Hi, I have a question about the change of peak position of NH3-TPD over HZSM-5 catalyst. The HZSM-5 catalyst were undergoing CO2 hydrogenation to DME reaction, different HZSM-5 catalysts with different reaction time were characterized by NH3-TPD. From the results, the peak position during the high temperature from 300-600 were shifted a little, from 500 to 450 C. I didn't modify the HZSM-5 catalyst using Na or other elements. Does anyboby know what usually cause the peak shift in the NH3-TPD of HZSM-5 catalyst?
Thanks.