Enhanced CO and C₃H₆
Conversions on Spinel Catalysts:

Impact of Catalyst
Architecture

Zhiyu Zhou, Michael P. Harold^*, Dan Luss^**

Department of
Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204

*
mharold@uh.edu, ** dluss@central.uh.edu

Three-way catalysts (TWC)
are used in gasoline vehicles to simultaneously eliminate CO, NOx and
hydrocarbons. Ceria has been widely utilized in TWC as the oxygen storage
material (OSM) for enhanced oxidation (of CO, H₂, HC)
and reduction (of NO and NO₂) since its introduction by Ford [1]. Recently researchers [2] reported that spinel-type mixed
metal oxides exhibit excellent oxygen storage capacity (OSC) and can reduce
precious group metal (PGM) loading requirements. We studied in a bench scale
reactor the abatement of CO and C₃H₆ over a series of (PGM)-OSM catalysts with near-stoichiometric feed under
steady-state and lean-rich modulation conditions. Mn_0.5Fe_2.5O₄was used as the model spinel and conventional zirconia stabilized ceria
(CZO) was used as a reference. The results show that the addition of spinel
leads to the enhanced CO and C₃H₆ conversion, especially
for C₃H₆. The dual-layer design (top PGM layer and bottom
spinel layer) results in the best performance while the single layer design
with direct deposited PGM onto OSM is the best for CZO. The study provides insight into the beneficial impact of oxygen storage
material and provides guidance for optimization of TWC catalyst formulation.

Monolith samples were provided by
CDTi Advanced Materials Inc., including PGM-only,
OSM-only and PGM-OSM catalysts. The studied OSMs include spinel (Mn_0.5Fe_2.5O₄)
and CZO. Two monolith different architecture PGM-OSM monolith samples were
used: the first is a single layer with PGM dispersed on OSM; the second is a
dual-layer with a top PGM layer and a bottom OSM layer. For brevity, the PGM-only sample was denoted as “PA” and the spinel-only
sample was denoted as “SA”. The PGM-spinel samples with single or dual layer
design were denoted as “PSS” or “PSD” respectively. All catalysts were pre-aged
in static air at 700 °C for 33 hrs before testing.

 A series simple
stoichiometric feed was used to study the light-off behavior of CO and C₃H₆.
Also, another complex stoichiometric feed was used to mimic the realistic
exhaust conditions. The complex feed had the composition of 500 ppm NO, 0.17% H₂,
0.52% CO, 400 ppm C₃H₆, 0.5% O₂, 7% CO₂,
7% H₂O. Both feeds had a space velocity of 70 k hr^-1. A
FTIR (Thermo Scientific, Nicolet 6700) measured the effluent concentrations in
the bench reactor system.

Figure 1 shows the C₃H₆
light-off curve over either PGM-only (PA) or spinel-only (SA) catalysts in a
simple stoichiometric feed with a ramp rate as 10 °C/min. Both SA and PA
result in similar T₅₀ value (temperature giving 50% conversion) at ~
400 °C.
In comparison with SA, a ~ 40 °C
lower T₈₀ value (temperature giving 50% conversion) is achieved for
PA. Although not shown here, the oxidation of C₃H₆ over
spinel is unaffected by the presence of CO while the oxidation of C₃H₆
over PA is inhibited by CO.

Figure 1. Propylene conversion as a
function of feed temperature on spinel-only and PGM-only catalysts [Conditions:
ramp rate: 10 C/min; feed: 400ppm C₃H₆, 0.18% O₂,
balance Ar].

Figure 2 shows the cycle-averaged CO
and C₃H₆ conversion over PGM-only (PA) and PGM-spinel
(PSD and PSS) samples using the stoichiometric steady feed from 200°C to 500°C.
The results obtained on PA is used as a reference. The addition of spinel (PSD
and PSS) promotes CO and C₃H₆ light-off, confirming the
promotional impact from spinel. In comparison to PA, a ~ 50 °C lower T₅₀
value is achieved for both CO and C₃H₆ conversion with
the single-layer design (PSS). The promotional impact from spinel is more
exaggerated with the dual-layer design, resulting in a ~ 70 °C lower T₅₀
value for both CO and C₃H₆ conversion. Although not shown
here, CO conversion is enhanced by addition of spinel and the dual layer design
(PSD) provides the largest promotional impacts.

Figure 2. Cycle-averaged CO and C₃H₆
conversion as a function of feed temperature on spinel contained samples with
PGM-only sample as reference. [Conditions: steady-state feed: 500ppm NO, 0.17%
H₂, 0.52% CO, 400 ppm C₃H₆, 7% CO₂,
7% H₂O, balance Ar].

The reactor data will be supplemented
with measurements of the cumulative and dynamic oxygen storage capacity of the
materials. The collective results will be interpreted with a phenomenological
mechanism.

Reference

1.      H.C. Yao, Y.F. Yu Yao, Journal of Catalysis, 86 (1984) 254-265

2.      S. Golden, Z. Nazarpoor, M.
Launois, R-F. Liu, P. Maram, Society of Automotive
Engineering. SAE 2016-01-0933 (2016)