Mesoporous zirconia nanobelts: Preparation, characterization and applications in catalytical methane combustion

Abstract

Mesoporous ZrO₂ nanobelts (MZNs) have been prepared by a calcination route of ZrS₃ nanobelts in air. The nanobelts prepared at 400–600 °C are the mixtures of tetragonal and monoclinic phases, and have well-distributed mesopores (pore diameter of about 3.4–3.6 nm). As the calcination temperature increased from 400 to 1200 °C, the structures changed from tetragonal to monoclinic phase, while the morphologies turned from regular nanobelts to bead-like nanowires, and the mesopores disappeared bit by bit. Fe-doped and Fe₂O₃-loaded MZNs have been prepared to compare the catalytic activities of Fe-doped, Fe₂O₃-loaded, and pure MZNs for methane combustion. The results showed that Fe₂O₃-loaded MZNs have rather high catalytic activity, suggesting its potential application in practice. Methane combustion data over the catalysts are well fitted by a first-order kinetic expression.

Introduction

Methane has a much larger greenhouse effect than carbon dioxide, and the concentration of methane in the atmosphere is increasing continuously. In order to eliminate methane emission from natural gas engines and power plants as well as petroleum and petrochemical industries, complete combustion of methane becomes a must. Noble metal catalysts show high activity for methane combustion at low temperature [1], [2], [3], [4], but limited due to their scarcity and high cost. So low-cost metal oxide or mixed metal oxide catalysts still are primary selection [5], [6]. Mn-, Co-, and Fe-stabilized ZrO₂ catalysts [7] and zirconia-supported La, Mn oxides and LaMnO₃ perovskites [8] were once reported to have high activity for methane combustion. Recent research shows that catalytic activity can be obviously tuned by changing morphologies of catalysts or catalyst supports [9], [10]. Therefore it is interesting and necessary to explore the possible shape effect of ZrO₂ as catalysts or catalyst supports.

Zirconia has been extensively used as catalysts or catalytic supports [11], [12], [13], [14], optical waveguides [15], gate dielectric in metal-oxide semiconductor (MOS) devices [16], high-performance ceramics [17], biological materials [11], and photon absorber and photocatalyst [18], [19] due to its valuable chemical and physical properties such as high melting point, high resistance to thermal shock, high dielectric constant, excellent wear resistance, biocompatibility, and wide gap band semiconductor [20]. In order to tailor these physical–chemical properties of ZrO₂, zirconia nanoparticles have been synthesized by sol–gel [21], [22], [23], hydrothermal/solvothermal [24], [25], [26], emulsion precipitation [27], and thermal decomposition [28] approaches. Zirconia nanowires [29] and rare-earth (RE)-doped zirconia nanobelts [30] have been prepared by an alumina template method and a pyrolysis of Zr(OH)₄: RE particles, respectively. ZrO₂ mesopore microfibers have been prepared by a Pluronic P-123 template-directed method [31], [32]. However, no ZrO₂ mesopore nanobelts have been reported to date.

Here we demonstrate a novel facile route to synthesize mesopore zirconia nanobelts (MZNs), neither triblock-copolymer nor surfactants are needed, while the nanobelts were prepared into Fe-doped and Fe₂O₃-loaded MZNs catalyst for methane combustion. Firstly, ZrS₃ nanobelts were prepared by a chemical-vapor-transport (CVT) of Zr powder and S powder at 650 °C. Then the ZrS₃ nanobelts were oxidated into mesoporous ZrO₂ nanobelts in air by changing calcination temperatures. When Fe was added in process of preparing ZrS₃ nanobelts, Fe-doped MZNs could be obtained. When the mesoporous nanobelts were impregnated in ferric nitrate solution, Fe₂O₃-loaded MZNs could be obtained. The research results showed that calcination temperatures have a great influence on the crystal structures, the morphologies and pore structures of ZrO₂ nanobelts, and that Fe₂O₃-loaded MZNs reveal rather high catalytic activity for methane combustion.