Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

Abstract

The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465 K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513 K. A change in the front propagation mechanism is proposed to explain the observed effects.

Fig. 1

Global kinetics of CO oxidation on Pt foil. (a) PEEM image of the clean Pt foil. The surface termination of the individual grains is indicated. (b) PEEM images of the oxygen-covered (top) and the CO-covered (bottom) Pt foil. The oxygen-covered surface appears dark in PEEM since adsorbed oxygen increases the work function of the Pt surface. (c) Hysteresis in the global CO₂ production rate at cyclic variation of the CO pressure at constant O₂ pressure of 1.3 × 10^− 5 mbar and at temperatures of 417 K (black squares), 453 K (red dots) and 477 K (blue triangles). The τ_A and τ_B values deduced from the hysteresis curves were used for the kinetic phase diagram in (d). (d) Global kinetic phase diagram of CO oxidation on polycrystalline Pt foil, obtained by mass spectroscopy.

Fig. 2

Local kinetics of CO oxidation on individual crystalline grains of a Pt foil. (a) Principle of the local kinetic measurements by PEEM. Left inset: Global CO₂ production rate versus CO partial pressure as measured by MS during catalytic CO oxidation at T = 417 K and p_O2 = 1.3 × 10^− 5 mbar. Right inset: Hysteresis plot of the local PEEM intensity of an individual Pt(100) facet versus CO partial pressure, as measured in situ by PEEM at the same reaction conditions. The selected domain is indicated in frame (1). The numbers along the hysteresis curve indicate the corresponding PEEM frames. (b) Corresponding kinetic phase diagram for a Pt(100) facet (black squares) in comparison to the global (MS-measured) diagram (red dots). (c) The same as in (b), but in comparison to a local diagram for a Pt(110) facet (red dots).

Fig. 3

Propagation of reaction fronts (here oxygen fronts) on polycrystalline Pt foil during catalytic CO oxidation at T = 417 K and p_O2 = 1.3 × 10^− 5 mbar (image frames recorded at an interval of 2 s). Regions of interest (two Pt(100) domains in comparison with a Pt(110) domain) are marked in the frame “0 s”. Note the different shapes of the propagating fronts on the (100) and (110) domains as marked in the frame “2 s”. In the frame taken at 6 s, the front positions at 0 s, 2 s, 4 s and 6 s are outlined in white.

Fig. 4

(a) Clean Pt foil imaged by PEEM. The AA’ line denotes the grain boundary between two (100) domains. (b) Snap-shot during the transition τ_B: anisotropic propagation directions of the reaction fronts on the neighboring (100) domains differ from each other. (c) Optical micrograph of the grain boundary region AA’. (d) and (e) AFM images of the neighboring (100) domains. (f) Line scan across the BB’ line in (e). The grooves imaged in (c)–(e) are schematically indicated in (a). The reaction front propagation directions (b) coincide with the directions of the grooves in (c) and are indicated in (a).

Fig. 5

Temperature dependence of the CO front propagation. (a) and (b) PEEM images of the CO fronts recorded during the transition τ_A at p_O2 = 1.3 × 10^− 5 mbar, at 417 K and 513 K, respectively. (c) and (d) Intensity line scans across the CO front along AA’ and BB’. (e) Temperature dependence of the front propagation velocities along (black squares) and across (red dots) the grooves on the (100) domain (see Fig. 4). The solid lines are fits using Luther's equation (see text).

Fig. 6

Oxygen front propagation at two different temperatures of 417 K and 489 K. (a) and (b) PEEM images of oxygen fronts during the transition τ_B at p_O2 = 1.3 × 10^− 5 mbar, at 417 K and 513 K, respectively. (c) and (d) Corresponding intensity line scans across the oxygen front along AA’ and BB’. The bright fringe between the CO- and oxygen-covered regions disappears at the higher temperature. (e) Temperature dependence of the ratio of the propagation velocities along (v_x) and across (v_y) the prolonged grooves on the (100) facet. The anisotropy decreases with increasing surface temperature.

Fig. 7

Scheme of the front propagation mechanism for: (a) CO fronts at all studied temperatures, (b) oxygen fronts at temperatures up to 465 K, (c) oxygen fronts at temperature above 465 K. The AFM image shown in Fig. 4 is used as a background.