Structural evolution of an intermetallic Pd-Zn catalyst selective for propane dehydrogenation

We report the structural evolution of Pd-Zn alloys in a 3.6% Pd-12% Zn/Al2O3 catalyst which is selective for propane dehydrogenation. High signal-to-noise, in situ synchrotron X-ray diffraction (XRD) was used quantitatively, in addition to in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) and extended X-ray absorption fine structure (EXAFS) to follow the structural changes in the catalyst as a function of reduction temperature. XRD in conjunction with DRIFTS of adsorbed CO indicated that the beta(1)-PdZn intermetallic alloy structure formed at reduction temperatures as low as 230 degrees C, likely first at the surface, but did not form extensively throughout the bulk until 500 degrees C which was supported by in situ EXAFS. DRIFTS results suggested there was little change in the surfaces of the nanoparticles above 325 degrees C. The intermetallic alloy which formed was Pd-rich at all temperatures but became less Pd-rich with increasing reduction temperature as more Zn incorporated into the structure. In addition to the b1-PdZn alloy, a solid solution phase with face-center cubic structure (alpha-PdZn) was present in the catalyst, also becoming more Zn-rich with increasing reduction temperature.