Hydrogen Adsorption, Reactivity, and Catalysis on Colloidal Iron Carbide Nanoparticles

Abstract

Supported iron carbide particles have long served as catalysts for CO hydrogenation (the Fischer–Tropsch synthesis, FTS) and continue to be attractive. Despite this, little is known about their chemistry. Reported here is a colloidal Fe_xC nanoparticle (NP) model system that allows direct observation of surface hydrogen and CO, as well as quantification of the surface H. Dodecylamine-capped Fe_xC NPs (DDA-Fe_xC NPs) were synthesized through solution-phase carburization of Fe NPs and form stable colloids in low-polarity organic solvents. Treatment of these colloids with H₂ or D₂ produced highly hydrogenated materials, and Fourier transform infrared (FTIR) spectra of DDA-Fe_xC–D_n showed that most of the D binds to carbides, with at least four distinct ν(C–D) modes. The surface C–H(D) bonds were reactive, transferring hydrogen to alkenes and other reagents in solution. Titration and ICP measurements showed a 0.17:1 ratio of added H:total Fe, or roughly 40 H per 1.8 nm DDA-Fe_xC NP. Conversely, CO was preferentially bound to surface Fe sites, with FTIR spectra showing a single broad ν(CO) that shifted with CO coverage or coadsorption of H₂. The DDA-Fe_xC NPs were active catalysts for both olefin hydrogenation and the FTS, under mild conditions and without catalyst pretreatment. The CO hydrogenation reactions yielded a broad distribution of long-chain linear paraffins and olefins. Though quantitative comparisons with typical FTS results are not possible because of our use of sealed batch reactors and other factors, the observations of high catalytic reactivity demonstrate the relevance of this model system to iron-carbide catalysis. Density functional theory (DFT) calculations on model slab surfaces with varying iron carbide stoichiometries revealed that the thermodynamically preferred surface adsorption sites are C for H_ads and Fe for CO_ads. A variety of binding sites and binding energies were found for each adsorbate. We are unaware of previous studies indicating that a diverse array of C–H bonds is the primary source of reactive H on iron carbides. Experimentally, the diversity of *C–H sites was evident in reactions with H atom donors and abstractors of different strengths, from both the reaction stoichiometries and IR spectra. The different surface–H binding energies correlate with the ν(C–D) stretching frequencies. These insights into complex iron carbide surfaces and catalysis could assist catalyst design, and they showcase the importance of stoichiometric studies of reaction intermediates.