Robert W. McCabe

Automotive exhaust catalysis

Abstract Research in the field of automotive exhaust catalysis has paralleled the broader growth in heterogeneous catalysis research—beginning in the 1960s, progressing through commercialization in the mid-1970s, and continuing today. The general trend has been one of increasingly complex catalyst formulations in response to increasingly stringent emission standards. Nowhere is this more evident than in the various means that have been employed to most effectively utilize the noble metal components. These efforts will continue, but with greater emphasis on optimizing catalyst formulations for lean-burn applications and reducing catalyst cost and complexity without sacrificing performance.

Twenty-five years after introduction of automotive catalysts: what next?

The union of catalysts and the automobile has been one of the greatest successes of heterogeneous catalysis over the last 25 years. Here, the history of automotive catalysis is briefly reviewed, followed by an assessment of where automotive catalysis stands today and where it is headed in the future. A key distinction between past automotive catalysis experience and that projected for the future is an increased focus on catalysts in upstream of power plant applications, such as on-board fuel processing units for fuel cell vehicles. Driven by ever tighter regulations, there will be continued research and development activity focused also on downstream applications (i.e. exhaust emission aftertreatment), especially for fuel-efficient, lean-burn vehicles, both diesel and spark-ignited.

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.

Characterization of phosphorus-poisoned automotive exhaust catalysts

Abstract Oil-derived contaminants on light-off catalysts from high mileage taxis were characterized by a variety of physical and chemical methods. The contaminants (mostly phosphates) deposit in a strong axial gradient from the front to the back of the monolithic catalyst channels. Two major forms of phosphorus contamination were observed: (1) an overlayer of Zn, Ca, and Mg phosphates, and (2) aluminum phosphate within the washcoat. The data also suggest the formation of cerium(III) phosphate. Laboratory catalytic reaction measurements on core samples from the taxi catalysts confirm a strong deactivating effect due to the phosphorus contamination. The activity improves dramatically after removing phosphorus via an oxalic acid wash.

Incorporation of La3+ into a Pt/CeO2/Al2O3 catalyst

The effects of La3+ incorporation into a Pt/CeO2/Al2O3 catalyst were investigated by a combination of activity, temperature-programmed reduction (TPR), oxygen storage capacity (OSC), noble-metal surface area, and X-ray diffraction (XRD) measurements. Incorporation of La3+ ions into the Al2O3, before CeO2 is added, promoted higher Pt and CeO2 dispersions. The oxygen storage capacity was also higher in the presence of La3+. This is attributed to a combination of Pt and CeO2 particle-size effects and possible blockage of the reaction between Al2O3 and CeO2. The XRD data show that La3+ forms LaAlO3 with Al2O3 and prevents α-Al2O3 formation after various heat treatments.

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.

Effect of calcination temperature on Al2O3-supported CeO2 : complementary results from XRD and XPS

Abstract X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been used to follow the effect of calcination temperature on CeO2 particle size, relative CeO2 concentration, and cerium oxidation state for CeO2 supported on γ-Al2O3. When the calcination temperature was increased from 500 to 900°C, XRD results indicated a decrease in the relative concentration of CeO2 at the highest loading (15 wt%), while XPS results reflected an apparent reduction of Ce4+toCe3+ at the lowest loading (5 wt%). Together these results suggest a transformation of a portion of the CeO2 particles to a more dispersed Ce3+-like species following high-temperature calcination. The reduction of ceria under these conditions is thermodynamically unfavorable. The apparent reduction of Ce4+ to Ce3+ upon calcination may be the result of an electronic effect, rather than a true reduction of the ceria.

NOx self-inhibition in selective catalytic reduction with urea (ammonia) over a Cu-zeolite catalyst in diesel exhaust

Abstract A NOx self-inhibiting effect was observed in the selective catalytic reduction (SCR) of NOx on a diesel engine over a Cu-zeolite catalyst with NH3 as the reductant (supplied either directly or as urea). The effect was strongest at low-temperatures (

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.