ARTICLE pubs.acs.org/JPCC
Interaction of a Nanosized Pd Catalyst with Active C from the Carbon Support: An Advanced in Situ XRD Study Walter Vogel† Department of Chemistry, National Central University, No. 300 Jung-Da Rd., Chung-Li, Taoyuan, Taiwan 32001
bS Supporting Information ABSTRACT: We have used in situ X-ray diffraction and line profile analysis to study the carburization of a 6 nm Pd catalyst supported on carbon at various temperatures under helium, hydrogen, and oxygen. A maximum of 10% C atoms was dissolved in the Pd lattice. This phase is stable at ambient conditions but decays above 400 °C. The rate of formation is limited by active C from the support and is slower by a factor of ∼100 compared to the supply of C atoms via CO decomposition. The bulk diffusion coefficient predicts an even faster rate, demanding a surface barrier to activate bulk diffusion. We find that the rate of formation depends on the surface to volume ratio. Carbon is nonuniformly distributed among individual Pd particles, but hydrogen fills interstitial sites to lift the irregularity. In the ternary phase PdCxHy, x and y are linearly linked. Oxygen reacts with interstitial carbon already below 160 °C.
1. INTRODUCTION Palladium catalysts are used in large-scale applications for hydrogenation of organic fine chemicals, aromatic hydrogenations, petroleum refining, the selective hydrogenation of acetylene to ethylene, the production of acetaldehyde by oxidation of ethene, the production of vinyl acetate and, to an increasing extent, in different kinds of automotive exhaust gas catalysts. Palladium has an outstanding importance in many catalytic reactions. For example, Pd nanoparticles have been extensively served as primary catalysts for many organic reactions such as Suzuki, Heck, and Stille coupling reaction.1,2 Pd-based nanostructured catalysts are also very active for electrocatalytic oxidation of formic acid,3,4 and for electroreduction of oxygen if alloyed with other metals.5,6 The use of palladium-based catalysts supported on carbon in fuel cell applications has been reviewed in ref 7. Only a few studies have been published so far on the structural implications of carbon interacting with nanosized palladium to form a carbide phase PdCx.8-15 The formation of carburized nanodivided Pd is not unlikely, especially in catalytic applications. The process of formation resembles that of the palladium hydride phase, as carbon occupies the octahedral sites of the fcc lattice8,12 and expands the host fcc lattice in proportion to the fraction of guest atoms. This can be expressed by a linear relation linking the lattice constant with the fraction x of carbon adsorbed in the Pd lattice:8,13,16 a ¼ a0 þ 0:69x ðÅÞ ð1Þ
Carbonaceous deposits on metal catalysts via decomposition of reactants can strongly affect the catalytic activity. It has been a paradigm for long that carbon solely acts as poison through coke formation. Recently it has been notified that under certain conditions carbon can act as a catalyst promoter, for instance in the partial hydrogenation of alkyne on Pd, which is of fundamental importance in industrial polyethylene production. A 1.4 nm thick carbon-rich phase is formed on the surface of the metal, identified by a fingerprint of this phase through the X-ray photoelectron spectrum (XPS) of palladium.21-23 This active phase contains carbon atoms located a few layers below the surface. Very recently, Garcia-Mota et al.24 have studied the interplay of carbon monoxide, hydride, and carbide in the selective hydrogenation on palladium and extended their experiments with density functional theory (DFT) calculation of slabs of the near-surface range of a Pd(111) surface. These subsurface carbon atoms are supplied from the gas phase via dissociative chemisorption of the reactants. For instance, palladium black has been shown to absorb a maximum of ∼13 atom % of carbon on its octahedral interstitial sites if exposed to gaseous carbon containing species as C2H2, C2H4, or CO.8 Very little is known about the direct interaction of carbon from the support to form a PdCx phase. Makkee et al.25 have used Pd supported on activated carbon for selective hydrogenolysis of CCl2F2. They observed an enhancement of activity and selectivity followed a pretreatment at 623 K in nitrogen. It was found that during reaction palladium is converted into palladium carbide.
Several authors reporting unusual lattice expansion of Pd nanoparticles may have not considered a possible interaction with carbon dissolved into the host Pd lattice.17-20
Received: August 28, 2010 Revised: October 21, 2010 Published: December 17, 2010
r 2010 American Chemical Society
1506
dx.doi.org/10.1021/jp108193v | J. Phys. Chem. C 2011, 115, 1506–1512
The Journal of Physical Chemistry C The presence of activated carbon species as, e.g., carbonilic groups induce the formation of a carburized palladium phase. In this case, carbon in its activated state is not supplied from the gas phase but from functional groups present at the surrounding carbon matrix. To the best of our knowledge, a detailed study of this process has only been published by Canton et al.14 The authors used synchrotron radiation and in situ X-ray diffraction (XRD) to study a 0.66 wt % Pd catalyst, supported on active carbon derived from coconut shell. Their study reveals a sudden expansion of the Pd lattice at 550 K, indicating the formation of a palladium carbide phase. This expansion recovers at T > 700 K to the value of the pure metal. The present work is aimed to get a better understanding of the thermodynamics of formation and decomposition of PdCx versus temperature in an inert ambient. Importantly, little is known about the interaction of both carbon and hydrogen to form mixed carbide-hydride phase of PdCxHy. In a recent review article about hydrogenation of ethyne on Pd catalysts, Borodzinski and Bond have pointed to the importance of unsolved questions related to the interaction of Pd with hydrogen- and carboncontaining species.26 We have used advanced laboratory-scale X-ray in situ techniques and precise line profile analysis to highlight palladium carburization in more detail. A Pd catalyst supported on commercial carbon (Vulcan XC72) with a metal loading of 20 wt % was used. It is believed that the higher metal loading will not affect the C-Pd interaction in a fundamental way.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A 20 wt % Pd/C electrocatalyst was used in this work. Details of the synthesis have been described in an earlier paper.27 In brief, palladium acetylacetonate and sodium citrate were dissolved in ethylene glycol and mixed with the carbon support (Vulcan XC-72R). After maintaining at a refluxing temperature of 140 °C for 4 h the solution was filtrated, washed, and dried at 70 °C in vacuum. 2.2. In Situ X-ray Diffraction. X-ray experiments were performed with a Guinier system (Huber), set to the D-45° transmission position. A focusing Johansson-type Ge(111) crystal delivers an intense, strictly monochromatic primary beam at a wavelength λ = 1.5406 Å. The standard sample holder is replaced by a homemade in situ cell described elsewhere.28,29 The samples in powder form are compressed with a hand press to pellets 5 15 0.2 mm, and sandwiched between 0.1 mm thick perforated, high-purity Be platelets. A pure carbon (Vulcan XC-72R) sample was used as a reference for the background correction. Up to three samples can be placed into the sample holder and exchanged by a stepper-controlled parallel displacement of the whole cell. Accurately calibrated gas mixtures can be passed through the cell via three mass flow controls (Bronkhorst). Under nonflow conditions, the gas pressure can be controlled between
