Effect of cobalt on the activity of nickel-based/magnesium-substituted hydroxyapatite catalysts for dry reforming of methane

Abstract

The dry reforming of methane (DRM) reaction can directly convert methane (CH4) and carbon dioxide (CO2) into syngas (H2+CO), which is a promising method for achieving carbon neutralization. In this study, a series of 3Ni-xCo/Mg1HAP alloy catalysts with different ratio were synthesized by the coprecipitation method, and the optimum Ni-Co ratio for the DRM reaction was studied. A series of characterization methods revealed that after Co was added, the formation of Ni-Co alloys increased the interactions between metals. However, an excess of Co inhibits the entry of Ni into the lattice of Mg1HAP, resulting in metal accumulation on the surface of the support. In addition, the introduction of Co improves the dispersion of Ni metal, which endows the catalyst with better catalytic activity and stability. Raman spectroscopy of the catalyst after the stability test showed that the addition of Co reduced the proportion of graphitic carbon, which was also the main reason for its improved stability.

1. Introduction

In recent years, the problem of global warming has attracted increasing amounts of attention, and methane and carbon dioxide, two greenhouse gases, are the main causes of global warming; moreover, carbon neutralization strategies have been widely implemented worldwide [1-7]. Dry reforming of methane is a very effective method. It directly uses carbon dioxide (CO2) and CH4, two greenhouse gases, and converts them into syngas (H2+CO) [8,9]. Syngas can be applied in the Fischer-Tropsch reaction (FTS) to produce methanol or other fuel resources, which can convert low-value-added chemicals into high-value-added chemicals [10,11].

Ni-based catalysts are considered to be the most practical potential dry reforming of methane (DRM) catalysts because they are less expensive and have the same catalytic activity as precious metals (Ir, Rh, Ru, Pt, and Pd) [12-17]. However, catalyst coking and active metal Ni sintering are the main challenges for the commercial application of Ni-based catalysts [18-20]. The sintering of metal Ni occurs mainly through the agglomeration of metal Ni into larger metal Ni particles under high-temperature reduction and high-temperature reaction conditions [21,22], while CH4 tends to decompose excessively on larger Ni particles (CH4 → C+4H*), resulting in carbon deposition [23-25]. Therefore, a useful way to solve the problems of carbon deposition and metal sintering is to reduce the size of metal nanoparticles and enhance the interaction between the metal and support [26,27]. A large number of published reports show that when the size of metal nanoparticles is less than a certain threshold, they can inhibit the nucleation of graphitized carbon and effectively prevent carbon deposition [28-30]. He et al. [31] prepared g-Al2O3-supported ultrafine Ni nanoparticle (3.7 nm) catalysts, which showed remarkable stability to CH4 and CO2 at 700 °C, and the CH4 conversion decreased by only 9.6 % after 20 h of reaction. Wang et al. [32] encapsulated bimetallic Ni nanoparticles (2.8 nm) in silicalite-2. They found that the reaction showed high resistance to carbon deposition even at 650 °C, where severe coking would occur. Metal sintering occurs due to the migration of metal nanoparticles, so it is highly important to enhance the interaction between the metal and support to improve the sintering resistance of metals. It has been proven that the alloying effect can induce stronger metal-support interactions in the reported literature. Because Ni and Co have similar electronic structures, it is very easy to form Ni-Co alloys.

One of the main reasons for the easy deactivation of Ni-based monometallic catalysts is that they have high activity against the C-H orbital of CH4 [23], which leads to the continuous accumulation of carbon species (C*) in the active sites, which eventually leads to deactivation of the catalysts. The addition of a second metal can effectively alleviate this situation, and the addition of a second metal will also improve the metal-support interaction [33,34]. The addition of the second metal promotes electron transfer between Ni and the second metal, which is beneficial for the formation of oxygen species (O*). O* oxidizes the C* produced by CH4 cracking into CO, thus preventing the accumulation of C* on the surface.