Work

The microstructures of promoted RANEY® nickel alloys and catalysts

Year: 2026

Type: article

Abstract: This study measured various effects of iron and chromium promotion on the microstructures of Raney(r) nickel alloys and catalysts. The following alloys and catalysts were studied: (i) unpromoted, (ii) multiple levels of Fe promotion, (iii) multiple levels of Cr promotion, and (iv) doubly-promoted with Fe and Cr. Microstructures and phase compositions of some initial alloys were characterized using backscattered scanning electron microscopy (SEM), electron microprobe, optical microscopy, and x-ray diffraction. Backscattered SEM and electron microprobe analysis showed the microstructure is composed of several phases: Al 3 Ni 2 , Al 3 Ni, promoter-rich phase(s) (which are absent in the unpromoted alloy), and a eutectic phase. The Fe and Cr promoters concentrate in the promoter-rich phases in these alloys. Based on XRD pattern and the phase diagram, we have assigned ‘Fe promoter phase 1’, present in some Raney nickel alloys promoted with ≥ 2 wt.% Fe, as the decagonite (Al 71 Ni 24 Fe 5 ) quasicrystal. This was a major finding, because decagonite was previously thought to be rare. Also, ‘Fe promoter phase 2’, present in all of the Fe-only promoted alloys, Al 9 Ni (2-y) Fe y (where y was 0.37 to 0.77) crystalline phase has been proposed, based on the findings of this study. A phi phase of composition Al 77.5 Cr 12 Ni 10.5 has been proposed for ‘Cr promoter phase’ (present in Cr-only promoted alloys) in this study. A ‘Cr, Fe promoter phase’ of approximate composition Al 75.5 Ni 13-14 Cr 6-7 Fe 4 was present in the Cr, Fe doubly-promoted alloys. The alloys were crushed into a powder and leached in aqueous sodium hydroxide solution. After leaching, the catalysts were washed and subsequently characterized. Nitrogen adsorption at 77 K was performed to extract the total surface area and pore size distribution of each catalyst. Carbon monoxide and hydrogen gas adsorption were performed to measure each catalyst’s metallic surface area. For some catalysts, bulk elemental composition was quantified using atomic absorption spectroscopy, and the surface compositions and oxidation states were studied using x-ray photoelectron spectroscopy (XPS). XPS results showed that Cr is oxidized to Cr(+III) during the leaching procedure to form a compound (likely chromium hydroxide) that increases the total surface area (while having negligible effect on the metallic surface area) and also leads to smaller pore diameters.

Source: RSC Advances

DOI: 10.1039/d6ra05868j