Carbon monoxide-free hydrogen production via low-temperature steam reforming of ethanol over iron-promoted Rh catalyst

Abstract

For the first time, a novel iron-promoted Rh catalyst is developed to produce CO-free H₂ through steam reforming of ethanol at low temperatures, between 623 and 673 K. The iron oxides in the vicinity of Rh sites reduce the CO adsorption on Rh sites and transfer the adsorbed CO from Rh to COO-formate species on Fe_xO_y for the subsequent water–gas shift reaction, resulting in a high H₂ yield, extremely low CO selectivity, and a long Rh life span.

Introduction

Hydrogen is used as a fuel for proton exchange membrane fuel cells (PEMFC) and phosphoric acid fuel cells (PAFC). It can be produced by on-site fuel reforming for stationary applications or onboard for automotive applications. A typical fuel processor is made up of different processing units such as fuel reformers and high-temperature and low-temperature water–gas shift (WGS) and CO cleanup reactors. WGS, as well as CO cleanup, is essential in fuel processors, since the electrodes can only tolerate about 1–2% CO for PAFC and less than 10 ppm for PEMFC [1], [2]. PEMFC and PAFC are classified as low-temperature fuel cells, operating in low-temperature ranges, 353–393 K and 453–493 K, respectively. Therefore, it will be ideal if CO-free hydrogen can be produced at low temperatures. This can in turn lower the cost of the fuel processor by reducing the stacks of heat exchanger, WGS, or even CO cleanup processing units [1], [2], [3].

Renewable hydrogen produced via steam reforming of bioethanol (SRE) (Eq. (1)) has gained intense interest in recent years for the above applications [2], [3], [4]:

$${\text{C}}_2{\text{H}}_5\text{OH}+3{\text{H}}_2\text{O}\to 2{\text{CO}}_2+6{\text{H}}_2$$

However, SRE is a complex process comprising many reactions that can be influenced by the properties of catalysts and the reaction conditions applied. Some of them are expressed in the following equations [5]:

$${\text{C}}_2{\text{H}}_5\text{OH}\to {\text{CH}}_3\text{CHO}+{\text{H}}_2$$

$${\text{C}}_2{\text{H}}_5\text{OH}\to {\text{C}}_2{\text{H}}_4+{\text{H}}_2\text{O}$$

$${\text{CH}}_3\text{CHO}\to {\text{CH}}_4+\text{CO}$$

$${\text{CH}}_3\text{CHO}+{\text{H}}_2\text{O}\to 2\text{CO}+3{\text{H}}_2$$

$$\text{CO}+{\text{H}}_2\text{O}\leftrightarrow {\text{CO}}_2+{\text{H}}_2$$

$${\text{CH}}_4+{\text{H}}_2\text{O}\leftrightarrow \text{CO}+3{\text{H}}_2$$

Therefore, it is possible to alter the hydrogen yield and product selectivities by choosing proper catalysts and reaction conditions. Furthermore, catalyst deactivation, which is usually caused by the accumulation of carbonaceous deposits, is a serious problem during SRE, especially at low temperatures [6]. Several reactions responsible for coke formation have been proposed, as shown in the following equations:

$$2\text{CO}\leftrightarrow {\text{CO}}_2+\text{C}$$

$${\text{CH}}_4\leftrightarrow 2{\text{H}}_2+\text{C}$$

$${\text{C}}_2{\text{H}}_4\to \text{polymers}\to \text{coke}$$

Coking can be mitigated at high temperatures. Thus, SRE is usually operated in the temperature range 823–1073 K to avoid coking and to obtain high hydrogen yield [6]. However, CO formation would increase correspondingly with increasing temperature (>823 K) due to the thermodynamically favored reverse WGSR (Eq. (6)). Therefore, in order to reduce the CO concentration significantly, the reaction should proceed at relatively low temperatures. To do so, one should optimize the reaction condition by balancing SRE and WGSR and develop coking-resistant catalysts that can perform at low temperatures. Indeed, methane produced from ethanol decomposition (Eqs. (2), (4)) cannot be converted to H₂ via steam reforming effectively (Eq. (7)) at low temperatures. Therefore, the yield of H₂ is lower than that at high reforming temperatures (above 773 K). However, the energy efficiency of low-temperature SRE could remain high if methane and carbon oxides could be separated from hydrogen and sent to the tail gas combustion chamber to generate heat for the reformer [2], [7].

Some supported noble metal catalysts have been reported for the low-temperature SRE [3], [7], [8], [9], [10], [11]. Basagiannis et al. reported that 0.5% Pt/Al₂O₃ was the best among catalysts of Rh, Ru, Pt, and Pd supported on Al₂O₃. However, the selectivity to CO was as high as 30% at 623 K [7]. In another study, a H₂ yield of 1.99 mol H₂/mol EtOH with a CO selectivity of 1.5% was achieved at 673 K over a 1.5% Pt/Ce_0.8Zr_0.2O₂ catalyst [3]. Roh et al. reported a H₂ yield of 4.3 mol H₂/mol EtOH with CO selectivity of 10% over a 2% Rh/Ce_0.8Zr_0.2O₂ catalyst at 723 K [8]. A bimetallic Rh–Pd/CeO₂ catalyst was reported to exhibit good performance at 777 K. However, the addition of Pd did not decrease the selectivity of CO, which remained as high as 42.8% at 580 K [9]. In one of our recent reports, 1% Rh supported on hydrothermally synthesized ZrO₂ could produce high H₂ yield in the temperature range 573–673 K; however, the selectivity to CO is about 30%, and catalyst deactivation was observed at temperatures below 673 K due to the accumulation of CO, carbonate, and CH_x on the catalyst surface [10]. Despite the high activity shown by noble metal catalysts for the SRE at low temperatures, CO is inevitable in the final products.

In this communication, we report a novel multifunctional iron-oxide-promoted catalyst, Rh–Fe/Ca–Al₂O₃, for the low-temperature SRE process. Significant increase in H₂ yield and extremely low CO selectivity can be achieved in the presence of iron promoter. A plausible reaction mechanism and the role of iron oxides are also discussed.