The performances and structure evolution of Pt-based catalysts for selective hydrogen combustion under propene-rich conditions
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摘要: 本工作研究了富丙烯气氛中Pt基催化剂的SHC性能和长期使用过程中活性相的演变规律。结果发现,Sn修饰的Pt/SiO2催化剂在丙烷和丙烯同时存在的条件下具有高的氢气氧化选择性(大于98%)。通过动力学研究,稳定性测试和结构表征发现,氧气的引入能够显著提高催化剂的结焦速率,形成的高度石墨化的焦炭,Pt的烧结,PtSn合金中Sn的氧化和偏析是导致长期稳定性测试中选择性下降的主要因素。Abstract: In the present study, the kinetic behaviour and active sites evolution processes of Pt-based catalysts were investigated. It was found that highly selective hydrogen combustion could be achieved over Sn modified Pt-based catalysts in presence of both propane and propene (over 98%). The stability tests, kinetic study and catalyst characterization revealed that the existence of oxygenated species is the reason for accelerated coking reactions. The formation of graphitized cokes serving as additional unselective active sites and the oxidation of tin in PtSn alloy phases are the primary reasons causing the catalytic selectivity loss during long-run tests under propene-rich condition.
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Figure 3 Activities and selectivites of Pt/SiO2, PtSn/SiO2 and Sn/SiO2 catalysts
SHC under condition: ((a), (b)) pC3H8 =12 kPa, pH2 = 6 kPa, total flow = 100 mL/min; ((c), (d)) pC3H8 =12 kPa, pC3H6 = 6 kPa, pH2 = 6 kPa, pO2 = 1.5 kPa, total flow =50 mL/min O2 in shown in the picture, balance is nitrogen, 550 °C
Figure 4 Activities and selectivites of Pt/SiO2 and PtSn/SiO2 catalysts (a): conversion of oxygen; (b): oxygen selectivity to water
Pt/SiO2_H denotes reacting in high C3H6 condition: pC3H8 =18 kPa, pC3H6 = 30 kPa, pH2 = 6 kPa, pO2 = 1.5 kPa; Pt/SiO2_L denotes reacting in low C3H6 condition: pC3H8 =18 kPa, pC3H6 = 6 kPa, pH2 = 6 kPa, pO2 = 1.5 kPa, total flow is 50 mL/min, balance is nitrogen, 550 °C
Figure 5 (a) Coke content of spent Pt/SiO2 catalyst at three reaction conditions after 10 h on stream at different conditions; ((b), (c)) TPO and Raman spectra of spent Pt/SiO2 catalyst; (d) relationship between propene partial pressure and coking rates in presence (this work) and absent of oxygen (data from ref. [24]); ((e), (f)) HRTEM images of spent Pt/SiO2 catalyst Condition A: pC3H8=18 kPa, pC3H6=30 kPa, pH2 = 6 kPa; Condition B: pC3H8 = 18 kPa, pC3H6 = 30 kPa, pH2 = 3 kPa ; Condition C: pC3H8 = 18 kPa, pC3H6 = 30 kPa, pH2 = 6 kPa, pO2=1.5 kPa. Balance is N2, total flow is 50 mL/min, 550 °C
Table 1 Physicochemical properties of Pt/SiO2, PtSn/SiO2 and SnO2/SiO2 catalysts
Sample Metal loading w/% Surface area/(m2·g−1) Pt dispersion/% $D_{\rm{chem}}^{\rm{a}} $/nm $d_{\rm{TEM}}^{\rm{b}} $/nm Pt/SiO2 0.35 232 59.5 1.9 2.3 PtSn/SiO2 (0.35) Pt-(0.22) Sn 221 43.4 2.6 2.8 SnO2/SiO2 0.35 235 − − − Note: a: determined by H2 chemisorption; b: determined by TEM analysis Table 2 Selective hydrogen combustion over Pt nanoparticles loaded on different supports
Sample O2 conversion/% O2 selectivity to H2O/% Coke contentc w/% initiala finalb initiala finalb PtK/SiO2 98.3 98.5 96.3 87.9 10.5 Pt/MgO 98.6 94.4 95.3 66.4 18.6 Pt/SiO2 95.1 96.6 94.5 83.6 39.1 Pt/ γ-Al2O3 98.6 94.8 85.8 87.6 43.8 Notes: a: recorded at 5 min; b: recorded at 25 h; c: detected by TGA -
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