Structural changes of PtPd nanoparticles in a natural gas oxidation catalyst were studied at elevated temperatures in air and low-oxygen conditions and in situ using environmental transmission electron microscopy (ETEM). The fresh catalyst shows <5 nm, PtPdOx particles on the gamma-Al2O3 support. At 700 degrees C, the noble metal oxide decomposes and Pt gets trapped by PdO particles followed by formation of metallic Pd and Pt containing particles. At 1000 degrees C, the particles had a metallic Pd and Pt containing core surrounded by PdO particles. In addition, the presence of <10 nm sized particles was always observed. The activity measurements indicate the decrease in activity at the elevated temperatures. ETEM studies showed significant mobility of the noble metal particles above 850 degrees C. Above 1100 degrees C, PtPd particles were mobile and smaller particles were trapped by larger ones by a particle coalescence mechanism. (C) 2017 Elsevier Inc. All rights reerved.