一直以来,以碳材料为载体负载的金属催化剂被广泛应用于甘油液相氧化反应.研究表明,催化剂活性与碳的孔径分布有关,随着碳载体微孔比例的增加,催化剂活性下降.此外,载体表面基团对金属活性有着重要影响.例如,载体表面含氧基团的吸电子作用可降低载体表面电子的流动性(电子密度和导电性),从而阻碍甘油氧化反应过程中OH–的吸附和再生,导致反应活性降低.因此,开发微孔比例小、富含负电性基团的碳载体成为甘油氧化过程中急需解决的问题之一.本文通过热解碳纳米管(MWCNTs)和三聚氰胺的混合物,在碳纳米管表面直接生长得到氮杂石墨烯(NG-MWCNTs),并采用SEM,N2吸附,TEM和XRD对所得复合材料进行了表征.实验发现,相比于单纯的MWCNTs和直接热解三聚氰胺所得的产物CNx,NG-MWCNTs具有更高的比表面积(173 m2/g)和更大的平均孔径.此外,NG-MWCNTs非常适合作为Pt催化剂的载体,Pt平均粒径可小至1.4±0.4 nm.所制备的Pt/NG-MWCNTs催化剂在甘油选择性氧化反应中具有很高的催化活性和甘油酸选择性(甘油转化率和甘油酸选择性分别可达64.4%和81.0%),且具有可重复使用性能.Pt/NG-MWCNTs催化剂优异的催化活性不仅与载体表面高分散的Pt有关,而且与N原子对Pt的给电子作用有关.
Selective oxidation of glycerol is a hot topic. Increased biodiesel production has led to glycerol oxi-dation over Au- and Pt-based catalysts being widely studied. However, Pt catalysts suffer from deac-tivation because of weak metal-support interactions. In this study, multi-walled carbon nanotube (MWCNTs)-pillared nitrogen-doped graphene (NG) was prepared by direct pyrolysis of melamine on MWCNTs, and the synthesized NG-MWCNT composite was used as the support for Pt. Character-ization results showed that the surface area (173 m2/g) and pore volume of the NG-MWCNT com-posite were greater than those of bare MWCNTs and the separated melamine pyrolysis product (CNx). Pt (1.4 ± 0.4 nm) dispersion on the NG-MWCNTs was favorable and the Pt/NG-MWCNT cata-lyst was highly active and selective in the oxidation of glycerol to glyceric acid (GLYA) in base-free aqueous solution. For example, the conversion of glycerol reached 64.4% with a GLYA selectivity of 81.0%, whereas the conversions of glycerol over Pt/MWCNTs and Pt/CNx were 29.0% and 31.6%, respectively. The unique catalytic activity of the Pt/NG-MWCNTs is attributed to well-dispersed Pt clusters on the NG-MWCNTs and the electron-donating effect of the nitrogen dopant in the NG-MWCNTs.
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