《Table 4 Catalytic properties of different catalysts after conducting reaction for 8 h》

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《Fabrication of the Core-Shell Structured ZSM-5@Mg(Al)O and Its Catalytic Application in Propane Dehydrogenation》

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To analyze the coke deposits on the catalyst，the TPO profiles of different catalysts are presented in Figure 8.In the TPO profiles，there were two successive peaks representing two different carbon deposits.The carbon deposits correponding to the first peak at low temperature were those located on the active centers of metals，while the second peak at high temperature represented those located on the external surface of the support[40].It can be easily seen from Figure 8 that there was a large amount of coke deposited on Pt Sn Na/ZSM-5，especially for the one located on the acitive centers.Our previous reports[13]showed that the coke deposited on the external surface of the support had little impact on the catalytic acitivity.This might be the main reason that the deactivation rate of Pt Sn Na/ZSM-5 was relatively high as shown in Figure 7.Compared with Pt Sn Na/ZSM-5，the coke deposition peaks of other Pt Sn Na/Z@MA-x catalysts on the active metal shifted towards the lower temperature region，suggesting that the deposited carbon became more reactive.Moreover，the quantitative analysis of coke in Table 2 clearly showed that the amount of coke formed over the series of Pt Sn Na/Z@MA-x catalysts was much lower than that formed over the Pt Sn Na/ZSM-5 catalyst（19.8%）.In addition，the content of Mg（Al）O species could also influence the amount of coke formed over the Pt Sn Na/Z@MA-x catalysts.As summarized in Table 2，the Pt Sn Na/Z@MA-3catalyst possessed the lowest coke amount（7.6%）.This outcome could be attributed to the weakened acidity in these catalysts after the introduction of Mg（Al）O shell in Z@MA-x materials.The intrinsic acidity of the support could affect the undesirable reaction such as cracking/isomerization，and olefins had turned out to be the primary precursors of the mechanism of coke formation[41].Therefore，based on the results of NH3-TPD analysis，a lower amount of acid sites in Z@MA-3 could be the main reason of less coke deposits on the catalyst.In addition，the relatively large Sext of Z@MA would facilitate the transportation of coke precursors，leading to a lower carbon amount and weaker carbon intensity in the catalyst[42].Moreover，the presence of Sn oxides in Pt catalyst would weaken the binding between hydrocarbons and metal in the catalyst[43-44].Thus，a much more amount of Sn oxides in the catalyst would promote the migration of coke precursor from the metal surface to the external surface of support，making more metallic active sites exposed.Combining with the results of XPS analysis，in the series of Z@MA catalysts，the coke precursor formed on the metal might migrate to the support and the acid sites[45].As a conclusion，in the Pt Sn Na/Z@MA-3，the coke precursor formed on the metal was highly suppressed and could be even migrated to the external surface of support，making the catalytic activity and stability of Pt Sn Na/Z@MA-3 the best.