Reactivity of Hydrogen with Solid-State Films of Alkylamine- and Tetraoctylammonium Bromide-Stabilized Pd, PdAg, and PdAu Nanoparticles for Sensing and Catalysis Applications

Hydrogen gas spontaneously adsorbs to Pd metal as atomic hydrogen and diffuses into the lattice to form PdH_x. We previously showed that films of hexanethiolate-coated Pd monolayer-protected clusters (MPCs) do not readily react with H₂ due to the strong chemical bonding of the thiolate to the Pd, which inhibits the reaction. Consequently, these films require ozone or heat treatment for reactivity to occur, which is inconvenient for sensing or catalysis applications. In this report, we describe the reactivity between H₂ and solid-state films of alkylamine-coated Pd, PdAg (10:1), and PdAu (10:1) MPCs and films of tetraoctylammonium bromide (TOABr)-stabilized Pd and PdAg (10:1) nanoparticles as determined by changes in film conductivity. Our data show that Pd nanoparticles coated with these more weakly coordinated amine or ammonium groups readily react with H₂ without any need for ozone or heat treatment. The conductivity of films of octylamine (C8NH₂)- or dodecylamine (C12NH₂)-coated Pd, PdAg, and PdAu MPCs increases irreversibly upon initial exposure to 100% H₂ to varying degrees and with different reaction kinetics and then exhibits stable, reversible changes in the presence of H₂ concentrations ranging from 9.6 to 0.08%. The behavior upon initial exposure to H₂ (conditioning) and the direction and magnitude of the reversible conductivity changes depend on the alkyl chainlength and alloy composition. Films of TOABr-coated Pd and PdAg nanoparticles show stable, reversible increases in conductivity in the presence of H₂ concentrations from 9.6 down to 0.11% without conditioning. Surface FTIR spectroscopy and atomic force microscopy (AFM) provide information about the organic monolayer and film morphology, respectively, following reactivity with H₂. This work demonstrates a simple approach toward preparing films of chemically synthesized Pd-containing nanoparticles with controlled reactivity to H₂ for sensing and catalysis applications.