Short communicationIdentification of step-edge sites on Rh nanoparticles for facile CO dissociation
Graphical abstract
Introduction
Supported metal nanoparticles are among the most used heterogeneous catalysts for important reactions such as steam methane reforming, ammonia synthesis, automotive exhaust gas clean-up and Fischer–Tropsch synthesis. Understanding the dependence of the rate of catalytic reactions on metal nanoparticle size remains one of the great challenges in heterogeneous catalysis [1], [2], [3], [4], [5]. Nanoparticles expose terraces, edges, corners, kinks and atomic steps. The different degree of coordinative unsaturation of these surface atoms leads to variations in chemical reactivity. Quantum-chemical calculations provide insight into the dependence of particular elementary reaction steps on surface topology. For the cleavage of σ-bonds (e.g. C–H bonds in CH4), low-coordinated corner and edge atoms are necessary [4], [5], [6], [7]. Dissociation of molecules with strong π-bonds (CO, NO and N2) is favored on step-edge sites because of the favorable geometry that relates to the strong overlap of the transition metal d-orbitals with the molecular π-bonds and the absence of surface metal atom sharing with the dissociating fragments in the transition state [4], [5], [8]. Below a critical size, nanoparticles do not contain step-edges and they will only expose terrace, edge and corner atoms [9], [10]. The highest density of step-edge sites will therefore occur on particles of intermediate size and the surface of large particles is dominated by terraces. A case in point of structure sensitivity of considerable commercial interest is the Fischer–Tropsch reaction, which employs metal nanoparticles to convert synthesis gas into transportation fuels and chemicals [11], [12], [13]. Step-edge sites are presumed to be the reaction centers for the dissociation of the carbon–oxygen bond in CO, which is one of the important steps in the overall Fischer–Tropsch reaction [14], [15], [16]. The strong dependence of CO conversion rate on metal nanoparticle size has been argued to relate to variation in the density of step-edge sites at the nanoparticle surface [9], [14], [17], [18], yet convincing evidence for this assertion is lacking.
Important contributions to the structure sensitivity issue in heterogeneous catalysis mainly derive from single crystal surface science studies. Ertl and co-workers convincingly demonstrated that step-edge sites on single-crystal surfaces are the reaction centers for NO dissociation [1]. Nørskov and co-workers proved that N2 dissociation also occurs on step-edge Ru sites [2]. While enumeration of step-edge sites on single-crystal models is straightforward, identification of step-edge sites on nanoparticles in technical catalysts is not possible yet. The earliest suggestion that steps at the nanoparticle surface are important for the activation of small molecules was made by Van Hardeveld and Van Montfoort [19], [20]. Based on the strong perturbation of the N–N bond in of adsorbed N2 on Ni, Pd and Pt nanoparticles observed in IR spectra, the authors claimed that the nanoparticle surface contains ensembles of surface atoms arranged in the form of a step. This unique B5 step-edge site comprises five surface metal atoms. Based on geometric considerations, Van Hardeveld concluded that B5 sites occur most frequently on the surface of ~ 2 nm particles [19], [21], [22]. This interpretation is supported by work of others [23], [24].
In this work, we demonstrate that N2 can be used as a semi-quantitative IR probe for coordinatively unsaturated sites that are a part of these step-edge sites. The choice for Rh in this study derives from the large activation barrier difference for CO dissociation on step-edge (~ 167 kJ/mol) and planar surfaces (300 kJ/mol) [5], [8], [25]. Accordingly, we expect the rate of CO dissociation on Rh nanoparticles to be proportional to the step-edge site density. As compared with Co and Fe that mainly produce long-chain hydrocarbons in the Fischer–Tropsch reaction, the use of Rh is interesting because in this way also oxygenates can be obtained [26]. Herein, we seek to correlate the IR intensity of N2 molecules perturbed by step-edge sites to the CO dissociation rate. We use a set of zirconia-supported Rh catalysts, which have been used before in studying the structure sensitivity in steam methane reforming [7]. The interpretation of the IR spectra is supported by DFT calculations of N2 adsorption on various Rh model surfaces.
Section snippets
Results and discussion
ZrO2-supported Rh nanoparticles were prepared by incipient wetness impregnation of Rh-nitrate on zirconia [7]. The Rh particle size was varied by changing the calcination temperature of the support, the Rh loading and the calcination/aging procedure of the final catalyst (see Table S1 in Supplementary material). In this way, a set of Rh nanoparticle catalysts was obtained comprising nanoparticles in the 1–8 nm range stabilized by a monoclinic zirconia support. Some of these catalysts were taken
Conclusions
IR spectroscopy of adsorbed N2 helps to identify step-edge sites on metallic Rh nanoparticles. N2 preferentially adsorbs on top of low-coordinated Rh surface atoms; this gives rise to an intense IR band at 2205 cm− 1. DFT shows that N2 IR probes low-coordinated surface atoms. The intensity strongly correlates with the rate of CO dissociation with maxima for Rh/ZrO2 observed at intermediate particle size. This implies that, at this intermediate size, nanoparticles contain additional corner and
Acknowledgments
The authors thank the Netherlands Organization for Scientific Research for access to the national high-performance computing facilities. EJMH also thanks financial support from a TOP grant of the Netherlands Organization for Scientific Research (NWO).
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