Stability of alloys under oxidizing and reducing conditions

ANDRÉS R. LEÓN GARZÓN; MARK SAEYS; ESTEBAN L. FORNERO; JENOFF DE VRIEZE

Santa Fe

Congreso; VI San Luis Conference on Surfaces, Interfaces and Catalysis; 2018

INTEC - Alexander von Humboldt Foundation

Bimetallic alloys are commonly found in catalytic applications and there are well-documented examples where the catalyst performance is enhanced after their adoption.1 Such is the case of Fe?Ni based catalysts for reforming reactions,2 and Pd-Zn for hydrogenation aplications.3 However, while alloys can improve the catalytic performance of such processes, numerous concerns arise, including: Under which conditions are they formed? Is temperature the only important factor? What happens with their stability under oxidizing or reducing conditions? Understanding the catalyst structure under reaction conditions is of vital importance in catalysts design and optimization. Nevertheless, as this concept is very challenging to determine via experimental methods only, first principles modeling could provide insight into the optimal reaction conditions and their effect on the alloy formation and/or segregation. In the present work, we use Density Functional Theory (DFT) to predict the stability of Fe-Ni and Pd-Zn alloys.Spin polarized DFT along with the vdW-DF2 functional, as implemented in the Vienna Ab initio Simulation Package (VASP), were used. For reactions involving Fe and Ni, a semi-empirical approach was used to calculate enthalpies and Gibbs free energies of reactions.4 Briefly, to correct self-interaction errors a Hubbard U correction term (DFT+U) was used to improve the relative stabilities of different oxides. Then, due to the errors in the energies of metallic phases using DFT+U, the experimental enthalpies of formation of the oxides with the lowest oxidation state were used to correct them. Similarly, the overbinding of O2 was corrected using the experimental enthalpy of formation of H2O. DFT energies were combined with calculated zero-point energies (ZPE), heat capacities and entropies to obtain Gibbs free energies as a function of pressure and temperature.Phase diagrams for bcc-Fe, fcc-Ni, FeO, Fe3O4, Fe2O3, NiO, Ni3Fe (fcc), NiFe (fcc) and FeNi (bcc) under H2/H2O and CO2/CO atmospheres were made. It was shown that, above 600 K, it is possible to completely reduce iron and nickel oxides under high concentration of H2 (TPR conditions) with the consequent formation of two stable alloys, Ni3Fe (fcc) and NiFe (fcc). On the other hand, the oxidation reaction of Fe-Ni alloys to Ni and Fe3O4 takes place above 900 K under TPO (CO2) conditions, without the presence of the FeO intermediate phase and without the formation of Fe2O3 (complete oxidation of iron) in the range 400-1000 K. These calculations are in complete agreement with experimental observations made for Fe-Ni catalysts via time-resolved in situ XRD measurements.2,4 Additionally, the reducibility of Fe3O4 with CH4 and the stability of the alloys under the presence of both CO2 and CH4 reagents were also investigated. Phase diagrams for fcc-Pd, PdZn, Pd2Zn and ZnO under H2/H2O atmospheres were made. It is well known that the stable bimetallic PdZn compound has a composition close to 1:1 ratio. However, after increasing the oxygen chemical potential, zinc atoms are oxidized and removed from the alloy resulting in a Pd2Zn alloy and ZnO. Further oxidation is possible at higher oxidizing conditions, which is in complete agreement with previous experimental observations.5 The examples shown here indicate that first principle calculations have proven to be a valuable tool in the assessment of the stability of alloy catalyst under oxidizing and reducing conditions.