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reaction. The time scales of elementary reactions on the catalyst\u2019s
active sites can themselves also vary due to the diffusion of
adsorbed species which is needed for reaction. On the other hand,
according to the transition state theory (Atkins [2]), the typical
[3], Luther et al. [14]), where cycling periods are several seconds
and amplitudes are tens of degrees Celcius.
The pulsed activation method is a type of dynamic operation in
which very fast temperature pulsing is used to induce chemical
reactions directly and locally as needed. In this method, we
distinguish two different temperature regimes which alternate
very fast in the reactor. The \ufb01rst regime is the base regime and the
second regime is the pulsed regime. Fig. 1 conceptually shows the
catalytic surface temperature during base and pulsed regimes in
* Corresponding author. Tel.: þ31 40 2473284; fax: þ31 40 2434582.
Contents lists available at
Applied Therma
ev
Applied Thermal Engineering 57 (2013) 180e187
E-mail address: l.ozkan@tue.nl (L. Özkan).
for these events ranges from 0.1 nm up to the reactor size (10 m).
Simultaneously, during a reaction the time scales involved range
from femto and pico seconds for the motion of valence electrons
and atoms in a molecule up to seconds/minutes or even in\ufb01nity if
the reactants end up in unwanted byproducts (Chorkendorff and
Niemantsverdriet [5]). In catalytic systems, the \u201cslow\u201d transport of
reactants toward the catalyst surface limits the \u201cfast\u201d conversion
rate of reactants. Additionally, transport of heat such as reaction
heat occurs at a rate substantially lower than speed of the actual
not in the scope of this paper. There also exist examples of dynamic
temperature operation, such as feed \ufb02ow temperature variation
(Dorawala and Douglas [6]) or the coolant temperature (Chang and
Schmitz [4]). However, the realization of temperature forcing was
not considered practical since \u201cAny large thermal inertia tends to
defeat the effect of sudden changes in this variable\u201d (Silveston et al.
[17]). With the improved heat transfer capabilities of microstruc-
ture devices, some of the recent research efforts are directed
toward forced temperature cycling inmicroreactors (Brandner et al.
1. Introduction
Chemical systems are characterize
and multi-scale nature. They natur
periodic phenomena and involve m
orders of magnitude in length and ti
example, one or more reactant mo
surface and adsorb onto an active sit
the reaction occurs and subsequently
the surface and desorb into the surro
1359-4311/$ e see front matter \ufffd 2012 Elsevier Ltd.
http://dx.doi.org/10.1016/j.applthermaleng.2012.06.03
eir nonlinear dynamics
hibit various types of
isms that span several
catalytic reactions as an
diffuse to the catalyst
tion center). At this site,
oducts move away from
phase. The length scale
requirement for chemical reactions is bringing molecules to an
energy level which exceeds the activation energy threshold.
The conventional way of providing energy to molecules is
conductive heating, i.e. increasing the temperature will increase
the number of molecules that are able to react. Conductive heating
is not selective and results in heating of both the reaction site with
bulk of the reactor. Recently, several mechanisms are being studied
in providing energy tomolecules for overcoming the energy barrier
namely, electromagnetic \ufb01elds, electric \ufb01elds, acoustic methods
and lasers (Zare [18], Zhang et al. [19], Durka et al. [7]) but these are
Pulsed activation in heterogeneous cata
J. Stolte a, L. Özkan a,*, P.C. Thüne b, J.W. Niemantsve
aDepartment of Electrical Engineering, Eindhoven Univ. of Technology, The Netherlands
bDepartment of Chemistry and Chemical Engineering, Eindhoven Univ. of Technology, T
a r t i c l e i n f o
Article history:
Received 29 September 2011
Accepted 19 June 2012
Available online 6 July 2012
Keywords:
Periodic operation
Temperature pulsing
Local heating
Heterogenous catalysis
a b s t r a c t
This paper describes a no
viewed as a form of period
reactions directly and local
will and within a time sc
principle experimental se
reactions. The temperature
have been reported before
journal homepage: www.els
All rights reserved.
5
sis
riet b, A.C.P.M. Backx a
etherlands
form of dynamic operation named pulsed activation method. It can be
peration in which very fast temperature pulsing is used to induce chemical
s needed. The main goal in this method is to activate catalytic reactions at
such that physical transport related dynamics cannot follow. A proof of
has been built to realize pulsed activation on heterogenous catalytic
the catalytic surface is pulsed at higher frequencies and amplitudes than
an example, oxidation of CO over a Pt catalyst is investigated.
\ufffd 2012 Elsevier Ltd. All rights reserved.
SciVerse ScienceDirect
l Engineering
ier .com/locate/apthermeng
traditional reactors, the time scale of these mechanisms are much
higher than reactions kinetics. When pulsed activation is used, the
reaction rate is negligible in the base state, but with each pulse
a certain amount of reactant conversion is achieved and after each
pulse the base conditions are restored again quickly. Through
variation of the amount of pulses per unit of time the reaction rates
can be controlled precisely and very nearly instantaneously
without the need for physical transport mechanisms. In this way,
each mechanism can be affected and optimized separately.
In this work, we consider the oxidation of CO as a test reaction
for the implementation of the pulsed activation method. CO
oxidation over platinum is one of the most studied reactions (Engel
and Ertl [8], Herz and Marin [11], Ertl et al. [9], Rinnemo et al. [16]).
It is of direct relevance in the removal of CO from waste gases, and
the removal of CO from the H2 streams for fuel cells. Platinum is an
active catalyst in this reaction, allowing studies with platinum
wires and foils. The reactionwill also run over a sputtered platinum
strip as it is presented in this study. The oxidation of CO has
[13,14]). In all these studies an increase in reactivity under periodic
al Engineering 57 (2013) 180e187 181
the ideal case of an instantaneous temperature switching. The base
regime is the regime the reactor is subjected for the majority of the
time during the dynamic operation. It is characterized by mild
conditions with negligible overall reaction rate. In the base regime,
the transport of molecules to and from the surface can occur, so that
reactants are available on the surface. When the desired surface
concentrations are achieved, the system can be switched to the
pulsed regime. In contrast, the pulsed regime is characterized by
locally very high temperatures. The switch from the base regime to
the pulsed regime is realized by introducing a burst of energy at
speci\ufb01c locations within the reactor. The release of energy is so fast
that the system does not have time to reach new equilibrium
conditions during the switching time. Therefore, at the start of the
pulse the system has different surface concentrations than those
which are associated with high energy steady state conditions. In
locations where there is a high energy density there is plenty of
energy available for chemical reactions to be activated so the
reaction rates are locally high. Note that the bulk of the reactor
material is not subjected to this high energy density at all and
continuously remains at or close to the mild conditions of the base
regime even during the pulsed regime.
In pulsed activation, the additional operation parameters in the
form of pulse frequency and the amplitude (energy content of pul-
ses) are introduced. Also, the reactor conditions