Research Article pubs.acs.org/acscatalysis
Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts Hiroaki Hirakawa,† Masaki Hashimoto,† Yasuhiro Shiraishi,*,†,‡ and Takayuki Hirai† †
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan ‡ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Ammonia (NH3) is an essential chemical in modern society, currently manufactured via the Haber−Bosch process with H2 and N2 under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO3−) contained in wastewater is one potential nitrogen source. Selective NO3−-to-NH3 transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH3 production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N2) via five-electron reduction of NO3−. Here, we report that unmodified TiO2, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO3−-to-NH3 reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO2 behave as reduction sites for NO3−. The surface defect selectively promotes eight-electron reduction (NH3 formation), while the Lewis acid site promotes nonselective reduction (N2 and NH3 formation). Therefore, the TiO2 with a large number of surface defects and a small number of Lewis acid sites produces NH3 with very high selectivity. KEYWORDS: photocatalysis, titanium dioxide, oxygen vacancies, nitrate anion, ammonia
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INTRODUCTION Ammonia (NH3) is an irreplaceable chemical for production of agricultural fertilizers and artificial fibers.1,2 It has also received much attention as a potential hydrogen carrier, because of its high hydrogen density (17.6 wt %) and low liquefying pressure (∼8 atm).3,4 NH3 is currently manufactured via the Haber− Bosch process from H2 and N2. The process, however, requires extremely high reaction pressure (>200 bar) and hightemperature (>673 K) conditions and involves a large amount of H2 produced by steam reforming of fossil fuels.5,6 A catalytic process to produce NH3 without H2 at atmospheric pressure and room temperature is desired for clean, safe, and sustainable NH3 synthesis. The high energy consumption in the Haber−Bosch process is ascribed to the high dissociation energy of the NN bond of N2 (941 kJ mol−1).7 Therefore, the use of an alternative nitrogen source other than N2 is one possible solution. Nitrate anion (NO3−) is one potential nitrogen source, because the NO bond has a relatively low dissociation energy (204 kJ mol−1),8 and it is contained in large amounts in agricultural waste produced by bacterial decomposition of fertilizers.9 NO3− is very toxic to the human body; its absorption into the body can cause several diseases, such as methemoglobinemia and non-Hodgkin’s lymphoma.10 Therefore, the maximum permissive level of NO3− in drinking water has been set at 45 ppm by the World Health Organisation (WHO).11 Selective NO3−-to© 2017 American Chemical Society
NH3 reduction, if promoted under ambient conditions, may therefore create a powerful green process for NH3 production via detoxification and recycling of waste resources. Photocatalysis on semiconductor catalysts is one promising method for this purpose, because it proceeds at atmospheric pressure and room temperature.12−14 Photocatalytic reduction of NO3− can be promoted with electron donors such as formic acid (HCOOH) as follows: the photoformed valence band holes (VB h+) oxidize HCOOH and produce CO2 and H+ (eq 1). HCOOH + 2h+ → CO2 + 2H+
(1) −
The conduction band electrons (CB e ) promote eightelectron reduction of NO3− (eq 2): NO3− + 9H+ + 8e− → NH3 + 3H 2O
(1.20 V vs NHE) (2)
As a result of this, NO 3 photocatalytically (eq 3):
−
can be converted to NH 3
NO3− + 4HCOOH + H+ → NH3 + 3H 2O + 4CO2 (3) Received: February 24, 2017 Revised: April 15, 2017 Published: April 19, 2017 3713
DOI: 10.1021/acscatal.7b00611 ACS Catal. 2017, 7, 3713−3720
Research Article
ACS Catalysis
Scheme 1. Schematic Drawing of (a) Rutile (110), (b) Anatase (110), and (c) Rutile (001) Surfaces,a and Catalytic Cycles for NO3− Reduction on (d) Surface Defects and (e) Lewis Acid Sites on TiO2
a
Here, the green spheres represent bulk O atoms, the black spheres represent Ti atoms, and the light green spheres represent surface O atoms.
Table 1. Properties of TiO2 Particles and Their Performance for Photocatalytic NO3− Reductiona Amount (μmol) entry
catalyst
1 2 3
JRC-TIO-1j ST-01k Aldrich anatase P25j Aldrich rutile Wako rutile MT-150Al JRC-TIO-6j Cu/P25m CuPd/P25n
4 5 6 7 8 9 10
crystalline phaseb A A A A83/R17 A31/R69 R R R
d
−
particle sizec (nm)
SBET (m2 g−1)
NO3 conversion (%)
16 7 36
71 217 67
27, 340o 172 276 11 21
59 28 15 125 100
e
NH3 (NH4+) selectivityg (%)
N2f
NH3 (NH4+)e
87 >99 95
14.9 20.2 19.0
13.6 9.5 9.2
99 79 76 >99
17.7 15.8 0.7 4.4
