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Adsorption and Decomposition of Methanol on Gold Nanoclusters Supported on a Thin Film of Al2O3/NiAl(100) Shrikrishna D. Sartale,† Hong-Wan Shiu,† Ming-Han Ten,† Won-Ru Lin,† Meng-Fan Luo,*,† Yin-Chang Lin,‡ and Yao-Jane Hsu*,‡ Department of Physics and Center for Nano Science and Technology, National Central UniVersity, 300 Jhongda Road, Jhongli 32001, Taiwan, and National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan ReceiVed: August 21, 2007; In Final Form: NoVember 12, 2007
Adsorption and decomposition of methanol on Au nanoclusters supported on an ordered ultrathin film of Al2O3/NiAl(100) have been investigated by means of their photoelectron spectra excited with synchrotron radiation and scanning tunneling microscopy. The results show adsorption sites on the clusters of at least two types: one catalyzes the decomposition of methanol via scission of the O-H bond into CO at a temperature as low as 120 K, whereas the other activates dehydrogenation only into methoxy. The CO from the decomposition remains on the surface up to 250 K, begins to desorb above 250 K, and decomposes further into elemental carbon around 350 K. All carbon species are removable from the surface at 450 K, a temperature much lower than on other metals studied. Regarding effects of morphology of the clusters on the reactivity, smaller Au clusters, of height 1-2 atomic layers, exhibit inactivity toward the decomposition of methanol.
1. Introduction Methanol has been extensively investigated because of its potential as a logistic fuel and a feed for a fuel cell.1 The presence of heteroatom bonds and their diverse mechanisms of decomposition on various surfaces and with involvement of stable surface intermediates make this molecule special for investigations in surface science. As part of various methanol conversionsssteam re-forming, partial oxidation, etc.sthe decomposition of methanol as a source of hydrogen has attracted intensive research. The adsorption and decomposition of methanol have thus been explored over various metal,2-16 alloy,17-19 and oxide20-23 surfaces, whereas there has been little research on reactions of methanol on metal nanoclusters supported by well-defined oxides, i.e., model catalysts.24-26 Pure gold (Au) had long been considered a poor catalyst for most reactions, mainly because of its weak interaction with most adsorbates, before Haruta et al. discovered that Au nanoparticles dispersed on transition-metal oxides exhibit extraordinary catalytic activity for the oxidation of CO.27 Much academic and industrial research has been subsequently devoted to catalysis on Au (see, for example, refs 28-31 and references therein). The supported Au nanoparticles exhibit a greater catalytic activity for other reactions including oxidation and hydrochlorination of hydrocarbons, water-gas shift, and NO reduction.29,32 The potential of Au catalysts in fuel cells and related processing of hydrogen fuel has attracted attention,31 but little is known about the decomposition of methanol on Au catalysts.33-35 For detailed mechanisms, model systems are in great demand, to which this work responds. Moreover, CO produced from complete dehydrogenation of methanol might adsorb on metallic catalysts and cause severe poisoning, such as that seen for Pt * To whom correspondence should be addressed. E-mail: mfl28@ phy.ncu.edu.tw (M.-F.L.);
[email protected] (Y.-J.H.). † National Central University. ‡ National Synchrotron Radiation Research Center.
catalysts in a direct methanol fuel cell. Our exploration of alternative catalysts, the supported Au clusters, may provide an opportunity to reduce such a poisoning effect. Our work has focused on the adsorption and decomposition of methanol on Au nanoclusters supported on an ordered Al2O3/ NiAl(100) thin film. The morphology and structure of the surface and clusters were characterized with a scanning tunneling microscope (STM) and reflection high energy electron diffraction (RHEED). The adsorption and temperature-dependent decomposition of methanol were characterized by photoelectron spectroscopy (PES) with synchrotron radiation. To elucidate the effects of cluster size and structure on the methanol decomposition, we grew the Au clusters by vapor deposition as a function of coverage (0.05-2.9 monolayer (ML)) and temperature (300570 K) of the substrate. The methanol was adsorbed at 120 K; we investigated the temperature dependence of methanol decomposition by increasing the sample temperature in a stepwise manner and recording a photoelectron spectrum of the reaction mixture on the surface at each temperature step. The behavior of methanol is reflected in the temporal variation of C 1s photoelectron spectra. The results show that adsorption sites on the clusters are of two kinds: one activates the decomposition via scission of the O-H bond into CO at a temperature below 120 K, whereas the other does not. The CO on the active sites remains on the surface below 250 K, begins to desorb above 250 K, and decomposes into elemental C (or mixed with some CHx) around 350 K. The methanol on the inert sites either desorbs or dehydrogenates into methoxy with increasing sample temperature. All carbon species can be desorbed from the surface at 450 K, a temperature much lower than for other metals, implying that carbon-induced poisoning can be readily suppressed on the Au clusters. The reactivity also exhibits a dependence on the morphology of the cluster: clusters with 1-2 atomic layers do not catalyze efficiently the methanol decomposition.
10.1021/jp0767011 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008
Methanol on Gold Nanoclusters
J. Phys. Chem. C, Vol. 112, No. 6, 2008 2067
2. Experimental Section All experiments were performed in ultrahigh vacuum (UHV) chambers with a base pressure in the 10-10 Torr regime. The NiAl(100) sample (MaTech GmbH) was polished to a roughness less than 30 nm and an orientation accuracy better than 0.1°. A thermocouple (type K) was spot-welded onto the side of the sample to monitor the temperature. The temperature difference between the sample and the measured values was smaller than 20 K. Through heating by electron bombardment and cooling with liquid nitrogen, the sample temperature was varied between 120 and 1500 K. To obtain a clean surface, the sample underwent alternative cycles of sputtering and subsequent annealing before each experiment. The cleanliness of the sample was monitored with Auger electron spectroscopy (AES), lowenergy electron diffraction (LEED), STM, and PES. An ultrathin θ-Al2O3 film was formed on oxidation of a NiAl(100) alloy surface at high temperature; descriptions of the formation of Al2O3 thin films appear in our previous articles.36-39 To achieve in this work a homogeneous crystalline Al2O3 surface with almost no NiAl facet,36,39 we refrained from protracted postoxidation annealing of oxide films. The sample was then quenched to a selected temperature for vapor deposition of Au from ultrapure Au wires in a Mo crucible heated by electron bombardment in a water-cooled commercial evaporator (Omicron EFM 3). The rate of deposition of Au was fixed at about 0.11 ML/min, calculated according to the coverage of Au prepared at room temperature. The Au coverages were estimated from the volumes of Au clusters on crystalline Al2O3 observed with a STM (1 ML corresponds to an area density 1.2 × 1015 atoms/cm2 of fcc Au(100) surface atoms). STM images (acquired with a RHK UHV 300 unit), constant-current topographies, were obtained at 90 K with a sample bias voltage typically 2.4-2.8 V and a tunneling current 0.8-1.2 nA. The STM tip consisted of an electrochemically etched tungsten wire. After Au deposition, the sample was cooled to the desired adsorption temperature 120 K. Methanol gas was admitted into the UHV chamber through a leak valve with a background pressure 1 × 10-8 Torr. The highly pure methanol (99.8%) was additionally purified by repeated freeze-pump-thaw cycles. We report methanol exposures in Langmuir units (1 L ) 10-6 Torr s). The PES experiments were performed at the U5-spectroscopy beamline at National Synchrotron Radiation Research Center (NSRRC) in Taiwan.40 The beamline is equipped with four spherical gratings to deliver soft X-ray photons with energy in a range 60-1400 eV. The total energy resolution, including the beamline and energy analyzer, was estimated to be better than 0.2 eV. A fixed photon energy 383 eV was used throughout these experiments. The beam was incident normal to the surface; photoelectrons were collected at an angle 58° from the surface normal. All photoelectron spectra presented here were first normalized to the photon flux. The binding energy (BE) in all spectra is referred to the substrate bulk Al 2p core level at 72.9 eV.41 After linear background subtraction, the spectra were fitted using mixed Gaussian-Lorentzian functions based on a nonlinear least-squares algorithm. 3. Results and Discussion 3.1. Morphology and Structure of the Supported Au Nanoclusters. Before reporting the adsorption and decomposition of methanol, we discuss briefly the effect of coverage and growth temperature on the morphology and electronic structures of Au nanoclusters on an Al2O3/NiAl(100) thin film. STM images reveal that, at a coverage
