ARTICLE pubs.acs.org/JPCC
Characterization of β-Palladium Hydride Formation in the Lindlar Catalyst and in Carbon-Supported Palladium Peter W. Albers* AQura GmbH, Rodenbacher Chaussee 4, D-63457 Hanau/Wolfgang, Germany
Konrad M€obus Evonik Industries, Business Line Catalysts, Rodenbacher Chaussee 4, D-63457 Hanau/Wolfgang, Germany
Christopher D. Frost and Stewart F. Parker ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom ABSTRACT: The proton dynamics of palladium catalysts on activated carbon, of palladium on calcium carbonate, and of lead-modified palladium on calcium carbonate (the Lindlarcatalyst), hydrogenated in situ to comparable equilibrium pressure, were measured by means of inelastic incoherent neutron scattering. Isolated primary particles of palladium of ∼3.2 nm size showed a lower degree of formation of β-phase hydride than 10 15 nm sized aggregates, which were formed from ∼3.7 nm primary particles of palladium. The addition of lead to the palladium on calcium carbonate in catalysts of identical primary particle size (3.7 nm) and shape caused a decrease in the β-phase hydride formation by a factor of 2. For the catalyst selectivity in the hydrogenation of triple to double bonds not only the presence of modifiers at the surface but also a reduced, moderated hydrogen storage function is of relevance.
1. INTRODUCTION Finely divided supported palladium particles are used as catalysts in many technical processes. Typical applications are the large scale production of hydrogen peroxide or the synthesis of fine chemicals, intermediates, vitamins, and pharmaceutical products.1 4 Despite the wide variety of applications and practical experience of these catalysts over many years, there is still considerable potential to raise catalyst performance and selectivity. Important parameters that can be adjusted and fine-tuned are the size, size distribution, and shape of the primary particles and the particle distribution on a support as appropriate for the particular application or process. An important aspect in this context is the interaction between hydrogen and finely dispersed palladium particles and, in particular: (1) the influence of decreasing primary particle size of palladium on hydrogen-uptake; the formation of α-phase hydride and the formation of the β-phase hydride; the degree of occupation of octahedral sites of the fcc lattice; changes of the long-range phase coherence in the 2 5 nm particle size regime and, therefore, an incomplete formation of the β-phase PdHx, and, correspondingly, changes of the phase diagram as revealed by hydrogen absorption measurements and X-ray diffraction on isolated, stabilized r 2011 American Chemical Society
Pd particles of 3 5 nm size as shown by Kirchheim and Reetz et al.5 8 (2) changes of the site occupation (100)/(111) by dissociatively chemisorbed hydrogen entities in the topmost atomic layer as a function of varying primary particle size according to the work of Van Hardeveld and Hartog.9 Because of its high sensitivity to the hydrogen nucleus, inelastic incoherent neutron scattering (IINS) is uniquely suitable to study hydrogen on and in metal systems and catalysts and the interactions between palladium and hydrogen in detail.10 16 In previous work of neutron scattering on Pd/C catalysts, we have measured 20%Pd/C (powder type catalyst with average primary particle sizes of ∼5 nm17) and 5%Pd/C (pellet-type catalyst supported on activated carbon derived from beech wood18 and containing a broad precious metal particle size distribution). For the powder catalysts with high precious metal loading as well as for the pellet-type catalyst with polydisperse properties,18 we could readily detect β-PdHx.17 In industrial applications, low palladium concentrations are preferred (below 5 wt % Pd/C), and it is suspected that this may Received: June 24, 2011 Revised: November 9, 2011 Published: November 10, 2011 24485
dx.doi.org/10.1021/jp205951c | J. Phys. Chem. C 2011, 115, 24485–24493
The Journal of Physical Chemistry C
ARTICLE
Table 1. Hydrogenation Catalyst Samplesa catalyst
metal content (wt %)
TEM DN (nm)
TEM SDN (nm)
TEM DA (nm)
IINS PeqH2 (mbar)
IINS sample weight (g)
H1
20%Pd/C
3.17
0.42
3.29
672
22.0
H2
5%Pd/C
2.48
0.47
2.65
600
16.0
H3
5%Pd/C
2.10
0.20
2.17
618
12.2
H4
5%Pd/C
3.49
1.04
4.12
598
11.0
L1
5%Pd/CaCO3
3.68
0.53
3.83
612
50.5
L2
5%Pd+3.5%Pb/CaCO3
3.67
0.86
4.06
616
55.0
a
H1 H4: palladium/activated carbon catalysts. L1 L2: palladium and Lindlar-type palladium/lead catalysts on calcium carbonate. DN and SDN: primary particle size (arithmetical average) and its standard deviation, DN = (Σnidi)/N; ni particle with diameter di; N: total number of particles. DA: primary particle size averaged over the surface, DA = (Σnidi3)/(Σnidi2). PeqH2: final equilibrium pressure of hydrogen in an IINS sample can measured at room temperature before quenching a can to liquid nitrogen temperature. Weight of the individual catalyst as sealed into the individual IINS can. The different weights are due to the different support materials and precious metal or lead loading.
be at or beyond the limits of sensitivity of neutron scattering experiments on TOSCA19 and its predecessor TFXA, where previous work17,18 was carried out. The MAPS spectrometer20 has been shown to have an order of magnitude greater sensitivity than TOSCA, which in principle makes the study of such catalysts feasible. The 10-fold sensitivity advantage of MAPS over TOSCA is essential to measure hydrogen/palladium interactions at low precious metal concentration and changing surface-to-bulk ratio of supported nanoparticles of decreasing size. To explore the suitability of IINS to probe such properties under improved conditions, we measured a set of different supported palladium catalyst: palladium-only catalysts (20%Pd/ C, 5%Pd/C, 5%Pd/CaCO3) and the Lindlar-type catalyst (5%Pd + 3.5%Pb/CaCO3). The Lindlar catalyst is used for the selective catalytic hydrogenation of triple bonds to double bonds, especially the stereoselective hydrogenation of alkynes to cis alkenes and other applications.21 24 The selection of the catalysts for this task was focused to study the proton dynamics, that is, the vibrational modes of interstitially dissolved protons of supported palladium particles in the 2 4 nm range of average primary particle size after in situ hydrogenation in the individual IINS sample can under the following aspects: - comparison of the proton dynamics of palladium and leadmoderated palladium, both supported on calcium carbonate - comparison of palladium as isolated supported primary particles and palladium as supported aggregates - comparison of 20% Pd/C catalyst versus 5% Pd/C catalysts - comparison of the properties of palladium particles supported on high surface area porous (>1000 m2/g, activated carbon) and low surface area (ca. 5 m2/g, calcium carbonate) nonporous material. The influence of size, shape, and composition of catalyst particles on the formation of β-palladium hydride is studied.
2. EXPERIMENTAL SECTION 2.1. Catalysts. Commercial palladium on activated carbon, palladium on calcium carbonate, and Lindlar-type hydrogenation catalysts (Evonik Industries, Business Line Catalysts25) were selected for the IINS measurements with respect to their average primary particle size and small primary particle size distribution, which were determined by transmission electron microscopy (TEM, Section 3.1, Table 1). Steam-activated and acid-washed wood-based, porous activated carbons were loaded with palladium in a standard procedure
(catalysts H1 H4).18 The Lindlar catalysts were prepared according to21 24 using a nonporous calcium carbonate support (catalysts L1-L2). Catalyst H2 was treated under hydrogen at 323 K for 2 h and catalyst H4 at 373 K for 12 h. Catalyst H3 was not heated under hydrogen. The effect of heating H2 and H4 under hydrogen on the primary particle size distribution is illustrated by the data in Table 1 (Section 3.1). The controlled heat treatment under hydrogen at increasing time of exposition caused particle growth and partly aggregation/agglomeration. 2.2. Transmission Electron Microscopy. A Jeol 2010F field emission transmission electron microscope (FE-TEM) was operated at 200 keV acceleration voltage. For spot analyses of the Lindlar catalyst energy-dispersive X-ray (EDX), spot analyses of the supported precious metal particles and the support particles were performed using a Noran SiLi detector with a 30 mm2 crystal and a Noran System Six device. A catalyst sample was dispersed in chloroform and transferred onto holey carbon foil supported by a 200 mesh copper grid. For statistical evaluation of the primary particle sizes of the supported primary particles, the I-TEM software of Soft Imaging Systems (SIS), M€unster, was utilized. The quality and stability of the TEM system was supervised, and the calibration of the instrument was performed using the Magical No. 641 standard (Norrox Scientific, Beaver Pond, Ontario, Canada). 2.3. X-ray Photoelectron Spectroscopy. For comparing the surface concentration of palladium and lead on the Lindlar-type catalysts, X-ray photoelectron spectroscopy (XPS) spectra were recorded using 150 W Mg Kα radiation. A Leybold MAX100 instrument with an EA200 electron energy analyzer was operated at 72 eV in the fixed analyzer transmission mode. 2.4. Sample Preparation and Inelastic Incoherent Neutron Scattering. Each catalyst was sealed in a thin-walled (0.5 mm) stainless-steel (1.4571) can that was closed by a top-flange with a stainless-steel pipe and a welded bellows valve (Nupro) via OFHC-copper (oxygen-free high conductivity) gasket (pressure and safety test certificate: RLI 533). A sealed can containing macroscopic amounts of catalyst (Table 1) was pumped down using a turbo-molecular pump that was backed by a dual stage rotary pump with a zeolite trap to avoid backdiffusion of oil and other potential molecular contaminants. Each catalyst was subjected to slow and careful cycles of hydrogenation (99.999% hydrogen) and dehydrogenation at room temperature to avoid fast local heating due to a spontaneous excessive release of the heat of the dissociative absorption of hydrogen in the palladium in fast-step dosing. No heating during degassing of a hydrogenated catalyst by pumping down a 24486
dx.doi.org/10.1021/jp205951c |J. Phys. Chem. C 2011, 115, 24485–24493
The Journal of Physical Chemistry C sample can was performed to avoid particle growth induced by heat under the presence of residual pressure of hydrogen and further reactions in the palladium/lead system. References on the Pd/Pb phase diagram26 and data on the hydrogen absorption isotherms of substitutional fcc Pd/Pb alloys (wires of varying Pd/ Pb composition) as measured by Flanagan et al.27 were focused on compact material. For the case of finely divided supported palladium particles of Lindlar-type catalysts, Palczewska et al. have shown by hydrogen absorption and desorption isotherms and X-ray diffraction that the hydrogen absorption properties can be dependent on the preparation conditions and thermal history and, therefore, the crystallite size, phase composition, and degree of alloying of a catalyst sample.28 In the present work, the exclusion of heating the catalyst samples in the presence of hydrogen was chosen to maintain the degree of dispersion and sample condition with respect to literature.28 30 Sermon reported that under the influence of hydrogen, particle growth of finely dispersed, unsupported palladium may occur even at temperatures as low as 330 K.29 This is in line with the observation of particle growth in preparing the catalysts H2 and H4 (Section 2.1). Martin et al. investigated sintering of supported palladium at 723 973 K in H2 and H2O vapor and under high vacuum.30 The hydrogen uptake of the supported palladium was monitored by capacitive pressure transducers (Baratron) using a known volume sample chamber. Four cycles of slow hydrogenation/dehydrogenation were applied to each sample. Because in these cycles the outgassing was restricted to room temperature, complete decomposition of the hydride phases was not achieved, and thus there is residual hydrogen in the samples. This procedure is suitable for qualitative comparison of the loading condition and the hydrogen capacity of a sample at the given final hydrogen pressure in the IINS sample can with respect to the known complete isotherms for finely divided palladium. From existing gas volumetric work on the absorption of hydrogen by nanoparticulate palladium, it is known that the hydrogen solubility in single-sized palladium clusters (stabilized by surfactant or polymer-embedding) does change as a function of primary particle size (2, 3, 5 nm).5 8 The solubility in the α-phase region is enhanced by a factor of 5 10 fold compared with bulk palladium,31 35 and the miscibility gap is narrowed compared with bulk palladium from 0.58 H/Pd for polycrystalline Pd at 20 °C down to 0.28 H/Pd for small Pdclusters.5 8 Hydrogen isotherms of nanosized palladium clusters resemble the bulk palladium hydrogen system above the critical point with enhanced low concentration solubility and reduced high concentration solubility and do not plateau but exhibit a sloped isotherm for 3.0 nm Pd H clusters compared with 5.2 nm Pd H clusters with a small plateau.5,7 Also, the hydrogen absorption behavior of nanocrystalline palladium (grain size
