VOLUME 83, NUMBER 14 PHYSICAL REVIEW LETTERS 4OCTOBER 1999
Dynamics on Microcomposite Catalytic Surfacces: The Effect of Active Boundaries
Stanislav Y. Shvartsman,
1
Eckart Shütz,
2
Ronald Imbihl,
2
and Ioannis G. Kevrekidis
1,
*
1
Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544
2
Department of Physical Chemistry and Electrochemistry, Universitaet Hannover, Callinstrasse 3-3a, D-30167 Hannover, Germany
(Received 7 May 1999)
We report experimental and theoretical studies of reaction dynamics on PtRh and PtTiO
2
microcomposite catalytic surfaces. Both steady and dynamic behaviors are dominated by reaction fronts
initiated at the interface between different catalytic components. Our analysis links the bifurcation
behavior of reactive composites with active boundaries to this transient phenomenon of front initiation.
PACS numbers: 82.40.Ck, 05.45.Jn, 82.40.Bj, 82.65.Jv
The presence of controlled heterogeneities in a reactive
medium introduces a new dimension to the complexity
of dynamic pattern formation. The shape and length
scales of the medium heterogeneity become nontrivially
linked to the intrinsic length and time scales of reac-
tion and transport in each active component. While the
resulting dynamic phenomenology becomes quickly too
rich to classify, understanding certain well-defined lim-
iting regimes can be useful in providing guidelines for
the design of composite reactive materials with nonlinear
properties.
In this Letter we address, through a combination of
experiments and modeling, a class of structured non-
linear materials: two-dimensional microcomposite cata-
lysts. Our goal is twofold: in addition to investigating
dynamics in structured bistable media [1], we take steps
towards understanding reactive properties of catalytic ma-
terials with several active components interdispersed at a
scale representative of patterns spontaneously forming in
these systems [2].
Consider a catalytic medium (a Pt single crystal sur-
face) on which a chemical reaction (NO reduction) oc-
curs. Instabilities due to kinetic nonlinearities are known
to lead to steady state multiplicity. We study the ef-
fect on these dynamics of active inhomogeneities: patches
with different kinetic/transport properties dispersed in, and
diffusively coupled to, the original catalyst matrix. A
systematic experimental study of inclusion size effects,
coupled with mechanistic and phenomenological model-
ing, lays the foundation for a comprehensive picture of
the dynamic and bifurcation behavior of the microdesigned
medium. In particular, we study composite ignition and
explore its bifurcation dependence on component geometry
(specifically, inclusion size). Dynamic imaging [with pho-
toelectron emission microscopy, (PEEM)] of NO reduc-
tion on microdesigned PtRh and PtTiO
2
catalysts shows
that reaction fronts originating at the matrix/inclusion
interface constitute key events in overall composite surface
behavior. Bistability (“kinetic hysteresis”) of both the ma-
trix and the inclusions of the composite surface lies at the
heart of the observed phenomenology. Inspection of the
“backbone” of the surface reaction mechanism,
NO
ads
1 ! N
ads
1 O
ads
R
1
,
2N
ads
! N
2
1 2 R
2
,
2H
ads
1 O
ads
! H
2
O 1 3 R
3
,
CO
ads
1 O
ads
! CO
2
1 2 R
4
,
reveals that empty sites , necessary for the initiation of
the rate limiting step R
1
, are regenerated in excess by the
subsequent steps R
224
. Bistability results from the bal-
ance between this autocatalytic generation of empty sites
and their removal due to chemisorption. At fixed external
conditions the surface can exist in two states differing in
the overall amount of empty sites. Large differences of
adsorbate coverages (especially O
ads
) between these states
give rise to a sharp contrast in PEEM, used here to visual-
ize the spatiotemporal catalytic dynamics: the unreactive/
reactive branches of the kinetic hysteresis appear dark/
bright in the PEEM image of the surface.
Kinetic bistability coupled with diffusion is responsible
for constant shape traveling concentration fronts on the sur-
face; as we illustrate here, controlled heterogeneities can
be designed to initiate such fronts. We design controlled
heterogeneities through a multistep lithographic process
leading to a surface on which areas of “bare” Pt100 are
surrounded by an 300 Å layer of either Rh or TiO
2
[3].
Figure 1 demonstrates that a step change in the operating
conditions can lead to a spatially nonuniform transient phe-
nomenon on the surface: here, a reaction front is gener-
ated at the boundary separating the catalytic components.
Changing P
H
2
from 2 3 10
26
to 7 3 10
26
mbar leads to
the development of a bright rim at the PtRh interface and
initiates a front propagating outwards from the boundary.
The front moving at velocities of 1mms leaves the Rh
surface in the reactive (PEEM bright) state.
Our analysis of boundary initiated transitions starts by
posing a reaction diffusion problem for a model PtRh mi-
crocomposite: We envision a periodic array of alternat-
ing PtRh stripes and focus on one-half of a unit cell of
such a “crystal.” Consistent mechanistic models for sur-
face chemistry of NO reduction on surfaces of both metals
[4], accounting for time evolution of coverages u of six
experimentally confirmed surface species (NO, N, O, H,
0031-9007 99 83(14) 2857(4)$15.00 © 1999 The American Physical Society 2857