J. Phys. Chem. C 2007, 111, 11619-11626
11619
Preparation of Coimpregnated Cu-Zn/SiO2 Catalysts: Influence of the Drying Step on Metallic Particle Size and on Cu0-ZnII Interactions Ste´ phanie Catillon-Mucherie,† Fatima Ammari,‡ Jean-Marc Krafft,† He´ le` ne Lauron-Pernot,*,† Raymonde Touroude,‡ and Catherine Louis† Laboratoire de Re´ actiVite´ de Surface, UMR 7609 CNRS, UniVersite´ Pierre et Marie Curie-Paris 6, 4 place Jussieu, 75252 Paris Cedex 05, France, and LMSPC, UMR 7515 CNRS, ECPM-UniVersite´ Louis Pasteur, 25 rue Becquerel, 67087 Strasbourg Cedex, France ReceiVed: March 8, 2007; In Final Form: May 7, 2007
Mixed copper-zinc catalysts supported on silica were prepared by incipient wetness impregnation of nitrate salt solution. The influence of the drying temperature on the properties of the final calcined and/or reduced material was investigated. It was shown that, as in the case of monometallic Cu or Zn systems, after a drying step performed at 25 °C, the nitrates are still present as an amorphous compound, whereas the drying step at 90 °C led to the formation of a poorly crystallized hydroxynitrate compound. From XRD and DTG measurements, it was concluded that the hydroxynitrate compound in the mixed systems is a copper-zinc hydroxynitrate [Cu2-x Znx(OH)3(NO3)] in which the two cations are closely associated. After reduction at 350 °C, a “memory effect” of the drying step was observed, and the intermediate formation of the mixed hydroxynitrate led to a better interdispersion of Cu0 and ZnII in the reduced material than when drying was performed at room temperature. This is attested by the higher selectivity to crotyl alcohol in the reaction of hydrogenation of crotonaldehyde. When a calcination step was performed before reduction, the formation of CuO particles partially destroyed the interaction between CuII and ZnII created during the drying step.
I. Introduction Mixed copper-zinc supported catalysts are used in industrial processes for methanol synthesis1,2 and are also potentially interesting/promising in other heterogeneous catalytic reactions as esters hydrogenolysis,3-6 low-temperature water gas shift,7 and methanol steam reforming.8-12 It is generally admitted that the interaction between Cu and Zn plays a role in the development of high catalytic activity for these reactions.13 Whereas the exact nature of the active copper sites for methanol synthesis remains a matter of discussion (metallic Cu14-16 or Cu0-Cu+ sites17), ZnII is known to improve the dispersion of Cu particles18,19 and to create active sites at the Cu-ZnOx interface through the migration of ZnO species onto the copper surface.20-23 In the case of zinc24 or copper25 supported on silica, prepared by incipient wetness impregnation, we have already reported the influence of the drying temperature on the nature of the supported phase after the drying step and after subsequent treatments at higher temperature. In the case of copper/silica,25 it was shown that small metal particles (∼ 30 Å) could be obtained in the reduced sample provided that the drying step was performed at room temperature. When the sample was dried around 100 °C as it is usually the case in most of the preparations, large metal particles were obtained after reduction. In the case of zinc/silica,24 it was shown that the formation of ZnO in the catalyst calcined at 450 °C could be avoided provided that the drying step was performed at a temperature lower than 70-80 °C. ZnO was obtained only when the drying temperature was higher than 90 °C, which allowed the inter* Corresponding author. E-mail:
[email protected]. † UMR 7609 CNRS. ‡ UMR 7515 CNRS.
mediate formation of zinc hydroxynitrate species. However, whatever the drying temperature (25 or 90 °C), the main phase formed after calcination at 450 °C was an amorphous zinc silicate of hemimorphite-type (Zn4Si2O7(OH)2·H2O). The aim of this work was to study the coimpregnation of copper and zinc nitrates over silica and determine the influence of the addition of zinc on the characteristics of Cu/SiO2 catalyst after each step of preparation (drying, calcination, reduction). We wanted to see whether there was evidence of a “memory effect” of the drying temperature on the final properties of the reduced material and to evaluate the influence of the addition of zinc on the properties of metallic copper particles and on the existence of Cu0-Zn2+ interactions after the reduction step. The dried materials were thus characterized by X-ray diffraction, infrared spectroscopy, and thermal analysis. The size of metallic copper particles in the final catalysts, either reduced directly after drying or reduced after an intermediate step of calcination, was investigated by TEM. In order to reveal copper-zinc interactions in the final catalyst, we chose to use the selective hydrogenation of crotonaldehyde as a model reaction as suggested by Abdullah et al.26 The general reaction scheme admitted for this reaction is given in Scheme 1. Crotonaldehyde CH3CHdCH2CHdO belongs to the RCHdCH2CHdO type of R,β unsatured aldehyde family. The first hydrogenation step of crotonaldehyde leads either to but-2-en-1-ol (crotyl alcohol) or to butanal, which is both thermodynamically and kinetically favored.27 Earlier studies reviewed by Claus28 showed that the selectivity to crotyl alcohol may be enhanced when the metallic phase Pt,29 Ru,30 or Co31 was doped by cations (such as Fe2+ or Sn2+) or when the metallic phase Pt32-35 or Co36 was supported on reducible supports (TiO2, CeO2, or ZnO). Ponec et al.37,38 showed that the promotion of the CdO bond hydrogenation
10.1021/jp0718956 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007
11620 J. Phys. Chem. C, Vol. 111, No. 31, 2007 SCHEME 1: Reaction Pathways for the Reaction of Crotonaldehyde Hydrogenation
arises from the creation of mixed metal-cation M10-M22+ sites, which lead to the formation of a Lewis acid-base interaction between the cationic species M22+ and the oxygen atom of the carbonyl group of crotonaldehyde. The selectivity to crotyl alcohol, which forms via the hydrogenation of the CdO bond rather than that of the CdC bond, can be controlled by the promoting effect of the cationic agent. Recently, Rodrigues et al.39 showed the promoting effect of zinc over copper-based catalysts for the hydrogenation of crotonaldehyde to crotyl alcohol, resulting from the formation of Cu0-ZnOx species. As a consequence, we can reasonably expect that the selectivity of the reaction of crotonaldehyde hydrogenation is a good tool to investigate Cu0-Zn2+ interaction in Cu-Zn/oxide catalysts. II. Experimental Section 1. Sample Preparation. The support used was a nonporous silica (Aerosil Degussa 380, 380 m2‚g-1 according to the manufacturer). Samples containing 10 wt % of Zn and 10 wt % of Cu were obtained by incipient wetness impregnation with zinc nitrate (Zn(NO3)2·6H2O Rectapur Prolabo) and with copper nitrate (Cu(NO3)2·3H2O Labosi). A total of 15 mL of an aqueous solution of zinc nitrate (0.5 mol‚L-1) and of copper nitrate (0.5 mol‚L-1) was mixed with 5 g of silica. After a thorough mixing, a blue gel was obtained. The sample was dried in air, either at room temperature or at 90 °C overnight (thereafter referred to as ICuZn25 and ICuZn90). Reference Cu and Zn samples were also prepared according to the same procedure, using an impregnating solution of 0.5 mol‚L-1 of zinc nitrate or copper nitrate, leading respectively to IZn25 and IZn90 and to ICu25 and ICu90 samples. After drying, some samples were calcined under flowing air (100 mL‚min-1) with a temperature ramp of 3 °C‚min-1 up to 450 °C, which was maintained for 1 h. Letter C was added to the sample codes to indicate that they have been calcined (e.g., ICuZn25C). All of the samples containing copper were reduced up to 350 °C under 5% v/v H2 in argon (25 mL‚min-1), with a heating ramp of 7.5 °C‚min-1. In such a case, the letter R was added to the sample code. (e.g., ICuZn25R or ICuZn25CR). Under such conditions of temperature, according to earlier TPR results (not shown here), only copper was reduced and not zinc. 2. Physico-Chemical Techniques. The X-ray diffraction (XRD) patterns were recorded on a Siemens diffractometer (D500) using Cu KR radiation. Phase identifications were performed by comparison with the tabulated Joint Committee on Powder Diffraction Standards (JCPDS) d spacing files. Thermal analyses (DTG) of the samples were obtained on a Seiko TG-DTA 320 module operated by a Seiko SSC5200 diskstation. Samples (20 mg) were heated (5 °C‚min-1) under air flow (100 mL.min-1). The nature of the effluents was analyzed by mass spectrometer (HPR20/DSMS from Hiden Analytical) through the profile of their ions of fragmentation (for water m/z ) 17 (OH+) and 18 (OH2+) and for nitrate ions m/z ) 30 (NO+), 44 (N2O+), and 46 (NO2+)).
Catillon-Mucherie et al. Diffuse reflectance IR (DRIFT) spectra of samples diluted in diamond (80 wt % of diamond powder with an average particle size of 6 µm) were recorded on a Bruker IFS 66 V spectrometer equipped with a DTGS detector in the 3800-600 cm-1 range (resolution 4 cm-1, 512 scans per spectrum). The DRIFT spectra were converted into Kubelka Munk units after subtraction of the spectrum of diamond treated in the same conditions. The samples were treated at 30 °C under flowing Ar (100 mL‚min-1), and the spectra were recorded after 2 h on flow. The average size of metal copper particles in the reduced samples was measured from the micrographs obtained by transmission electron microscopy (TEM, JEOL 100 CXII). The average particle diameter was deduced from the following formula: dm ) ∑nidi/∑ni, where ni is the number of particles of diameter di. The detection limit of the particles diameter was ≈10 Å. 3. Crotonaldehyde Hydrogenation. A crotonaldehyde hydrogenation reaction was performed in a glass flow reactor, operating at atmospheric pressure as described elsewhere.40 Hydrogen was purified from traces of oxygen and water through Pt/Al2O3 catalyst, zeolite, and then MnO traps. A total of 100 µL of crotonaldehyde (Fluka), used as received thus containing 0.1-0.3% butanal, was kept at 0 °C in a saturator. Before catalytic experiments, each catalyst (50-100 mg) was reduced in situ at 350 °C under H2 flow (30 mL‚min-1 for 15 min at RT, heating (8 °C‚min-1), 1 h at 350 °C). Reactions were carried out with a flow rate of crotonaldehyde of 0.247 mol‚s-1 and the appropriate reaction temperature was chosen to maintain the conversion below 7% to limit secondary reactions and over 4% to minimize the errors. The reaction products were analyzed on line by gas-liquid FID chromatography (GLC, with a 30 m long and 0.5461 mm diameter DBWax column (J&W Scientific) heated at 85 °C). The reaction activities (µmol‚s-1‚gCu-1) were calculated according to the following formula: A ) RF/ω, where R is the crotonaldehyde conversion, F, the crotonaldehyde flow rate in mol‚s-1, and ω, the mass of copper in g. The selectivities to the different productsscrotyl alcohol, butanal, butanol, and hydrocarbonsswere expressed as the molar ratio of the selected product over all products formed. In the range of conversion considered in this paper, 4%-7%, only the two primary hydrogenated products, butanal and crotyl alcohol, were detected. Therefore, the selectivity to crotyl alcohol will be only reported in the figures, the complement to 100% being the selectivity to butanal. III. Results 1. Characterization after the Drying Step. 1.1. Color Changes upon Drying. Although the Zn/SiO2 samples are white whatever the drying temperature, the color of the Cu/SiO2 samples is variable. It was shown previously that ICu25 is blue, whereas ICu90 is green-blue,25 and this change was attributed to the formation of copper hydroxynitrate Cu2(OH)3(NO3) upon drying at 90 °C. When copper and zinc are coimpregnated on silica, the same phenomenon is observed: ICuZn25 is pale blue and ICuZn90 is green-blue. 1.2. X-ray Diffraction. As already shown in earlier papers,24,25 no diffraction lines are visible for the ICu25 and IZn25 samples, except the broad band characteristic of amorphous silica (patterns not shown), but during drying at 90 °C, poorly crystalline phases form (Figure 1). Diffraction lines of ICu90 and IZn90 correspond to copper nitrate hydroxide Cu2(OH)3(NO3) and zinc nitrate hydroxide Zn(OH)(NO3), respectively.
Cu0-ZnII Interactions in Coimpregnated Cu-Zn/SiO2
Figure 1. X-ray diffractograms of ICuZn25 and ICuZn90 samples and ICu90 and IZn90 references.
The same effect of the drying temperature is observed for the ICuZn samples: only the broad band characteristic of silica is observed on the ICuZn25 diffractogram (Figure 1), and diffraction lines are observed for the ICuZn90 sample. The two first diffraction lines (2θ ) 12.9° and 25.8°) are similar to those of the Cu2(OH)3(NO3) crystalline structure. The interlayer distance d of about 6.89 Å, calculated from the Bragg law for the (100) plane, is consistent with that of the copper hydroxynitrate, which is equal to 6.91 Å.41 The intensity of the diffraction lines of ICuZn90 relative to that of silica is twice as low as those of ICu90. This seems to indicate that in mixed Cu-Zn catalysts the crystallinity of the copper hydroxynitrate is lowered. On the other hand, it can be noticed that the diffraction lines characteristics of zinc hydroxynitrate Zn(OH)(NO3) observed in the IZn90 system are not present in the mixed ICuZn90 sample. Thus, from the XRD results, it may be concluded that, after drying the coimpregnated Cu-Zn/SiO2 sample at room temperature, the supported phase is still probably a mixture of amorphous nitrates as in the case of ICu25 and IZn25. Drying the mixed system at 90 °C mainly leads to a copper hydroxynitrate phase less crystallized than in the case of ICu90, and zinc phase is not detected. 1.3. DRIFT Spectroscopy. The DRIFT spectra of ICu90, ICuZn25, and ICuZn90 are reported in Figures 2 and 3 for the region corresponding to the νOH vibration bands and to the νNO3 vibrations bands, respectively. In order to interpret these results, Table 1 gathers the expected vibrations of nitrate group depending on its coordination mode,42 and Table 2 gathers published data on zinc and copper hydroxynitrates.43,44 The spectrum obtained for ICuZn25 shows only a tiny νOH vibration band of silanols groups (SiOH) at 3740 cm-1 and a broad band around 3400-3500 cm-1 due to OH stretching vibrations of amorphous silica, and exhibits two strong νNO3 bands at 1510 and 1290 cm-1 (Figure 3). These bands have already been observed on IZn2524 and attributed to the lowering of the D3h symmetry of the NO3- free anion into the C2V symmetry giving rise to the splitting of the O-NO2 stretching band (Table 1). This lowering of symmetry can be due either to the coordination of the nitrate species to the metallic cation or to the adsorption on the silica support. It can be noted that no band characteristic of the hydroxynitrates is visible in the νOH region (Figure 2) or in the NO vibration range (Figure 3). As a consequence, one can propose that the supported phase in
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11621
Figure 2. DRIFT spectra of ICu90, ICuZn25 and ICuZn90 in the νOH region, recorded after 2 h at 30 °C under flowing argon (100 mL‚min-1).
Figure 3. DRIFT spectra of ICu90, ICuZn25 and ICuZn90 in the νNO3 region, recorded after 2 h at 30 °C under flowing argon (100 mL‚min-1).
ICuZn25 consists of amorphous zinc and copper nitrate dispersed on the silica surface. For ICu90, the bands of Cu2(OH)3(NO3), expected from literature data (Table 2), are observed, superimposed with amorphous silica contributions: a band at 3542 cm-1 is clearly seen in the νOH region (Figure 2) and the two bands at 1420 and 1352 cm-1 (Figure 3) can be attributed to the ν4(NO2) and ν1(NO2) vibration of nitrate group. In addition, a broad contribution can be seen around 1370 cm-1, i.e., in a region where the free nitrates give the ν3(NO2) vibration (Table 1). As the transformation of copper nitrate into copper hydroxynitrate during drying at 90 °C may be accompanied by HNO3 evolvement, the DRIFT spectrum of HNO3 impregnated on silica was recorded under comparable conditions (not shown). It shows a broad band centered at 1355 cm-1 and a shoulder at 1480 cm-1. Thus, the DRIFT spectrum of ICu90 can be explained by the formation of Cu2(OH)3(NO3) and additional adsorption of HNO3 on the silica. In the νOH region, the spectrum of ICuZn90 shows the same contribution at 3542 cm-1 as that of ICu90 but less intense, which may correspond to the presence of a quite disordered copper hydroxynitrate. In this region, no noticeable contribution attributable to the zinc hydroxynitrates can be observed, but the formation of Zn(OH)(NO3)·H2O cannot be definitely excluded (Table 2). Compared to the spectrum of ICuZn25, new bands are present in the νNO3 region between 1350 and 1450 cm-1 (Figure 3). They are located in the same region as those
11622 J. Phys. Chem. C, Vol. 111, No. 31, 2007
Catillon-Mucherie et al.
TABLE 1: Expected Vibrations Bands of Nitrate Group Depending on Its Coordination Mode (from ref 42) NO3- in D3h symmetry
a
-O-NO2 in C2V symmetry
type
no.
assignment
frequency (cm-1)
type
no.
assignment
frequency (cm-1)
A′1 A′′2 E′ E′
ν1 ν2 ν3 ν4
NO stretch Out-of-plane NO2 stretch NO2 bend
1050a 831 1390 720
A1 B2 A1 B1 A1 B1
ν2 ν6 ν1 ν4 ν3 ν5
NO stretch out-of-plane NO2 stretch NO2 stretch NO2 bend NO2 bend
1034-970 800-781 1290-1253 1531-1481 ∼739 ∼713
This mode is inactive in Infrared spectroscopy.
TABLE 2: Expected Wavenumbers (cm-1) of the Vibrations Bands of Copper and Zinc Hydroxynitrates in the νOH and Nitrate Vibration Regions Cu2(OH)3(NO3)a
Zn(OH)(NO3)‚H2Ob
Zn3(OH)4(NO3)2b
Zn5(OH)8(NO3)2b
attribution
3544 3424
3542 3487 3297 (broad) 1440 1369
3516 (broad)
3578 3533 3488 (broad) 1491 1345
νOH νOH νOH ν4 (NO2 stretch) ν1 (NO2 stretch)
1424 1340 a
1500 1336
From ref 44; only the strong bands have been reported. b From ref 43; only the strong bands have been reported.
obtained on ICu90 but with different relative intensities. The small band at 1430 cm-1 can be attributed to the ν1(NO2) vibration of the nitrate group localized in a structure similar to that of Cu2(OH)3NO3 (Table 2). The main band at 1366 cm-1 could result from the contribution of the ν4(NO2) vibration of the nitrate group and maybe of that of free nitrate ions generated by HNO 3 adsorption on the surface as in the case of ICu90. Moreover, the two strong νNO3 bands at ≈1510 and ≈1290 cm-1 already observed on ICuZn25 indicate that amorphous metallic nitrates are still present. Hence, according to the DRIFT results, ICuZn90 would contain copper hydroxynitrate, copper or zinc nitrates compounds, but no zinc hydroxynitrate. 1.4. DTG Analysis. The thermal analysis profiles of ICu25, IZn25 and ICuZn25 are reported in Figure 4. Thermal decomposition of ICu25 and of IZn25 occurs in two steps with a first peak around 135 °C, which is accompanied by the evolution of water and nitrous oxide (detected by MS analysis), and a second peak at 280 °C for ICu25 and around 330 °C for IZn25, associated with the evolution of only nitrous oxide. These results obtained on supported monometallic nitrates are in agreement with the data on thermal decomposition of bulk nitrates.45,46 The profile of thermal decomposition of the mixed ICuZn25 sample can be described as the superimposition of those of copper and zinc nitrates with a first peak at 135 °C and a broad one at 315 °C. This result is consistent with the interpretations of the XRD and DRIFT results, i.e., that ICuZn25 consists of a mixture of copper and zinc nitrate supported on silica. Figure 5 reports the results of thermal analysis obtained on the samples dried at 90 °C. The DTG curve obtained for ICu90 strongly differs from that of ICu25 (Figure 4). Indeed, that of ICu90 shows a single thin and intense peak at 215 °C. As the thermal decomposition of bulk copper hydroxynitrate is known to occur in one step under inert45,47 or oxidative atmosphere,48,49 this result confirms the XRD and DRIFT results that ICu90 consists mainly of supported copper hydroxynitrate. The profile of thermal decomposition of IZn90 shows roughly the two same peaks as for IZn25 (around 135 and 300 °C, Figure 4) plus a third one at 195 °C. According to earlier study,24 such a sample contains a mixture of zinc nitrate and zinc hydroxynitrate (Zn(OH)NO3·H2O) after drying at 90 °C. Zn(OH)NO3· H2O is known to decompose in two steps under inert gas50 or vacuum,51 first into Zn3(OH)4(NO3)2 between 60 and 90 °C and then into ZnO between 160 and 190 °C. In our study, the
TABLE 3: TEM Measurements of the Average Size and Size Distribution of the Copper Particles in the Samples Reduced up to 350 °C (R for Samples Reduced after Drying and CR for Samples Calcined before Reduction) “small” particles
sample
drying step temp.
daverage (Å)
size distribution (Å)
ICu25R ICu90R ICuZn25R ICuZn90R ICuZn25CR ICuZn90CR
25 °C 90 °C 25 °C 90 °C 25 °C 90 °C
30 48 34 46 25 34
10-55 10-55 10-55 10-90 10-75 10-75
“large” particles daverage (Å)
size distribution (Å)
330
85-1100
160
100-230
240
100-700
presence of zinc nitrate in the IZn90 sample does not allow us to distinguish the first step of Zn(OH)NO3·H2O decomposition, but the peak at 195 °C can be attributed to the second peak of decomposition of the zinc hydroxynitrate. From the DTG profiles of the ICu90 and IZn90 reference samples, we can now assign the peaks of decomposition of the ICuZn90 system, observed at 135, 240, and 300 °C: • On the basis of the DTG profile of ICu25, IZn25 (Figure 4) and IZn90 (Figure 5), the peaks at 135 and 300 °C can be attributed to the two steps of decomposition of copper and zinc nitrates or also of those of zinc hydroxynitrate supported on silica. However, according to DRIFT spectroscopy and XRD, the sample do not contain zinc hydroxynitrate. • The peak at 240 °C can be attributed to the thermal decomposition of copper hydroxynitrate, already clearly identified by XRD and DRIFTS. The higher temperature of the peak compared to that of ICu90 (215 °C) can be tentatively explained by the presence of zinc in the sample, which delays the decomposition of copper hydroxynitrate by 25 °C. 2. Characterization after the Reduction Step. 2.1. Metal Particle Size in the Reduced ICuZn Samples. After reduction, the ICu and ICuZn samples are black. The TEM results reported in Table 3 clearly show that the size of the metal copper particles in reduced ICu and ICuZn samples depends on the drying temperature. Indeed, ICu25R and ICuZn25R contain only small copper particles, of 30 and 34 Å of average size, respectively. The presence of zinc has therefore no detectable effect on metallic copper particle size. In contrast, the samples dried at 90 °C exhibit a bimodal distribution of size with a set of small particles with an average size of 48 Å for ICu90R and 46 Å for
Cu0-ZnII Interactions in Coimpregnated Cu-Zn/SiO2
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11623
Figure 6. Comparison of the activity (white bars) and selectivity to crotyl alcohol (black bars) of the ICu25R, ICu90R, ICuZn25R, and ICuZn90R samples reduced up to 350 °C.
Figure 4. DTG profiles of ICu25, IZn25 and ICuZn25 samples; the gaseous species desorbed detected by MS are (I) H2O; (II) H2O + NO; (III) NO.
Figure 7. X-ray diffractograms of ICu25C, ICu90C, ICuZn25C, and ICuZn90C samples, calcined at 450 °C.
Figure 5. DTG profiles of ICu90, IZn90 and ICuZn90 samples; the gaseous species desorbed detected by MS are (I) H2O; (II) H2O + NO; (III) NO.
ICuZn90R and a set of large particles with an average size twice as small for ICuZn90R (160 Å) as for ICu90R (330 Å). The size distribution of the large particles in ICuZn90R is also more narrow than in ICu90R. The dependence of the copper particle size with the drying temperature on the Cu-Zn samples is consistent with former results obtained on monometallic copper systems,25 and the presence of Zn in the samples has an influence only in the case of the sample dried at 90 °C; it lowers the average size and size distribution of the large copper metal particles. 2.2. Catalytic ReactiVity in SelectiVe Hydrogenation of Crotonaldehyde. Figure 6 shows the activity of Cu/SiO2 and Cu-Zn/SiO2 catalysts in hydrogenation of crotonaldehyde and the selectivity to crotyl alcohol for a conversion lower than 7% and a temperature of reaction of 80 °C. Two main results can be drawn from these data: • The introduction of Zn on the catalyst has a strong effect on the catalytic properties; it lowers the activity of the catalysts, and increases the selectivity to crotyl alcohol.
• This effect is more pronounced when the sample is dried at 90 °C than for the sample dried at 25 °C. As the samples ICu25R and ICuZn25R possess the same particle sizes and very different catalytic behaviors, it can be deduced that the lowering of activity accompanied by an enhanced selectivity to crotyl alcohol is due to a close interaction between ZnII and copper particles. 3. Influence of the Calcination Step before Reduction. 3.1. Characterization after the Calcination Step. After calcination under air up to 450 °C, the Cu/SiO2 samples are gray-black whatever the drying temperature, and both ICu25C and ICu90C exhibit a diffraction pattern similar to that of bulk CuO (Figure 7). In contrast, the drying temperature of the Cu-Zn/SiO2 samples has drastic consequences on the nature of the copper species obtained after calcination. Whereas ICuZn90C is grayblack and exhibits a diffraction pattern characteristic of CuO as in the case of the Cu/SiO2 samples plus very tiny ZnO peaks, the ICuZn25C sample is green and does not show any diffraction pattern (Figure 7). 3.2. Characterization after Calcination and Reduction. 3.2.1. Size of the Copper Metallic Particles. The X-ray diffraction patterns of the samples reduced after calcination are given in Figure 8. The samples containing only copper show the characteristic lines of metal copper. The same results are obtained with ICuZn90CR with the occurrence of very small lines that can be attributed to ZnO (this is also an indication that the reduction temperature of 350 °C only leads to the reduction of copper). The diffractogram of ICuZn25CR is drastically different as it shows almost no diffraction lines, except the band of amorphous silica.
11624 J. Phys. Chem. C, Vol. 111, No. 31, 2007
Catillon-Mucherie et al. effect of the calcination step is similar: it increases the activity and reduces the selectivity to crotyl alcohol compared to the catalysts reduced after drying. These results may indicate that the interaction between copper and zinc is weaker when a calcination step is performed before reduction. IV. Discussion
Figure 8. X-ray diffractograms of ICu25CR, ICu90CR, ICuZn25CR, and ICuZn90CR samples, calcined at 450 °C then reduced up to 350 °C.
Figure 9. Comparison of the activity (white bars) and selectivity to crotyl alcohol (black bars) of the samples directly reduced up to 350 °C to those calcined at 450 °C then reduced up to 350 °C. (a) ICuZn25R and ICuZn25CR at a reaction temperature of 80 °C. (b) ICu90R and ICuZn90CR at a reaction temperature of 100 °C.
The TEM results (Table 3) show that ICuZn25CR contains only small particles, whereas ICuZn90CR presents a bimodal distribution of particle sizes. These results are roughly the same as those obtained when the ICuZn samples are directly reduced. As in the case of Cu/SiO2,25 the XRD and TEM results show that the drying temperature of the Cu-Zn/SiO2 samples has drastic consequences on the size of the copper particles when the samples are calcined before reduction. 3.2.2. Catalytic Reactivity in Selective Hydrogenation of Crotonaldehyde. The catalytic properties of the ICuZn samples calcined then reduced were compared to those directly reduced after drying. In order to limit the crotonaldehyde conversion to value lower than 7%, the temperature of the reaction had to be adjusted. The reactivity of the samples dried at 25 °C was studied at 80 °C (Figure 9a), whereas that of the samples dried at 90 °C was studied at 100 °C (Figure 9b). In both cases, the
1. Influence of the Drying Conditions on the Nature of the Copper-Zinc Species. The X-ray diffractograms (Figure 1), DRIFT spectra (Figures 2 and 3), and thermal analysis profiles (Figures 4 and 5) obtained on Cu-Zn/SiO2 dried at 25 and 90 °C show the influence of the drying temperature on the nature of the species formed during this step. The comparison of the results obtained on ICuZn25 to those obtained on ICu25 and IZn25 that contain copper and zinc nitrate, respectively, leads to the conclusion that ICuZn25 consists of amorphous copper and zinc nitrate dispersed on the surface of silica: • The pale blue color of ICuZn25 is similar to that of ICu25; • No X-ray lines are visible as in IZn25 and ICu25; • The DRIFT spectrum is characteristic of metallic nitrates; • The thermal decomposition profile can be described as the contribution of the profiles of the monometallic systems IZn25 and ICu25. Thus, drying the Cu-Zn/SiO2 mixed sample at room temperature does not lead to chemical transformations of the precursors: zinc and copper nitrates. The same characterization techniques applied to ICuZn90 show that the samples contains a poorly crystallized phase of copper hydroxynitrate. This is attested by the fact that: • The color of the sample is green-blue as in the case of copper hydroxynitrate in ICu90; • The DRIFT spectrum shows the vibration bands expected for supported copper hydroxynitrate; • The XRD pattern and the DTG peak at 240 °C are close to those obtained for ICu90, which also contain copper hydroxynitrate. In addition, ICuZn90 contains nitrates as attested by the DRIFT spectrum. These nitrates can be those of zinc (DTG peaks around 135 and 300 °C) and maybe also those of residual copper nitrate (one cannot exclude that DTG peaks of copper nitrate are hidden by those of zinc nitrate). No zinc hydroxynitrate is detected by DRIFTS. However some differences in the characteristics of copper species in ICuZn90 and in ICu90 must be outlined. The diffractogram of the mixed Cu-Zn/SiO2 dried at 90 °C shows the presence of a crystalline phase with a lamellar structure similar to that of the copper hydroxynitrate, with a similar interlayer distance, but if one carefully examines the diffraction lines between 30 and 50° (Figure 1), they correspond neither in position nor in intensity to those observed for ICu90. Moreover, the temperature of the DTG peak assigned to copper hydroxynitrate (240 °C) is 25 °C higher than that of ICu90 (215 °C). We may infer that these features rather correspond to that of a mixed copper-zinc hydroxynitrate formed upon drying at 90 °C. It could result from the modification of the structure of copper hydroxynitrate due to the presence of zinc into the (120) and (121) planes of the lamellar layer. This hypothesis is based on the examination of the literature data concerning this type of compound. Copper-zinc hydroxynitrates are not listed in the JCPDS files, but three papers directly related to this topic could be found.48,52 It appears that coprecipitation of copper and zinc nitrates with NaOH leads to a mixed copper-zinc hydroxynitrate single phase with the same
Cu0-ZnII Interactions in Coimpregnated Cu-Zn/SiO2 type of structure as Cu2(OH)3(NO3). For example, Mannoorettonnil et al.48 followed the incorporation of Zn into the copper hydroxynitrate. On this basis of chemical analysis and thermogravimetryresults,theyproposedtheformationofCu2-xZnx(OH)3(NO3) (with 0 < x < 0.5) and showed that the presence of zinc in the structure of copper hydroxynitrate led to the increase of the decomposition temperature up to 20 °C. So, our interpretation is that in co-impregnated Cu-Zn/SiO2 catalyst, the crystalline structure formed during the drying at 90 °C is a mixed Cu-Zn hydroxynitrate and, in view of the thermal analysis that zinc stabilizes the mixed lamellar structure. Thus in line with the results of Manoorettonil et al.48 described above, we may propose the insertion of Zn2+ in the host structure of copper hydroxynitrate. The zinc, which is not involved into the mixed Cu,Zn compound, would remain on the support surface as zinc nitrate species (amorphous). As mentioned above, we cannot rule out the possible presence of copper nitrate or that of zinc hydroxynitrate. 2. Influence of the Drying Conditions on the Size of Copper Metal Particles and on Cu-Zn Interactions after Reduction. 2.1. ICuZn Sample Reduced Directly after Drying. The ICu and ICuZn samples reduced directly after drying can be classified as follows: (i) those dried at 25 °C, which give small metal particles after reduction, whether they contain zinc or not; (ii) those dried at 90 °C, which may give small and large particles of copper after reduction, whose average size and size distribution depend on the presence of zinc (Table 3). As it was shown by Toupance et al.,25 the coexistence of small and large metal particles was proposed to arise from the reduction of amorphous copper nitrate and copper hydroxynitrate, respectively. Hence, as already observed several times in other systems,53,54 the metal particle size is directly dependent of the nature of the precursor present on the support before reduction. For the sample containing zinc, the reduction of the sample dried at 90 °C (ICuZn90) also leads to small and large particles, but with a narrower distribution of the large particles (100-230 Å) than in ICu90 (85-1100 Å; Table 3). So, one can deduce that the formation of the mixed copper-zinc hydroxynitrate during the drying step at 90 °C is responsible for the better dispersion of copper metal particles after reduction. It is proposed that the insertion of Zn2+ ions into the structure of the copper hydroxynitrate leads to the “fragmentation” of the copper precursor and to the formation of smaller copper particles during the reduction. Concerning the existence of copper-zinc interactions, the activities in the reaction of crotonaldehyde hydrogenation show (Figure 6) that Zn inhibits the formation of butanal and, to a lesser extent, that of crotyl alcohol. This behavior cannot be attributed to a modification of particle size since in ICu25R and ICuZn25R the Cu particles have the same size. Moreover, in the case of the zinc-free samples, ICu90R to ICu25R, the activity of ICu90R is lower due to the larger copper particle sizes, but the lower activity is not accompanied by an increase of selectivity to crotyl alcohol. Thus, the selective inhibition of butanal formation in the mixed system can be attributed to copper-zinc interactions. Of course the exact nature of this interaction (partial coverage of copper by oxidized zinc species or pure electronic interaction as discussed by Rodrigues et al.39) cannot be drawn from these data and would require further characterization of the final catalyst in the metallic state. We also observe that the existence of copper-zinc interactions depends on the drying temperature and, therefore, on the nature of the species presents after drying. Indeed, the selective inhibition of butanal production is more pronounced for the Cu-
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11625 Zn catalyst dried at 90 °C, containing mixed copper-zinc hydroxynitrate than for the sample dried at 25 °C, containing copper and zinc nitrates dispersed on silica. The formation of the mixed copper-zinc hydroxynitrate after drying at 90 °C, leads to an interdispersion between the copper and the zinc in the catalyst at the reduced state. Thus, one can posit that there is a “memory effect” of the proximity of the copper and zinc into the copper hydroxynitrate formed during the drying step at 90 °C, on copper-zinc interactions in the reduced catalyst. 2.2. ICuZn Sample after Calcination. The “memory effect” of the drying step and the influence of the addition of zinc are also observed after the calcination. If ICuZn90C is very similar to ICu90C in terms of XRD pattern and metallic particles size, ICuZn25C is very different from ICuZn90C and from ICu25C (Figure 7): the supported phase is amorphous in ICuZn25C while the other two samples exhibit the characteristic lines of CuO, and it shows a green color very different from the others, which are gray black. Such a color has been already observed by Van der Grift et al.54 for Cu/SiO2 samples prepared by ion-exchange or homogeneous deposition-precipitation, followed by drying at 130 °C and calcination at 430 °C. On the basis of TPR experiments performed on dried and calcined samples and by comparison with TPR profiles of bulk copper hydroxysilicates, Van der Grift et al. proposed the formation of a chrysocolla-like structure (Cu8(OH)12(Si4O10)2. 8H2O) during the drying step of the samples, which decomposes upon calcination into copper oxide of tenorite-like structure, according to another study performed by Van Oosterwyck-Gastuche,55 dispersed over the silica support. From our characterizations (olive-green color and no diffraction lines in XRD), we anticipate that copper in ICuZn25C sample has an environment similar to that in the samples prepared by Van der Grift et al. So, the effect of zinc when the Cu-Zn sample is dried at room temperature then calcined is to favor the dispersion of CuII ions on the silica, even if our results do not show evidence of the formation of chrysocolla-like structure. There is no evidence for an influence of zinc on the size of CuO particles in ICu-Zn/90 after calcination, as the XRD peaks of CuO in ICuZn90C are very similar in intensity and broadness to those of ICu90C (Figure 7). Very small ZnO peaks are also visible on the ICuZn90C diffractogram as in the case of IZn90C.24 In our earlier study,24 we showed that the formation of ZnO arose from the decomposition of zinc hydroxynitrate. The tiny peaks of ZnO in ICuZn90C may be an indirect proof for the presence of zinc hydroxynitrate in the dried sample, even though zinc hydroxynitrate was not detected by IR and XRD. Moreover, in the same earlier study,24 a XAFS study combined to IR showed that in IZn90C, the zinc nitrate present as the main zinc compound after drying transformed into zinc silicate of hemimorphite-type (Zn4Si2O7(OH)2·H2O) due to a reaction with silica during calcination. Such a compound may exist also on ICuZn90CR because zinc nitrate is still present on ICuZn90. 2.3. ICuZn Sample Reduced after Calcination. The effect of the drying temperature and of the presence of zinc is also maintained after reduction when calcination is performed in between: smaller particles are obtained when the drying step is performed at room temperature and when zinc is associated to copper. However, from the catalytic results (Figure 9), it seems that the calcination step decreases the interactions between copper and zinc that are created during drying: the catalysts are less selective to crotyl alcohol and more active than when the samples are directly reduced (Figure 9, panels a and b); they show a catalytic behavior close to that of Cu/SiO2 catalysts.
11626 J. Phys. Chem. C, Vol. 111, No. 31, 2007 It may be inferred that the calcination step, which leads to the formation of particles of CuO, which are small in ICuZn25C, limits the interaction between CuII and ZnII that exist after drying when the cations are dispersed on the silica as amorphous nitrates or as mixed hydroxynitrate. Conclusion The characterization study of the Cu-Zn/SiO2 samples prepared by co-impregnation of the metallic nitrates, performed after drying, calcination and reduction step leads to the following conclusions: • Drying the samples at room temperature leads to a mixture of amorphous metallic nitrates on SiO2, whereas a drying at 90 °C leads to the formation of a mixed hydroxynitrate Cu2-2xZn2x(OH)3(NO3). • The samples reduced directly after drying keep the memory of the drying step temperature. Small copper particles are obtained after the sample has been dried at room temperature whereas, as already observed for pure copper samples, the formation of an intermediate hydroxynitrate upon drying at 90 °C gives a population of larger particles after reduction. • The interactions between copper and zinc, evaluated on the basis of catalytic performances in crotonaldehyde hydrogenation, are favored by the formation of the mixed hydroxynitrate as if the intimate association between the two metals created in the hydroxynitrate was maintained after reduction. • These effects are less pronounced when the samples are calcined before reduction as if calcination, leading to CuO particles, limits the proximity between copper and zinc created upon drying. • The exact nature of these interactions cannot be described on the basis of these data, but would require further work. References and Notes (1) Klier, K. AdV. Catal. 1982, 31, 243. (2) Waugh, K. C. Catal. Today 1992, 31, 243. (3) Brands, D. S.; Poels, E. K.; Krieger, A. A.; Makarova, O. V.; Weber, C.; Veer, S.; Bliek, A. Catal. Lett. 1996, 36, 175. (4) Brands, D. S.; Poels, E. K.; Bliek, A. Appl. Catal. A 1999, 184, 279. (5) Brands, D. S.; U-A-Sai, G.; Poels, E. K.; Bliek, A. J. Catal. 1999, 186, 169. (6) Poels, E. K.; Brands, D. S. Appl. Catal. A 2000, 191, 83. (7) Chinchen, G. C.; Spencer, M. S.; Waugh, K. C.; Whan, D. A. J. Phys. Chem., Farafay Trans. 1987, 83, 2193. (8) Agrell, J.; Birgersson, H.; Boutonnet, M. J. Power Sources 2002, 106, 249. (9) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl. Catal. A 1999, 179, 21. (10) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl. Catal. A 1999, 179, 31. (11) Zhang, X. R.; Shi, P.; Zhao, J.; Zhao, M.; Liu, C. Fuel Proc. Technol. 2003, 83, 183. (12) Matter, P. H.; Ozkan, U. S. J. Catal. 2005, 234, 463. (13) Van den Berg, M. W. E.; Polarzb, S.; Tkachenkoa, O. P.; Klementieva, K. V.; Bandyopadhyayc, M.; Khodeira, L.; Giesc, H.; Muhlera, M.; Gru¨nerta, W. J. Catal. 2006, 241, 446.
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