Effects of TiO2 Size, Surface Area, and Phase on Congo Red Photocatalytic Degradation, Papers of Chemistry

Download Effects of TiO2 Size, Surface Area, and Phase on Congo Red Photocatalytic Degradation and more Papers Chemistry in PDF only on Docsity! Journal of Molecular Catalysis A: Chemical 242 (2005) 48–56 Photodegradation of Congo Red catalyzed by nanosized TiO2 Rajeev K. Wahi, William W. Yu ∗, Yunping Liu, Michelle L. Mejia, Joshua C. Falkner, Whitney Nolte, Vicki L. Colvin Department of Chemistry, Rice University, 6100 Main Street, Houston, TX 77005, USA Received 6 June 2005; accepted 27 July 2005 Available online 6 September 2005 Abstract The effects of various physical properties (particle size, shape, surface area, crystal structure, and phase composition) on the photocatalytic performance of nanosized TiO2 were studied through the photodegradation of Congo Red. Kinetic results showed that anatase TiO2 was a superior photocatalyst to rutile TiO2 due to the inherent difference in the two crystal structures. Anatase TiO2 nanorods with predominantly (1 0 1) surface exhibited low activity because the non-dissociative adsorption of H2O to this surface retarded the generation of OH• radicals required for facile photocatalytic oxidation. It was found for the first time that the shape of TiO2 nanocrystals significantly affected their photocatalytic activities. The previously reported anatase–rutile synergetic effect in commercialized mixed-phase TiO2 (Degussa P25) was not observed in this study, perhaps, due to poor contact between the two phases and the extremely small sizes, as well as the not-optimized phase compositions in the current work. © 2005 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Congo Red; TiO2; Nanoparticle; Catalysis 1. Introduction Nanoscale materials possess distinctive properties differing from their molecular and bulk forms, such as quantum confine- ment [1,2], superparamagnetism [3], superior catalytic perfor- mances including the high selectivity and high reactivity [4–7]. The use of nanocrystalline TiO2 in the photocatalytic oxidation of organic molecules represents a promising remediation strat- egy for wastewater systems. In the past decades, hundreds of reports have been published on TiO2-mediated destruction of organic pollutants such as polychlorobiphenyls, toluene, surfac- tants, pesticides and their precursors, herbicides, phenols and phenolic compounds, carboxylic acids, halogenated hydrocar- bons, aromatic sulfides, and organic dyes [8–17]. Given the enormous variety of organic compounds that can be photocat- alytically degraded by TiO2, there has naturally been a great interest in finding ways to improve the efficiency of TiO2 in such processes. In general, the efficiency or activity of a photocatalyst increases with its adsorptive capacity and the extent of charge ∗ Corresponding author. Tel.: +1 713 348 3489; fax: +1 713 348 2578. separation upon photoinduced generation of electron–hole (e−/h+) pairs. Small particle size and high specific surface area tend to increase the photocatalytic activity of TiO2 due to the large number of surface sites for adsorption and subsequent des- orption of reactant molecules, although there might be limits to this beneficial effect since the recombination rate of e−/h+ may be high in extremely small particles [18–21]. Efficient charge separation is particularly critical since photogenerated holes are required for oxidation of adsorbents, usually through the medi- ation of an adsorbed OH− group that reacts with a hole to form the powerful oxidant OH• radical [22]. It is widely accepted that the anatase TiO2 is a more effi- cient photocatalyst than the rutile TiO2 because of the former’s relatively high adsorptive affinity for organics and the superior hole-trapping ability [22–27]. However, it has also been shown that a commercialized mixture of the two phases (Degussa P25) exhibited much superior photocatalytic activity to pure-anatase and rutile TiO2 [28–35]. This enhanced activity in mixed-phase catalysts results from a synergetic effect involving prolonged separation of photogenerated electrons and holes through inter- facial electron transfer from the conduction band of rutile phase to the trapping states of anatase phase [33]. Despite the vast body of knowledge about the effects of E-mail address: wyu@rice.edu (W.W. Yu). TiO2’s physical properties on its photocatalytic behavior, a num- 1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2005.07.034 R.K. Wahi et al. / Journal of Molecular Catalysis A: Chemical 242 (2005) 48–56 49 Scheme 1. Molecular structure of Congo Red. ber of ambiguities questions remain. It is unclear that if the particle size (and therefore, the specific surface area) plays any effect when anatase is usually a better photocatalyst than rutile [33]. A second concern is whether the anatase–rutile synergy for Degussa P25 TiO2 (particle size ∼ 20 nm) would also be observed in other mixed-phase TiO2 systems, especially those with ultrafine particle sizes (<10 nm). It is also not clear that if the relative amounts of anatase and rutile in TiO2 catalysts would affect this synergetic effect. And, the influence of nanocrystal shape and dominant crystallographic surfaces on the photocat- alytic activity of TiO2 has not been thoroughly investigated. For example, it has been found that ZnO-mediated photocatalysis is more efficient on the (1 0 −1 0) surface than the (0 0 0 2) surface [36,37]. If TiO2 were found to exhibit face-dependent activity, it would provide a valuable new way to optimize the photocatalytic performance of TiO2 through the tuning of surface orientation. And thus, one could design a TiO2 photocatalyst with dominant crystal faces that would be best for photocatalysis. Such a strat- egy would be especially useful to prepare oriented TiO2 thin films photocatalyst. A series of photocatalysis experiments with different particle size, specific surface area, and phase composition of nanosized TiO2 were studied in this work in order to assess the effects of each factor on the photocatalytic activity of both single- and mixed-phase photocatalysts. Several techniques were modified to synthesize TiO nanocrystals that allowed different physical p v m s a o b n 2 u o f t d f rather than harmful nitrates. Thus, photocatalysis is a promising way to eliminate Congo Red and similar organic pollutants from the environment. 2.1. Chemicals An anatase–rutile mixture TiO2 powder (P25) was kindly supplied by Degussa Corporation. Ultrapure water (18.2 M
cm) was obtained from a Millipore purification sys- tem. Tetramethylammonium hydroxide (TMAH), titanium(IV) isopropoxide (99.999%), titanium(IV) chloride (99.995%), Congo Red (85%), and 2-propanol (99.5%) were purchased from Aldrich; titanium(IV) ethoxide (97%) was from Fluka; dry ethanol was from Pharmco; hydrochloric acid, sodium hydrox- ide, and sodium chloride were from Fisher. All chemicals were used as-received. 2.2. Preparation of nanosized TiO2 photocatalyst powders The reaction conditions used to prepare nine different nano- sized TiO2 powders used in this study were summarized in Table 1. Symbol A for TiO2 powder (catalyst) means anatase phase, R means rutile phase, AR means the mixture of anatase and rutile phases, and Rod means rod-like shapes (though they are anatase).2 roperties to be tuned independently. This is in contrast to pre- ious studies in which calcination was almost the only means to ake TiO2 nanocrystals, with the result that changes in particle ize were accompanied by changes in such as phase composition nd vice versa [38]. In addition, the effects of particle morphol- gy on photocatalytic performance were studied for the first time y comparing the activities of isotropic and anisotropic anatase anocrystals with different proportions of (1 0 1) surface sites. . Experimental Photodecomposition of aqueous Congo Red (Scheme 1) was sed as a model reaction to characterize photocatalytic behavior f different nanosized TiO2. Congo Red is a recalcitrant azo dye ound in textile wastewater. It has been the subject of several pho- ocatalysis studies including TiO2 [17,39–42]. Photocatalytic egradation of Congo Red proceeds in a way that nitrogen atoms rom the azo functionality are liberated as molecular nitrogen2.2.1. Anatase spherical dots of nanosized TiO2 powders Anatase spherical dots of nanosized TiO2 with different par- ticle sizes (A1, A2, and A3) were prepared by hydrothermal method. A known amount of ultrapure water was mixed with ethanol and heated in a 450 mL Monel autoclave (Parr Instru- ments). Once the mixture in the autoclave reached the desired temperature (see Table 1), a dry ethanol solution of titanium(IV) ethoxide was transferred into the autoclave. The total reaction volume was 100 mL and the molar ratio of water to titanium(IV) ethoxide was 20:1. The reaction mixture was stirred at a constant temperature for 2 h and then quenched by putting the autoclave into a coldwater bath. The product was filtered, washed twice with pure water, and dried overnight at 60 ◦C. 2.2.2. Ultrafine rutile and mixed-phase nanosized TiO2 powders One TiO2 powder of pure rutile (R1) and two anatase–rutile mixtures (AR1 and AR2) were prepared using a modified method reported by Cheng et al. [43] Typically, titanium(IV) chloride (47.0 g, 0.25 mol) was added slowly to ultrapure water (450 mL) to produce a white suspension. Hydrochloric acid (37.5%, 50 mL) was added to the suspension as a peptizing agent, and stirred overnight to get a clear solution. The peptized TiO2 suspension was adjusted to the desired pH (pH 1 for R1, pH 0.25 for AR1 and AR2) by addition of aqueous sodium hydroxide; then it was heated and refluxed for 2 h. The TiO2 particles were precipitated out of the suspension by adding solid sodium chlo- ride. The TiO2 powder was collected by centrifugation, washed twice with ultrapure water, and dialyzed in several successive ultrapure water baths for a total time of 24 h. The final powder was dried overnight at 60 ◦C. 52 R.K. Wahi et al. / Journal of Molecular Catalysis A: Chemical 242 (2005) 48–56 Fig. 3. DTA profile of A2 nanosized TiO2 photocatalyst. The endothermic peak near 100 ◦C corresponds to the evaporation of adsorbed water, while the absence of a sharp exothermic peak between 350 and 450 ◦C indicates the 100% crys- tallinity (i.e., no amorphous phase) of the nanosized TiO2 powder. BET surface areas ranged from 28 to 251 m2g−1 in the 10 nanosized TiO2 powders studied here. Generally, powder with smaller particle size had higher surface area. But R1 and AR2 had only about 50% surface area (110–123 m2g−1) of A1 (251 m2g−1) with similar particle size (∼ 5 nm), largely due to the extensive aggregation of those particles (Fig. 4). The absence of any characteristic TMAH peaks in the FTIR spectrum of anisotropic Rod1 indicated that the TMAH surfac- tant was completely removed from the nanocrystal surfaces by washing and dialyzing (Fig. 5). The same spectrum as Rod1 was also observed for Rod2 (not shown). 3.2. Photocatalytic decomposition of Congo Red over nanosized TiO2 photocatalysts The photocatalytic activity of each TiO2 catalyst was quanti- fied using the half-life time (t1/2) for a 10 mg L−1 (14 mol L−1) Congo Red solution and the Langmuir–Hinshelwood rate con- stant (k). The values of t1/2 and k were expressed on both a unit mass basis (t1/2,m, km) and a unit surface area basis (t1/2,s, ks). Activities evaluated on a unit mass basis reflected largely Fig. 5. FTIR spectra of TMAH and Rod2. The absence of characteristic TMAH resonances in Rod2 confirms that TMAH was completely removed from nanorod surfaces. the influence of surface area on a catalyst’s performance, while activities expressed on a unit surface area basis (i.e., specific activities) reflected the effects of physical properties other than surface area. The Congo Red decomposition curves indicate that the A2, A3 and P25 catalysts had similar activity (t1/2,m = 1.5–3.5 min g) on a mass basis, while the mixed-phase powders AR1 and AR2 had a little longer half-life time (t1/2,m ∼= 5 min g) (Fig. 6 and Table 2). In contrast, the photocatalytic reaction was consider- ably slower (t1/2,m = 15–30 min g) over Rod1 and Rod2 anatase nanorods. And, very slow photodegradations of Congo Red were observed over rutile R1 (t1/2,m = 120 min g). The mass- based activities (t1/2,m) of the TiO2 catalysts increased in the order R1 < A1 < Rod2 < Rod1 < AR1 ∼= AR2 < P25 < A3 ( A2. The surface area-based activities (t1/2,s) of TiO2 catalysts were also calculated and had basically the same trend as the mass-based activities (Table 2). The initial rate (r0) of Langmuir–Hinshelwood analysis pro- vides an additional quantitative comparison of the photocat- alytic activity of TiO2. An important issue to address first, however, is the initial Congo Red concentration (C0) limits anosiFig. 4. TEM pictures of n zed TiO2 photocatalysts. R.K. Wahi et al. / Journal of Molecular Catalysis A: Chemical 242 (2005) 48–56 53 Fig. 6. Photodegradation of Congo Red catalysed by nanosized TiO2. to the applicability of the Langmuir–Hinshelwood model in different systems. This can be assessed by examining the lin- earity of a plot of r−10 versus C −1 0 for each catalyst. Fig. 7 shows curves of r0 − C0 (Fig. 7a) and r−10 − C−10 (Fig. 7b) for two TiO2 catalysts (AR1 and R1) over a wide range of initial Congo Red concentrations tested. The concentration ranges over which the reciprocal plots (Fig. 7c) were linear in accordance with Langmuir–Hinshelwood model were specified in Table 2. Langmuir–Hinshelwood model were good over wide ranges for highly active photocatalysts, e.g., A2, A3, and P25, presumably, because the generation rates of OH• in those TiO2 catalysts were fast enough to meet the unimolecular Langmuir–Hinshelwood model’s assumption of a large, essentially constant concentra- tion of OH• relative to the Congo Red concentration. On the other hand, low-activity catalysts such as R1 generated OH• very slowly; the unimolecular Langmuir–Hinshelwood model’s requirement was only met when C0 was low. The rate constant k and the adsorption equilibrium constant K were obtained through linear fits (Fig. 7c). For most TiO2 catalysts, the order of photocatalytic activity quantified by rate constant k was generally in agreement with the order of activity quantified by t1/2 (Table 2). In cases where the k value con- tradicted the t1/2 result, t1/2 was assumed to be more reliable measurement of photocatalytic activity and used in the follow- Fig. 7. Langmuir–Hinshelwood analysis of Congo Red photodegradation: (a) r0 vs. C0 on a unit surface area basis; (b) reciprocal plots on a unit surface area basis; (c) fitting the linear portions of the reciprocal plots to determine the parameters k and K. Table 2 K catalysts S −1) kmd (mol g−1 min−1) kse (mol m−2 min−1)/10−2 K (mol g−1) P 4.08 8.15 0.235 A – – – A 5.11 3.34 0.206 A – – – R 1.60 1.46 0.027 A 3.57 1.54 0.199 A – 2.5 – R 1.80 1.90 0.068 R 0.47 1.67 0.230 shelwood analysis applicability.inetic parameters in Congo Red photodegradation over nanosized TiO2 photo ample t1/2,ma (min g) t1/2,sb (min m2)/102 Maximum C0c (mol L 25 3.5 1.75 30 1 40 100 – 2 1.5 2.30 ≥40 3 1.75 1.46 – 1 120 – ∼5 R1 5 11.6 ≥40 R2 5 6.15 – od1 15 14.2 15 od2 30 8.40 20 a Half-life time expressed on a unit mass basis. b Half-life time expressed on a surface area basis. c Maximum initial Congo Red concentration for unimolecular Langmuir–Hin d Rate constant expressed on a unit mass basis. e Rate constant expressed on a surface area basis. 54 R.K. Wahi et al. / Journal of Molecular Catalysis A: Chemical 242 (2005) 48–56 ing discussion, because the determination of t1/2 required fewer calculations and approximations than the determination of k. 3.2.1. Effects of particle size and surface area The kinetic data for the spherical dot-shaped anatase cata- lysts (A1, A2, and A3) indicated that particle size and surface area were the two most important factors influencing the photo- catalytic activity of nanocrystalline TiO2 catalyst (Table 2). The t1/2,m for the 15.1 nm (A3) and 10.1 nm (A2) nanodots were both about 1.5 min g, while t1/2,m for the 5.5 nm nanodots (A1) was 40 min g. Thus, 10–15 nm nanodots with low to intermediate sur- face areas (80–150 m2 g−1) were about 25 times more active than 5.5 nm nanodots with very high surface area (251 m2 g−1). This shows that despite the larger number of surface sites available for Congo Red adsorption onto smaller nanodots, the increased recombination rate of e−/h+ in very small particles resulted in an overall decrease in photocatalytic activity. The 10.1 nm nan- odots of A2 (t1/2,m = 1.5 min g) were slightly more active than the 15.1 nm nanodots of A3 (t1/2,m = 1.75 min g), indicating that A2’s larger surface area contributed more in photocatalytic activ- ity than electron–hole separation when the particle was big enough (>10 nm). It appears to be an optimal particle size of about 10 nm for a maximum photocatalytic activity of anatase nanodots. 3.2.2. Effect of phase composition 3 s t b f r b n t p i f ( t c a t t a a L l 3 c e t t a ( ence of two phases in P25 resulted in a remarkable increase in photocatalytic activity over pure-anatase TiO2. However, this synergetic effect was not apparent in the ultra- fine anatase–rutile mixture AR1 and AR2 (∼5 nm) despite their high surface areas (123–232 m2 g−1). They were even less active than pure-anatase A2 and A3. The t1/2 and Langmuir–Hinshelwood rate constant k supported this obser- vation. There are several possible reasons for the absence of syner- getic effect in our anatase–rutile mixtures. First, the photocat- alytic activity of such mixtures depends in part on the relative amounts of these two phases. Bacsa and Kiwi reported that a highest activity for p-coumaric acid degradation was observed for an anatase/rutile ratio of 70/30 [29]. It is possible that AR1 exhibited low photocatalytic activity because of not-optimized anatase/rutile ratio (40/60). However, AR2 performed almost the same activity as AR1 even its anatase/rutile ratio (70/30) was close to that of P25 (80/20) and equal to the optimal com- position reported by Bacsa and Kiwi. A more important factor is likely the anatase–rutile interface in these two catalysts. The enhancement of activity in Degussa P25 has been attributed to efficient electron transfer from rutile to anatase, which increases the charge separation needed for efficient photocatalytic reaction at the particle surfaces [33]. Well-contacted interfaces between the anatase and rutile phases would thus be essential for an appreciable increase in photo- c a w A a 3 c w a w s t ( o R d a w n t p c d v a d c H .2.2.1. Single-phase catalysts: anatase versus rutile. The ingle-phase anatase catalysts exhibited widely varied activities hat appeared to depend not only on particle size and surface area ut also on the surface (crystal) structure. It can be concluded rom t1/2, data that anatase TiO2 is a better photocatalyst than utile TiO2. The t1/2, values for the anatase TiO2 were found to e about 2–100 times smaller than that of the 5.4 nm R1 rutile anodots, indicating that the surface of anatase is better to pho- ocatalysis than the surface of rutile. Thus, the generally superior hotocatalytic activity of anatase is not merely a result of the typ- cally easy-achieved larger surface areas, but rather a result of the undamental differences between the anatase and rutile surfaces crystal structures). Specifically, the poor activity of R1 suggests hat even a moderately high surface area (110 m−2 g−1) cannot ompensate for the inefficient hole trapping and low adsorptive ffinity that typically related with rutile’s inherent crystal struc- ure. The adsorption isotherms in Fig. 1 showed the low adsorp- ive affinity of rutile TiO2 for Congo Red, thus, lowered the ctivity of R1 in photocatalytic degradation of Congo Red. In ddition, the low apparent adsorption constant K obtained from angmuir–Hinshelwood analysis for R1 (K = 0.027) confirms its ow adsorptive affinity. .2.2.2. Synergetic effect of anatase–rutile mixed-phase TiO2 atalysts. Our experiments confirmed the strong synergetic ffect in enhancing the photocatalytic activity of P25. In the erms of t1/2 and k, it was found that P25, an anatase–rutile mix- ure with an intermediate surface area (50 m2 g−1), was more ctive than A2, a pure-anatase powder with higher surface area 153 m2 g−1), by a factor of 2.5. Thus, it is clear that the pres-atalytic activity for any mixed-phase catalysts [32,48]. But the natase–rutile interfacial contacts in AR1 and AR2 may not be ell controlled as P25. And the smaller size (5 nm) in AR1 and R2 may not be good for the electron-hole separation for a high ctivity (see Section 3 for A1). .2.3. Effect of morphology on the activity of anatase TiO2 atalysts TiO2 nanorods (Rod1 and Rod2) performed significantly orse than the anatase nanodots (A2 and A3) (Table 2). Specific ctivities, as quantified by t1/2, and k, indicated that nanorods ere 5–10 times less active than the 10–15 nm nanodots. FTIR pectroscopy confirmed the complete removal of surfactant from he nanorods samples prior to the photocatalytic experiments Fig. 5), so surfactant poisoning or retarding could be ruled ut as a reason for the poor performance of the Rod1 and od2 catalysts. The surface areas accounted partially for the ifferences in their per mass activities, but cannot account for ll the activity difference. A (1 0 1) surface dominated TiO2 as obtained by selective binding of the tetramethylammo- ium cation (N(CH3)4+) to the (1 0 1) surface, which inhibited he growth in the [1 0 1] direction and promoted the growth in erpendicular directions [44]. The higher the surfactant con- entration, the more pronounced this anisotropic effect. So, the ominated (1 0 1) surface sites in Rod1 and Rod2 seemed unfa- orable to the photocatalytic activity of TiO2. The deleterious effect of (1 0 1) surfaces on photocatalytic ctivity could be derived from the water adsorption modes to ifferent anatase surfaces. Theoretical calculations have indi- ated that water molecules tend to adsorb dissociatively (i.e., as + and OH−) to anatase (0 0 1) but non-dissociatively (i.e., as