The Design Of Carbon Based Catalysts

Energy is the material base of human activity, is the important subject of the sustainable development. Improving energy efficiency and developing new energy are effective means to solve the problem of energy crisis. So the development of low-cost, high efficient and high selective catalyst is necessary. Carbon-based materials are earth-abundant and very cheap. Due to the excellent physical, chemical and mechanical properties, carbon-based materials have achieved more and more attention and been applied in many fields. In this paper, different carbon-based nanomaterials were prepared through mild methods and the catalytic activity in the catalysis system was also explored. This thesis introduced the preparation, photocatalytic and electrocatalytic properties of gold nanoparticles/carbon nanotubes(CNTs), Au depositing on C3N4, Ag3PW12O40/C3N4, CDots-C3N4, NH2-CDots and Mn O2-C3N4 composites, and further explore the photo-/electric-catalytical mechanism of the catalysts. The detailed works included the following parts:1. Gold nanoparticles/carbon nanotubes(CNTs) composites(Au-in-CNTs) were prepared by in-situ reduction of HAu Cl4 with ultraviolet lamp radiation, which was introduced into CNT channels utilizing the capillary forces of CNTs aided by ultrasonication. The structure and properties were characterized by UV-Vis, Raman spectra and HRTEM image. A series of catalytic experiments suggested that the catalyst can catalytic oxidize cyclohexane efficiently using air as oxidant at 60 ℃. The photo-catalytic mechanism is also studied by SECM, RRDE and so on. The research showed that compared to Au nanoparticles loading on the outer wall of CNTs, the catalyst which Au nanoparticles confined in carbon nanotubes is higher photocatalytic active for the oxidation of cyclohexane with 14.64 % conversion of cyclohexane and a high selectivity of 86.88 % of cyclohexanol.2. Au depositing on C3N4(C3N4/Au) were prepared by in-situ reduction of HAu Cl4 on C3N4 with ultraviolet lamp radiation, which was prepared by pyrolysis of urea at 550 °C. The C3N4/Au nanocomposites exhibit the highest activity with about 10.54 % conversion of cyclohexane and 100% selectivity to cyclohexanone without any oxidant(H2O2 or O2). The catalysts were characterized by XRD, UV-Vis, TEM and so on. The catalytic mechanism is researched by electrochemical tests. Based on the results, under visible light, H2 O was oxidized to H2O2 by C3N4 and then decomposed into hydroxyl radicals(HO·) by Au nanoparticles, which serves as a strong oxidant for catalytic oxidation of cyclohexane.3. Ag3PW12O40/C3N4 nanocomposites were combined H3PW12O40 with C3N4 which was obtained by pyrolysis of urea for photo-catalytic oxidization of hydrocarbon(cyclohexane and cyclooctene). Through the electrochemical tests, we found that H2 O was oxidized to H2O2 on the surface of C3N4 via a two-electron pathway and then decomposed into hydroxyl radicals(HO·) by catalysis of Ag3PW12O40, which serves as a strong oxidant for catalytic oxidation of cyclohexane and cyclooctene. The photo-catalytic tests show that the conversion based on cyclohexanone and cyclooctene can achieve 8.62 % and 41.26 % after 48 h at 60 ℃ under NUV, respectively. Meanwhile, the selectivity of selective oxidation cyclohexane and epoxycyclooctane can be up to > 99.0 % and 77.2 %, respectively.4. CDots were synthesized by a typical electrochemical method followed by hydrothermal treatment with ammonia. CDots-C3N4 composites were then prepared by heating a mixture of ammonia treated CDots and urea powder at 550 °C. The CDots-C3N4 catalyst achieves the aim of overall water splitting by visible light without sacrificial agent. The catalyst was characterized by TEM, UV-Vis, BET, FTIR, Ramam and XPS methods. The research showed that the reduction level for H2 is positioned below the CB of CDots-C3N4, and the oxidation level for H2 O to H2O2 or O2 is above the VB; these bands are properly positioned to permit transfer of electrons and holes respectively for water splitting. Meanwhile, further increase of the CDots concentration could enhance the light absorption by introducing sub-band states, the maximum absorption wavelength can be reached 620 nm, which is helpful for improving light absorbance and the photocatalytic efficiency. By increasing the quantity of CDots, the catalyst can achieve a high QE=16 % at l=420±20 nm, QE=6.2 % at l=580±15 nm and STH=2 %. The catalytic mechanism is researched by electrochemical tests(RRDE and SECM) and isotope labelling method. This study shows that H2 O was oxidized to H2O2 by the catalysis of CDots-C3N4 under light, then decomposed into O2 on the surface of CDs. This ‘metal-free’ material has shown no obvious decay of QE(≥50 days).5. To verify the catalytic mechanism of CDots-C3N4, we used Mn O2 to replace the CDots as the peroxide decomposing catalyst. The final C3N4/Mn O2 photocatalyst was obtained via an in-situ oleinic acid reduction of KMn O4. The structure and composition were determined by TEM, EELS, XRD, UV-Vis and so on. From the photo-catalytic tests, we can see this catalyst also achieve high catalytic activity for water splitting under visible light, QE=3.82 %(420±20 nm). The catalytic mechanism also proved to be a two-electron water oxidation process by the electrochemical tests, the H2O2 generated decomposed into O2 fast by Mn O2 nanoparticles.6. Carbon nanoparticles also exhibit high catalytic activity for electrocatalytic reaction. CDs were synthesized by a typical electrochemical method followed by mixing the CDs with ammonia in different proportions, then transferred into a Teflon-lined stainless steel autoclave and heated at 160 °C for 12 h, afterwards, the NH2-CDs was obtained. NH2-CDs catalyst shows high catalytic activity for oxygen evolution reaction(OER)(generated a current density of 16.5 m A cm-2 at a potential of 1.6 V versus RHE). By controlling reaction condition, we achieved NH2-CDots with different doped types and content. The study showed that the OER activity of the NH2-CDs catalysts improved with increasing contents of amidogen-N. Compared to other carbon materials modified with amidogen-N, CDs exhibit higher catalytic activity due to abundant terminating species existing. For the photogenerated electrons and protons properties of CDs, with the addition of SPR property of Au nanoparticles, NH2-CDots/Au composites were designed. This catalyst showed higher catalytic activity(generated a current density of 39.5 m A cm-2 at a potential of 1.6 V versus RHE under visible light).