Time resolved operando spectroscopic study of the origin of phosphorus induced chemical aging of model three-way catalysts Pd/Al2O3

Abstract

The origin of phosphorus (P) induced chemical aging of model three-way catalyst Pd/Al2O3 is studied by transient CO and NO pulses. The fresh catalyst Pd/Al/F (calcined at 500°C for 4h) is subjected to thermal treatment in air at 700°C for 5h with (7.5P/Pd/Al) and without (Pd/Al/700) P poison. The catalyst surface and reaction dynamics during CO/NO pulses at 300°C are captured in a single shot by time resolved operando energy dispersive-extended X-ray absorption fine structure (ED-EXAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS). The results reveal the occurrence of Pd redox cycle during the temporal analysis of the catalysts by CO/NO pulses. However a secondary process, presumably PdCx formation, is hampered under these experimental conditions on the P aged catalyst 7.5P/Pd/Al. Additionally on 7.5P/Pd/Al, reactions involving Pd redox cycle are not as efficient as on Pd/Al/700 as evident from ED-EXAFS, MS and DRIFTS data. Furthermore, DRIFTS data show a larger population of isocyanate species (reflected by bands at 2180, 2228 and 2250cm−1) on Pd/Al/700 than on 7.5P/Pd/Al that exhibits only a single band at 2260cm−1. The missing low frequency bands at 2180 and 2228cm−1 are attributed to the clogging of the surface Pd sites by P and formation of AlPO4 species at the expense of a fraction of surface Al3+ sites of the support, respectively. Consequently, the production of CO2 (and also N2) is higher on Pd/Al/700 than on 7.5P/Pd/Al as evident by MS data confirming that the chemical aging caused by P is more detrimental than thermal aging for three-way catalytic efficiency of the model Pd/Al2O3 as reported in [S.K. Matam, et al., Catal. Today 184 (2012) 237].