ABSTRACT
The structure and evolution of Pd species in Pd-exchanged zeolite materials intended for use as passive NO x adsorbers were examined under various pretreatment conditions. Using in situ CO-diffuse reflectance infrared spectroscopy, Pd structures were characterized after 500 °C pretreatments in inert (Ar), water (1-2% H2O in Ar), oxidizing (air), and reducing (H2, CO) atmospheres. Two zeolites of similar Si/Al ratios but different framework topologies (Beta, CHA) were found to show different distributions of Pd species, depending on the reducing agent used. Reduction in H2 (500 °C; 10% H2 in Ar) followed by re-oxidation (500 °C; air) led to higher amounts of single-site Pd ions on Pd-CHA than Pd-Beta, whereas high-temperature reduction in CO (500 °C; 1000 ppm CO in Ar) followed by re-oxidation (500 °C; air) led to significant loss of ionic Pd on both Pd-CHA and Pd-Beta, albeit H2 temperature-programmed reduction and XPS experiments suggest that this phenomena may be limited to surface Pd. High-temperature treatments with water (500 °C; 1-2% H2O in Ar) are shown to form either Pd metal or PdO particles, with Pd-Beta being more susceptible to these effects than Pd-CHA. This work suggests that the effects of CO are especially problematic with respect to the durability of these materials in passive NO x adsorption applications, especially in the case of Beta zeolite.
ABSTRACT
Direct analysis in real time mass spectrometry (DART-MS) was used to analyze ionic liquids (ILs) containing either imidazolium or phosphonium cations combined with different types of inorganic and organic anions. Ionic liquids were directly inserted into the ionization source using a glass probe without dissolution into organic solvents. Mass spectra of the ILs were collected in both positive and negative mode with a linear ion-trap instrument. The intact cation of the compound was typically the dominant peak in positive mass spectra and cluster ion formation was present. Some individual anions were not readily observed in the negative mass spectra (based on the type of anion); however, the mass difference of adjacent cluster ions equal the mass of a complete IL and the anion mass could be verified by subtracting the known cation mass. The degree and intensity of the cluster ion formations was found to be dependent on the nature of the specific ILs as well as the DART temperature gas stream.