Promotion of activation ability of N vacancies to N2 molecules on sulfur-doped graphitic carbon nitride with outstanding photocatalytic nitrogen fixation ability

硫掺杂石墨相氮化碳的光催化固氮性能中氮空穴对氮气分子活化的提升作用（英文）

Nitrogen vacancies and sulfur co-doped g-C3N4 with outstanding N2 photofixation ability was synthesized via dielectric barrier discharge plasma treatment. X-ray diffraction, ultraviolet–visible spectroscopy, N2 adsorption, scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and temperature-programmed desorption were used to characterize the as-prepared catalyst. The results showed that plasma treatment cannot change the morphology of the as-prepared catalyst but introduces nitrogen vacancies and sulfur into g-C3N4 lattice simultaneously. The as-prepared co-doped g-C3N4 displays an ammonium ion production rate as high as 6.2 mg·L^-1·h^-1·gcat^-1, which is 2.3 and 25.8 times higher than that of individual N-vacancy-doped g-C3N4 and neat g-C3N4, respectively, as well as showing good catalytic stability. Experimental and density functional theory calculation results indicate that, compared with individual N vacancy doping, the introduction of sulfur can promote the activation ability of N vacancies to N2 molecules, leading to promoted N2 photofixation performance.

地球上的氮元素十分丰富并主要以氮气形式存在.然而,由于氮的化学及生物惰性,大多数生物体不能直接吸收氮.因此,人工固氮过程被称为继光合作用之后的第二大化学反应.目前工业上常用的固氮技术为Haber-Bosch法,该方法能耗高且使用的原料氢气十分危险.因此,寻找更加节能、绿色、安全的固氮技术迫在眉睫.石墨相氮化碳作为一种非金属半导体光催化剂近年来受到研究者青睐.氮化碳化学性质稳定,能带宽度适中,且耐酸碱腐蚀.然而,氮化碳的反应活性位较少,量子效率较低,而且由于光能的能量密度较低,反应物N2分子难以活化,因此光催化固氮性能并不理想.有研究报道称晶格缺陷能作为反应活性位活化氮气分子,提高光催化固氮性能.本文采用介质阻挡放电等离子体法制备了硫和氮空穴共掺杂的石墨相氮化碳催化剂,考察了硫的引入对催化剂光催化固氮性能的影响. XRD, UV-Vis和氮气吸附结果表明,掺杂未改变催化剂的比表面积,但使得氮化碳催化剂晶格发生了细微扭曲,且改变了催化剂的电子结构,降低了能带宽度.通过XPS结果推测了硫元素与氮空穴的相对位置,并确定硫元素取代了氮元素,以S–C键的形式掺入到催化剂晶格中.采用N2-TPD法分析了催化剂对氮气分子的吸附能力,发现氮空穴是氮气分子的化学吸附位.硫的引入能促进氮空穴对氮气分子的吸附与活化能力. PL光谱结果证实硫和氮空穴掺杂均对电子空穴对的分离有促进作用.不同气氛下的PL光谱结果表明,光电子能通过氮空穴从催化剂转移至吸附的氮气分子上. DFT计算结果显示,氮气分子吸附在氮空穴上,吸附后的N≡N键长为1.331 A,小于非吸附状态的N≡N键长(1.157 A).而引入硫以后,氮气分子同样吸附在氮空穴上,N≡N键长进一步增大到1.415 A,且氮气分子的吸附能也显著降低.这些结果都说明硫的引入提高了氮空穴对氮气分子的吸附和活化作用.此外,态密度图结果显示,光电子通过氮空穴从催化剂向吸附的氮气分子转移,且引入硫之后吸附的氮气分子上的电子密度明显提高,进一步证实硫的引入对氮气分子的活化作用.