Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H₂ from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions.

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The search for more efficient heterogeneous catalysts remains critical to the chemical industry. The Sabatier principle of maximizing catalytic activity by optimizing the adsorption energy of the substrate molecule could offer pivotal guidance to otherwise random screenings. Here we show that the chemical shift value of an adsorbate (formic acid) on metal colloid catalysts measured by (13)C nuclear magnetic resonance (NMR) spectroscopy in aqueous suspension constitutes a simple experimental descriptor for adsorption strength. Avoiding direct contact between the (13)C atom and the metal surface eliminates peak broadening that has confounded prior efforts to establish such correlations. The data can guide rational design of improved catalysts, as demonstrated here for the cases of formic acid decomposition and formic acid electro-oxidation reactions.

Ein Katalysator zeigt sich von seiner besten Seite: Die polare (002)‐Fläche von ZnO‐Nanoplättchen geht im Vergleich zu anderen Flächen eine viel stärkere elektronische Wechselwirkung mit Cu‐Nanopartikeln ein (siehe Bild; CB=Leitungsband, VB=Valenzband) und bietet eine höhere Selektivität in der katalytischen Hydrierung von CO2 zu Methanol. Dieser Befund könnte für die Entwicklung neuer Nanokatalysatoren für die CO2‐Hydrierung genutzt werden.

A remarkable effect from the modification of electronic and geometric properties of Pd nanoparticles by the use of polymer pendant groups bound to the surface of palladium nanoparticles is reported. The degree of electron promotion to the Pd nanoparticles under ambient conditions was found to be dependent on the availability of the lone pair electrons of the pendant groups.

A remarkable promotion: Functional groups added onto single-wall carbon nanotubes (SWNTs) can significantly influence the activity of a noble metal for formic acid oxidation. Phenolate groups on SWNTs under alkaline conditions can double the activity of 20 % w/w Pd compared to unmodified SWNTs. This catalyst has 14 times higher activity than the commercial benchmark catalyst (10 % w/w Pd on Vulcan).

Ceria has been widely used as support in electrocatalysis for its high degree of oxygen storage, fast oxygen mobility, and reduction and oxidation properties at mild conditions. However, it is unclear what are the underlying principles and the nature of surface involved. By controlling the growth of various morphologies of ceria nanoparticles, it is demonstrated that the cubic-form of ceria, predominantly covered with higher energy polar surface (100), as support for Pd gives much higher activity in the electrocatalytic oxidation of formic acid than ceria of other morphologies (rods and spheres) with low-indexed facets ((110) and (111)). High-resolution transmission electron spectroscopy confirms the alternating layer-to-layer of cations and anions in (100) surface, and the electrostatic repulsion of oxygen anions within the same layers gives intrinsically higher oxygen vacancies on this redox active surface in order to reduce surface polarity. Density functional theory calculations suggest that the properties of fast oxygen mobility to reoxidize the CO-poisoned Pd may arise from the overdosed oxygens on these ceria surface layers during electro-oxidation hence sustaining higher activity.

Formic acid adsorption on ruthenium nanoparticles of different sizes allows differentiation of differently bound formate species by solution (13)C NMR spectroscopy (see picture). The chemical shifts are comparable to those of organometallic analogues, thus indicating that formate can act as a probe to distinguish surface features of metallic nanoparticles in solution with good quantification and resolution.