Local catalytic ignition during CO oxidation on low-index Pt and Pd surfaces: a combined PEEM, MS, and DFT study

Abstract

Shedding light on light-off: : Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis.

Figure 1

a) Scheme of the experiment: The CO oxidation reaction on polycrystalline Pd (Pt) foil is simultaneously monitored by MS and PEEM. Three different domains, Pd(110), Pd(100), and Pd(111) are identified in the PEEM image. b) Ignition (red squares) and extinction curves (black triangles) on Pd foil, as CO₂ production rate measured globally by MS during cyclic variation of the sample temperature (rate: 0.5 K s⁻¹) at constant ρ_CO=5.8×10⁻⁶ mbar and ρ_o2 = 1.3×10⁻⁵ mbar. 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. c) 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.

Figure 2

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; open symbols) with the diagram obtained from cyclic variation of p_CO (from the poisoning/reactivation curves in the right insets; filled symbols). The dashed regions indicate the range of bistability.

Figure 3

Palladium versus platinum in CO oxidation. a) Comparison of the global kinetic phase diagrams (by MS) at constant oxygen pressure (ρ_o2=1.3×10⁻⁵ mbar) of polycrystalline Pt (filled red squares and circles) and Pd (black squares and circles). (An improved temperature measurement method was used in comparison to Ref. , with the corresponding correction being applied to the Pt data.) 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.

Figure 4

a) Simulated kinetic phase diagrams for Pd(111) and Pt(111) at ρ_o2=1.3×10⁻⁵ mbar, as well as a simulated p_CO hysteresis curve for Pt(111) at 417 K (left inset) and a simulated ignition/extinction curve for Pd(111) at p_CO=5.8×10⁻⁶ mbar (right inset). b) Simulated local kinetic phase diagrams of Pt(110), Pt(100), and Pt(111) at ρ_o2=1.3×10⁻⁵ mbar.