No abstract

The comparative catalytic activities of iron phosphides, Fe x P (x = 1-3), have been established with phase-pure material grown by Chemical Vapor Deposition (CVD) from single-source organometallic precursors. This is the first report of the preparation of phase-pure thin films of FeP and Fe 2 P and their identity was established with scanning-electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. All materials were deposited on fluorine-doped tin oxide (FTO) for evaluation of their activities towards the hydrogen evolution reaction (HER) of water splitting in 0.5 M H 2 SO 4. HER activity follows the trend Fe 3 P > Fe 2 P > FeP, with Fe 3 P having the lowest overpotential of 49 mV at a current density of 10 mA cm-2. Density functional theory (DFT) calculations are congruent with the observed activity trend with hydrogen binding favoring the iron-rich terminating surfaces of Fe 3 P and Fe 2 P over the iron-poor terminating surfaces of FeP. The results present a clear trend of activity with iron-rich phosphide phases outperforming phosphorus rich phases for hydrogen evolution. The films of Fe 2 P were grown using Fe(CO) 4 PH 3 (1), while the films of FeP were prepared using either Fe(CO) 4 P t BuH 2 (2) or the new molecule {Fe(CO) 4 P(H) t Bu} 2 (3) on quartz and FTO. Compound 3 was prepared from the reaction of PCl 2 t Bu with a mixture of Na[HFe(CO) 4 ] and Na 2 [Fe(CO) 4 ] and characterized by single-crystal X-Ray diffraction, ESI-MS, elemental analysis, and 31 P/ 1 H NMR spectroscopies. Films of Fe 3 P were prepared as previously described from H 2 Fe 3 (CO) 9 P t Bu (4).

Developing stable and efficient bifunctional catalysts for overall water splitting into hydrogen and oxygen is a critical step in the realization of several clean-energy technologies. Here we report a robust and highly active electrocatalyst that is constructed by deposition of the ternary metal phosphide FeMnP onto graphene-protected nickel foam by metal-organic chemical vapor deposition from a single source precursor. FeMnP exhibits high electrocatalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Utilizing FeMnP/GNF as both the anode and the cathode for overall water splitting, a current density of 10 mA cm-2 is achieved at a cell voltage of as low as 1.55 V with excellent stability. Complementary density functional theory (DFT) calculations suggest that facets exposing both Fe and Mn sites are necessary to achieve high HER activity. The present work provides a facile strategy for fabricating highly efficient electrocatalysts from earth-abundant materials for overall water splitting.

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here,w ee xamine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods.U sing ac ombination of state-of-the-art characterization, catalyst testing,and density functional theory calculations,weshow that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)-Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis,are the most energetically favorable sites for reaction pathwaysl eading to aromatics.C alculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal as ite-specific preference for stabilizing Lewis acids,w hich is qualitatively consistent with experimental measurements.T hese findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, whichc an have am arked impact on catalyst performance relative to conventional H-ZSM-5.

The shape selectivity of ZSM‐5 (MFI type) catalysts is ideal for the production of C6–C8 aromatics. Developing high‐performance zeolite catalysts with improved selectivity to aromatics, particularly from diversified (non‐petroleum) feedstocks, has broad commercial appeal. Non‐oxidative coupling (NOC) of ethylene was examined over Ag‐ZSM‐5 catalysts at 400 °C and shows that Ag+ sites promote dehydroaromatization with enhanced selectivity to toluene and xylenes. Metal exchange of H‐ZSM‐5 results in Ag zoning wherein Ag+ site density is higher on the exterior of ZSM‐5 particles. Catalyst performance was characterized with varying Ag loading as well as the use of methane co‐feed. Aromatic selectivity is about 60 % on Ag‐ZSM‐5 compared to 20 % on H‐ZSM‐5, which is qualitatively consistent with density functional theory (DFT) showing that ethylene forms strong complexes with Ag+ (Lewis acid) sites. DFT calculations also reveal that ethylene activation on H+ (Brønsted acid) sites is more energetically favorable, and likely constitutes the first mechanistic step in ethylene‐to‐liquids (ETL) reactions. Ag‐ZSM‐5 is thus identified as an effective catalyst for low‐temperature ETL reactions that has the potential to outperform conventional metal‐exchanged zeolites.