Formation and Characterization of PdZn Alloy: A Very Selective

ARTICLE pubs.acs.org/JPCC

Formation and Characterization of PdZn Alloy: A Very Selective Catalyst for Alkyne Semihydrogenation Min Wei Tew,† Herman Emerich,‡ and Jeroen A. van Bokhoven*,†,§ †

ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland SNBL/ESRF BP 220F-38043 Grenoble Cedex, France § Paul Scherrer Institute, 5232 Villigen, Switzerland ‡

bS Supporting Information ABSTRACT: The formation of a PdZn alloy from a 4.3% Pd/ZnO catalyst was characterized by combined in situ high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy (XAS). Alloy formation started already at around 100 °C, likely at the surface, and reached the bulk with increasing temperature. The structure of the catalyst was close to the bulk value of a 1:1 PdZn alloy with a L1o structure (RPdPd = 2.9 Å, RPdZn = 2.6 Å, CNPdZn = 8, CNPdPd = 4) after reduction at 300 °C and above. The activity of the gas-phase hydrogenation of 1-pentyne decreased with the formation of the PdZn alloy. In contrast to Pd/SiO2, no full hydrogenation occurred over Pd/ZnO. Over time, only slight decomposition of the alloy occurred under reaction conditions.

1. INTRODUCTION In recent years, the potential of a PdZn alloy to replace conventional catalysts in several important catalytic reactions has been explored. Improved thermal stability and selectivity to the desired products were reported, for example, for the autothermal reforming of dimethyl ether,1 methanol steam reforming,2,3 the watergas shift reaction,4 and the hydrogenation of alkenes and alkadienes.5 X-ray diffraction showed that the formation of a PdZn alloy from Pd/ZnO starts at about 200 °C and increases with temperature.2 It was postulated that during reduction H2 spills over from the Pd metal, thus reducing ZnO and forming the PdZn intermetallic phase. It was proposed that the Zn metal, which decorates the Pd particles, significantly dilutes the available palladium sites and, consequently, diminishes the number of active sites on the surface. Palladium supported on Al2O3 or SiO2 is a poor catalyst for the hydrogenation of esters, in contrast to the PdZn alloy, which reacted at much lower hydrogen pressure than that required in conventional catalysis.6 Hydrogenation of crotonaldehyde over a PdZn alloy led to similar amounts of crotyl alcohol and butyraldehyde, while only little crotyl alcohol was produced over Pd/Al2O3 and Pd/SiO2.7 During the hydrogenation of acetonitrile, the PdZn alloy showed 99% selectivity to ethylamine compared to less than 5% for the palladium black.8 Higher activity of the hydrogenation of isoprene9 and esters of carboxylic acids over PdZn alloy compared to monometallic palladium was also reported.6,10 During the hydrogenation of alkynes, the desired hydrogenation to alkene occurs; there is a small extent of hydrogenation to alkanes. Alkynes must be removed from a feed of alkenes to avoid poisoning of the polymerization catalyst.11 Oxide-supported r 2011 American Chemical Society

palladium is, so far, the most commonly used catalyst. Although one of the best, palladium is not an ideal catalyst because of its limited selectivity and long-term stability. Thus, the catalyst is selectively poisoned through, for example, the addition of lead to palladium supported on calcium carbonate (Lindlar catalyst) or cofeeding the carbon monoxide during the reaction to avoid overhydrogenation.12,13 Palladium provides spacious active sites for side reactions such as oligomerization. Restricting the size of the active sites in a palladium-containing hydrogenation catalyst has proven to be effective in increasing selectivity and the long-term stability of the catalyst.14,15 This is achievable by using an alloy-based catalyst. Compared to a monometallic palladium catalyst, palladiumbased alloy catalysts do not generally form a hydride phase such as β-PdH.16 Isolation of surface sites through alloying changes the electronic and geometric structure of the active sites. This characteristic is of interest in the petroleum refineries and polymer industries because it is often associated with various undesirable side reactions such as total hydrogenation1720 and the formation of oligomers, which occur during the hydrogenation of alkynes.21,22 Previous researchers attributed the suppression of total hydrogenation of 1,3-butadiene to the very low tendency of hydrogen chemisorption of the PdZn alloy.5 The PdZn alloy may, therefore, also be very selective in catalyzing the hydrogenation of alkynes.

Received: October 28, 2010 Revised: February 9, 2011 Published: April 12, 2011 8457

dx.doi.org/10.1021/jp1103164 | J. Phys. Chem. C 2011, 115, 8457–8465

The Journal of Physical Chemistry C

ARTICLE

Here we report the formation the PdZn alloy from Pd/ZnO, and the structural properties of the PdZn alloy established by means of X-ray absorption spectroscopy (XAS) and highresolution X-ray diffraction (HRXRD). The catalytic performance of the PdZn alloy in the gas-phase hydrogenation of 1-pentyne was explored by gas-phase analysis. XAS was also employed to study the stability of the catalyst structure under reaction conditions.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Catalysts. Pd/ZnO was prepared by incipient-wetness impregnation. An amount of 10 g of ZnO (Nanoscale, Kansas, 65 m2/g) was impregnated with 3.0 g of 14% Pd(NO3)2 (Johnson Matthey) in 1 g of water. The catalyst was dried overnight, calcined at 250 °C for five h, and reduced at 300 °C for 30 min and then cooled in pure H2. 2.2. Characterization of the Catalyst. The palladium weight loading was determined by inductively coupled plasma mass spectrometry. The particle size distribution of the as-synthesized catalyst was characterized using scanning transmission electron microscopy (STEM), performed on a Tecnai F30 microscope by field emission cathode (FEI) operated at 300 kV. The material was dispersed in ethanol, and a few drops were placed on a perforated carbon foil supported on a copper grid. STEM images were recorded with a high-angle annular dark field (HAADF) detector to obtain images with atomic number (Z) contrast.23 The metal particles were thus revealed as bright spots. Typically, STEM images of 10 regions of a catalyst were imaged for analysis of the particle size. 2.3. Gas-Phase Hydrogenation. Catalytic experiments were performed in a continuous-flow, fixed-bed tubular quartz reactor. The as-synthesized Pd/ZnO was diluted 100 times with ZnO, which was previously pretreated in the same way as the catalysts. In each experiment, sieved fractions (45 < x < 60 mesh) of the diluted catalyst (0.05 g) were used. Prior to the reaction, the catalyst was reduced in situ to 150, 200, 300, or 400 °C at a rate of 2 °C/min and left for 1 h in a flow of pure hydrogen (30 mL/min), referred to as PdZnO150, PdZnO200, PdZnO300, and PdZnO400, respectively. The Pd/ZnO was then cooled to the desired reaction temperature. 1-Pentyne (Fluka, 99% purity) was introduced into the system in a stream of helium in a saturator at 0 °C. The 1-pentyne/helium flow was mixed with hydrogen before reaching the catalyst bed. The H2/1-pentyne ratio was 4. The reaction products were analyzed with an Agilent 6890A gas chromatograph (GC) equipped with a flame ionization detector and an HP-5 (50 m 0.32 mm) column. The activity and selectivity of the reaction were determined as a function of temperature. The temperature of the reactor was changed as follows: 45 °C f 50 °C f 60 °C f 70 °C f 75 °C f 80 °C f 90 °C f 100 °C f80 °C f60 °C f50 °C. Data were collected at each temperature. The reactions were performed at a constant reaction temperature of 45, 75, or 100 °C. During the reactions, the first point was measured after an induction period of 50 min to achieve a stable performance. Blank experiments with ZnO reduced to 150, 200, 300, and 400 °C showed no reactivity. On the basis of XAS results, the sample reduced at 100 °C closely resembled that of metallic palladium (see below); however, we cannot exclude the presence of undetected amounts of reduced zinc at the surface. Thus, we have added catalytic data of hydrogenation of 1-pentyne over a 1.9% Pd/SiO2 of 2.8 nm,24 performed at 75 °C under identical conditions for

Figure 1. Temperature-programmed reduction scheme used to activate the catalyst.

comparison. The Pd/SiO2 catalyst was diluted 100 times with silica. Sieved fractions (45 < x < 60 mesh) of the diluted Pd/ SiO2(0.05 g) were used, and the catalyst was reduced at 150 °C prior to the reaction. The catalyst was labeled as Pd/SiO2150 here and after. The conversion (X) and selectivity (S) of the products were calculated according to the following equations X ¼

moltotal molproduct 100% moltotal

SproductA ¼

AproductA 3 WproductA 100% molall products

ðiÞ ðiiÞ

where Ai is the GC peak area and Wi the calibrated mole coefficient. Wi was 1 for 1-pentene, trans-2-pentene, cis-2-pentene, and pentane but 0.5 for the oligomers, assuming that the oligomers consist mainly of hydrocarbons with 10 carbon atoms (C10), as suggested by the retention time in the GC. 2.4. In Situ X-ray Absorption Spectroscopy and HighResolution X-ray Diffraction. The changing structure of the Pd/ZnO during high-temperature reduction was characterized by combined in situ HRXRD and XAS at the Swiss-Norwegian Beamline (SNBL, BM01B) of the European Synchrotron Radiation Facility (Grenoble, France). A double-crystal Si(111) monochromator was used for XAS experiments and a Si(111) channel cut monochromator for HRXRD experiments. XRD patterns were recorded by a robust two-circle diffractometer, with each circle having a high-precision encoder mounted on the rotary axis. The diffractometer is equipped with six counting channels, which enable the simultaneous collection of six patterns with a small offset of 1.1° in 2θ for fast data collection. A Si-111 analyzer crystal is mounted in front of each detector (NaI scintillation counter), resulting in an intrinsic resolution (fwhm) of approximately 0.01° at a wavelength of 0.5 Å. A batch reactor and a plugflow cell were used.25,26 The results show that there are no fundamental differences and are, therefore, presented without further details. Figure 1 shows the reaction scheme, on which the high-temperature reduction of the Pd/ZnO is based. The as-synthesized Pd/ ZnO was heated at 5 °C/min in 4% H2/He to 100 °C and then in helium to produce PdZnO100-He. Thereafter, the system was switched back to 4% H2/He, and the catalyst was heated in steps of 5 °C/min up to 350 °C to form the alloy. During the reduction, 8458

dx.doi.org/10.1021/jp1103164 |J. Phys. Chem. C 2011, 115, 8457–8465

The Journal of Physical Chemistry C diffraction patterns were collected at 35, 100, 250, 300, and 350 °C. Transmission X-ray absorption spectroscopy at the Pd K edge was performed during the same experiment as the collection of the powder diffraction patterns. EXAFS patterns were recorded using ionization chambers as detectors, after a dwell time of 15 min at each reduction temperature. Collection of an EXAFS spectrum took about 30 min and of an XANES spectrum about 10 min. Transmission X-ray absorption spectroscopy at the Pd K edge was also performed at the Dutch-Belgian Beamline (DUBBLE, BM26) of the European Synchrotron Radiation Facilities with a double-crystal Si(111) monochromator. In the plug-flow cell, the Pd/ZnO was reduced in hydrogen at 400 °C at 2 °C/min, left for an hour, and cooled to a reaction temperature of 45 °C. An amount of 3 mg of pure Pd/ZnO catalyst was used, a compromise to achieve reasonable conversion and goodquality XAS data. XANES spectra were taken during the reduction, and three EXAFS spectra were taken at constant temperature before introducing 1-pentyne into the system. Thereafter, the products were analyzed by gas chromatography. Three EXAFS measurements were taken after stable conversion was achieved.

Figure 2. STEM images of (a) as-synthesized Pd/ZnO and (b) Pd/ ZnO after reduction in pure hydrogen at 350 °C. The black arrows indicate the possible PdZn alloy formed.

ARTICLE

The data were normalized and analyzed according to the standard procedures of the XDAP software package (version 2.3).27 Experimentally calibrated theoretical references, obtained with the FEFF8 code, were used to obtain S02, F, and δ.28 A Pd foil was the PdPd reference. A PdZn reference file was generated with FEFF8 with interatomic distances taken from the literature.29,30 The coordination parameters were obtained by a least-squares fit in r space, k2-weighted Fourier transforms.

3. RESULTS The palladium loading of the Pd/ZnO was 4.3 wt %. The STEM image of the as-synthesized Pd/ZnO (Figure 2(a)) showed an evenly distributed particle size from 1 to 3 nm. According to the TEM images, the average particle size of PdZn, indicated by the black arrows in Figure 2(b), after reduction of high temperature had grown to around 67 nm. However, the sample measured was already exposed to air, and thus the metal could have become oxidized and may not represent the real particle size under reaction conditions. 3.1. Hydrogenation of 1-Pentyne. 3.1.1. Activity and Selectivity as a Function of Temperature. Figure 3(a) shows the

conversion of 1-pentyne at different temperatures over Pd/ ZnO, previously reduced at 150, 200, 300, and 400 °C. Overall conversion increased with temperature. For PdZnO300 and PdZnO400, the overall conversion was similar and nearly twice as low as that of PdZnO200 and three times lower than PdZnO150. Compared to heating, conversion was lower during cooling at the same reaction temperatures. The differences were greater for PdZnO150 and PdZnO200 than for PdZnO300 and PdZnO400. Figures 3(b)(d) list the corresponding product selectivity. All four catalysts had similar product selectivity, reaching a maximum of 85% selectivity to pentenes. Full hydrogenation was totally suppressed in any case. During cooling, the product selectivity was more or less similar to that during heating. The lower conversion observed during cooling might be due to coke formation. Figure S1(a) (Supporting Information) compares the conversion of 1-pentyne at different temperatures over

Figure 3. (a) Conversion of 1-pentyne over Pd/ZnO reduced at different temperatures, as a function of temperature: PdZnO150 (red [), PdZnO200 (yellow 9), PdZnO300 (blue 2), and PdZnO400 (green b). Corresponding selectivity to (b) pentenes, and (c) pentane, and (d) oligomers. The dashed lines represent the cooling curves. 8459



ARTICLE

Figure 4. Conversion of 1-pentyne over Pd/ZnO, reduced at different temperatures under differential conditions ((a), (e), and (i)): PdZnO150 (red [), PdZnO200 (yellow 9), PdZnO300 (blue 2), and PdZnO400 (green b). Corresponding pentene selectivity at (b) 45 °C, (f) 75 °C, and (j) 100 °C. Corresponding pentane selectivity at (c) 45 °C, (g) 75 °C, and (k) 100 °C. Corresponding oligomer selectivity at (d) 45 °C, (h) 75 °C, and (l) 100 °C.

PdZnO150 and Pd/SiO2150. At increasing temperature, the conversion of the two catalysts was similar at below 70 °C. At above 70 °C, the conversion of Pd/SiO2150 was double or even nearly triple that of PdZnO150. The conversion of the two catalysts during cooling was rather similar and lower than that during heating. Figures S1 (b), (c) and (d) (Supporting Information) show the corresponding selectivity to pentenes, pentane, and oligomers. The selectivity to pentenes of PdZnO150 was constantly higher than that of Pd/SiO2150 during increasing temperature. During cooling the selectivity to pentenes of PdZnO150 remained similar to that during heating, while that of Pd/SiO2150 appeared to be rather scattered. In comparison to PdZnO150, which did not show full hydrogenation, Pd/SiO2150 showed selectivity to pentane in between 5 and nearly 20%. The selectivity to oligomers was similar for both catalysts up to 90 °C.

3.1.2. Activity and Selectivity at Low Conversion. Catalyst performance of PdZnO150, PdZnO200, PdZnO300, and PdZnO400 was investigated under differential conditions at 45, 75, and 100 °C. Pentane did not form in any of the experiments (Figure 4 (c), (g), and (k)). Figure 4(a) shows that, at a reaction temperature of 45 °C, the initial conversion was 4, 4, 3, and 2% for PdZnO150, PdZnO200, PdZnO300, and PdZnO400, respectively. After 100 min, the conversion of PdZnO200 decreased, while the conversion of the others remained nearly constant. Although the data showed significant scatter, the selectivity of PdZnO300 and PdZnO400 to pentenes (Figure 4 (b)) increased with time and surpassed that of PdZnO150 and PdZnO200 after 150 min. The reverse was true for the selectivity to oligomers (Figure 4(d)). At 75 °C, the conversion for PdZnO150 and PdZnO200 decreased, while those of PdZnO300 8460



ARTICLE

Figure 5. In situ XRD patterns of the sample during and after reduction in a plug-flow cell: ZnO (9), PdZn (1), and aluminum window (b). (a) RT, (b) 100 °C, (c) 250 °C, (d) 300 °C, and (e) 350 °C.

and PdZnO400 increased over time (Figure 4(e)). Throughout the reaction period, the conversion was higher for PdZnO150 and PdZnO200 than for PdZnO300 and PdZnO400. PdZnO150 and PdZnO200 showed constantly higher selectivity to pentene than PdZnO300 and PdZnO400 (Figure 4(f)). Figure 4(i) gives the conversion of the catalysts at 100 °C. The initial conversion of all samples decreased before stabilizing at around 100 min. For PdZnO300 and PdZnO400, such a trend was opposite to that observed at 45 and 75 °C. The corresponding product selectivity is shown in Figure 4(j) and (l). PdZnO150 and PdZnO200 showed consistently higher selectivity to pentene than PdZnO300 and PdZnO400. Figure S2(a) (Supporting Information) compares the conversion of 1-pentyne at 75 °C over PdZnO150 and Pd/SiO2150. The conversion was around 12% for Pd/SiO2150 and 6% for PdZnO150. Figures S2(b), (c) and (d) (Supporting Information) show the corresponding selectivity to pentenes, pentane, and oligomers. Pd/SiO2150 was around 4% selective to pentane, compared to 0% of PdZnO150. PdZnO150 showed 24% higher selectivity to oligomers than Pd/SiO2150. 3.2. Alloy Formation: XRD. Figures 5(a)(e) show the in situ XRD patterns collected under different conditions. The support was characterized with three reflections ((100), (002), and (101)) at 2θ = 31.6°, 34.3°, and 36.1°.31 The reflection at 2θ = 38.4° originated from the aluminum window of the plug-flow cell. The presence of the PdZn alloy was revealed by the appearance of two reflections ((111) and (200)) at 2θ = 41.2° and 44.1°.32 They were first observed at 250 °C (Figure 5(c)) as a very broad band centered at 2θ = 41.2°. There were no reflection bands of the palladium metal at 2θ = 40.2°. The reflections of the alloy were sharper at 2θ = 41.2° and 44.1° when the temperature was increased to 350 °C (Figure 5(e)), indicative of particle growth and supporting the TEM results. 3.3. Alloy Formation: XAS. Figure 6(a) compares the XANES spectra of the catalyst collected during reduction to the spectrum of the palladium foil (Figure 6a (dashed black)). After reduction in 4% H2/He at 100 °C and switching to helium, the spectrum of PdZnO100He (Figure 6a (solid black)) showed an absorption edge at around 24 350 eV, with near-edge features with a maximum peak at 24 369, 24 391, and 24 431 eV. The shape and the edge position resembled those of the palladium foil and were characteristic of metallic palladium.33 As the temperature increased to 250 °C (Figure 6a, (red)), 300 °C (Figure 6a (blue)), and 350 °C (Figure 6a (green)), the maxima at

Figure 6. (a) Pd K edge XANES spectra of Pd/ZnO before and after reduction at high temperature. k2-weighted (b) chi function and corresponding Fourier transforms (c). PdZnO100He in helium at 100 °C after reduction at 100 °C (solid black). In 4% H2/He: Pd/ZnO at 250 °C (red), at 300 °C (blue), and at 350 °C (green). Pd K edge XANES of foil (dashed black) is shown for comparison.

24 369 eV became narrower, while the others shifted slightly to lower energy, which suggests structural change from palladium to PdZn. The overall amplitude of the oscillations decreased. At 350 °C the spectrum (Figure 6a (green)) no longer resembled that of the palladium foil; it consisted of two maxima, a narrow maximum at around 24 395 eV and a broader, featureless maximum at around 24 380 eV. Simultaneous in situ HRXRD suggested that these spectral changes were associated with the structural modification of the original catalyst due to the formation of the PdZn alloy. Figures 6(b) and (c) give the k2-weighted chi functions and the Fourier transforms of the catalyst, collected during reduction, 8461



ARTICLE

compared to the palladium foil (black dashed line). Figure 7 shows some of the Fourier transforms and the best fit (red dashed line). Table 1 gives the structural parameters determined from the fitting. The chi function of the PdZnO100He (Figure 6b, (black)) was low in intensity, which suggests that the palladium particles were small.34 The intensity of the peak at k ∼2.8, 4.3, and 5.5 Å1 shifted upward to higher k, while the intensity at above k = 6.0 Å1 shifted downward, in comparison to the intensity of the palladium foil (Figure 6b (dashed line)). The corresponding Fourier transforms of the foil and the PdZnO100He (Figure 6b (black)) showed peaks at R = 2.05 and 2.50 Å. During the EXAFS fitting, the best fit for PdZnO100He (Figure 7(i)) and Table 1 (condition i) included a PdZn shell. The PdPd bond distance (RPdPd) was 2.73 Å, and the coordination number (CNPdPd) was 5.5. The PdZn shell included a contribution of the PdZn bond distance (RPdZn) of 2.56 Å and a coordination number (CNPdZn) of 1.4. This suggests that the alloy had already formed. The PdPd scattering consists of two contributions, one of the palladium and one of the alloy. As the reduction temperature increased from 100 to 350 °C, the chi function changed significantly (Figure 6b (black to green)) at k from 2.5 to 6 Å1. The intensity at k ∼3.3, 4.6, and 5.7 Å1 gradually decreased and shifted to lower k as the temperature was increased. At the same time, the intensity of a new feature at k = 5.0 Å1 steadily increased. At higher k, the spectra were similar, but the intensity decreased due to a higher DebyeWaller factor at high temperature. The corresponding Fourier transforms (Figure 6c (black to green)) show that the

two peaks at R = 2.05 and 2.50 Å gradually merged to one peak, centered at around 2.25 Å at 350 °C. EXAFS fitting (Table 1 (conditions iiv)) revealed that the palladiumpalladium distance increased with temperature up to about 2.88 Å, while the RPdZn remained at around 2.56 Å. The PdPd coordination decreased to 2.4, while that of PdZn increased to 4.9. The RPdPd and RPdZn were close to that of the intermetallic PdZn alloy. The CNPdZn/CNPdPd ratio of 2 suggests that a 1:1 PdZn alloy formed with a face-centered tetragonal structure.35,36 The effect of high temperature was reflected by the higher DebyeWaller factor. 3.5.3. XAS under Reaction Conditions. Figure 8 shows the conversion of 1-pentyne and the corresponding selectivity to pentenes and oligomers over PdZnO400 as a function of time at 45 °C. The plot was based on the GC data collected simultaneously during the XAS experiments. The conversion was almost constant at around 6% throughout the experiment. The corresponding selectivity was around 80% to pentenes and around 20% to oligomers. These data are similar to those under differential conditions (Figures 4(a)(d)). Figure 9(a) displays the XANES spectra of PdZnO400 collected at 45 °C. The XANES spectra before (Figure 9(a) (solid black)) and during hydrogenation for 100 min (Figure 9(a, solid red)) were very similar to those of the PdZn alloy (Figure 6(a) (green)), with a narrow first maximum and a rather flat region after the edge. Figures 9(b) and (c) show the corresponding k2weighted chi functions and the Fourier transforms of the data. The chi functions before (Figure 9(b) (black)) and during hydrogenation (Figure 9(b) (red)) show a high-intensity PdZn feature at around k = 5.0 Å1, suggesting that the PdZn alloy was at least reasonably stable during the reaction. The k2-weighted Fourier transforms (Figures 9(c), (i and ii)) collected at 45 °C

Figure 7. Fitted k2-weighted Fourier transforms. (i) PdZnO100He in helium at 100 °C after reduction at 100 °C and (ii) at 350 °C in 4% H2/ He. The fittings are shown as a red dashed line.

Figure 8. Conversion of 1-pentyne (9) and corresponding selectivity to pentene (red b), pentane (blue [), and oligomers (green 2) over PdZnO400 at 45 °C.

Table 1. Structural Parameters during High-Temperature Reduction, Determined by EXAFS Fitting of the Pd K Edgea conditions i ii iii iv

PdZnO100He/helium (after reduction) 250 °C/4% H2/He 300 °C/4% H2/He 350 °C/4% H2/He

scat. pair

CN

R (Å)

DWF (103 Å2)

E0 (eV)

PdPd

5.5

2.73

5.6

7.5

PdZn

1.4

2.56

6.3

1.0

PdPd

2.7

2.82

15.0

6.9

PdZn

3.5

2.54

10.2

2.6 6.2

PdPd

2.3

2.82

10.4

PdZn

4.3

2.55

12.5

1.0

PdPd PdZn

2.4 4.9

2.88 2.56

10.0 12.5

7.2 0.3

k -weighting; fitting range, 2.913.0.

a 2

8462



ARTICLE

parameters during the reaction were similar, with the exception of a small increase in CNPdPd at shorter distance.

4. DISCUSSION

Figure 9. Pd K edge XAS spectra of PdZnO400 before and during hydrogenation of 1-pentyne at 45 °C. (a) Pd K edge XANES: before hydrogenation in pure H2 at 45 °C (solid black) and during hydrogenation in the reaction mixture at 45 °C (solid red). The spectrum of palladium foil (dashed black line) is included as a reference. (b) Corresponding k2-weighted Chi function: before hydrogenation at 45 °C in pure H2 (black) and during hydrogenation in the reaction mixture at 45 °C (red). (c) Corresponding Fourier transform: (i) before hydrogenation at 45 °C in pure H2 and (ii) during hydrogenation in the reaction mixture at 45 °C. The fittings are shown as red dashed lines.

showed a peak centered between 2 and 2.5 Å, indicative of the PdZn alloy. Table 2 gives the structural parameters extracted from the EXAFS fitting. Before the reaction, the catalyst had a CNPdPd of 2.5 at RPdPd of 2.91 Å. The CNPdZn was 5.5 with a RPdZn of 2.62 Å. Clearly, the PdZn alloy was the dominating species. The

4.1. Formation of the PdZn Alloy. In situ Pd K edge XANES and EXAFS showed that reduction at 100 °C triggers the formation of the PdZn alloy but probably only at the surface. XRD does not detect this transformation (Figure 5), perhaps because the formation of PdZn starts at the surface.3 The differences in activity and selectivity already observed at low amounts of alloy formation also suggest that the surface is modified. The dissociated hydrogen atom spills over from the Pd metal to the metal support interface and reduces ZnO to form the PdZn intermetallic phase. Although we could not prove the occurrence or absence of surface spillover from our data, such an idea is supported by numerous reports.3741 The alloy then grows into the metallic core, where the amount of alloy and its crystallinity increase with temperature at the expense of the metallic palladium. Because the particles are detected by XRD, particle growth occurs during this process. The presence of palladium hydride was normally indicated by a lattice expansion of up to 2.5% in the Pd K edge EXAFS analysis.42 We were not able to draw a conclusion about the formation of hydride prior to alloy formation because the formation of the PdZn alloy already started at a temperature as low as 100 °C, thus it is rather difficult to identify exactly at which point hydride formation and alloy formation exactly occurred. During the reduction, changes in the PdPd distance revealed by EXAFS analysis were much larger than that normally caused by hydride and thus cannot be used to determine the formation of hydrides. Because of the presence of both alloy and reduced palladium, it is not possible to differentiate changes of the Pd:Zn ratio during reduction. A CNPdZn/CNPdPd ratio of around 2 was obtained only at a reduction temperature of 300 °C and higher, the temperature at which XRD detects the alloy. The RPdPd and RPdZn of ∼2.82 and ∼2.55 Å, respectively, obtained at this temperature, were close to the bulk values for a 1:1 PdZn alloy with a L1o structure (RPdPd = 2.9 Å, RPdZn = 2.6 Å, CNPdZn = 8, CNPdPd = 4).35 4.2. Activity and Selectivity of Pd/ZnO. The kinetic data at 45, 75, and 100 °C showed a decrease in the activity of the catalyst at all three reaction temperatures with increasing reduction temperature of the catalysts (Figures 4(a), (e), and (i)). This may be due to the sintering of the particle into larger ones, as indicated by the STEM image of Pd/ZnO after reduction in pure hydrogen at 350 °C (Figure 2(c)), as well as to the dilution of the palladium sites of the surface, which causes a decrease in the total active surface and, consequently, the overall catalytic activity. An increase in activity, observed especially at 75 °C, might be due to the partial decomposition of the PdZn. We speculate that this enhances the surface concentration of active sites. Similar effects were observed for other palladium-based alloys such as PdCu and PdAg.4345 A decrease in the activity may also be due to coke formation. The error of the measurements was a few percent. In a certain conversion range the data showed significant scatter, which we established to be caused by oscillation of the reaction, similar to that previously observed.20 The amount of alloy, which increased with increasing reduction temperature, had a pronounced effect on selectivity. In situ XAS and XRD showed that PdZnO400 consists of a PdZn alloy. Under reaction conditions at 45 °C, its structure changed only 8463



ARTICLE

Table 2. Structural Parameters of PdZnO400 before and during Hydrogenation of 1-Pentyne at 45 °C As Determined by EXAFS Fitting of the Pd K Edgea

i ii

CN

DWF (103 Å2)

E0 (eV)

2.91

7.0

10.7

2.62 2.89

3.2 8.1

4.9 9.8

5.7

3.1

R (Å)

conditions

scat. pair

45 °C/H2 (after reduction at 400 °C)

PdPd

2.5

45 °C/H2/He/1-pentyne

PdZn PdPd

5.5 3.2

PdZn

5.6

2.61

k -weighting; fitting range, 2.513.0.

a 2

slightly, as indicated by the PdPd and PdZn contributions. The activity and selectivity remained almost constant (Figure 8). Note that the technical success of a palladium-based catalyst in hydrogenation is measured by its ability to lower the alkyne content to about zero without further hydrogenation of the alkene to alkane and to avoid oligomer formation. Previous work on other palladium-based alloys ascribed the suppression of side reactions such as full hydrogenation and oligomerization to the limited hydrogen chemisorption capacity of the zinc-decorated palladium catalysts or to the isolation of the active site.6 Our previous work showed that hydride is not necessarily the main factor that determines the selectivity of hydrogenation.46 Intrinsic selectivity depends on the relative bond strength of the alkynes and alkenes and that the activation barrier for hydrogenation is less affected by a change in surface activity.4749 In this context, an active and selective catalyst must, therefore, bind alkyne strongly and alkene weakly. In comparison to Pd/SiO2, Pd/ZnO showed entirely suppressed full hydrogenation and higher selectivity to oligomers. The formation of the PdZn bond changes the electronic property of palladium. A decrease in the electron population of the Pd(4d) orbitals was attributed to Pd(4d)Zn(4p) charge transfer and Pd(4d)Pd(5s,5p) rehybridization.30 These electron transfers reduce electronelectron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The calculated electronic structure of the PdZn alloy was very similar to that of copper.50,51 It was even proposed that such a structure leads to comparable activity and selectivity between the PdZn alloy and copper in catalyzing methanol steam reforming. Also, a change in the electron density of Pd due to the presence of Zn reduced the CO chemisorption ability of Pd. The Pd(4d)CO(2π) bonding interaction was weakened, and the PdCO bonds of PdZn surfaces are 5067 kJ/mol weaker than those on pure Pd surfaces. The heat of adsorption of alkynes on palladium is higher than that of alkenes,52,53 whereas the calculated heat of adsorption of alkenes on copper was lower than on palladium.47 The overall changes of electronic properties from Pd to PdZn may bring a stronger affect to the adsorption of alkenes than of alkynes. This enables faster desorption of 1-pentene and leads to enhanced selectivity to semihydrogenation. This was shown by the very high selectivity to propene and butene during the selective hydrogenation of propyne and butyne over Cu/SiO2, over which complete hydrogenation was not detected.54,55 Therefore, we propose that the PdZn alloy, with a similar electronic structure to copper, will show similar selectivity during the hydrogenation of 1-pentyne, as observed in this study. Reduction of Pd/TiO2 at high temperature changed the electronic structure of the catalyst, which favored rapid desorption of styrene and thus enhanced the overall selectivity during

the liquid-phase semihydrogenation of phenylacetylene.56 Without reduction at 500 °C, the activity and selectivity trends were similar to those on other supported palladium catalysts reported in the literature, where the TOF and the selectivity to styrene increased with increasing particle size. Palladium-based catalysts are known to form oligomers during the hydrogenation of alkynes.57,58 The formation of the oligomer can occur by the formation of CC bonds by the recombination of neighboring intermediates, by insertion of a C2 unit into an existing PdC bond, or by the participation of free-radical types of vinyl intermediates.59,60 Although other palladium-based alloys showed that a suppression of oligomerization is possible by isolation of the active site, our PdZn alloy showed conversion to the oligomer. We cannot draw a definite conclusion about the mechanism of the oligomer formation. However, bearing in mind that the PdZn alloy has a similar electronic structure to that of copper and that copper is used as a polymerization catalyst,61 selectivity to oligomers is not surprising

5. CONCLUSION In situ HRXRD, XANES, and EXAFS revealed that the formation of the PdZn alloy started at a reduction temperature as low as 100 °C; alloy formation increased with temperature at the expense of metallic palladium. Above 300 °C, the crystalline PdZn alloy, with a structure close to the bulk 1:1 PdZn alloy with a L1o structure, was observed. The PdZn alloy was reasonably stable during the hydrogenation of 1-pentyne. A decrease in activity was observed due to dilution of the site at the surface and an increase in particle size with increasing reduction temperature. In comparison to Pd/SiO2, full hydrogenation was entirely suppressed on Pd/ZnO. There was a very high selectivity to pentenes, thus the suppression of total hydrogenation was attributed to the electronic structure of the PdZn alloy, similar to that of copper, and the concomitant isolation of the active sites. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ41 44 632 55 42. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the Swiss National Science Foundation (SNF) for financial support and the BM26 (DUBBLE) 8464


The Journal of Physical Chemistry C and BM01B (SNBL) beamlines of the European Synchrotron Radiation Facilities for the beam time and technical support. The authors also thank Dr. Jeffrey T. Miller from the Chemical Technology Division of Argonne National Laboratory for the samples, Dr. Frank Krumeich of the ETH Zurich for measuring the TEM micrographs, and the ETH workshop for constructing the in situ cells.

’ REFERENCES (1) Nilsson, M.; Jansson, K.; Jozsa, P.; Pettersson, L. J. Appl. Catal. B: Environ. 2009, 86, 18. (2) Iwasa, N.; Masuda, S.; Ogawa, N.; Takezawa, N. Appl. Catal. A: Gen. 1995, 125, 145. (3) Foettinger, K.; Van Bokhoven, J. A. Submitted. (4) Bollmann, L.; Ratts, J. L.; Joshi, A. M.; Williams, W. D.; Pazmino, J.; Joshi, Y. V.; Miller, J. T.; Kropf, A. J.; Delgass, W. N.; Ribeiro, F. H. J. Catal. 2008, 257, 43. (5) Sarkany, A.; Zsoldos, Z.; Furlong, B.; Hightower, J. W.; Guczi, L. J. Catal. 1993, 141, 566. (6) Wehner, P. S.; Gustafson, B. L. J. Catal. 1992, 135, 420. (7) Iwasa, N.; Takizawa, M.; Arai, M. Appl. Catal. A: Gen. 2005, 283, 255. (8) Iwasa, N.; Yoshikawa, M.; Arai, M. Phys. Chem. Chem. Phys. 2002, 4, 5414. (9) Zakumbaeva, G. D.; Naidin, V. A.; Dagirov, T. S.; Litvuakova, E. N. Russ. J. Phys. Chem. 1987, 61, 801. (10) Gustafon, B. L., Wehner, P. S., Nelson, G. O., Mercer, P. N. U.S. Patent, 1989. (11) Bond, G. C. Metal-Catalyzed Reactions of Hydrocarbons, Fundamental and Applied Catalysis Series; Springer-Verlag: New York, 2005; pp 395435. (12) Lindlar, H.; Dubuis, R Org. Synth. Collect. 1973, 5, 880. (13) Derrien, M. L. Stud. Surf. Sci. Catal. 1986, 27, 613. (14) Osswald, J.; Giedigkeit, R.; Jentoft, R. E.; Armbr€uster, M.; Girgsdies, F.; Kovnir, K.; Ressler, T.; Grin, Y.; Schl€ogl, R. J. Catal. 2008, 258, 210. (15) Osswald, J.; Giedigkeit, R.; Jentoft, R. E.; Armbr€uster, M.; Girgsdies, F.; Kovnir, K.; Ressler, T.; Grin, Y.; Schl€ogl, R. J. Catal. 2008, 258, 219. .; Sarkany, A.; Varga, M. J. Mol.Catal. A: Chem. 2001, (16) Molnar, A 173, 185. (17) Doyle, A. M.; Shaikhutdinov, S. K.; Jackson, S. D.; Freund, H. J. Angew. Chem., Int. Ed. 2003, 42, 5240. (18) Teschner, D.; Vass, E.; H€avecker, M.; Zafeiratos, S.; Schn€orch, P.; Sauer, H.; Knop-Gericke, A.; Schl€ogl, R.; Chamam, M.; Wootsch, A.; Canning, A. S.; Gamman, J. J.; Jackson, S. D.; McGregor, J.; Gladden, L. F. J. Catal. 2006, 242, 26. (19) Khan, N.; Shaikhutdinov, S.; Freund, H. Catal. Lett. 2006, 108, 159. (20) Teschner, D.; Borsodi, J.; Wootsch, A.; Revay, Z.; Havecker, M.; Knop-Gericke, A.; Jackson, S. D.; Schlogl, R. Science 2008, 320, 86. (21) Borodzinski, A. Catal. Lett. 1999, 63, 35. (22) Nikolaev, S. A.; et al. Russ. Chem. Rev. 2009, 78, 231. (23) Pennycook, S. J. Ultramicroscopy 1989, 30, 58. (24) Tew, M. W.; Miller, J. T.; van Bokhoven, J. A. J. Phys. Chem. C 2009, 113, 15140. (25) Kampers, F. W. H.; Maas, T. M. J.; Vangrondelle, J.; Brinkgreve, P.; Koningsberger, D. C. Rev. Sci. Instrum. 1989, 60, 2635. (26) Weiher, N.; Bus, E.; Gorzolnik, B.; Moller, M.; Prins, R.; van Bokhoven, J. A. J. Synchrotron Radiat. 2005, 12, 675. (27) Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. Top. Catal. 2000, 10, 143. (28) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B 1998, 58, 7565. (29) Waser, J.; Levy, H. A.; Peterson, S. W. Acta Crystallogr. 1953, 6, 661.

ARTICLE

(30) Rodriguez, J. A. J. Phys. Chem. 1994, 98, 5758. (31) Perez-Lopez, O. W.; Farias, A. C.; Marcilio, N. R.; Bueno, J. M. C. Mater. Res. Bull. 2005, 40, 2089. (32) Park, M. H.; Wang, L. C.; Palmstrom, C. J. J. Appl. Phys. 1997, 81, 2720. (33) Davis, R. J.; Landry, S. M.; Horsley, J. A.; Boudart, M. Phys. Rev. B 1989, 39, 10580. (34) Frenkel, A. I.; Hills, C. W.; Nuzzo, R. G. J. Phys. Chem. B 2001, 105, 12689. (35) PdZn, ICDD card # 06-0620. (36) Pearson, W. B. Pearson’s Handbook of Crystallographic Data for Intermetallic Phases; American Society for Metals: Metals Park, 1985. (37) Wehner, P. S.; Tustin, G. C.; Gustafson, B. L. J. Catal. 1984, 88, 246. (38) Hong, C. T.; Yeh, C. T.; Yu, F. H. Appl. Catal. 1989, 48, 385. (39) Zsoldos, Z.; Sarkany, A.; Guczi, L. J. Catal. 1994, 145, 235. (40) Iwasa, N.; Ogawa, N.; Masuda, S.; Takezawa, N. Bull. Chem. Soc. Jpn. 1998, 71, 1451. (41) Iwasa, N.; Takezawa, N. Top. Catal. 2003, 22, 215. (42) Mccaulley, J. A. J. Phys. Chem. 1993, 97, 10372. (43) Leviness, S.; Nair, V.; Weiss, A. H.; Schay, Z.; Guczi, L. J. Mol. Catal. 1984, 25, 131. (44) Guczi, L.; Schay, Z.; Stefler, G.; Liotta, L. F.; Deganello, G.; Venezia, A. M. J. Catal. 1999, 182, 456. (45) Zhang, Q.; Li, J.; Liu, X.; Zhu, Q. Appl. Catal. A: Gen. 2000, 197, 221. (46) Tew, M. W.; Van Bokhoven, J. A. Submitted (47) Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Sorensen, R. Z.; Christensen, C. H.; Norskov, J. K. Science 2008, 320, 1320. (48) Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Sørensen, R.; Christensen, C.; Nørskov, J. Angew. Chem., Int. Ed. 2008, 120, 9439. (49) Bond, G. C. Catalysis by Metals; Academic Press: New York, 1962. (50) Chen, Z.-X.; Neyman, K. M.; Gordienko, A. B.; Rosch, N. Phys. Rev. B 2003, 68, 075417. (51) Tsai, A. P.; Kameoka, S.; Ishii, Y. J. Phys. Soc. Jpn. 2004, 73, 3270. (52) Conn, J. B.; Kistiakowsky, G. B.; Smith, E. A. J. Am. Chem. Soc. 1939, 61, 1868. (53) Taylor, G. F.; Thomson, S. J.; Webb, G. J. Catal. 1968, 12, 150. (54) Koeppel, R. A.; Wehrli, J. T.; Wainwright, M. S.; Trimma, D. L.; Cant, N. W. Appl. Catal. A: Gen. 1994, 120, 163. (55) Wehrli, J. T.; Thomas, D. J.; Wainwright, M. S.; Trimm, D. L.; Cant, N. W. Appl. Catal. 1990, 66, 199. (56) Weerachawanasak, P.; Mekasuwandumrong, O.; Arai, M.; Fujita, S. I.; Praserthdam, P.; Panpranot, J. J. Catal. 2009, 262, 199. (57) Thomson, S. J.; Webb, G. J. Chem. Soc., Chem. Commun. 1976, 526. (58) McGregor, J.; Gladden, L. F. Appl. Catal. A: Gen. 2008, 345, 51. (59) Sheridan, J. J. Chem. Soc. 1945, 470. (60) Bond, G. C.; Sheridan, J. Trans. Faraday Soc. 1952, 48, 651. (61) Galletti, A. M. R. Copper catalysts for olefin polymerization. Handbook of Transition Metal Polymerization Catalysts; John Wiley and Sons: New York, 2010; pp 497511.

8465


Formation and Characterization of PdZn Alloy: A Very Selective

Recommend Documents