Article pubs.acs.org/JPCC
Microstructural Changes of Supported Intermetallic Nanoparticles under Reductive and Oxidative Conditions: An in Situ X‑ray Absorption Study of Pd/Ga2O3 Andreas Haghofer,† Karin Föttinger,*,† Maarten Nachtegaal,‡ Marc Armbrüster,§ and Günther Rupprechter† †
Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9/BC, A-1060 Vienna, Austria Paul Scherrer Institute, CH-5232 Villigen, Switzerland § Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, D-01187 Dresden, Germany ‡
S Supporting Information *
ABSTRACT: In this work, the structure and stability of Pd− Ga intermetallic nanoparticles under various reactive conditions was investigated by combining in situ X-ray absorption spectroscopy (XAS), FTIR of CO adsorption, and XRD. By in situ XAS we followed in detail the formation of Pd−Ga intermetallic compounds (IMC) upon reduction of Pd/Ga2O3, which was observed to be a rather slow process that depends on the availability of reduced Ga formed by the atomic H provided by Pd. Using crystal structures of a variety of Pd−Ga IMCs, we have identified Pd2Ga as the compound that is formed during reduction at 623 K. In contrast to Pd/Ga2O3, βhydride formation did not occur once Pd2Ga particles are formed, as evidenced by the absence of lattice expansion in hydrogen atmosphere. However, XAS revealed that Pd2Ga is not stable in oxygen already at room temperature. Although XRD showed no bulk structural modification, CO adsorption on an oxygen exposed catalyst detected a metallic Pd surface, partly decorated with oxidic Ga. Only in situ XAS provided clear indications on the structural modification occurring upon oxygen exposure, showing that the overall state of the sample is a mixture of Pd or a Ga-depleted IMC and Pd2Ga. Based on these observations, Ga segregation from the surface-near region to the surface, followed by oxidation, was concluded. The intermetallic surface is easily reformed by reduction, due to remaining Pd at the surface activating H2.
1. INTRODUCTION Supported bimetallic nanoparticles play an important role in many catalytic reactions (e.g., three way catalysis, reforming reactions, fuel cell electrocatalysis, and hydrogenations1), due to the possibility of achieving enhanced catalytic properties via alloy formation, very different from those of the constituting metals.2 In most cases, the aim of using alloys in catalysis is to control the selectivity through a combination of geometric and electronic factors.3−5 A well-known example is the industrially important semihydrogenation of acetylene to ethylene, a reaction for which PdAg alloys show superior selectivity and stability when compared to monometallic Pd. Several explanations have been put forward, among them the “active site isolation” concept, which explains the enhanced selectivity in terms of a smaller ensemble size of Pd. The absence of extended Pd ensembles can influence the preferred adsorption geometry of intermediates and diminish the supply of dissolved hydrogen. The role of H adsorbed on the surface versus that absorbed in the bulk as Pd-β-hydride has also been proposed to be responsible for selective (via surface H) versus unselective (via β-hydride) hydrogenation.6,7 On monometallic Pd © 2012 American Chemical Society
catalysts, it was recently shown that the selective hydrogenation catalyst is not metallic Pd but a Pd−C phase that efficiently blocks the exchange of H between bulk and surface.8 A similar situation was envisioned for Pd modified not by C but in a more controlled manner by introducing Ga as a second metal.9 The concept of using structurally well-ordered Pd−Ga intermetallic compounds, i.e., single-phase compounds with a strongly modified electronic and geometric structure, as hydrogenation catalysts has been studied in detail in recent years, yielding catalysts outperforming the industrially employed Pd−Ag system.10−13 One of the drawbacks of many catalysts based on disordered substitutional alloys, however, is the tendency of segregation of one of the components to the surface, in extreme cases leading to bimetallic particles with a monometallic surface, paralleled by a loss of the advantageous catalytic properties of the bimetallic material. In contrast, intermetallic compounds, which by Received: June 21, 2012 Revised: September 17, 2012 Published: September 19, 2012 21816
dx.doi.org/10.1021/jp3061224 | J. Phys. Chem. C 2012, 116, 21816−21827
The Journal of Physical Chemistry C
Article
The bulk intermetallic compound Pd2Ga used as a reference in X-ray absorption spectroscopy was synthesized according to ref 13. Briefly, Pd and Ga were melted in a ratio of 2:1 (Pd, ChemPur 99.95%; Ga, ChemPur 99.99%) under Ar in a high frequency furnace, followed by subsequent annealing in an evacuated quartz glass ampule at 1073 K for one week. After cooling to ambient temperature, samples were powdered using an agate mortar and the single-phase nature of the sample was confirmed by XRD (Huber, Image plate Guinier camera G670, Cu Kα radiation, Ge(111) monochromator). 2.2. X-ray Diffraction (XRD). XRD experiments were performed on a STOE Theta/Theta diffractometer (Cu Kα radiation, secondary monochromator, scintillation counter) operating in reflection scan mode. Diffraction patterns were recorded with a step size of 0.02° and a time per step of 2 s. For the in situ measurement the calcined supported sample was reduced in a flow of 25% H2/He (100 mL/min) at 673 K using an Anton Paar XRK 900 high temperature gas cell and cooled to 303 K before recording diffractograms. The ex situ measurement was performed on the same sample after exposure to ambient air at 303 K for 36 h. 2.3. X-ray Absorption Spectroscopy (XAS): Data Acquisition and Analysis. In situ XAS spectra were recorded at the Pd K-edge (E0 = 24350 eV) at the SuperXAS beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). The spectra were recorded “on-the-fly” in transmission mode using Ar filled ionization chambers and with a time resolution of approximately 1 min. The calcined Pd/Ga2O3 or Pd/Al2O3 sample was pressed into a selfsupporting wafer (6 mm in diameter, m ≈ 30 mg) and placed into a cylindric stainless steel sample holder, which is mounted into the ring-shaped furnace of a small, stainless steel transmission flow cell with aluminum windows. The sample temperature is measured by a type K thermocouple at the sample holder (Agreement between measured temperature and actual temperature within the sample pellet was determined by calibration measurements). Gases and gas mixtures were introduced into the system by calibrated mass flow controllers using 8 mL/min 21% O2/He for oxidation, 40 mL/min 10% H2/N2 for reduction, and 32 mL/min pure He flow for inert conditions. XAS measurements of Pd foil and bulk Pd2Ga powder as references were performed in air at 303 K. Pd/Al2O3 and Pd/Ga2O3 LTR were measured in H2/N2 at 423 and 373 K, respectively, after reduction at these respective temperatures for 15 min. The elevated measurement temperature was chosen in order to decompose Palladium-β-hydride. Pd/Ga2O3 HTR was measured at 303 K in H2/N2 after reduction at 623 K for 30 min. Spectra in section 3.1.3 were recorded in situ during reduction in a TPR-type experiment. The sample was heated at a rate of 5 K/min from 373 to 623 K in H2/N2 and spectra were recorded every 7 K. All spectra in section 3.2 were recorded at 303 K, first in H2/N2 and then after flushing in He for 10 min. Samples were pretreated by reduction at 423 K (Pd/Al2O3) and 623 K (Pd/Ga2O3 HTR). In section 3.3, spectra of a 623 K reduced sample were recorded at 303 K in He and in O2/He. Subsequently, the temperature was increased to 573 K in O2/He before cooling back to 303 K and recording another set of spectra. X-ray absorption data were processed and analyzed using the IFEFFIT20 software package. Data processing was done by fitting a linear and a polynomial function to the pre- and postedge region, respectively, resulting in a normalized μ(E). An atomic background was then removed to isolate the EXAFS
definition possess a defined crystal structure that is different from the constituting metals and often exhibit more covalent binding, may be more resistant to segregation than random alloys. This provides further motivation for us to study the stability of Pd−Ga nanoparticles. Another potential application of Pd−Ga intermetallic compounds that has also been a topic of great interest is the steam reforming of methanol (MSR).14−17 Analogous to the related Pd−Zn system, which has been studied more extensively as a potential replacement of the industrially applied Cu catalysts,18,19 Pd−Ga has shown superior catalytic properties as compared to unmodified supported Pd. Although it is generally agreed that the formation of intermetallic compounds from metallic Pd and Ga (originating e.g. from reduced parts of the Ga2O3 support) is a prerequisite for achieving the desired selectivity in MSR, a number of questions remain open. These include details of the formation mechanism, the identity of the alloy/intermetallic compound that is formed under certain conditions and, most importantly, the stability of the intermetallic nanoparticles under various pretreatment and reaction conditions. The catalytic properties in methanol steam reforming of the same Pd/Ga2O3 sample used in the present work were reported in a previous publication.17 A strong influence of the reduction temperature applied prior to the reaction was observed. The selectivity to CO2 and H2 formation in MSR at 523 K developed with increasing reduction temperature, from 2% when reduced at 303 K, up to about 90% after reduction at 673 K. Experiments at varying reaction temperatures and flow rates and thus different conversion levels indicated that the selectivity is mainly influenced by the state of the catalyst surface (metallic or intermetallic) and not by the conversion.17 This strong dependence of the catalytic performance on the pretreatment and as a consequence on the structure and surface composition of the catalyst is an apparent motivation for detailed investigations of the structural properties and the process of intermetallic formation. In this work, we utilized in situ X-ray absorption spectroscopy to explore in detail the formation process, structure and stability of Pd−Ga bimetallic particles formed upon H2 reduction starting from Pd/Ga2O3. Based on the EXAFS spectra of Pd/Ga2O3 and Pd2Ga/Ga2O3, hydride formation in hydrogen atmosphere could be excluded on the latter. The stability under oxygen was determined by combining XAS with XRD and FTIR of adsorbed CO, which provide complementary information. Although XRD showed no bulk structural changes, CO adsorption indicated a strongly modified surface composition. Application of XAS provided clear indications on the structural modification occurring upon oxygen exposure, resulting in a mixture of Pd or a Ga-depleted IMC and Pd2Ga.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Pd/Ga2O3 catalysts were prepared as described in a preceding publication17 by incipient wetness impregnation of commercial β-Ga2O3 (Alfa Aesar, 99.99% purity, particle size
