Progetti di ricerca

Few-metal atoms aggregates deposited on solid supports, usually oxides, are close to the optimal limit of size/activity ratio in heterogenous catalysis. Downsizing to the sub-nano dimension allows to save precious material and maximise the ratio of active metal atoms in the aggregates. The… Leggi tutto aim of the present project is to study the growth, aggregation, chemical nature and reactivity of sub-nanometric metal clusters at the surfaces of ultrathin oxide films. The motivation for using ultrathin films rather than bulk surfaces is that they allow for additional degrees of freedom, permitting a fine tuning of the cluster properties. Indeed, for thicknesses of few monolayers the properties of the clusters' atoms may be influenced, e.g., by the strain in the oxide film due to the lattice mismatch with the metal support and by charge transfer occurring via electron tunnelling from the metal support through the ultrathin film. The presence of oxygen at the metal/oxide interface may also play a role in determining the oxidation state, the morphology and the stability of the metal clusters at the oxide surfaces. We propose to investigate aggregates of different metal atoms (Ni and Pd) deposited on the surface of MgO films supported on Ag(100). The clusters will be characterised with state-of-the-art experimental surface science techniques such as scanning probe microscopies (STM and AFM) and electron based spectroscopies (XPS, STS, HREELS). Experimental results will be supported by first-principles DFT based calculations (acronyms defined in the main text). The catalytic activity will be investigated for three reactions: CO2 methanation, methane oxidation to methanol, and steam reforming. These reactions are of paramount relevance to green chemistry and fine synthesis, since they deal with the usage of harmful greenhouse gases (CO2, CH4) and with the prototypical activation of very stable C-O and C-H bonds. We aim at exploring the reactivity of Ni and Pd clusters both toward reduction (CO2 methanation) and oxidation (methane oxidation and steam reforming) reactions, based on the different affinity to Ministero dell'Università e della Ricerca MUR - BANDO 2022 oxygen displayed by Ni and Pd and aiming at performing different processes on the same, well-defined catalytic system. In some selected cases, we plan to study, within this project, also the morphology, stability and reactivity of bimetallic NiPd clusters. The strength of the SUMCAR project relies on the possibility to define the active particles at the atomic level by combining microscopy, spectroscopy and atomistic simulations, and to follow the evolution of the adsorbed reactant molecules through the reactions. The project benefits of previous consolidated common works carried out by the team to characterise the spontaneous oxidation of Ni nanoclusters on MgO/Ag(100). Preliminary promising results display the tendency of these aggregates to strongly activate CO molecules under relatively mild thermal conditions.

Bimetallic nanoclusters (bNCs) are at the forefront of research in chemical physics for their peculiar structural, electronic and catalytic properties, which are different with respect to those of bulk materials and alloys and of mono-metallic nanoparticles. The capability to control… Leggi tutto their size and composition allows to tune their properties and to optimize the use of precious metals, which are often of high cost and difficult supply. In addition, their catalytic properties can be influenced by the presence of the solid support under two aspects: i) the anchoring of the nanoparticles to the substrate enhances the stability of the catalyst through the life cycle and prevents adverse aggregation phenomena; ii) the chemical interaction with the support can modify the catalyst and represents a promising tunable factor to enhance the particle’s activity. In particular, the interaction with the support may affect shape, charge state and electronic structure of the metal cluster. Current front-line research focuses on ultrasmall particles, that represent the ultimate limit for the optimal use of precious materials due to their extremely large surface to volume ratio. In this limit, moreover, the simplicity of the NC shape leads to a higher selectivity, while a larger fraction of atoms are located at the cluster/substrate interface and are chemically modified by their bonding with the substrate. In the Bi-NANO project we intend to study the physical and chemical properties of bNCs used for the conversion of biomasses. If derived from wood and agricultural residues, biomasses are often rich in lignin, cellulose and hemicellulose, which are an excellent base for the production of fuel and chemicals, but have a large oxygen content. The latter can be reduced by dehydration and hydrodeoxygenation, for which reactions bNCs are promising catalysts. We propose to investigate the processes underlying the chemical reactions of high economic and environmental relevance mentioned above on well-defined model systems. This research will provide elements for a knowledge-based optimization of the current protocols and hints for the design of new active systems. For this reason, we will focus our attention on Pt-Zn, Ru-Cu and Cu-Pt bNCs of ultra-small size, i.e. up to a few tens of atoms, supported on MgO or graphene. We will study the interaction of these systems with very simple reactants (e.g. ethylene glycol and benzaldehyde) that contain, however, some of the functional groups (OH, C=O) involved in the deoxidation processes occurring in biomass conversion. Our analysis will take advantage of the complementary competences of the research team, which combines the most advanced experimental methods of surface and material science (scanning probe microscopy, photoemission and vibrational electron-based spectroscopies, thermal desorption spectroscopy) with up-to-date density functional theory calculations.