(144f) Spatiotemporal Behavior of Multifunctional Pt/Pd/Zeolite-Beta Diesel Oxidation Catalysts for Low-Temperature Oxidation of Hydrocarbon Mixtures

Spatiotemporal Behavior of Multifunctional
Pt/Pd/Zeolite-BETA Diesel
Oxidation Catalysts for Low-Temperature Oxidation of Hydrocarbon Mixtures

Gregory S. Bugosh, Rachel L.
Muncrief, Michael P. Harold

Department of Chemical and Biomolecular Engineering, University of Houston, Houston,
TX

The first
aftertreatment reactor for diesel engine exhaust is typically the Diesel
Oxidation Catalyst (DOC). The primary function of the DOC is the efficient,
complete oxidation of engine-out carbon monoxide (CO) and unburned hydrocarbons
(HCs). A mixture of Pt and Pd is an effective oxidation catalyst but challenges
remain during low temperature operation, which is the trend for emerging
emissions regulations. The DOC operates at very high conversion once sufficient
temperature has been achieved. However, during the ?cold start? most of the
hydrocarbons pass through the DOC unreacted and tailpipe emissions may exceed
allowable limits. New diesel engines designed to produce less thermal NOx can result
in lower exhaust temperatures, compounding the challenge. In addition, some newer
drive cycles include periods of low engine load that can lead to the DOC falling
below the light-off temperature (quenching) and subsequently higher hydrocarbon
emissions. 

One
approach to solving the low-temperature hydrocarbon emission problem is to
incorporate a zeolitic component into the DOC washcoat which serves as a trap
of higher molecular weight hydrocarbons emitted at low temperatures below their
ignition temperature.  This transient
approach stores the hydrocarbons until the exhaust temperature increases to
sufficient level to cause their release and light-off. While many commercial
DOCs contain a zeolite hydrocarbon trapping component, much of the research has
been proprietary. There are relatively few studies in the open literature
exploring fundamental issues regarding hydrocarbon trapping and its effect on
DOC operation.

In this
study we conduct a systematic study of the spatiotemporal behavior of a series
of model Pt/Pd/Zeolite DOC catalysts.  The
catalysts contain varying precious metal (0 to 120 g/ft³Pt/Pd) and zeolite-Beta content
(0 to 1 g/in³). The objectives are to (i) utilize spatially-resolved
mass spectrometry to measure the transient profiles of reacting species, (ii)
elucidate the kinetic and transport coupling associated with the
multi-component catalysts, and (iii) provide data for model development, and
subsequent structured DOC and reactor design and optimization.

A
dedicated bench-flow reactor system was developed for this study comprising
mixture flow control, the reactor, and combined mass spectrometry and FTIR
spectroscopy. Model hydrocarbon adsorption/desorption and light-off
temperatures (T₅₀) were examined using a temperature ramp. Experiments
to determine steady-state oxidation reaction kinetics were also carried
out.  Reaction rates showed negative
order dependence as increased reactant concentration reduced the rate of
oxidation at steady-state temperatures.  This
result is also revealed through light-off temperatures, which are elevated as
reactant concentration is increased. Steady-state propene oxidation conversion for the 60 g/ft³
PGM, no zeolite sample are shown in Figure 1.

Spatially
resolved mass spectrometry measurements were performed with the use of a quartz
capillary inserted along the channel of the monolith (Fig. 2). These local
concentration measurements provide essential information about the reaction
progression along the catalyst length. At lower temperatures when the reaction
is kinetically limited, the negative order dependence on reactant concentration
is observed, especially in the front section of the monolith. The reaction rate
for the 50 ppm propene reactant feed is faster (more reactant consumed) in the
front 1 cm of the monolith channel. When the 50 ppm propene feed is nearly used
up, then the higher concentration feeds begin to catch
up in terms of total reactant consumed (Figure 3).

Fig. 1. 60 g/ft³ PGM, no
zeolite, space velocity ≈ 56,000 hr^-1, 10% oxygen (balance
argon), propene feed = 50, 100,200, 400 ppm.

Fig. 2. Depiction of experimental setup for spatially
resolved measurements.

Fig. 3. Sample of
spatially resolved measurements for propene oxidation.