Elucidating the interaction between Ni and CeOx in ethanol steam reforming catalysts: A perspective of recent studies over model and powder systems

Abstract

Bulk metallic nickel is a poor catalyst for the reforming of hydrogenates being deactivated by the deposition of coke. In contrast, Ni-ceria is an active system for the catalytic extraction of H 2 from the ethanol steam reforming reaction (ESR, C 2 H 5 OH + 3H 2 O ↔ 2CO 2 + 6H 2). Numerous studies, with model (well-defined crystal surfaces) and technical (high surface area powders) catalysts, have been devoted to understand the fundamental role of each catalyst component, the performance of adjacent sites in the metal-oxide interface, and the complex mechanistic steps that convert two oxygenated reactants (ethanol and H 2 O) into H 2. The size and low loading of Ni on ceria facilitate metal-oxide support interactions that probably enhance the reactivity of the system. To establish the precise role of both Ni and Ce is challenging. However it is clear that both Ni and Ce are associated with the dissociation of H 2 O (OH+H), while ceria readily adsorbs and partially dissociates ethanol (i.e. ethoxy formation). The most difficult step of CC bond dissociation likely occurs only on Ni or at the NiCe interface. H 2 O and OH remain as important agents for the prevention of excess C build up during the C-H/C-C dissociation process. Often, deactivation upon C build up, is a direct result of Ni sintering and decoupling of the NiCe interactions. One strategy to maintain good activity and stability is to protect the NiCe interaction, and this can be achieved through the use of solid solutions (Ce 1-x Ni x O 2-y) or by employing stabilizing agents such as W (Ni x W y Ce z O 2). In this article, we present and discuss the most recent work for the ESR reaction and show the important role of ceria which participates directly in the reaction and also enhances catalytic activity through metal-support interactions.

Effect of Supports and Promoters on the Performance of Ni‐Based Catalysts in Ethanol Steam Reforming

“…To cope with this, the reaction conditions need to be optimized and the catalyst must be improved by modification. To improve the catalyst, one can use different Ni loadings and different supports, promoters can be added, and the catalyst can be synthesized by different techniques to obtain small Ni particle sizes and a high dispersion of Ni on the catalyst [28,58,59,62,63,129,[131][132][133]. The above-mentioned efforts have been tried in order to achieve both the high Ni surface area and the high Ni dispersion that play key roles in the ESR reaction for the highest hydrogen yield [28,[62][63][64][65][66].…”

Modern Ni and Pd–Ni Catalysts Supported on Sn–Al Binary Oxide for Oxy‐Steam Reforming of Methanol

“…Typical catalytic systems extensively used in reforming of alcohol are copper‐based systems supported on mono‐ and binary oxides . The high activity of nickel catalysts in the steam reforming reaction to hydrogen generation was reported in the literature . Furthermore, copper catalysts are promoted by palladium in order to improve selectivity to hydrogen generation due to the ability to improve the reducibility of the active phase component and formation of intermetallic alloys .…”

“…[8][9][10][11][12][13] The high activity of nickel catalysts in the steam reforming reaction to hydrogen generation was reported in the literature. [14][15][16][17][18] Furthermore, copper catalysts are promoted by palladium in order to improve selectivity to hydrogen generation due to the ability to improve the reducibility of the active phase component and formation of intermetallic alloys. [8] Bobadilla et al [19] investigated the influence of Sn addition on catalytic activity of Ni/CeO 2 -MgO-Al 2 O 3 catalysts in steam reforming of methanol.…”

Estudo da reação de oxidação de etanol em ânodos de células a combustível SOFC

“…Conforme pode-se observar, na faixa de temperatura de interesse para este trabalho, apresenta maior peso na formação de carbono a reação de "cracking" do combustível 44 . Especula-se que um dos motivos práticos que favorecem a formação de carbono sobre o catalisador é a carência de espécies oxigenadas nas proximidades das espécies carbonáceas adsorvidas 45,46 as quais, em princípio, promoveriam de imediato a oxidação e dessorção do carbono antecipadamente ao crescimento de sua cadeia, vindo evitar a formação do coque. Tendo em vista este entendimento sugerese que uma maneira de se evitar a formação de carbono no ânodo é justamente aumentar a concentração dessas espécies oxigenadas.…”

“…Neste sentido tem sido proposta a utilização de uma camada pré-anódica, depositada sobre o ânodo de Ni (denominada de camada protetora, num sistema denominado de reforma direta interna), a qual contém um material com as características desejadas. Com relação a este material, alguns autores utilizam-se de cerâmica ionômera GDC como fonte de O 2ao sítio catalítico 45,53,54 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62H2 X Na própria figura, neste caso, é indicada a direção de aumento da frequência. Desta maneira, sabe-se que a região do diagrama com dados para os menores valores de impedância real é a região de altas frequências enquanto a região com os maiores valores para a impedância real é a região de baixas frequências.…”