PdZn Surface Alloys as Models of Methanol Steam Reforming Catalysts: Molecular Studies by LEED, XPS, TPD and PM-IRAS

The formation and stability of PdZn/Pd(111) surface alloys have been studied, with emphasis on their interaction with CO, methanol and D2O, applying complementary techniques such as low energy electron diffraction, X-ray photoelectron spectroscopy, temperature programmed desorption (TPD), and polarization–modulation infrared reflection absorption spectroscopy. PdZn surface alloys represent well-suited model systems for technological methanol steam reforming (MSR) catalysts. It could be shown that upon Zn deposition on Pd(111) at or below room temperature non-interacting Zn layers are formed first, that subsequently transform to PdZn surface alloys upon annealing above 473 K. At annealing temperatures above approximately 623 K the surface alloy starts to decompose, finally restoring the clean Pd(111) surface. TPD spectra reveal that methanol was decomposing to a significant amount on Pd(111), yielding CO and CHx (apart from H2), a process that did not occur on the PdZn surface alloys (i.e. methanol desorbed molecularly). This difference in part explains the improved catalytic properties (selectivity and stability) of PdZn catalysts for the MSR reaction.