Improving the sintering resistance of Ni/Al2O3 steam-reforming catalysts by promotion with noble metals

• Ir and Rh addition to Ni/α-Al2O3 improves sintering resistance of the catalyst.
• Synergistic effect of Ni combined with Ir and Rh promoters also contributes to increased activity.
• Ru addition to Ni/α-Al2O3 is not as effective as Ir or Rh.
• The Ni-Ru/Al2O3 catalyst performs worse than pure Ru catalyst only after aging.

The role of Rh, Ir and Ru promoters on the structural properties and catalytic behavior of Ni/α-Al2O3 catalyst during steam methane reforming (SMR) was investigated. The catalysts were subjected to aging treatments at 800 °C and PH2O=22 bar gPH2O=22 bar g in order to induce sintering of the metal nanoparticles and alloying between Ni and the promoters. The crystallite sizes and the extent of alloying were determined in the reduced and aged catalysts by combination of in situ extended X-ray absorption fine structure (EXAFS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (STEM-XEDS) mapping. The results reveal that the degree of alloying between Ni and Rh or Ir promoters increases during the aging process, as a result of the high mobility of the nickel species and diffusion into the FCC structures of Rh and Ir. Moreover, the formation of Ni–Rh and Ni–Ir alloy particles has a positive effect in mitigating the sintering of the nickel particles. In contrast, sintering inhibition effect of Ru was poorer due to its lower miscibility in nickel as well as its lower sintering resistance under the aging conditions employed. Based on density functional theory (DFT) calculations the diffusion of the Ni atoms into the lattice structure of Ru is energetically not favorable, whereas Ni atoms are equally stable in the first and second surface layers of Ir and Rh. Hence, a synergistic effect between Ni and Rh or Ir is achieved due to the alloy formation leading to conservation of larger active metal surface area and consequently to superior SMR activity with respect to the monometallic systems.

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