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Abstract
The introduction of dopant atoms into metal nanoparticles is an effective way to control the interaction with adsorbate molecules and is important in many catalytic processes. In this work, experimental and theoretical evidence of the influence of Pd doping on the bonding between small cationic AuN+ clusters and CO is presented. The CO adsorption is studied by combining low-pressure collision cell reactivity and infrared multiple photon dissociation spectroscopy experiments with density functional theory calculations. Measured dissociation rates of cluster-CO complexes (N ≤ 21) allow the estimation of cluster-CO binding energies, showing that Pd doping increases the CO adsorption energy to an extent that is size-dependent. These trends are reproduced by theoretical calculations up to N = 13. In agreement with theory, measurements of the C-O vibrational frequency suggest that for the doped PdAuN-1+ (N = 3-5, 11) clusters, CO adsorbs on an Au atom, while for N = 6-10 and N = 12-14, CO interacts directly with the Pd dopant. A pronounced red-shifting of the C-O vibrational frequency is observed when CO interacts directly with the Pd dopant, indicating a significant back-donation of electron charge from Pd to CO. In contrast, the blue-shifted frequencies, observed when CO interacts with an Au atom, indicate that σ-donation dominates the Au-CO interaction. Studying such systems at the sub-nanometre scale enables a fundamental comprehension of the interactions between adsorbates, dopants and the host (Au) species at the atomic level.
Highlights
Fundamental aspects of molecular adsorption and catalytic activity of nanoalloys can be investigated by studying small gas-phase clusters.[15,16,17,19] The ability to tune the size, composition and charge state of clusters in the gas phase enables elucidating the specific role each of these parameters play in different reactions
This is performed by using the Birmingham Parallel Genetic Algorithm (BPGA)[64] combined with Density Functional Theory (DFT)
For the clusters with CO bound to Pd (N ≥ 6, except for N = 11), the lowered νCO frequencies suggest electron charge donation from the d-orbitals of Pd to CO, in agreement with the Blyholder model. This agrees with previous calculations on the Pd–CO system, which predicted a decrease in νCO upon adsorption, smaller than for other transition metals.[103]. For those sizes where CO binds to the Pd dopant, the enhanced adsorption energy of CO upon doping can be rationalized by the significant electron donation from Pd d-states to the 2π* orbital of CO
Summary
Fundamental aspects of molecular adsorption and catalytic activity of nanoalloys can be investigated by studying small gas-phase clusters.[15,16,17,19] The ability to tune the size, composition and charge state of clusters in the gas phase enables elucidating the specific role each of these parameters play in different reactions. From a theoretical point of view, small clusters at the sub-nanometre scale can be studied at a high level of theory.[15] The possibility of combining state-of-the-art experiments with high-level calculations allows the direct comparison of findings, by which one can gain an important. Paper understanding of interactions and processes at the atomic scale.[18,19] sub-nanometre clusters themselves often exhibit higher catalytic performance (activity and/or selectivity)[20] compared to their bulk and even nanoscale counterparts.[21,22] For example, AuN clusters (N = 8, 13 and 20) supported on MgO and Mg(OH)[2] surfaces have shown high catalytic activity for CO oxidation.[23,24,25]
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