Heterogeneous palladium catalysts with high atom-efficiency offer practical value for selective hydrogenation. Here, we prepared atomically dispersed Pd species supported on gamma-AlOOH nanosheets (Pd/gamma-AlOOH), which contain clusters and dominated single atoms. Even with an extremely low amount of Pd loading, the catalytic activity of 0.1 Pd/gamma-AlOOH leads to 50.3% of succinic acid conversion at 4h. At the time, the succinic acid conversion reaches 90.7% and 97.1%, and the selectivity to gamma-butyrolactone is high up to 96.9% and 93.8% over 0.5 Pd/gamma-AlOOH and 1.0 Pd/gamma-AlOOH , respectively. The supported Pd single atom (Pd-1/gamma-AlOOH ) Exhibits 30- and 1100-fold increase in the catalytic activity compared with the supported clusters Pd-13/gamma-AlOOH and Pd-55/gamma-AlOOH for the selective hydrogenation of succinic acid to gamma-butyrolactone, respectively. Analysis of the structural properties confirms that Pd single atoms on gamma-AlOOH (0 1 0) surface have high adsorption energy, which is in agreement with catalytic efficiency. In addition, Pd single atoms in the catalyst 0.2 Pd/gamma-AlOOH account for 86.6% of the overall catalytic activity and can serve as the most efficient catalytic active sites. (C) 2016 Published by Elsevier B.V.