(610d) Kinetic Studies On the Stability of Pt for NO Oxidation: Effect of Sulfur and Long Term Aging

NO oxidation over Pt supported catalysts plays a key role in developing NOx abatement technologies for lean-burn engines, such as the NOx storage reduction process (NSR). Given the increasing market prices for Pt and other noble metals broad commercialization of NSR catalysts still requires great improvements in performance especially in terms of durability and resistance to poisons. In that regard, the presence of sulfur in fuel and lubricants constitutes a potential source for catalyst degradation by poisoning platinum or storage sites. The latter effect has been reported in previous studies where sulfur was observed to accumulate on the alkali component in the form of sulfates, thus inhibiting the NOx storage capacity. However, the former aspect of sulfur poisoning during NSR operation has not been assessed in great detail.

The work reported here focused on studying the effect of sulfur poisoning on the NO oxidation kinetics (apparent activation energy and power rate law reaction orders) over a series of Pt/SBA-15 silica catalysts with varying particle sizes (ca 2-9 nm). Sulfur was introduced by ex-situ treatments under reducing (430 ppm H₂S/20%H₂/N₂) and oxidizing (Air, 538 ppm H₂S/N₂) environments. In agreement with previous work in our group the rate of NO oxidation was found to be close to first order with respect to both reactants NO and O₂, while it was close to negative first order with respect to the product NO₂. We also observed a strong dependency of the turnover rate (TOR) on the particle size, with TORs on large particles (9 nm) sixty times higher rate than on the smaller particles (2 nm). The addition of sulfur did not affect the kinetic parameters for NO oxidation on any of the samples studied. Sulfur pretreatments showed only a slight effect on the TOR and the deactivation profile (within a factor of 2) with changes being more pronounced as the Pt particle size increased. As platinum is known to exhibit great resistance to sulfur, x-ray absorption spectroscopy (XAS) experiments were performed on the samples sulfided in H₂S/H₂. The results confirmed the presence of Pt-S bonds on the 2 nm and 4 nm particles which completely became Pt-O after 1h of exposure to a standard NOx mixture. Interestingly a second reduction of the sample completely reversed the Pt-S backscattering implying that under oxidation conditions Pt is not regenerated, but sulfur remains in the particle possibly forming Pt-O-SOx species. A similar behavior was observed for the used catalyst which indicates that Pt-S interactions are not cancelled by prolonged exposures to strongly oxidizing environments. X-ray photoelectron spectroscopy (XPS) measurements indicate that under oxidation conditions sulfur is present as sulfates on Pt. Reduction was able to form sulfides on Pt, consistent with our XAS findings. The minimal change in TOR on sulfided samples is explained as a combination of sulfur migration and S-poisoning on open surfaces that are inactive regardless of the presence of sulfur. Therefore, we turned our attention into understanding the potential source of deactivation and long term instability.

Proper determination of kinetic parameters required substantial aging time periods under NOx to account for deactivation. During the course of experiments a drastic initial drop in rate was observed within 2 hours followed by a slow (but continuous) decrease. Over extended times on stream (TOS>40h) a 20-40 kJ mol^-1 increase in Eapp was observed to occur on Pt/Al₂O₃ and Pt/SBA-15 catalysts. While the initial TOR was fully recovered after reduction in H₂, the presence of oxygen during start-up shut down procedures was able to maintain the state of the catalyst after reaction and suppress deactivation. In-situ time-resolved XAS experiments showed that the fraction of Pt²⁺ decreases sharply during the first 10 min and stayed constant. The fraction of metallic platinum is strongly dependent on particle size. Longer exposures to NOx (18h) showed no change in oxidation state on the large particles, but an increase in Pt⁴⁺ for the small particles. This suggest that changes in kinetics with TOS are a not a consequence of bulk oxidation, but surface phenomena. High Resolution Transmission Electron Microscopy (HRTEM) showed no evident change in surface structure upon aging. We propose that the instability of Pt catalysts for NO oxidation can be related to a reversible over-oxidation or surface restructuring of Pt that slowly occurs during reaction. Overall, our results suggest that sulfur has a minimal effect on the NO oxidation reaction but it is the oxidation of platinum that causes the long term instability.