Syngas production via CO₂ reforming of methane over noble metal (Ru, Pt, and Pd) doped LaAlO₃ perovskite catalyst

Highlights

• LaAlO₃ based perovskite catalysts were synthesized by solution combustion synthesis.

• Ru, Pt and Pd substituted LaAlO₃ are active catalysts for DRM.

• Stability study shows LaAl_0.98Ru_0.02O_3-δ is highly active and coke resistant.

• Synergetic effect between Ru and Al offers high sintering stability.

Abstract

Recently perovskite materials have received extensive attention for dry reforming of methane (DRM) reaction due to their stable ordered crystalline structure. In this work, materials with ionic substitutions of Ru, Pt, and Pd in LaAlO₃ perovskite were synthesized using solution combustion synthesis route. The characterizations of catalysts were carried out by XRD, TEM, XPS, STEM, and BET surface area analyser. H₂-TPR was performed to examine the strong metal support interaction between active metal and support that improve the resistance of the perovskite structure. The catalytic activity of these catalysts was examined for DRM. Stable conversion of both CH₄ and CO₂ were observed on stability test experiments for 50 h over Ru substituted LaAlO₃ catalyst. This indicates that it is a highly stable and coke resistant catalyst for DRM. TEM and STEM results show the deactivation of Pt and Pd incorporated LaAlO₃ due to sintering and carbon deposition. The apparent activation energy was determined from Arrhenius plot which was found to be 86 kJ mol⁻¹ and 72 kJ mol⁻¹ over LaAl_0.98Ru_0.02O_3-δ for CH₄ and CO₂ consumption, respectively. DRM reaction mechanism was proposed via formation of oxycarbonates over LaAl_0.98Ru_0.02O_3-δ.

Introduction

Syngas (CO + H₂), is a mixture of hydrogen and carbon monoxide, a key intermediate for the production of hydrogen, ammonia and value-added chemicals such as Fischer-Tropsch derived liquid fuels, and methanol [[1], [2], [3]]. Production of syngas via reforming of hydrocarbons has gained much attention in the past decade. The most common employed hydrocarbon source is methane. Syngas can be produced via various technologies such as steam reforming (SRM), dry reforming (DRM), partial oxidation (POM) and auto thermal reforming (ARM) from methane. Predominately they differ in production of CO/H₂ ratio, oxidant used, and energies of respective technology. Syngas production via DRM, which utilizes the most important greenhouse gases CH₄ and CO₂, has received growing scientific interest in the recent past. The dry reforming of methane is represented as

$$\begin{array}{cc}\hfill C{H}_4+C{O}_2\leftrightarrow 2CO+2H_2\hfill & \hfill \Delta H_{298K}^o=247kJmo{l}^{-1}\hfill \end{array}$$

DRM is a highly endothermic reaction that has to be conducted at high temperature and it yields H₂/CO ratio close to unity, which is favourable for the industrial production of oxygenates, methanol and long chain hydrocarbons [4,5]. Although DRM is environmental friendly, it has not been commercialized due to deactivation of catalyst by coke deposition and sintering of active metal at high temperature during the reaction. Thermodynamics of DRM indicates that the coke formation is usually associated with high temperature, feed ratio of CH₄/CO₂, and the nature of catalyst [6,7]. The most prominent carbon formation pathways are methane decomposition and carbon monoxide disproportionation. Encapsulated carbon is found to be responsible for catalyst deactivation while amorphous carbon is released in the form of CO [8]. Thus, catalysts are highly sensitive for methane reforming reactions due to coke formation. The activation of both CH₄ and CO₂ in DRM possess relatively higher activation energies compared to individual CH₄ and CO₂ activation. Therefore, development of catalyst with high activity, selectivity, and resistance to carbon formation for DRM is essential.

DRM has been studied extensively over Ni based catalysts and serious stability problem of those catalysts due to coke deposition was reported [9,10]. Both stability and catalytic activity of catalysts are equally important for high temperature reforming reactions. Noble metals with various supports are highly active and stable catalysts for dry reforming of methane due to high resistance to coke formation. The observed activity order of noble metal is as follows Rh > Ru > Ir > P t > Pd [[11], [12], [13], [14], [15], [16]]. The relatively inexpensive and most active noble metal Ru based catalysts were explored for DRM [[17], [18], [19]]. It is usually accepted that the activation of both CH₄ on active metal and CO₂ on support takes place. Literature emphasizes that the support is also as important as the active metal for dry reforming of methane. The promising support should be thermally stable and promote more active sites. Moreover, the support also plays a crucial role in catalytic reactions by offering certain key properties such as acid-base characteristics, high oxygen storage capacity, reducibility and strong metal–support interaction for high activity by minimizing the carbon deposition. Ioannis et al. [15] studied the effect of support on the activity, selectivity, and resistance to carbon deposition over Rh supported catalysts. They observed that ceria–zirconia support minimized carbon formation by maintaining Rh in metallic state compared to alumina and alumina-ceria-zirconia. Several supports and combined such as SiO₂, Al₂O₃, CeO₂, La₂O₃ have been investigated extensively for DRM [[20], [21], [22], [23], [24]]. CO₂ activation depends on acid-base characteristics of support that could be activated by forming formates and oxy-carbonates on acidic support (Al₂O₃) and basic support (La₂O₃) respectively [25]. Li et al. [26] have studied the DRM over Ru supported Mg-Al oxides. The higher catalytic activity of Ru/MgO and Ru/Mg₃ (Al)O was speculated due to the strong basicity of the support with more available Ru° surface atoms.

In addition to conventional supports, crystalline mixed oxides such as perovskite, pyrochlores, fluorite, are employed for DRM due to their thermal stability at high temperatures [15,27,28]. The effect of noble metal substitution at B-site of LaZr₂O₇ has been studied for DRM by Pakhare et al. [29] and they reported that higher resistance to coke deposition was observed for 1%Ru substituted LaZr₂O₇ compared to 0.5 %Ru/Al₂O₃. Moreover, the activation of CO₂ takes place at the La site by forming La₂O₂CO₃ that further react with carbon formed from methane to produce CO. Mixed oxides such as perovskite (ABO₃) type material have been studied as promising support for DRM due to their structure and ability to facilitate high metal dispersion [[30], [31], [32]]. Among the perovskite ceramic material, LaAlO₃ becomes attractive for reforming reactions [[33], [34], [35]]. However, there are only few studies on noble metal incorporated LaAlO₃ for DRM. LaAlO₃ was synthesized using various methods such as sol–gel, hydrothermal, and solution combustion synthesis [36,37]. In the present study, LaAlO₃ and noble metal substituted LaAlO₃ have been synthesized using solution combustion synthesis, which is a highly versatile and efficient technique to synthesize a variety of materials. Characterizations such as XRD, XPS, TEM and STEM have performed thoroughly for synthesized catalysts. DRM reactions were carried out over these catalysts and reaction mechanism was proposed.