Enhanced NO, CO and C₃H₆
Conversions on Precious Group Metal Catalysts: Impact of Oxygen Storage
Material and 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 simultaneous abatement of NO, CO and C₃H₆
over a series of precious group metal (PGM)-OSM catalysts with
near-stoichiometric feed under steady-state and lean-rich modulation conditions.
The studied OSMs include the model spinel Mn_0.5Fe_2.5O₄
and conventional zirconia stabilized ceria (CZO). The results show that NO, CO,
C₃H₆ conversions are enhanced by the addition of OSM (either
CZO or spinel). While the single layer design with direct deposited PGM onto
OSM results in the best performance for CZO, the dual-layer design is the best
for spinel. 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 and
PGM-OSM catalysts. The studied OSMs include spinel (Mn_0.5Fe_2.5O₄)
and CZO. The PGM-only monolith sample had a single layer formulation with
PGM dispersed on alumina support.   Two 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 each of
the samples the PGM loading is 30 g/ft³
with a Pt:Pd ratio of 19:1. For PGM-OSM catalysts, the OSM layer had a loading
of 100 g/L with the composition of 22.5 wt% OSM/Al₂O₃. For
brevity, the PGM-only sample was denoted as “PA”. The PGM-spinel samples with
single or dual layer design were denoted as “PSS” or “PSD” respectively. The
PGM-CZO samples with single or dual layer design were denoted as “PCS” or “PCD”
respectively.

The steady stoichiometric
feed had a space velocity of 70k hr^-1 and a feed composition of 500
ppm NO, 0.17% H₂, 0.52% CO, 400 ppm C₃H₆, 0.5%
O₂, 7% CO₂, 7% H₂O and the balance Ar. For
modulated feeds, the stoichiometric number S_N (S_N=) was varied with an amplitude of ±0.018 and a cycle timing was 2s
lean/ 2s rich. A FTIR (Thermo Scientific, Nicolet 6700) measured the effluent
concentrations in the bench reactor system.

Figure 1 shows the cycle-averaged NO
and C₃H₆ conversion over PGM-only (PA) and PGM-CZO (PCD
and PCS) samples using the stoichiometric steady feed from 200°C to 500°C. The results
obtained with PA are used as a reference for CZO-containing samples. Both PCD
and PCS have a lower NO and C₃H₆ light-off, confirming
the promotional impact from CZO. In comparison to PA, the promotion from
dual-layer design is rather limited, with a ~ 25 °C lower T₅₀ value (temperature
giving 50% conversion) for both NO and C₃H₆ conversion. Further,
the single-layer design with direct PGM deposition on CZO results in a dramatic
decrease (~ 100 °C)
for NO and C₃H₆ light-off temperature. Although not shown
here, the CO conversion is enhanced by addition of CZO and the single layer
design (PCS) provides the largest promotional impacts.

Figure 1. Cycle-averaged NO and C₃H₆
conversion as a function of feed temperature on CZO 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].

Figure 2 shows the cycle-averaged NO
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.
Similarly, the results obtained on PA is used as a reference. The addition of
spinel (PSD and PSS) promotes NO and C₃H₆ light-off,
suggesting the promotional impact from spinel. In comparison to PA, a ~ 50 °C lower T₅₀
value is achieved for both NO 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 NO 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 NO 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 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)