9040
J. Phys. Chem. C 2008, 112, 9040–9044
MgO-Supported Rhodium Particles and Films: Size, Morphology, and Reactivity P. Dudin,† A. Barinov,† L. Gregoratti,† D. Scaini,† Y. B. He,‡ H. Over,‡ and M. Kiskinova*,† Sincrotrone Trieste, Area Science Park, I-34012 BasoVizza-Trieste, Italy, and Department of Physical Chemistry, Justus-Liebig-UniVersity, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany ReceiVed: February 29, 2008; ReVised Manuscript ReceiVed: March 25, 2008
The chemical transformations of supported Rh particles, ranging in size from a few micrometers to a few nanometers, and nanocrystalline Rh films have been studied under identical oxidizing and reduction conditions by means of scanning photoelectron microscopy (SPEM), which allows determination of the chemical state of single Rh microparticles. Comparing the oxidation states attained by the Rh particles revealed substantial reactivity differences with particle size, as well as variations in the reactivity of particles with similar dimensions and within the same particle. The results are interpreted in terms of the variable morphology of the particles as verified by secondary electron microscopy and atomic force microscopy. 1. Introduction The key issue in understanding the reaction mechanisms in heterogeneous catalysis is the identification of the catalytically active state of the catalyst, which develops under the specific reaction conditions. In oxidation reactions on late transition metal catalysts, surface oxide or bulk oxide phases have been identified with the catalytically active states.1–3 For instance, extensive studies of oxidation/reduction of Rh single crystal and vicinal surfaces have provided clear evidence for the formation of Rh surface oxide, mediating the formation of a stoichiometric bulk oxide phase.3–6 The vicinal surfaces undergo the largest morphological changes, due to the large density of undercoordinated atoms at the step edges. This structural complexity represents the most important chain in the bottom-up approach to bridge the “materials gap” between model catalysts and supported metal particles.7–12 However, recent reports have demonstrated that one cannot always find a direct correlation between the reactive properties of supported particles and those of extended surfaces. The polyhedral equilibrium shape of small supported particles, defined by minimizing the surface energy and truncated at the interface via the adhesion energy, is too idealized for reality.9 The specific activities of particles smaller than 5 nm is mostly related to the dramatic increase of the relative density of the edges, corners, steps, and facets exposing an appreciable number of coordinatively unsaturated atoms at the surface,13 as well as to surface stress and increased surface mobility.14 In addition, quantum size effects and the altered properties of the interfacial region between the particles and the support may exert a stronger effect in smaller particles, where wetting of the support might prevail over those of the predicted equilibrium shape.15 A survey of the literature shows that there are no general size-reactivity trends. For example, with the oxidation/reduction behavior of Rh supported catalysts, the Rh particle size effect points toward lower activity of the larger particles during oxidation but no size dependence was observed during reduction.16 In fact, the size dependence should be correlated to the morphology of the supported particles, which can vary between * To whom correspondence should be addressed. Phone: +39-0403758549. Fax: +39-040-3758565. E-mail:
[email protected]. † Sincrotrone Trieste. ‡ Justus-Liebig-University.
particles of the same size and dynamically change under reaction conditions. Significant structural alteration accompanying the oxidation and reduction processes and the dynamic response of Rh particles under variable oxidation/reduction conditions have been monitored by in situ transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS).17,18 The dynamic changes during oxidation/reduction cycles may also lead to nonreversible aggregation and sintering processes with distinct alteration of the reactivity.9,18,19 Both formation of Rh2O3 and RhO2 phases were evidenced depending on the reaction temperature and oxygen potential. The oxidation process was accompanied by progressive sintering producing progressively larger particles.20 In this X-ray photoelectron microscopy study we address the size and morphology effects comparing the reactivity in the oxidizing and reducing environment of Rh nanoparticles, microparticles, and nanocrystalline films grown on MgO supports. We were able to unravel the relative activity of the different Rh particles under identical reaction conditions, as well as the lateral variations in the oxidation states within the same microparticle. 2. Experimental Details Model supported catalysts were prepared by pulsed laser deposition (PLD) of Rh with a Nd:YAG laser operated at 10 Hz repetition rate and 10 ns pulse width. The ablated rhodium target was placed in the focal spot of the 10 cm lens illuminated with both 532 and 1064 nm harmonics of the laser. The calibration with 1 µm aperture and power meter estimated the fluence at 10 J/cm2, which is fairly above the laser ablation limit. Due to the rather high fluence we were able to produce rhodium film decorated with microparticles, which is essential for the whole experiment since we can study with scanning photoelectron microscopy (SPEM) the local reactivity of single supported Rh particles as a function of size under identical reaction conditions. PLD prepared Rh particles represent a novel procedure to prepare model supported catalysts. We used as supports a MgO(100) single crystal sample or ultrathin 2-3 nm MgO films grown on a W(110) substrate. Before Rh deposition the MgO(100) sample was cleaned in situ by Ar ion sputtering followed by annealing at 1100 K in 10-6 mbar oxygen to restore the stoichiometry and to smoothen the
10.1021/jp8017953 CCC: $40.75 2008 American Chemical Society Published on Web 05/24/2008
Size, Morphology, and Reactivity of Rh Particles
J. Phys. Chem. C, Vol. 112, No. 24, 2008 9041
TABLE 1: Compilation of Binding Energy (BE) Values of Rh 3d5/2 and O 1s (eV) for Single Crystalline Rhodium, Rhodium Nanoparticles, Transient “Surface oOxide” and the Two Stoichiometric Oxides, Rh2O3 and RhO2 Taken from the Literature compound
Rh 3d5/2 (eV)
Rh metal Rh nanoparticles RhOx on Rh singlecrystal surface Rh2O3 RhO2
307.2 307.0-308.5 308.0 ( 0.1
two components: 529.9 and 528.9 (surf) ( 0.1
23 22 3, 24
308.2-308.6 308.6-309.4
two components: 529.9 (dominant) and ∼531.2 two components: ∼529.8 and 531.1 (comparable intensity)
20, 25, 26 26
O1s (eV)
surface. Smooth and stable up to 600 K, MgO films were prepared by growth of an epitaxial Mg layer on an atomically clean W(110) sample at 350 K, followed by deposition of Mg in ambient oxygen (3 × 10-6 mbar). The deposition of Mg was carried out in the ultrahigh-vacuum-connected preparation chamber of the SPEM station, which is equipped with a lowenergy electron diffraction (LEED) and Auger spectrometer. By monitoring the dependence of the Mg KLL and W MNN Auger signals with deposition time, a clear slope break was observed with completion of the first Mg layer, which was used for calibration of the deposition time versus film thickness. The advantage of the MgO films over the MgO(100) sample is that they are conductive enough to eliminate the charging problems in the photoemission studies. After deposition of Rh on MgO, samples were annealed at 500 K in H2 environment which resulted in Rh 3d5/2 spectra indicative of a pure metallic state. Ex situ characterization of a large set of samples by atomic force microscopy (AFM) and secondary electron microscopy (SEM) allowed us to establish the deposition procedures for reproducible fabrication of two types of samples. Both types contain isolated, randomly distributed large microparticles (from ∼0.3 to a few micrometers). The difference is in the areas surrounding the microparticles, namely, a 15-20 nm thick nanocrystalline Rh film (sample type 1) or dispersed nanoparticles with dimensions from a few nanometers to a few tens of nanometers(sample type 2). Following the established growth recipes (varying the deposition time at constant laser fluence) we prepared and characterized the two types of samples in the experimental station of the SPEM at the ESCA microscopy beamline.21 We used an atomic oxygen plasma source for the oxidation, since the oxidation of Rh by molecular oxygen at pressures lower than 10-4 mbar is kinetically hindered by the oxygen dissociation. The oxidation with atomic oxygen was carried out at 450-500 K, with the pressure in the measurement station being kept below 10-6 mbar. The upper limit of the atomic oxygen flux in the vicinity of the sample surface was estimated to be ∼ 1013 atoms/s × cm2. The overall oxygen dose was controlled by the exposure time. After oxidation detailed SPEM characterization of the oxidation state was carried out of various microparticles and their surrounding areas covered by a nanocrystalline film (type 1) or isolated nanoparticles (type 2). To explore the oxide activity, in situ reduction in ambient H2 (pressures up to 5 × 10-6 mbar) was performed. As fingerprints of the oxidation states, we used the Rh 3d5/2 and O 1s spectra, which have distinct components for the transient “surface oxide” and the two stoichiometric oxides, Rh2O3 and RhO2. The Rh 3d5/2 and O 1s binding energy positions of these oxidation states used in the present study are summarized in Table 1. Note the spread in the binding energy (BE) values for the two oxide phases, the higher BE values being measured for supported Rh nanoparticles.20,22
refs
Figure 1. AFM (a) and SPEM (b) images illustrating the distribution of microparticles on large areas and the shape of single particles. The higher resolution AFM distinguishes particles consisting of several aggregates. (c) SEM images representing the three typical morphologies of the microparticles and of the nanocrystalline film.
3. Results and Discussion The typical morphologies of the Rh deposits on the MgO supports under investigation are illustrated by the AFM, SPEM, and SEM images in Figure 1. The randomly distributed Rh particles, with dimensions ranging between ∼0.2 µm and a few micrometers are well-distinguishable by AFM and with inferior resolution also with SPEM. The representative SEM images in Figure 1c show typical surface morphologies of the microparticles. The round microparticles in SPEM and AFM images expose different grains (from a few tens to a few hundreds of nanometers), whereas the elliptical ones appear more facetted or rougher. Undoubtedly such differences in the surface morphology should affect the reactivity even at micrometer scales. Depending on the duration of the Rh deposition the rest of the surface was covered either by a nanocrystalline Rh film of thickness of ∼12-15 Å (Figure 1c) or randomly distributed Rh nanoparticles of sizes not exceeding ∼20 nm. The relative reactivity of the Rh microparticles and the nanocrystalline films (sample type 1) or randomly distributed nanoparticles (sample type 2) in surrounding areas was determined by comparing the corresponding Rh 3d5/2 and O 1s spectra measured after exposure to oxygen and following reduction in H2. The oxidation and reduction were carried out at temperatures ∼500 K, far below 620 K when significant alteration of surface morphology, spreading, and coalescence can occur.20 Figure 2 shows representative Rh 3d5/2 spectra of a selected isolated microparticle and from the discontinuous 1-2 nm nanocrystalline film around the particle after being exposed to oxygen and following H2 treatment. The Rh spectral features corresponding to different oxidation states are identical for the film and the microparticle. The dominant RhOx component in both Rh 3d5/2 spectra indicates relatively mild oxidation limited to formation of a surface oxide, which precedes the growth of
9042 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Figure 2. Rh 3d5/2 spectra of a nanocrystalline film (a) and a selected microparticle (b) taken after 30 min exposure to atomic oxygen at 490 K (bottom spectrum in each panel) and following 10 min exposure to H2 (top spectrum in each panel. PH2 ) 10-6 mbar).
a thicker oxide phase.24 Closer inspection of the RhOx weight in the deconvoluted Rh 3d5/2 spectra discloses that the particle is covered by a thinner surface oxide layer. However, in the Rh 3d5/2 spectra of the particle already a feature assigned to a Rh2O3-like phase emerges. After the subsequent H2 treatments at 500 K the selected microparticle undergoes almost complete reduction, whereas the nanocrystalline film is only partially reduced. This points toward different activity of the formed oxides, most likely related to differences in the particle and film morphology with respect to size. Exploring different areas of the discontinuous 1-2 nm nanocrystalline film, surrounding the microparticles, we measured almost identical spectra, indicating a homogeneous oxidation and reduction at our probing scales. Similar results were recently obtained for the oxidation/reduction of ultrathin nanocrystalline Ru/RuO2 films on silicon.27 On the contrary, as discussed below, the randomly dispersed Rh microparticles exposed to identical oxidation conditions exhibit significant variations in the attained oxidation state. Figure 3 shows the Rh 3d5/2 spectra taken from a microparticle and a microprobe area covered with dispersed nanoparticles on a sample type 2. The Rh 3d5/2 spectrum of the microparticle after oxidation shows the formation of surface oxide. After the first H2 treatment it evolved into the typical spectrum of metallic Rh, measured also before oxidation. It contains even a weak surface component, supposing exposed faces with a defined structure. The Rh 3d5/2 spectra from the nanoparticles show distinct shape and energy differences. This can be attributed to the following well-known reasons: (i) size- and environmentdependent core level energy shifts below certain critical particle dimensions (
