In Situ XAS/SAXS Study of Al2O3-Coated PtGa Catalysts for Propane Dehydrogenation

The effects of applying an alumina (Al2O3) coating by atomic layer deposition, with a thickness of about similar to 1 nm, on two PtGa catalysts (PtGa/MgAl2O4 and Pt/MgGaAlO) were explored for the propane dehydrogenation reaction (PDH). The combined application of small angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS) was employed on the samples to gain insight into the effect of this coating on catalyst stability. The coating restricted the mobility of surface metal nanoparticles, thereby preventing sintering of the catalyst. On the PtGa/MgAl2O4 catalyst, the presence of the coating hindered the alloy formation between Pt and Ga, while it did not negatively affect the formation of an alloy for the Pt/MgGaAlO catalyst as the Ga is delivered from the support. The SAXS and XAS findings were reflected in the PDH activity tests. The alumina-coated PtGa/MgAl2O4 performed worse than its uncoated counterpart due to the limited alloy formation in the presence of the coating. The coated and uncoated Pt/MgGaAlO catalysts were tested for PDH after 1, 5, and 10 H-2/O-2 redox cycles to see the effect of the coating on activity and stability. In general, the coating reduces the total amount of carbon formation, and the rate of deactivation for the coated sample is slower than for the uncoated counterpart. A higher thickness of coating led to reduced activity due to increased blockage of active sites but at the same time drastically reduced total carbon formation at similar conversions. The coated and uncoated Pt/MgGaAlO samples were then subjected to another 30 redox cycles (40 in total) and subsequently examined with HAADF STEM. Through particle size distribution of STEM images, it is determined that the coating reduced the extent of sintering of the sample. The direct correlation between increased coating thickness and lower extent of sintering of the surface metal nanoparticles was confirmed.