illustrate the ignition process: Frame (1) inactive, CO covered surface; Frame (2)—ignition begins on (110) domains; Frame (3)—ignition continues on (100) domains; Frame (4)—oxygen covered, active surface. b) Laterally resolved ignition/extinction measurements: local PEEM intensity for the individual (110), (100), and (111) domains during the same cyclic temperature scan as in (b). The vertical dashed line indicates the turning point from heating to cooling. From .

Fig. 5

Ignition/extinction measurements versus pressure variations: global (a) and local (b) kinetic phase diagram illustrating the CO oxidation reaction on polycrystalline Pd foil (a) and on a single Pd(100) domain of the Pd foil (b). Note the agreement of the transition points τ_A* and τ_B* obtained at varying T (from the ignition/extinction curves shown in the left insets) with the diagram obtained via cyclic variation of p_CO (from the poisoning/reactivation curves in the right insets). The dashed regions indicate the range of bistability. From .

Fig. 6

Palladium versus platinum in CO oxidation. a) Comparison of the global kinetic phase diagrams (by MS) at constant oxygen pressure (p_O2 = 1.3 × 10^− 5 mbar) of polycrystalline Pt (filled red squares and circles) and Pd (black squares and circles). Open circles are ignition points for Pt. b) Corresponding local kinetic phase diagrams for individual Pt(hkl) domains (left) and Pd (right), obtained by local PEEM intensity analysis. Open symbols are the local ignition and extinction points. From .

Fig. 7

CO oxidation on individual (hkl) domains of a sputtered Pd foil in comparison with the smooth surface. (a) Local kinetic phase diagrams of the individual Pd(110) — (red), Pd(100) — (black) and Pd(111) — (blue) domains of an additionally sputtered Pd foil, in comparison with the (100) and (111) domain of an annealed Pd foil, at constant p_O2 = 1.3 × 10^− 5 mbar and different constant temperatures. The local kinetic phase diagrams of the sputtered surface are shifted together and towards higher CO pressures, as compared to the local kinetic phase diagrams of the annealed sample (see Fig. 5a). Frames 1–3: transition τ_B at 405 K on the additionally sputtered Pd foil, p_CO = 5.9 × 10^− 5 mbar. (b) The reaction fronts propagate across the grain boundaries. The independency of the individual domains is lifted. From .

Fig. 8

Comparison of the catalytic behavior of a Pt(100) domain of supporting Pt foil and of a Pd agglomerate on the same foil: a) hysteresis-like PEEM intensity plots measured locally for the Pt(100) domain during cyclic variation of the CO pressure at constant temperature of 473 K and a constant partial oxygen pressure of 1.3 × 10^− 5 mbar. b) the same, but for Pd agglomerate. The insets show chosen PEEM frames for particular characteristic parts of the hysteresis loop. Note the significantly different ranges of bistability (CO pressure values between the τ_A and τ_B for Pt domain and Pd agglomerate). From .

Fig. 9

Component-specific kinetic phase diagrams: a) diagrams for Pt(100) and Pt(110) domains of the supporting Pt foil from in comparison with the diagram for the supported Pd agglomerates; and b) the same diagram as in (a) for the supported Pd agglomerates in comparison with diagrams for the ordered Pd(111) surface and sputtered (highly defected) Pd(111) surface (the latter two adapted from [13]). From .