Materials for Clean H₂ Production from Bioethanol Reforming

Introduction

Hydrogen is one of the most important resources in providing food, fuel, and chemical products for our everyday life. Sustainable catalytic hydrogen production from bioethanol has gained significant attention in recent years due to globally diminishing fossil fuel supplies, which have necessitated the search for new chemical feedstocks. Ethanol has been considered as one of the most promising renewable energy resources since it has low toxicity and can be safely stored and easily transported. Bioethanol is the ethanol produced by fermentation of biomass materials including sugar cane, corn, and lignocellulose. Catalytic steam reforming (SR) of ethanol has been extensively investigated for hydrogen production in the past decade. These SR reactions are commonly carried out using heterogeneous catalysts, which include an active phase of nanosized metal particles on an oxide support. This article reviews the catalytic process for bioethanol reforming with a focus on supported nanoscale rhodium catalysts.

Thermodynamics for Ethanol Steam Reforming (SR)

The overall SR of ethanol can be simplified as: C₂H₅OH+ 3H2O → 6H₂ + 2CO₂. A typical reaction mechanism for ethanol SR reaction is presented in Figure 1. Ethanol conversion by SR may include^1-3:

1. ethanol dehydrogenation to form acetaldehyde
2. acetaldehyde decarbonylation to produce CH₄ and CO
3. CH₄ reforming, i.e., CH₄ + H₂O → CO + 3H₂
4. the production of H2 through a water-gas-shift (WGS) reaction: CO + H₂O → CO₂ + H₂.

The goal for the SR process is to obtain a high yield of H₂ and a high selectivity toward CO₂, since a total of 6 moles of H₂ can be obtained from ethanol for every 2 moles of CO₂ produced. The CO generated in the SR process is undesired, as it can be poisonous to the catalysts or the electrode in fuel cell applications. Therefore, the demand of high CO₂ selectivity makes it necessary to couple SR reaction with WGS reaction.

Depending on the catalysts and reaction conditions, the actual ethanol SR reaction may undergo different reaction pathways such as ethanol dehydration and decomposition pathways to generate undesired byproducts of CO, CH₄, and C₂H₄, etc. that may lead to coke formation and consequently lower H₂ production. The main reactions that may contribute to coke formation are: ethanol dehydration to ethylene followed by polymerization to coke; Boudouard reaction, i.e., carbon monoxide decomposition to form carbon dioxide and coke; and methane decomposition to form coke and hydrogen. In addition, catalyst sintering under SR reaction conditions can also lead to deteriorated catalytic performance by increasing the active phase particle sizes and decreasing the amount of available surface active sites.

Catalysts play a decisive role in the reaction stability, ethanol conversion, and H₂ selectivity for ethanol SR reactions. It is, therefore, crucial to develop suitable catalysts that will maximize H₂ yield, and inhibit CO production and coke formation that will deactivate catalysts.

Rh-supported Catalysts

Supported transition metal catalysts such as Co and Ni catalysts have been investigated for ethanol reforming reactions.^4-6 However, compared to transition metal catalysts, supported noble metal catalysts have shown greatly enhanced performance at much lower metal loadings for the SR of ethanol (Table 1). Aupretre et al.⁷ investigated the catalytic performance of γ-Al₂O₃ supported Rh, Pt, Pd, and Ru catalysts with metal loadings of 0.67−1 wt% for the ethanol SR reaction at 700 °C. The 1 wt% Rh catalyst proved to be the most active and selective of these noble metals. Breen et al.⁸ also reported that Rh/Al₂O₃ ( Product No. 212857) showed a higher ethanol conversion than Al₂O₃ ( Product No. 202606) supported Pd, Pt, and Ni catalysts under the same reaction conditions from 400−700 °C. Frusteri et al.¹ studied MgO ( Product No. 529699) supported Rh, Pd, Ni, and Co catalysts in the SR of simulated bioethanol at 650 °C. Again, the Rh/MgO showed the best performance in terms of reaction activity and stability. Furthermore, the Rh/MgO catalyst was most resistant to coke formation and metal sintering. In contrast, Ni, Co and Pd catalysts deactivated mainly due to metal sintering. Rioche⁹ et al. reported the SR of ethanol from 664−779 °C over Pt, Pd and Rh catalysts supported on Al₂O₃ and CeZrO₂ ( Product No. 634174). Only the Rh-based catalysts efficiently converted the ethanol over the temperature range investigated. The Ptand Pd-based catalysts were significantly less active.-The high SR reaction activity of the Rh catalysts is due to the fact that Rh favors C−C bond cleavage compared to other noble metals. Duprez et al.⁷ proposed a bi-functional mechanism where the hydrocarbon would be activated on the metal phase while the water molecule would be activated on the support as hydroxyl groups. Owing to this effect, oxide supports with enhanced surface oxygen mobility are expected to have improved SR reaction activity.

Support Effects

Rhodium is the most active metal for ethanol SR (Table 1). However, the support also plays a significant role in determining the catalytic performance of the catalysts for ethanol SR reaction (Table 2). The use of CeO₂−ZrO₂, a redox mixed oxide, led to higher H₂ yields as compared to the Al₂O₃-supported catalysts.⁹ Roh et al.¹⁰ studied Rh catalysts on various supports, including Al₂O₃, MgAl₂O₄ ( Product No. 677396), ZrO₂ ( Product No. 204994), and ZrO₂−CeO₂ for ethanol SR reaction. For the 1 wt% Rh catalysts, at temperatures ≤450 °C, the reaction activity was found to decrease in the order: Rh/ZrO₂−CeO₂ > Rh/Al₂O₃ > Rh/MgAl₂O₄ > Rh/ZrO₂. The 1% Rh/ZrO₂−CeO₂ catalyst exhibited the highest CO₂ selectivity up to 550 °C. At 450 °C, the CO₂ selectivity of 52% was at least 3 times higher than all other tested catalysts. Furthermore, the 1% Rh/ZrO₂−CeO₂ catalyst exhibited the highest H₂ yield among all the catalysts at temperatures below 500 °C.

Generally, a good SR catalyst support should favor Rh stability with minimum Rh sintering. In addition, water should be activated on the support as mobile surface hydroxyl groups to promote reaction with CHxOy fragments adsorbed on Rh species. Finally, it should favor ethanol dehydrogenation to acetaldehyde instead of ethanol dehydration to ethylene in order to prevent coke formation. In fact, the acidic Al₂O₃ support is very active for the dehydration reaction leading to the formation of ethylene,⁸ which is undesirable in ethanol SR. Over a CeO₂ support, acetaldehyde can be formed via the dehydrogenation of ethanol.³ CeO₂ ( Product No. 211575) can promote the redox reaction of rhodium due to its high oxygen storage capacity associated with the fast Ce⁴⁺/Ce³⁺ redox process.¹⁰ ZrO₂, with its high thermal stability and tunable acidic and basic properties,¹¹ has also been used as a catalyst support for ethanol SR reactions. Zhong et al.¹² studied Rh catalysts supported on ZrO2-based oxides; in their study, pure ZrO₂ resulted in higher H₂ yield compared to the binary metal oxide supports of ZrO₂ with Al₂O₃, La₂O₃, or Li₂O at 300 °C. The addition of ZrO₂ to CeO₂ can enhance the oxygen storage capacity, the reducibility of Ce⁴⁺, and thermal stability of CeO₂.¹³ It is possible that ZrO₂−CeO₂ may transfer mobile oxygen species to Rh to enhance CH₄ reforming and subsequent H₂ production, while the oxygen vacancies would be replenished by water molecules from the redox cycle during the SR of ethanol.^7,10 Among the investigated supports, CeO₂, MgO, and ZrO₂ are the most suitable catalyst supports for effective ethanol SR with Rh catalysts.

A. H₂ yield (g h^-1 g^-1 catalyst)
B. Ethanol conversion based on CO + CO₂ formation
C. CO₂ yield

A. H₂ produced/ethanol feed (mol/mol)
B. CO₂ yield
C. 99 wt% ZrO₂ + 1 wt% La₂O₃
D. 99 wt% ZrO₂ + 1 wt% Al₂O₃

Preparation Method Effects

The catalyst preparation methods can also affect the catalytic performance of the resulting catalysts. Aupretre et al.¹⁴ prepared a MgAl₂O₄ spinel support by solid-solid reaction between MgO and Al₂O₃. Compared with the Al₂O₃-supported catalysts, the spinel-supported catalysts exhibited a slightly higher basicity; whereas the surface acidity was strongly reduced. The MgAl₂O₄ was impregnated with different Rh precursor salts (nitrate, chloride, acetate) to obtain the supported Rh catalysts, which gave less acidic catalysts with greatly enhanced ethanol SR performances compared with the Rh catalysts supported on Mg^xNi^1-xAl₂O₄/Al₂O₃. In this case, the Rh precursor salts had a substantial impact on the properties of the final catalysts. The use of a Rh nitrate precursor dramatically increased the overall acidity of the catalyst, while a Rh acetate precursor gave the catalyst with lowest acidity. Rh chloride resulted in catalysts with moderate acidity, allowing the preparation of well-dispersed Rh catalysts that were very active and stable for ethanol SR. Zhong et al.¹² prepared ZrO₂ supports using a co-precipitation method (ZrO₂−CP) and a hydrothermal method (ZrO₂−HT). A significant enhancement in catalytic activity, selectivity, and stability was achieved by the ZrO₂−HT-supported Rh catalyst compared with its ZrO₂−CPsupported counterpart at temperatures below 400 °C. Relatively strong Lewis acidic sites were found on the Rh/ZrO₂−HT catalyst, which was responsible for the strong absorption of ethanol and the subsequent C−H cleavage. In contrast, on the Rh/ZrO₂−CP catalyst, the ethanol adsorption was weak and the C−C cleavage was the dominating reaction. Recent studies¹⁵ have shown the effectiveness of the HT approach coupled with an optimized calcination treatment on the preparation of a nanosized, hydrothermally stable ZrO₂ support of less than 20 nm (Figure 2). Pt deposited on this ZrO₂ support showed enhanced activity for WGS reaction compared to a series of commercial ZrO₂-supported Pt catalysts. This ZrO₂ support may have great potential in bioethanol SR application.

Promoter Effects

The introduction of alkali or alkali earth metal species into the supported Rh catalysts can improve the catalyst stability and performance. Roh et al.¹⁶ studied the deactivation of Rh/CeO₂−ZrO₂ catalysts for ethanol SR at 350 °C with a high space velocity of 480,000 cm³/g·h. Ethanol conversion decreased from 70% to 6% within 5 h due to coke deposition. The addition of 0.5% potassium increased the catalyst stability at the same reaction temperature of 350 °C. Chen et al.¹⁷ introduced Ca to neutralize the acidity of Al₂O₃, which helped decrease the undesired reaction pathway of the ethanol dehydration. Furthermore, the addition of Fe into this Ca−Al₂O₃-supported Rh catalyst allowed the transfer of CO from Rh to Fe species for subsequent WGS reaction to produce CO-free H₂, which improved the Rh catalyst stability. Indeed, the acidic and basic properties of the supports strongly affect the SR reaction selectivity toward acetaldehyde and ethylene.¹⁴ Basic sites on the supports benefit the ethanol dehydrogenation step in acetaldehyde formation while the acidic sites need to be neutralized to decrease the ethylene formation.

Loading Effects

Catalyst loading also affects ethanol SR reaction activity. Roh et al.¹⁰ discovered that, for ethanol SR at 450 °C, the H₂ yield based on per mole of ethanol decreased in the order: 2% Rh/ZrO₂−CeO₂ > 1% Rh/ZrO₂− CeO₂ > 3% Rh/ZrO₂−CeO₂, possibly due to differences in metal dispersion. Ethanol SR has been more extensively studied over the temperature range 600−700 °C since H₂ selectivity is increased at these high temperatures. Liguras et al.18 reported the effect of metal loading on catalytic performance over Ru/γ−Al₂O₃ catalysts. They observed that the ethanol full conversion temperature was 825 °C for the 0.5% Rh catalyst, which was lowered to 775 °C in the case of the 2.0% Rh catalyst. Furthermore, the selectivities toward H₂ and CO₂ were substantially enhanced with increasing Rh loading. Additionally, the 2.0% Rh/Al₂O₃ catalyst fully converted ethanol without the formation of acetaldehyde or ethylene at temperatures close to 800 °C. The observed differences in catalytic performance were attributed to the increased number of active sites but not the differences in particle sizes with increasing metal loadings, since the rhodium particle sizes were ~2.1 nm for all these Rh/Al₂O₃ catalysts.

Conclusions

In the present review, an overview of previous ethanol steam reforming (SR) studies focusing on Rh has been given. Rh has been identified as the most active metal for ethanol SR reaction. Catalyst support, catalyst preparation method, metal additive promoter, and Rh loading all play important roles in optimizing performance of the ethanol SR reaction. The catalyst performance characteristics suggest a strong metal−support interaction. The ethanol SR reaction network is complex and water-gasshift (WGS) reaction also plays an important role in the overall reforming process. Undesirable reaction pathways such as ethanol dehydration and CH₄ decomposition can lead to coke deposition and, consequently, deactivate the catalyst. The use of a binary support system such as MgO−Al₂O₃ and CeO₂−ZrO₂, and the introduction of hetero-metal species may inhibit carbon deposition leading to better Rh dispersion and, consequently, improve catalyst performance for clean H₂ production from bioethanol. Finally, the introduction of new nanosized hydrothermally prepared catalyst support materials presents a promising avenue toward increased catalytic performance for ethanol SR.

Acknowledgment

This material is based upon work supported as part of the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.