Junjie Zhang,Zhourong Xiao,Li Wang,Xiangwen Zhang,Guozhu Li
Graphical The key role of Ni valence state on the catalytic SMR performance of Ni/Al 2 O 3 is reported. Ni 0 /Ni 2+ ratio on Ni/Al 2 O 3 is individually regulated by changing the reduction temperature while maintaining the states of nickel particles and support. A deep understanding is gained on how the Ni valence state of Ni/Al 2 O 3 affects the catalytic SMR performance. Steam methane reforming (SMR) is the most mature technology for hydrogen production in industry, and alumina supported nickel (Ni/Al 2 O 3 ) is the most widely used catalyst. Herein, the key role of Ni valence state on the catalytic SMR performance of Ni/Al 2 O 3 is reported. The ratio of Ni 0 /Ni 2+ on Ni/Al 2 O 3 is individually and effectively tuned by changing the reduction temperature after wet impregnation. Catalytic performance of Ni/Al 2 O 3 is greatly dependent on the surface concentration of Ni 0 and Ni 2+ . The catalyst reduced at 675 °C possesses balanced Ni 0 and Ni 2+ sites and exhibits superior catalytic performance, on which 60 % of methane is stably converted at 600 °C. CH 4 -TPD and CO-TPD profiles demonstrate that appropriate Ni 0 /Ni 2+ ratio can promote the adsorption of CH 4 and inhibit the moderate-strength adsorption of CO, which leads to increased SMR activity and selectivity.
Spinel-structured NiMn 2 O 4 ceramics, with different valence Ni sources, were originally prepared using Ni 2 O 3 and NiO as raw materials, and the effects of different valence Ni sources on their electrical properties were first investigated. XRD patterns show that both Ni 2 O 3 -based and NiO-based NiMn 2 O 4 ceramics are single cubic spinel structures. SEM/EDS images indicate that the NiMn 2 O 4 ceramics exhibited high density at the experiment-determined sintering temperatures. XPS results and Raman drifts prove that the Ni valence-induced changes in Mn ions at B sites played a significant role in the electrical properties and thermal stability of NiMn 2 O 4 ceramics. Compared with NiO-based NiMn 2 O 4 , the resistivity at 25°C ( ρ 25°C ) of Ni 2 O 3 -based NiMn 2 O 4 increased dramatically from 3109 to 106958 Ω cm, the thermal constant ( B 25 / 50 ) increased from 3264 to 4473 K, and the resistance shifts after annealing for 1000 h at 150°C decreased from 0.80% to 0.74%. The investigation of the relationship between the material properties and valence of Ni sources has provided a new and effective way for designing the spinel-structured negative temperature coefficient (NTC) materials by modulating the valence of ions at A sites in the raw materials.
Chun Kong,Chuang Zhi,Zirui Wu,Wenqiang Yang,Juan Yang,Zhongti Sun
NiFe-(oxy)hydroxides are the most active transition metal oxide electrocatalysts for oxygen evolution reaction (OER) under the alkaline media. Herein, we controllably manipulated oxygen vacancy (V O )-tunable NiFe-(oxy) hydroxides that their OER performances possessed a volcano-type relationship with V O concentration, positively-correlated with Ni 3+ /Ni 2+ ratio. Theoretical simulations further unearthed the enhanced activation and dissociation of H 2 O by the inserting of V O . As a result, the optimal sample featuring the Ni 3+ /Ni 2+ ratio of 30.3 % and V O of 23.8 % exhibited the overpotential of 243 mV at the current density of 100 mA cm –2 , simultaneously lasting 120 h durability without any attenuation, exceding the most reported NiFe-(oxy)hydroxides. This work offers an innovative view to understand the OER performance using hypervalent Ni ratio induced by V O defects.
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