High-Entropy Alloys as Catalysts for the CO2 and CO Reduction Reactions: Experimental Realization

Conversion of carbon dioxide into selective hydrocarbon using a stable catalyst remains a holy grail in the catalysis community. The high overpotential, stability, and selectivity in the use of a single-metal-based catalyst still remain a challenge. In current work, instead of using pure noble metals (Ag, Au, and Pt) as the catalyst, a nanocrystalline high-entropy alloy (HEA: AuAgPtPdCu) has been used for the conversion of CO2 into gaseous hydrocarbons. Utilizing an approach of multimetallic HEA, a faradic efficiency of about 100% toward gaseous products is obtained at a low applied potential (−0.3 V vs reversible hydrogen electrode). The reason behind the catalytic activity and selectivity of the high-entropy alloy (HEA) toward CO2 electroreduction was established through first-principles-based density functional theory (DFT) by comparing it with the pristine Cu(111) surface. This is attributed to the reversal in adsorption trends for two out of the total eight intermediates—*OCH3 and *O on Cu(111) and HEA surfaces

Magnetic states at the surface of alpha Fe2O3 thin films doped with Ti, Zn, or Sn

The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped (T_M~200 K) and Sn-doped (T_M~300 K) cases, while Zn and Ti-doped samples were always in the high and low temperature phases, respectively. In contrast to what has been reported for bulk hematite doped with the tetravalent ions Sn4+ and Ti4+, for which T_M dramatically decreases, these dopants substantially increase T_M in thin films, far exceeding the bulk values. The normalized Fe LII-edge dichroism for T<T_M does not strongly depend on doping or temperature, except for an apparent increase of the peak amplitudes for T<100 K. We observed magnetic field-induced inversions of the dichroism peaks. By applying a magnetic field of 6.5 T on the Ti-doped sample, a transition into the T>T_M state was achieved. The temperature dependence of the critical field for the Sn-doped sample was characterized in detail. It was demonstrated the sample-to-sample variations of the Fe LIII-edge spectra were, for the most part, determined solely by the spin orientation state. Calculations of the polarization-depedent spectra based on a spin-multiplet model were in reasonable agreement with the experiment and showed a mixed excitation character of the peak structures.Comment: 8 pages, 8 figure

Formic acid and methanol electro-oxidation and counter hydrogen production using nano high entropy catalyst

Renewable harvesting of clean energy using the benefits of multi-metallic catalytic materials of high entropy alloy (HEA, equimolar CueAgeAuePtePd) from formic acid with minimum energy input has been achieved in the present investigation. The synergistic effect of pristine elements in the multimetallic HEA drives the electro-oxidation reaction towards non-carbonaceous pathway. The atomistic based simulations based on DFT rationalize the distinct lowering of the d-band center for the individual atoms in the HEA as compared to the pristine counterparts. Further this catalytic activity of the HEA has also been extended to methanol electro-oxidation to show the unique capability of the novel catalyst. The nanostructured HEA, prepared using a combination of casting and cryomilling techniques can further be utilized as the fuel cell anode in the direct formic acid/methanol fuel cells (DFFE)

Ultra-Low Temperature CO Oxidation Activity of Octahedral Site Cobalt Species in Co3O4 Based Catalysts: Unravelling the Origin of Unique Catalytic Property

Co3O4 with spinel structure shows CO oxidation activity at very low temperature under dry conditions. This study aims at finding the origin of the unique catalytic activity of Co species in Co3O4 based oxides. Although, octahedral site Co3+ species have been reported to be active in Co3O4 based catalysts, there is no solid explanation as to why Co is so special as compared with other metals like Fe having similar redox states. In this study, mainly, three model spinel catalysts including MnCo2O4, MnFe2O4, and CoCr2O4 have been chosen. A detailed analysis of bulk and crystal surface structure, surface properties of the catalysts, and redox properties of the active metals has been performed to understand the unusual catalytic activity. Low-temperature CO oxidation activity decreases in the following order: MnCo2O4 ≫ MnFe2O4 > CoCr2O4. It indicates that the Co2+ species in a tetrahedral site (in CoCr2O4) remains inactive for low-temperature catalytic activity, while Co3+ in an octahedral site (in MnCo2O4) is active in Co3O4 based catalysts. This result is corroborated with CoFe2O4 which shows a higher activity than CoCr2O4, as it has partial occupation of the octahedral site. Fe, being a weak redox metal, does not show low-temperature activity, although crystallite facets of MnCo2O4 and MnFe2O4 catalysts are predominantly exposed in the (100) and (110) lattice planes, which contain quite similar concentrations of Co3+ and Fe3+ species in both. The intensity of the redox peak for CO oxidation involving a Co3+/Co2+ couple in MnCo2O4 indicates a highly favorable reaction, while a nonresponsive behavior of Co species is observed in CoCr2O4. As expected, MnFe2O4 is proven to be weak, giving a much lower intensity of electrochemical CO oxidation. Both CO- and H2-TPR indicate a much higher reducibility of Co species in MnCo2O4 as compared with Co species in CoCr2O4 or Fe in MnFe2O4