Effect of supports on Pd–Cu bimetallic catalysts for nitrate and nitrite reduction in water

Catalysis Today 185 (2012) 81–87

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Effect of supports on Pd–Cu bimetallic catalysts for nitrate and nitrite reduction in water Kenji Wada a , Tomoaki Hirata a , Saburo Hosokawa a , Shinji Iwamoto b , Masashi Inoue a,∗ a b

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan Department of Chemistry and Chemical Biology, Gunma University, Tenjin, Kiryu 376-8515, Japan

a r t i c l e

i n f o

Article history: Received 31 May 2011 Received in revised form 15 July 2011 Accepted 24 July 2011 Available online 20 August 2011 Keywords: Nitrate reduction Nitrite reduction Titania Pd–Cu catalyst Glycothermal method

a b s t r a c t The activities of the Pd–Cu catalysts supported on various metal oxides for the reduction of nitrate and nitrite ions were compared at a constant pH. Among them, the catalysts supported on titania xerogel prepared by a glycothermal method showed the highest performance for nitrate reduction. For the reduction of nitrite ions, Pd–Cu/CeO2 showed the highest catalytic activity; however, significant dissolution of the components of the catalyst was observed during the reaction at a relatively low pH range. On the other hand, a single-component Pd catalyst supported on titania xerogel showed excellent activity for the reduction of nitrite ion and could be used at a wide range of pH. For these catalysts, the product selectivity for N2 from nitrite ions was markedly improved by decreasing the partial pressure of H2 in the feed.

1. Introduction Recently, the nitrate pollution of groundwater has attracted much attention, and the establishment of effective methods for denitrification of water is highly demanded, because nitrate in drinking water causes methemoglobinemia in infants. The WHO recommends the maximum nitrate concentration of 50 mg/L in drinking water [1], while expansion of the pollution has been predicted with growing food production [2]. This situation has led the development of the nitrate removal technologies to the key issue. Among various nitrate removal methods, ion exchange and reverse osmosis denitrification processes have been already put in practical use [3,4]. However, condensed aqueous nitrate solutions are formed as effluents from these processes. Therefore, the development of the catalytic nitrate reduction process is highly desired and has been widely studied since the discovery of noble metal-containing bimetallic catalysts by Hörold et al. [5]. In particular, Pd–Cu catalysts are most promising, since they show high activities and high N2 selectivities [5,6]. Besides carbonaceous supports [6–10], various metal oxides such as Al2 O3 [5,11–21], SiO2 [5,7], Nb2 O5 [21], ZrO2 [22,23], TiO2 [24–28], zeolites [29,30], and hydrotalcites [31–33] have been applied as a support of the Pd–Cu catalysts. In addition, there are several reports on the success-

∗ Corresponding author. Tel.: +81 75 383 2478; fax: +81 75 383 2479. E-mail address: [email protected] (M. Inoue). 0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.07.021

© 2011 Elsevier B.V. All rights reserved.

ful reduction of nitrate promoted by single noble metal catalysts supported on TiO2 [34,35], SnO2 [36,37], and CeO2 [11,38,39]. However, there are a limited number of reports which systematically investigate the effects of supports of the Pd–Cu catalysts under the identical conditions, i.e. at constant pH [7,11], since the reaction conditions were optimized by each of the supported catalysts. On these backgrounds, in the present study the activities of Pd–Cu catalysts supported on various metal oxides were compared for the reduction of nitrate and nitrite ions at constant pH. Remarkably, Pd–Cu catalysts supported on titania xerogel prepared by the glycothermal method [40] were found to show the highest activity for the reduction of nitrate. Effects of the supports on the properties of surface copper species as well as dissolution behavior of the components of the catalysts were investigated in detail.

2. Experimental 2.1. Preparation of supported catalysts Supported catalysts were prepared by an impregnation method. To a HCl solution (0.5 N) of palladium(II) chloride (8.3 mg, 0.047 mmol) and copper(II) nitrate trihydrate (19 mg, 0.079 mmol), 0.49 g of support was added. After drying at 80 ◦ C, the resulting powder was calcined in air at 400 ◦ C for 1 h to afford the Pd–Cu/support catalysts. The loading levels of Pd and Cu were 1 wt% as metals unless otherwise noted. All the catalysts were reduced in

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Table 1 Hydrogenation of nitrate ions over Pd–Cu catalysts supported on various oxides.a Catalyst

Pd–Cu/TiO2 (JRC-TIO-4) Pd–Cu/ZrO2 Pd–Cu/CeO2 Pd–Cu/Al2 O3 Pd–Cu/SiO2

BET S.A. (m2 /g)

Rate constant

Support

Catalyst

k1 + k4 + k5 (10−2 /min)

(k2 + k3 )/k1

49 97 63 110 254

47 77 62 115 190

2.7 0.65 0.56 0.26 0.32

0.60 0.13 >5 0.33 0.05

a Reaction conditions: 40 ◦ C; 1.6 mmol/L (100 ppm) 100 mL/min; catalyst, 20 mg/200 mL; NaCl, 32.5 mmol/L.

NO3 − ;

pH = 6;

H2 ,

a flow of 20 vol% H2 in Ar (100 mL/min) at 400 ◦ C (heating rate, 10 ◦ C/min) for 30 min just before the catalytic runs. 2.2. Catalytic tests Catalytic runs were carried out at 40 ◦ C under atmospheric pressure in a five-necked Pyrex flask equipped with a pH-stat (SUGAI CHEM, U-702, U-101; HORIBA, F-52), a sampling port, a H2 gas inlet, and a reaction gas outlet. In typical runs, 200 mL of deionized water containing sodium nitrate (27 mg, 1.6 mmol/L, 100 ppm as NO3 − ), NaCl (32.5 mmol/L), and 20 mg of reduced catalyst was added to the reactor. In the case of the reduction of nitrite, sodium nitrite (15 mg, 1.1 mmol/L, 50 ppm as NO2 − ) was added in place of sodium nitrate. Then, a H2 flow (100 mL/min) was passed through the reaction mixture with stirring. The pH value of the solution was adjusted by the pH-stat with a titrant of 0.2 N HCl, and NaCl was added in order to minimize the influence of increasing Cl− concentration by the titration. The concentrations of NO3 − , NO2 − , NH2 OH, and NH4 + were measured by ion chromatography (detector: Shodex CD-5, Column: Shodex Y-521, NI-424). Gaseous products (N2 and N2 O) were analyzed by a quadrupole mass spectrometer (Pfeiffer Vacuum: Omnistar GSD 301 O1). Further experimental details are noted in Supplemental information. 3. Results and discussion 3.1. Effects of supports on the activity of Pd–Cu catalysts for nitrate reduction Metal oxide supports of the Pd–Cu catalysts markedly affected the catalytic activity for nitrate reduction at pH = 6, as shown in Fig. 1. BET surface areas of the catalysts and supports are noted in Table 1. The loading level of palladium was 1 wt%. The copper loading was also fixed at 1 wt% based on the preliminary study on the optimization of loading levels of copper species (see Supplemental information, Fig. S1). Of the catalysts examined, a titania-supported catalyst showed the highest activity in terms of the NO3 − conversion. There was a poor correlation between BET surface areas of the catalysts and the NO3 − conversions. As can be seen in Fig. 1(ii), the concentration of NO2 − formed on the Pd–Cu/TiO2 (JRC-TIO-4) catalyst markedly increased at the initial stage of the reaction, and then decreased after 50 min of the reaction. The major product after 60 min was NH4 + . These results clearly indicate that nitrite is an intermediate to the final products such as NH4 + , N2 O, and N2 . Therefore, the nitrate reduction can be explained by the reaction pathways shown in Scheme 1(a), where k1 –k5 are the pseudofirst order rate constants. In most cases, the conversion of nitrate obeyed the pseudo-first-order kinetics, and the logarithmic plots gave k1 + k4 + k5 . Table 1 also shows (k2 + k3 )/k1 obtained by the curve fitting on the nitrite concentration. In the presence of the Pd–Cu/CeO2 catalyst, the concentrations of nitrite were very low,

Fig. 1. Concentration of (i) nitrate, (ii) nitrite, and (iii) ammonium ion during the nitrate reduction over Pd–Cu/CeO2 (), Pd–Cu/TiO2 (JRC-TIO-4) (䊉), Pd–Cu/ZrO2 (♦), Pd–Cu/Al2 O3 (JRC-ALO-8) (), and Pd–Cu/SiO2 (Cabosil) (). Dotted lines show the simulated curves. Reaction conditions: 40 ◦ C; 1.6 mmol/L (100 ppm) NO3 − ; pH = 6; H2 , 100 mL/min; catalyst, 20 mg/200 mL; NaCl, 32.5 mmol/L.

indicating that this catalyst was very active for the reduction of nitrite ion to ammonium ion (see below). Large amounts of nitrites were accumulated during the course of the reaction over the catalysts supported on ZrO2 , SiO2 , and Al2 O3 , suggesting that these catalysts have relatively poor activities for nitrite reduction. The activities of Pd–Cu catalysts supported on various titanias were compared. The rate constants are summarized in Table 2, and the time courses of the reactions are shown in Fig. 2(a). The catalysts supported on glycothermally prepared titania xerogel [40] (designated as TiO2 (XG)-800, 800 indicates the calcination temperature of xerogel in degree Celsius) showed the highest activity in terms of the conversion of nitrate. The Pd–Cu supported on TiO2 (JRC-TIO-4) showed slightly lower activity. The Pd–Cu/TiO2 (JRC-TIO-6) catalyst showed a poor activity, which indicates that rutile titania is not suitable as a support. The catalyst supported on anatase ST-01 showed

K. Wada et al. / Catalysis Today 185 (2012) 81–87

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Fig. 2. Concentration of (i) nitrate, (ii) nitrite, and (iii) ammonium ion during the nitrate reduction over the Pd–Cu catalysts supported on (a) various titanias and (b) titania xerogel calcined at various temperatures. For (a), Pd–Cu/TiO2 (ST-01) (), Pd–Cu/TiO2 (XG)-800 (), Pd–Cu/TiO2 (JRC-TIO-4) (䊉), and TiO2 (JRC-TIO-6) (). For (b), Pd–Cu/TiO2 (XG)-400 (), Pd–Cu/TiO2 (XG)-600 (䊉), Pd–Cu/TiO2 (XG)-800 (), and TiO2 (XG)-1000 (♦). Dotted lines show the simulated curves. Reaction conditions: 40 ◦ C; 1.6 mmol/L (100 ppm) NO3 − ; pH = 6; H2 , 100 mL/min; catalyst, 20 mg/200 mL; NaCl, 32.5 mmol/L.

a moderate activity in spite of large surface area of the support. For this catalyst, a significant decrease of surface area caused by the calcination at 400 ◦ C for 1 h after the impregnation of palladium and copper precursors was observed. The incorporation of a part of Table 2 Hydrogenation of nitrate ions over Pd–Cu catalysts supported on various titanias.a Catalyst

TiO2 (JRC-TIO-4) TiO2 (JRC-TIO-6) TiO2 (ST-01) TiO2 (XG)-1000 TiO2 (XG)-800 TiO2 (XG)-600 TiO2 (XG)-400

Phaseb

A+R R A A+R A A A

BET S.A. (m2 /g)

Rate constant

Support

Catalyst

k1 + k4 + k5 (10−2 /min)

(k2 + k3 )/k1

49 86 234 9 56 71 78

47 67 141 16 53 68 77

2.7 0.10 0.51 2.8 6.7 2.0 1.5

0.61 >5 2.1 1.0 0.20 1.7 2.6

a Reaction conditions: 40 ◦ C; 1.6 mmol/L (100 ppm) 100 mL/min; catalyst, 20 mg/200 mL; NaCl, 32.5 mmol/L. b A, anatase; R, rutile.

NO3 − ;

pH = 6;

H2 ,

metallic species in bulk of titania, which concomitantly occurred accompanying the sintering of the catalyst support, would be one of the reasons for its moderate activity. Effects of calcination temperature of titania xerogel were examined. As shown in Table 2 and Fig. 2(b), the xerogel calcined at 800 ◦ C was found to be most suitable as a support. The lower activity of Pd–Cu/TiO2 (XG)-1000 is probably due to its very low surface area. Furthermore, the XRD analysis showed that TiO2 (XG)1000 contains a small amount of the less effective rutile phase (Supplemental information, Fig. S2), although titania xerogel synthesized by the glycothermal method has a high thermal stability during the phase transition [40]. Note that the reactions with all the TiO2 (XG)-supported catalysts showed the maximum nitrite concentrations at similar reaction time in spite of different activity for nitrate reduction. This suggests that the reduction of nitrate was promoted at the expense of the rate of the nitrite reduction. The effects of pH of the aqueous phase on the nitrate reduction over Pd–Cu/TiO2 (XG)-800 and Pd–Cu/CeO2 , which showed excellent performance for the nitrate reduction as demonstrated

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k4

(a)

NO3-

k1

(b)

N2 k2

NO2-

NO2-

N2

Pt2+ (Pd2+)

H2

k2 NO2- k 3

k3

k5

NH4+

NH4+

N2 or NH4+

Pt (Pd)

Scheme 3. Proposed reaction mechanisms for nitrite reduction [8].

k2

(c)

NO2-

k6

N2

NH2OH k 7 k3

NH4+

Scheme 1. Assumed reaction pathways for (a) nitrate reduction, (b) nitrite reduction over Pd–Cu catalysts, and (c) nitrite reduction over Pd catalysts.

above, were examined (see Supplemental information, Fig. S3 and Tables S1 and S2). Marked improvement of the nitrate conversion was observed by increasing pH from 4 to 6 in the presence of Pd–Cu/TiO2 (XG)-800. Similar trend was reported by Gao et al. in their experiment with titania-based Pd–Cu catalysts with higher Pd loading [25]. However, further increase in pH decreased the rate for the conversion of nitrite to final products, leading to the accumulation of large amounts of nitrite. The conversion rate of nitrate in the presence of Pd–Cu/CeO2 also increased with increasing pH from 4 to 9. 3.2. Effect of the supports on the reduction behavior of surface Cu specie on titania It is generally accepted that the copper or other metals are indispensable as a promoter of the catalyst for the reduction of nitrate [5]. Reaction mechanisms involving a redox cycle of copper species have been proposed [8,12]. As shown in Scheme 2, the first step in the nitrate reduction is considered to be the reaction of Cu0 and NO3 − , leading to the formation of nitrite and Cu2+ . The oxidized Cu2+ is reduced to Cu0 by the action of hydrogen atoms formed on noble-metal species by the dissociative adsorption of molecular hydrogen. On the other hand, the reduction of nitrite is considered not to proceed on copper species but take place on noble-metal species (Scheme 3) [12]. The Pd–Cu bimetallic catalysts used in the present work would also act in similar manners as shown in Schemes 2 and 3 for the reduction of nitrate and nitrite ions, respectively. One can expect that nitrate reduction with these catalysts is quite sensitive to the state of the supported copper species. Therefore, effects of titania support on surface copper species were examined by the H2 -TPR study of single-component copper samples. As shown in Fig. 3, copper species on the supports of the highly active Pd–Cu catalysts tend to be reduced at relatively

NO3-

NO2N2 or NH4+

Cu2+

Pt

Cu

Pt-H

low temperatures: The TPR profiles of copper species supported on TiO2 (XG)-800 and JRC-TIO-4 showed distinct peaks at below 150 ◦ C. On the other hand, copper species on TiO2 (JRC-TIO-6) and TiO2 (ST-01) were reduced at higher temperatures than the CuO standard sample. The presence of the interaction with the supports which hampers the reduction of surface Cu species would be one reason for the poor catalytic activity of TiO2 (JRC-TIO-6)- and TiO2 (ST-01)-supported catalysts. Effects of the calcination temperature of titania xerogel on the H2 -TPR profiles were also examined (see Supplemental information, Figs. S4 and S5). Again, there is a good relationship between the reduction temperature of copper species and the activity of the Pd–Cu catalysts: copper species supported on TiO2 (XG)-800 that is the most suitable support among those examined were reduced at the lowest temperature. 3.3. Dissolution behavior of supported metals and supports during nitrate reduction Dissolution of supported metals and/or support metal oxides, which sometimes reported to occur for the catalytic reduction of nitrate, could be a serious problem in the practical uses [7]. Therefore, the leaching of catalyst components into the aqueous phase was investigated under the following conditions: pH = 6; initial nitrate concentration, 10,000 ppm; catalyst, 1 g/200 mL; reaction time, 180 min. After the reaction, the solid catalyst was removed from the reaction mixture by filtration, and the filtrate was analyzed by ICP-AES (Table 3). Pd–Cu/TiO2 is a suitable catalyst for the nitrate reduction, because only a small amount of copper was dissolved even in the presence of high concentration of nitrate ion. On

H2

Scheme 2. Proposed reaction mechanisms for nitrate reduction [8].

Fig. 3. H2 -TPR profiles of Cu (1 wt%) supported on various titanias. Heating rate, 2 ◦ C/min; sample loading, 0.1 g; 2% of H2 in Ar (30 mL/min).

K. Wada et al. / Catalysis Today 185 (2012) 81–87

Support

TiO2 (XG)-800 TiO2 (JRC-TIO-4) ZrO2 Al2 O3 (JRC-ALO-8) SiO2 (Cabosil) CeO2 CeO2 b CeO2 b (pH = 8)

Dissolved amount (%)a Pd

Cu

Support

0 0 0 0 0 4.5 4.3 0

0.2 0.1 0.2 6.8 3 15 17 0

0 0 0 0 3.9 53 48 0

a Reaction conditions: 40 ◦ C; 0.16 mol/L (10000 ppm) NO3 − ; pH = 6; H2 , 100 mL/min; catalyst, 1 g/200 mL; NaCl, 32.5 mmol/L; reaction time 180 min. b Catalyst 0.4 g/200 mL, 32 mmol/L (2000 ppm) NO3 − .

NO2- Concentration (mmol/L)

(i)

Table 3 Dissolved amount of supported metals and carriers.

the other hand, dissolution of the components of Pd–Cu/CeO2 was significant at pH = 6. Note that the solubility product of Ce(OH)4 at 25 ◦ C is 2 × 10−48 (mol/L)5 [41]. Accordingly, Ce4+ is difficult to be dissolved at pH = 6. In contrast, the solubility product of Ce(OH)3 at 25 ◦ C is 1.6 × 10−20 (mol/L)4 , indicating that Ce3+ in ceria can be dissolved easily at pH = 6. Ce4+ species on the surface of Pd–Cu/CeO2 would be reduced to Ce3+ (see above) and then dissolved to the aqueous phase during the reaction. The dissolution of supported metals and cerium species were not observed at pH = 8. This means that the Pd–Cu/CeO2 catalyst has to be used under basic conditions. Note that Epron et al. reported that dissolution of palladium and cerium species was not detected by ICP-AES analysis during the nitrate reduction over the Pd/CeO2 catalyst without the pH control [38]. In their case, pH of the solution might rise just after the reaction started so that dissolution of ceria would be suppressed. 3.4. Effect of the support on nitrite reduction over supported Pd–Cu and Pd catalysts As demonstrated in Figs. 1 and 2, the nitrate reduction proceeds via a common intermediate, nitrite ion. Therefore, effects of the supports of Pd–Cu (Fig. 4) and Pd (Fig. 5) catalysts on the reduction of nitrite ion were examined at pH = 8. Among the catalysts examined, CeO2 -supported ones showed the highest activity. TiO2 (XG)-800-supported Pd catalysts were found to be the secondbest catalysts. Note that the formation of hazardous NH2 OH was observed during the nitrite reduction over Pd/CeO2 (see Fig. 5(c)). On the other hand, there was no sign of NH2 OH formation over Pd–Cu/CeO2 and Pd/TiO2 (XG)-800. Based on the above observations, the nitrite reduction is considered to be explained by the reaction pathways shown in Scheme 1(b) and (c) for the Pd–Cu and Pd catalysts, respectively. The conversion of nitrite obeyed the pseudo-first order kinetics, and k2 + k3 or k2 + k3 + k6 and k7 /k6 estimated by the simulations are shown in Supplemental information, Tables S3 and S4. Note that for the Pd/CeO2 catalyst decay plot of nitrite deviated from the first-order kinetics toward zero-order kinetics, suggesting that the supply of H2 to the catalyst controls the kinetics because of the quite rapid surface reaction. Effect of pH on the nitrite reduction activities of the Pd–Cu catalysts supported on TiO2 (XG)-800 and CeO2 were examined (see Supplemental information, Fig. S6). Contrary to the case of nitrate reduction, the rates of nitrite reduction for both the catalysts decreased with increasing pH of the solution. However, taking severe leaching of the catalyst components at a low pH range into account, Pd–Cu/CeO2 could be used only at high pH. Therefore, in terms of the catalytic activity as well as the inhibition of the formation of harmful byproducts and leaching of metallic species, Pd/TiO2 (XG)-800 are considered to be the most promising for nitrite reduction.

NH4+ Concentration (mmol/L)

(ii)

85

1.25 1.00 0.75 0.50 0.25 0.00

0

30

60 90 Time (min)

120

0

30

60 90 Time (min)

120

1.25 1.00 0.75 0.50 0.25 0.00

Fig. 4. Concentration of (a) nitrite and (b) ammonium ion during the nitrite reduction over Pd–Cu/CeO2 (), Pd–Cu/TiO2 (XG)-800 (), Pd–Cu/TiO2 (JRC-TIO-4) (䊉), Pd–Cu/ZrO2 (♦), Pd–Cu/Al2 O3 (JRC-ALO-8) (), and Pd–Cu/SiO2 (Cabosil) (). Dotted lines show the simulated curves. Reaction conditions: 40 ◦ C; pH = 8; 1.1 mmol/L (50 ppm) NO2 − ; H2 , 100 mL/min; catalyst, 20 mg/200 mL.

The effect of pH on the nitrate and nitrite reduction can be explained on the basis of Schemes 2 and 3. Since both nitrate and nitrite reduction consume proton, low pH is favored from the thermodynamical point of view. However, nitrate reduction on Pd–Cu/TiO2 did not take place at low pH. Nitrate is reduced by Cu0 , and oxidized Cu2+ is reduced by hydrogen atom formed by the action of Pd. The reduction becomes difficult at low pH because the reaction creates proton. On the other hand, nitrite reduction obeyed thermodynamics because Cu species do not participate to the nitrite reduction. It was reported that N2 selectivity of nitrate reduction was improved by controlling the partial pressure of H2 gas in the feed [5]. In our present study, the feed of 100% H2 gas might excessively reduce nitrite ion to NH4 + . To improve the selectivity for N2 , nitrite reduction was performed in a flow of 2 vol% H2 /Ar balance gas (100 mL/min) in the presence of Pd/CeO2 , Pd–Cu/CeO2 , or Pd/TiO2 (XG)-800 (see Table 4 and Supplemental information, Fig. S7). The nitrite concentration decreased linearly versus time, and the reduction rate of nitrite is considered to be limited by H2 feed because the conversions of nitrite with these three catalysts were almost identical with each other. Although the nitrite reduction rate was significantly decreased by dilution of H2 feed, the selectivity to molecular nitrogen increased drastically. In particular, Pd–Cu/CeO2 showed the highest nitrogen selectivity of 70%, while the NH4 + formation was suppressed to 7%. As a result, it was confirmed that the nitrogen selectivity was able to be improved by controlling the H2 feed [5]. As discussed above, there was good correlation between the reduction temperature of surface Cu species and the nitrate reduction activities: copper species on better supports, namely titania,

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Table 4 Hydrogenation of nitrite ions over Pd–Cu and Pd catalysts supported on various carriers. Catalyst

Pd/CeO2 Pd–Cu/CeO2 Pd/TiO2 (XG)-800 a b

Partial pressure of H2 (atm)

1 0.02 1 0.02 1 0.02

Rate constant (10−2 /min)

Conv.a (%)

k2 + k3 + k6

NO3

13 0.27 5.9b 0.30b 3.0 0.31

100 28 100 29 99 33

−

Selectivitya (%) NH2 OH

NH4 +

N2

N2 O

0 7 0 0 0 0

73 14 91 7 97 16

19 33 6 70 2 64

8 46 3 23 1 20

Reaction conditions: 40 ◦ C; 1.1 mmol/L (50 ppm) NO2 − ; pH = 8; gas flow rate, 100 mL/min; catalyst, 20 mg/200 mL; reaction time 120 min. k2 + k3 .

were reduced at lower temperatures. This trend is quite reasonable since nitrate reduction is considered to proceed via the redox cycle of copper species. It was reported that copper oxides supported on titania were reduced into metallic Cu even at room temperature [25]. The activity of the titania-supported catalyst was markedly affected by the crystal structure and calcination temperature of titania: copper species on anatase were reduced at lower tempera-

tures than those on rutile titania. Similar effects of crystal structure of titania on the reduction behavior of copper oxides have been reported [42]. Remarkably, the H2 -TPR profile of Cu species supported on TiO2 (XG)-800 showed a distinct reduction peak at the lowest temperature without peaks at higher temperatures due to the bulk CuO [43] as noted in Supplemental information, Figs. S4 and S5, indicating the high dispersion of copper species on the support. Furthermore, it was often reported that the promotional effects of titania on the reduction of PdO to Pd(0), which would facilitate the nitrite reduction [44]. Titania xerogel calcined at 800 ◦ C (TiO2 (XG)-800) is composed of well-crystallized nanoperticles of anatase and does not contain the rutile phase. At present, we deduce that these features of TiO2 (XG)-800 promote the reduction of surface copper and/or palladium species, eventually increasing the catalytic activity toward both nitrate and nitrite reduction.

4. Conclusions Comparable study on the effects of metal oxide supports, namely TiO2 , CeO2 , ZrO2 , SiO2 , and Al2 O3 , upon the activities of the Pd–Cu or Pd catalysts for the reduction of nitrate or nitrite ions was performed at the constant pH ranging from 4 to 10. Among the supports examined, TiO2 exhibited the most suitable performance. The activity of the titania-supported catalyst is markedly affected by the preparation method, crystal structure, and calcination temperature of titania. In particular, a titania xerogel synthesized by the glycothermal method followed by the calcination at 800 ◦ C (TiO2 (XG)-800) is the most effective as a support of the Pd–Cu catalyst for nitrate reduction. Titania xerogels markedly promote the reduction of surface copper species, which is the key factor for the excellent catalytic activity. No significant leaching of the catalyst components was observed for titania-supported catalysts at a wide range of pH, whereas ceria-supported catalysts can be used only at high pH because of severe dissolution of the components of the catalysts. For nitrite reduction, ceria-supported Pd or Pd–Cu catalysts showed the highest activity, while the Pd catalyst supported on TiO2 (XG)-800 also showed a good activity. For these catalysts the selectivity for N2 is markedly improved by lowering the partial pressure of H2 . The present work clearly demonstrates the effectiveness of titania xerogels prepared by the glycothermal method as a support for both the reduction of nitrate and nitrite ions. Further optimization of the preparation methods of the catalysts and/or the supports for nitrate/nitrite reduction is now in progress.

Acknowledgement Fig. 5. Concentration of (a) nitrite, (b) ammonium ion, and (c) hydroxylamine during the nitrite reduction over Pd/CeO2 (), Pd/TiO2 (XG)-800 (), Pd/TiO2 (JRC-TIO-4) (䊉), Pd/ZrO2 (♦), Pd/Al2 O3 (JRC-ALO-8) (), and Pd/SiO2 (Cabosil) (). Dotted lines show the simulated curves. Reaction conditions: 40 ◦ C; pH = 8; 1.1 mmol/L (50 ppm) NO2 − ; H2 , 100 mL/min; catalyst, 20 mg/200 mL.

This work was supported in part by a grant-in aid for Scientific Research (No. 19651036) from the Ministry of Education, Sports, Culture, Science, and Technology, Japan.

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