Probe Molecules

Heterogeneous catalysts are often complex materials, consisting of metal centers dispersed on a support with a large variety of chemical and physical environments. While advances in characterization techniques have enabled to access valuable information on materials, direct structural information of surface sites, that are involved in catalysis, often remains inaccessible, even with state-of-the-art techniques and instrumentations. In that context, “probe-molecules”, spectroscopically addressable molecules that interact with specific (surface) sites, can play an important role to extract information, albeit indirect, on (catalyst) surface sites, e.g. strengths and types of surface acid sites, e.g. Brønsted vs. Lewis. With the goal to have a more detailed understanding of surfaces and ultimately an atomistic description, the developments of improved detection methods and novel probes are still needed and will be discussed below.

CO-IR

A classical approach to address surface sites with probe molecules is infrared spectroscopy (IR). Notable examples are the use of pyridine, CO and CO2 to probe the types and strengths of surface acid or reactive sites. The well-known IR signatures of the interaction of these probe molecules with surface sites enable to highlight specific surface sites, evidencing for instance alloying for metal nanoparticles. This approach is also powerful to monitor reaction intermediates as it can be combined with in situ or Operando approaches, providing detailed information about possible reaction mechanisms (Figure 1). [1-4]

Solid-state NMR in combination with probe molecules is also a powerful tool to interrogate the nature of surface sites, using the change of chemical shift of probe molecules upon interaction with surface sites. Notably, the use of DNP SENS5 helps to access high quality NMR spectra. Furthermore, chemical shift analysis of the NMR signatures, using computational methods, make possible to understand the origin of chemical shift, to reconstruct electronic structure and to even extract the strength of the interaction of the probe molecules with the surface sites [6,7]

¹⁵N Pyridine

As discussed above, the support acidity, i.e. Lewis- and Brønsted acidities are crucial parameters in catalysis which often determines the metal-support interaction and further the catalytic transformation itself. The NMR signature of adsorbed ¹⁵N-pyridine on surfaces provide information regarding its coordination and interaction strength with Lewis acid sites, or evidence Brønsted acidity. Such an approach has enabled to evidence the distribution of acid sites on alumina (Al₂O₃) [8] or to highlight the unexpected presence of Brønsted acid sites on MgCl₂ due to adsorbed water on strong Lewis acid sites. [9] The same approach has also helped to evidence the formation of Brønsted acidic sites on titanosilicalite-1 (TS-1) catalyst upon exposure to water.[10]

¹⁰⁹Ag

While probing the presence of Brønsted acid sites, access to the speciation and the strength of these sites has been more challenging. Recent development by our group has shown that ¹⁰⁹Ag NMR chemical shift of L-Ag-X is directly proportional to the gas phase acidity of H–X. [11] Thus, the reaction of L-Ag-Mesityl with surface O-H group and measuring the 109Ag NMR of the resulting surface-bound O-Ag(I)-L motif enables to access this information.

 

¹³C

N-heterocyclic carbenes (NHCs) are ubiquitous ligands in organometallic chemistry and have been more recently introduced in surface science. Notably, the interaction of the sp2-hydridized carbene carbon of NHC ligands with metal sites (or protons) yields a very large chemical shift window (up to ca. 100 ppm) depending on the nature and the strength of that interaction, in sharp contrast to what is observed for the corresponding sp3-hydridized ³¹P in phosphane ligands (10-20 ppm). This feature has enabled us to distinguish surface sites on supported coinage metal sites in single-sites and nanoparticles; in particular helping to distinguish the signatures of mono- and bis-ligated metal cations (Cu, Ag and Au).12-14

As for IR, one can also interrogate the nature of reactive intermediates in CO2 hydrogenation reaction using ex situ solid-state NMR spectroscopy. For instance, reaction of supported metal nanoparticles with mixture of CO2 and H2 at various pressures followed by a rapid quenching and analysis of the resulting solids enable to monitor the formation of surface formate and their conversion to surface methoxide intermediates. Such studies enable to track the formation and the stability of these intermediates to relate catalyst structure and catalytic performance, i.e. selectivity towards methanol formation for instance.1-2