LETTER pubs.acs.org/JPCL
Dynamic Structure of a Working Methanol Steam Reforming Catalyst: In Situ Quick-EXAFS on Pd/ZnO Nanoparticles Karin F€ottinger,*,†,‡ Jeroen A. van Bokhoven,*,‡,§ Maarten Nachtegaal,§ and G€unther Rupprechter† †
Institute of Materials Chemistry, Vienna University of Technology, 1060 Vienna, Austria Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland § Paul Scherrer Institute, 5232 Villigen, Switzerland ‡
bS Supporting Information ABSTRACT: Combining time-resolved X-ray absorption spectroscopy (XAS) with simultaneous mass spectrometric activity measurements, both performed during catalytic methanol steam reforming, we have studied the dynamic changes of Pd nanoparticles supported on ZnO. The formation of the catalytically active phase PdZn was unambiguously monitored in real time. We thus directly prove for the first time that the PdZn alloy formed in the course of the catalytic reaction is responsible for the excellent selectivity to H2 and CO2. Alloying started at the nanoparticle surface and proceeded “inwards” toward the particle center. On the basis of extended X-ray absorption fine structure (EXAFS) analysis, the extent of PdZn alloy formation was estimated. Alloying was reversible. In contact with oxygen, PdZn segregated into palladium metal and ZnO. The XAS data were corroborated by Fourier transform infrared (FTIR) spectroscopy. It is demonstrated that the application of operando quickEXAFS is a powerful tool for studying nanomaterials and their dynamic adaptation to reactive environments. SECTION: Surfaces, Interfaces, Catalysis
H
ydrogen may be one of the most important future energy sources. Methanol steam reforming (MSR) yielding H2 and CO2 is among the most promising processes for on-board hydrogen production for fuel cells, with MSR operating at relatively low reforming temperatures (523-623 K; CH3OH þ H2O f CO2 þ 3 H2). Currently, Cu supported on ZnO is commercially used as an MSR catalyst, but this catalyst suffers from serious drawbacks, such as low thermal stability, tendency to sintering, and, moreover, Cu is pyrophoric.1 Iwasa and coworkers2-6 reported excellent MSR performance for Pd/ZnO, exhibiting nearly the same activity as the Cu-based catalyst but with much better long-term stability. Since then, Pd/ZnO has attracted growing attention as an alternative catalyst for industrial MSR. In contrast to Pd nanoparticles on inert (nonreactive) supports, which rather catalyze methanol decomposition to CO and H2 (CH3OH f CO þ 2 H2), Pd supported by ZnO shows excellent selectivity to CO2 and H2. The substantially different catalytic behavior was attributed to the formation of a PdZn alloy or intermetallic compound (IMC) upon hydrogen reduction.2-7 PdZn is widely accepted as the active phase for MSR, but this is r 2011 American Chemical Society
based on ex situ (postreduction and postreaction) characterization of the catalysts by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).2,3,5,8,9 Comprehensive in situ studies of the structure and composition are lacking so far, but they would be required to unambiguously prove that alloy formation is indeed relevant during catalytic process conditions. Conant et al.10 applied in situ XRD for studying the reduction of Pd/ZnO/Al2O3 in hydrogen. However, the alloy composition could not be identified conclusively due to broad reflections and a high background of the alumina support. Bollmann et al.11 applied ex situ X-ray absorption spectroscopy (XAS) to study Pd/ZnO/Al2O3 water gas shift catalysts after reduction, and they found an increasing extent of alloy formation with increasing amount of Zn added. On the basis of density functional theory (DFT) calculations12,13 and measurements of valence band spectra by XPS and ultraviolet photoelectron spectroscopy Received: December 31, 2010 Accepted: February 4, 2011 Published: February 10, 2011 428
dx.doi.org/10.1021/jz101751s | J. Phys. Chem. Lett. 2011, 2, 428–433
The Journal of Physical Chemistry Letters
LETTER
industrial applications). The resulting mean Pd aggregate size was ∼30 nm, as determined by CO chemisorption (after reduction at room temperature) and by transmission electron microscopy (TEM). Figure 1 shows an image of a clearly polycrystalline Pd particle after calcination (623 K) in air (subsequent reduction did not change the size of the aggregates). The presence of smaller domains a few nanometers in size as constituents of the large aggregates could potentially affect the Zn diffusion into the Pd. Due to the low surface area of ZnO (∼2-3 m2/g) and the high Pd loading, the aggregates were rather large. Nevertheless, the high Pd loading enabled acquisition of higher quality EXAFS spectra. EXAFS and Fourier transform IR spectroscopy using CO as the probe molecule (CO-FTIR) indicated that the particles were mostly metallic with only the surface being partially oxidized. In the following, the evolution of the active phase, as monitored by EXAFS and FTIR, will be discussed. Figure 2A (trace a) shows the Pd K near-edge region of the Pd/ZnO catalyst upon pretreatment in inert gas (He) at 623 K. Prior to high temperature reduction (623 K), Pd was in the metallic state. The X-ray absorption near-edge structure (XANES) spectrum resembled that of Pd metal, and fitting of the EXAFS region indicated a mean Pd-Pd coordination number of about 7.5 at a mean PdPd atomic distance of ∼2.75 Å. When the catalyst was exposed to reactive conditions, that is, the MSR reaction mixture (CH3OH/ H2O) at 423 and 523 K, the XANES spectra remained unaltered though (not shown), indicating that the catalyst structure/ composition did not change in this temperature range. However, when the reaction temperature was increased to 623 K, a technologically relevant reaction temperature, the XANES spectra gradually changed with time-on-stream, as illustrated by the arrow in Figure 2A (traces b). In particular, the spectral features at around 24.390 and 24.430 keV decreased in intensity, and an additional feature at about 24.410 keV appeared. These spectral changes were paralleled by a pronounced change in reaction selectivity from methanol decomposition (CO þ H2) to steam reforming (CO2 þ H2), as clearly identified by a decrease of the signal of CO (mass 28) and an increase of the CO2 signal (mass 44) with time-on-stream (cf. Figure 4). The overall conversion of methanol remained constant, though. It is wellknown that methanol decomposition is the main reaction occurring on Pd black, as well as on Pd nanoparticles supported on various (inert) carriers such as SiO2 or activated carbon.2,4,5 The change in selectivity toward steam reforming can be explained by the successive transformation of the (metallic) Pd nanoparticles to PdZn alloy nanoparticles, which were highly selective toward CO2 and H2. Comparison to a theoretical reference spectrum of PdZn, obtained using the ab initio scattering code FEFF8,19 based on crystallographic input parameters of a 1:1 PdZn ordered IMC, confirmed that the spectral changes were the result of the formation of the selective PdZn alloy (see Supporting Information). The observed dynamic adaptation of the catalyst (surface) structure to the gaseous reaction environment is in accordance with a study of Conant et al.10 who reported that the reactivity of a Pd/ZnO/Al2O3 catalyst was independent of the catalyst pretreatment. This observation was attributed to the formation of a specific equilibrium surface state during the reaction, which solely depended on the actual reaction conditions but not on the applied pretreatment. Similarly, Tao et al.20 reported dynamic changes of the surface composition of bimetallic PtRh nanoparticles under different reaction environments, applying
Figure 1. TEM image of calcined Pd/ZnO.
(UPS),12,14 a modification of the electronic properties upon alloy formation was suggested as the cause for the strongly altered reactivity, affecting the stability of various reaction intermediates. Studying a PdZn/Pd(111) model catalyst by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), Jeroro et al.15,16 observed a large effect on the reactivity already at low Zn coverages (
