Palladium (Pd) clusters on zirconia (ZrO2) and single Pd atoms on ceria (CeO2) exhibited high catalytic activity and selectivity for decarbonylation of furfural to furan without additives in the liquid phase. To study the active size of Pd and changes in chemical states or structures during the reaction, in-situ X-ray absorption fine structure (XAFS) measurements were conducted. The size of Pd clusters consisting of less than 10 Pd atoms was maintained on ZrO2 during the reaction. In contrast, single Pd atoms were aggregated during heating and only Pd clusters consisting of 13 atoms were present after the reaction. According to the in-situ XAFS results, the size of Pd particles did not gradually increase to Pd-13 clusters. Instead, Pd-13 clusters were partly formed from the beginning of the reaction, and the proportion of Pd-13 clusters increased with time while keeping the size of Pd(13)( )clusters. This result suggests that a single Pd atom is inactive, but Pd clusters are active for decarbonylation. Ab-initio calculation revealed that ZrO2 (1 1 1) surface had lower adsorption and migration energies than CeO2 (111), implying that Pd was easily diffused on the surface and stabilized as small Pd clusters. (C) 2019 Elsevier Inc. All rights reserved.