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Sn-doped V2O5 nanoparticles as catalyst for fast removal of ammonia in air via PEC and PEC-MFC
Journal Pre-proofs Sn-doped V2O5 nanoparticles as catalyst for fast removal of ammonia in air via PEC and PEC-MFC Cai Yan, Lifen Liu PII: DOI: Reference:
S1385-8947(19)33153-5 https://doi.org/10.1016/j.cej.2019.123738 CEJ 123738
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 September 2019 19 November 2019 5 December 2019
Please cite this article as: C. Yan, L. Liu, Sn-doped V2O5 nanoparticles as catalyst for fast removal of ammonia in air via PEC and PEC-MFC, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123738
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Sn-doped V2O5 nanoparticles as catalyst for fast removal of ammonia in air via PEC and PEC-MFC Cai Yan, Lifen Liu*
MOE，key lab of industrial Ecology and Environmental Engineering, School of Ocean Science and Technology, Dalian University of Technology, China. * Corresponding Author
Email: [email protected]
Highlights
Catalyst activity of V2O5 is increased by optimal Sn-doping.
Average NH3 removal rate in PEC and PEC-MFC is about 4 and 8 times PC.
Oxygen vacancies generate active species for NH3 decomposition.
High surface area and rich pores caused fast NH3 adsorption.
Abstract Effective removal of ammonia from air at room temperature is rarely reported by photocatalysis, PEC (photo-electro-catalysis) or PEC-MFC (microbial fuel cell), but realized by a Sn-doped V2O5 nanocatalyst. The doping ratio of Sn influenced catalytic activity and 1 wt%Sn-V2O5 exhibited optimal degradation (96.4% NH3) and
competitive stability. By integrating PC with electro-catalysis and microbial fuel cells, the removal is enhanced (nearly complete ammonia removal). The structural and electronic properties of the Sn-doped V2O5, effect of Sn doping, the active species for ammonia degradation are investigated. The Sn-doping decreased the size of the nanoparticles and increased the oxidizing capacity and number of active sites. Oxygen vacancies played key roles in ammonia oxidation. This research provides an innovative and stable room temperature catalyst for air purification and malodor control.
Keywords Sn doped V2O5; Ammonia gas removal; Oxygen vacancies; Appropriate dopant concentration;
1. Introduction As we all know, a wide range of malodorous gases have negative or serious effects[1]. Gaseous ammonia (NH3) is extremely toxic as an odorous and corrosive pollutant, with possible emission from nitrogen fertilizer in agriculture, livestock breeding and selective/catalytic DeNOx process[2]. It is not only harmful to animals and human beings, but also leads to the formation of haze and particulate matter[3][4], damaging the ecological environment[5]. Adsorption and selective catalytic oxidation (SCO) have been developed to control emission of NH3, but general SCO cannot remove NH3 efficiently at room
temperature[6][7]. Therefore, it is very important to develop effective environmental remediation technologies to solve the problems, it is a hot topic that requires cross-disciplinary research of environmental science, nanomaterials and applied nano-catalysis chemistry. Photocatalytic oxidation (PCO) air purification is considered green, and environmental-friendly. It uses light energy to degrade various pollutants into harmless terminal products such as CO2 or H2O and other small molecular substances, by exciting nanocatalysts with sufficient activities[8][9]. Many researchers studied ammonia oxidation by constructing heterostructure with TiO2[10][11],
depositing
noble
metal
nanoparticles
to
improve
the
activity[1][12][13][14]. Recently, Transition metal oxide, vanadium pentoxide (V2O5) with low bang gap energy (2.3 eV) has attracted much attention due to the excellent physicochemical properties[15][16][17][18]. The V2O5 can more effectively use solar light energy[19]. Its problems about easy recombination of photo-generated carriers and small specific surface area need to be solved. Cation doping can improve the properties of V2O5, by introducing extraneous ions into the V2O5 lattice to adjust the ion occupation and the electronic structure of the host V2O5[20][21][22]. Among them, Sn doping is preferred, because V5+ and V4+ inside V2O5 and abundant oxygen vacancies on the surface can activate surface metal cations and surface oxygen species by Sn doping, increases oxidizing activity and electrochemical properties[23]. The doping of Sn could also inhibit the formation of nanobelt morphology in V2O5, making the composite nanoparticles smaller[24]. Studies have shown that Sn-V2O5 is not only a
promising material for fabricating high sensitivity room-temperature optical ammonia sensor, but also an excellent LIB cathode material[23] [24] [25], but it is rarely used to remove ammonia. At present, photo-electrocatalysis (PEC) is and will be one of the most promising technologies for effective air purification given its numerous advantages including environmental compatibility, energy efficiency, and safety[26][27][28]. In PEC process, photo-induced electrons can be extracted to the outer circuit by the effect of a potential, thereby effectively suppressing the recombination of photo-induced electron-hole pairs and increasing the photocatalytic performance without secondary pollution[29]. Studies have shown that PEC can remove indoor HCHO more quickly and effectively in comparison with EC and PC[30]. It is worth noting that low-voltage electricity is simply needed to accelerate the electron-hole separation to enhance the efficiency of photocatalysis[31]. MFC technology can achieve bacterial oxidation of gaseous pollutants generating a certain electric current as a result[32]. This technology has been considered to be a potential win-win both energy and environmental. Therefore, MFC is used to provide this low voltage required for the PEC process. It has been proven that the air purification system with photo-electrochemical catalysis integrated in microbial fuel cell is more effective[33], in which light energy and bioenergy are simultaneously utilized. However, there is no study yet reporting the removal of ammonia from air in this kind of system. So, we prepared single V2O5 or Sn-V2O5 nanoparticles with different proportions of Sn doped using simple sol-gel method and tested this Sn-V2O5 in the PEC-MFC
integrated system, where adsorption and photo-electro-catalysis of ammonia was successfully realized using a photo-electro-catalytic membrane electrode module with Sn-V2O5. Integrating this cathode with microbial fuel cell, increased the ammonia removal to nearly 99%. 2. Experimental 2.1. Materials Vanadium pentoxide (V2O5), Tin (IV) chloride pentahydrate (SnCl4•5H2O), 30% Hydrogen peroxide solution (H2O2), Ammonium Hydroxide(NH3•H2O), and Potassium sulphate (K2SO4), All substances used in this study were analytical grade and were used without further purification. Deionized water was used in all experiments. 2.2. Preparation of catalysts The pure V2O5 and Sn doped V2O5 photo-catalysts with 0.5–6 wt. % Sn doping were prepared via sol–gel process. The detailed method was similar to that reported by Nitu Singh et al.[25] In short, Vanadium pentoxide (V2O5) and tin (IV) chloride pentahydrate (SnCl4•5H2O) were used as precursors of vanadium (V) and tin (Sn), respectively. For the synthesis of pure V2O5, 2.7282g V2O5 powder was added into a 50 ml mixture of DI water and H2O2 under stirring to form vanadium pentoxide solution. The volume ratio of DI water to H2O2 is 3:1. The obtained solution was repeatedly diluted under ultrasonic agitation until the color of solution changes to transparent brick-red. Specifically, the above vanadium pentoxide solution was stirred using
magnetic stirrer for 20 min and then sonicated for 20 min at 50 °C. The concentration of V2O5 was further diluted by adding 128 ml of deionized water and the solution was further sonicated till brownish-red colored V2O5 gel was obtained. Subsequently to the gel, 357.7 ml of water was again added with stirring to make it further diluted. Then, the obtained solution of V2O5 was evaporated under vigorous stirring in water bath at 95 °C. The remaining was dried at 70°C for 12h and finally V2O5 was obtained. All prepared solids were sieved to the particle size < 0.054 mm（300 mesh）. Sn-doped V2O5 nanoparticles (0.5, 1, 2, 4 and 6 wt% of Sn) could be synthesized using almost the same process. The required amount of SnCl4•5H2O aqueous solution was added into the V2O5 solution prior to water bath evaporation. The rest of procedures were the same as described above. 2.3. Materials characterization The synthesized Sn-doped V2O5 nanoparticles were examined using various techniques
for
characterizing
the
crystalline
microstructures,
optical
and
photoluminescence properties. The purity and the crystalline structure of the as-prepared photocatalysts were examined by powder X-ray diffractometer (XRD-7000S, Shimadzu, Japan) with Cu Kα (λ=0.15406 nm) radiation while the voltage and electric current were maintained at 40 kV and 40 mA. The X-ray diffraction (XRD) patterns were obtained over the 2θ range from 10° to 80° with a scan step of 5° min-1. The transmission electron microscope (TEM) analyses were performed by a Tecnai G2 F30 S-TWIN field emission electron microscope at an accelerating voltage of 200 kV. The elemental composition was determined by energy
dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy (XPS) was measured on a Scanning X-ray Microprobe (ESCALAB™ 250Xi), and the spectra were calibrated to the C 1s peak at 284.6 eV. The UV–vis diffuse reflection spectra (DRS) were obtained with Ba2SO4 as a reference with a diffuse reflectance UV–vis Spectrophotometer (LAMBDA 950, Perkin Elmer Management Co, Ltd). The pore structure and surface area were investigated on an automatic physical adsorption apparatus (Autosorb-iQ-C, USA). The microstructure evaluation of the synthesized nanoparticles was done by (AFM) atomic force microscopy (Dimension ICON, Bruker). 2.4. PCO activity tests The photocatalytic activity tests for the decomposition of ammonia were performed in a sealed stainless steel reactor at ambient temperature. The tests include injection of ammonia solution and volatilization, circulation and mixing, photocatalytic degradation, and ammonia gas concentration detection. The stainless steel reactor, shaped like a lunch box, has a volume of 3 L. In the reaction, air was used as a background gas, and a fixed volume of ammonium hydroxide was injected into the closed chamber from the inlet by a gas phase injector. The reactor was equipped with a fan to prompt the valorization and even mixing of ammonia, and to reach the desired concentration required for the test. The temperature in the reactor can be controlled to reduce the light heating effect. First, we conducted a blank control experiment in which NH3 was exposed to light in the absence of photocatalyst, and we observed that both the self-decomposition of NH3 and the adsorption of NH3
by the reactor were negligible (Fig. S1). Both the quartz membrane module loaded with photocatalyst and a small fan were placed inside the reactor before the experiment. Photocatalyst (8 mg) was coated onto stainless steel with silica sol, and then used as the membrane cathode module. The other electrode was constructed by immersing a copper rod in an electrolyte solution. Inside the fuel cell constructed by above membrane module, the proton exchange membrane provided a pathway for the migration and transport of protons, allowing protons to pass through the membrane from the anode to the cathode, and electrons to transfer through the external circuit. The same structure was adopted when the microbial fuel cell was integrated, except that the bio-anode replaced the Cu anode, with carbon rod and electrogenic microorganism. Ammonium hydroxide was used as the source of ammonia during the experiment. At the beginning of each degradation experiment, 2 μl of ammonium hydroxide was injected into the reactor, meanwhile, turned on the fan inside to evaporate the ammonia quickly and to mix evenly till the concentration of ammonia gas reached about 100 ppm. The reaction started, once the light source was turned on. The concentration of ammonia gas was detected using an ammonia gas portable detector every 10 minutes. The NH3 removal efficiency was used to evaluate photocatalytic performance according to the following equation: 𝜂=
𝐶0 ― 𝐶𝑡 𝐶0
× 100%
(1) All data were measured three times. The accuracy of measurements was verified
by series of repeated experiments, and the relative error of ammonia removal rate was less than 10%. 3. Results and discussion 3.1. Structures and morphology of the catalysts The crystallographic structure of the as-synthesized samples was confirmed by the X-ray diffraction patterns (XRD). As shown in Fig. 1, all the samples show the peaks located at 22.48°, 26.12°, 31.06°, 34.38°, 47.32°, 51.4°, and 61.07°, which are indexed to (1 0 1), (1 1 0), (3 0 1), (3 1 0), (6 0 0), (0 2 0), (3 2 1) facet of orthorhombic V2O5 (JCPDS No. 41-1426), demonstrating that the prepared vanadium pentoxide has an orthorhombic structure. When the dopant concentration increases from 0.5 wt% to 2 wt%, it seems that the first SnO2 phase appeared, and the second phase was formed at 4~6 wt%. This is clearly demonstrated by the sharp diffraction peaks of SnO2. The dopant concentration of Sn4+ influences the crystalline features of the composites.
Fig. 1. XRD patterns of V2O5 and Sn-V2O5 (0.5 - 6 wt %). The composition and the chemical states of 1wt% Sn doped V2O5 with the best
performance were further clarified by XPS analysis. Fig. 2 displays elemental composition and corresponding valence states. Fig. 2a shows the XPS survey spectra of 1wt% Sn doped V2O5, containing information of V, O, and Sn elements with sharp peaks at binding energies of 530.6 eV (O 1s), 518.8 eV (V 2p), 488.4eV(Sn 3d), 42.8 eV(V 3p), and 631.6 eV (V 2s), revealing the presence of dopant Sn, host elements and its oxidation states compared to the pure V2O5[34]. Fig. 2b shows the XPS spectra of O 1s with two prime diffraction peaks of 530.6 eV and 531.3 eV. The peak of 530.6 eV is attributable to the lattice oxygen of V2O5 and the peak of 531.3 eV corresponds to surface V-OH. Fig. 2c illustrates the dopant Sn 3d peaks in which two predominant peaks located at 486.5 eV and 495 eV are consistent with the binding energies of Sn 3d5/2 and Sn 3d3/2 orbitals respectively, which is in agreement with thoroughly oxidized Sn4+ state of Sn dopant reported in literature[35]. One detailed observation is the shift of binding energy of Sn to lower energies, compared to SnO2[36]. It can thus be argued that chemical environment of Sn atoms in the composites is a little different from that in SnO2, which indicates that the tin was successfully doped[37]. The V 2p spectrum (Fig. 2d) exhibits the V 2p3/2 peak at 517.2 eV and V 2p1/2 peak at 524.5 eV, respectively. The difference in binding energy between them is approximately 7.3 eV, which conforms to electronic states of V2O5[38]. The V 2p and O 1s orbits shifted 0.3 eV and 0.4 eV to lower energies, respectively. These shifts may be attributed to the fact that the presence of oxygen vacancies on the surface increases the density of electron clouds around the metal cations, thereby reducing the binding energy. Hence, the presence of elements and
state of chemical environment such as Sn4+ doping of vanadium pentoxide is verified by XPS spectra.
(a)
(b)
(c)
(d)
Fig. 2. (a)XPS survey scan (b) O 1s (c) Sn 3d (d) V 2p XPS spectra of 1 wt% Sn-V2O5. In Fig. 3 the TEM micrographs of V2O5 and Sn-V2O5 nanocomposite, Fig. 3a represents orthorhombic structure V2O5 crystal, the dark areas are from aggregated nanoparticles. 1 wt%Sn-V2O5 nanoparticles are irregular shaped and slightly agglomerated (Fig. 3b, c). In high resolution TEM (Fig. 3d, e), the inter-planar distances of 0.261 nm, 0.288 nm and 0.341 nm correspond to the (3 1 0), (3 0 1) and (1 1 0) lattice planes of V2O5, respectively. The results are consistent with the SAED data (Fig. 3f) and XRD results (Fig.1). Additionally, The EDS mapping of
1wt%Sn-V2O5 illustrates the doping of Sn and uniform distribution of Sn on the surface of V2O5 (Fig. 3g, h, i). The above results show that doping of Sn on V2O5 causes changes in the morphology, and increases in the total specific surface area. Consequently, the Sn doping modification may enhance photocatalytic reactivity.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
O
(h)
V (i)
Sn
Fig. 3. TEM images of (a) V2O5; (b) &(c) 1 wt%Sn-V2O5 nanocomposite; (d) & (e) HRTEM images; (f) SAED pattern; and (g) & (h) & (i) corresponding EDS elemental mapping of 1 wt%Sn-V2O5. 3.2. Optical and Photo-electrochemical properties In Fig. 4, the UV-Vis spectrum shows that the V2O5 and Sn-V2O5 samples possess wide light absorption. The absorption intensity in 385-414 nm region is nearly equivalent to that in 532-800 nm region due to the O 2p valence band and the empty V 3d orbital in the conduction band[39][40]. And as the doping of Sn increases, the light absorption intensity decreases with gradient. This may be due to the
hypochromic effect caused by doping of Sn. The calculated band gaps of V2O5 and Sn-V2O5 from absorption spectra by Tauc plot were estimated by the following Eqs.: 1
(αhν)𝑛 = 𝐴(hν ― Eg)
(2)
Where α is the absorption coefficient, h is incident photon energy, a Planck's constant, A is a constant, ν is the frequency of light, Eg is the band gap energy, and n = 1/2 and 2 for direct and indirect band gap materials, respectively. The tauc plots for V2O5 and Sn-V2O5 are shown in the inset. The Eg values are obtained by extrapolating the linear portion of the curves to the x axis ((αhυ) 2 = 0). Compared with pure V2O5, the band gaps of the Sn-V2O5 samples are reduced. This may be due to the activation of the oxygen vacancies on the surface of the V2O5 by Sn, which narrows the band gaps. An increase in the band gaps of Sn-V2O5 with the augment of dopant could be due to the atomic hybridization between the Sn, V and O atoms, giving rise to the splitting of the energy levels nearby the Fermi level[41]. The band-gap is consistent with the values reported in literature and such nano-material with this band-gap range could be used in potential optoelectronic devices and in sensing applications[42].
Fig. 4. UV-Vis absorption spectra of composite samples compared with bare V2O5
and the Tauc’s plots (inset) for all the samples. The electrochemical performance of the catalytic electrode was investigated by cyclic voltammetry using an electrochemical workstation equipped with three electrodes in 0.5 M K2SO4 solution. Fig. 5a displays the CV curves of electrodes with V2O5 and Sn-V2O5 at scanning rate of 50 mV s-1 (vs. Hg/HgCl2 reference electrode) over -0.65 to 1.15 V. In Fig. 5, clearly, the peak of photocurrent density is highest for 1 wt% Sn-V2O5, meaning the significant enhancement in the electrochemical activity. The increased current density is attributed to Sn capturing photo-excited electrons, effectively separating photo-generated electrons-holes, and increasing the photo electrochemical catalytic activity. The doped Sn improved charge transfer kinetics[43], consistent with the improved photocurrent response. On the contrary, the reduction peak of 6 wt%Sn-V2O5 is weakest, probably by formed tin dioxide. To investigate the detailed photoelectric response of pristine and Sn doped V2O5 samples. The photocurrents were measured for 100 seconds under light on/off state. V2O5 has lower photocurrent intensity than other Sn-doped samples (Fig. 5b). This states the improvement in separation of photogenic electron-hole in the composites[44][45][46]. The 1 wt%Sn-V2O5 photo anode exhibits the highest photocurrent intensity. As already explained, the enhanced photocurrent response in 1wt%Sn-V2O5 must be due to the synergetic effect of optimized dopant concentration, reduced size of nanoparticles and favorable band position, improved electrical conductivity, and photo stability compared with other samples[47]. But 4~6 wt%Sn-V2O5 samples exhibit weaker photoelectric response, which may be caused by
forming e-/h+ recombination center.
(a)
(b)
Fig. 5. (a) CVs of V2O5 and Sn-V2O5 in 0.5 M K2SO4 electrolyte at -0.65 to 1.15 V. (b) Transient photocurrent response of pure V2O5 and Sn-V2O5 samples； 3.3. Textural properties Fig. 6 shows the N2 adsorption-desorption isotherms for as-prepared pure V2O5 and Sn-V2O5 samples, with the inset showing the pore size distribution. The isotherms of these samples can be roughly categorized into two types: the former being an aggregation of pure V2O5 and 2 to 6 wt%Sn-V2O5; the latter being a combination of 0.5 to 1 wt%Sn-V2O5 from Fig. 6. The isotherms are of classical IV adsorption isotherm indicating the presence of mesopores[48][49]. The curves present a H2 type hysteresis loop at relative pressure P/P0 = 0.4–0.9, which is characteristic of materials with high degree of pore size uniformity[48]. The presence of these mesopores allows the rapid diffusion of various reactants and products in the photocatalytic process and enhances the reaction rate[50]. However, the isotherms of the latter show type III isotherm indicating the formation of micropores[51]. This is due to the narrowing of
pores caused by the addition of Sn and thus rendering the microporosity[50]. The absence of hysteresis loop further confirms the presence of micropores. From the pore size distribution of all samples (Fig. 6 inset), wide distribution can be witnessed (1.1–130 nm). The BET, average pore size and pore volume of as-prepared samples are listed in Table 1. It is possible that the number of active sites may increase when the Vp is the highest and when there are more smaller pores, so that the catalytic activity of 1 wt%Sn-V2O5 was highest[52][53].
Fig. 6. N2 adsorption-desorption isotherms and pore size distribution curves (inset) of different samples; Table 1. Textural properties of different samples. Samples
SBET (m2 g–1)
Vp (cm3 g–1)
dp (nm)
V205
89.42
0.110
3.510
0.5 wt%Sn-V2O5
64.49
0.470
2.130
1 wt%Sn-V2O5
79.64
0.512
2.004
2 wt%Sn-V2O5
97.86
0.116
3.507
4 wt%Sn-V2O5
101.85
0.123
3.512
6 wt%Sn-V2O5
93.62
0.108
3.511
Notes: SBET: BET specific surface area; Vp: pore volume; dp: average pore size; 3.4. PCO performance in removal of gas NH3 The prepared pure and various Sn doped Sn-V2O5 photo catalysts were tested in the photocatalytic degradation of NH3. The degradation performance of the prepared catalysts on ammonia gas over time is shown in Fig. 7a. The removal of ammonia gas was 80% or even higher in 5-10 minutes over an appropriate Sn doped V2O5, but when the loading of Sn reaches 6%, the removal is significantly lower than the pure V2O5. The order from fast to slow of photocatalytic degradation of ammonia by six catalyst samples is: 1 wt%Sn-V2O5>0.5 wt%Sn-V2O5, 2 wt%Sn-V2O5, and 4 wt% Sn-V2O5 > 6 wt%Sn-V2O5, V2O5. The degradation -time profile of ammonia over 1 wt%Sn-V2O5 in different catalytic conditions is shown in Fig. 7b. The catalyst can adsorb nearly 80% of ammonia gas under dark adsorption conditions, and the removal of ammonia gas was further enhanced by starting photo irradiation, electrochemical catalysis and integrating PEC-MFC (more than 99%). The ammonia was degraded rather than adsorbed, verified by tests that in dark condition, when the concentration of ammonia was becoming stable, reaching adsorption-desorption equilibrium, then the light source was turned on to start photo-catalysis. Subsequently, the temperature of the reactor was increased by external heating, and the concentration of the ammonia gas had no significant increase (Fig. S2).
The kinetic behaviors of as-prepared samples for degradation of NH3 in different systems were further investigated. As illustrated in Fig. 7c, all of them fit well with pseudo-second order correlation: 1 𝐶0
1
(3)
― 𝐶 = 𝑘𝑡
Where C is the concentration of NH3 remaining in the reactor at the reaction time of t, C0 is the initial concentration at t = 0, and k is the degradation apparent rate constant. The k values of different systems are shown in Fig. 7d. According to the magnitude of k, the photoelectric-coupled microbial fuel cell catalytic system (PEC-MFC for short) degrades ammonia gas much faster than other systems. The photo light is necessary for improving the catalytic efficiency. Compared with the previous works on photocatalytic oxidation (PCO) of NH3 over various TiO2-based catalysts (Table 2), it can be found that Sn-V2O5 exhibited the highest activity, and in the PFC-MFC system, the kinetic rate constant is much higher (8 times PCO).
(a)
(b)
(c)
(d)
Fig. 7. (a) The photocatalytic activities of as-prepared samples for NH3 degradation at room temperature; (b) Degradation of NH3 by different systems over 1%Sn-V2O5; (c) The pseudo-secondary reaction kinetics of NH3 degradation; (d) The apparent rate constant for NH3 degradation. Table 2. Comparison with other studies on PCO of NH3 over TiO2-based catalysts. Catalyst Catalysts
weight (mg)
NC@TiO2[1]
Anatase TiO2{001}[12]
500
70
Degradation method
PCO
PCO
Light C (ppm)
source
50
PCO
100μL NH3·H2O
500
25 W UV lamp
50
photo-SCO 120
fixed bed flow
1000
system Sn-V2O5(this study) Sn-V2O5(this study)
PCO
100
(2h)
PFC-MFC
100
43% (2h)
300 Xe
70%
lamp
(5h)
LED lamp
8
(35 min)
Hg lamp
3W 8
100%
95%
UV
activity (mol/m3·g·h)
500 W
lamp JRC-TIO-8[11]
η (%)
(W)
500W TiO2(400℃)[14]
Catalyst
91% (35min)
3w LED
96%
lamp
(15min)
——
0.15
Rate constant 0.67 min-1 0.034 min-1
0.009
——
0.052
——
0.87
2.14
0.0024 L·mol-1·min-1 0.018 L·mol-1·min-1
The relationship between the degradation efficiency of ammonia and the initial concentration of ammonia and the electricity production were further studied (Fig. 8). It is discovered that the degradation efficiency of the ammonia gas is always high even if the ammonia concentration is different, and the final ammonia degradation rate is close to 100 % when the initial ammonia concentrations was 50, 100, 150 and
200 ppm, which further confirms the high catalytic activity of the catalytic material. The details that cannot be ignored are that the degradation efficiency of the catalyst is slightly different within the first 15 minutes when the initial concentrations vary. The current generated during the experiment was also examined. The current generated by the photo-electro-catalytic system at 15 minutes (the current is stable at this time) with different the initial concentrations of ammonia gas are shown in Fig. 8b. The steady current generated when treating different ammonia concentrations increases as the concentration increases, this feature implies the potential of the catalyst as an effective sensor.
(a)
(b)
Fig. 8. (a) Degradation efficiency of ammonia with initial concentrations of 50, 100, 150 and 200 ppm of ammonia over 1%Sn-V2O5, (b) The current generated by the photoelectrocatalytic system at 15 minutes when the initial concentration of ammonia gases is different. 3.5. The stability of as-prepared photocatalyst To evaluate the stability of catalyst and provide reference for the practical application, we carried out a series of repeated experiments degrading ammonia over
the 1%Sn-V2O5. The photocatalytic activity of the 1%Sn-V2O5 sample has no apparent decrease or deactivation (Fig. 9a, stable removal >96.03%) even after four consecutive cycles of degrading NH3. This reveals the superior stability of photocatalyst in practical application. To check the stability of the material, the 1% Sn-V2O5 sample after 4th cycled experiments was characterized by XPS (Fig. 9b), which did not show any structural, valency or compositional changes. The above results reveal the high activity, high chemical and structural stability of the 1% Sn-V2O5 sample, as an effective photocatalytic material.
(a)
(b)
Fig. 9. (a) The repeated photo-electrochemical catalytic experiments of 1%Sn-V2O5 photocatalyst for degradation of NH3; (b) The XPS spectra of the 1%Sn-V2O5 sample before and after 4th cycles experiments 3.6. The reaction mechanism for PCO of NH3 Based on the aforementioned results and analysis, a reasonable photocatalytic mechanism (Fig. 10) is proposed to explain the enhancement in the photocatalytic activity of Sn-V2O5 nanocomposite. The Sn doping can improve the photocatalytic activity of V2O5 nanoparticles, from two main aspects. One is the increase in
separation rate of photo-induced charge carriers of V2O5 after doping Sn. It has been verified that the metal nanoparticles doped on the semiconductor surface can act as the electron mediator to obtain the high charge transfer rate and separation rate of electron-hole pairs[54]. The other is that an increase in the ability to bind photo induced carriers to form excitons for surface states. Generally, when doping low-valence ionic metal element, a large number of oxygen vacancies are generated because nanocrystals cannot reach electrically neutral status through cationic valence. EPR spectroscopy further confirms the presence of oxygen vacancies (Fig. 10a). V2O5 and 1 wt%Sn-V2O5 display a strong EPR signal at the g=2.002, which can be attributed to the response of oxygen vacancies[55]. The EPR signal of the 1 wt%Sn-V2O5 was much stronger than that of V2O5. The increased oxygen vacancies facilitate the formation of surface states between valence band and conduction band. The oxygen vacancies easily bind electrons to form excitons. Thus, the excitation energy level near the bottom of the conduction band can come into being[56]. Meanwhile, the doping of Sn formed SnO2 and V2O5 composite nanoparticles. The close potential difference between SnO2 and V2O5 makes migration of photoelectrons easier from V2O5 surfaces to SnO2 conduction band, which reduces the likelihood of recombination of photo-induced electrons on V2O5 surfaces by oxygen vacancies. But an excess amount of dopant could act as the recombination center of photo-induced electron-hole pairs, which is caused by the instantaneous accumulation of photo-induced electrons. Therefore, doping an appropriate amount of Sn is the key to improving the photocatalytic activity of V2O5 nanoparticles. In addition, the doped Sn
NP can serve as active sites. And the band-gap of V2O5 is reduced from 2.3 to 1.74 eV after doping, which helps to generate more charge carriers under light irradiation. At the same time, the PEC and PMFC systems can further inhibit the recombination of photogenic electron-hole and enhance the catalytic efficiency of degradation of ammonia gas.
(a)
(b)
Fig. 10. (a) EPR spectra of pure V2O5 and 1 wt%Sn-V2O5; (b) Schematic diagram of explanation for the photocatalytic activity improvement of V2O5 nanoparticles with an appropriate amount of Sn doping. To further investigate the reaction mechanism, the concentration of nitrogen oxides was measured and no nitrogen oxides were produced. According to the above explanation and the related literature[57][58], the degradation of ammonia by oxygen vacancies in Sn-doped V2O5 material can be understood from the following equations: 𝑉 5 + + 𝑂2 ―
ℎ𝜈
𝑉4 + + 𝑂 ― +𝑉𝑂
(4) (5)
𝑂2 +𝑉𝑂→ 𝑂2― (ads) 𝑂2― + 𝑒 ― → 2𝑂 ―
(6)
2𝑂
-
→ 𝑂2 ―
(7) 2𝑁𝐻3 +3𝑂 ― → 𝑁2 +3𝐻2O + 3𝑒 ―
(8)
The specific process is as follows: Under the simulated sunlight illumination, the catalyst absorbs light energy, the VB electrons of V2O5 are excited to the CB, creating holes in the VB and generating oxygen vacancies while oxidizing ammonia gas by the active oxygen O- overflowed from the catalyst. In the process of degrading ammonia gas by active oxygen species, part of the lattice oxygen O2- is converted to active oxygen O- and the gas phase oxygen is continuously adsorbed to the oxygen vacancies (𝑉𝑂) to generate molecular adsorption oxygen O2-(ads) which is further converted into O- and lattice oxygen O2-. In this way, active oxygen can be supplied incessantly to ensure the continuous reaction. On the other hand, active oxygen is converted into hydrogen peroxide by binding electrons, generating hydroxyl radicals. At the same time, the holes in the VB oxidize water molecules in the air to hydroxyl radicals. The hydroxyl radicals oxidize ammonia gas to nitrogen and water molecules. 4. Conclusions In this work, V2O5 and Sn-V2O5 nanoparticles synthesized by sol-gel method enabled an innovative gas-solid PEC-MFC system, where the influence of parameters on the degradation performance was evaluated in the stainless steel reactor at room temperature. It is known from the characterization results that the size of nanoparticles reduced and the number of mesopores increased as well as the electron-hole separation rate for 1% Sn-V2O5, which resulted in the best performance for photocatalytic degradation of gaseous ammonia. The UV-vis spectrum and the
steady current at different initial concentrations of ammonia demonstrate the potential applications of this composite in optoelectronic devices and sensors. NH3 can be basically removed in the PEC-MFC system, and the kinetic behaviors fit well with pseudo-second order correlation with a rate constant of 0.018 L·mol-1·min-1. Tin doping increase the surface oxygen vacancies of the catalyst, which combine light-induced carriers to form excitons, thereby improving the performance of photocatalysis, electrocatalysis and PEC-MFC. The electric circuit enhanced activity of oxygen vacancies and catalytic oxidation of ammonia. This study contributes to the researches of V-based catalysts by constructing catalytic integrated system to eliminate contaminants.
Acknowledgments This study was supported by the National Natural Science Foundation of China (No. 21677025) and Basic Research Foundation of Dalian University of Technology (Project DUT17ZD223).
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Graphical Abstract
S1385-8947(19)33153-5 https://doi.org/10.1016/j.cej.2019.123738 CEJ 123738
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 September 2019 19 November 2019 5 December 2019
Please cite this article as: C. Yan, L. Liu, Sn-doped V2O5 nanoparticles as catalyst for fast removal of ammonia in air via PEC and PEC-MFC, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123738
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Sn-doped V2O5 nanoparticles as catalyst for fast removal of ammonia in air via PEC and PEC-MFC Cai Yan, Lifen Liu*
MOE，key lab of industrial Ecology and Environmental Engineering, School of Ocean Science and Technology, Dalian University of Technology, China. * Corresponding Author
Email: [email protected]
Highlights
Catalyst activity of V2O5 is increased by optimal Sn-doping.
Average NH3 removal rate in PEC and PEC-MFC is about 4 and 8 times PC.
Oxygen vacancies generate active species for NH3 decomposition.
High surface area and rich pores caused fast NH3 adsorption.
Abstract Effective removal of ammonia from air at room temperature is rarely reported by photocatalysis, PEC (photo-electro-catalysis) or PEC-MFC (microbial fuel cell), but realized by a Sn-doped V2O5 nanocatalyst. The doping ratio of Sn influenced catalytic activity and 1 wt%Sn-V2O5 exhibited optimal degradation (96.4% NH3) and
competitive stability. By integrating PC with electro-catalysis and microbial fuel cells, the removal is enhanced (nearly complete ammonia removal). The structural and electronic properties of the Sn-doped V2O5, effect of Sn doping, the active species for ammonia degradation are investigated. The Sn-doping decreased the size of the nanoparticles and increased the oxidizing capacity and number of active sites. Oxygen vacancies played key roles in ammonia oxidation. This research provides an innovative and stable room temperature catalyst for air purification and malodor control.
Keywords Sn doped V2O5; Ammonia gas removal; Oxygen vacancies; Appropriate dopant concentration;
1. Introduction As we all know, a wide range of malodorous gases have negative or serious effects[1]. Gaseous ammonia (NH3) is extremely toxic as an odorous and corrosive pollutant, with possible emission from nitrogen fertilizer in agriculture, livestock breeding and selective/catalytic DeNOx process[2]. It is not only harmful to animals and human beings, but also leads to the formation of haze and particulate matter[3][4], damaging the ecological environment[5]. Adsorption and selective catalytic oxidation (SCO) have been developed to control emission of NH3, but general SCO cannot remove NH3 efficiently at room
temperature[6][7]. Therefore, it is very important to develop effective environmental remediation technologies to solve the problems, it is a hot topic that requires cross-disciplinary research of environmental science, nanomaterials and applied nano-catalysis chemistry. Photocatalytic oxidation (PCO) air purification is considered green, and environmental-friendly. It uses light energy to degrade various pollutants into harmless terminal products such as CO2 or H2O and other small molecular substances, by exciting nanocatalysts with sufficient activities[8][9]. Many researchers studied ammonia oxidation by constructing heterostructure with TiO2[10][11],
depositing
noble
metal
nanoparticles
to
improve
the
activity[1][12][13][14]. Recently, Transition metal oxide, vanadium pentoxide (V2O5) with low bang gap energy (2.3 eV) has attracted much attention due to the excellent physicochemical properties[15][16][17][18]. The V2O5 can more effectively use solar light energy[19]. Its problems about easy recombination of photo-generated carriers and small specific surface area need to be solved. Cation doping can improve the properties of V2O5, by introducing extraneous ions into the V2O5 lattice to adjust the ion occupation and the electronic structure of the host V2O5[20][21][22]. Among them, Sn doping is preferred, because V5+ and V4+ inside V2O5 and abundant oxygen vacancies on the surface can activate surface metal cations and surface oxygen species by Sn doping, increases oxidizing activity and electrochemical properties[23]. The doping of Sn could also inhibit the formation of nanobelt morphology in V2O5, making the composite nanoparticles smaller[24]. Studies have shown that Sn-V2O5 is not only a
promising material for fabricating high sensitivity room-temperature optical ammonia sensor, but also an excellent LIB cathode material[23] [24] [25], but it is rarely used to remove ammonia. At present, photo-electrocatalysis (PEC) is and will be one of the most promising technologies for effective air purification given its numerous advantages including environmental compatibility, energy efficiency, and safety[26][27][28]. In PEC process, photo-induced electrons can be extracted to the outer circuit by the effect of a potential, thereby effectively suppressing the recombination of photo-induced electron-hole pairs and increasing the photocatalytic performance without secondary pollution[29]. Studies have shown that PEC can remove indoor HCHO more quickly and effectively in comparison with EC and PC[30]. It is worth noting that low-voltage electricity is simply needed to accelerate the electron-hole separation to enhance the efficiency of photocatalysis[31]. MFC technology can achieve bacterial oxidation of gaseous pollutants generating a certain electric current as a result[32]. This technology has been considered to be a potential win-win both energy and environmental. Therefore, MFC is used to provide this low voltage required for the PEC process. It has been proven that the air purification system with photo-electrochemical catalysis integrated in microbial fuel cell is more effective[33], in which light energy and bioenergy are simultaneously utilized. However, there is no study yet reporting the removal of ammonia from air in this kind of system. So, we prepared single V2O5 or Sn-V2O5 nanoparticles with different proportions of Sn doped using simple sol-gel method and tested this Sn-V2O5 in the PEC-MFC
integrated system, where adsorption and photo-electro-catalysis of ammonia was successfully realized using a photo-electro-catalytic membrane electrode module with Sn-V2O5. Integrating this cathode with microbial fuel cell, increased the ammonia removal to nearly 99%. 2. Experimental 2.1. Materials Vanadium pentoxide (V2O5), Tin (IV) chloride pentahydrate (SnCl4•5H2O), 30% Hydrogen peroxide solution (H2O2), Ammonium Hydroxide(NH3•H2O), and Potassium sulphate (K2SO4), All substances used in this study were analytical grade and were used without further purification. Deionized water was used in all experiments. 2.2. Preparation of catalysts The pure V2O5 and Sn doped V2O5 photo-catalysts with 0.5–6 wt. % Sn doping were prepared via sol–gel process. The detailed method was similar to that reported by Nitu Singh et al.[25] In short, Vanadium pentoxide (V2O5) and tin (IV) chloride pentahydrate (SnCl4•5H2O) were used as precursors of vanadium (V) and tin (Sn), respectively. For the synthesis of pure V2O5, 2.7282g V2O5 powder was added into a 50 ml mixture of DI water and H2O2 under stirring to form vanadium pentoxide solution. The volume ratio of DI water to H2O2 is 3:1. The obtained solution was repeatedly diluted under ultrasonic agitation until the color of solution changes to transparent brick-red. Specifically, the above vanadium pentoxide solution was stirred using
magnetic stirrer for 20 min and then sonicated for 20 min at 50 °C. The concentration of V2O5 was further diluted by adding 128 ml of deionized water and the solution was further sonicated till brownish-red colored V2O5 gel was obtained. Subsequently to the gel, 357.7 ml of water was again added with stirring to make it further diluted. Then, the obtained solution of V2O5 was evaporated under vigorous stirring in water bath at 95 °C. The remaining was dried at 70°C for 12h and finally V2O5 was obtained. All prepared solids were sieved to the particle size < 0.054 mm（300 mesh）. Sn-doped V2O5 nanoparticles (0.5, 1, 2, 4 and 6 wt% of Sn) could be synthesized using almost the same process. The required amount of SnCl4•5H2O aqueous solution was added into the V2O5 solution prior to water bath evaporation. The rest of procedures were the same as described above. 2.3. Materials characterization The synthesized Sn-doped V2O5 nanoparticles were examined using various techniques
for
characterizing
the
crystalline
microstructures,
optical
and
photoluminescence properties. The purity and the crystalline structure of the as-prepared photocatalysts were examined by powder X-ray diffractometer (XRD-7000S, Shimadzu, Japan) with Cu Kα (λ=0.15406 nm) radiation while the voltage and electric current were maintained at 40 kV and 40 mA. The X-ray diffraction (XRD) patterns were obtained over the 2θ range from 10° to 80° with a scan step of 5° min-1. The transmission electron microscope (TEM) analyses were performed by a Tecnai G2 F30 S-TWIN field emission electron microscope at an accelerating voltage of 200 kV. The elemental composition was determined by energy
dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy (XPS) was measured on a Scanning X-ray Microprobe (ESCALAB™ 250Xi), and the spectra were calibrated to the C 1s peak at 284.6 eV. The UV–vis diffuse reflection spectra (DRS) were obtained with Ba2SO4 as a reference with a diffuse reflectance UV–vis Spectrophotometer (LAMBDA 950, Perkin Elmer Management Co, Ltd). The pore structure and surface area were investigated on an automatic physical adsorption apparatus (Autosorb-iQ-C, USA). The microstructure evaluation of the synthesized nanoparticles was done by (AFM) atomic force microscopy (Dimension ICON, Bruker). 2.4. PCO activity tests The photocatalytic activity tests for the decomposition of ammonia were performed in a sealed stainless steel reactor at ambient temperature. The tests include injection of ammonia solution and volatilization, circulation and mixing, photocatalytic degradation, and ammonia gas concentration detection. The stainless steel reactor, shaped like a lunch box, has a volume of 3 L. In the reaction, air was used as a background gas, and a fixed volume of ammonium hydroxide was injected into the closed chamber from the inlet by a gas phase injector. The reactor was equipped with a fan to prompt the valorization and even mixing of ammonia, and to reach the desired concentration required for the test. The temperature in the reactor can be controlled to reduce the light heating effect. First, we conducted a blank control experiment in which NH3 was exposed to light in the absence of photocatalyst, and we observed that both the self-decomposition of NH3 and the adsorption of NH3
by the reactor were negligible (Fig. S1). Both the quartz membrane module loaded with photocatalyst and a small fan were placed inside the reactor before the experiment. Photocatalyst (8 mg) was coated onto stainless steel with silica sol, and then used as the membrane cathode module. The other electrode was constructed by immersing a copper rod in an electrolyte solution. Inside the fuel cell constructed by above membrane module, the proton exchange membrane provided a pathway for the migration and transport of protons, allowing protons to pass through the membrane from the anode to the cathode, and electrons to transfer through the external circuit. The same structure was adopted when the microbial fuel cell was integrated, except that the bio-anode replaced the Cu anode, with carbon rod and electrogenic microorganism. Ammonium hydroxide was used as the source of ammonia during the experiment. At the beginning of each degradation experiment, 2 μl of ammonium hydroxide was injected into the reactor, meanwhile, turned on the fan inside to evaporate the ammonia quickly and to mix evenly till the concentration of ammonia gas reached about 100 ppm. The reaction started, once the light source was turned on. The concentration of ammonia gas was detected using an ammonia gas portable detector every 10 minutes. The NH3 removal efficiency was used to evaluate photocatalytic performance according to the following equation: 𝜂=
𝐶0 ― 𝐶𝑡 𝐶0
× 100%
(1) All data were measured three times. The accuracy of measurements was verified
by series of repeated experiments, and the relative error of ammonia removal rate was less than 10%. 3. Results and discussion 3.1. Structures and morphology of the catalysts The crystallographic structure of the as-synthesized samples was confirmed by the X-ray diffraction patterns (XRD). As shown in Fig. 1, all the samples show the peaks located at 22.48°, 26.12°, 31.06°, 34.38°, 47.32°, 51.4°, and 61.07°, which are indexed to (1 0 1), (1 1 0), (3 0 1), (3 1 0), (6 0 0), (0 2 0), (3 2 1) facet of orthorhombic V2O5 (JCPDS No. 41-1426), demonstrating that the prepared vanadium pentoxide has an orthorhombic structure. When the dopant concentration increases from 0.5 wt% to 2 wt%, it seems that the first SnO2 phase appeared, and the second phase was formed at 4~6 wt%. This is clearly demonstrated by the sharp diffraction peaks of SnO2. The dopant concentration of Sn4+ influences the crystalline features of the composites.
Fig. 1. XRD patterns of V2O5 and Sn-V2O5 (0.5 - 6 wt %). The composition and the chemical states of 1wt% Sn doped V2O5 with the best
performance were further clarified by XPS analysis. Fig. 2 displays elemental composition and corresponding valence states. Fig. 2a shows the XPS survey spectra of 1wt% Sn doped V2O5, containing information of V, O, and Sn elements with sharp peaks at binding energies of 530.6 eV (O 1s), 518.8 eV (V 2p), 488.4eV(Sn 3d), 42.8 eV(V 3p), and 631.6 eV (V 2s), revealing the presence of dopant Sn, host elements and its oxidation states compared to the pure V2O5[34]. Fig. 2b shows the XPS spectra of O 1s with two prime diffraction peaks of 530.6 eV and 531.3 eV. The peak of 530.6 eV is attributable to the lattice oxygen of V2O5 and the peak of 531.3 eV corresponds to surface V-OH. Fig. 2c illustrates the dopant Sn 3d peaks in which two predominant peaks located at 486.5 eV and 495 eV are consistent with the binding energies of Sn 3d5/2 and Sn 3d3/2 orbitals respectively, which is in agreement with thoroughly oxidized Sn4+ state of Sn dopant reported in literature[35]. One detailed observation is the shift of binding energy of Sn to lower energies, compared to SnO2[36]. It can thus be argued that chemical environment of Sn atoms in the composites is a little different from that in SnO2, which indicates that the tin was successfully doped[37]. The V 2p spectrum (Fig. 2d) exhibits the V 2p3/2 peak at 517.2 eV and V 2p1/2 peak at 524.5 eV, respectively. The difference in binding energy between them is approximately 7.3 eV, which conforms to electronic states of V2O5[38]. The V 2p and O 1s orbits shifted 0.3 eV and 0.4 eV to lower energies, respectively. These shifts may be attributed to the fact that the presence of oxygen vacancies on the surface increases the density of electron clouds around the metal cations, thereby reducing the binding energy. Hence, the presence of elements and
state of chemical environment such as Sn4+ doping of vanadium pentoxide is verified by XPS spectra.
(a)
(b)
(c)
(d)
Fig. 2. (a)XPS survey scan (b) O 1s (c) Sn 3d (d) V 2p XPS spectra of 1 wt% Sn-V2O5. In Fig. 3 the TEM micrographs of V2O5 and Sn-V2O5 nanocomposite, Fig. 3a represents orthorhombic structure V2O5 crystal, the dark areas are from aggregated nanoparticles. 1 wt%Sn-V2O5 nanoparticles are irregular shaped and slightly agglomerated (Fig. 3b, c). In high resolution TEM (Fig. 3d, e), the inter-planar distances of 0.261 nm, 0.288 nm and 0.341 nm correspond to the (3 1 0), (3 0 1) and (1 1 0) lattice planes of V2O5, respectively. The results are consistent with the SAED data (Fig. 3f) and XRD results (Fig.1). Additionally, The EDS mapping of
1wt%Sn-V2O5 illustrates the doping of Sn and uniform distribution of Sn on the surface of V2O5 (Fig. 3g, h, i). The above results show that doping of Sn on V2O5 causes changes in the morphology, and increases in the total specific surface area. Consequently, the Sn doping modification may enhance photocatalytic reactivity.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
O
(h)
V (i)
Sn
Fig. 3. TEM images of (a) V2O5; (b) &(c) 1 wt%Sn-V2O5 nanocomposite; (d) & (e) HRTEM images; (f) SAED pattern; and (g) & (h) & (i) corresponding EDS elemental mapping of 1 wt%Sn-V2O5. 3.2. Optical and Photo-electrochemical properties In Fig. 4, the UV-Vis spectrum shows that the V2O5 and Sn-V2O5 samples possess wide light absorption. The absorption intensity in 385-414 nm region is nearly equivalent to that in 532-800 nm region due to the O 2p valence band and the empty V 3d orbital in the conduction band[39][40]. And as the doping of Sn increases, the light absorption intensity decreases with gradient. This may be due to the
hypochromic effect caused by doping of Sn. The calculated band gaps of V2O5 and Sn-V2O5 from absorption spectra by Tauc plot were estimated by the following Eqs.: 1
(αhν)𝑛 = 𝐴(hν ― Eg)
(2)
Where α is the absorption coefficient, h is incident photon energy, a Planck's constant, A is a constant, ν is the frequency of light, Eg is the band gap energy, and n = 1/2 and 2 for direct and indirect band gap materials, respectively. The tauc plots for V2O5 and Sn-V2O5 are shown in the inset. The Eg values are obtained by extrapolating the linear portion of the curves to the x axis ((αhυ) 2 = 0). Compared with pure V2O5, the band gaps of the Sn-V2O5 samples are reduced. This may be due to the activation of the oxygen vacancies on the surface of the V2O5 by Sn, which narrows the band gaps. An increase in the band gaps of Sn-V2O5 with the augment of dopant could be due to the atomic hybridization between the Sn, V and O atoms, giving rise to the splitting of the energy levels nearby the Fermi level[41]. The band-gap is consistent with the values reported in literature and such nano-material with this band-gap range could be used in potential optoelectronic devices and in sensing applications[42].
Fig. 4. UV-Vis absorption spectra of composite samples compared with bare V2O5
and the Tauc’s plots (inset) for all the samples. The electrochemical performance of the catalytic electrode was investigated by cyclic voltammetry using an electrochemical workstation equipped with three electrodes in 0.5 M K2SO4 solution. Fig. 5a displays the CV curves of electrodes with V2O5 and Sn-V2O5 at scanning rate of 50 mV s-1 (vs. Hg/HgCl2 reference electrode) over -0.65 to 1.15 V. In Fig. 5, clearly, the peak of photocurrent density is highest for 1 wt% Sn-V2O5, meaning the significant enhancement in the electrochemical activity. The increased current density is attributed to Sn capturing photo-excited electrons, effectively separating photo-generated electrons-holes, and increasing the photo electrochemical catalytic activity. The doped Sn improved charge transfer kinetics[43], consistent with the improved photocurrent response. On the contrary, the reduction peak of 6 wt%Sn-V2O5 is weakest, probably by formed tin dioxide. To investigate the detailed photoelectric response of pristine and Sn doped V2O5 samples. The photocurrents were measured for 100 seconds under light on/off state. V2O5 has lower photocurrent intensity than other Sn-doped samples (Fig. 5b). This states the improvement in separation of photogenic electron-hole in the composites[44][45][46]. The 1 wt%Sn-V2O5 photo anode exhibits the highest photocurrent intensity. As already explained, the enhanced photocurrent response in 1wt%Sn-V2O5 must be due to the synergetic effect of optimized dopant concentration, reduced size of nanoparticles and favorable band position, improved electrical conductivity, and photo stability compared with other samples[47]. But 4~6 wt%Sn-V2O5 samples exhibit weaker photoelectric response, which may be caused by
forming e-/h+ recombination center.
(a)
(b)
Fig. 5. (a) CVs of V2O5 and Sn-V2O5 in 0.5 M K2SO4 electrolyte at -0.65 to 1.15 V. (b) Transient photocurrent response of pure V2O5 and Sn-V2O5 samples； 3.3. Textural properties Fig. 6 shows the N2 adsorption-desorption isotherms for as-prepared pure V2O5 and Sn-V2O5 samples, with the inset showing the pore size distribution. The isotherms of these samples can be roughly categorized into two types: the former being an aggregation of pure V2O5 and 2 to 6 wt%Sn-V2O5; the latter being a combination of 0.5 to 1 wt%Sn-V2O5 from Fig. 6. The isotherms are of classical IV adsorption isotherm indicating the presence of mesopores[48][49]. The curves present a H2 type hysteresis loop at relative pressure P/P0 = 0.4–0.9, which is characteristic of materials with high degree of pore size uniformity[48]. The presence of these mesopores allows the rapid diffusion of various reactants and products in the photocatalytic process and enhances the reaction rate[50]. However, the isotherms of the latter show type III isotherm indicating the formation of micropores[51]. This is due to the narrowing of
pores caused by the addition of Sn and thus rendering the microporosity[50]. The absence of hysteresis loop further confirms the presence of micropores. From the pore size distribution of all samples (Fig. 6 inset), wide distribution can be witnessed (1.1–130 nm). The BET, average pore size and pore volume of as-prepared samples are listed in Table 1. It is possible that the number of active sites may increase when the Vp is the highest and when there are more smaller pores, so that the catalytic activity of 1 wt%Sn-V2O5 was highest[52][53].
Fig. 6. N2 adsorption-desorption isotherms and pore size distribution curves (inset) of different samples; Table 1. Textural properties of different samples. Samples
SBET (m2 g–1)
Vp (cm3 g–1)
dp (nm)
V205
89.42
0.110
3.510
0.5 wt%Sn-V2O5
64.49
0.470
2.130
1 wt%Sn-V2O5
79.64
0.512
2.004
2 wt%Sn-V2O5
97.86
0.116
3.507
4 wt%Sn-V2O5
101.85
0.123
3.512
6 wt%Sn-V2O5
93.62
0.108
3.511
Notes: SBET: BET specific surface area; Vp: pore volume; dp: average pore size; 3.4. PCO performance in removal of gas NH3 The prepared pure and various Sn doped Sn-V2O5 photo catalysts were tested in the photocatalytic degradation of NH3. The degradation performance of the prepared catalysts on ammonia gas over time is shown in Fig. 7a. The removal of ammonia gas was 80% or even higher in 5-10 minutes over an appropriate Sn doped V2O5, but when the loading of Sn reaches 6%, the removal is significantly lower than the pure V2O5. The order from fast to slow of photocatalytic degradation of ammonia by six catalyst samples is: 1 wt%Sn-V2O5>0.5 wt%Sn-V2O5, 2 wt%Sn-V2O5, and 4 wt% Sn-V2O5 > 6 wt%Sn-V2O5, V2O5. The degradation -time profile of ammonia over 1 wt%Sn-V2O5 in different catalytic conditions is shown in Fig. 7b. The catalyst can adsorb nearly 80% of ammonia gas under dark adsorption conditions, and the removal of ammonia gas was further enhanced by starting photo irradiation, electrochemical catalysis and integrating PEC-MFC (more than 99%). The ammonia was degraded rather than adsorbed, verified by tests that in dark condition, when the concentration of ammonia was becoming stable, reaching adsorption-desorption equilibrium, then the light source was turned on to start photo-catalysis. Subsequently, the temperature of the reactor was increased by external heating, and the concentration of the ammonia gas had no significant increase (Fig. S2).
The kinetic behaviors of as-prepared samples for degradation of NH3 in different systems were further investigated. As illustrated in Fig. 7c, all of them fit well with pseudo-second order correlation: 1 𝐶0
1
(3)
― 𝐶 = 𝑘𝑡
Where C is the concentration of NH3 remaining in the reactor at the reaction time of t, C0 is the initial concentration at t = 0, and k is the degradation apparent rate constant. The k values of different systems are shown in Fig. 7d. According to the magnitude of k, the photoelectric-coupled microbial fuel cell catalytic system (PEC-MFC for short) degrades ammonia gas much faster than other systems. The photo light is necessary for improving the catalytic efficiency. Compared with the previous works on photocatalytic oxidation (PCO) of NH3 over various TiO2-based catalysts (Table 2), it can be found that Sn-V2O5 exhibited the highest activity, and in the PFC-MFC system, the kinetic rate constant is much higher (8 times PCO).
(a)
(b)
(c)
(d)
Fig. 7. (a) The photocatalytic activities of as-prepared samples for NH3 degradation at room temperature; (b) Degradation of NH3 by different systems over 1%Sn-V2O5; (c) The pseudo-secondary reaction kinetics of NH3 degradation; (d) The apparent rate constant for NH3 degradation. Table 2. Comparison with other studies on PCO of NH3 over TiO2-based catalysts. Catalyst Catalysts
weight (mg)
NC@TiO2[1]
Anatase TiO2{001}[12]
500
70
Degradation method
PCO
PCO
Light C (ppm)
source
50
PCO
100μL NH3·H2O
500
25 W UV lamp
50
photo-SCO 120
fixed bed flow
1000
system Sn-V2O5(this study) Sn-V2O5(this study)
PCO
100
(2h)
PFC-MFC
100
43% (2h)
300 Xe
70%
lamp
(5h)
LED lamp
8
(35 min)
Hg lamp
3W 8
100%
95%
UV
activity (mol/m3·g·h)
500 W
lamp JRC-TIO-8[11]
η (%)
(W)
500W TiO2(400℃)[14]
Catalyst
91% (35min)
3w LED
96%
lamp
(15min)
——
0.15
Rate constant 0.67 min-1 0.034 min-1
0.009
——
0.052
——
0.87
2.14
0.0024 L·mol-1·min-1 0.018 L·mol-1·min-1
The relationship between the degradation efficiency of ammonia and the initial concentration of ammonia and the electricity production were further studied (Fig. 8). It is discovered that the degradation efficiency of the ammonia gas is always high even if the ammonia concentration is different, and the final ammonia degradation rate is close to 100 % when the initial ammonia concentrations was 50, 100, 150 and
200 ppm, which further confirms the high catalytic activity of the catalytic material. The details that cannot be ignored are that the degradation efficiency of the catalyst is slightly different within the first 15 minutes when the initial concentrations vary. The current generated during the experiment was also examined. The current generated by the photo-electro-catalytic system at 15 minutes (the current is stable at this time) with different the initial concentrations of ammonia gas are shown in Fig. 8b. The steady current generated when treating different ammonia concentrations increases as the concentration increases, this feature implies the potential of the catalyst as an effective sensor.
(a)
(b)
Fig. 8. (a) Degradation efficiency of ammonia with initial concentrations of 50, 100, 150 and 200 ppm of ammonia over 1%Sn-V2O5, (b) The current generated by the photoelectrocatalytic system at 15 minutes when the initial concentration of ammonia gases is different. 3.5. The stability of as-prepared photocatalyst To evaluate the stability of catalyst and provide reference for the practical application, we carried out a series of repeated experiments degrading ammonia over
the 1%Sn-V2O5. The photocatalytic activity of the 1%Sn-V2O5 sample has no apparent decrease or deactivation (Fig. 9a, stable removal >96.03%) even after four consecutive cycles of degrading NH3. This reveals the superior stability of photocatalyst in practical application. To check the stability of the material, the 1% Sn-V2O5 sample after 4th cycled experiments was characterized by XPS (Fig. 9b), which did not show any structural, valency or compositional changes. The above results reveal the high activity, high chemical and structural stability of the 1% Sn-V2O5 sample, as an effective photocatalytic material.
(a)
(b)
Fig. 9. (a) The repeated photo-electrochemical catalytic experiments of 1%Sn-V2O5 photocatalyst for degradation of NH3; (b) The XPS spectra of the 1%Sn-V2O5 sample before and after 4th cycles experiments 3.6. The reaction mechanism for PCO of NH3 Based on the aforementioned results and analysis, a reasonable photocatalytic mechanism (Fig. 10) is proposed to explain the enhancement in the photocatalytic activity of Sn-V2O5 nanocomposite. The Sn doping can improve the photocatalytic activity of V2O5 nanoparticles, from two main aspects. One is the increase in
separation rate of photo-induced charge carriers of V2O5 after doping Sn. It has been verified that the metal nanoparticles doped on the semiconductor surface can act as the electron mediator to obtain the high charge transfer rate and separation rate of electron-hole pairs[54]. The other is that an increase in the ability to bind photo induced carriers to form excitons for surface states. Generally, when doping low-valence ionic metal element, a large number of oxygen vacancies are generated because nanocrystals cannot reach electrically neutral status through cationic valence. EPR spectroscopy further confirms the presence of oxygen vacancies (Fig. 10a). V2O5 and 1 wt%Sn-V2O5 display a strong EPR signal at the g=2.002, which can be attributed to the response of oxygen vacancies[55]. The EPR signal of the 1 wt%Sn-V2O5 was much stronger than that of V2O5. The increased oxygen vacancies facilitate the formation of surface states between valence band and conduction band. The oxygen vacancies easily bind electrons to form excitons. Thus, the excitation energy level near the bottom of the conduction band can come into being[56]. Meanwhile, the doping of Sn formed SnO2 and V2O5 composite nanoparticles. The close potential difference between SnO2 and V2O5 makes migration of photoelectrons easier from V2O5 surfaces to SnO2 conduction band, which reduces the likelihood of recombination of photo-induced electrons on V2O5 surfaces by oxygen vacancies. But an excess amount of dopant could act as the recombination center of photo-induced electron-hole pairs, which is caused by the instantaneous accumulation of photo-induced electrons. Therefore, doping an appropriate amount of Sn is the key to improving the photocatalytic activity of V2O5 nanoparticles. In addition, the doped Sn
NP can serve as active sites. And the band-gap of V2O5 is reduced from 2.3 to 1.74 eV after doping, which helps to generate more charge carriers under light irradiation. At the same time, the PEC and PMFC systems can further inhibit the recombination of photogenic electron-hole and enhance the catalytic efficiency of degradation of ammonia gas.
(a)
(b)
Fig. 10. (a) EPR spectra of pure V2O5 and 1 wt%Sn-V2O5; (b) Schematic diagram of explanation for the photocatalytic activity improvement of V2O5 nanoparticles with an appropriate amount of Sn doping. To further investigate the reaction mechanism, the concentration of nitrogen oxides was measured and no nitrogen oxides were produced. According to the above explanation and the related literature[57][58], the degradation of ammonia by oxygen vacancies in Sn-doped V2O5 material can be understood from the following equations: 𝑉 5 + + 𝑂2 ―
ℎ𝜈
𝑉4 + + 𝑂 ― +𝑉𝑂
(4) (5)
𝑂2 +𝑉𝑂→ 𝑂2― (ads) 𝑂2― + 𝑒 ― → 2𝑂 ―
(6)
2𝑂
-
→ 𝑂2 ―
(7) 2𝑁𝐻3 +3𝑂 ― → 𝑁2 +3𝐻2O + 3𝑒 ―
(8)
The specific process is as follows: Under the simulated sunlight illumination, the catalyst absorbs light energy, the VB electrons of V2O5 are excited to the CB, creating holes in the VB and generating oxygen vacancies while oxidizing ammonia gas by the active oxygen O- overflowed from the catalyst. In the process of degrading ammonia gas by active oxygen species, part of the lattice oxygen O2- is converted to active oxygen O- and the gas phase oxygen is continuously adsorbed to the oxygen vacancies (𝑉𝑂) to generate molecular adsorption oxygen O2-(ads) which is further converted into O- and lattice oxygen O2-. In this way, active oxygen can be supplied incessantly to ensure the continuous reaction. On the other hand, active oxygen is converted into hydrogen peroxide by binding electrons, generating hydroxyl radicals. At the same time, the holes in the VB oxidize water molecules in the air to hydroxyl radicals. The hydroxyl radicals oxidize ammonia gas to nitrogen and water molecules. 4. Conclusions In this work, V2O5 and Sn-V2O5 nanoparticles synthesized by sol-gel method enabled an innovative gas-solid PEC-MFC system, where the influence of parameters on the degradation performance was evaluated in the stainless steel reactor at room temperature. It is known from the characterization results that the size of nanoparticles reduced and the number of mesopores increased as well as the electron-hole separation rate for 1% Sn-V2O5, which resulted in the best performance for photocatalytic degradation of gaseous ammonia. The UV-vis spectrum and the
steady current at different initial concentrations of ammonia demonstrate the potential applications of this composite in optoelectronic devices and sensors. NH3 can be basically removed in the PEC-MFC system, and the kinetic behaviors fit well with pseudo-second order correlation with a rate constant of 0.018 L·mol-1·min-1. Tin doping increase the surface oxygen vacancies of the catalyst, which combine light-induced carriers to form excitons, thereby improving the performance of photocatalysis, electrocatalysis and PEC-MFC. The electric circuit enhanced activity of oxygen vacancies and catalytic oxidation of ammonia. This study contributes to the researches of V-based catalysts by constructing catalytic integrated system to eliminate contaminants.
Acknowledgments This study was supported by the National Natural Science Foundation of China (No. 21677025) and Basic Research Foundation of Dalian University of Technology (Project DUT17ZD223).
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Graphical Abstract