Catalyst preparation

A novel Al₂O₃/CeO₂/ZrO₂ mixed oxide was developed in-house. The material was prepared by a co-precipitation method. For comparison purposes, a conventional CeO₂/ZrO₂ mixed oxide was prepared following the same co-precipitation method, then blended with γ-Al₂O₃ powder. Pd-loaded catalyst powders were made by impregnating Pd onto the Al₂O₃/CeO₂/ZrO₂ mixed oxide and the (Al₂O₃ + CeO₂/ZrO₂) mixture, respectively. The Pd loading was 1 wt% in both samples. Fully formulated TWCs were prepared using the Al₂O₃/CeO₂/ZrO₂ mixed oxide and the (Al₂O₃ + CeO₂/ZrO₂) mixture, respectively, as the Pd support. Full experimental details and characterisation data are published elsewhere (6).

Light-off Activity of Palladium Catalysts Supported on Al₂O₃/CeO₂/ZrO₂ Mixed Oxide

The Al₂O₃/CeO₂/ZrO₂ mixed oxide and the (Al₂O₃ + CeO₂/ZrO₂) mixture were evaluated as support materials for Pd. The Pd-impregnated catalysts were redox aged at 1050°C for 36 hours prior to the tests. Light-off activity of the aged catalysts was measured in a gas mixture containing a stoichiometric amount of HC/CO/NO/O₂ without perturbation and the results are plotted in Figure 1.

The Al₂O₃/CeO₂/ZrO₂ mixed oxide supported Pd catalyst shows rapid light-off for HC/CO and reaches 100% conversion at temperatures above 320°C. As a comparison, the (Al₂O₃ + CeO₂/ZrO₂) mixture supported Pd catalyst shows more gradual HC/CO light-off, and does not reach 100% conversion until the temperature goes above 390°C. As Pd catalysts are in general relatively inactive for NOx reduction, it is not surprising that neither of the two catalysts show appreciable NOx conversion in these tests. In a fully formulated TWC, a separate Rh component will be incorporated into the formulation to enhance the NOx performance. Nevertheless, the Al₂O₃/CeO₂/ZrO₂ mixed oxide supported Pd catalyst is clearly more active than the (Al₂O₃ + CeO₂/ZrO₂) mixture supported Pd catalyst and is therefore considered more suitable for applications with low exhaust temperatures.

To understand why the Al₂O₃/CeO₂/ZrO₂ mixed oxide supported catalyst has better light-off activity, the 1050°C/36 h redox aged powder catalysts were analysed by transmission electron microscopy (TEM). The dark field images together with the elemental images of Ce and Pd are shown in Figure 2. Comparing the two dark field images in combination with the Ce elemental images, it is apparent that the CeO₂/ZrO₂ mixed oxide particles on the Al₂O₃/CeO₂/ZrO₂ sample are about one order of magnitude smaller than the CeO₂/ZrO₂ mixed oxides in the (Al₂O₃ + CeO₂/ZrO₂) sample. The elemental images of Pd further indicate that high Pd dispersion is maintained on the 1050°C/36 h redox aged Al₂O₃/CeO₂/ZrO₂ sample, whereas noticeable Pd sintering has occurred on the (Al₂O₃ + CeO₂/ZrO₂) sample as evidenced by the two large Pd particles in the image.

The results above suggest that depositing CeO₂/ZrO₂ mixed oxides directly on alumina supports can minimise the sintering of the CeO₂/ZrO₂ mixed oxides, hence improving their thermal stability and OSC properties. The material can also maintain Pd in high dispersion even after severe ageing. All these features contribute to the superior light-off activity of the Al₂O₃/CeO₂/ZrO₂ supported Pd catalyst.

Engine Dynamometer Evaluation of TWC Supported on the Al₂O₃/CeO₂/ZrO₂ Mixed Oxide

A TWC was formulated using the Al₂O₃/CeO₂/ZrO₂ mixed oxide as the Pd support in combination with a Rh component. As a reference, a conventional TWC using the (Al₂O₃ + CeO₂/ZrO₂) mixture as the Pd support and the same Rh component was also prepared. Both catalysts were coated on ceramic monolith substrates with a cell density of 62 cells cm^–2 and a wall thickness of 64 μm (or 400 cpsi and 2.5 mil). The dimension of the substrates is 10.6 cm in diameter and 7.8 cm in length. The pgm loadings of the catalysts were kept at relatively low levels (1.34 g l^–1 Pd and 0.07 g l^–1 Rh) to better differentiate their performance. The catalysts were aged on a 4.6 l gasoline engine under 4-mode conditions for 100 hours with the catalyst bed temperatures averaging 925°C. The aged catalysts were evaluated on a separate 4.6 l gasoline engine that was capable of changing the air-to-fuel ratio from 13.5 to 15.5 with a perturbation frequency of 1 Hz and amplitude of 0.5. The CO/NOx crossover conversion and the corresponding HC conversion measured at a space volume of 112,000 h^–1 at 400°C and 350°C are summarised in Table I. At 400°C, both catalysts achieve high NOx/CO/HC conversions. At 350°C, the Al₂O₃/CeO₂/ZrO₂ mixed oxide supported catalyst still maintains high NOx/CO/HC conversion efficiency. The (Al₂O₃ + CeO₂/ZrO₂) mixture supported catalyst, however, is nearly inactive.

The engine dynamometer evaluation results on fully formulated TWCs are in line with the laboratory reactor results on the supported Pd powder catalysts. Both demonstrate that the Al₂O₃/CeO₂/ZrO₂ mixed oxide developed in this study can offer significant advantages for applications with low exhaust temperatures.

Vehicle Evaluation of TWC Systems Based on the Al₂O₃/CeO₂/ZrO₂ Mixed Oxide

A 2010 model year vehicle equipped with an advanced 3.5 l GTDI engine and a turbo charger was selected to evaluate the performance of a TWC system based on the Al₂O₃/CeO₂/ZrO₂ mixed oxide. Compared to other vehicles in the same class with traditional naturally aspirated engines, this vehicle represents approximately 20% better fuel efficiency and 15% reduction of GHG emissions. As a result, the exhaust temperature of the vehicle is also substantially lower.

Catalyst systems with either the Al₂O₃/CeO₂/ZrO₂ mixed oxide or the (Al₂O₃ + CeO₂/ZrO₂) mixture as the Pd support were evaluated. Prior to vehicle evaluation, the TWC systems were aged under 4-mode conditions to simulate the end of their useful life performance. The non-methane hydrocarbon (NMHC) and NOx emissions of the aged systems under federal test procedure (FTP) testing cycles are summarised in Table II. While the two systems show comparable NOx performance, the Al₂O₃/CeO₂/ZrO₂ mixed oxide based system clearly demonstrates better HC conversion and 7 mg mile^–1 lower NMHC emissions from the tailpipe.

The cumulative tailpipe total HC (THC) emissions of the two systems during the cold start period are shown in Figure 3. The majority of the THC is emitted in the initial 250 seconds while the temperature of the catalyst system is warming up. During this period, the Al₂O₃/CeO₂/ZrO₂ mixed oxide based TWC system has approximately 25% lower tailpipe HC emissions than the (Al₂O₃ + CeO₂/ZrO₂) mixture based system.