Well dispersible and stable single atom catalysts (SACs) with hydrophilic features are highly desirable for selective hydrogenation reactions in hydrophilic solvents towards important chemicals and pharmaceutical intermediates. A general strategy is reported for the fabrication of hydrophilic SACs by cation-exchange approach. The cation-exchange between metal ions (M = Ni, Fe, Co, Cu) and Na + ions introduced in the skeleton of metal oxide (TiO 2 or ZrO 2 ) nanoshells plays the key role in forming M 1 /TiO 2 and M 1 /ZrO 2 SACs, which efficiently prevents the aggregation of the exchanged metal ions. The as-obtained SACs are highly dispersible and stable in hydrophilic solvents including alcohol and water, which greatly facilitates the catalysis reaction in alcohol. The Ni 1 /TiO 2 SACs have been successfully utilized as catalysts for the selective C=C hydrogenation of cinnamaldehyde to produce phenylpropanal with 98% conversion, over 90% selectivity, good recyclability, and a turnover frequency (TOF) of 102 h −1 , overwhelming most reported catalysts including noble metal catalysts.

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