ABSTRACT
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
II. Catalytic CO Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
A. Reaction Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
B. Experimental: CO and O2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
1. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
2. Adsorption of CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
3. Adsorption of O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
C. Detection of Surface Intermediates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
D. Effect of Subsurface Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
E. Steady-State Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
F. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
III. Oscillations and Chemical Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
A. Experimental Techniques on an Atomic Scale . . . . . . . . . . . . . . . . . . . . . . . . 170
B. Phase Transitions: Pt-tip Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
C. Subsurface Oxygen: Pd-tip Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
D. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
IV. Mathematical Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
A. General Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
B. Pd: Modeling of Oscillations and Wave Patterns . . . . . . . . . . . . . . . . . . . . . . 176
C. Pt : Modeling of Oscillations and Wave Patterns . . . . . . . . . . . . . . . . . . . . . . . 180
D. Spatio-Temporal Chaos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
V. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
The catalytic oxidation of CO over platinum group metals is relatively simple and also important
from the ecological viewpoint. In addition, this reaction exhibits a rich kinetic behavior,
including regimes with sustained kinetic oscillations (for reviews, see [1-12]). Great interest in
self-oscillatory phenomena in catalytic reaction over metal surfaces is for a large part caused by the
possibility to perform more effectively the catalytic processes using the unsteady-state operation.
The CO and H2 oxidation on metals (Pt, Pd) is a nonlinear system, in which temporal and
spatial organization becomes possible [1,2]. In the oscillatory regime, the reaction mixture period-
ically affects the properties of metal surfaces. As there is a synergy between the concentrations of
the adsorbed species and the structure of the surface throughout those oscillations, and as the differ-
ent products often display different oscillation cycles, and are also objected by changes in surface
phases, valuable information can be extracted about the mechanism of such reactions from kinetic
and characterization studies on the surface species. In the last decades, CO oxidation reaction has
became a model for testing the newest physical methods for studying the structure and composition
of catalysts. Specifically, it has been reported that the mechanisms of oscillatory oxidation reactions
are connected with a periodic change of surface structure (from a reconstructed hexagonal phase to
the unreconstructed surface in surface structure on Pt(1 0 0)), with subsurface oxygen formation (at
least on Pd(1 1 0)), and with the “explosive” nature of interactions between adsorbed species [1-3].
A common feature in all these mechanisms is the spontaneous periodical transitions of the metal
from inactive to highly active states. Since the first discovery of a relationship between reconstruc-
tion and kinetic oscillations in CO oxidation on Pt(1 0 0) by Ertl [1], this has become one of the
most extensively investigated oscillatory systems in heterogeneous catalysis. The use of spatially
resolved (1 mm) photoelectron emission microscopy made it then possible to discover the formation of chemical waves on the surfaces of Pt and Pd single crystals [1]. Real metal and
support catalysts usually consist of nanosized metal particles on which different crystal planes
are exposed. The important question is, can a small supported particle be compared with macro-
scopic single-crystal surfaces that are normally used in surface science studies. Recent experi-
mental work has shown that field electron microscopy (FEM), which has a sharp tip with a
lateral resolution of 20 A˚, can also serve as an in situ catalytic flow reactor for the study of these oscillations [3,13].
