The sample was firstly equilibrate at 30 C, then the sample was heated with 20 C/min to 60 C. Subsequently, the samplewas kept at 60 C for another 3 min with a nitrogen flow of 50 mL/min and then heated to 170 C at a rate of 20 C/min, and the gas was switched to oxygen at a flowrate of 50mL/min.