Vanadia-based catalyst

H. Eckert and I. E. Wachs. J. Phys. Chem. 93,6796, 1989. NMR studies of vanadia-based catalysts and model compounds. 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

Spengler, ]. Anderle, F. Bosch, E. Grasselli, R. K. Pillep, B. Behrens, R Lapina, O. B. Shubin, A. A. Eberle, H. ]. Knozinger, H. Antimony Oxide-Modified Vanadia-Based Catalysts—Physical Characterization and Catalytic Properties. J. Phys. Chem. B 2001, 105, 10772-10783. 

Typical catalysts for SCR include supported vanadia, and iron or copper supported on zeolite. Here the application of a model to the design and understanding of vanadia catalyst systems is presented. Over the vanadia-based catalyst system, a Rideal-Eley approach has been adopted by most workers in the field, in which the first step is ammonia adsorption on the catalyst. This stored ammonia can then either react with NOx or be desorbed. Some important contributions to the SCR modelling literature are Andersson et al. (1994), Lietti and Forzatti (1994), Dumesic et al. (1996), Lietti et al. 

One of the primary uses for an accurate SCR model is in the sizing of catalyst systems. The model has been shown to be accurate for the prediction of NOx emissions and conversions, as is shown in Table I for 267 mm diameter vanadia-based catalysts of various lengths. Excellent agreement was obtained between the model and the experimental data, especially for the 17 L system. 

As discussed at the beginning of this section, the difficulty of activating short alkanes causes selectivity problems in these oxidation reactions. Intermediates formed in the, usually, consecutive reaction mechanisms are less stable than propane against further oxidation, and total oxidation and cracking occur as side reactions limiting the selectivity compared with the first vanadia-based catalysts... 

Molecular strucmre and reactivity of vanadia-based catalysts for propane oxidative dehydrogenation smdied by in sim Raman spectroscopy and catalytic activity measurments. Journal of Catalysis, 111 (2), 293-306. 

All these studies have demonstrated the usefulness of calorimetry for studying the surface characteristics of vanadia-based catalysts. Among the surface properties of vanadia catalysts of relevance to their catalytic activity in selective oxidation reactions, acidity is one of the most significant. 

Several proposals have been advanced in the literature concerning the mechanism of the SCR reaction over vanadia-based catalysts They converge in suggesting that the... 

As mentioned earlier, the DeNO reaction over vanadia-based catalysts is accompanied, in the case of sulfur-containing gas, by the oxidation of SO2 to SO3 This aspect has... 

Runs of toluene oxidation depicted in Table 3 also indicate improved activity of catalyst 1. Similar conversion of toluene on promoted and non-promoted catalysts 1 and 3 was reached at 400°C and 500°C accordingly. Selectivity to benzaldehyde was also higher on the promoted catalytic system however, the total performance of this catalyst was lower than results reported for the vanadia based catalysts [13]. 

The production of 1,3-butadiene needs catalysts that cannot adsorb butadiene strongly. Actually, this occurs on Mg ferrite and distinguishes the behavior of this material from that of vanadia-based catalysts, that allow the oxidation of butenes to maleic anhydride and actually adsorb butadiene strongly [12]. Our data suggest that the predominant pathway to total oxidation is competitive with respect to the key alkoxide elemination step, more than successive to it, over MgFe204. 

In recent years, much attention has been devoted to the oxidative dehydrogenation (ODH) of light paraffins [1] and alcohols to aldehydes [2]. Among all the explored catalysts, supported vanadia-based catalysts have been seen promising both in terms of activity and selectivity. A large number of factors may determine their catalytic performances such as (i) the nature,... 

Using isotopically labelled reactants, it has been demonstrated that on both vanadia-based catalysts and noble metals, the two nitrogen atoms of N2 arise one from NO and the other from ammonia. V20s-based catalysts also catalyse the reduction of NO2 in the presence of oxygen ... 

Steady-State Kinetics of the SCR Reaction. The kinetics of the standard SCR reaction over vanadia-based catalysts has been investigated by several authors (64,66,67,86,87,92,93), and different kinetic rate expressions have been proposed. Most of them refer to steady-state conditions, but kinetic studies performed under transient conditions have been reported as well (94-102). 

A global, dynamic kinetic model of NO-NO2/NH3 reactions for vanadia-based catalysts, with high level of detail, closely reflecting all the mechanistic steps discussed in Reference (15), was first published by Chatteijee and coworkers (130). The model could adequately describe the effects of the NO2/NOX feed ratio on transient and steady-state NO conversions, as well as on the selectivity of the SCR process to NH4NO3 and N2O, in the relevant range of temperatures for diesel exhaust after-treatment. 

Bench-scale flow reactor experiments are an effective way of examining the main performance features of the SCR reaction system on various catalysts. In this section, we review these features for Fe-based catalysts as a backdrop to considering more fundamental kinetics and mechanistic studies in Sect. 11.3 and transport effects in Sect. 11.4. The selective catalytic reduction of NOx by ammonia on Fe-ZSM-5 catalyst has been studied in detail by various research groups [19-22, 26, 27, 34-42]. The results from earlier studies of vanadia-based catalysts have underpinned the more recent studies of zeolite-based catalysts. For example, Koebel et al. [3, 6, 43] carried out a detailed study of the SCR chemistry on V-based catalysts. Nova et al. [5, 8, 44, 45] studied the chemistry of SCR over V-based catalyst and proposed a mechanism for the fast SCR reaction. To this end, the data here are by no means unique but are intended to highlight the important trends. 

The resulting LH type model builds on this concept by considering that adsorbed NH3 reacts with surface NOx species. It is noted that NH3 adsorption on Fe-zeolite systems is not inhibited by the co-adsorption of water. A simple interpretation of this key observation is that the adsorption of NH3 and H2O occur on different sites. NH3 adsorption on protonated zeolites is known to occur on the Bronsted acid sites, and that has led Tronconi, Nova, and coworkers among others to propose for vanadia-based catalysts the exchange of NH3 between two types of sites. Applying this concept for Fe-zeolite catalysts gives ... 

Progress has been made towards the development of monolith reactor models that predict SCR performance under both steady state and transient operation. Guth-enke et al. [33]. provided a thorough review of SCR reactors. Most of the earlier work in this area was done for the more established Vanadia-based catalysts and involved the use of global kinetic models [81-83]. More recent works by Nova et al. provided detailed transient model for the SCR reaction system on Vanadia-based catalyst [8, 45]. Olsson and coworkers developed both global and detailed kinetic models for NH3-SCR reactions on Cu-ZSM-5 catalysts [14, 15, 49]. More recent works have communicated models for NH3-SCR reactions on Fe-zeoUte catalysts [25, 57, 76, 84]. 

Several kinetic models regarding the reduction of NO with NH3 have been reported in the Uteratme for various catalyst technologies Tronconi et al. [29] and Chatteijee et al. [30] for Vanadia-based catalysts, Malmberg et al. [21] and Schuler et al. [31] for Fe-zeolite catalysts and Olsson et al. [32] for Cu-zeohte catalysts. Neglecting detailed kinetic mechanisms and reaction intermediates, which are beyond the scope of this chapter, a similar global modeling approach can be used for the common SCR catalytic materials. The major reactions, considered in this model, are given below. 

El-Korso, S., Khaldi, L, Bedrane, S., Choukchou-Braham, A., Thibault-Starzyk, F., and Bachir, R. (2014) Liquid phase cyclohexene oxidation over vanadia based catalysts with tert-butyl hydroperoxide epoxidation versus allylic oxidation. /. Mol Catal A Chem., 394, 89-96. 

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