Characterization by in situ X-ray powder diffraction of Au-Pd nanoalloy catalysts for NO decomposition

MICROSCOPIE:RX+ING+TEG:ATU:LBU:FBS:YSC:FRMNational audienceThe direct decomposition of NOx at moderate temperatures is a challenge over noble metal based catalysts because of the poisoning of the metal surface by oxygen. Au is inactive for NO direct decomposition, but displays oxidation resistance. The objective of this work is to obtain durable activity for NO direct decomposition by combining inert Au with an active metal such as Pd, which is known to deactivate. Pd-Au/SiO2 nanoparticles were prepared using a surface redox method. TEM-EDX analyses showed that most particles (3-24 nm in size) contained both metals, though the composition was not homogeneous. In situ XRPD analyses initially displayed poorly crystalline Au, Pd and possibly a Pd-Au alloy phase. Calcination in air yielded PdO and a Au-rich alloy phase, whose composition (Au0.8Pd0.2) seems to be the limiting composition under oxidizing conditions. Under H2, β-PdHx (at 25°C) or metallic Pd (with a thermal expansion at 450°C) were obtained besides Pd-rich alloyed phases. After removing H2 pressure, PdHx decomposed into metallic Pd. The Au-modification of the parent Pd/SiO2 yielded a stable activity. The post-reaction XRPD pattern showed a significant fraction of metallic Pd and alloyed phases in addition to PdO