Elsevier

Catalysis Today

Volume 75, Issues 1–4, 3 July 2002, Pages 331-338
Catalysis Today

Influence of NH3 and NO oxidation on the SCR reaction mechanism on copper/nickel and vanadium oxide catalysts supported on alumina and titania

https://doi.org/10.1016/S0920-5861(02)00055-XGet rights and content

Abstract

The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N2O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H2-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO2 and N2O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO2 intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO2 species could be related to the N2O formation.

Introduction

The selective catalytic reduction (SCR) of NOx with ammonia has been extensively studied as a catalytic technology for NOx elimination from stationary sources [1]. Nevertheless, there is still an open discussion concerning the chemical and mechanistic aspects involved in this process, mainly those related with N2O formation. The superiority of NH3 as a reductant for the SCR reaction is due to the fact that all other reducing agents are susceptible to oxidation by O2 instead of reacting with NOx [2].

A great number of catalysts systems have been investigated for this reaction, for example supported noble metals and supported Fe2O3 or Cr2O3. But for industrial applications copper [3], [4] and vanadium supported catalysts have shown excellent results [5], [6], [7]. The addition of small quantities of NiO to CuO/Al2O3 catalysts enhance the segregation of copper on the surface and the Ni2+ leads to a Cu2+ redistribution with an increase in the tetrahedral site population by Cu2+ [8]. Thus, copper/nickel alumina catalysts have shown superior performances in NOx elimination from the tail gas in nitric acid plants [3], [9], [10].

When SO2 is present in the flue gas from power generation units, sulphur resistance is among the catalysts requirements, and titania (anatase) is used extensively as a support due to its resistance to sulphur oxide poisoning [11]. Another widely used support is γ-alumina due to the high surface area compared with TiO2.

Two types of reaction mechanisms have been proposed in the literature over these catalytic systems namely Eley–Rideal and Langmuir–Hinshelwood. The Eley–Rideal type process does not involve strongly adsorbed NO species [12], whereas in the Langmuir–Hinshelwood mechanism adsorbed NO is suggested to play an important role in the reaction [13].

In a previous paper [14], the performance of copper/nickel and vanadium catalysts in a monolithic form supported on titania (anatase) and γ-alumina, at low temperature 180–230 °C for different NO/NOx and NH3/NOx ratios was studied. The operation conditions selected for the catalytic activity measurement were close to those found for lower pressure nitric acid plants tail gases. For this application, where SO2 is not present, vanadium content can be higher, obtaining a more active catalyst for NOx elimination at lower temperature [6], [15].

The aim of this work was to clarify the influence of ammonia and nitric oxide, oxidation on the SCR of NO by ammonia on these catalytic systems, using powder samples and operating in a wider temperature range (180–450 °C). To obtain more information about the behaviour of these systems, H2-TPR, NH3-TPD and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) NH3 oxidation experiments were carried out.

Section snippets

Catalysts preparation

Copper/nickel (CuNiAl, CuNiTi), and vanadium oxide (VTi, VAl) monolithic catalysts were prepared as described elsewhere [14]. Monolithic supports based on titania and alumina with natural silicates as binders, were manufactured in our laboratories by extrusion. After heat treatment of the support, the catalysts were prepared by impregnation of the monoliths with aqueous solutions of copper nitrate, nickel nitrate or vanadyl sulphate. The monolithic catalysts were crushed in order to carry out

SCR results

The variations of the NO conversion and the N2O outlet as a function of the temperature are shown in Fig. 1 for all samples. VTi catalyst had higher activity than VAl at temperatures lower than 300 °C, while CuNiAl catalyst had higher activity than CuNiTi over the whole temperature range. VAl catalysts maintains activity near to 95% even at 400 °C. N2O formation during SCR reaction was detected at temperatures above 300 °C for all the catalysts except for CuNiTi, where N2O started to be formed at

Summary

As a result, the difference in the behaviour of NO and NH3 oxidation, could explain the different catalytic activity in the SCR reaction between vanadia and copper–nickel oxide catalysts. The chemistry of NO reduction by ammonia in the absence of catalyst is known as the following [28]:NH3+OHNH2+H2ONH2+NON2+H2ONH2+OHNH+H2ONH+O2NO+OH

When the temperature is high enough (800 °C), a H/O radical pool is built due to the presence of O2 and H2O, which results in appreciable amounts of OH radicals

Acknowledgements

We are grateful to CICYT (projects AMB97-0946 and 2FD97-0035) and CECA-EU (project 7220-ED/093) for supporting this work. We also thank Dr. M. Yates for valuable discussions.

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