Reaction Mechanism of Ammonia Decomposition to Nitrogen and


Jan 10, 2012 - The reaction mechanism of ammonia decomposition to nitrogen and hydrogen over platinum loaded titanium oxide photocatalyst was ...
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Reaction Mechanism of Ammonia Decomposition to Nitrogen and Hydrogen over Metal Loaded Titanium Oxide Photocatalyst Hayato Yuzawa,†,§ Takamasa Mori,† Hideaki Itoh,‡ and Hisao Yoshida*,† †

Department of Applied Chemistry, Graduate School of Engineering and ‡Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan ABSTRACT: The reaction mechanism of ammonia decomposition to nitrogen and hydrogen over platinum loaded titanium oxide photocatalyst was investigated through various reaction tests, as well as ESR and FT-IR spectroscopies. The photoformed hole on the titanium oxide oxidizes NH3 to form amide radical (•NH2) and proton. The amide radicals produce hydrazine (N2H4), and it can be further decomposed to form nitrogen and hydrogen. On the other hand, the photoformed electron migrates to platinum nanoparticles through the conduction band of the titanium oxide and reduces the proton to yield hydrogen. The metals with larger work function, such as platinum, can provide more effective cocatalysts. In this photocatalytic reaction system, water molecules were essential for the continuous reaction progress. An in situ FT-IR study clarified that water restricted the accumulation of inactive byproduct, ammonium ion (NH4+), on the titanium oxide surface. titanium oxide.30 However, the reaction mechanism to explain these observations has not been clarified in detail. Thus, in the present study, we investigated the photocatalytic ammonia decomposition of both gaseous and aqueous ammonia on metal loaded titanium oxide in detail, and successfully clarified the mechanism.

1. INTRODUCTION Ammonia is one of the major nitrogen-containing pollutants in wastewater.1 Although ammonia conversion processes such as break-point chlorination and biological nitrification were established,1 the resulting compounds (chloramine compounds (NH3−xClx, x = 1−3) and NO3−, respectively) are still harmful and useless. On the other hand, ammonia decomposition (NH3 → 1/2N2 + 3/2H2) is more preferable, since harmless nitrogen and useful hydrogen can be yielded. However, conventional catalytic decomposition processes consume a lot of energy.2−4 Therefore, a more environmentally benign process has been desired. Photocatalysis has received much attention because of its potential to utilize photoenergy for production of photoexcited electron (e−) and hole (h+), which can promote even uphill reactions at room temperature and even in water.5−13 On metal-loaded titanium oxide photocatalyst as one of the most investigated photocatalysts, it is believed that the loaded metal promotes the reductive reaction and receives the photoexcited electrons from the conduction band of titanium oxide, while the titanium oxide surface promotes the oxidative reaction with the photoformed holes. Photocatalytic conversion of ammonia has also been intensively investigated. Most of the studies have concerned ammonia oxidation to N2, N2O, NO2, and NO3− over titanium oxide,14−23 platinum loaded titanium oxide,24,25 titanate nanotube,26,27 and Ru(bpy)32+.28 On the contrary, only a few studies focused on the photocatalytic ammonia decomposition to nitrogen and hydrogen.29,30 Li et al. reported that the decomposition of gaseous and aqueous ammonia could proceed with stoichiometrical molar ratio of the products (H2:N2 = 3:1) over NiO and RuO2 loaded SrTiO3 or BaTiO3.29 Nemoto et al. reported the optimum condition (platinum loading amount and reaction pH) for the decomposition of ammonia in aqueous solution over platinum loaded © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Reagents. Titanium oxide samples, JRC-TIO-4 (anatase/rutile, 50 ± 15 m2 g−1), JRC-TIO-6 (rutile, 100 m2 g−1), and JRC-TIO-8 (anatase, 338 m2 g−1), were supplied by Catalysis Society of Japan, and they were referred to as TiO2(A/R), TiO2(R), and TiO2(A), respectively, in this paper. Precursors of loading metal were chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Wako, 99.9%), ammonium hexachlororhodate monohydrate ((NH4)3RhCl6·H2O, Kishida, 99.9%), palladium nitrate (Pd(NO3)2, Kishida, 99.9%), ruthenium chloride (RuCl3, Kishida, 99.9%), gold chloride acid tetrahydrate (HAuCl4·4H2O, Kishida, 99%), nickel nitrate (Ni(NO3)2, Wako, 98%), and copper nitrate (Cu(NO3)2, Wako, 99.9%). Other reagents were methanol (Kishida, 99.8%), gaseous ammonia (Sogo, 99.999%), aqueous ammonia (Kishida, conc. 15 M), ammonium sulfate (Kishida, 99.5%), hydrazine (Kishida, >80%), acetone (Kishida, 99.5%), and sodium hydroxide (Wako, 96%). These reagents were used as received. 2.2. Preparation and Characterization of Metal Loaded Titanium Oxide Photocatalysts. Metal loaded titanium oxide was prepared by a photodeposition method for platinum, rhodium, palladium, and gold and an impregnation Received: October 12, 2011 Revised: November 28, 2011 Published: January 10, 2012 4126

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Figure 1. Time course of production rate for hydrogen (red circles) and nitrogen (blue triangles) and molar ratio of the produced hydrogen to nitrogen (green squares) in the photocatalytic ammonia decomposition by using the Pt(0.1)/TiO2(A) sample with flowing water vapor (25 μmol/min) (a) and without flowing water vapor (b). Reactor, a fixed-bed flow reactor; catalyst, 0.6 g; flow rate of ammonia, 1 mL/min (45 μmol/min); argon, 20 mL/min; water vapor, 25 μmol/min; irradiated light intensity, 50 and 150 mW/cm2 when measured at 254 ± 10 and 365 ± 15 nm, respectively.

and water (0−20 μL, corresponding to 0−1.1 mmol) were introduced, the photocatalyst was photoirradiated for 3 h. Gas products were collected with a gastight syringe and analyzed by a GC-TCD (Shimadzu, GC-8A). To estimate the photocatalytic reaction rate upon the wavelength of light from λ1 to λ2 nm (λ1 < λ2), the production rate was measured by using each cutoff filter, which cut off the light of wavelengths shorter than λ1 and λ2 nm, respectively, and the difference between them was adopted. Photocatalytic decomposition of aqueous ammonia was carried out in the closed system. The Pt(0.1)/TiO2(A) sample (0.2 g) in the quartz cell was photoirradiated with the Xe lamp from beneath in air atmosphere, and then, the gas phase was purged by Ar. After an aqueous ammonia solution (15 M, 2 mL) was introduced, the photocatalyst was photoirradiated from the Xe lamp for 3 h at 308 K in Ar atmosphere with magnetic stirring. In order to quantify produced hydrazine (N2H4) by stoichiometric conversion to acetone azine ((CH3)2CNNC(CH3)2), acetone (5 mL) was added to the resulting liquid phase. Gas phase products were analyzed by a GC-TCD (Shimadzu, GC-8A), and liquid phase products were analyzed by a GC-MS (Shimadzu, QP-5050A). 2.4. Spectroscopic Study. ESR measurement was carried out with an X-band spectrometer (JEOL, JES-TE200). The photocatalyst sample was stirred in distilled water at room temperature for 24 h and dried at 383 K for 24 h. The obtained sample was granulated to the size of 400−600 μm and heated at 773 K for 3 h. Before the measurement, 0.3 g of the sample in a quartz cell was calcined at 673 K for 90 min under oxygen (80 Torr) atmosphere and evacuated at 673 K for 30 min, followed by introduction of gaseous ammonia into the cell at room temperature. After adsorption equilibrium of the gaseous ammonia was achieved, the cell was sealed and the ESR spectra were recorded. The measurements under photoirradiation were carried out with a 500 W ultrahigh-pressure Hg lamp. In situ diffuse reflectance FT-IR spectra of adsorbed species on the catalyst surface were recorded with a JASCO FT/IR6100. The Pt(0.1)/TiO2(A) sample in the sample holder was photoirradiated from the Xe lamp in air atmosphere, and heated at 473 K for 2 h under flowing He to clean up the catalyst surface. Gaseous ammonia (1 mL/min (45 μmol/min)) diluted by He (20 mL/min) was introduced into the cell until the adsorption equilibrium was obtained (130 min), and then, the spectra were recorded under the various conditions. All of the spectra were measured under the flow of ammonia (1 mL/min (45 μmol/min)) diluted by He (20 mL/min). Water vapor (25 μmol/min) was supplied from the saturator with He carrier.

method for nickel and copper. In the former method, the titanium oxide powder (4 g) was dispersed into an aqueous methanol solution (400 mL, methanol concentration 25%) of the metal precursor in a beaker with vigorous stirring, followed by photoirradiation from the top with a 300 W Xe lamp, which emitted both UV and visible light, in air atmosphere for 3 h. Then, the suspension was filtered off with suction, washed with water, and dried at 323 K overnight. The loading amount of platinum was from 0.05 to 1.0 wt %, and that of the other metals was 0.1 wt %. In this paper, x wt % metal loaded titanium oxide is described as M(x)/TiO2. To examine the influence of specific surface area for the Pt(0.1)/TiO2(A) sample, the TiO2(A) sample was calcined at 773−1123 K for 1−5 h before loading the platinum. In the latter method, the titanium oxide powder (4 g) was dispersed into an aqueous solution of the metal precursor (100 mL), followed by evaporation of water on a hot plate with stirring. The obtained sample was calcined at 773 K for 1 h and reduced at 673 K in a flow of hydrogen (20 mL/min) for 1 h. Powder X-ray diffraction was recorded at room temperature with a Rigaku diffractometer RINT 2500 using nickel filtered copper Kα radiation, and the average crystallite diameter of titanium oxide particles was calculated by the Scherrer equation. The average platinum particle size on the Pt/TiO2(A) samples was estimated by using a CO-pulse method in a flow system after the reductive pretreatment with hydrogen at 423 K for 20 min. The specific surface area of the prepared photocatalysts was calculated by the BET method from the amount of adsorbed nitrogen at 77 K measured with a Quantachrome Monosorb. The diffuse reflectance UV−vis spectrum of the Pt(0.1)/TiO2(A) sample was measured with a JASCO V-570 spectrophotometer. 2.3. Photocatalytic Reaction Test. Photocatalytic decomposition of gaseous ammonia was carried out in a fixed-bed flow reactor, or a closed reactor connected with a vacuum line. In the former system, a quartz cell (60 × 20 × 1 mm3) was filled with 0.6 g of the prepared photocatalyst, which was granulated to the size of 400−600 μm and photoirradiated with the 300 W Xe lamp at 323 K in a flowing mixture of ammonia gas (1 mL/min (45 μmol/min)) and Ar (20 mL/min). Water vapor (25 μmol/min) was introduced from a saturator with Ar carrier. Obtained gas products were analyzed by an online GCTCD (Shimadzu, GC-8A). In the latter system, the prepared photocatalyst sample (0.2 g) in another quartz cell was photoirradiated from beneath (18 cm2) by the Xe lamp in air atmosphere for 1 h followed by evacuation at 473 K for 2 h to clean up the catalyst surface. After ammonia gas (3.9 mmol) 4127

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Table 1. Photocatalytic Decomposition of Gaseous Ammonia on the Pt(0.1)/TiO2 Samplesa production rated (μmol min−1) entry

photocatalyst

1 2 3 4 5 6 7

Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A)e Pt(0.1)/TiO2(A/R) Pt(0.1)/TiO2(R)

calcination temperature (K) 773 773 923 1123

calcination time (h)

BET S.S.A.b (m2 g−1)

crystallite diameterc (nm)

H2

N2

molar ratio H2/N2

255 92 82 37 5.8 45 84

7 9 17 19 26 20 15

5.6 4.3 3.9 1.9 0.33 2.8 0.10

1.8 1.4 1.3 0.63 n.d.f 0.91 n.d.f

3.1 3.1 3.0 3.0

1 5 5 5

3.1

a

As for the reaction conditions, see the caption of Figure 1. bBET specific surface area of the Pt(x)/TiO2(A) samples calculated by the amount of N2 adsorption at 77 K. cCalculated by the Scherrer equation. dProduction rate measured after continuous reaction for 3 h. The detection limits of hydrogen and nitrogen were 0.002 and 0.02 μmol/min, respectively. eCrystal phase of titanium oxide was transformed from anatase to rutile. fn.d. = not detected.

facts indicate that the deactivation of the TiO2(A) sample was derived from the reduction of the Ti4+ to Ti3+ by photoformed electron as often reported in the literature.33,34 Thus, it was confirmed that, although the TiO2(A) sample has the ability to decompose ammonia, it cannot consume the photoformed electron efficiently without the deposited platinum nanoparticles. From the above results, both the platinum on the titanium oxide and water molecules were essential for the continuous photocatalytic reaction. The platinum would contribute to accelerate the reaction by the effective consumption of the photoformed electron, as reported in the literature.35 The role of water will be discussed later. 3.2. The Reaction Field. Since the structure of the titanium oxide is often one of the controlling factors for the photocatalytic reaction, the photocatalytic activity of the samples with various crystal phases and specific surface areas was investigated. Table 1 and Figure 2 show the results of photocatalytic

The spectra shown were obtained by using the spectrum of the pretreated Pt(0.1)/TiO2 (A) sample as the reference.

3. RESULTS AND DISCUSSION 3.1. Photocatalytic Decomposition of Ammonia. Figure 1 shows the results of the photocatalytic reaction tests. When the Pt(0.1)/TiO2(A) sample was used in a flow of ammonia and water vapor, nitrogen and hydrogen were constantly produced (Figure 1a). Although the production rate varied with time in the initial period, the reaction rate became steady and it constantly lasted at least 72 h. In addition, the molar ratio of the products was stoichiometric, i.e., 3. Photolysis of gaseous ammonia produced very slowly upon photoirradiation without the photocatalyst. No reactions occurred in the dark with the photocatalyst. These facts confirmed that the photocatalytic decomposition of ammonia proceeds as shown in eq 1. hν NH3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 1 2 N2 + 3 2 H2 Pt/TiO2 ,H2O

(1)

The change of Gibbs free energy (ΔG298K ° ) of this reaction is 33 kJ/mol,31 which means that the photocatalyst can convert the photoenergy into chemical energy through this reaction.32 As shown later, the deposited platinum nanoparticle is the active site on the photocatalyst. Under these conditions, the turnover frequency (TOF) per surface platinum atom and the conversion of ammonia under the optimum conditions in the present reactor were 2.3 min−1 and 9.6% (contact time: 3.0 s), respectively, supporting that the reaction photocatalytically proceeded. In the case of the Pt(0.1)/TiO2(A) sample without flowing water vapor, although the nitrogen and hydrogen were also produced with the ideal molar ratio (Figure 1b), the reaction rate gradually decreased with the reaction time. Thus, it was found that water molecules were required for the continuous photocatalytic reaction. When the TiO2(A) sample was used without a flow of water vapor, nitrogen and hydrogen were initially obtained with 100 times lower rate, 0.064 μmol/min. The molar ratio of hydrogen to nitrogen for the initial products was 2.8, which was a little smaller than the stoichiometrical ratio of 3. However, their production rates rapidly dropped down to an undetectable level within 1 h and the color of the TiO2(A) sample changed from white to pale blue. The color was however returned to white upon exposure to air. The same result was also obtained in the reaction by using the TiO2(A) sample with water vapor. These

Figure 2. The hydrogen production rate over the various Pt(0.1)/ TiO2 photocatalysts. Red, green, and blue circles correspond to the TiO2(A), TiO2(A/R), and TiO2(R) samples, respectively. The open symbols, two filled symbols, and a half-filled symbol indicate anatase, rutile, and anatase/rutile mixture, respectively. As for the reaction conditions, see the caption of Figure 1.

reaction test on the samples with various titanium oxides. Among the investigated platinum loaded titanium oxide samples, the highest product yield was obtained on the platinum loaded anatase, i.e., the Pt(0.1)/TiO2(A) sample with the high surface area (Table 1, entry 1). Among the various Pt(0.1)/TiO2(A) samples, the hydrogen production rate increased with increasing BET specific surface area of the sample (Table 1, entries 1−4); especially the hydrogen production rate was proportional to the 4128

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specific surface area in the range below 100 m2 g−1 (Figure 2, solid line). This tendency suggests that the surface active sites exist with a certain density and quality for these Pt(0.1)/TiO2(A) samples, as reported, e.g., for photocatalytic decomposition of acetic acid36 and photocatalytic degradation of toluene37 over titanium oxide. However, on the uncalcined Pt(0.1)/TiO2(A) sample with a high specific surface area of 255 m2g−1, the hydrogen production rate was slower than the expected one from the proportional relationship (Figure 2, dotted line). This might be because the uncalcined TiO2(A) sample had a larger quantity of defect sites than the additionally calcined samples. The hydrogen yield on the Pt(0.1)/TiO2(A/R) sample was slightly higher compared to the sample consisting of TiO2(A) calcined at 923 K, although they had a crystallite size and specific surface area similar to each other (Table 1, entries 6 and 4). This might be because the junction of anatase and rutile would enhance the charge separation.38,39 On the other hand, when the Pt(0.1)/TiO2(R) sample was used, the production rate was much lower than those on the Pt(0.1)/TiO2(A) and Pt(0.1)/TiO2(A/R) samples (Table 1, entry 7). This result was consistent with the fact that photocatalytic activity of rutile is generally lower than that of anatase.40 Therefore, the crystal phase, the crystallite size, and the quantity of the surface active sites of titanium oxide were the controlling factors for the reaction activity. This confirms that the titanium oxide surface provided the reaction field. 3.3. Active Sites for Reduction. To elucidate the role of loading metal cocatalyst, the reaction was carried out with various M(0.1)/TiO2(A) samples (Table 2 and Figure 3a).

original catalytic property of these loaded metals for the hydrogen production but the charge separation ability for the photoformed hole and electron pair, as reported in the literature.42 In other words, the loaded metal with larger work function, i.e., lower Fermi level, would more easily accept the photoexcited electron from the conduction band (−0.16 V vs NHE,43 Figure 3c) to promote the reductive reaction (eq 2).

2H+ + 2e− → H2

Figure 4 shows the effect of platinum loading amount (0.05− 1.0 wt %) on the reaction rate after the continuous reaction for 3 h. Among the samples with various platinum loadings, the Pt(0.1)/TiO2(A) sample gave the highest reaction rate. On the Pt(0.05)/TiO2(A) sample, although the reaction rate was enhanced compared to the bare TiO2(A), the molar ratio of hydrogen and nitrogen was 2.7 (Figure 4a), which was lower than the stoichiometrical ratio, and the reaction rate was gradually decreased to 60% of the initial reaction rate for 3 h. This phenomenon was similar to the results on the bare TiO2(A), as described above. Thus, 0.05 wt % platinum for the TiO2(A) would not be enough quantity for the efficient consumption of the photoformed electron. On the other hand, the larger amount of platinum (0.5, 1.0 wt %) than 0.1 wt % provided a lower reaction rate than that for the Pt(0.1)/TiO2(A) sample, although the stoichiometrical molar ratio of the produced hydrogen to nitrogen was conserved for at least 5 h. In these samples, the average diameter of platinum particles was larger than that for the Pt(0.1)/ TiO2(A) sample, as shown in Figure 4b. It is known that the work function of metal generally decreases with increasing size of metal particles (Figure 4b).44 Thus, the lower reaction rate would be derived from the decrease in the work function of the loaded platinum nanoparticles, as discussed above. Note that the platinum nanoparticles having such a large work function actually exhibited the high photocatalytic activity through the reduction of proton (eq 2). This means that the actual potential of the platinum nanoparticles upon photoirradiation would be higher than the redox potential of the adsorbed protons on them. As other possibilities, the metal particles would partially block the incident light and might contribute the decrease of the photocatalytic activity, as proposed in the literature,35 or the number of the surface platinum atoms might reflect the activity. In the present study, the optimum photocatalyst for the ammonia decomposition was the 0.1 wt % platinum-loaded anatase with high specific surface area, i.e., the Pt(0.1)/TiO2(A) sample. The high activity of the sample would originate from the large surface area of titanium oxide, which would provide the large reaction field, and the loaded platinum nanoparticles with the large work function, which would contribute to the effective separation of the photoformed electron. 3.4. Photoexcitation. Figure 5 shows the action spectrum and DR UV−vis spectrum for the Pt/TiO2(A) sample. As shown, the action spectrum of the decomposition of gaseous ammonia was almost consistent with the shape of the DR UV− vis spectrum except for the region around 370−400 nm. This indicates that the photocatalytic decomposition of gaseous ammonia would mainly proceed through the bandgap excitation of titanium oxide. In the region around 385 nm, the production rate was slightly higher than the corresponding light absorption. This region of light can contribute to the activation of ammonia chemisorbed on the surface Ti site of titanium oxide through the excitation from the N 2p orbital of ammonia to the Ti 3d orbital, which was reported in the photocatalytic

Table 2. Effect of the Loading Metal on the Photocatalytic Decomposition of Gaseous Ammonia over the M(0.1)/ TiO2(A) Samplesa production ratec (μmol min−1) loading metal

work functionb (eV)

H2

N2

molar ratio H2/N2

Pt Rh Pd Au Ni Cu none

5.41 5.02 5.02 4.81 4.75 4.56

6.5 2.6 1.5 0.11 0.05 0.02 n.d.d

2.2 0.87 0.51 0.04 0.02 n.d.d n.d.d

3.0 3.0 3.0 2.8 2.8

(2)

a

As for the reaction conditions, see the caption of Figure 1. bWork function of bulk metal from ref 41. cThe production rate was measured after continuous reaction for 3 h. The detection limits of hydrogen and nitrogen were 0.002 and 0.02 μmol/min, respectively. dn.d. = not detected.

Each photocatalyst promoted the reaction almost constantly at higher reaction rate than the bare titanium oxide to produce nitrogen and hydrogen with almost stoichiometrical product ratio. Although the nitrogen yield on the Cu(0.1)/TiO2(A) sample was lower than the detection limit, the reaction rate would be higher than that on the bare titanium oxide. Among these M(0.1)/ TiO2(A) samples, the metal having larger work function was effective for the gaseous ammonia decomposition and the platinum most promoted the reaction (Table 2). In addition, the work function of bulk metal41 and logarithmic of the hydrogen production rate was almost a linear relationship, as shown in Figure 3b. This relationship suggests that the variation of their photocatalytic activity would be mainly derived from not the 4129

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Figure 3. Relationship between the hydrogen production rate over the M(0.1)/TiO2(A) samples and work function of corresponding bulk metal (a), the logarithmic plot (b), and the standard potentials for the band edge positions of titanium oxide and the Fermi level of each loading metal (c). As for the reaction conditions, see the caption of Figure 1.

Figure 5. DR UV−vis spectrum of the Pt(0.1)/TiO2(A) sample diluted by MgO and action spectrum for the photocatalytic ammonia decomposition on the sample. Each wavelength region of the irradiation light (see the text) was indicated by horizontal bars. As for the other reaction conditions, see the caption of Figure 1.

restrict the chemisorption of ammonia on the Ti sites and thus the contribution of this photoexcitation path would be minor. In other words, in our present study, the ammonia adsorbed on the surface hydroxyl group of titanium oxide would mainly react with the photoformed holes from the photoexcited titanium oxide. 3.5. Reaction Intermediates. In the photocatalytic ammonia decomposition over platinum loaded titanium oxide, NH3 and NH4+ should be considered as the candidates for the reactive species, since both species exist in aqueous solution in equilibrium (eq 3).

Figure 4. Relationship between the amount of platinum loading and (a) the production rate over the various Pt(x)/TiO2(A) samples, (b) the average diameter of platinum nanoparticles calculated by the COpulse method and the work function calculated according to refs 41 and 44. The red circles, blue triangles, green squares, orange diamonds, and purple inverse triangles correspond to the production rate for hydrogen, that for nitrogen, the molar ratio of hydrogen to nitrogen, the average diameter of platinum nanoparticles, and the work function of platinum nanoparticles, respectively. As for the reaction conditions, see the caption of Figure 1.

NH3 + H2O ⇄ NH4 + + OH−

(3)

In the case of aqueous ammonia solution (15 M) employed in the present study, the molecular NH3 predominantly exists (15 M) while ammonium ion, NH4+, hardly exists (1.6 × 10−2 M) according to the equilibrium constant (Kb = 1.8 × 10−5)46 at

reaction with ammonia in the absence of flowing water.45 In the present study, the water vapor flowing with ammonia would 4130

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Table 3. Photocatalytic Decomposition of Aqueous Ammonia on the Pt(0.1)/TiO2(A) Samplea products (μmol) entry

photocatalyst

1 2 3

Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A)

4e 5f

Pt(0.1)/TiO2(A)

reagent ammonia (NH4)2SO4 (NH4)2SO4 NaOHd ammonia ammonia

NH3b (mmol)

NH4+ b (mmol) −2

pH

H2

N2

N2H4

molar ratio H2/N2 3.0

30 5.3 × 10−3 7.5

3.2 × 10 25 17.5

13.2 5.6 8.9

304 15.2 164

101 n.d.c 51

1.4 n.d.c 1.0

30 30

3.2 × 10−2 3.2 × 10−2

13.2 13.2

2.2 n.d.c

n.d.c n.d.c

0.4 n.d.c

3.2

a

Reactor, a closed system; catalyst, 0.2 g; aqueous ammonia (ammonium salt) solution, 2 mL; irradiated light intensity, 3 and 26 mW/cm2 (entry 1) and 7 and 53 mW/cm2 (entries 2−4) when measured at 254 ± 15 and 365 ± 15 nm, respectively; reaction time, 3 h. The detection limits for hydrogen and nitrogen, and hydrazine were 0.1, 1.0 μmol, and 0.1 μmol, respectively. bCalculated by equilibrium constant at 298 K. cn.d. = not detected. dThe content of sodium hydroxide was 0.77 mmol. ePhotochemical reaction without the photocatalyst. fCatalytic reaction carried out at 313 K without photoirradiation.

exhibited a small signal indicated with g = 1.972 and 1.982, which was assignable to Ti3+ as reported in the literature.34,45 Upon photoirradiation, the signal intensity of Ti3+ increased (Figure 6b). This clearly shows that the photoformed electrons in the TiO2(A/R) sample were captured by Ti4+ in the conduction band (eq 4).

298 K. To clarify the reactive species, two kinds of the photocatalytic reactions were carried out in the closed system: one was the reaction in an aqueous ammonia solution (the contents of NH3 and NH4+ were 30 and 3.2 × 10−2 mmol, respectively, in the solution of 2 mL, Table 3, entry 1), and another was that in an aqueous ammonium sulfate solution (the contents of NH3, NH4+, and SO42− were 5.3 × 10−3, 25, and 12.5 mmol, respectively, in the solution of 2 mL, Table 3, entry 2). As a result, the reaction rate of the photocatalytic decomposition of NH3 was much higher than that of the photocatalytic decomposition of NH4+ (Table 3, entries 1 and 2). In an additional reaction by using another solution of the same concentration of SO42− and less concentration of NH4+ (the contents of NH3, NH4+, and SO42− were 7.5, 17.5, and 12.5 mmol, respectively, in the solution, Table 3, entry 3), the reaction rate was higher than that of entry 2, indicating that the SO42− scarcely reduces the reaction rate. Thus, it is clear that the dominant reactive species for the ammonia decomposition was not NH4+ but NH3 in this reaction system. This result was consistent with the results in photocatalytic ammonia oxidation in the literature.18 Further, the current result was also consistent with the fact that ammonium ion was an inactive species for the oxidation by hydroxyl radical (•OH), which is one of the strong oxidants, as reported.47 To clarify the active intermediates produced through the activation of NH3, ESR measurement was carried out. Figure 6

Ti4 + + e− → Ti 3 +

(without Pt)

(4)

Excess formation of Ti3+ would result in the release of the lattice oxygen and the formation of a color center, which would be the reason for the color change of titanium oxide during the photocatalytic reaction test, as described above. In addition, a new signal indicated with g = 2.006 and g = 2.009 was observed (Figure 6b), which was assignable to surface amide radical (•NH2).45 These results suggest that the amide radical would be the dominant active species produced from NH3 through the reaction with the photoformed hole (eq 5) or surface hydroxyl radical (•OHs, eqs 6 and 7).

NH3 + h+ → •NH2 + H+

(5)

− OHs + h+ → •OHs

(6)

NH3 + •OHs → •NH2 + H2O

(7)

The Pt(0.1)/TiO2(A/R) sample also exhibited the signal of amide radical upon photoirradiation (Figure 6d). This indicates that the active species is the amide radical also in the presence of the platinum. However, the signal of Ti3+ in the Pt(0.1)/ TiO2(A/R) sample hardly increased in intensity upon photoirradiation (Figure 6c and d). This result indicates that the deposited platinum nanoparticles efficiently received the photoformed electron from the conduction band of the TiO2(A/R) sample (eq 8), as described above, and the reduction of Ti4+ (eq 4) would be restricted.

Pt(nanoparticle) + e− → Ptδ−(nanoparticle)

(8)

3.6. Successive Reactions. When the product analysis was carried out with the addition of acetone in order to detect hydrazine (N2H4) as acetone azine (Table 3), the formation of acetone azine was detected, indicating that hydrazine was formed, although the amount was much smaller than that for the nitrogen and hydrogen (Table 3, entry 1). This suggests that the amide radical as the first intermediate would form hydrazine over platinum loaded titanium oxide, as shown in eq 9.

Figure 6. ESR spectra of the TiO2(A/R) sample adsorbing ammonia before photoirradiation (a) and under photoirradiation (b) and the Pt(0.1)/TiO2(A/R) sample adsorbing ammonia before photoirradiation (c) and under photoirradiation (d).

shows the ESR spectra of adsorbed ammonia on the surface of the TiO2(A/R) sample and the Pt(0.1)/TiO2(A/R) sample. The bare TiO2(A/R) sample before photoirradiation (Figure 6a)

2 • NH2 → N2H4 4131

(9)

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Table 4. Photocatalytic Decomposition of Aqueous Hydrazine on the Samplesa products (μmol) entry

photocatalyst

reagent

amount of reagent (mmol)

H2

N2

1 2b 3b 4d 5

Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) TiO2(A)

hydrazine hydrazine hydrazine hydrazine ammonia

2.5 2.5 2.5 2.5 5.0

319 12.4 n.d.c 0.8 77.6

181 7.3 n.d.c n.d.c 26.7

Pt(0.1)/TiO2(A)

N2H4

molar ratio H2/N2 1.8 1.7

0.6

2.9

a

Reactor, a closed system; catalyst, 0.2 g; aqueous hydrazine (or ammonia) solution, 2 mL; irradiated light intensity, 7 and 53 mW/cm2 when measured at 254 ± 10 and 365 ± 15 nm, respectively; reaction time, 1.5 h. The detection limits for hydrogen and nitrogen, and hydrazine were 0.1, 1.0 μmol, and 0.1 μmol, respectively. bCatalytic reaction carried out at 313 K without photoirradiation. cn.d. = not detected. dPhotochemical reaction without the photocatalyst.

Even in the case of photochemical reaction without the photocatalyst, a very small amount of the hydrazine was detected (Table 3, entry 4). In contrast, the hydrazine was not detected without photoirradiation (Table 3, entry 5) Photocatalytic reaction of aqueous hydrazine was carried out as listed in Table 4. With the Pt(0.1)/TiO2(A) sample, the hydrazine was decomposed into hydrogen and nitrogen (Table 4, entry 1). The ratio of hydrogen and nitrogen was 1.8, which was less than the ideal one, 2.0. This might be due to the low purity of hydrazine used in this study (>80%). When the reaction was carried out without photoirradiation, only small amounts of products were obtained over the Pt(0.1)/TiO2(A) sample (Table 4, entry 2), and no products were detected over the bare TiO2(A) sample (Table 4, entry 3). In the case of photochemical reaction without the photocatalyst, only a very small amount of hydrogen was obtained, suggesting that photochemical reaction hardly contributes to the hydrazine activation (Table 4, entry 4). These results indicate that the hydrazine would be activated by the photocatalysis (major), and by the catalysis (minor) on the platinum sites. The hydrazine would be converted to diazene (N2H2) as the next intermediate through hydrogen abstraction (eq 10).

N2H4 → N2H2 + H2

Figure 7. The photocatalytic gaseous ammonia decomposition on the Pt(0.1)/TiO2(A) sample under transient conditions. The red circles, blue triangles, and green squares correspond to the production rate for hydrogen, that for nitrogen, and the ratio of produced hydrogen to nitrogen, respectively. After the continuous reaction without water vapor for 4 h, water vapor (25 μmol/min) was added to the reaction system with the ammonia gas. As for the reaction conditions, see the caption of Figure 1.

gested that the deactivation of photocatalyst would originate from a reversible phenomenon that could be restricted by water. In addition, to know the effect of the amount of water on the initial stage of the reaction, the photocatalytic decomposition of gaseous ammonia was carried out with a strict amount of water in the batch reactor at low conversion level by using the dry Pt(0.1)/TiO2(A) sample pretreated under evacuation at 473 K for 2 h (Table 5). As shown, the product yields did not

(10)

It is known that the disproportionation into hydrazine and nitrogen and self decomposition into hydrogen and nitrogen can proceed very easily under ambient temperature and pressure, as shown in eqs 11 and 12.48,49

2N2H2 → N2 + N2H4

(11)

N2H2 → N2 + H2

(12)

Table 5. Effect of the Amount of Water on the Photocatalytic Decomposition of Gaseous Ammonia on the Pt(0.1)/ TiO2(A) Sample in the Closed Systema substrates (mmol)

In this way, the hydrazine would be eventually decomposed into nitrogen and hydrogen. Since the amount of produced hydrazine was much less than those of nitrogen and hydrogen, the reactivity of the hydrazine was expected to be higher than that of ammonia. When the decomposition of the aqueous ammonia was examined under the same conditions using the same quantity of nitrogen atom as that for the aqueous hydrazine decomposition (Table 4, entries 1 and 5), the yield of nitrogen and hydrogen was much less than that for the decomposition of aqueous hydrazine. This result supports that the hydrazine was more reactive than the ammonia. 3.7. Role of H2O. Figure 7 shows the photocatalytic reaction test under the transient conditions in the absence and presence of water vapor. In the first period without water vapor for 4 h, the reaction rate gradually decreased. When water vapor was additionally introduced into the reactor, the reaction rate was recovered within 1 h and became constant. Thus, it is sug-

products (μmol)

entry

ammonia

water

H2

N2

molar ratio H2/N2

1 2 3

3.9 3.9 3.9

0 0.3 1.1

130 121 133

39 42 46

3.3 2.9 2.9

a

Reactor, a closed system; catalyst, 0.2 g; irradiated light intensity, 7 and 53 mW/cm2 when measured at 254 ± 10 and 365 ± 15 nm, respectively; reaction time, 3 h. Before the photocatalytic reaction, the catalyst was evacuated at 473 K for 2 h.

drastically vary with the presence or the amount of water (Table 5, entries 1−3), where turnover number (TON) per surface platinum atom was 125−138, indicating that water would not accelerate the reaction and would not be necessary to complete the photocatalytic cycles during the initial stage. This means that water would be important to promote the photocatalytic cycles for a long time. 4132

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Figure 8. In situ FT-IR spectra of the Pt(0.1)/TiO2(A) sample with preadsorbed ammonia under photoirradiation from 0 to 60 min without flowing water vapor (a), in the dark from 60 to 150 min with flowing water vapor (b), and under photoirradiation from 150 to 270 min with flowing water vapor (c), and the time course of the band intensity at 1457 cm−1 during these procedures (d). (i), (ii), and (iii) in part d correspond to the band intensity for parts a, b, and c, respectively. All of the spectra were measured under the flow of ammonia (1 mL/min (45 μmol/min)) diluted by He (20 mL/min). The spectra shown were obtained by using the spectrum of the pretreated Pt(0.1)/TiO2 (A) sample as the reference. Irradiated light intensities were 50 and 150 mW/cm2 when measured at 254 ± 10 and 365 ± 15 nm, respectively.

Figure 8 shows in situ FT-IR spectra recorded under some transient conditions in a flow of ammonia (1 mL/min, corresponding to 44.5 μmol/min) diluted by He (20 mL/min). After the adsorption of ammonia (Figure 8a, 0 min), absorption bands due to ammonia were observed at 3360 and 3251 cm−1, assignable to the N−H stretching vibration of ammonia adsorbed on the surface Lewis acid sites,50−52 at 1600 and 1215 cm−1, assigned to deformation vibration of the adsorbed ammonia,50−52 and at 3000−3600 cm−1 (very broad), corresponding to adsorbed ammonia through hydrogen bond. A negative absorption band was also observed at 3670 cm−1, assignable to the surface hydroxyl group, since ammonia would be adsorbed on the surface hydroxyl group of titanium oxide through hydrogen bond. Further, a broad and large absorption band at 1457 cm−1 was observed, assignable to deformation of adsorbed ammonium ion.50,51 When the photoirradiation was carried out under the flowing ammonia diluted by He without water vapor (Figure 8a), the broad absorption band around 3600 cm−1 decreased in intensity, corresponding to a decrease of adsorbed ammonia by the photocatalytic ammonia decomposition. The initial reaction rate on the surface would be higher than the rate of additional ammonia adsorption. In addition, the absorption bands at 3360 and 3251 cm−1 clearly appeared, and the intensity of the absorption bands at 1600 and 1215 cm−1 increased, corresponding to the increase of the adsorbed ammonia on the surface Lewis acid sites. Probably, a part of the surface hydroxyl group was replaced by ammonia under photoirradiation under

the dry conditions. Further, the intensity of the absorption band at 1457 cm−1 increased. Thus, it is confirmed that the ammonium ion inactive for the photocatalytic decomposition increased under the dry conditions. This result suggests that the deactivation of the photocatalytic decomposition of ammonia without water vapor (Figure 1b) would be derived from the accumulation of the inactive ammonium ion, which would be produced from proton and ammonia. Furthermore, an absorption band at 1341 cm−1 increased in intensity with irradiation time. This peak was assigned to the NH2 deforming vibration of hydrazine as reported in the literature.52 When the photoirradiation was stopped and water vapor was introduced with ammonia in He carrier, the broad absorption band around 3600 cm−1 gradually recovered with the increase of adsorbed ammonia and/or water (Figure 8b). In addition, the absorption band for the adsorbed ammonium ion at 1457 cm−1 increased in intensity from 60 to 90 min and unchanged from 90 to 150 min. This would be because the equilibrium of the reaction between ammonia and water (eq 3) was shifted to the right by the introduction of water vapor. In these spectra, the intensity of absorption bands assignable to adsorbed water seems low, although it is difficult to distinguish the absorption bands for water from those for ammonia. Considering the reaction conditions (ammonia, 45 μmol/min; water vapor, 25 μmol/min), this would be because the dominant adsorbate on the surface might be ammonia. Then, 150 min later, the photoirradiation was again started in the flowing mixture of both ammonia and water with He. As a 4133

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Figure 9. Proposed dominant reaction path of the photocatalytic ammonia decomposition on platinum loaded titanium oxide in the presence of water (a) and in the absence of water (b).

where the ammonium ion would be reduced to NH3 and •H and the proton would be reduced to •H by the photoexcited electron. Since the adsorbed cation species would be not strictly static but somewhat dynamic on the surface, the role of water might be just an improvement of the mobility. Thus, even a small amount of adsorbed water more or less than monolayer would be enough to improve the mobility, although there is no clear evidence for the amount of the water on the surface in this condition. The improvement of ionic conductivity by addition of water was also reported on a singlecrystalline titanium oxide.54 3.8. Proposed Reaction Mechanism of Photocatalytic Ammonia Decomposition. Through the above investigations, a proposed reaction mechanism of the photocatalytic ammonia decomposition over platinum loaded titanium oxide in the presence of water molecules can be summarized as shown in Figure 9a. Ammonia is adsorbed on the hydroxyl group or the Lewis acid site of titanium oxide (Figure 9a, i), where the former would be a major reactive species under the present conditions. Although the ammonia adsorbed on the Ti cation can be activated by visible light through the electron transfer from N to Ti, this contribution is minor under the present conditions. These are the equilibrium between adsorbed ammonia and ammonium ion. Although the ammonium ion is

result, most of the absorption bands decreased in intensity (Figure 8c), suggesting that the adsorbed ammonia reacted and the inactive ammonium ion was eliminated by the photoirradiation in the presence of water (Figure 8d). As described above, the ammonium ion could not be oxidized by a hole even in the presence of water (eq 13).

NH4 + + h+ → (no reaction)

(13)

In contrast, ammonium ion can be reduced into ammonia by the electron having enough potential (eq 14).53

NH4 + + e− → NH3 + •H

(14)

The present result shows that the ammonium ion could be reduced effectively with the formation of hydrogen radical under photoirradiation in the presence of water over the Pt/TiO2(A) photocatalyst. (In this case, the ammonium ion functioned as a proton carrier.) However, water could not take part in this reductive reaction of ammonium ion. In addition, water did not accelerate the ammonia decomposition, as described above (Table 5). Therefore, the water would act as not the reactant but a kind of solvent to transport the unfavorably formed ammonium ion, and also proton, from the titanium oxide surface to the deposited platinum nanoparticle sites, 4134

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inactive, it can be transferred to the platinum sites and reduced to form NH3 and •H (eq 14) in the presence of water. Thus, the ammonium ion is not accumulated on titanium oxide in the presence of water. The titanium oxide is photoexcited by UV-irradiation to produce hole and excited electron; the former migrates to the surface O atom through the valence band, and the latter migrates to the platinum nanoparticle sites through the conduction band (ii). The photoformed hole oxidizes the adsorbed NH3 to form amide radical (•NH2) and proton (iii), which was confirmed by ESR (Figure 6). The amide radicals would produce hydrazine (iv). The protons are transferred by the aid of water from the surface of titanium oxide to the platinum nanoparticle sites and reduced by the photoformed electron to form hydrogen (v). The hydrazine can be decomposed to diazene and hydrogen mainly by photocatalysis and partly by catalysis (vi). The diazene is converted to hydrogen and nitrogen through the self decomposition (vii), or to nitrogen and hydrazine through the disproportionation (viii). These processes shown in Figure 9a would compose the dominant photocatalytic reaction cycle in the presence of water, which can proceed continuously for a long time. As another possible mechanism, the amide radical might be further oxidized by the photoformed holes to form NH or N species on the surface, followed by their coupling to produce nitrogen finally, although no evidence for the formation of these species was obtained in the present study. Since the ammonia decomposition through the production of hydrazine is considered to occur more easily than that through the formation of NH or N species, the dominant reaction mechanism would be the proposed mechanism in Figure 9a. When the amount of water molecules is not enough in the reaction system, the proton produced by the photocatalysis is trapped by ammonia to form ammonium ion (Figure 9b). The ammonium ion on the titanium oxide cannot easily migrate to the platinum site without water, and the accumulation of them would inhibit the oxidation of ammonia on the titanium oxide surface.

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-52-789-4609. Fax: +81-52-789-3178. E-mail: [email protected] apchem.nagoya-u.ac.jp. Notes

§ Research Fellow of the Japan Society of Division for the Promotion of Science.



ACKNOWLEDGMENTS This work was supported by Grant-in-aid for JSPS fellows (22-7799) and the fund for doctoral students in Nagoya University. We thank Professor J. Kumagai (Nagoya University) and Professor S. Yamazoe (Ryukoku University) for the ESR measurements.



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4. CONCLUSION The reaction mechanism for the photocatalytic ammonia decomposition to nitrogen and hydrogen over platinum loaded titanium oxide was clarified. The titanium oxide surface is the important reaction field, and the loaded platinum nanoparticles receive the photoformed electron to act as the reduction sites. The metals with larger work function, such as platinum, can provide more effective cocatalysts. The dominant reaction path was proposed as follows: the photoformed hole oxidizes NH3 to form amide radical and proton, and the amide radical produces hydrazine, which can be decomposed to form nitrogen and hydrogen, while the photoformed electron reduces the protons to yield hydrogen. In addition, this reaction system requires the presence of water, which functions as a kind of solvent, for the continuous reaction. Ammonium ion can be formed during the reaction as an unfavorable byproduct. Although it inhibited the reaction in the dry condition, the presence of water restricted the accumulation of the ammonium ion on the titanium oxide surface and provided the continuous progress of the reaction. 4135

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