Bimetallic reductants are frequently more reactive toward organohalides than unamended iron and can also alter product distributions, yet a molecular-level explanation for these phenomena remains elusive. In this study, surface characterization of six iron-based bimetallic reductants (Au/Fe, Co/Fe, Cu/Fe, Ni/Fe, Pd/Fe, and Pt/Fe) revealed that displacement plating produced a non-uniform overlayer of metallic additive on iron. Batch studies demonstrated that not all additives enhanced rates of 1,1,1-trichloroethane (1,1,1-TCA) reduction nor was there any clear periodic trend in the observed reactivity (Ni/Fe approximately Pd/Fe > Cu/Fe > Co/ Fe > Au/Fe approximately Fe > Pt/Fe). Pseudo-first-order rate constants for 1,1,1-TCA reduction (kobs values) did, however, correlate closely with the solubility of atomic hydrogen within each additive. This suggests absorbed atomic hydrogen, rather than galvanic corrosion, is responsible for the enhanced reactivity of bimetallic reductants. In addition, all additives shifted product distributions to favor the combined yield of ethylene plus ethane over 1,1-dichloroethane. In rate-enhancing bimetallic systems, branching ratios between 1,1-dichloroethane and the combination of ethylene and ethane were uniquely dependent on kobs values, indicating an intimate link between rate-determining and product-determining steps. We propose that our results are best explained by an X-philic pathway involving atomic hydrogen with a hydride-like character.

The interaction of an X-ray-modified self-assembled monolayer with a mixture of atomic and molecular oxygen (O/O 2 ) has been studied using in situ X-ray photoelectron spectroscopy. Initially the reaction dynamics are dominated by the incorporation of new oxygen containing functionality at the vacuum/film interface. At intermediate O/O 2 exposures, when a steady-state concentration of C-O, CdO, and O-CdO groups has been established, the production of volatile carbon-containing species, including CO 2 , is responsible for etching the hydrocarbon film. Upon prolonged O/O 2 exposures, O atoms penetrate to the film/substrate interface, producing Au 2 O 3 and sulfonate (RSO 3 ) species. Under steady-state conditions, the thickness of the hydrocarbon film was reduced with an efficiency of ≈7.4 × 10 -4 Å/impingent O atom while the average penetration depth of O atoms within the hydrocarbon film was determined to be ≈5.5 Å.

Although iron-based bimetallic reductants offer promise in treating organohalides, the influence of additive mass loading and two-dimensional surface coverage on reductant reactivity has not been fully elucidated. In this study we examine 1,1,1-trichloroethane reduction by Cu/Fe bimetals as a function of Cu loading and surface coverage. Information from a suite of complementary techniques (X-ray photoelectron spectroscopy, Auger electron spectroscopy, and cross-sectional energy-dispersive X-ray spectroscopy) indicates that displacement plating produces a heterogeneous metallic copper overlayer on iron. The dependence of pseudo-first-order rate constants (k(obs) values) for 1,1,1-trichloroethane reduction on Cu loading exhibits two distinct regimes. At Cu loadings less than 1 monolayer equivalent (approximately 10 micromol Cu/g Fe), a pronounced increase in k(obs) is associated with a corresponding increase in the two-dimensional surface coverage of Cu. A weaker dependence of k(obs) on Cu mass is exhibited at loadings in excess of 1 monolayer equivalent, which we ascribe to an increase in the volume of the metallic overlayer. The observed relationship between k(oba) and loading suggests that 1,1,1-trichloroethane reduction occurs on the Cu surface rather than at the interface between the Cu overlayer and the iron substrate.

The low-temperature (<150 K) oxidation of nitrided iron surfaces exposed to oxygen and water was probed using X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and mass spectrometry (MS). During exposure of nitrided iron surfaces to oxygen, iron oxynitride (Fe x N y O z ), nitrosonium ions (NO+), as well as nitrite/nitrito- and nitrate-type species were observed. The production of nitrite/nitrito and nitrate species is taken as evidence for the existence of oxygen insertion chemistry into the iron nitride lattice under these low-temperature oxidation conditions. No molecular nitrogen was produced during reactions with oxygen or water in contrast to oxidation studies on other transition metal nitrides. Upon annealing the oxidized overlayer, nitrogen desorbs exclusively as nitric oxide (NO) between 250 and 400 K. In contrast to oxygen, the reactivity of nitrided iron surfaces toward water was limited to the production of adsorbed N−O species.

Anticipating which pollutants are amenable to treatment by iron-based bimetallic reductants requires an understanding of the mechanism(s) driving pollutant reduction. Here, batch studies with six bimetals (Au/Fe, Co/Fe, Cu/Fe, Ni/ Fe, Pd/Fe, and Pt/Fe) and four oxidants (alkyl polyhalides, vinyl polyhalides, alkynes, and water) explored the influence of the electron acceptor on reductant reactivity. Bimetals exhibited disparate reactivity toward some oxidant classes. For example, Pt/Fe enhanced rates of cis-dichloroethylene reduction, but it inhibited the reduction of several alkyl polyhalides. Moreover, the rate increase for vinyl polyhalide reduction by Ni/Fe (approximately 100-fold) and Pd/Fe (approximately 1000-fold) was far greater than that measured for alkyl polyhalides (approximately 10-fold), and reactivity toward vinyl polyhalides exhibited a more pronounced dependence on Ni and Pd loadings than did reactivity toward alkyl polyhalides. These results suggest that the reactions of alkyl and vinyl polyhalides with iron-based bimetals involve different active reductants. Neither rates of alkyl nor vinyl polyhalide reduction correlated with rates of iron corrosion by water, contrary to expectations if galvanic corrosion was primarily responsible for organohalide reduction. Trends observed for the hydrogenation of 2-butyne did mirror the sequence we identified for 1,1,1-trichloroethane reduction, consistent with a role for atomic hydrogen as the principal electron donor in these two systems.