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Sodium promotion strongly enhances the performance of Pd/γ-Al2O3catalysts operated under simulated “three-way” conditions over a range of stoichiometry from fuel rich to fuel lean. The optimally promoted catalyst (7.0% w/w Na) exhibits a ∼50°C lower light-off temperature than the unpromoted sample when tested under stoichiometric feed conditions. Sodium promotion is also beneficial under both oxidising and reducing conditions. Under reducing conditions the alkali prevents poisoning by hydrocarbon deposits, therefore maintaining high NO conversions. Under oxidising conditions sodium significantly reduces the formation of N2O (nitrogen selectivity, SN2=80% over the optimally promoted sample compared to ∼45% over the unpromoted catalyst at 325°C). The improvement in activity and selectivity is understandable in terms of Na-induced changes in the adsorption strength of the various reactants on the Pd surface, accompanied by enhanced NO dissociation.

The catalytic performance of Pt dispersed on γ-Al2O3is very strongly promoted by sodium in the temperature range 200–500°C under simulated exhaust conditions at the stoichiometric point. Remarkable and unprecedented effects on NO, CO, and C3H6conversions, as well as enhanced selectivity towards N2, are found. Addition of Na causes a significant decrease in the temperature of 50% conversion (T50) for NO, CO, and C3H6. For optimally promoted Pt this temperature is ∼100° lower than for unpromoted Pt. Similar decreases are observed for the temperatures required for 100% conversion of NO, CO, and C3H6. The selectivity towards N2is improved from ∼75% over unpromoted Pt to ∼100% over the optimally Na-promoted catalyst. These effects are understandable in terms of the effects of sodium on the relative adsorption strengths of reactant species.

Sodium promotion strongly enhances both the activity and the nitrogen selectivity of Rh/γ-Al2O3catalysts in the reduction of NO by propene. The optimally promoted catalyst (7.3% w/w Na) exhibits a ∼3 times higher TOF and 90% N2selectivity (at 375°C) as compared to 53% for the unpromoted catalyst under the same conditions. Na also suppressed the formation of CO and HCN. At sufficiently high Na loadings the system exhibits poisoning. The improved performance and kinetic behaviour are understandable in terms of Na-induced enhancement of NO adsorption and dissociation on the Rh surface, in competition with propene and its decomposition products.

XAFS, XRD, and TEM measurements on dispersed Pt/Al2O3catalysts have elucidated the promotional effect of SO2in propane oxidation over these systems. At low loadings (0.05 wt%), Pt/Al2O3catalysts initially contain small (<20 Å) stoichiometric β-PtO2particles. Catalyst sulphation is accompanied by reduction and concomitant sintering of these oxidic particles, resulting in metallic Pt clusters exhibiting similar structural and reactive properties to their counterparts formed at higher loading (9 wt%). The formation of stable SO4species on both alumina support and on the Pt surface facilitates the dissociative chemisorption of propane onto both support and metal. Evidence is presented for the role of interfacial sulphate in the activation and subsequent oxidation of propane viaa bifunctional mechanism.

XRD, XPS/XAES, TPR analysis and catalytic testing have been applied to Na/W/Mn/SiO2methane coupling catalysts and to corresponding formulations without one or more of Na, Mn, and W. We find a clear correlation between catalyst performance and support structure in the final calcined material. Amorphous silica yields active but very unselective catalysts. Crystalline SiO2(α-cristobalite) generates active and highly selective catalysts—especially with respect to the formation of ethylene. We demonstrate that the presence of Na is essential for the anomalous low temperature silica → cristobalite support phase transition to occur. The structural, catalytic, and spectroscopic results indicate that Na plays a dual role as both structural and chemical promoter.