Promoting effect of trace Pd on hydrotalcite-derived Ni/Mg/Al catalyst in oxidative steam reforming of biomass tar

•Addition of trace noble metals enhances the catalytic performance of Ni/Mg/Al.•Optimized PdNi catalyst with Pd/Ni = 0.0023 shows the best catalytic performance.•Palladium atoms are isolatedly dispersed on the surface of Ni metal particles.•Palladium enhances the catalyst reducibility and dispersion of Ni metal particles.

Promoting effects of trace noble metals (Pd, Pt, Au, Ru, Rh and Ir) on the activity and stability of the Ni catalyst prepared from the hydrotalcite-like compounds containing Ni, Mg, and Al were investigated in steam reforming and oxidative steam reforming of tar derived from pyrolysis of biomass. The optimum loading of Pd was 0.05 wt% (0.05PdNi catalyst; molar ratio of Pd/Ni = 0.0023), and this catalyst showed higher performance than the other noble-metal-added catalysts with the same noble metal/Ni molar ratio. Characterization results indicated that the addition of 0.05 wt% Pd enhanced the reducibility of Ni species and the dispersion of Ni metal particles simultaneously, which can be attributed to the highest catalytic performance. The monometallic Ni catalyst (0PdNi) totally lost the catalytic activity due to the oxidation of catalytically active Ni metal particles by oxygen after two hour stability test in oxidative steam reforming. In contrast, the oxidation of Ni metal particles did not occur on 0.05PdNi catalyst, and the 0.05PdNi catalyst showed stable catalytic performance during the two hour stability test, which can be due to the formation of highly dispersed Pd atoms on the surface of Ni metal particles on the 0.05PdNi catalyst as suggested by Pd K-edge EXAFS analysis.

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