Catalysis Communications

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1 Catalysis Communications 1 (9) Contents lists available at ScienceDirect Catalysis Communications journal homepage: The relation between surface composition of Pd Cu/ACC catalysts prepared by selective deposition and their denitrification behavior Uri Matatov-Meytal, Moshe Sheintuch * Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 3, Israel article info abstract Article history: Received 2 April 8 Received in revised form 6 October 8 Accepted 9 October 8 Available online 24 November 8 Keywords: Bimetallic catalysts Copper Palladium Activated carbon cloth support Selective deposition Hydrogenation Nitrates Pd Cu catalysts supported on activated carbon cloth (Pd Cu/ACC), prepared by selective catalytic deposition, showed a high activity (92% conversion) and selectivity (ca 93%) in the liquid phase hydrogenation of nitrates to nitrogen. From this study, first knowledge on the surface composition of a family of Cu 2 wt%pd/ac bimetallic catalysts and their remarkable influence on the catalytic behavior in nitrate hydrogenation could be obtained. The results of the surface composition determination by the combination of CO chemisorption and HRSEM reveal a specific orientation for the Cu metal deposition on the core Pd crystallites, probably of Pd Cu bimetallic islets and free Pd atoms. The main differences between samples with different Cu loadings are attributed to changes in these two metallic phases over the surface of metallic particles and they are reflected in the activity and selectivity of nitrate hydrogenation. The results reveal that TOF Cu, i.e., the number of nitrate ions consumed per second per active surface Cu seems to be independent of surface composition, while TOF Pd was found to increase with surface Cu, indicating the bifunctional nature of the Cu Pd sites responsible for the nitrate reduction by red ox reaction. Also, the surface ratio of Pd Cu sites that catalyze nitrate-to-nitrite hydrogenation, to Pd sites, which catalyze the hydrogenation of nitrite to nitrogen, should be optimal. This might offer novel possibilities for the design of fibrous catalyst showing better performance. Ó 8 Elsevier B.V. All rights reserved. 1. Introduction In the field of hydrogenation, Pd-based bimetallic catalysts are often used in order to improve activity, selectivity and stability of a single component catalyst [1]. Among others, a great interest has been paid to the Pd Cu systems for the hydrogenation of nitrates to nitrogen for the denitrification of contaminated water [2] 2NO 3 ðaqþþ5h 2ðgÞ!N 2 ðgþþ2oh ðaqþþ4h 2 O Synthesis of these materials is typically carried out either by simultaneous co-impregnation of both metal salts onto a catalyst support or by successive steps of metal salt addition. For either of these preparative methods it is virtually impossible to ensure the formation of only bimetallic particles [3]. An alternative approach for the preparation of bimetallic catalysts is the use of selective deposition methodology, in which, for example, the second metal salt is decomposed onto the first metal(s) by a controlled reaction catalyzed by the pre-existing metal [4]. We have reported that activated carbon cloth supported Pd Cu catalysts (Pd Cu/ACC), prepared by the selective deposition of Cu to a pre-reduced Pd/ACC, showed a high activity (92% conversion) and selectivity (ca 93%) in liquid phase hydrogenation of nitrates to * Corresponding author. Tel.: ; fax: address: cermsll@techunix.technion.ac.il (M. Sheintuch). ð1þ nitrogen [5]. One reason for this performance of Pd Cu/ACC catalysts (as well as Pd/ACC for nitrite reduction [6]) may be related to the unique surface and pore size properties of this novel fibrous carbon material used as support. ACCs have almost uniform micropores which are open directly to outer surface [7]. However, the way in which the additive Cu modifies the properties of the Pd in Pd Cu/ACC catalysts is not yet fully elucidated. When one metal is deposited on the another, one observes a number of different phenomena. The deposited metal may form islands on the substrate or may alloy into the first or deeper layers. It is also not clear if surface segregation of one element occurs, or if the two elements are randomly distributed on the surface of the catalyst. The Pd/Cu atomic ratio, the support used and the preparation conditions applied may affect Pd Cu metallic surface in a different way. Therefore, a detailed surface characterization of the prepared bimetallic catalysts and their relation with the reaction rate (and selectivity) are of importance. However, as far as we know, only a few researchers [4,8 12] have studied the surface composition/catalytic behavior of the Pd Cu denitrification catalysts (supported mainly on alumina) and no study of surface composition of Pd Cu catalysts supported on carbon materials [13,14] for nitrate hydrogenation has been reported. In this contribution, we present a complete study for the determination of surface composition Pd Cu particles supported on /$ - see front matter Ó 8 Elsevier B.V. All rights reserved. doi:1.116/j.catcom

2 1138 U. Matatov-Meytal, M. Sheintuch / Catalysis Communications 1 (9) ACC, aimed to relate the Pd Cu/ACC catalysts activity and selectivity during nitrate hydrogenation reaction (1) with their surface composition. Surface compositions of Pd Cu/ACC catalysts were determined in this work by means of high-resolution scanning electron microscopy (HRSEM) and CO chemisorption. Kinetic studies here were performed with a continuous-flow reactor which assures higher reproducibility; most other studies in the literature were performed in a batch reactor. 2. Experimental Activated carbon cloth (ACC , from Kynol TM ), used as support in this work, was woven from 4 to 6 cm long threads from elementary fibers of 1 lm in diameter, with BET specific surface area of 15 m 2 /g. First, the ACC was pre-treated in an aqueous solution of HNO 3 (4.5 wt%) and was then rinsed in distilled water. Two weight percent Pd/ACC was obtained by incipient wetness impregnation method using H 2 PdCl 4 (dihydrogen tetrachloropalladate (II)) from solution of PdCl 2 (pure, Fluka) in hydrochloric acid. After impregnation, cloth was heated first at 9 C overnight to eliminate solvent and calcined at 3 C for 5 h in flowing argon, and then was washed to remove chloride ions, which form the decomposition of H 2 PdCl 4 during calcination. Final concentration of chloride ions in rinsing waters was detected using AgNO 3 / HNO 3. After drying at 1 C for 2 h, the samples were reduced at C for 1 h under flowing hydrogen. In the second step, a given amount of 2 wt%pd/acc was placed on a rotating drum and immersed into a solution of copper formate (Cu(HCO 2 ) 2, reagent grade Aldrich) at required concentration under flowing argon. The copper formate catalytically decomposes only at the surface of Pd particles even at room temperature, according to [15] generating the metallic Cu selectively only at the metallic Pd surface. Since the decomposition of copper ions is determined by the metal already present on the support, only bimetallic particles are formed, and this preparation technique can be regarded as selective deposition by a controlled catalytic surface reaction. Cu loading on the Pd surface can be varied by modification of deposition kinetic parameters such as deposition time or concentration of copper formate. The solution was regularly checked spectrophotometrically at k max = 778 nm for the presence of copper ions in solution. Then cloth was separated from the liquid and dried overnight under argon stream at 9 C. The real metal loadings of the samples were determined by inductively coupled plasma emission spectrometry (ICP-ES Perkin Elmer Optima 3 DV instrument). The surface observation of metallic particles on prepared samples was carried out with field emission gun high-resolution digital scanning electron microscope HR SEM LEO 982 (co-operation Zeiss Leica). CO chemisorption measurements were performed on an ASAP 1 Chemi Micromeritics apparatus. The samples were dried under vacuum (1 6 Torr) at 15 C over 3 min, reduced at 1 C (1 C min 1 ) for 1 h and evacuated prior taking the CO isotherms at room temperature over the pressure range 5 3 Torr of CO. Under these conditions, the adsorption of CO on Cu o was considered to be largely physical in nature [16,17]. Nitrate hydrogenation tests were performed using laboratory scale flow reactor system described previously [18]. In a typical run, a solution of nitrate (1.8 mmol/l from NaNO 3, reagent grade, Aldrich) in distilled water was fed into the gas/water saturator, in which H 2 (Orgim, Israel) was dissolved in feed solution under pressure. The concentrations of nitrate and nitrite ions were monitored using a liquid ion chromatography (761 Compact IC, Methrom instrument) with electro-conductivity detection (anion analytical column METROSEP A SUPP 4 (4 25 mm); the mobile phase was carbonate/bicarbonate effluent and sulfuric acid regenerant). Ammonium ions concentrations were measured spectrophotometrically with Nessler reagent at k max = 5 nm. 3. Results and discussion 3.1. Catalyst performance Catalytic performance is characterized by nitrate conversion X NO3 and by the selectivity to nitrogen and to nitrite or to ammonium ions (S i ) defined as X NO3 ¼ð1 C NO3 =C NO 3 Þ ð3þ S i ¼ C i =ðc NO 3 C NO3 Þ ð4þ S N2 ¼ 1 S NO2 S NH4 ð5þ where C NO 3 and C NO3 are measured influent and effluent molar concentrations of nitrates, respectively, C i and S i are the effluent concentrations and selectivities to nitrite and ammonium ions, respectively. The selectivity to N 2 accounts, possibly for some conversion to N 2 O and NO. Table 1 shows steady state performance data (after at least 6 h of experimentation time) for nitrate hydrogenation over 2 wt%pd/acc and 2 wt%pd Cu/ACC catalysts at 25 C, at a flow rate of F =.25 ml/s nitrate solution and with catalyst weight of W cat = 9.5 g. The obtained data show that while Pd is inactive for denitrification (as well as Cu itself [2 4]), the activity of Pd Cu/ACC catalysts studied is very high. It is noteworthy that activity was enhanced by the addition of only.28 wt% Cu and change in the content of Cu had a significant effect on the activity and selectivity. CuðHCO 2 Þ 2! Cu þ 2CO 2 þ H 2 ð2þ 3.2. Catalyst characterization The HRSEM micrographs (Fig. 1, left column) show that the monometallic 2 wt%pd/acc catalyst exhibits small particles, of 2 6 nm in size, with a semispherical morphology. The bimetallic samples, on the other hand, contain relatively bigger and irregularly shaped particles. Representative particle size distributions based on the measured sizes of 9 1 individual particles from HRSEM micrographs, also show that with increasing Cu content the particle size distributions become heterogeneous and broader (Fig. 1, right column). The average crystallite sizes d s (nm) were calculated from d s ¼ P ni P ni d 3 i d 2 i where n i is the number of particle with diameter d i. The dispersion of palladium D Pd was estimated from CO chemisorption results under the assumption that CO chemisorbed was partly in a linear form and partly in a bridged form, and the average stoichiometry of the chemisorption is Pd:CO = This assumption was based on the literature data [19,]. Negligible CO adsorption on Cu is assumed (and is discussed below). Thus, Table 1 Summary of Pd Cu/ACC catalysts performance for nitrate hydrogenation. Sample NO 3 conversion (%) Selectivity (mol%) N 2 NO þ 2 NH þ 4 2 wt%pd 2 wt%pd.28 wt%cu wt%pd.59 wt%cu wt%pd 1. wt%cu wt%pd 1.8 wt%cu ð6þ

3 U. Matatov-Meytal, M. Sheintuch / Catalysis Communications 1 (9) wt%Pd/ACC wt%Cu wt%Cu wt%Cu wt%Cu Fig. 1. HRSEM images and typical size distributions for 2 wt%pd/acc and 2 wt%pd Cu/ACC samples. D Pd ¼ V COM Pd X Pd Co 22; 414f Pd ð7þ where M Pd is the atomic weight of Pd (16.4 g/mol), f Pd = m Pd /m cat is the weight fraction of Pd; the constant 22,414 refers to molar density (cm 3 /mol), and X Pd CO is the chemisorption stoichiometry. Table 2 lists the average particle diameter, CO chemisorption and the Pd dispersion for various Cu 2 wt%pd/acc samples showing that the selective deposition of Cu led to a significant increase of the average particle size d s. As expected, copper loading led to diminished chemisorbed CO expressed as (V CO,cm 3 /g), confirming that Cu blocks Pd sites and that, in turn, led to decline in Pd dispersion from 26.3 to 1.1%. We want to relate now the surface composition of the supported Pd Cu particles, expressed by the fraction of palladium (X Pds ) and of copper (X Cus ) atoms, Table 2 Experimental and calculated data of 2 wt%pd and Cu 2 wt%pd samples. Sample HRSEM CO chemisorption d s D Cu Pd (%) V CO (cm 3 /g) D Pd (%) 2 wt%pd wt%pd.28 wt%cu wt%pd.59 wt%cu wt%pd 1. wt%cu wt%pd 1.8 wt%cu

4 11 U. Matatov-Meytal, M. Sheintuch / Catalysis Communications 1 (9) n Pds X Pds ¼ ðn Pds þ n Cus Þ ¼ 1 X Cus to the bulk composition [21] n Pd X Pd ¼ ðn Pd þ n Cu Þ where n i and n is (i = Pd, Cu) denote the total and surface atoms of each kind. Eq. (8) can be rewritten as n Pd X Pds ¼ ðn Pd þ n Cu Þ ðn Pd þ n Cu Þ ðn Pds þ n Cus Þ n Pds 1 ¼ X Pd D Pd n Pd D Pd Cu ð8þ ð9þ ð1þ where the second factor, (n Pd + n Cu )/(n Pds + n Cus ), is the inverse of the overall metal dispersion (1/D Cu Pd ), that is the ratio between the number of surface atoms to the total number of atoms (Pd and Cu). The third factor, (n Pds /n Pd ), is the palladium dispersion D Pd. The D Cu Pd, could be calculated from D Pd Cu ¼ a d s v Pd X Pd þ v CuX Cu d Pd d Cu ð11þ where a is a constant that characterizes the ratio of surface to bulk atoms and is set here to be a = 3; i.e., semispherical particles. The mean volumes occupied by a metal atom in the solid bulk are m Pd =.147 and m Cu =.1183 nm 3, and the mean surface areas of a metal atom, are d Pd =.793 nm 2 and d Cu =.685 nm 2, respectively []. The data in Fig. 2 show a marked surface impoverishment in palladium, and hence a marked surface enrichment in copper for ACC-supported Pd Cu particles compared with their bulk. This difference is expected as a consequence of preparation procedure used and from thermodynamic consideration. Theoretical predictions attribute the segregation of Cu to its lower surface free energy (1.84 J m 2 at 273 C) compared with that of palladium (2.9 J m 2 at 273 C) and to the difference in the atomic radii of Cu and Pd [21]. Since all the palladium and copper atoms in the catalyst are contained in bimetallic particles, the total number of palladium and copper atoms in one bimetallic particle (n Pd + n Cu ) 1 can be estimated as follows: ðn Pd þ n Cu Þ 1 ¼ pd 3 =½6ðX Pd m Pd þ X Cu m Cu ÞŠ ð12þ while the number of palladium (n Pd ) and copper (n Cu ) atoms follows from the definitions ðn Pd Þ 1 ¼ X Pd ðn Pd þ n Cu Þ 1 ¼ð1 X Cu Þðn Pd þ n Cu Þ 1 Surface composition, (atom/atom) Bulk composition, (atom/atom) Fig. 2. Surface composition vs. bulk composition for Pd Cu/ACC catalysts. Cu Pd ð13þ The total number of surface palladium and surface copper atoms in one bimetallic particle should be estimated as ðn Pds þ n Cus Þ 1 ¼ D Pd-Cu ðn Pd þ n Cu Þ 1 ð14þ while the number of surface palladium (n Pds ) and surface copper (n Cus ) atoms follows from Eq. (7): ðn Cus Þ 1 ¼ X Cus ðn Pds þ n Cus Þ 1 Using these data we can estimate the Cu dispersion (D Cu ) D Cu ¼ðn Cus Þ 1 =ðn Cu Þ 1 ð15þ ð16þ and the surface density of metal atoms (P (Pds + Cus), atom/nm 2 ), that is the number of surface Pd and Cu atoms normalized to the particle surface unit, is P ðpdsþcusþ ¼ðn Pds þ n Cus Þ1=pd 2 s ð17þ These calculated values summarized in Table 3 show that increasing X Cus (from.38 to.86) leads to increasing surface density P (Pds + Cus), indicating the intimate contact between both metals, probably, due to the formation of Pd Cu intermetallic bonds [22]. Simultaneously, increasing X Cus led to a decline of D Cu, indicating the partial blockade of Cu by Cu atoms. Generally two kinds of explanations were given to explain the specific patterns of bimetallic catalysts [1], which were also been adapted to the behavior of Pd Cu systems. The first type of interpretation is geometric (or ensemble) effect. According to this concept, Cu is located on the Pd surface and the role of Cu is the separation and stabilization of Pd Cu ensembles. The site isolation could explain the decreasing activity during nitrate hydrogenation with increasing Cu content on Pd Cu/ACC catalyst (Table 1). The second interpretation is the so-called electronic effect [1] that emphasizes the modification of electronic density of Pd due to a partial positive charge transfer from Cu species and/or the formation of Pd Cu alloys with specific electronic structure. The picture of the electronic and geometrical modifications of Pd Cu systems upon formation of Pd Cu intermetallic bonds using quantum chemical calculations on Pd 1 Cu 12 and Pd 4 Cu 6 clusters was studied in [23], which under the constrained space orbital variation (SCOV) method shows that charge transfer occurs from Cu(4sp) orbital to the Pd(5sp) ones, (Pd(4d)? Pd(5sp) rehybridization), while almost negligible change occurs in the Cu(3d) population. It was found that surface Pd atoms carry on a small but noticeable negative charge. Computations in our group [24] showed that the Pd Cu clusters geometry depends mainly on their composition: i.e., clusters enriched with Pd atoms prefer 3D structures while increasing Cu contents favors planar configurations forming larger clusters. Ensembles of Cu atoms, which form less than two Pd Cu bonds, tend to arrange into local planar islets within bimetallic clusters. These results and the calculations for larger clusters [25] indicate that the formation of planar-shaped copper islets becomes energetically favorable for the ensembles, in which Cu atoms form intermetallic bonds. Therefore, characterization results backed by reported computational ones lead to the conclusion that the selective deposition of Cu atoms on Pd particles provides close contact between the Table 3 Calculated data for copper dispersion and the surface density of metal atoms. Sample X Cus (n Pd + n Cu ) 1 (n Pds + n Cus ) 1 (n Cus ) 1 D Cu (%) 2 wt%pd.28 wt%cu wt%pd.59 wt%cu wt%pd 1. wt%cu.72 41, wt%pd 1.8 wt%cu , P (Pds + Cus) (atom/nm 2 )

5 U. Matatov-Meytal, M. Sheintuch / Catalysis Communications 1 (9) two metals and that forms particles made of Pd core Pd Cu islands on their surface. These results find some resonance in the finding of Radnik et al. [26] that reported the formation of core shell structures in Pd Cu catalysts, supported on titania, that were developed for the gas phase acetoxylation of toluene. The main differences between samples with different amounts of added Cu are attributed to changes in the two metallic phases (Pd Cu bounded atoms and free Pd atoms) over the surface of metallic particles. These differences are reflected in the activity and selectivity of nitrate hydrogenation, as discussed below. The turnover frequency (TOF), i.e., the number of nitrate ions consumed per second per active surface metal (either Pd or Cu, denoted as TOF Pd and TOF Cu ) were calculated as TOF Pd ¼ rm Pd f Pd D Pd TOF Cu ¼ rm Cu f Cu D Cu ð18þ ð19þ where M Pd and M Cu are the atomic weights, and f Cu = m Cu /m cat is the weight fraction of Cu. The overall nitrate disappearance rate r (mmol NO 3 /s g catalyst) was computed from the nitrate conversion-contact time dependence r ¼ðC NO 3 X NO3 ÞF=W cat ðþ The data in Table 4 show that TOF Pd increases with X Cus, while TOF Cu, within experimental error, does not vary significantly. This effect is best explained by the bifunctional nature of the Cu Pd sites present on the surface of these catalysts: it is known that the Cu metal is responsible for the nitrate reduction by red ox reaction, but using Cu alone deactivate rapidly [4]; presence of the Pd metal is required to regenerate Cu by means of activated (chemisorbed) hydrogen [27]. Quantum chemical calculations on Pd Cu clusters suggest that edge and corner sites in Pd crystallites are highly active for hydrogenation [28]. Thus, these sites are probably favorable for deep hydrogenation of nitrite to form ammonium ions more than on terrace sites of the Pd particles. This may explain the observed reaction selectivity trends in Table 1. We should now justify the assumption that CO adsorption on Cu is negligible. A recent study characterizing CO adsorption on various metals calculated by the cluster model at the B3PW91/ RPBE//LANL level of the theory [29] led to adsorption energy of 26 3 kcal/mole on Pd for the on-top position. Use of standard dynamic expressions led to a desorption specific rate constant (k des ) of s 1 and a ratio k ads /k des of adsorption to desorption constants at p =1 5 Pa (i.e., 1 bar) of for indirect adsorption and of for direct adsorption (with sticking coefficient of.5). Either value suggests complete coverage at this pressure and even at much lower pressure. Unpublished data for Cu using both B3PW91 and RPBE functionals led to adsorption energy of 1 to 16 for on-top position (in kcal/mol) compared with 11 to 17 observed in experiments. Using these values the adsorption on Cu (expressed as k ads /k des )isby smaller than that on Pd, justifying the assumption stated above. Table 4 Calculated turnover frequency data. Sample X Cus 1 3 TOF Pd (s 1 ) 1 3 TOF Cu (s 1 ) 2 wt%pd 2 wt%pd.28 wt%cu wt%pd.59 wt%cu wt%pd 1. wt%cu wt%pd 1.8 wt%cu Conclusions First knowledge on the surface composition of a family of Cu- 2 wt%pd/acc bimetallic catalysts prepared by selective deposition method and their remarkable influence on the catalytic behavior in nitrate hydrogenation in water could be obtained from this study. The results of the surface composition determination by the combination of CO chemisorption and HRSEM reveal a specific orientation for the Cu metal deposition on the core Pd crystallites, which surfaces consisted, probably, of Pd Cu bimetallic islets and free Pd atoms. The main differences between samples with different Cu loadings are attributed to changes in these two metallic phases over the surface of metallic particles and they are reflected in the activity and selectivity of nitrate hydrogenation. The results reveal that TOF Cu, i.e., the number of nitrate ions consumed per second per active surface Cu seems to be independent of surface composition, while TOF Pd was found to increase with surface Cu, indicating the bifunctional nature of the Cu Pd sites responsible for the nitrate reduction by red ox reaction. 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