ArticleEnhanced ambient ammonia photosynthesis by Mo-doped Bi5O7Br nanosheets with light-switchable oxygen vacancies
Graphical Abstract
Enhanced ambient ammonia photosynthesis is achieved using Mo-doped Bi5O7Br nanosheets; these catalysts exhibit sufficient numbers of light-switchable oxygen vacancies serving as active sites, an optimized electronic structure, and a suitable absorption edge, resulting in enhanced N2 fixation photoactivity.
Introduction
Ammonia (NH3) is one of the most vital commodity chemicals in the modern chemical industry and represents a key precursor for the production of fuel, fertilizers, and potential energy carriers [1, 2, 3]. The common method used for the industrial production of NH3 is the energy-intensive Haber–Bosch process (673–873 K and 15–25 MPa) [4, 5], which unsustainably employs natural gas as hydrogen (H2) feedstock, with enormous energy consumption from fossil fuels [6, 7] leading to large amounts of carbon dioxide (CO2) emissions [8, 9]. To address these problems, a variety of research efforts have been directed toward the discovery of a sustainable and economical process for NH3 production.
Introducing solar energy or electricity in the nitrogen (N2) fixation process could reduce the associated energy input and CO2 emissions [10, 11]. Among various methods, photocatalytic N2 fixation is regarded as one of the most appealing approaches for directly promoting the reduction of N2 to NH3 under atmospheric pressure and room temperature in water. However, the efficiency of most traditional photocatalysts is still unsatisfactory, mainly due to the difficult bond dissociation of the inert N2 molecules [12, 13]. The N≡N bond possesses a huge energy of 940.9 kJ mol−1 [14], which leads to a weak binding of N2 to the catalytic material and further results in an inefficient electron transfer from the photocatalyst to the N2 antibonding orbitals [15]. A promising strategy to increase the efficiency of N2 photofixation involves introducing electron-donating centers as catalytic activation sites for optimizing the N2 adsorption properties and improving the photoexcited charge transport in the catalysts.
Oxygen vacancies (OVs) represent the most widely studied type of surface defects for N2 fixation [16, 17]. On one hand, OVs can be easily created because of their relatively low formation energy [18]; on the other hand, OV can assist photocatalysts to gain exciting N2 fixation photoactivity by virtue of their superior N2 capture and activation properties [19]. Therefore, a semiconductor with a sufficient number of OVs may achieve an improved N2 fixation performance. Transition metal (TM) doping is another widely investigated practical strategy to improve the N2 fixation photoactivity [20], because the TM species possess the advantageous ability of binding (and even functionalizing) inert N2 at low temperatures, owing to their empty and occupied d orbitals, which can form TM-N2 interactions via “acceptance-donation” of electrons [21]. Mo, as a critical component of the catalytic center in the complex Mo-dependent nitrogenase [22, 23], has attracted much attention for N2 fixation [24]. Hao et al. [25] reported that edge-exposed unsaturated Mo atoms served as catalytic centers for N2 fixation, resulting in high NH3 evolution rates that benefited from the steady N2 adsorption by the Mo-based catalytic centers and the enhanced electron transfer between the exciton donor and the reactant. In this context, OV-rich and Mo-doped materials would be ideal candidates for N2 photofixation.
Recent research has shown that layered bismuth oxybromide (BiOBr) materials exhibit outstanding performance in photocatalytic N2 fixation, because of their suitable band gaps and unique layer structures [26]. In the case of BiOBr-based semiconductors, such as Bi3O4Br [27] and Bi5O7Br [28], it has been shown that OVs with sufficient localized electrons on their surface facilitate the capture and activation of inert N2 molecules. For example, Di et al. reported a novel thin structure consisting of single-unit-cell Bi3O4Br nanosheets with abundant surface defects, which improved the photocatalytic N2 fixation efficiency by simultaneously enhancing the N2 adsorption energy and promoting the bulk and surface charge separation [27].
Herein, we report that the introduction of OVs and Mo dopants into Bi5O7Br nanosheets can markedly improve the N2 fixation photoactivity. The modified photocatalysts showed an optimized conduction band position, enhanced light absorption, as well as improved N2 adsorption and charge carrier separation, whose combination contributed to increase the N2 fixation photoactivities. In particular, the Mo-doped Bi5O7Br nanosheets containing OVs displayed volcano-type curves of N2 photofixation activities as a function of the Mo content. The 1 mol% Mo-doped Bi5O7Br sample achieved an NH3 generation rate of 122.9 μmol g−1 h−1, 3.8 times higher than that of blank Bi5O7Br, and exhibited excellent stability with negligible photoactivity loss after five cycles.
Section snippets
Chemicals
Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), sodium molybdate (Na2MoO4), potassium bromide (KBr), hydrochloric acid (HCl), ammonium chloride (NH4Cl), para-(dimethylamino) benzaldehyde (C9H11NO), and ethanol (C2H6O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nessler's reagent was supplied by Macklin Reagent (Shanghai, China). Polyvinyl pyrrolidone (PVP) was supplied by Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents were used directly,
Results and discussion
The Mo-doped Bi5O7Br nanosheets were prepared via the feasible hydrothermal process schematically shown in Fig. 1(a); in which Mo was introduced in the Bi5O7Br lattice using Na2MoO4 as the metal salt precursor. The obtained samples are denoted as Mo-Bi5O7Br-x, where x is the molar percentage of doped Mo. The X-ray diffraction (XRD) patterns of the crystalline phase of the Mo-doped Bi5O7Br samples are displayed in Fig. S1. The diffraction peaks at 24.46°, 30.18°, 32.64°, 46.76°, 51.18°, and
Results and discussion
The Mo-doped Bi5O7Br nanosheets were prepared via the feasible hydrothermal process schematically shown in Fig. 1(a); in which Mo was introduced in the Bi5O7Br lattice using Na2MoO4 as the metal salt precursor. The obtained samples are denoted as Mo-Bi5O7Br-x, where x is the molar percentage of doped Mo. The X-ray diffraction (XRD) patterns of the crystalline phase of the Mo-doped Bi5O7Br samples are displayed in Fig. S1. The diffraction peaks at 24.46°, 30.18°, 32.64°, 46.76°, 51.18°, and
Results and discussion
The Mo-doped Bi5O7Br nanosheets were prepared via the feasible hydrothermal process schematically shown in Fig. 1(a); in which Mo was introduced in the Bi5O7Br lattice using Na2MoO4 as the metal salt precursor. The obtained samples are denoted as Mo-Bi5O7Br-x, where x is the molar percentage of doped Mo. The X-ray diffraction (XRD) patterns of the crystalline phase of the Mo-doped Bi5O7Br samples are displayed in Fig. S1. The diffraction peaks at 24.46°, 30.18°, 32.64°, 46.76°, 51.18°, and
Conclusions
In summary, we have demonstrated that the photoactivity of Bi5O7Br nanosheets for N2 reduction to NH3 can be greatly improved by introduction of light-induced OVs and Mo dopants in Bi5O7Br. The Mo-doped Bi5O7Br nanosheets showed volcano-type photoactivities for N2 reduction as a function of the Mo content. In particular, Mo-Bi5O7Br-1 nanosheets with suitable absorption edges, abundant light-switchable surface OVs, and improved charge carrier separation exhibited optimal and stable catalytic
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This work was supported by the Natural Science Foundation of China (21872029, 22072023, U1463204), the Program for National Science and Technology Innovation Leading Talents (00387072), the 1st Program of Fujian Province for Top Creative Young Talents, the Program for Leading Talents of Fujian Universities, the Award Program for Minjiang Scholar Professorship, and the Natural Science Foundation of Fujian Province (2017J07002, 2019J0106).
Available online 20 August 2021