Thermal and adsorbate effects on the activity and morphology of size-selected Pdn/TiO2 model catalysts
Abstract
Model catalysts containing size-selected Pdn (n = 1,2,4,7,10,16,20,25) deposited on rutile TiO2(110) deactivate during repeated CO oxidation temperature-programmed reaction (TPR) cycles, and the deactivation process has been probed using a combination of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), low-energy ion scattering (ISS), temperature-dependent ion scattering (TD-ISS), annealing experiments, and temperature-programmed desorption following exposure to CO and O2 reactants. Results from such experiments suggest the cluster deactivation proceeds via an alloy-like, strong metal-support interaction (SMSI) effect that chemically modifies the clusters via electronic interactions between the supported metal atoms and Ti from the support. Threshold measurements show that this effect detrimentally affects CO-oxidation activity prior to the formation of an encapsulating overlayer by severely weakening the CO
Pd bond strengths for binding configurations on top of the clusters. Oxidation appears to provide means of partially restoring the clusters to their initial state, but after sufficient exposure to reducing environments and elevated temperatures, all Pdn become covered by an overlayer and begin to electronically and chemically resemble freshly deposited atoms, which are completely inactive towards the probe reaction. In addition, we find evidence of oxygen spillover induced by co-adsorbed CO during TPRs for all active Pdn clusters.
Introduction
Supported catalysts containing particles with sizes of a few nanometers or less often have interesting, and potentially useful chemical properties [1], [2], [3], [4], [5], [6], [7], [8], [9], however, such catalysts often deactivate rapidly due to the low stability of small particles with respect to sintering and other diffusion processes [10], [11], [12], [13], [14]. Stability is a particularly critical issue for catalysts containing particles with fewer than ~ 50 atoms (i.e., clusters), both because chemical properties can vary significantly for even single atom changes in cluster size [1], [2], [3], [4], [7], [15], and because such clusters may sinter rapidly under reaction conditions [16]. Some of the general mechanisms by which catalysts may deactivate include site poisoning [9], [17], particle restructuring/sintering leading to active site loss and changes in electronic structure [7], [10], [16], [18], [19], [20], support degradation [13], and changes in the nature of the particle-support interaction [4].
For reducible metal-oxide supports, like TiO2, deactivation can occur via a particular poisoning mechanism, which is often included as a form of strong metal-support interaction (SMSI) [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. In this mechanism, first noted by Tauster et al. [21], exposure to reducing environments is thought to result in the liberation of interfacial Ti species, which then interact with the catalytic metal particles in such a way that results in their deactivation via either blocking or other detrimental changes to the active-site chemistry. Typically, this behavior is marked by decreased CO uptake, which can be reversed after exposure to oxidizing conditions [26]. Following sufficient reduction, an encapsulating Ti-containing layer is often observed over the particles [23], [24], [27], [29], [30]. The nature of, and mechanism by which, this film appears is a matter of some debate, with some suggesting that alloying of Ti with the metal particles acts to create the eventual overlayer [21], [23], [29], and others suggesting that it is merely a non-interacting TiO(2-x) film that grows over the particles as the surface is reduced [27], [31], [32], [33], [34], [35].
Recently, we reported a study of CO oxidation over Pdn/TiO2(110) (n ≤ 25) where both CO desorption and CO2 production varied non-monotonically with cluster size, and the overall activity was strongly correlated with shifts in the electronic properties of the Pd clusters, as probed by X-ray photoelectron spectroscopy (XPS) of the Pd 3d core level [7]. This variation in Pdn activity was shown to depend largely on size-dependent efficiencies for dissociatively binding O2 [36]. Both theory [37] and experiment [38] have shown that these small Pdn/TiO2(110) catalysts have two distinct Pd-associated CO binding arrangements. CO binds strongly (1–1.4 eV) in sites on top of the clusters, but there are also sites around the cluster periphery bound by 0.5–0.9 eV, and theory suggests that these correspond to CO bound to the edge of the clusters with the CO axis roughly parallel to the surface. Experimentally, CO bound in these two types of sites desorbs as broad features peaking at ~ 430 K and ~ 200 K, respectively.
Here, we explore the effects of CO oxidation on the morphology and electronic properties of Pdn/TiO2 samples, with the goal of understanding the factors that result in the rapid loss of activity during repeated reaction cycles. We recently reported a study of analogous Pdn/alumina/Ta(110) model catalysts [39], and the observation that activity is quite stable in that system suggests that the TiO2 support plays a major role in the deactivation process of the samples discussed herein. Given the general difficulties and efforts associated with the stabilization of small supported clusters [12], [40], [41], and the strong evidence of SMSI in Pd/TiO2 catalysts, it will be of interest to determine if the support interaction is strong enough to stabilize the very small and highly dispersed Pdn against surface diffusion and sintering. Such stabilization could allow for practical application of small cluster catalysts in reactions (i.e. CO2 hydrogenation) where formation of the SMSI state is known to increase activity and/or selectivity [42], [43], [44].
Section snippets
Apparatus
The experimental apparatus, which has been described in detail previously [45], [46], consists of a mass-selected ion deposition beamline attached to an ultrahigh vacuum (UHV) chamber with a base pressure of ~ 1.5 × 10− 10 Torr. The setup allows for in situ sample preparation and characterization by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS), low energy He+ ion scattering (ISS), and temperature-programmed
Deactivation in repeated temperature-programmed reaction cycles
Fig. 1 shows 13CO (bottom) and 13C16O18O (top) desorption during a series of three sequential TPR experiments for Pd7/TiO2 (left) and Pd20/TiO2 (right) samples. Pd7 and Pd20 were chosen as examples of relatively inactive and active catalysts [7], respectively, but qualitatively similar behavior was observed for all clusters studied. Prior to each TPR run, the samples were first exposed to 10 L 18O2 at 400 K, and then exposed to 5 L 13CO at 180 K. The O2 exposure and temperature (Toxidation) were
Discussion
Based on the results presented above, we find that the source of Pdn/TiO2 catalyst deactivation under our CO-oxidation conditions is related to a change in the chemical nature of the clusters, and that this change is most likely related to interactions with the support after exposing the samples to reducing conditions. The changes to cluster chemistry have been identified based on their subsequent reactivity and are marked by a decreased ability to strongly bind CO, a related decrease in the
Conclusion
From differences in ISS spectra after heating oxidized samples to 500 K, with and without first exposing them to saturation CO doses at 180 K, we have been able to propose an oxygen spillover pathway to more fully account for the loss of O reactant from the active sites during CO-oxidation TPR. This pathway, which is catalyzed by Pdn at T < 500 K, appears to be brought about by PdO bond destabilization, resulting from co-adsorption of CO on the clusters.
By tracking changes of the Pdn cluster
Acknowledgments
This work was supported by a MURI grant from the Air Force Office of Scientific Research (AFOSR), grant number FA9550-08-1-0400. The UPS results were obtained under funding from Air Force Office of Scientific Research (AFOSR) Basic Research Initiative Grant FA9550-12-1-0481.
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Mass-selected supported cluster catalysts: Size effects on CO oxidation activity, electronic structure, and thermal stability of Pd<inf>n</inf>/alumina (n ≤ 30) model catalysts
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