(304d) Spatiotemporal Studies of Methane Conversion On Pt/Pd Catalytic Monoliths

Spatiotemporal
Studies of Methane Conversion on Pt/Pd Catalytic Monoliths

Gregory S. Bugosh, Michael P. Harold

University of Houston

Houston, TX 77204

The current price advantage toward
natural gas over petroleum is leading to increased utilization to power
vehicles with compressed natural gas (CNG) and liquefied natural gas (LNG).
Methane is the primary component of natural gas and some small but significant
amount of it survives the combustion chamber and ?slips? into the exhaust.
Among the hydrocarbons, methane (CH₄) is particularly difficult to oxidize
due to its low reactivity. Since methane is a greenhouse gas with global
warming potential 20 times that of carbon dioxide (CO₂), work must
be done to ensure its emission is at low as practicably possible. Moreover,
methane emissions have historically been ignored in the design and operation of
the three-way catalytic converter. When methane conversion is added to the list
of required reactions, this necessarily changes the design and operation of
conventional emission control devices and will complicate the behavior of
emerging emission control systems.

In
the current study we examine the conversion of methane in model feeds
representative of exhaust mixtures of a natural gas powered vehicle run under stoichiometric
or lean feed conditions.  Many
fundamental questions are raised about the performance and design of precious
metal catalysts for methane oxidation.  The
experiments were carried out in a bench-flow reactor system equipped with a
mass spectrometer having spatially resolved sampling.  This technique enables measurement of
chemical species concentration profiles along the length of the monolith
channel.

During
scouting measurements of the oxidation of hydrocarbons with a diesel oxidation
catalyst (DOC) comprising Pt/Pd/Al₂O₃washcoated
monolith, an increase in hydrocarbon conversion was observed with decreasing O₂
concentration near the stoichiometric point. 
 A peak in the methane conversion occurred
at intermediate O₂ feed concentration, which interestingly exceeded
the methane conversion in excess O₂ (10%).  The conversion peak could be shifted by
varying the oxygen and co-reductant (CO) concentrations.  However, when O₂ levels were
decreased below a point there was not enough available to complete the oxidation
and the conversion declined.  Similar
results had been reported in the early 90's when investigating natural gas
engine emission aftertreatment at GM [1] and at Ford [2]. The peak in conversion
was explained to be a result of oxygen blocking the access of methane to the
active catalyst sites as inlet oxygen concentrations increase. The existence of
the methane conversion maximum complicates the design of a catalyst capable of
achieving a high methane conversion over a wide range of conditions.

In
order to better understand the origin of the peak in methane conversion at low O₂
concentration, spatially resolved profiles were obtained. The spatial
dependence of the reacting species provides new insight about the coupling between
the conversions of CH₄ and CO in stoichiometric levels of O₂.
Typical results are shown in Figure 1 for a feed gas containing 800 ppm CH₄,
2000 ppm CO, 2000 ppm O₂ and a feed temperature of 482^oC.
 The CO first reacts with the O₂,
which forms CO₂ leading to a reduction in the O₂
concentration. When the O₂ concentration is at a sufficiently low
level, CH₄ conversion commences with a notable increase in H₂
and CO, presumably formed by the partial oxidation of CH₄ and steam
reforming.  Since methane is the target
species for removal in this case, due to its low reactivity, the production of
H₂ and CO can be tolerated, since they can be easily removed in a
subsequent reactor with addition of air. Adjusting the O₂ and CO
feed concentrations, as well as the inlet gas temperature, results in different
concentration profiles, and this information is very helpful toward the
eventual design of a methane removal system.

These
and other integral conversion data will be presented over a range of feed
temperatures and compositions in order to elucidate the catalyst
performance.  Additional kinetics
experiments provide important information about the underlying mechanism. Supporting
unsteady state (dynamic) experiments will be described that address the methane
conversion under conditions encountered during warmup
and transient operation.

Fig.
1. Spatially resolved species measurements through the length of the monolith
channel.

References

1.     
 Oh et. al, Journal of Catalysis, 132, 287-301, 1991.

2.     
Subramanian
et. al, Ind. Eng. Chem. Res.,
31, 2460-2465, 1992.