Investigation of Metal–Support Interactions and Coking Behavior over Ni/CexZr1−xO2–Al2O3 Catalysts for Ethanol Steam Reforming

Abstract

With its abundant, renewable, and eco-friendly attributes, hydrogen stands out as a key contender in addressing climate change challenges and attaining carbon neutrality objectives. As the transition to H2 energy is inevitable in the future, various studies have contributed to the research on H2 global production, storage, and utilization. From the perspective of sustainable development, the reforming of low-carbon bio-alcohols derived from biomass fermentation emerges as a pivotal avenue for hydrogen production, offering nearly zero net carbon dioxide emissions. In particular, Ethanol Steam Reforming (ESR) garners significant attention, leveraging well-established ethanol production technologies from biomass and capitalizing on ethanol's advantages in terms of transport and toxicity for a prospective carbon-neutral society. The first research investigates selective Ni locations over Ni/CeZrOx-Al2O3 catalysts at different Ni loading contents and their influences on reaction pathways in ethanol steam reforming (ESR). Depending on the Ni loading contents, the added Ni selectively interacts with CeZrOx-Al2O3, resulting in the stepwise locations of Ni over CeZrOx-Al2O3. This behavior induces a remarkable difference in hydrogen production and coke formation in ESR. The selective interaction between Ni and CeZrOx for 10-wt.% Ni generates more oxygen vacancies in the CeZrOx lattice. The Ni sites near the oxygen vacancies enhance reforming via steam activation, resulting in the highest hydrogen production rate of 1863.0 mmol/gcat ·min. Incontrast, for 15 and 20-wt.% Ni, excessive Ni is additionally deposited on Al2O3 after the saturation of Ni-CeZrOx interactions. These Ni sites on Al2O3 accelerate coking from the ethylene produced on the acidic sites, resulting in a high coke amount of 19.1 mgc/gcat ·h (20Ni/CZ-Al). For a deeper understanding of coke formation routes over Ni-based catalysts in ESR, in the second research, we focused on optimizing the active sites distribution via the metal-support interactions and investigating the coke precursor gasification in Ni/CexZr1-xO2–Al2O3 (CZA) catalysts for ethanol steam reforming. We found that the optimized active sites distribution could minimize coke formation by enabling efficient gasification of a coke precursor on Ni. In the 20Ni/40CZA catalyst, the crystalline CZ exhibited a strong interaction with Ni, maintaining a uniform distribution of highly dispersed Ni nano­particles and creating abundant oxygen vacancies on CZ, even at 20 wt% of Ni loading. Consequently, the resistance to coking over 20Ni/40CZA was significantly enhanced due to the efficient delivery of active oxygen atoms from steam on the abundant oxygen vacancies to the coke precursor on the Ni surface, resulting in fast gasification of the coke precursor. Finally, we reduced the variables in the support and only investigated the metal–support interactions over Ni/CeO2–ZrO2 (Ni/CZ) catalysts for ESR. Different Ni contents (5, 10, and 15 wt%) were loaded onto the CZ support using two catalyst preparation methods, namely one-pot, and successive impregnation methods. Using the one-pot method, the Ni species over the Ni/CZ catalysts strongly interacted with the CZ support, resulting in smaller Ni nanoparticles with abundant oxygen vacancies. In contrast, the successive impregnation method mainly generated an isolated Ni phase at a high Ni content, resulting in large Ni clusters with limited oxygen vacancies. The poor Ni dispersion over the Ni/CZ catalysts prepared by the successive impregnation method induced low ethanol conversions and hydrogen production rates. In addition, the large Ni clusters over the Ni/CZ catalysts prepared by the successive impregnation method achieved a fast-coking behavior, thereby increasing the coke amount. Further, coke gasification was demonstrated to dominate the Ni surface with the Ni/CZ catalysts prepared by the one-pot method because of the high Ni dispersion and abundant oxygen vacancies.