Understandings in surface metal exsolution of NiFeZn ternary alloy/spinel in ethanol steam reforming: The key to hydrogen production

[Display omitted] •Surface metal exsolution of NiFeZn ternary alloy/spinel was found in ethanol steam reforming;•The formation of NiFeZn nanoalloy contributed to efficient C–C and C–H bond breaking;•Active surface oxygen species played a roles in C2 products formation;•The relationship between H2 production and coke deposition over polymetallic catalyst was revealed. Ethanol steam reforming (ESR) has been recognized as a preferred method for producing hydrogen on a flexible scale. Much effort has been focused on developing a non-noble metal catalyst that can alternate noble metals. Polymetallic catalyst are a type of promising catalyst candidate for hydrogen production because of their adjustable catalytic performance, whereas their catalytic mechanism is still unclear. In this work, quinary metal oxides NiFeMgAlZnOx and CoFeMgAlZnOx were synthesized by a polyol hydrothermal method that was tactically designed for hydrogen production via ESR. Transition metals Ni [3d74s2] and Co [3d84s2] with similar electronic configurations were selected as active components, while Mg, Fe, Zn and Al were employed to form the spinel structure for enhancing the catalyst’s stability. Catalyst performances under different operating conditions were evaluated and compared, and the catalytic mechanism of the MFeMgAlZnOx was revealed. The active sites (surface oxygen species and metal NPs) on NiFeMgAlZnOx and CoFeMgAlZnOx were analyzed and identified, respectively. Interestingly, it was discovered that NiFeZn nanoalloy was generated during the ESR, which formed a supported-like NiFeZn/spinel structure for efficient C–H and C–C bond breaking. The H2 production rate was directly related to the formed NiFeZn nanoalloy. On the other hand, surface oxygen species were the only active site type of CoFeMgAlZnOx that took part in the ESR reaction before 600 °C, which contributed to ethanol convert into C2 products and a low H2 production rate because of its weak C–C bond breaking. It was found that differences in hydrogen production and the coke deposition pathways in ESR were mainly due to the differences in active species.