Comparison of bimetallic Co-Ru nanoparticles supported on highly porous activated carbonized polyacrylonitrile with monometallic ones in ethanol steam reforming

Highlights

• Simultaneous formation of porous carbon support and metal nanoparticles was proposed.

• Comparison of mono- and bimetallic nanocatalysts in the ESR reaction was carried out.

• IR-PAN-Co-Ru catalyst provided a hydrogen yield of 5.6 mole per mole of EtOH.

Abstract

A method for simultaneous formation of mono- (Co, Ru) or bimetallic (Co-Ru) nanoparticles and a highly porous carbon support is proposed. The obtained IR-PAN-Co, IR-PAN-Ru and IR-PAN-Co-Ru samples based on pyrolyzed polyacrylonitrile are characterized by a specific surface area within the range of 1683–2174 m² g^-1, which strongly depends on the nature of the metal used. It has been shown that the presence of cobalt leads to the graphitization of amorphous carbon and the formation of carbon shells around the mono- and bimetallic nanoparticles. The average size of metallic nanoparticles for all three samples ranged within 12–20 nm. The comparison of the samples revealed a dramatic advantage of the bimetallic catalyst in the ethanol steam reforming (ESR). The IR-PAN-Co-Ru sample has shown the highest hydrogen yield, which was 5.6 moles per mole of EtOH at 550 °C. To evaluate the stability of the catalysts, the catalytic test was carried out for 17 h after which no deactivation of the catalysts was observed. The spent catalysts were characterized by the same techniques as the as-prepared ones. It was found that there are no significant changes in the structure of the catalysts after the ESR reaction. The presence of filamentous carbon in the spent cobalt-based catalysts has been observed, which does not lead to deactivation in the time period studied.

Introduction

The increasing consumption of energy by industry, transport, and households poses a number of potential and actual challenges for scientists and researchers. Thus, one of the problems is pollution of the environment by emissions into the air, water and soil of the products of fuel combustion from non-renewable sources [1]. Currently, the main energy carriers are hydrocarbons, the combustion of various types of which leads to the emission of large amounts of carbon dioxide into the atmosphere [2]. It is believed that the transition to alternative energy sources will help to solve environmental problems and become the basis of energy in the case of exhaustion of non-renewable sources. Thus, hydrogen is considered to be the most promising energy carrier due to the fact that it has a high energy density per unit mass and when burned, the reaction product is water, which makes it more environmentally friendly than hydrocarbons. Various sources are used to produce hydrogen, including ethanol. At the same time, bioethanol produced from agricultural wastes, as a non-toxic source with high hydrogen content, is considered to be preferable [3], [4], [5].

Catalytic steam reforming of ethanol (ESR) is currently receiving much attention because of the high hydrogen yield that can be achieved [6], [7]. The basic ESR reaction yields six hydrogen molecules from one alcohol molecule (1):

С₂Н₅ОН + 3Н₂О ↔ 2СО₂ + 6Н₂ ΔH⁰_298 K = +173 kJ mol⁻¹

The main reaction is accompanied by a number of side reactions (2)-(10):

C₂H₅OH + H₂O ↔ 2 CO + 4 H₂

CO + H₂O ↔ CO₂ + H₂

C₂H₅OH ↔ CH₃CHO + H₂

C₂H₅OH ↔ C₂H₄ + H₂O

C₂H₅OH ↔ CH₄ + CO + H₂

C₂H₅OH ↔ 1/2CO₂ + 3/2CH₄

C₂H₅OH ↔ C + CO + 3 H₂

C₂H₅OH ↔ 2 C + H₂O + 2 H₂

C₂H₅OH ↔ CH₄ + C + H₂O

As a result, hydrogen, carbon oxides, methane, and acetaldehyde can be the products of the ESR reaction. Along with the temperature of the ESR reaction, one of the important conditions for catalysis is the ratio of water to alcohol. The molar ratio of water to EtOH is usually chosen as 3:1 or higher, since its decrease causes undesirable coke formation and a drop in selectivity to hydrogen [8], [9]. Otherwise, the hydrogen yield increases as the H₂O/alcohol ratio is raised [7]. Coke formation is one of the main problems of reforming reactions [6]. The formation of C-C bonds at the catalytic site leads to the formation of a carbon layer on the metal surface due to possible side reactions such as ethylene polymerization, ethanol and acetaldehyde cracking, methane decomposition, and Boudoir reaction [10], [11]. This eventually leads to deactivation of the catalyst. In order to increase the resistance of catalysts to coking, the heating rate and temperature during ESR, catalyst composition, acid-base properties of the catalyst support, and the addition of promoters such as sodium or potassium compounds [6], [12], [13] are varied.

A wide range of transition (Co, Ni, Cu, Fe, Zn) and noble (Rh, Pd, Pt, Ru) metals are studied as catalysts in ESR [14]. As a rule, the noble metals are the most active in hydrogen production reactions. For instance, Rh and Ru are reported to be the most active ESR catalysts. However, recent studies show that more available transition metals can also exhibit high catalytic activity [15], [16]. The use of bimetallic systems carries a number of advantages over monometallic nanoparticles. Most obviously, an addition of a transition metal to a noble metal reduces the cost of the catalyst and makes large-scale production reasonable. Therewith, bimetallic systems demonstrate a synergistic effect resulting in an increase in catalytic activity and in stability [17], [18]. Cobalt, which can break C-C bonds, has been actively investigating as a ESR catalyst both as an individual supported metal and in combination with other metals (Co-Ni, Pt-Co, Rh-Co) [19], [20], [21], [22], [23]. Ruthenium is also known for its high catalytic activity in many chemical reactions, including ESR [24], [25]. However, it is worth noting that there are very few works aimed at the study of cobalt-ruthenium alloy-based nanoparticles [26].

One of the standard routes to obtain metal nanoparticles immobilized on the support is the deposition of nanoparticles or metal compounds on a ready-made support: oxide or carbon [27], [28]. Oxide supports, such as Al₂O₃, SiO₂, MgO, CeO₂, La₂O₃, Y₂O₃, etc., are widespread and are studied due to their facile availability and relative low-cost. These supports can positively influence the activity and selectivity of metal catalysts [29]. However, under some conditions they are unstable, which negatively affects the activity of the catalyst. The disadvantages of oxide supports include a very small number of options for their modification for tuning their properties. Thus, one of the main approaches of the modification is an addition of various oxide components to change the acidity of the main support surface in order to reduce the coke formation process [10]. Moreover, it is usually difficult to recover metal catalysts from oxide supports employed for reuse, which is especially important for noble metals because of their cost. Therefore, the use of carbon supports looks more promising in some cases due to their resistance to acid and alkaline environments, the tolerance to elevated temperatures, adjustable porosity, and the possibility of obtaining such supports in various forms (pellets, fibers, powder) [28], [30]. It should also be noted that the properties of carbon supports can be varied by the alteration of their structure and morphology, as well as by the modification of the surface with functional groups [31], [32]. In addition, carbon supports allow the extraction of metal from spent catalysts by their combustion in air.

Carbon materials prove to be good support for ESR catalysts. For instance, B.L. Augusto et al. note catalyst stability and low degree of deactivation [28]. Carbon supports are able to prevent sintering of metal particles, which is also a problem in high-temperature catalytic reactions, due to immobilization of metal particles in a rigid carbon matrix. Activated carbon materials characterized by the porous structure have a special place among the carbon types of supports due to their very large specific surface area, which allows the access of reagents to the catalytic centers and their homogeneous distribution over the volume.

To date, the most common method for the synthesis of metal-carbon catalysts is impregnation of the prepared carbon support with solutions of metal salts followed by reduction by chemical or thermal methods [24]. This approach requires separate stages of the support formation and the introduction of the metallic filler nanoparticles. In the present work, we propose a one-step method for obtaining a highly porous carbon material based on carbonized polyacrylonitrile (PAN) with bimetallic Co-Ru nanoparticles distributed in it. The main feature of the developed approach is the simultaneous formation of a highly porous carbon support and Co-Ru alloy nanoparticles. The catalytic properties of the synthesized metal-carbon nanocomposites with bimetallic nanoparticles were compared to those of similar nanocomposites with monometallic Co and Ru nanoparticles in the ESR reaction. It is worth noting that there are not many studies aimed at studying the catalytic properties of metal-carbon nanocomposites in the ESR reaction. Also, to the best of our knowledge there are no publications studying the bimetallic Co-Ru system on carbon supports in this reaction.