Surface characterization studies of CuO-CeO₂-ZrO₂ catalysts for selective catalytic reduction of NO with NH₃

Highlights

• The CeO₂-ZrO₂-CuO catalysts show excellent NH₃-SCR activity at low temperature.

• The catalyst prepared by co-precipitation method shows the best SCR activity.

• The SCR activity and chemical state are obviously effected by preparation method.

• The synergistic effect between Cu and Ce contributes to the high SCR activity.

• The high dispersion of copper species contributes to the excellent SCR activity.

Abstract

A series of CuO-CeO₂-ZrO₂ catalysts were prepared by different methods and applied to the selective catalytic reduction of NO with NH₃ reaction at low temperature. The results showed that the SCR activities, morphology, particles dimension, and the surface chemical state of CuO-CeO₂-ZrO₂ catalysts were obviously influenced by the preparation method. The SCR performance results showed that the CuO-CeO₂-ZrO₂ catalyst prepared by co-precipitation method presented the best activity in the temperature range of 125–180 °C. The characterization results showed that the Ce⁴⁺, Ce³⁺, Cu²⁺ and Cu⁺ species were coexistence in the CuO-CeO₂-ZrO₂ catalysts, and the Cu species mainly existed as Cu²⁺. It was also found that the high surface area, the synergistic effect between copper and ceria, enhanced acidity and the highly dispersed copper species were responsible for the high SCR activity of the CuO-CeO₂-ZrO₂ catalyst.

Introduction

Nitrogen oxides (NO_x) are the major atmospheric pollutants that cause acid rain, photochemical smog, ozone depletion and global warming, mainly from the combustion of fossil fuels such as coal-fired power plants (stationary source) and motor vehicle engine (mobile source). The selective catalytic reduction of NO with NH₃ (NH₃-SCR) is an efficient way to convert NO_x into N₂ from stationary sources. The commercial catalysts for this process are V₂O₅-WO₃/TiO₂ and V₂O₅-MoO₃/TiO₂ oxides [1]. Although the vanadium-based catalyst presents high SCR activity and excellent resistance of SO₂, this catalyst system also suffers from disadvantages such as the narrow activity temperature window of 300–400 °C and the toxicity of vanadium pentoxide [2]. Therefore, it is necessary to develop the SCR catalyst system with high activity at low temperature (<200 °C).

In the recent years, Ce-based catalysts have been reported to have excellent activity in the low-temperature SCR reaction and attracted increasing attention due to the high oxygen storage capacity (OSC) and strong redox property of CeO₂ [3], [4], [5], [6]. In order to promote the SCR activity, some other transition metals such as Fe, Ti, and Cu are often doped into ceria based catalyst [1], [7], [8]. Jiang et al. [9] have reported that the carbon fiber supported CuO-CeO₂ showed high activity for urea-SCR of NO at room temperature. Guo et al. [10] have found that CuO-CeO₂ catalyst presented excellent low temperature SCR performance and the state of Cu species had great impact on the SCR activity of CuO-CeO₂ catalyst. Gao et al. [11] reported that Ce-Cu-Ti catalyst could remain at high performance in the presence of H₂O and SO₂. Zirconia (ZrO₂) has a high surface acidity and a high thermal stability. During the past several years, ZrO₂ is widely used as a support in catalysts [12], [13]. Adding ZrO₂ to CeO₂ has been done to stabilize ceria against high temperature sintering and prevent the decline of oxygen storage capacity (OSC) [14]. The high OSC can promote the oxidization of NO to NO₂, which would increase the catalytic activity for reduction of NO with NH₃, especially in the range of low temperatures [15], [16]. The SCR catalysts, such as CuO/CeO₂/ZrO₂ [17], CuO-CoO_x/Ce_0.67Zr_0.33O₂ [18], CuO/WO₃/Ce_0.5Zr_0.5O₂ [19] and WO₃/CeO₂-ZrO₂ [20], have been reported that these catalysts show high NH₃-SCR reaction activities in the temperature range of 200–450 °C. However, few reports have been focused on the CeO₂-ZrO₂ based catalysts for NH₃-SCR process at low temperatures (120–220 °C).

It is well-known that the catalytic activity of a catalyst depends not only on the content and chemical components, but also on the particles dimension, morphology and properties of the catalyst. Furthermore, the catalyst preparation method also has great influence on the particles dimension, morphology, physico-chemical properties and the dispersion state of the active components. Different preparation methods such as incipient wetness impregnation [21], [22], co-precipitation [23], [24], hydrothermal synthesis [25] and sol–gel method [26], [27] are often used to obtain catalysts with different physico-chemical properties. These preparation methods act as an important role in promoting the performance of catalyst materials.

In this work, we aim to investigate the NH₃-SCR catalytic activity of the CuO-CeO₂-ZrO₂ systems and the effect of preparation methods on the structure, redox behavior of the systems. SEM, BET specific surface area, XRD, H₂-TPR, NH₃-TPD and XPS were performed to study the structures morphological and physico-chemical properties of the CuO-CeO₂-ZrO₂ catalysts. The work tried to provide basic knowledge for NH₃-SCR catalysis in CuO-CeO₂-ZrO₂ systems.