The CoPOM complex was synthesized according to literature. Briefly, Na2WO42H2O (50.89 g), NaHPO47H2O (4.60 g) and Co(NO3)26H2O (9.98 g) were dissolved in 50 mL deionized water in a 200 mL round-bottom flask. The pH was adjusted to 7 by HCl under magnetic stirring. The solution was then stirred and refluxed at 100 °C for 2 h and cooled down to room temperature. CoPOM was finally obtained by recrystallization and washed by deionized water. The purity of CoPOM is confirmed by Attenuated total reflection Fourier transform infrared (ATR-IR) spectroscopy.