(608f) Elucidation of Heterogeneous Catalysis Reaction Mechanisms Using a SCT Shock Tube Reactor

Elucidation of heterogeneous catalysis reaction
mechanisms using a SCT shock tube reactor

Hope Connolly,
Marco Castaldi

Columbia University, New York, NY 10027 (USA)

hhc2119@columbia.edu

With increasing energy demands and awareness of global
climate change, it has become necessary to develop methods to reduce the impact
of fossil fuels while the hydrogen economy is developed for full-scale
implementation. Crucial to this new fossil fuel approach are two reactions: 1)
catalytic partial oxidation of methane to syngas and 2) reforming of carbon
dioxide by methane instead of steam. Both of these reactions produce syngas, a
combination of hydrogen and carbon monoxide of importance as a hydrogen source
for fuel cells. The use of a SCT shock tube reactor at is being used to study
these fast heterogeneous reactions in order to overcome the shortcomings of
previous studies which have yet to confirm detailed reaction mechanisms. It is
located at ATK/GASL in Ronkonkoma, NY. In the reactor, a fast-acting valve is
remotely released, allowing the high-pressure N2 driver gas to propagate a shock
wave through the reaction test gas mixture surrounding the SCT catalyzed
screen. A rarefaction wave quickly follows, bringing the temperature and
pressure down as quickly as it was increased, and quenching the intermediates
for detection and identification.

First, this set-up allows analysis of
the entire catalytic cycle of both surface reactions and gas-phase products and
intermediates. In pure surface and continuous flow systems, short lived
intermediates released from the surface may react and not be observed in the
final product gases. Secondly, the reactor looks at the entire surface in aggregate,
not limited to a small subsection of the catalyst surface. Furthermore, it
operates under actual reacting temperatures and pressures, enabling insight for
easy scale-up to a flow reactor. Lastly, the SCT shock tube reactor is able to
carry out experiments without transport effects. The microsecond step change
induced by the shock wave and the 3.64x10⁵ K/s¹⁷ quenching time
eliminates the reaction of unreacted material downstream. Precise control of
temperature, pressure and reaction time allows previously unseen insight into
the progression of the reaction, by quenching the reaction at varying stages of
completion. Wiesz-Prater criterion and Damkohler numbers have been analyzed to
ensure that operations are taking place in a kinetically-controlled regime and
without external diffusion mass transfer resistance, respectively.

Initial tests have shown 3 promising conclusions.
First, a shock wave passed through the uncatalyzed SCT mesh screen element
showed no signs of attenuation, maintaining its shape and strength throughout
the interaction. Proof of the robustness of the mesh screen and the absence of
deterioration of the shockwave were necessary before moving on to further
experiments. Next, we passed a test gas mixture (60% nitrogen, 20% oxygen, 20%
methane) and an inert gas through both a catalyzed and uncatalyzed
Pt/γ-alumina SCT element. Only the reactive test gas and the catalyzed
screen combination exhibited a prolonged pressure increase, indicative of the
predicted reaction taking place. Finally, we observed a kinetic time delay of
0.005 s, similar to the calculated kinetic time of 0.0051 s. This delay is
shorter than a homogeneous ignition delay.

        This
presentation will show results from reforming CH₄-CO₂ mixtures.
The mixture range was varied from 20% CH₄ to 70% CH₄
(balance CO₂) in reactant concentration at temperatures between 500 and
1000K for pressures spanning 1 to 10 atm.  Product gas composition as a
function of temperature, mixture concentration and pressure will be discussed
with an emphasis on a reaction sequence understanding.