PNNL

Abstract

In this study PtK/TiO2 (K: 2, 5, 10, 15, 20 wt%) and Pt/ K2Ti6O13 LNT catalysts have been synthesized, characterized and tested. The aim of this study was to investigate whether the interactions between the selected supports and K can decrease the mobility of K, with the purpose of obtaining stable NOx storage performance. NOx storage capacity tests and XRD analysis have been coupled to correlate structure and NOx storage properties. The effect of K loading and thermal aging on NOx storage performance have been addressed, as well as an analysis of the formation/decomposition of stored NOx species over corresponding Pt-free samples with FT-IR, TPD and XRD techniques. NOx storage-reduction tests of PtK/TiO2 catalysts indicate that 10 wt% K-loaded samples show the highest NOx storage capacity, registered at 300 °C. Comparative results for varying K-loaded samples highlights a composition dependence for the temperature at which maximum NOx storage capacity is attained and of NOx uptake yields. A low K utilization for the NOx storage has been observed. Especially at high K contents (K= 20 wt%), this result can be attributed, in part, to the low surface area of the support material (decreasing K dispersion), and to the depletion of the K storage phase via reaction between K and TiO2. In fact XRD analyses demonstrate that K and TiO2 react already during the initial catalyst synthesis calcination process, by forming a “K2Ti6O13-like” potassium titanate at temperatures as low as 550 °C. This reaction is promoted by increasing the K content and calcination temperatures. Activity, XRD and FT-IR measurements have demonstrated that poorly crystalline potassium titanates have storage properties, although formation/crystallization decreases the NOx stored amounts especially after aging treatments, as the K incorporates into the TiO2 structure and, therefore, is no longer available to store NOx. Agreement among all collected results highlights that the reaction of K with TiO2 to form potassium titanates can lead to a positive increase in the stability of K, but also loss of performance as K becomes inaccessible for NOx storage. The latter may well mean that K-based LNTs will not be able to meet durability requirements for vehicle emission control applications.