Studying the Formation of Ni₃C from CO and Metallic Ni atT= 265 °C in Situ Using Ni K-Edge X-ray Absorption Spectroscopy

Metallic Ni nanopowder (Ni 0 ) was monitored during 23 h of carburization (2CO + 3Ni 0 → Ni 3 C + CO 2, T = 265 °C) using Ni K-edge X-ray absorption spectroscopy. X-ray diffraction analysis made afterward at room temperature revealed 28 ± 3% Ni 3 C among 72% unreacted Ni 0 . The χ( k ) data recorded during carburization showed small changes at low k indicative of carbon backscattering. The identification of carbon was possible with wavelet transform analysis after eliminating the integral contribution from the unreacted Ni 0 phase using experimental χ( k ) data collected during methanation (CO + 3H 2 → CH 4 + H 2 O) at T = 265 °C. The Fourier-transformed χ( k ) data recorded during carburization revealed destructive interference between signals from Ni atoms in slightly different (Ni 0, Ni 3 C) environments. The interference effect mainly lowered the peak amplitude of the first two Ni−Ni coordination shells compared to metallic Ni at T = 265 °C and it propagated very slowly with increasing carburization run time. In simulation of the amplitude lowering of the first Ni−Ni peak by destructive interference as a function of the carburization run time, it followed that the carbon atoms migrate into the Ni 0 particle lattice according to the diffusion-induced grain boundary motion advocated in the literature.