The valence state of the Zr-P-Cr catalyst was further characterized by XPS. As shown in ... a, the peaks are corresponded to the binding energy of Zr,Cr,p and O,which indicates that the catalyst is composed of the four elements and no other impurity is observed. As shown in ... b, the binding energy of Cr 2p3/2 and Cr 2p 1/2 are observed at 577.3 and 586.8 eV, respectively, which are the characteristic binding energy of Cr3+. In addition, the molar ratio of Cr:Zr:P on the catalysts surface can also be determined by XPS technology, and the molar ratio of Cr:Zr:P on the catalysts surface was determined to be 1:8.02:9.1.