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.

The inelastic scattering of gas-phase OH radicals from a liquid hydrocarbon and a liquid perfluorinated polyether (PFPE) has been investigated. The surfaces examined were the potentially reactive, branched hydrocarbon squalane (C 30 H 62 , 2,6,10,15,19,23-hexamethyltetracosane) and the inert PFPE Krytox 1506 (F-[CF(CF 3 )-CF 2 O] 14ave -CF 2 CF 3 ). Superthermal OH was formed by 355-nm laser photolysis of a low pressure of HONO above the liquid surface. Laser-induced fluorescence (LIF) was used to determine the relative yields and nascent translational and rotational distributions of OH (V′ ) 0). The time-of-flight profiles from both liquids can be resolved, at least empirically, into two components. The dominant, faster component is consistent with direct, inelastic scattering. It has a higher average translational energy from PFPE than from squalane. This faster OH also has a higher Boltzmann-like rotational temperature for PFPE (655 ( 45 K) than for squalane (473 ( 27 K), in both cases considerably hotter than the incoming OH. For both liquids, there is also a slower component, with characteristics consistent with a thermalized, trapping-desorption mechanism. This is a higher proportion for squalane (0.22 ( 0.02) than for PFPE (0.09 ( 0.01). These results are consistent with squalane being the "softer" surface, exhibiting more efficient momentum transfer than PFPE, and more able to temporarily trap OH. Relative to PFPE, around half (0.49 ( 0.04) of the OH molecules that collide with squalane are lost, presumably due to reaction forming H 2 O. These results are compared with previous studies of the scattering of inert gas species from both squalane and PFPE. The reactive branching fraction of OH on squalane is discussed in the context of previous observations of enhanced reactivity at the gas-liquid interface.

The reactivity of photolytically generated, gas-phase, ground-state atomic oxygen, O((3)P), with the surfaces of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([NTf(2)]) ionic liquids has been investigated. The liquids differ only in the length of the linear C(n)H(2n+1) alkyl side chain on the cation, with n = 2, 4, 5, 8, and 12. Laser-induced fluorescence was used to detect gas-phase OH v' = 0 radicals formed at the gas-liquid interface. The reactivity of the ionic liquids increases nonlinearly with n, in a way that cannot simply be explained by stoichiometry. We infer that the alkyl chains must be preferentially exposed at the interface to a degree that is dependent on chain length. A relatively sharp onset of surface segregation is apparent in the region of n = 4. The surface specificity of the method is confirmed through the nonthermal characteristics of both the translational and rotational distributions of the OH v' = 0. These reveal that the dynamics are dominated by a direct, impulsive scattering mechanism at the outer layers of the liquid. The OH v' = 0 yield is effectively independent of the bulk temperature of the longest-chain ionic liquid in the range 298-343 K, also consistent with a predominantly direct mechanism. These product attributes are broadly similar to those of the benchmark pure hydrocarbon liquid, squalane, but a more detailed analysis suggests that the interface may be microscopically smoother for the ionic liquids.

The relative reactivity of the liquid surface of a long-chain, partially branched hydrocarbon (squalane, C 30 H 62 ) with gas-phase O( 3 P) atoms has been measured as a function of liquid temperature. The O( 3 P) atoms were generated with a superthermal velocity distribution by 355 nm photolysis of NO 2 . Laser-induced fluorescence was used to detect the relative branching into specific OH product vibrational states. The yield of OH(V′)0) proves significantly less dependent on liquid surface temperature than the yield of OH(V′)1). Time-of-flight measurements of the escaping OH provide partially resolved product translational energy distributions. These profiles also differ between OH vibrational states. OH(V′)1) shows overall longer arrival times, but with a clear trend toward earlier times as the surface temperature is increased. OH(V′)0) shows little detectable variation of the distribution of arrival times over the range of temperatures investigated (263-333 K). We discuss the interpretation of these findings, taking account of earlier experimental work, which has indicated significant contributions from distinct "direct" and "trapping-desorption" reaction mechanisms, and new molecular dynamics simulations of the surface structure. There are a number of factors that may contribute, including both energetic and structural effects. It is not possible on the basis of the current evidence to discriminate conclusively between them. Nevertheless, we conclude, on balance, that structural effects may well be the more important. In particular, higher temperatures are predicted to promote more open structures. We speculate that this may enable more OH(V′)1) to escape before it is either vibrationally relaxed or, less probably, undergoes vibrationally enhanced reaction to produce H 2 O.

We have investigated the reactivity of photolytically generated ground-state, gas-phase atomic oxygen O(3P) with the surfaces of two related ionic liquids, 1-ethyl-3-methylimidazolium ([emim]) bis(trifluoromethyl-sulfonyl)imide ([NTf2]) and 1-dodecyl-3-methylimidazolium ([C12mim]) [NTf2]. The liquids differ only in the length of the alkyl side-chain on the cation. Laser-induced fluorescence was used to detect the gas-phase OH radicals formed at the gas−liquid interface. The considerable yield of OH from [C12mim][NTf2] in comparison to the undetectable amount from [emim][NTf2] cannot simply be explained by the difference in stoichiometry. We propose that this “chemical probe” method therefore provides strong evidence for the preferential accumulation of longer alkyl chains at the gas−liquid interface.