ZrO2

Jun 3, 2009 - The results showed that ZrO2 support calcined at 573 K was unstable during the preparation of the bimetallic catalyst. Increasing calcin...
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Ind. Eng. Chem. Res. 2009, 48, 8356–8363

Catalytic Hydrogenation of Aqueous Nitrate over Pd-Cu/ZrO2 Catalysts Zhaoyi Xu,† Liqiang Chen,† Yun Shao,† Daqiang Yin,‡ and Shourong Zheng*,† State Key Laboratory of Pollution Control and Resource Reuse, School of the EnVironment, Nanjing UniVersity, Nanjing 210093, People’s Republic of China, and Key Laboratory of Yangtze RiVer Water EnVironment, Ministry of Education, College of EnVironmental Science and Engineering, Tongji UniVersity, Shanghai 200092, People’s Republic of China

ZrO2 supports with different properties were prepared, and the hydrogenation of aqueous nitrate catalyzed by ZrO2 supported Pd-Cu bimetallic catalysts was investigated. The results showed that ZrO2 support calcined at 573 K was unstable during the preparation of the bimetallic catalyst. Increasing calcination temperature led to the increase of particle size of ZrO2 support. In addition, using ZrO2 calcined at 973 K as the support resulted in the increase of metal particle sizes and the increased content of bimetallic ensembles at the expense of monometallic Pd. The bimetallic catalyst with ZrO2 calcined at 773 K as the support exhibited higher catalytic activity and N2 selectivity for the reduction of aqueous nitrate as compared to the bimetallic catalyst with ZrO2 calcined at 573 or 973 K as the support. The activity and selectivity of ZrO2 supported bimetallic catalysts for nitrate reduction also depended on the Pd/Cu ratio. Decreasing Pd/Cu ratio led to the decrease of the amount of monometallic Pd sites and to the increase of the content of Pd-Cu ensembles. The bimetallic catalyst with Pd/Cu ratio of 4/1 showed the optimum activity and N2 selectivity for the reduction of aqueous nitrate. 1. Introduction Groundwater serves as an important source of drinking water in the world. However, the increasing nitrate pollution in groundwater has emerged as a particular concern during the past decade. As nitrate in drinking water may induce serious health risks to human health, the United States Environmental Protection Agency (USEPA), the World Health Organization (WHO), and the European Community have regulated the maximum nitrate level in drinking water lower than 10 mg NO3-N/L, 25 and 50 mg/L, respectively.1,2 To comply with the regulations, a variety of treatment methods have been developed to remove nitrate from drinking water. Catalytic denitrification has provided an economical and promising approach for the removal of aqueous nitrate, which has been considered to be superior to typical biological treatment as well as ion exchange and reverse osmosis processes.3-9 In the catalytic denitrification process, supported bimetallic catalysts, containing noble metal and second metal, were generally believed to be effective catalysts for catalytic hydrogenation of aqueous nitrate, although nitrate reduction catalyzed by supported monometallic Pd or Pt catalysts was also reported.10,11 The activity and selectivity of the bimetallic catalyst for aqueous nitrate reduction were found to be dependent on catalyst component,12,13 preparation methods,13-15 reaction conditions,16 and the interaction between metal and support.17,18 Pd, Rh, Ir, Ru, Ag, or Pt was usually adopted as the noble metal and Cu, Ni, Sn, or In as the second metal in the bimetallic catalyst.12,13,19,20 Among the bimetallic catalysts, Pd-Cu/Al2O3 was the most intensively investigated catalyst for nitrate reduction in water. Recent results revealed that Pd-Sn/Al2O3 and Pd-In/Al2O3 exhibited higher catalytic activity and N2 selectivity for nitrate reduction as compared to Pd-Cu/Al2O3.12,13 For a typical bimetallic catalyst, the presence of bimetallic ensemble, consisting of noble metal and second metal, and pure * To whom correspondence should be addressed. Tel.: +86-2583595831. Fax: +86-25-83707304. E-mail: [email protected]. † Nanjing University. ‡ Tongji University.

noble metal domain is believed to be essential for selective nitrate reduction. The bimetallic ensemble serves as the active site for nitrate reduction to nitrite and the pure noble metal domain as the active site for subsequent nitrite reduction to nitrogen and ammonia. Therefore, precise control of metal dispersion over the support to obtain an appropriate content of bimetallic ensemble and noble metal domain in the supported bimetallic catalyst is of particular importance for the selective reduction of aqueous nitrate. In general, the content and component of bimetallic ensemble and noble metal domain in the catalyst can be tuned by varying the loading of noble metal and second metal, adjusting the ratio of noble metal to second metal, and using different preparation methods.7,8 The catalytic behaviors of bimetallic catalysts can also be adjusted by using different supports. Membrane,20,21 fiber,22 and cloth23 were used as the supports in bimetallic catalysts to circumvent the diffusion problem. Deganello et al. selected pumice as the support to change the electronic properties of Pd sites to enhance nitrate reduction.24 Hydrotalcite supported Pd-Cu catalyst exhibited high catalytic activity and selectivity due to the decreased mass transfer limitation by adsorption of nitrate in the interlayer of hydrotalcite.25,26 Roveda et al. used acrylic resins as supports in Pd-Sn bimetallic catalysts to provide the buffering environment.27 The Pd-Cu bimetallic catalyst with ZrO2 as the support was found to be more active as compared to SnO2 supported Pd-Cu catalyst.11 In addition, the catalytic behaviors of ZrO2 supported Pd-Sn and Pd-In bimetallic catalysts were found to be different from the catalysts with Al2O3 as the support. It is noteworthy that for a particular support the metal dispersion as well as the catalytic behaviors of supported metallic catalysts were found to be closely linked to the properties of the support.28,29 However, the dependence of the catalytic behaviors of supported bimetallic catalysts for catalytic denitrification on support properties has been seldom addressed. In this Article, ZrO2 supports with different properties were prepared, and the hydrogenation of aqueous nitrate catalyzed by different Pd-Cu/ZrO2 catalysts was investigated to evaluate

10.1021/ie9005854 CCC: $40.75  2009 American Chemical Society Published on Web 06/03/2009

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the influence of support properties on the catalytic behaviors of Pd-Cu/ZrO2 catalysts for nitrate reduction. 2. Experimental Section 2.1. Catalyst Preparation. The ZrO2 precursor was prepared by the precipitation method. Typically, 400 mL of 0.5 M ZrOCl2 · 8H2O aqueous solution was added dropwise to 150 mL of 2 M ammonia solution under vigorous stirring followed by aging at room temperature for 2 h. The ZrO2 precursor was obtained by filtration, repeated washing with distilled water, and drying at 378 K for 6 h. Different ZrO2 supports were prepared by calcining the ZrO2 precursor at 573, 773, or 973 K for 4 h. ZrO2 supports calcined at 573, 773, and 973 K are referred to as Zr573, Zr773, and Zr973, respectively. Monometallic or bimetallic catalysts were prepared using the conventional wet impregnation method. The ZrO2 support was impregnated by H2PdCl4 or/and CuCl2 solution followed by drying at 373 K for 2 h, calcining at 573 K for 2 h under air, and subsequently reducing at 573 K under H2 atmosphere for 4 h. The resulting monometallic or bimetallic catalyst is denoted as Pd(x)-Cu(y)/Zrz, where x, y, and z are Pd content (wt %), Cu content (wt %), and the calcination temperature of ZrO2 support, respectively. 2.2. Catalyst Characterization. BET specific surface areas of the catalysts were measured using the nitrogen adsorption method on a Micromeritics ASAP 2200 instrument. Prior to the measurement, the sample was pretreated at 573 K under vacuum (1.33 Pa) for 1 h. Transmission electron microscopy (TEM) images of the samples were collected on a Hitachi H-800 transmission electron microscope. X-ray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku D/max-RA powder diffraction-meter. The average particle sizes of the samples were calculated from XRD patterns using the Scherrer equation.30 The contents of tetragonal phase in ZrO2 supports and ZrO2 supported bimetallic catalysts were calculated according to eq 1:31 Xt (%) )

I(111)t × 100 I(111)t + I(111)m + I(1¯11)m

(1)

where Xt, I(111)t, I(111)m, and I(1j11)m are the percentage of tetragonal ZrO2 in the sample, the intensity of (111) reflection of tetragonal ZrO2, the intensity of (111) reflection of monoclinic ZrO2, and the intensity of (1j11) reflection of monoclinic ZrO2, respectively. Prior to H2 reduction, the calcined samples were characterized using temperature programmed reduction (TPR), which was performed on a homemade apparatus consisting of a gas chromatograph with a TCD detector. Typically, 100 mg of the sample was pressed into wafers, broken into small platelets, and charged into a quartz reaction tube. The sample was preheated at 473 K in a He flow for 2 h. After being cooled to room temperature, the sample was heated from room temperature to 773 K under a flow gas consisting of 10% H2 in Ar (40 mL/min) at a ramping rate of 5 K/min. H2 consumption of the sample was monitored by the online GC and was normalized on the basis of sample mass. 2.3. Catalytic Denitrification. Catalytic tests were carried out at room temperature and atmospheric pressure in a fournecked flask batch reactor equipped with a pH-stat, H2 inlet and outlet, and a sampling port. Typically, 300 mg of the catalyst was added into the reactor containing 116 mL of distilled water followed by purging the reaction system with a H2 flow (60 mL/min) for 1 h under vigorous stirring. Next, 34 mL of 441

Figure 1. XRD patterns of ZrO2 supports and ZrO2 supported bimetallic catalysts. Table 1. Properties of ZrO2 Supports and ZrO2 Supported Bimetallic Catalysts

sample Zr573 Pd(5)-Cu(1.25)/Zr573 Zr773 Pd(5)-Cu(1.25)/Zr773 Zr973 Pd(5)-Cu(1.25)/Zr973

BET (m2/g)

ZrO2 particle size (nm)

91.1 32.0 7.2

XRD 9.4 9.7 26.9 27.3 57.4 59.1

TEM 10-20 20-40 60-80

tetragonal phase content (%) 45.9 20.2 11.2 9.1 4.0 3.9

mg/L nitrate solution was added rapidly. Solution pH was adjusted using 0.2 M HCl solution during the reaction process. Samples were taken at selected intervals, and the catalyst particles were removed using a 0.45 µm filter. The residual concentration of nitrate in the filtrate was analyzed using a UV/ vis spectrophotometer at a wavelength of 220 nm. The nitrite concentration was determined using a UV/vis spectrophotometer at 540 nm using the naphthylamine analytical procedure. The concentration of ammonium ion was analyzed using the Nessler method.32 The activity of the catalyst was evaluated using either the average activity or the initial activity. The average activity is defined as the specific removal rate of nitrate after complete reduction of nitrate, and the initial activity is defined as the specific removal rate of nitrate within the initial 2 min. 3. Results and Discussion 3.1. Characterization of ZrO2 Supports and ZrO2 Supported Catalysts. XRD patterns of ZrO2 supports and ZrO2 supported bimetallic catalysts are compared in Figure 1. The average particle sizes of ZrO2 supports and ZrO2 in bimetallic catalysts were estimated using the Scherrer equation, and the composition of the crystalline phases in ZrO2 supports and ZrO2 in bimetallic catalysts was calculated using eq 1. The results are listed in Table 1. The ZrO2 support calcined at 573 K consisted of both monoclinic and tetragonal phases, and the relatively low intensity of the diffraction peaks suggested the existence of amorphous phase in Zr573. Calcination of ZrO2 at 773 and 973 K led to the partial transformation of metastable tetragonal ZrO2 into monoclinic ZrO2. Garvier concluded that an excess of surface energy in small crystallites could stabilize tetragonal ZrO2.33 The phase transformation of ZrO2 results from dehydroxylization of ZrO2 during the thermal treatment process.34 After impregnation and H2 reduction, a marked change was observed in Pd(5)-Cu(1.25)/Zr573 as compared to Zr573. In Zr573, the content of tetragonal phase was determined to be

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Figure 2. Transmission electron micrographs of (a) Pd(5)-Cu(1.25)/Zr573, (b) Pd(5)-Cu(1.25)/Zr773, and (c) Pd(5)-Cu(1.25)/Zr973.

45.9%. However, the content of tetragonal phase in Pd(5)Cu(1.25)/Zr573 was found to be 20.2%, which was markedly lower as compared to that in Zr573. For other catalysts, almost identical ratios of tetragonal to monoclinic phase were observed before and after the impregnation and reduction processes, indicating minor changes in the ZrO2 supports. This suggests that ZrO2 support calcined at 573 K is unstable and stable ZrO2 supports can be obtained after calcination at high temperature. In addition, increasing the calcination temperature of ZrO2 supports from 573 to 973 K led to an increase of their average particle sizes from 9.4 to 57.4 nm. BET measurements revealed that the specific surface areas of Pd(5)-Cu(1.25)/Zr573, Pd(5)-Cu(1.25)/Zr773, and Pd(5)-Cu(1.25)/Zr973 were 91.1, 32.0, and 7.2 m2/g, respectively. This indicated that increasing the calcination temperature led to a decrease in the specific surface areas and an increase of the average particle sizes of ZrO2 supports. TEM images and histograms of metal particle size distribution of Pd(5)-Cu(1.25)/Zr573, Pd(5)-Cu(1.25)/Zr773, and Pd(5)Cu(1.25)/Zr973 are compiled in Figures 2 and 3, respectively. For Pd(5)-Cu(1.25)/Zr573, the particle size of ZrO2 support was found to be approximately 10-20 nm. The particle sizes of ZrO2 supports in Pd(5)-Cu(1.25)/Zr773 and Pd(5)-Cu(1.25)/ Zr973 were determined to be 20-40 and 60-80 nm, respectively, which was in agreement with XRD results. For Pd(5)-Cu(1.25)/Zr573 and Pd(5)-Cu(1.25)/Zr773, a narrow particle size distribution of metals was observed. The average particle sizes of metals in Pd(5)-Cu(1.25)/Zr573 and Pd(5)Cu(1.25)/Zr773 were calculated to be 2.3 and 2.6 nm, respectively. For Pd(5)-Cu(1.25)/Zr973, however, a wide particle size distribution was observed, and the metal particle size varied from 1.0 to 30 nm with an average value of 12.5 nm. This demonstrated that metallic species can be well dispersed over the ZrO2 support calcined at low temperature, which is attributed to the larger specific surface area and a higher content of tetragonal ZrO2 in Pd(5)-Cu(1.25)/Zr573 and Pd(5)-Cu(1.25)/ Zr773 than that in Pd(5)-Cu(1.25)/Zr973.35 The TPR profiles of the calcined Cu(1.25)/Zr773 and Pd(5)/ Zr773 are presented in Figure 4. For Cu(1.25)/Zr773, only one H2 consumption peak was observed at 523 K. Chary et al. studied the reduction behaviors of CuO/ZrO2 and found that the dispersion of CuO over ZrO2 was related to the copper loading, and multiple reduction peaks were observed for CuO/ ZrO2 with a high copper loading.36 The H2 consumption peak at low reduction temperature is ascribed to the reduction of the highly dispersed Cu species in an octahedral environment, and the peak at high reduction temperature is attributed to the reduction of bulk Cu cluster to metallic Cu. Therefore, the single

H2 consumption peak in TPR profile of calcined Cu(1.25)/Zr773 implies that metallic Cu is highly dispersed over ZrO2 surface under our experimental conditions. In contrast to calcined Cu(1.25)/Zr773, different reduction behaviors of the calcined Pd(5)/Zr773 sample were observed. In the TPR profile of the calcined Pd(5)/Zr773 sample, a small reduction peak and a negative H2 consumption peak were observed at 354 and 359 K, respectively. The H2 reduction peak at 354 K probably results from reduction of bulk PdOx, and the presence of the negative peak is indicative of the decomposition of Pd-β-hydride,37 which suggests that the Pd species on the surface of ZrO2 are susceptible to reduction even below room temperature. In addition, metallic Pd on the surface of ZrO2 is prone to activating and storing H2 at room temperature. To clearly testify to the nature of Pd-Cu ensembles on the surface of ZrO2, TPR of calcined bimetallic catalysts with varied ratio of Pd to Cu was conducted, and the TPR profiles of the calcined samples are compiled in Figure 5. For calcined Pd(5)-Cu(0.625)/Zr773, a negative H2 consumption peak at 351 K and broad H2 consumption peaks ranging from 373 to 423 K were observed. The negative peak, characteristic of the decomposition of Pd-β-hydride, demonstrates the presence of monometallic Pd in Pd(5)-Cu(0.625)/Zr773. The broad H2 consumption peaks are ascribed to the reduction of Pd-Cu ensembles on ZrO2 surface.37,38 Sun et al. concluded that Pd-Cu ensembles could be formed with a wide range of compositions at low reduction temperature via either the diffusion of reduced Cu species into Pd particles or the direct reduction of mixed Pd-Cu species during the reduction process.39 In parallel, FernndezGarca et al. studied KL zeolite supported Pd-Cu bimetallic catalyst and concluded that Pd and Cu could generate Pd-Cu alloy over KL zeolite and no significant metal enrichment was observed.40 It is noteworthy that the intensity of the negative peak in the TPR profile of calcined Pd(5)-Cu(0.625)/Zr573 was markedly lower as compared to that of calcined Pd(5)/Zr773. Batista et al. attributed the lower H2 uptake over Pd-Cu/Al2O3 than Pd/Al2O3 to the decrease of Pd-Pd pairs upon the addition of Cu.41 Therefore, it can be concluded that increasing Cu content leads to a substantially decreased amount of monometallic Pd sites in bimetallic catalyst and a portion of Pd is combined with Cu to form bimetallic Pd-Cu ensembles. In addition, the H2 consumption peak characteristic of highly dispersed Cu species was not observed in the TPR profile of Pd(5)-Cu(0.625)/Zr773. In contrast, broad H2 consumption peaks at lower temperatures were observed, suggesting a facile reduction of Cu species in the presence of metallic Pd due to the split-over H2 from Pd metal.42 Further increasing the Cu content led to the decrease of the negative peak and to the

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Figure 4. TPR profiles of calcined Pd(5)/Zr773 and Cu(1.25)/Zr773.

Figure 5. TPR profiles of calcined Pd-Cu bimetallic catalysts with varied Pd/Cu ratio.

Figure 3. Histograms of metal particle size distribution in (a) Pd(5)-Cu(1.25)/ Zr573, (b) Pd(5)-Cu(1.25)/Zr773, and (c) Pd(5)-Cu(1.25)/Zr973.

increase of the broad multiple peaks. In addition, the broad multiple peaks shifted to higher reduction temperatures for Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773. This demonstrates the gradually decreased amount of monometallic Pd sites and increased content of Pd-Cu bimetallic ensembles on ZrO2 surface. It is noteworthy that in Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773 the negative reduction peaks were not observed. In parallel, TPR of the H2 reduced samples confirmed the absence of the negative reduction peaks in Pd(5)-Cu(2.5)/ Zr773 and Pd(5)-Cu(5)/Zr773. Pintar et al. characterized Al2O3 supported Pd-Cu catalysts using TPR and attributed the disappearance of the negative peak to the absence of monome-

tallic Pd in supported Pd-Cu bimetallic catalysts.43 In parallel, Lambert et al. studied 1,2-dichloroethane hydrodechlorination over Pd-Cu/SiO2 and found that Pd only presented in the form of Pd-Cu domains in Pd-Cu/SiO2 with Pd/Cu of 1.2.44 This suggests that at low Pd/Cu ratio the monometallic Pd is absent and metals only exist in the form of Pd-Cu ensembles in ZrO2 supported bimetallic catalysts. It should be emphasized that multiple H2 consumption peaks at different reduction temperatures can be attributed to Pd-Cu bimetallic ensembles with different compositions, and the H2 reduction peak at higher reduction temperature is characteristic of the Pd-Cu ensemble with lower Pd/Cu ratio, evidenced by the fact that the H2 reduction peaks in Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/ Zr773 were observed at markedly higher reduction temperatures as compared to Pd(5)-Cu(0.625)/Zr773 and Pd(5)-Cu(1.25)/ Zr773. The results of TEM, XRD, and BET showed that increasing the calcination temperature of ZrO2 support led to the decrease of the specific surface areas of the catalysts and increase of the particle sizes of both ZrO2 support and metals, which may influence the composition of metals on the surface of the support. Therefore, TPR of calcined Pd-Cu bimetallic catalysts with ZrO2 supports calcined at different temperatures was performed, and the TPR profiles of the samples are described in Figure 6. Similar reduction profiles were obtained for the samples, and all TPR profiles presented negative peaks at 355 K, indicating the presence of monometallic Pd on ZrO2 surface. The intensity of the negative peak in calcined Pd(5)-Cu(1.25)/ Zr773 was slightly higher than that in Pd(5)-Cu(1.25)/Zr573 and markedly higher than that in Pd(5)-Cu(1.25)/Zr973, suggesting that the amount of monometallic Pd sites in the catalysts decreases as follows: Pd(5)-Cu(1.25)/Zr773 > Pd(5)-Cu(1.25)/Zr573 > Pd(5)-Cu(1.25)/Zr973. In addition,

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Figure 6. TPR profiles of calcined Pd(5)-Cu(1.25)/Zr573, Pd(5)-Cu(1.25)/ Zr773, and Pd(5)-Cu(1.25)/Zr973.

Figure 7. Nitrate hydrogenation catalyzed by (], [) Pd(5)-Cu(0.625)/ Zr773, (2, ∆) Pd(5)-Cu(1.25)/Zr773, (b, O) Pd(5)-Cu(2.5)/Zr773, and (9, 0) Pd(5)-Cu(5)/Zr773. Filled symbols denote nitrate concentration, and open symbols denote ammonia concentration.

slightly larger H2 consumption peaks of Pd(5)-Cu(1.25)/Zr973 at reduction temperature ranging from 373 to 423 K were observed as compared to Pd(5)-Cu(1.25)/Zr573 and Pd(5)Cu(1.25)/Zr773, indicative of a slightly higher content of Pd-Cu ensembles in Pd(5)-Cu(1.25)/Zr973. This can be explained by the fact that the majority of Pd is combined with Cu to generate Pd-Cu ensembles in Pd(5)-Cu(1.25)/Zr973. It should be emphasized that the average particle sizes of the metals in Pd(5)-Cu(1.25)/Zr573 and Pd(5)-Cu(1.25)/Zr773 were 2.3 and 2.6 nm, respectively, suggesting a slightly higher dispersion of Pd and Pd-Cu ensembles over Zr573 than Zr773. Thus, it could be expected that Pd(5)-Cu(1.25)/Zr573 contains a higher content of monometallic Pd and Pd-Cu ensembles as compared to Pd(5)-Cu(1.25)/Zr773, which is inconsistent with the experimental results. Considering that Zr573 is unstable and impregnation and reduction processes resulted in a marked change in Zr573 structure, the low amount of monometallic Pd sites and Pd-Cu ensembles in Pd(5)-Cu(1.25)/Zr573 is tentatively attributed to the embedment of a portion of monometallic Pd and Pd-Cu ensembles into ZrO2 during the impregnation and reduction processes. 3.2. Influence of Metal Ratio on Nitrate Reduction. The reduction of aqueous nitrate catalyzed by a series of bimetallic catalysts with varied Pd/Cu ratio is compiled in Figures 7 and 8, respectively, and the nitrate reduction activities are compared in Figure 9. For the catalysts with identical Pd content, nitrate reduction activity was found to be dependent on the copper loading. A typical volcano type dependence of the activity of the catalyst on the Pd/Cu ratio was observed, and the optimum

Figure 8. Concentration profiles of nitrite as a function of nitrate reduction time in the presence of ([) Pd(5)-Cu(0.625)/Zr773, (2) Pd(5)-Cu(1.25)/ Zr773, (b) Pd(5)-Cu(2.5)/Zr773, and (9) Pd(5)-Cu(5)/Zr773.

Figure 9. Catalytic activity as a function of Pd/Cu ratio. Filled symbols denote initial activity, and open symbols denote average activity.

Pd/Cu ratio was found to be 4/1, which is in good agreement with nitrate reduction over Al2O3 supported Pd-Cu bimetallic catalysts.8,45 In general, the activity of the supported bimetallic catalyst for nitrate reduction is correlated to the content of bimetallic ensembles.8 For bimetallic catalysts with Pd/Cu ratio of 8/1 and 4/1, the increased activity with the increase of Cu content in the catalyst is due to the increased content of Pd-Cu ensembles, which is reflected by the fact that the intensity of the reduction peaks ranging from 373 to 423 K in TPR profile of Pd(5)Cu(1.25)/Zr773 was markedly higher than that of Pd(5)Cu(0.625)/Zr773 (see Figure 5). At a Pd/Cu ratio lower than 4/1, increasing Cu content led to declined activities of the catalysts for nitrate reduction. For Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773, the markedly higher intensities of the reduction peaks characteristic of Pd-Cu ensembles were observed, demonstrating the higher content of Pd-Cu ensembles than those in Pd(5)-Cu(0.625)/Zr773 and Pd(5)-Cu(1.25)/ Zr773. In addition, the negative H2 consumption peak was not observed in Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773. The absence of monometallic Pd in the catalysts inevitably leads to the buildup of nitrite during nitrate reduction (see Figure 8). Deganello et al. concluded that increased nitrite concentration during nitrate reduction resulted in the competitive adsorption of nitrite with nitrate over Pd-Cu ensembles, which led to the decreased adsorption amount of nitrate over Pd-Cu ensembles and eventually to the decreased activities of the catalysts.24 It is noteworthy that the initial activities of Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773 also decreased with the increase of Cu content, although the content of Pd-Cu ensembles increased with the increase of Cu content. Considering that the concentration of nitrite was relatively low at the initial reaction stage, the declined initial activity of the bimetallic catalyst directly

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reflected the declined catalytic activity of Pd-Cu ensembles for nitrate reduction with the decrease of Pd/Cu ratio. Berndt et al. concluded that the lower catalytic activity of Pd-Sn/Al2O3 with higher Sn content for the denitrification of nitrate resulted from the decrease in the concentration of suitable Pd-Sn ensembles.14 Sa´ and Vinek further pointed out that at high Cu content H2 activated on Pd sites could not reduce all Cu atoms present on the surface.45 This suggests that for Pd-Cu ensembles with high Cu content a portion of Pd-Cu ensembles are irreversibly inactivated after the redox reaction between Cu and nitrate at the initial reaction stage and only suitable Pd-Cu ensembles serve as the authentic active sites for nitrate reduction,46 resulting in the low catalytic activity of Pd-Cu ensembles with high Cu content. The generation and decay of nitrite intermediate was also found to be related to Cu content in the catalyst. A minor amount of nitrite was detected during nitrate reduction over Pd(5)Cu(0.625)/Zr773 and Pd(5)-Cu(1.25)/Zr773, due to the presence of a high amount of monometallic Pd sites on the surface of the catalysts. However, the buildup of nitrite and a marked time lag in nitrite reduction was observed during nitrate reduction over Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773. For the reduction of nitrite, Pru¨sse and Vorlop concluded that only monometallic Pd sites were responsible for nitrite reduction due to the strong adsorption of nitrite onto monometallic Pd sites.8 Pintar et al. pointed out that nitrite could be reduced over both monometallic Pd sites and Pd-Cu ensembles, and the selectivity of nitrite reduction toward N2 over monometallic Pd sites was higher as compared to that over Pd-Cu ensembles.47 Our results revealed that monometallic Pd was not present in Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773, suggesting that both nitrite and nitrate can be reduced over Pd-Cu ensembles. Although the nitrate reduction rate over Pd(5)-Cu(1.25)/Zr773 was markedly higher as compared to that over Pd(5)-Cu(5)/ Zr773, nitrite concentration was markedly lower during nitrate reduction, implying that nitrite reduction over monometallic Pd is markedly fast as compared to that over Pd-Cu ensembles. In addition, the ammonia selectivity was related to the content of Cu in the bimetallic catalyst. For Pd(5)-Cu(1.25)/Zr773, a higher content of Pd-Cu ensembles and lower amount of monometallic Pd sites was found as compared to Pd(5)Cu(0.625)/Zr773. This results in fast reduction of aqueous nitrate and slightly higher nitrite concentration during nitrate reduction. Because of the lower amount of monometallic Pd sites in Pd(5)-Cu(1.25)/Zr773 and slightly higher nitrite concentration, a higher nitrite coverage can be expected on the surface of monometallic Pd sites than that in Pd(5)-Cu(0.625)/Zr773, which eventually leads to the facile generation of N-N over the surface of monometallic Pd and to a lower ammonia selectivity. For Pd(5)-Cu(2.5)/Zr773 and Pd(5)-Cu(5)/Zr773, the reduction of both nitrate and nitrite mainly occurs over Pd-Cu ensembles due to the absence of monometallic Pd sites. In addition, the dilution effect of Cu in Pd-Cu ensembles decreases the possibility of the generation of N-N due to the absence of contiguous Pd sites in Pd-Cu ensembles,8,24 resulting in a high ammonia selectivity. 3.3. Influence of ZrO2 Support on Nitrate Reduction. Characterization results showed that the change of support properties concomitantly led to a marked difference in the nature of supported bimetallic catalysts, which could also result in different catalytic behaviors of these catalysts for nitrate reduction. The reaction profiles for nitrate reduction over Pd(5)-Cu(1.25)/ Zr573, Pd(5)-Cu(1.25)/Zr773, and Pd(5)-Cu(1.25)/Zr973 at

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Figure 10. Effect of ZrO2 support on the activity and selectivity of nitrate hydrogenation. Filled symbols denote average activity, and open symbols denote ammonia formation.

Figure 11. Concentration profiles of nitrite as a function of nitrate reduction time in the presence of ([) Pd(5)-Cu(1.25)/Zr573, (9) Pd(5)-Cu(1.25)/ Zr773, and (2) Pd(5)-Cu(1.25)/Zr973.

pH 6.0 are compiled in Figures 10 and 11. For Pd(5)-Cu(1.25)/ Zr773, nitrate was completely converted within 20 min, whereas the complete conversion of aqueous nitrate over Pd(5)-Cu(1.25)/ Zr573 and Pd(5)-Cu(1.25)/Zr973 was achieved after reduction for about 30 and 35 min, respectively, indicative of a slightly higher catalytic activity of Pd(5)-Cu(1.25)/Zr773 as compared to Pd(5)-Cu(1.25)/Zr573 and Pd(5)-Cu(1.25)/Zr973. Upon the complete conversion of nitrate, 7.6, 7.0, and 10.9 mg/L of ammonia were formed as the byproduct of nitrate reduction over Pd(5)-Cu(1.25)/Zr573, Pd(5)-Cu(1.25)/Zr773, and Pd(5)Cu(1.25)/Zr973, respectively. Nitrite intermediate was also detected for all catalysts during the nitrate reduction process (see Figure 11). However, the generation and decay profiles of nitrite intermediate were found to be different for Pd(5)Cu(1.25)/Zr573, Pd(5)-Cu(1.25)/Zr773, and Pd(5)-Cu(1.25)/ Zr973. For Pd(5)-Cu(1.25)/Zr573 and Pd(5)-Cu(1.25)/Zr773, nitrite concentration was fast increased at the initial stage and gradually decreased during nitrate reduction, whereas for Pd(5)-Cu(1.25)/Zr973 a time-lag in the profile of nitrite decay revealed a markedly slower nitrite reduction rate as compared to that over Pd(5)-Cu(1.25)/Zr573 and Pd(5)-Cu(1.25)/Zr773. At identical metal loading, the content of monometallic Pd and Pd-Cu ensembles was dependent on the nature of ZrO2 support. For Pd(5)-Cu(1.25)/Zr773, the slightly higher nitrate reduction activity as well as the lower nitrite concentration and ammonia selectivity was mainly ascribed to the higher amount of Pd-Cu ensembles and monometallic Pd sites than that in Pd(5)-Cu(1.25)/Zr573. The amount of monometallic Pd sites in Pd(5)-Cu(1.25)/Zr973 was found to be markedly lower as compared to Pd(5)-Cu(1.25)/Zr773 and Pd(5)-Cu(1.25)/Zr573, leading to a marked time lag in nitrite reduction. Therefore,

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the lower nitrate reduction activity is due to the generation of higher nitrite concentration during nitrate reduction, which inhibited the reduction of nitrate by competitive adsorption. The markedly high ammonia selectivity can be attributed to the reduction of a majority of nitrite over Pd-Cu ensembles due to the low amount of monometallic Pd sites present in Pd(5)-Cu(1.25)/Zr973. 4. Conclusions The catalytic behaviors of ZrO2 supported Pd-Cu bimetallic catalysts for the hydrogenation of aqueous nitrate are related to the properties of ZrO2 supports and Pd/Cu ratio. High dispersion of metals can be achieved over ZrO2 calcined at low temperature, which results in a high content of monometallic Pd. Increasing Cu content in the bimetallic catalysts leads to the decreased amount of monometallic Pd sites and to the increase of the content of Pd-Cu ensembles as well as the change of the composition of Pd-Cu ensembles. The nitrate reduction rate depends on the content and the composition of Pd-Cu ensembles. The bimetallic catalyst containing a high content of Pd-Cu ensembles with high Pd/Cu ratio exhibits a high catalytic activity for nitrate reduction. In addition, the decay profile of nitrite is linked to the amount of monometallic Pd sites in the bimetallic catalyst, which influences the nitrate reduction rate and N2 selectivity. The optimized ratio of monometallic Pd to bimetallic Pd-Cu ensembles in ZrO2 supported bimetallic catalyst is essential for the selective reduction of nitrate toward N2. Finally, the present results further suggest that the catalytic behaviors of supported bimetallic catalysts for catalytic hydrogenation of aqueous nitrate can be optimized via adjusting the properties of the support. Acknowledgment Financial support from the Natural Science Foundation of China (No. 20677026) and Scientific and Technical Supporting Programs (2006BAC02A15) is gratefully acknowledged. Literature Cited (1) United States EnVironmental Protection Agency. National Primary Drinking Water Regulations; Contaminant Specific Fact Sheets; EPA: Washington, DC. (2) Bontoux, L.; Bournis, N.; Papameletiou, D. The IPTS Rep. 1996, 6, 7. (3) Pintar, A.; Batista, J.; Levec, J.; Kajiuchi, T. Kinetics of the Catalytic Liquid-Phase Hydrogenation of Aqueous Nitrate Solutions. Appl. Catal., B 1996, 11, 81. (4) Lecloux, A. J. Chemical, Biological and Physical Constrains in Catalytic Reduction Processes for Purification of Drinking Water. Catal. Today 1999, 53, 23. (5) Ho¨rold, S.; Vorlop, K.-D.; Tacke, T.; Sell, M. Development of Catalysts for a Selective Nitrate and Nitrite Removal from Drinking Water. Catal. Today 1993, 17, 21. (6) Vorlop, K.-D.; Tacke, T. Erste Schritte auf dem Weg zur edelmetallkatalysierten Nitrat-und Nitrit-Entfernung aus Trinkwasser. Chem.-Ing.Tech. 1989, 61, 836. (7) Pintar, A. Catalytic Processes for the Purification of Drinking Water and Industrial Effluents. Catal. Today 2003, 77, 451. (8) Pru¨sse, U.; Vorlop, K.-D. Supported Bimetallic Palladium Catalysts for Water-phase Nitrate Reduction. J. Mol. Catal. A 2001, 173, 313. (9) Tacke, T.; Vorlop, K.-D. Wissenschaftliche Kurzbeitra¨ge Kinetische Charakterisierung von Katalysatoren zur selektiven Entfernung von Nitrat und Nitrit aus Wasser. Chem.-Ing.-Tech. 1993, 65, 1500. (10) Epron, F.; Gauthard, F.; Barbier, J. Catalytic Reduction of Nitrate in Water on a Monometallic Pd/CeO2 Catalyst. J. Catal. 2002, 206, 363. (11) Gavagnin, R.; Biasetto, L.; Pinna, F.; Strukul, G. Nitrate Removal in Drinking Waters: the Effect of Tin Oxides in the Catalytic Hydrogenation of Nitrate by Pd/SnO2 Catalysts. Appl. Catal., B 2002, 38, 91.

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ReceiVed for reView April 11, 2009 ReVised manuscript receiVed May 15, 2009 Accepted May 22, 2009 IE9005854