H.-W. Jen

Characterization of model automotive exhaust catalysts: Pd on ceria and ceria–zirconia supports

Model Pd automotive three‐way catalysts were prepared with high‐surface‐area Zr‐rich ceria–zirconia powders as support materials, aged for 12 h at 1050°C under redox conditions simulating automotive exhaust gases, and characterized by a combination of techniques including oxygen storage capacity (OSC) measurements. Differences in OSC amongst aged catalysts made with materials of similar ceria–zirconia compositions, but produced by different processes, were weakly correlated with differences in support surface area. Evidence of encapsulation of Pd particles was found in most of the aged catalysts. The promotional effect of zirconia on ceria reducibility, well known from previous temperature‐programmed H2 reduction studies, was apparent in Ce 3d core‐level spectra from X‐ray photoelectron spectroscopy measurements, which showed that both X‐ray‐induced and H2 reduction become increasingly facile with increasing zirconia content. A slight Ce enrichment at the surface of the fresh catalysts made with the more Zr‐rich powders was found to increase upon aging.

NO2 formation over Cu-ZSM-5 and the selective catalytic reduction of NO

The extent of the selective catalytic reduction (SCR) of nitric oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO2 intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the NO2 itself is reduced only back to NO. Thus, for the selective reduction of NO2 to N2 (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O2 reaction to form NO2 parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al2O3 or on H-ZSM-5. The coppercontaining zeolite is also active in the decomposition of NO2 back to NO and O2 while the other catalysts are much less active. The inhibiting effect of water on the NO + O2 catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an NO2 intermediate.

Coarsening of Pt particles in a model NOx trap

The effects of temperature and atmosphere on the coarsening of Pt particles in a model NOx trap (Pt/BaO/Al2O3) were examined by XRD, TEM, and CO chemisorption. The main finding, that the most significant particle growth occurs at elevated temperatures under oxidizing conditions, is relevant to NOx trap durability.

Characterization of model automotive exhaust catalysts: Pd on Zr‐rich ceria–zirconia supports

Model Pd automotive three‐way catalysts were prepared with high‐surface‐area Zr‐rich ceria–zirconia powders as support materials, aged for 12 h at 1050°C under redox conditions simulating automotive exhaust gases, and characterized by a combination of techniques including oxygen storage capacity (OSC) measurements. Differences in OSC amongst aged catalysts made with materials of similar ceria–zirconia compositions, but produced by different processes, were weakly correlated with differences in support surface area. Evidence of encapsulation of Pd particles was found in most of the aged catalysts. The promotional effect of zirconia on ceria reducibility, well known from previous temperature‐programmed H2 reduction studies, was apparent in Ce 3d core‐level spectra from X‐ray photoelectron spectroscopy measurements, which showed that both X‐ray‐induced and H2 reduction become increasingly facile with increasing zirconia content. A slight Ce enrichment at the surface of the fresh catalysts made with the more Zr‐rich powders was found to increase upon aging.

Encapsulation of Pd particles by ceria-zirconia mixed oxides

Redox aging above 1000°C of initially high-surface-area (of order 100 m2/g) cerium-rich ceria-zirconia mixed oxides containing a few wt% Pd results in the loss of most of the surface area and concomitant encapsulation of a substantial fraction of the Pd. The encapsulated Pd exists in the form of metallic particles on the order of 10 nm in diameter held under a large (as much as 3.6 GPa) compressive stress by the mixed oxide. This stress, evidently arising from changes in lattice parameter of the mixed oxide, induced by aging, produces a volume contraction of Pd, which appears as a shift of Pd-peak positions in XRD measurements. Subsequent reduction and reoxidation of the mixed oxide in turn relieves and restores the stress on the Pd particles, which remain metallic throughout, demonstrating that this portion of the Pd is effectively lost for catalytic purposes.

Leaching of Ba2+ in NOx Traps

It has been found that crystalline BaCO3 replaces crystalline BaAl2O4 in prototypical NOx traps upon exposure to liquid water. This phenomenon can be understood in terms of acid–base chemistry, where Ba2+ is leached from the aluminate, precipitating as a carbonate, under the influence of weak carbonic acid naturally present due to the equilibrium between CO2 in the air and the water. Consequences of such a redistribution of Ba2+ in a NOx trap could include a decrease in surface area of the trapping material and a loss of contact between the trapping material and the catalytic components, such as Pt. Indeed, preliminary testing has shown that exposure of fully formulated NOx traps to liquid water results in the loss of approximately half of the initial trapping capacity.

Chemisorption of alkenes on copper-exchanged ZSM-5 zeolite

On Cu-ZSM-5 zeolite at room temperature, propane is physisorbed, while propene shows characteristics of chemisorption. The chemisorption mode has certain advantages for the catalytic control of automotive emissions. Alkane and alkene adsorption equilibria and kinetics are compared on Cu-ZSM-5 and on ZSM-5 free of alumina. The results are discussed in terms of the Langmuir adsorption isotherm.

Effect of SO2 on the oxygen storage capacity of ceria-based catalysts

We have examined the effect of SO2 poisoning on a series of catalysts having Pd supported on ceria, alumina, and ceria–zirconia. For pre-exposure of 20 ppm SO2 at 673 K, we observed no changes in the light-off curves for CO oxidation on Pd/alumina. This pre-exposure of SO2 to Pd/ceria resulted in a significant upward shift in the light-off curve, so that the poisoned Pd/ceria catalyst exhibited similar rates to that of Pd/alumina. Similar upward shifts were observed for the water–gas-shift reaction upon exposure of Pd/ceria or Pd/ceria–zirconia samples to SO2. However, pulse-reactor data with alternating CO and O2 pulses showed that SO2 poisoning actually increased the amount of oxygen that could be transferred to and from the catalyst over the entire temperature range that was examined. The implication of these results for understanding the effect of SO2 poisoning and the measurement of OSC are discussed.

Partial encapsulation of Pd particles by reduced ceria-zirconia

Direct observation of metal-oxide interfaces with atomic resolution can be achieved by cross-sectional high-resolution transmission electron microscopy (HRTEM). Using this approach to study the response of a model, single-crystal thin film automotive exhaust-gas catalyst, Pd particles supported on the (111) ceria-zirconia (CZO) surface, to a redox cycle, we have found two distinct processes for the partial encapsulation of the Pd particles by the reduced CZO surface that depend on their relative crystallographic orientations. In the case of the preferred orientation found for Pd particles on CZO, Pd(111)[110]∕∕CZO(111)[110], a flat and sharp metal/oxide interface was maintained upon reduction, while ceria-zirconia from the adjacent surface tended to accumulate on and around the Pd particle. In rare cases, Pd particles with other orientations tended to sink into the oxide support upon reduction. Possible mechanisms for these encapsulation processes are proposed.

Aging-Induced Metal Redistribution in Bimetallic Catalysts

Alloy particles were detected by XRD in bimetallic catalysts, made from physical mixtures of powders comprising distinct metal-support combinations (e.g., Pd on alumina and Rh on ceria-zirconia), following high-temperature redox aging. The morphology of the catalysts was examined by TEM, and the compositional structure of some of the alloy particles was determined. Two different effects of the redistribution of metals on oxygen-storage capacity were identified, one due to Pd enrichment of the surface of Pd-Rh alloy particles and the other due to loss of contact between metal and ceria-zirconia.