Experimental studies on the catalytic behavior of alloy and core-shell supported Co-Ni bimetallic nano-catalysts for hydrogen generation by hydrolysis of sodium borohydride

Abstract

Monometallic (Co) and bimetallic (Co-Ni and Co-Cu) oxides catalysts supported on the almond based activated carbon (AC) were prepared by the heterogeneous deposition-precipitation method. The activity of these catalysts was evaluated as a function of reaction temperature, NaOH, and NaBH4 concentration. Several analysis methods including XRD, XPS, FTIR, TEM, FESEM, ICP-OES, and BET were applied to characterize the structure of prepared samples. Well-dispersed supported bimetallic nano-catalysts with the size of particles below 20 nm were formed by using nickel and copper oxides as a promoter which was confirmed by XRD and TEM techniques. Surface composition of alloy and core-shell cobalt-nickel oxides catalysts was analyzed by ICP-OES which was in a good agreement with nominal content during catalyst preparation. The performance of bimetallic cobalt-nickel oxides catalysts indicated the synergic effect between cobalt and nickel in comparison with monometallic and bimetallic cobalt-copper samples for hydrogen production. Maximum hydrogen generation rate was measured for the supported core-shell catalyst as named Ni1/Co3/AC. The reaction rate increased with increasing the temperature of the alkaline solution as a significant parameter while other operating conditions were kept constant. The optimal values for NaOH and NaBH4 content were calculated to be 10 wt % for both variables at 30 °C. Hydrogen production rates were calculated to be 252.0, 310.8 and 658.8 mL min−1.g−1 by applying Co3/Ni1/AC, Co3-Ni1/AC (alloy) and Ni1/Co3/AC at 30 °C in 5 wt % NaBH4 and 5 wt % NaOH solutions, respectively. Obtained activation energy (50 kJ mol−1) illustrated that the suitable catalysts were synthesized for hydrogen generation. The experimental study showed that the hydrolysis of NaBH4 was a zero-order type reaction with the respect to the sodium borohydride concentration. A semi empirical kinetic model was derived at the various temperatures and NaOH concentrations.