The Role of Defects in the Local Reaction Kinetics of CO Oxidation on Low-Index Pd Surfaces

Abstract

The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10–5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.

Analysing the Reaction Kinetics for Individual Catalytically Active Components: CO Oxidation on a Pd Powder Supported by Pt Foil

“…sputtered surfaces performed in our recent study showed that ion bombardment of the Pd surface leads to formation of three-dimensional islands, with a very high density of steps and edges [13]. The formed ''mound''-like islands have uppermost terraces with the diameters of 2-3 nm, and the slope angles are between 10°and 20°, corresponding to terrace widths of 0.6-1.2 nm.…”

“…ion bombardement, i.e. with an artificially defected surface [13]. The comparison shows that the reactive behaviour of the Pd agglomerates differs significantly from that of the smooth ordered Pd(111) domain, with the CO poisoning (upper boundary of the bistability region) occurring for Pd powder at a considerably higher CO partial pressure than for the smooth Pt(111) surface.…”

“…Such a model system is well suitable for proving the possibility to study the reaction kinetics not only in a spatially-resolved but also in a component-distinctive way since the parameter ranges of catalytic activity for CO oxidation differ significantly for Pd and Pt [8]. In addition, the work function changes caused by CO and oxygen adsorption on Pt and Pd surfaces lead to differing contrasts in PEEM during the CO oxidation: an oxygen covered Pt surface appears as ''dark'' [7,12] in contrary to the Pd surface [8,13] which appears as ''bright'' upon oxygen adsorption. This allows an easy differentiation between the Pt support and Pd powder particles down to the resolution limit of the available PEEM (a few lm in our case).…”

Hydrogen Oxidation on Stepped Rh Surfaces: µm-Scale versus Nanoscale

“…Since the step density is known to influence the catalytic behaviour, a Rh sample with a step density which is closer to that of the tip surface was prepared by Ar + sputtering of the present Rh foil (cf. our recent STM observation of the Ar + created steps and defects on Pd [34]). The corresponding phase diagram for such an artificially defected Rh surface is also included in Fig.…”

“…The ROI for which the PEEM intensity was registered, is marked in (a); c τ A and τ B values for different temperatures in the range of 433-493 K and constant oxygen pressure of 7.7 × 10 −7 mbar, plotted in the p H2 vs 1/T coordinates (kinetic phase diagram). Regions of high and low catalytic activity and of bistability are marked partially filled with sputtered Pd, the reaction fronts were able to cross the grain boundaries [34].…”