Molecular tunability of surface-functionalized metal nanocrystals for selective electrochemical CO2 reduction

Organic ligands are used in homogeneous catalysis to tune the metal center reactivity; in contrast, clean surfaces are usually preferred in heterogeneous catalysis. Herein, we demonstrate the potential of a molecular chemistry approach to develop efficient and selective heterogeneous catalysts in the electrochemical CO2 reduction reaction (CO2RR). We have tailor-made imidazolium ligands to promote the CO2RR at the surface of hybrid organic/inorganic electrode materials. We used silver nanocrystals for the inorganic component to obtain fundamental insights into the delicate tuning of the surface chemistry offered by these ligands. We reveal that modifying the electronic properties of the metal surface with anchor groups along with the solid/liquid interface with tail groups is crucial in obtaining selectivities (above 90% FE for CO), which are higher than the non-functionalized Ag nanocrystals. We also show that there is a unique dependency of the CO2RR selectivity on the length of the hydrocarbon tail of these ligands, offering a new way to tune the interactions between the metal surface with the electrolyte and reactants.

Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst

Pierrick Berruyer, David Benjamin Roger Antoine Gajan

The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

AMER CHEMICAL SOC2022

State-to-state surface scattering of methane studied by bolometric infrared laser tagging detection

Bo-Jung Chen

State-to-state molecule/surface scattering experiments prepare the incident molecules in a specific quantum state and measure the quantum state distribution of the scattered molecules. The comparison of state resolved experiments with theory can serve as stringent tests of the molecule/surface interaction potential and of the scattering dynamics. The overall motivation is to develop a predictive understanding of the molecule/surface interactions and reactions needed for the understanding and optimization of heterogeneous catalysis. In this thesis, I describe the design characterization, and first applications of a new apparatus, dedicated to performing state-to-state surfaces scattering using the bolometric infrared laser tagging detection (BILT). An important advantage of the BILT detection method over other state-resolved detection techniques such as resonant multi-photon ionization (REMPI), is its applicability to any molecule with an infrared active vibrational mode and a rotationally resolved vibrational gas phase spectrum. For example, BILT detection can be used to detect important molecules such as methane and carbon dioxide for which REMPI detection is not possible. Our new state-to-state scattering machine features a liquid helium-cooled bolometer detector installed on a rotatable lid allowing independent variation of the incident and the scattering angle. With this capability, one can study energy transfer such as the conversion of translational to rotational or vibrational energy as well as vibrational energy redistribution for molecules colliding with a well-defined single crystal surface at a wide variety of scattering geometry. To demonstrate the capabilities of the BILT machine, I studied the rotationally inelastic scattering of CH4 from Ni(111). The results show that rotational excitation depends not only on the kinetic energy along the surface normal but also on the parallel component although with lower efficiency. The extent of rotational excitation is found to increase with increasing surface temperature. The conversion efficiency appears to be higher for low velocity component normal to the surface. The observations indicate a mechanism of rotational excitation by phonon annihilation with the probability related to the relative velocity of the incoming molecules and surface atoms. Using the experimentally determined formula which takes into account the conversion efficiency of the normal, the parallel component of incident kinetic energy, and the surface thermal energy to the rotational energy, the mechanism of rotational excitation of CH4 scattering from Ni(111) is quantitatively unraveled. Besides rotational inelastic scattering, I report very distinct behavior of vibrationally inelastic scattering of CH4 from clean Ni(111) and Gr-Ni(111). Vibrational energy transfer to the surface dramatically changes when a clean Ni(111) surface is covered with a single layer of graphene. Theoretical calculations based upon reaction path Hamiltonian suggest that the probability of the vibrational energy transfer is related to the catalytic activity of the surface impact sites. Therefore, by monitoring the fate of the initially prepared vibrational energy, the state-to-state surface scattering technique can potentially serve as a probe of the catalytic activity of surfaces.

EPFL2022

The native oxide skin of liquid metal Ga nanoparticles prevents their rapid coalescence during electrocatalysis

Raffaella Buonsanti, Laia Castilla Amorós, Anna Loiudice, Valery Okatenko, Dragos-Constantin Stoian, Jan Vávra

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke-poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs drive the electrochemical CO2 reduction reaction (CO2RR) while remaining well-separated from each other. The combination of electrochemical, microscopic and spectroscopic techniques, including operando X-Ray absorption, indicates that the unique properties of the native oxide skin of the Ga NPs account for their resistance to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.

2022

State-to-state surface scattering of methane studied by bolometric infrared laser tagging detection

Bo-Jung Chen

EPFL2022