Investigations of the Dispersion of Pd in H-ZSM-5

Abstract

The state of Pd dispersion in Pd–H-ZSM-5 was investigated by means of infrared spectroscopy and temperature-programmed desorption spectroscopy, using CO and NO as probe molecules. Following oxidation in O2at 773 K of a freshly prepared, low-loaded sample (Pd/Al=0.048), most of the Pd is found to be present as isolated cations, viz. Pdn+(n=1–3). CO reduction at 773 K leads to the formation of Pd particles that fill the local pore space of the zeolite and are inaccessible to CO. Following CO reduction, oxidation in O2for 2 or 8 h at 773 K only partially redisperses the Pd. During treatment of a reduced sample in 5000 ppm NO for 60 min at 673 K, N2O and N2are produced in an amount sufficient to oxidize all the Pd to PdO. Characterization of the state of Pd dispersion after NO pretreatment shows that all of the Pd has been redispersed as cations. Reoxidation of the reduced sample in NO is accompanied by a decrease in the infrared band due to Brønsted acid groups in an amount nearly equivalent to 2H+/Pd. Subsequent reduction of the sample regenerates the Brønsted acid band intensity. These observations suggest that isolated Pd cations may reside as Z−H+(PdO)H+Z−in association with pairs of Al sites in the zeolite that are next-nearest neighbors. Following oxidation in O2at 773 K, a high-loaded sample of Pd–H-ZSM-5 (Pd/Al=0.48) is found to have nearly the same concentration of highly dispersed Pd cations as the low-loaded sample, with the balance of the Pd in this case being present as small particles of PdO. Treatment of a reduced, high-loaded sample of Pd–H-ZSM-5 in NO at 773 K redisperses all of the Pd in structures that are proposed to be Z−H+[(PdO)(NO)]H+Z−and Z−H+[(PdO)(NO)]. However, upon removal of the adsorbed NO by treatment in O2at 773 K, the Z−H+[(PdO)(NO)] species revert to PdO, but the Z−H+[(PdO)(NO)]H+Z species remain stable as Z−H+(PdO)H+Z−. The present results suggest that maintenance of Pd in a high state of dispersion as isolated Pdn+cations in the presence of O2at elevated temperatures requires pairs of next-nearest-neighbor Al sites in the zeolite.