Synthesis and characterization of supportless Ni-Pd-CNT nanocatalyst for hydrogen production via steam reforming of methane

Highlights

• Addition of MWCNTs increased the specific surface area from 251.2 to 611.3 m²/g.

• Pd clusters dispersion improved by the addition of MWCNTs.

• The activation energy of redox reactions decreased by addition of the MWCNTs.

• Methane conversion increased to 85% at 850 °C when using the Ni-Pd-0.1CNT catalyst.

Abstract

Supportless Ni-Pd-0.1CNT foamy nanocatalyst with specific surface area of 611.3 m²/g was produced by electroless deposition of nickel, palladium and multiwall carbon nanotube (MWCNT) on interim polyurethane substrate. Application of temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) data into Kissinger (Redhead) kinetic model showed lessening of their activation energies due to Pd and CNT addition. Presence of foamy Ni/SiC caused 8% higher steam reforming of methane; while Ni-Pd-0.1CNT presence resulted in 22% higher methane conversion. The catalytic behavior of the samples was described by morphological and compositional studies which were carried out by transmission electron microscope (TEM), field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS) and atomic absorption spectrometer (AAS) pondered with Brunauer–Emmett–Teller (BET), TPR, TPO and X-ray diffraction (XRD).

Introduction

During the last decade, hydrogen production industry attracted significant funding sources because of the wide application range and environmental concerns like climate change and the greenhouse effect. Due to the about three times higher energy density than the oil, along with cleanness, hydrogen has been recognized as a promising source of energy [1], [2], [3], [4]. Hydrogen-energy processing chain includes three significant steps of production, storage, and repowering. All three steps require serious attention and improvement [4], [5]. The need for higher production efficiency is considered as a fundamental issue in hydrogen generation. Several green production methods like electrochemical, solar and biomass have been introduced recently [6], [7]. According to the irreversible nature of hydrogen storage, production by means hydrolysis reactions of hydrides has drawn exceptional attention due to the safety and amenability to mild reaction conditions [8], [9], [10], [11], [12]. However, neither of these approaches still shows promising efficiency results for industrial-scale production [13]. Therefore, fossil fuels are used predominantly for hydrogen production in several sectors. Due to the inevitable need for H₂ production from hydrocarbons and low conversion values (<70%) of these techniques, a huge emission of greenhouse gases occurs annually by these industries [11], [12], [13], [14]. An immediate solution to overcome the issues mentioned above is taking advantage of low-cost nanotechnology production methods along with designing new catalytic structures for increasing hydrocarbons conversion and decreasing the greenhouse gases emission. Steam reforming of methane (SMR) is a common approach for industrial-scale production of the reducing gases (H₂ and CO) through the following reactions [14], [15], [16], [17], [18], [19]:

$${\text{CH}}_{4\left(\text{g}\right)}{+\text{H}}_2{\text{O}}_{\left(\text{g}\right)}\to {\text{CO}}_{\left(\text{g}\right)}{+3\text{H}}_{2\left(\text{g}\right)}\Delta H_{298}^{\circ }=206\text{kJ}/\text{mol}$$

$${\text{CH}}_{4\left(\text{g}\right)}{+2\text{H}}_2{\text{O}}_{\left(\text{g}\right)}\to {\text{CO}}_{2\left(\text{g}\right)}{+4\text{H}}_{2\left(\text{g}\right)}\Delta H_{298}^{°}=165\text{kJ}/\text{mol}$$

$${\text{CH}}_{4\left(\text{g}\right)}{+\text{CO}}_{2\left(\text{g}\right)}\to {2\text{CO}}_{\left(\text{g}\right)}{+2\text{H}}_{2\left(\text{g}\right)}\Delta H_{298}^{°}=247\text{kJ}/\text{mol}$$

$${\text{CO}}_{\left(\text{g}\right)}{+\text{H}}_2{\text{O}}_{\left(\text{g}\right)}\to {\text{CO}}_{2\left(\text{g}\right)}{+\text{H}}_{2\left(\text{g}\right)}\Delta H_{298}^{°}=-41\text{kJ}/\text{mol}$$

The efficiency of this process depends on the type of the available catalyst. As it comes from the reactions, the conversion of methane to hydrogen is considerably endothermic and requires high temperatures (>700 °C) to take place which is both economically and environmentally unfavorable [19], [20]. Nickel-based catalysts are widely used for SMR due to the activity, affordability, selectivity, and stability for hydrogen production, hydrogenation and hydrogen absorption [21], [22], [23], [24]. Despite the industrial importance of nickel catalysts, only a few studies emphasized on practical and inexpensive production methods leading to the synthesis of nanostructured nickel catalysts that can maintain their superior catalytic properties and performance at high temperatures. Many of the introduced synthesis methods are either expensive or impractical for industrial purposes. Furthermore, competitive materials like ruthenium, rhodium, and platinum are scarce and unaffordable for reforming of methane [25], [26]. Accordingly, designing a novel nickel catalyst that can perform efficiently under the working circumstances with a high conversion percentage is necessary. Electroless deposition of nickel which is a significantly low-cost method can be utilized to prepare nanostructured and highly porous nickel catalysts. The amount of precursor used in this method is considerably lower than the conventional nanocatalysts production techniques. Moreover, a uniform dispersion of coating is achievable by controlling the processing parameters like pH [26], [27], [28], [29], [30].

An ideal catalyst must behave as an electron relay which simplifies the electron transfer between the donor and the acceptor [21]. Metal particles are lovely from this respect. However, their size has a significant effect on their catalytic behavior [20], [21], [22], [23]. Redox properties of the bulk and nano-sized particles are entirely different [24], [25], [26]. While metal particles are helpful in heterogeneous catalysis, the most entangled issues in this realm are ascribed to their electronic contributions [24], [27], [28], [29]. To improve the catalytic properties of metallic species, carbon nanotubes and noble metals (Pd, Au, Ag, etc.) can be utilized. Carbon nanotubes have enormous specific surface area [14], [15] good electrical conductivity [16], [17], inertness and high stability even at acidic, basic and hot environments [18], [19]. MWCNTs have coaxial nanotube cylinders with carbon atoms in hexagonal lattice arrangements [6] which can adsorb metallic atom wanderers [20]. Electron transfer is a significant stage in redox reactions [21]. While the huge difference in acceptor/donor potentials limits transmission of the electrons in the redox reactions [22], multi-walled carbon nanotubes can improve the catalytic efficiency by lowering of the redox activation energies [15], [31] and facilitation of the required electron transfer. On the other hand, the existence of the noble metals on the surface of catalyst can increase the efficiency and conversion value in hydrogen production through SMR process [32], [33], [34], [35].

Another critical factor in the catalytic enhancement of metallic species is the selection of support or substrate. It has been claimed that the type of support can affect the efficiency of catalysts in many aspects. Although ceramic oxides have tend to react with the catalyst active sites [30], most researchers have exclusively considered oxide supports for Ni catalysts [26], [31], [32], [33]; while minor attention has been paid to polymeric substrates [30], [34]. While inexpensive, simple synthesis of polymers is surprisingly impressive, they can be used as a temporary substrate to increase the efficiency and performance of nickel catalysts [30]. In this context, polymeric support can be eliminated by appropriate heat treatment after the deposition of metal on the substrate, to create a highly porous catalyst with the strikingly high surface area. In this case, the final product is an interconnected nickel network with considerably high efficiency (Fig. 1). Currently, the maximum achieved hydrogen yield by Ni-Pd catalysts for steam reforming of methane has been reported to be 62% at 700 °C [36]. Meanwhile, the results obtained in same reaction condition for Ni-Pd-0.1CNT nanocatalyst shows significantly higher conversion value of 76.5%.

Current study deals with the electroless production and then characterization of Ni nanocatalysts improved by using Pd and MWCNTs on polyurethane support. The polymeric support eventually evaporates under controlled conditions. The aim of this study is to achieve maximum specific surface area with superb reactant interactions, most effective catalytic effects and making a comparison with the same active materials on a ceramic support (SiC) as well as an industrial catalyst. The highly porous products show enhanced catalytic effects which are noticeably superior to previous reports [36], [37], [38], [39].