(147f) Elucidating the Role of the Pt/ba Interface during NOx Storage and Reduction on Model Pt/bao Catalysts

Clayton, R. D., Caterpillar Inc.
Balakotaiah, V., University of Houston

NOx storage and reduction (NSR) is an elegant technology to reduce NOx in the exhaust of lean burn vehicles. It is inherently periodic and it relies on both a precious metal (PM) function and a NOx storage function (NS) to carry out the desired chemical reduction. The PM (typically Pt and Rh) and storage compound (typically BaO) must work in synergy during both the storage and reduction steps. During storage, the PM catalyzes NO to NO2 which then transports to the NS, forming stored nitrates. During reduction, the PM serves initially as an oxidation catalyst, with the reductant scavenging surface oxygen, enabling subsequent NO decomposition and reduction chemistry to occur. The role of the PM/NS interface, such as Pt/Ba, is considered the primary location of both during NOx storage and reduction with multiple spillover and reaction processes. Moreover, PM sintering is considered to be an important deactivation mechanism. To this end, the design of the NSR catalyst in terms of the PM and NS loadings, the PM dispersion, and the PM/NS interfacial perimeter is critical.

In this study a systematic study of NSR in bench-scale monolith and TAP reactors is conducted to elucidate the interfacial processes during the regeneration of Pt/Ba with H2. A series of model Pt/Ba catalysts of fixed Pt and BaO loadings and varied Pt dispersion was used to compare the effects of these variables on the catalyst performance in terms of NOx conversion and product selectivities. The bench-scale reactor was used to conduct simulated storage and reduction cycling of realistic duration and conditions (space velocity, temperature, feed composition, etc.). The TAP reactor was used to conduct precise interrogation of the Pt/Ba catalysts to compare the rates of key steps during reductant/NO pump-probe experiments.

A typical series of bench-scale reactor experiments compared the performance of three catalysts of 3%, 10%, and 50% Pt dispersion for fixed total Pt loading (2.6 wt.%) and BaO loading (15 wt.%). The 50% dispersion catalyst exhibited superior storage and NOx conversion over a wide range of temperatures. This underscores the importance of the Pt/Ba coupling during storage as well as during the regeneration step. A considerably higher N2 selectivity was observed as well, which is the result of the higher NOx storage. The higher storage is effective in converting intermediate NH3 to N2 at temperatures exceeding 200 oC. A more complex effect of dispersion on the ammonia production was observed. For example, except at low temperature (125 oC) and high temperature (350 oC), the 10% dispersion catalyst produced the most NH3. Moreover, the ammonia selectivity shows an interesting maximum at an intermediate temperature for the 10% dispersion catalyst. These and other data will be interpreted in detail in order to elucidate the effect of dispersion. For example, we have carried out experiments in which the amount of NOx stored is fixed for the three catalysts. This helps to isolate effects that are due solely to the regeneration chemistry.

The TAP reactor is used for both powder and monolith catalysts to identify rate limiting steps during the regeneration. Detailed mass balances are used to construct reaction pathways and to elucidate the role of the Pt/Ba interface. A phenomenological picture will be discussed that explains the bench-scale and TAP findings.