(484c) Syntheses of Pd Doped CaCoxZr1-XO3-? novel Perovskite-Type Redox Materials for Automotive Emissions Control

Syntheses of Pd doped CaCo_xZr_1-xO_3-dnovel perovskite-type redox materials for automotive
emissions control

Qinghe Zheng and Marty Lail*

Energy Technology Division, RTI International,
Durham, NC 27709, United States

*mlail@rti.org

Introduction

        In recent years the family of rare
earths, including cerium, experienced a rapid increase in price and decrease in
availability, upsetting the markets and initiating a search for methods to
reduce their concentrations in the oxygen storage materials (OSMs). Meanwhile,
with the rapid increase of vehicle population and more stringent automotive
emissions regulation, which requires as high as 96% conversion of all
pollutants, improvements of three-way catalyst (TWC) performance by broadening
operation window of the stoichiometric air-to-fuel ratios are of high
interest.  Furthermore, TWC, including
the active metal (PGM metals) and the support materials (with Ce_xO_y-ZrO₂-type
OSM incorporated), may experience deactivation under fuel cut-off, an engine
mode for enhanced fuel economy but exposes catalyst to high temperature (c.a.
> 1000 ^oC) lean oxidative condition.

        Perovskite-type materials, with
general chemical composition of ABO_3-d, where A and B are cations that may have multiple valence states,
can exhibit significant swings in oxygen non-stoichiometry d through the reaction with gas phase oxygen. Perovskites possess not only
excellent redox property but also high chemical/structural stability.
Preliminary development of perovskite-type OSMs by selecting transition metals
with low cost, large abundance and availability, remarkable redox properties,
and good thermal stability would be a reasonable research strategy. Further
research effort would require the development of advanced TWCs by incorporation
of PGM group metals into the novel perovskite structures with outstanding OSC
capacity. Enhanced catalyst activity and stability were previously reported by
incorporating noble metals into the perovskite structures, which stabilize the
metal against sintering, reaction with the support, and volatilization.

        The present
study focuses on a systematic screening of advanced perovskites as potential substitutes
for commercial CZO in TWC. Novel perovskite-type materials CaCo_xZr_1-xO_3-dwere
synthesized for the first time and reported here to show outstanding redox
property and oxygen mobility than the current state-of the-art perovskite
materials and the commercial CZO. Pd-doped CaCo_xZr_1-xO_3-d showed profound catalytic activity towards NMHC
and CO oxidation. The study
aims at providing a comprehensive comparison of the oxygen storage/release
capacities of the current state-of-the-art perovskite materials, and preliminary
results for the development of the performing perovskite materials in
automotive applications.

Experimental

        CaCo_xZr_1-xO_3-d and other perovskites
were synthesized using Pechini method. CO- and H₂-
temperature programmed reduction (CO- and H₂-TPR) measurements, and
dynamic CO-air redox cyclic tests were performed to examine the CO and oxygen
storage capacities of the as-synthesized materials, in comparison with the
commercial CZO and CeO₂. X-ray fluorescence (XRF), X-ray diffraction
(XRD), and BET surface area measurements were used to characterize the studied
materials. The kinetic data of the oxygen intake of the best performed perovskite
materials is included. The HC
and CO catalytic oxidation activities of perovskiteswith different amount of Pd
doping were evaluated under simulated automotive exhaust conditions.

Results and Discussion

        H₂-TPR
profiles of the as-synthesized perovskites and commercial ceria-based oxygen
storage material (OSM) samples are presented by Figure 1(a). The novel CaCo_xZr_1-xO_3-dperovskites exhibit higher reducibility than
commercial Ce-based OSMs,
as represented by higher H₂ consumption (4.1~13.4 times higher) at
lower reduction temperatures (50~250 ^oC
lower). Specifically, the H₂ consumption of Co_xZr_1-xO_3-d increases with Co content. Doping of Pd into the perovskite structure further enhances the
reducibility by lowering the reduction temperature. Catalytic oxidation activities of Pd doped perovskite OSMs are shown in Figure 1(b) and 1(c).
Pd doped Co_xZr_1-xO_3-d exhibit
excellent activity for CO and HC (C₃H₆) oxidation.
Complete combustion of CO and HC was observed. The T₅₀ for C₃H₆
oxidation was as low as 240 °C. The HC oxidation activities of all the Pd doped CaCo_xZr_1-xO_3-d TWCs are comparable to, if
not higher than, state-of-the-art TWCs. These promising data clearly confirm
the excellent potential for the proposed perovskite TWCs.

Figure 1. (a) H₂
consumption as a function of reduction temperature during H₂-TPR for
CaCo_xZr_1-xO_3-d –type perovskites with or
without Pd doping, in comparison with commercial CeO₂
and CeO₂-ZrO₂ OSMs. Catalytic oxidation activity of (b)
CO and (c) C₃H₆ using simulated exhaust feed over CaCo_0.55Zr_0.4Pd_0.05O_3-d perovskite-type TWC. The
simulated exhaust feed contains 600-660 ppmv C₃H₆,
0.75-0.825 vol-% CO, 0.675-0.75 vol-%
O₂, 19.5 vol-% CO₂, balanced N₂,
at WHSV=16.69 h^-1. The catalytic oxidation activity was measured
under different stoichiometric numbers (SN) calculated as SN=2 [O₂]
/ ([CO] + 9 [C₃H₆]). 

Selected References

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[2] Tanaka,
H.; Taniguchi, M.; Uenishi, M. et al. Angew. Chem. Int. Ed.
2006, 45 (36), 5998–6002.

[3]
You, R.; Zhang, Y.; Liu, D. et al. J.
Phys. Chem. C 2014, 118 (44), 25403–25420.