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Microstructure and photocatalytic properties of nanostructured TiO2 and TiO2–Al coatings elaborated by HVOF spraying for the nitrogen oxides removal
Materials Science and Engineering A 417 (2006) 56–62
Microstructure and photocatalytic properties of nanostructured TiO2 and TiO2–Al coatings elaborated by HVOF spraying for the nitrogen oxides removal Filofteia-Laura Toma a , Ghislaine Bertrand a,∗ , Sang Ok Chwa a , Didier Klein a , Hamlin Liao a , Cathy Meunier b , Christian Coddet a a
Laboratoire d’Etudes et de Recherches sur les Mat´eriaux, les Proc´ed´es et les Surfaces (LERMPS), Universit´e de Technologie de Belfort-Montb´eliard (UTBM), Site de S´evenans, 90010 Belfort Cedex, France b CREST-UMR CNRS 6000, Pˆ ole Universitaire des Portes du Jura, 25211 Montb´eliard, France Received in revised form 1 July 2005; accepted 1 September 2005
Abstract This paper deals with the study of the photocatalytic behaviours of TiO2 and TiO2 –10 wt.% Al coatings elaborated by high-velocity oxygen fuel (HVOF) spraying using spray-dried nanosized feedstock powders. In the HVOF flame, the powders were injected by two methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. outside the torch nozzle. Scanning electron microscopy and X-ray diffraction were employed to characterize the microstructure and the crystalline structure of the TiO2 -based coatings. The photocatalytic activities were evaluated by the nitrogen oxides removal and compared with that of the initial powders. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of injection method. The TiO2 coatings, as well as, the TiO2 –Al coatings elaborated using the external powder injection presented a better photocatalytic activity (the nitrogen oxides conversion rates were about 25–42% for NO and 14–18% for NOx ) than those obtained by the conventional HVOF process (<5%). Besides, TiO2 coatings containing aluminium particles had an enhanced photocatalytic activity than those of un-doped titania deposits. © 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 ; TiO2 –Al; HVOF spraying; Photocatalysis; Nitrogen oxides
1. Introduction The photocatalytic process that allows the decomposition of organic compounds and removal of harmful gases is among the most studied methods to solve the major problems of air and water pollution [1]. Titanium dioxide is one of the most important photocatalyst used for such applications. The photocatalytic activity of TiO2 depends on various parameters such as crystalline phases, particle size, morphology and heat treating conditions, etc. [2–4]. The nanostructured TiO2 materials containing a high amount of anatase phase provide a better photocatalytic activity for the pollutants degradation. Thermal spray technology is an effective method to obtain nanostructured TiO2 coatings starting from nanopowders. In
∗
Corresponding author. Tel.: +33 3 84 58 32 40; fax: +33 3 84 58 32 86. E-mail address: [email protected] (G. Bertrand).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.09.112
our previous work [5], it was observed that the use of nanoparticles of anatase TiO2 in atmospheric plasma spraying did not permit to elaborate coatings with an effective photocatalytic performance, because of the phase transformation from anatase to rutile that occurs even at low plasma power. Lee et al. [6] have also reported that the plasma spraying of TiO2 nanoparticles did not allow to obtain coatings with a good photocatalytic efficiency for the decomposition of organic pollutants. Moreover, Ohmori and coworkers [7–9] observed that the photocatalytic activity strongly depends on the thermal spray process and the spray conditions. This work deals with the study of the photocatalytic properties of titanium dioxide coatings elaborated by the high velocity oxy-fuel (HVOF) spraying technique using agglomerated TiO2 based nanopowders as feedstock materials. The aim is to prevent the phase transformation and to maintain the nanosize of the grains. The photocatalytic efficiency of the elaborated coatings was evaluated by the nitrogen oxides removal. The nitrogen
F.-L. Toma et al. / Materials Science and Engineering A 417 (2006) 56–62
oxides (NOx ≈ NO + NO2 ) are major air pollutants and play an important role in atmospheric chemistry [10]. They participate to the formation of the acid rain, the greenhouse effect (in synergy with sulphur oxides) and the photochemical pollution (in presence of carbon monoxide and volatile organic compounds). On the human health, nitrogen oxides can cause very serious respiratory problems. 2. Materials and experimental procedure 2.1. Powders preparation In the present study, anatase TiO2 powder (ST 01, Ishihra Sangyo, Japan) with an average grain size of 7 nm was used. In order to prepare spherical agglomerated micro-sized particles to be used as a powder feedstock in thermal spraying, the spraydrying technique using polyvinyl alcohol as organic binder was carried out (the detailed conditions of the spray drying are property of POWREX, Japan). Aluminium powders with an average particles size of 6 m were also added to obtain an agglomerated composite (TiO2 –10 wt.% Al) feedstock material. The powders were sieved to obtain a size distribution ranging from 10 to 50 m. The TiO2 Degussa P25 powder (from Degussa AG) was considered as a reference in the photocatalytic tests. This powder contains two crystalline phases, i.e. anatase (in proportion of about 82 vol.%) and rutile. P25 consists of spherical nanoparticles with an average diameter between 25 and 50 nm. 2.2. HVOF spraying Thermal spraying was carried out using a Sulzer–Metco CDS 100 HVOF gun with natural gas, methane, as fuel gas. In the flame, the powders were injected by two different methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. from outside of the nozzle torch, as depicted in Fig. 1. In the case of the external injection, the distance between the particles injection and the flame axis was 7.7 mm. The powder feed rate was fixed at 20 g min−1 and the powder was carried to the HVOF flame by N2 at a flow rate of 9.5 slpm. The spraying parameters are summarized in Table 1. The XC 10 mild steel plates (60 mm × 70 mm × 2 mm) sand-blasted with corundum particles were used as substrates.
Table 1 HVOF spraying parameters Parameters
Unit
Value
CH4 fuel flow rate O2 gas flow rate Volumic rate (CH4 /O2 ) N2 gas flow rate Nozzle Speed torch N2 carrier gas flow rate Powder feed rate Spray distance Number of passes
slpm slpm
100 400 0.25 50 3 30 25 20 150 10
slpm inches mm s−1 slpm g min−1 mm
2.3. Characterization The morphology of the powders and the microstructures of the coatings were examined using a JEOL JSM-5800LV scanning electron microscope (SEM) and elemental analysis was also carried out by energy dispersive X-ray spectroscopy (EDS). X-rays diffraction (performed with a X’Pert Philips diffractometer) using the Cu K␣ radiation was employed to determine the anatase to rutile ratio in the powders and HVOF sprayed coatings. Scan step was 0.02◦ s−1 with a step time of 0.5 s in the 20–90◦ 2θ range. The volume percentage of anatase was determined according to the following relation [11]: CA =
8IA 8IA + 13IR
(1)
where IA and IR are the X-ray intensities of the anatase (1 0 1) and the rutile (1 1 0) peaks, respectively. The crystallite size was evaluated from the X-ray diffraction patterns based on the Scherrer formula as shown in the following equation [12]: t=
0.9λ B cos θ
(2)
with B the corrected peak width at half maximum intensity, λ the X-ray wavelength and θ the Bragg diffraction angle. The photocatalytic activity was evaluated by the nitrogen oxides degradation performed in an environmental test chamber set-up, described elsewhere [13]. The photocatalytic tests were realized over the sprayed coatings and powders. The photocatalytic efficiency was evaluated as the ratio of the photocatalytic conversion of nitrogen oxides concentrations and determined by the following relations: [NO]removed (%) =
[NO]initial − [NO]UV × 100 [NO]initial
[NOx ]removed (%) =
Fig. 1. Schematic diagram of the powder injection: (a) conventional process— internal injection and (b) external injection.
57
[NOx ]initial − [NOx ]UV × 100 [NOx ]initial
(3a) (3b)
where [NO]removed (%) and [NOx ]removed (%) are the conversions in the concentration of NO and NOx in the presence of the catalyst and UV irradiation; [NO]initial and [NOx ]initial represent the values of the NO and NOx concentrations without UV irradiation; [NO]UV and [NOx ]UV are the values of the NO and NOx concentrations under UV irradiation.
F.-L. Toma et al. / Materials Science and Engineering A 417 (2006) 56–62
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Fig. 2. Internal morphologies of agglomerated feedstock materials observed by SEM and EDS analysis: (a) TiO2 powder and (b) TiO2 –10 wt.% Al powder.
3. Results and discussions 3.1. Characterization of the spraying powders The internal morphologies of the TiO2 and TiO2 –10 wt.% Al powders are shown in Fig. 2. The cross-section analysis of these agglomerated powders shows that the nanosized primary particles form agglomerated clusters that loosely bound each other after the spray-drying process. The Al particles seem well embedded by the TiO2 particles as it is confirmed by EDS analysis (Fig. 2(b)). The average size diameter (D50 ) of the agglomerated particles is about 20 m for the spray-dried TiO2 powder and about 30 m for the TiO2 –10 wt.% Al composite powder (Fig. 3). Fig. 4 displays the XRD patterns of the primary anatase nanoparticles and of the spray-dried TiO2 and TiO2 –Al powders. Only the anatase phase was identified in the agglomerated powders and, as expected, no increase in the crystallite size was found after the spray-drying process (7–8 nm).
tified. The SEM images of the cross-section and the surface microstructure can be observed in Fig. 6. The TiO2 deposit, densely structured, contains principally partially/non-melted particles as well as the initial nano-agglomerated particles anchored in the coating structure. When the external injection of the powder is employed, the heat transfer from the flame to the
3.2. Coatings morphology The microscopic analysis showed that the morphologies of the sprayed coatings strongly depend on the manner in which the powder was injected in the flame. The TiO2 deposits elaborated by the conventional HVOF process are porous (a porosity ratio estimated by image analysis at 12%) and characterized by a layered microstructure, which can be observed in most thermally sprayed deposits (Fig. 5). The coatings are mainly built-up by the melted nanoparticles impacting on the substrate that flatten to form splats, which consecutively pile on top of the others. Some partially molten particles embedded in the coating structure are also observed. The surface analysis shows the presence of fine particles presenting the initial features of the agglomerated TiO2 nanopowder. When the TiO2 nanoparticles were radially injected in the flame, outside the nozzle torch, i.e. with the external powder injection, the lamellar structure of the coatings was hardly iden-
Fig. 3. Particle size distribution curves of studied powders: (a) TiO2 and (b) TiO2 –10 wt.% Al.
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Fig. 4. XRD analysis of powders.
Fig. 6. SEM micrographs of TiO2 coating obtained by external injection of the powder in the HVOF flame: (a) cross-section and (b) top surface.
particles is not sufficient to ensure the droplets melting before impinging on the substrate. The SEM morphology of the deposit obtained by the external injection of TiO2 –10 wt.% Al powder is presented in Fig. 7. The coating has a porous structure (with 14% porosity ratio, measured by image analysis [14]), including partial and nonmolten TiO2 particles. Al particles, identified by elemental EDS analysis, appear like elongated “splats” (average diameter size of about 5 m) as depicted in Fig. 8. Therefore, Al particles are fully melted due to their significantly low melting temperature and high reactivity and are considered to be well distributed in the coating structure. Moreover, the addition of metallic Al particles in the agglomerated spray-dried TiO2 powder increases the spraying efficiency and a thicker coating (around 30 m) was obtained compared with that of the TiO2 deposit (10 m of thickness) elaborated under the same HVOF spraying conditions. It was affirmed that the aluminium plays a role as a binding agent [8], which increases the cohesion of the TiO2 nanoparticles in the coating. A schematic mechanism of TiO2 –Al composite coating obtained by thermal spraying has already been proposed elsewhere [14]. 3.3. Coatings crystalline structure Fig. 5. SEM micrographs of TiO2 coating obtained by conventional HVOF process: (a) cross-section and (b) top surface.
The crystalline structure of the elaborated HVOF-coatings was studied by XRD analysis. The results show that the passage
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Fig. 7. The cross-section micrography (a) and surface morphology (b) of TiO2 –Al coating elaborated by external powder injection.
of the nanoparticles in the flame involves a modification of the chemical state of the particles with regard to the one of the initial anatase nanopowder, which depends on the kind of powder and HVOF process. Thus, the coating obtained from the TiO2 powder sprayed by the conventional HVOF process contains the rutile and anatase phases, with an anatase ratio of about 12.6 vol.%. The crystalline size of the anatase/rutile increases from 7/0 nm (as determined in the agglomerated TiO2 powder) to 80/90 nm. When the non-conventional HVOF was used to obtain the TiO2 deposit, the structural transformation was less important, wherein an anatase ratio of about 65.6 vol.% was determined, with a crystalline size of anatase particles of 18 nm. The rutile phase disposes of grains with an average size of 122 nm. Fig. 9 shows the XRD pattern of the TiO2 –Al sprayed coatings obtained by the non-conventional HVOF process. The analysis shows that titania was almost in form of anatase (92:8 anatase to rutile (vol.%)) with nanosized crystallites (9 nm). In our previous work [14], it was observed the presence of nonstoechiometric oxides of the Ti–O system (Ti3 O5 and Magneli phases) when the TiO2 –Al composite coating was elaborated by plasma spraying; such sub-stoechiometry was not identified in the HVOF deposit. The Al peaks were also identified although some oxidation of the aluminium was observed through peaks attributed to an aluminium oxide Al2 O3 . It was observed that the anatase ratio in the TiO2 –Al coating was more important than that of un-doped TiO2 coating probably because part of the flame heat was used to melt and maybe to vaporise the aluminium particles, thus avoiding the phase transformation from anatase to rutile.
Fig. 8. EDS analysis of the TiO2 –Al coating microstructure.
F.-L. Toma et al. / Materials Science and Engineering A 417 (2006) 56–62
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Fig. 9. XRD pattern of the TiO2 –Al coating sprayed by non-conventional HVOF process.
3.4. Evaluation of the photocatalytic properties The HVOF TiO2 -based coatings were tested in the photocatalysis to evaluate their efficiencies for the nitrogen oxides removal and compared with that of the agglomerated feedstock powders. The results of the photocatalytic tests are illustrated in Fig. 10. In the case of the agglomerated spray-dried powders no significant difference in the photocatalytic activities was observed. The values of the conversion ratio ranged between 30 and 32% for NO and 16–18% for NOx , practically identical with that of the reference TiO2 Degussa P25. In contrast, the photocatalytic activity of the sprayed coatings was different and depends on the spraying conditions: powder injection and nature of the material feedstock. TiO2 coatings elaborated by the conventional HVOF process exhibited poor performance for the nitrogen oxides removal (lower than 5%). When the agglomerated TiO2 powder is conventionally injected in HVOF process, the thermal treatment of the anatase nanoparticles in the flame involves the structural transformation into rutile (the most stable phase at high temperature but less active in the photocatalysis). In the case of the external injection of the powder in the HVOF flame, the TiO2 coating presents a higher photocatalytic performance in the nitrogen oxides removal, with conversion ratios of about 25% for NO and 14% for NOx . The different photocatalytic performance in the pollutants degradation was principally correlated with the crystalline structure of the titania coating. As previously presented [5,15], the anatase phase is a key parameter in the photocatalytic activity and ensures a higher decomposition of nitrogen oxides. A better degradation of pollutant was obtained in the presence of the nonconventional HVOF coating due to an important anatase ratio (65.6%) than that determined in the standard HVOF TiO2 coating (12.6%). Once again, the critical role played by the anatase phase in the photocatalytic degradation of nitrogen oxides was pointed out. A significant improvement of the nitrogen oxides photocatalytic removal was identified on the TiO2 –Al composite coatings elaborated by the modified HVOF spraying. The conversion ratio of NO was about 42% and that of NOx around 20%, higher than
Fig. 10. Photocatalytic activity of the titanium dioxide-based materials in form of powders (a) and HVOF-sprayed coatings (b); int.: internal injection of the powder; ext.: external injection of the powder.
that obtained on the initial agglomerated powder or in the presence of the Degussa P25 reference powder. Similar results, i.e. an increased photocatalytic activity of Al-doped TiO2 coatings, were obtained by Matsusaka et al. [8]. Several studies were performed to explain the role of the aluminium and its compounds versus the titania photocatalytic activity [16–18]. It is generally presumed that the active species responsible for the photocatalytic degradation of pollutants are the hydroxyl radicals (HO• ) due to their very high oxidation potential. The surface of the metallic oxides is able to chemisorb water vapour that can be removed from the surface only by drastic means. The water is adsorbed by hydrogen bond or by dissociation when the surface hydroxyl groups are formed. UV-light excitation of TiO2 (wavelength between 360 and 380 nm) generates electrons and holes in the conduction and valence bands, respectively (Fig. 11). These species diffuse to the oxide surface where they are trapped by adsorbed water and oxygen molecules. The oxidation of water or HO ions by photogenerated holes produces hydroxyl radicals that react with the adsorbed pollutants to oxidize them. The electron/hole pairs recombination
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The presence of the co-sprayed Al particles allows to retain the initial nanosized anatase phase of the titania particles. Furthermore, the composite coating ensured the best photocatalytic performance in the pollutants degradation (42% for NO and 18% for NOx ), higher than that of the initial agglomerated powder or that of the reference powder (TiO2 Degussa P25). Nevertheless, detailed surface investigations are still necessary to clarify the role of the aluminium particles in the enhancement of the photocatalytic performance of titanium dioxide deposits. Acknowledgements Fig. 11. Principle of the photocatalysis over the TiO2 particles.
and the diminution of HO• concentration on the TiO2 surface involve the decrease of the photocatalytic efficiency. To avoid this phenomenon, several solutions have been proposed such as the doping of the titania with different materials to favour the electron–hole separation and to modify the interfacial electron/hole transfer. In this manner, it was affirmed that the aluminium particles significantly decrease the electron density of the excited states, enhancing the holes density at the surface of the photocatalyst [16]. In our case, no notable improvement of the photocatalytic decomposition of nitrogen oxides was observed over the agglomerated TiO2 –10 wt.% Al powder compared with that of un-doped titania powder. In contrast, the pollutants were notably removed over the TiO2 –Al sprayed coating. This enhanced activity is probably the result of the elimination of impurities existing in the initial agglomerated powder (i.e. the organic binder used in the powder spray-drying process) and a cleaning-up of the particles surface when crossing the flame. Such cleaning may improves the contact between the Al and TiO2 particles and thus, aluminium could contribute to the charges separation during the ultraviolet irradiation enhancing the photocatalytic activity of the coating. However, surface investigations are still necessary to clarify the photocatalytic performance of TiO2 –Al composite coating. 4. Conclusions The HVOF process was used to prepare coatings from TiO2 and TiO2 –10 wt.% Al agglomerated nanopowders. In the flame, the particles were injected by two different methods: internal injection as in a conventional HVOF system and external injection at the exit of the torch nozzle. The coatings prepared by standard HVOF are characterized by a lamellar structure as observed in common thermally sprayed coatings. The photocatalytic ability for the nitrogen oxides degradation was lower than 5% correlated with a reduced anatase ratio. The deposits obtained by external injection of the powder feedstock presented a specific structure with a high ratio of non-melted particles and an important amount of anatase phase. In this case, the conversion ratio of nitrogen oxides was between 14 and 25%.
The authors are grateful to Pr. A. Ohmori (Osaka University, Japan) for agglomerated TiO2 and TiO2 –Al powders delivery and S. Lamy (LERMPS) for SEM analysis. References [1] D.F. Ollis, H. Al-Ekabi, Photocatalytic Purification and Treatment of Water and Air Proceedings, Elsevier, 1993. [2] N.K Negiski, Appl. Surf. Sci. 121/122 (1997) 417–420. [3] K. Tanaka, T. Hisanaga, A.P. Rivera, in: D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air Proceedings, Elsevier, 1993, pp. 169–178. [4] S. Matsuda, H. Hatano, A. Tsutsumi, Chem. Eng. J. 82 (2001) 183– 188. [5] F.-L. Toma, G. Bertrand, S.-O. Chwa, D. Klein, P. Nardin, A. Ohmori, ITSC 2004 Thermal Spray Solutions: Advances in Technology and Application, DVS-Verlag, Osaka, Japan, 2004 (CD-rom version). [6] C. Lee, H. Choi, C. Lee, H. Kim, Surf. Coatings Technol. 173 (2003) 192–200. [7] A. Ohmori, K. Nakade, Y. Yamamoto, in: C. Moreau, B. Marple (Eds.), Thermal Spray 2003: Advancing the Science and Applying the Technology, Pub. ASM Int., Materials Park, OH, USA, 2003, pp. 189– 193. [8] S. Matsusaka, A. Ohmori, Y. Toki, in: C.C. Berndt, K.A. Khor, E.F. Lugscheider (Eds.), Thermal Spray 2001: New surfaces for a New Millennium, Pub. ASM Int., Material Park, OH, USA, 2001, pp. 87–92. [9] A. Ohmori, H. Shoyama, S. Matsusaka, K. Ohashi, K. Moriya, C.J. Li, in: C. Berndt (Ed.), Thermal Spray: Surface Engineering via Applied Research, Proceedings of the 1st International Thermal Spray Conference, Pub. ASM Int., 2000, pp. 319–323. [10] G. Brasseur, Physique et chimie de l’atmosphere, Masson, Paris, 1982. [11] N. Berger-Keller, G. Bertrand, C. Filiatre, C. Meunier, C. Coddet, Surf. Coatings Technol. 168 (2003) 281–290. [12] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, 1978. [13] F.-L. Toma, N. Berger-Keller, G. Bertrand, D. Klein, C. Coddet, in: C. Moreau, B. Marple (Eds.), Thermal Spray 2003: Advancing the Science & Applying the Technology, Pub. ASM Int., Materials Park, OH, USA, 2003, pp. 1331–1335. [14] S.O. Chwa, D. Klein, F.L. Toma, G. Bertrand, H. Liao, C. Coddet, A. Ohmori, Surf. Coatings Technol. 194 (2005) 215–224. [15] F.-L. Toma, G. Bertrand, D. Klein, C. Coddet, Environ. Chem. Lett. 2 (3) (2004) 117–121. [16] R. Bauer, G. Waldner, H. Fallmann, S. Hager, M. Klare, T. Krutzler, S. Malato, P. Maletzky, Catal. Today 53 (1999) 131–144. [17] P. Knauth, G. Auer, Solid State Ionics 147 (2002) 115–121. [18] T.K. Kim, M.N. Lee, S.H. Lee, Y.C. Park, C.K. Jung, J.-H. Boo, Thin Solid Films 475 (2005) 171–177.
Microstructure and photocatalytic properties of nanostructured TiO2 and TiO2–Al coatings elaborated by HVOF spraying for the nitrogen oxides removal Filofteia-Laura Toma a , Ghislaine Bertrand a,∗ , Sang Ok Chwa a , Didier Klein a , Hamlin Liao a , Cathy Meunier b , Christian Coddet a a
Laboratoire d’Etudes et de Recherches sur les Mat´eriaux, les Proc´ed´es et les Surfaces (LERMPS), Universit´e de Technologie de Belfort-Montb´eliard (UTBM), Site de S´evenans, 90010 Belfort Cedex, France b CREST-UMR CNRS 6000, Pˆ ole Universitaire des Portes du Jura, 25211 Montb´eliard, France Received in revised form 1 July 2005; accepted 1 September 2005
Abstract This paper deals with the study of the photocatalytic behaviours of TiO2 and TiO2 –10 wt.% Al coatings elaborated by high-velocity oxygen fuel (HVOF) spraying using spray-dried nanosized feedstock powders. In the HVOF flame, the powders were injected by two methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. outside the torch nozzle. Scanning electron microscopy and X-ray diffraction were employed to characterize the microstructure and the crystalline structure of the TiO2 -based coatings. The photocatalytic activities were evaluated by the nitrogen oxides removal and compared with that of the initial powders. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of injection method. The TiO2 coatings, as well as, the TiO2 –Al coatings elaborated using the external powder injection presented a better photocatalytic activity (the nitrogen oxides conversion rates were about 25–42% for NO and 14–18% for NOx ) than those obtained by the conventional HVOF process (<5%). Besides, TiO2 coatings containing aluminium particles had an enhanced photocatalytic activity than those of un-doped titania deposits. © 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 ; TiO2 –Al; HVOF spraying; Photocatalysis; Nitrogen oxides
1. Introduction The photocatalytic process that allows the decomposition of organic compounds and removal of harmful gases is among the most studied methods to solve the major problems of air and water pollution [1]. Titanium dioxide is one of the most important photocatalyst used for such applications. The photocatalytic activity of TiO2 depends on various parameters such as crystalline phases, particle size, morphology and heat treating conditions, etc. [2–4]. The nanostructured TiO2 materials containing a high amount of anatase phase provide a better photocatalytic activity for the pollutants degradation. Thermal spray technology is an effective method to obtain nanostructured TiO2 coatings starting from nanopowders. In
∗
Corresponding author. Tel.: +33 3 84 58 32 40; fax: +33 3 84 58 32 86. E-mail address: [email protected] (G. Bertrand).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.09.112
our previous work [5], it was observed that the use of nanoparticles of anatase TiO2 in atmospheric plasma spraying did not permit to elaborate coatings with an effective photocatalytic performance, because of the phase transformation from anatase to rutile that occurs even at low plasma power. Lee et al. [6] have also reported that the plasma spraying of TiO2 nanoparticles did not allow to obtain coatings with a good photocatalytic efficiency for the decomposition of organic pollutants. Moreover, Ohmori and coworkers [7–9] observed that the photocatalytic activity strongly depends on the thermal spray process and the spray conditions. This work deals with the study of the photocatalytic properties of titanium dioxide coatings elaborated by the high velocity oxy-fuel (HVOF) spraying technique using agglomerated TiO2 based nanopowders as feedstock materials. The aim is to prevent the phase transformation and to maintain the nanosize of the grains. The photocatalytic efficiency of the elaborated coatings was evaluated by the nitrogen oxides removal. The nitrogen
F.-L. Toma et al. / Materials Science and Engineering A 417 (2006) 56–62
oxides (NOx ≈ NO + NO2 ) are major air pollutants and play an important role in atmospheric chemistry [10]. They participate to the formation of the acid rain, the greenhouse effect (in synergy with sulphur oxides) and the photochemical pollution (in presence of carbon monoxide and volatile organic compounds). On the human health, nitrogen oxides can cause very serious respiratory problems. 2. Materials and experimental procedure 2.1. Powders preparation In the present study, anatase TiO2 powder (ST 01, Ishihra Sangyo, Japan) with an average grain size of 7 nm was used. In order to prepare spherical agglomerated micro-sized particles to be used as a powder feedstock in thermal spraying, the spraydrying technique using polyvinyl alcohol as organic binder was carried out (the detailed conditions of the spray drying are property of POWREX, Japan). Aluminium powders with an average particles size of 6 m were also added to obtain an agglomerated composite (TiO2 –10 wt.% Al) feedstock material. The powders were sieved to obtain a size distribution ranging from 10 to 50 m. The TiO2 Degussa P25 powder (from Degussa AG) was considered as a reference in the photocatalytic tests. This powder contains two crystalline phases, i.e. anatase (in proportion of about 82 vol.%) and rutile. P25 consists of spherical nanoparticles with an average diameter between 25 and 50 nm. 2.2. HVOF spraying Thermal spraying was carried out using a Sulzer–Metco CDS 100 HVOF gun with natural gas, methane, as fuel gas. In the flame, the powders were injected by two different methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. from outside of the nozzle torch, as depicted in Fig. 1. In the case of the external injection, the distance between the particles injection and the flame axis was 7.7 mm. The powder feed rate was fixed at 20 g min−1 and the powder was carried to the HVOF flame by N2 at a flow rate of 9.5 slpm. The spraying parameters are summarized in Table 1. The XC 10 mild steel plates (60 mm × 70 mm × 2 mm) sand-blasted with corundum particles were used as substrates.
Table 1 HVOF spraying parameters Parameters
Unit
Value
CH4 fuel flow rate O2 gas flow rate Volumic rate (CH4 /O2 ) N2 gas flow rate Nozzle Speed torch N2 carrier gas flow rate Powder feed rate Spray distance Number of passes
slpm slpm
100 400 0.25 50 3 30 25 20 150 10
slpm inches mm s−1 slpm g min−1 mm
2.3. Characterization The morphology of the powders and the microstructures of the coatings were examined using a JEOL JSM-5800LV scanning electron microscope (SEM) and elemental analysis was also carried out by energy dispersive X-ray spectroscopy (EDS). X-rays diffraction (performed with a X’Pert Philips diffractometer) using the Cu K␣ radiation was employed to determine the anatase to rutile ratio in the powders and HVOF sprayed coatings. Scan step was 0.02◦ s−1 with a step time of 0.5 s in the 20–90◦ 2θ range. The volume percentage of anatase was determined according to the following relation [11]: CA =
8IA 8IA + 13IR
(1)
where IA and IR are the X-ray intensities of the anatase (1 0 1) and the rutile (1 1 0) peaks, respectively. The crystallite size was evaluated from the X-ray diffraction patterns based on the Scherrer formula as shown in the following equation [12]: t=
0.9λ B cos θ
(2)
with B the corrected peak width at half maximum intensity, λ the X-ray wavelength and θ the Bragg diffraction angle. The photocatalytic activity was evaluated by the nitrogen oxides degradation performed in an environmental test chamber set-up, described elsewhere [13]. The photocatalytic tests were realized over the sprayed coatings and powders. The photocatalytic efficiency was evaluated as the ratio of the photocatalytic conversion of nitrogen oxides concentrations and determined by the following relations: [NO]removed (%) =
[NO]initial − [NO]UV × 100 [NO]initial
[NOx ]removed (%) =
Fig. 1. Schematic diagram of the powder injection: (a) conventional process— internal injection and (b) external injection.
57
[NOx ]initial − [NOx ]UV × 100 [NOx ]initial
(3a) (3b)
where [NO]removed (%) and [NOx ]removed (%) are the conversions in the concentration of NO and NOx in the presence of the catalyst and UV irradiation; [NO]initial and [NOx ]initial represent the values of the NO and NOx concentrations without UV irradiation; [NO]UV and [NOx ]UV are the values of the NO and NOx concentrations under UV irradiation.
F.-L. Toma et al. / Materials Science and Engineering A 417 (2006) 56–62
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Fig. 2. Internal morphologies of agglomerated feedstock materials observed by SEM and EDS analysis: (a) TiO2 powder and (b) TiO2 –10 wt.% Al powder.
3. Results and discussions 3.1. Characterization of the spraying powders The internal morphologies of the TiO2 and TiO2 –10 wt.% Al powders are shown in Fig. 2. The cross-section analysis of these agglomerated powders shows that the nanosized primary particles form agglomerated clusters that loosely bound each other after the spray-drying process. The Al particles seem well embedded by the TiO2 particles as it is confirmed by EDS analysis (Fig. 2(b)). The average size diameter (D50 ) of the agglomerated particles is about 20 m for the spray-dried TiO2 powder and about 30 m for the TiO2 –10 wt.% Al composite powder (Fig. 3). Fig. 4 displays the XRD patterns of the primary anatase nanoparticles and of the spray-dried TiO2 and TiO2 –Al powders. Only the anatase phase was identified in the agglomerated powders and, as expected, no increase in the crystallite size was found after the spray-drying process (7–8 nm).
tified. The SEM images of the cross-section and the surface microstructure can be observed in Fig. 6. The TiO2 deposit, densely structured, contains principally partially/non-melted particles as well as the initial nano-agglomerated particles anchored in the coating structure. When the external injection of the powder is employed, the heat transfer from the flame to the
3.2. Coatings morphology The microscopic analysis showed that the morphologies of the sprayed coatings strongly depend on the manner in which the powder was injected in the flame. The TiO2 deposits elaborated by the conventional HVOF process are porous (a porosity ratio estimated by image analysis at 12%) and characterized by a layered microstructure, which can be observed in most thermally sprayed deposits (Fig. 5). The coatings are mainly built-up by the melted nanoparticles impacting on the substrate that flatten to form splats, which consecutively pile on top of the others. Some partially molten particles embedded in the coating structure are also observed. The surface analysis shows the presence of fine particles presenting the initial features of the agglomerated TiO2 nanopowder. When the TiO2 nanoparticles were radially injected in the flame, outside the nozzle torch, i.e. with the external powder injection, the lamellar structure of the coatings was hardly iden-
Fig. 3. Particle size distribution curves of studied powders: (a) TiO2 and (b) TiO2 –10 wt.% Al.
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Fig. 4. XRD analysis of powders.
Fig. 6. SEM micrographs of TiO2 coating obtained by external injection of the powder in the HVOF flame: (a) cross-section and (b) top surface.
particles is not sufficient to ensure the droplets melting before impinging on the substrate. The SEM morphology of the deposit obtained by the external injection of TiO2 –10 wt.% Al powder is presented in Fig. 7. The coating has a porous structure (with 14% porosity ratio, measured by image analysis [14]), including partial and nonmolten TiO2 particles. Al particles, identified by elemental EDS analysis, appear like elongated “splats” (average diameter size of about 5 m) as depicted in Fig. 8. Therefore, Al particles are fully melted due to their significantly low melting temperature and high reactivity and are considered to be well distributed in the coating structure. Moreover, the addition of metallic Al particles in the agglomerated spray-dried TiO2 powder increases the spraying efficiency and a thicker coating (around 30 m) was obtained compared with that of the TiO2 deposit (10 m of thickness) elaborated under the same HVOF spraying conditions. It was affirmed that the aluminium plays a role as a binding agent [8], which increases the cohesion of the TiO2 nanoparticles in the coating. A schematic mechanism of TiO2 –Al composite coating obtained by thermal spraying has already been proposed elsewhere [14]. 3.3. Coatings crystalline structure Fig. 5. SEM micrographs of TiO2 coating obtained by conventional HVOF process: (a) cross-section and (b) top surface.
The crystalline structure of the elaborated HVOF-coatings was studied by XRD analysis. The results show that the passage
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Fig. 7. The cross-section micrography (a) and surface morphology (b) of TiO2 –Al coating elaborated by external powder injection.
of the nanoparticles in the flame involves a modification of the chemical state of the particles with regard to the one of the initial anatase nanopowder, which depends on the kind of powder and HVOF process. Thus, the coating obtained from the TiO2 powder sprayed by the conventional HVOF process contains the rutile and anatase phases, with an anatase ratio of about 12.6 vol.%. The crystalline size of the anatase/rutile increases from 7/0 nm (as determined in the agglomerated TiO2 powder) to 80/90 nm. When the non-conventional HVOF was used to obtain the TiO2 deposit, the structural transformation was less important, wherein an anatase ratio of about 65.6 vol.% was determined, with a crystalline size of anatase particles of 18 nm. The rutile phase disposes of grains with an average size of 122 nm. Fig. 9 shows the XRD pattern of the TiO2 –Al sprayed coatings obtained by the non-conventional HVOF process. The analysis shows that titania was almost in form of anatase (92:8 anatase to rutile (vol.%)) with nanosized crystallites (9 nm). In our previous work [14], it was observed the presence of nonstoechiometric oxides of the Ti–O system (Ti3 O5 and Magneli phases) when the TiO2 –Al composite coating was elaborated by plasma spraying; such sub-stoechiometry was not identified in the HVOF deposit. The Al peaks were also identified although some oxidation of the aluminium was observed through peaks attributed to an aluminium oxide Al2 O3 . It was observed that the anatase ratio in the TiO2 –Al coating was more important than that of un-doped TiO2 coating probably because part of the flame heat was used to melt and maybe to vaporise the aluminium particles, thus avoiding the phase transformation from anatase to rutile.
Fig. 8. EDS analysis of the TiO2 –Al coating microstructure.
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Fig. 9. XRD pattern of the TiO2 –Al coating sprayed by non-conventional HVOF process.
3.4. Evaluation of the photocatalytic properties The HVOF TiO2 -based coatings were tested in the photocatalysis to evaluate their efficiencies for the nitrogen oxides removal and compared with that of the agglomerated feedstock powders. The results of the photocatalytic tests are illustrated in Fig. 10. In the case of the agglomerated spray-dried powders no significant difference in the photocatalytic activities was observed. The values of the conversion ratio ranged between 30 and 32% for NO and 16–18% for NOx , practically identical with that of the reference TiO2 Degussa P25. In contrast, the photocatalytic activity of the sprayed coatings was different and depends on the spraying conditions: powder injection and nature of the material feedstock. TiO2 coatings elaborated by the conventional HVOF process exhibited poor performance for the nitrogen oxides removal (lower than 5%). When the agglomerated TiO2 powder is conventionally injected in HVOF process, the thermal treatment of the anatase nanoparticles in the flame involves the structural transformation into rutile (the most stable phase at high temperature but less active in the photocatalysis). In the case of the external injection of the powder in the HVOF flame, the TiO2 coating presents a higher photocatalytic performance in the nitrogen oxides removal, with conversion ratios of about 25% for NO and 14% for NOx . The different photocatalytic performance in the pollutants degradation was principally correlated with the crystalline structure of the titania coating. As previously presented [5,15], the anatase phase is a key parameter in the photocatalytic activity and ensures a higher decomposition of nitrogen oxides. A better degradation of pollutant was obtained in the presence of the nonconventional HVOF coating due to an important anatase ratio (65.6%) than that determined in the standard HVOF TiO2 coating (12.6%). Once again, the critical role played by the anatase phase in the photocatalytic degradation of nitrogen oxides was pointed out. A significant improvement of the nitrogen oxides photocatalytic removal was identified on the TiO2 –Al composite coatings elaborated by the modified HVOF spraying. The conversion ratio of NO was about 42% and that of NOx around 20%, higher than
Fig. 10. Photocatalytic activity of the titanium dioxide-based materials in form of powders (a) and HVOF-sprayed coatings (b); int.: internal injection of the powder; ext.: external injection of the powder.
that obtained on the initial agglomerated powder or in the presence of the Degussa P25 reference powder. Similar results, i.e. an increased photocatalytic activity of Al-doped TiO2 coatings, were obtained by Matsusaka et al. [8]. Several studies were performed to explain the role of the aluminium and its compounds versus the titania photocatalytic activity [16–18]. It is generally presumed that the active species responsible for the photocatalytic degradation of pollutants are the hydroxyl radicals (HO• ) due to their very high oxidation potential. The surface of the metallic oxides is able to chemisorb water vapour that can be removed from the surface only by drastic means. The water is adsorbed by hydrogen bond or by dissociation when the surface hydroxyl groups are formed. UV-light excitation of TiO2 (wavelength between 360 and 380 nm) generates electrons and holes in the conduction and valence bands, respectively (Fig. 11). These species diffuse to the oxide surface where they are trapped by adsorbed water and oxygen molecules. The oxidation of water or HO ions by photogenerated holes produces hydroxyl radicals that react with the adsorbed pollutants to oxidize them. The electron/hole pairs recombination
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The presence of the co-sprayed Al particles allows to retain the initial nanosized anatase phase of the titania particles. Furthermore, the composite coating ensured the best photocatalytic performance in the pollutants degradation (42% for NO and 18% for NOx ), higher than that of the initial agglomerated powder or that of the reference powder (TiO2 Degussa P25). Nevertheless, detailed surface investigations are still necessary to clarify the role of the aluminium particles in the enhancement of the photocatalytic performance of titanium dioxide deposits. Acknowledgements Fig. 11. Principle of the photocatalysis over the TiO2 particles.
and the diminution of HO• concentration on the TiO2 surface involve the decrease of the photocatalytic efficiency. To avoid this phenomenon, several solutions have been proposed such as the doping of the titania with different materials to favour the electron–hole separation and to modify the interfacial electron/hole transfer. In this manner, it was affirmed that the aluminium particles significantly decrease the electron density of the excited states, enhancing the holes density at the surface of the photocatalyst [16]. In our case, no notable improvement of the photocatalytic decomposition of nitrogen oxides was observed over the agglomerated TiO2 –10 wt.% Al powder compared with that of un-doped titania powder. In contrast, the pollutants were notably removed over the TiO2 –Al sprayed coating. This enhanced activity is probably the result of the elimination of impurities existing in the initial agglomerated powder (i.e. the organic binder used in the powder spray-drying process) and a cleaning-up of the particles surface when crossing the flame. Such cleaning may improves the contact between the Al and TiO2 particles and thus, aluminium could contribute to the charges separation during the ultraviolet irradiation enhancing the photocatalytic activity of the coating. However, surface investigations are still necessary to clarify the photocatalytic performance of TiO2 –Al composite coating. 4. Conclusions The HVOF process was used to prepare coatings from TiO2 and TiO2 –10 wt.% Al agglomerated nanopowders. In the flame, the particles were injected by two different methods: internal injection as in a conventional HVOF system and external injection at the exit of the torch nozzle. The coatings prepared by standard HVOF are characterized by a lamellar structure as observed in common thermally sprayed coatings. The photocatalytic ability for the nitrogen oxides degradation was lower than 5% correlated with a reduced anatase ratio. The deposits obtained by external injection of the powder feedstock presented a specific structure with a high ratio of non-melted particles and an important amount of anatase phase. In this case, the conversion ratio of nitrogen oxides was between 14 and 25%.
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