Efficient Nitrogen Fixation Catalyzed by Gallium Nitride Nanowire Using Nitrogen and Water

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThe highly efficient and low-temperature fixation of elemental nitrogen (N2) into ammonia (NH3) is a highly desirable process concerning the well-being for the entire humanity. The process utilizes and transforms extremely abundant atmospheric N2 to produce various important products such as clean fuels, fertilizers, pharmaceuticals, pesticides, explosives, dyes, bleaches, and rocket propellants (). However, the stable N≡N triple bond in N2 renders extreme chemical inertness. Up to now, the predominant method for N2 fixation relies heavily on the Haber-Bosch process, developed a century ago, and requires harsh reaction conditions of 500°C–600°C, 20–50 MPa, and excessive flammable hydrogen (H2) gas. Such process directly consumes more than 1% of the world's annual energy supply (). Furthermore, due to the impractical energy requirement of the traditional electric water-splitting process, H2, as the necessary feedstock for current N2 fixation industries, is still predominantly produced from non-renewable fossil resources (). Although the development of photochemistry in recent years has been enabling N2 fixation at ambient conditions (), using photoexcited electrons generated from photosensitizing semiconductor catalysts such as TiO2 (, , , , , , ; , , ) (including its derivatives such as IIA-oxide-TiO2 ternary compound) (, href="#bib32" rid="bib32" class=" bibr popnode">Li et al., 1983, href="#bib46" rid="bib46" class=" bibr popnode">Oshikiri et al., 2014, href="#bib51" rid="bib51" class=" bibr popnode">Rusina et al., 2001), bismuth oxide (href="#bib4" rid="bib4" class=" bibr popnode">Bai et al., 2016, href="#bib33" rid="bib33" class=" bibr popnode">Li et al., 2015, href="#bib34" rid="bib34" class=" bibr popnode">Li et al., 2016, href="#bib58" rid="bib58" class=" bibr popnode">Wang et al., 2017a, href="#bib59" rid="bib59" class=" bibr popnode">Wang et al., 2017b), CdS (href="#bib5" rid="bib5" class=" bibr popnode">Banerjee et al., 2015, href="#bib8" rid="bib8" class=" bibr popnode">Brown et al., 2016, href="#bib9" rid="bib9" class=" bibr popnode">Cao et al., 2016, href="#bib21" rid="bib21" class=" bibr popnode">Hu et al., 2016a, href="#bib22" rid="bib22" class=" bibr popnode">Hu et al., 2016b, href="#bib23" rid="bib23" class=" bibr popnode">Hu et al., 2016c, href="#bib29" rid="bib29" class=" bibr popnode">Khan et al., 1983, href="#bib28" rid="bib28" class=" bibr popnode">Khan and Rao, 1990, href="#bib37" rid="bib37" class=" bibr popnode">Liu et al., 2016, href="#bib40" rid="bib40" class=" bibr popnode">Miyama et al., 1980, href="#bib54" rid="bib54" class=" bibr popnode">Sun et al., 2017, href="#bib60" rid="bib60" class=" bibr popnode">Wu et al., 2013, href="#bib63" rid="bib63" class=" bibr popnode">Zhang et al., 2016), and carbonaceous materials (such as graphene (href="#bib38" rid="bib38" class=" bibr popnode">Lu et al., 2016, href="#bib62" rid="bib62" class=" bibr popnode">Yang et al., 2017), diamond (href="#bib65" rid="bib65" class=" bibr popnode">Zhu et al., 2013), and graphitic carbon nitride (href="#bib10" rid="bib10" class=" bibr popnode">Cao et al., 2017, href="#bib14" rid="bib14" class=" bibr popnode">Dong et al., 2015, href="#bib21" rid="bib21" class=" bibr popnode">Hu et al., 2016a, href="#bib22" rid="bib22" class=" bibr popnode">Hu et al., 2016b, href="#bib23" rid="bib23" class=" bibr popnode">Hu et al., 2016c, href="#bib24" rid="bib24" class=" bibr popnode">Hu et al., 2017, href="#bib36" rid="bib36" class=" bibr popnode">Liang et al., 2017, href="#bib58" rid="bib58" class=" bibr popnode">Wang et al., 2017a, href="#bib59" rid="bib59" class=" bibr popnode">Wang et al., 2017b)), still these catalysts either give highly limited N2 conversion rate (even with large excessive N2 flowing of >1 L·min⁻¹) or pose stability problems and undergo photo-self-decomposition during reaction (href="#bib20" rid="bib20" class=" bibr popnode">Hoshino, 2001). Such dilemma has been keeping recent N2 fixation researches from gaining further practical impact. Therefore, an innovative design that offers the solution and inspires the potential photo-fixation advancement toward a more sustainable future remains highly desirable.Ever since the first single-crystalline growth in 1969 (href="#bib39" rid="bib39" class=" bibr popnode">Maruska and Tietjen, 1969), nitride semiconductors, represented by gallium nitride (GaN), have attracted much research attention for their extreme stability (melting point >2,500°C), which at the same time enables more than six magnitudes of voltage carrying enhancement and dissipation inhibition compared with conventional oxide semiconductors (href="#bib49" rid="bib49" class=" bibr popnode">Pearton, 1997). With those superior properties, the nitride semiconductor is believed to serve as the semiconductor of the future (href="#bib1" rid="bib1" class=" bibr popnode">Akasaki, 2002). We have demonstrated the superior performance of GaN and its derivatives to carry out highly efficient catalytic photo-water-splitting (href="#bib15" rid="bib15" class=" bibr popnode">Fan et al., 2015, href="#bib31" rid="bib31" class=" bibr popnode">Kibria et al., 2016, href="#bib57" rid="bib57" class=" bibr popnode">Wang et al., 2011, href="#bib61" rid="bib61" class=" bibr popnode">Xu et al., 2018). In early 2017, we reported the preliminary results about GaN as an efficient photosensitizer to conduct ruthenium(Ru)-catalyzed photochemical N2 fixation, which achieved record catalytic conversion rate (href="#bib35" rid="bib35" class=" bibr popnode">Li et al., 2017). Nevertheless, the rate per gram of catalyst was still limited. In addition, the fixation still requires the flammable and non-sustainable H2 gas as the reductant with scarce noble metal as the catalyst. We then contemplated the feasibility to eliminate the use of hydrogen by using water as reductant to conduct highly efficient GaN-catalyzed N₂ fixation. Herein, we would like to report photochemical N₂ fixation using only N₂ and water catalyzed by GaN nanowire (NW).