Hybrid catalysts for the selective catalytic reduction of NO by NH3: The influence of component separation on the performance of hybrid systems

https://doi.org/10.1016/j.apcatb.2015.09.028Get rights and content

Highlights

  • Component distance dominating synergies in mixtures of oxidation and NH3–SCR catalysts.

  • Shortest distance brings best performance.

  • Operation mode involving NO2 intermediate rejected.

  • Critical intermediate might be HNO2.

Abstract

The selective catalytic reduction (SCR) of NO by NH3 was investigated over physical mixtures of an oxidation catalyst with an SCR catalyst (“hybrid catalyst”) in which the components were arranged in variable distance to each other: pressed in mixed particles after grinding the mixture, mixing particles of individual components, and in two-layer and four-layer beds. Using Fe-ZSM-5 or V2O5–WO3/TiO2 as SCR components and Mn2O3, Mn–Cu and Ce–Zr mixed oxides as oxidation components, it was observed that the pronounced synergies described in literature are observed only when the components are mixed within the particles. The synergy is strongly attenuated in beds of component particles and fades away completely in the layered structures. This observation is at variance with earlier ideas according to which the oxidation component catalyses the formation of NO2 which opens a fast SCR reaction path over the SCR component. Instead, a more labile critical intermediate is more likely, e.g., HNO2, which may be formed over the oxidation component and proceeds to the SCR sites for further reaction along routes discussed in recent literature. The new understanding is in accordance with earlier observations that correlation between NO oxidation activities of oxidation components and performace of the resulting hybrids was observed to be coarse and to include remarkable outliers.

Introduction

Although the selective catalytic reduction of NO by NH3 is a mature technology for purification of flue gases from stationary sources and is presently being commercialized at large scales in exhaust gas treatment of Diesel vehicles [1], research dedicated to the creation of ever more active, selective and stable catalysts remains to be important and attractive. While the installations in cars employ metal-exchanged zeolites (Cu, Fe) or promoted VOx–TiO2 catalysts depending on the application, there has been much effort to present alternative options based, for instance on supported or bulk mixed Mn or Ce oxides [2]. However, Mn-based catalysts tend to release undesired side products (N2O) and to suffer from deactivation. Ce-based catalysts often fail to perform well at low temperatures, where significant conversion is needed due to the low exhaust temperatures of modern Diesel engines.

The disclosure of catalyst combinations by Stakheev et al. [3] has recently opened a novel, productive route in this development. These “hybrid” systems combine an oxidation catalyst with an SCR catalyst (up to now Fe zeolites) in mechanical mixtures, which results in much higher activities than the SCR component would achieve alone. The explanation proposed for this relies on the well-known fact that the SCR of NO/NO2 mixtures (Eq. (1)) is much faster than the SCR of NO alone (Eq. (2)).NO + NO2 + 2 NH3  2 N2 + 3H2O, “fast SCR”4 NO + 4 NH3 + O2  4 N2 + 6H2O, “standard SCR”

It is therefore inferred that part or the NO is oxidized to NO2 over the oxidation component, which opens fast SCR as a new reaction channel for the system. This is actually what happens in equipment for urea-SCR where Diesel oxidation catalysts upstream urea injection are upgraded in a way that they produce sufficient NO2 from the NO present, which allows the SCR catalyst to operate via fast SCR (Reaction (1)). In the hybrid catalysts, the oxidation component is no longer separated from the SCR catalyst, it faces the full reaction mixture including NH3, which is easily oxidized as well.

In a study with a variety of oxidation components we have recently explored these synergies for a number of them when combined with Fe-ZSM-5 [4]. Huge activities and promising stabilities were observed with Mn-containing binary and ternary oxides. With Ce–Zr-oxide, synergistic effects were found as well, but the activity remained smaller. However, while N2O and NO2 were formed in inacceptable extent at different reaction temperatures over Mn-containing systems, the hybrid with Ce–Zr-oxide was completely selective for N2. Notably, an attempt to relate the observed synergistic effects to NO oxidation rates measured with the oxidation components resulted in a rather approximate correlation only while signficant contradictions remained in detail. Therefore, the consecutive scheme of standard SCR proceeding via NO oxidation and fast SCR could not be unequivocally proven for the hybrid catalysts.

In mechanical mixtures, the spatial separation of components is a promising approach to study the functions of the components separately. Chemical stability of the molecule which carries the reaction from one component to the other one is an indispensible requirement for this approach. The stability of NO2 is evident from the success of the commercial scheme with NO2 formation (far) upstream of the SCR catalyst. The present paper communicates the result of studies devoted to confirm the applicability of this approach to our hybrid catalysts. Its failure suggests that NO2 may not be the mediating component. To enlarge the experimental basis for the discussion, the hybrid concept will be initially extended to the use of V2O5–WO3/TiO2 as the SCR component.

Section snippets

Catalyst preparation

The preparation of most catalyst components has been described earlier [4]. Fe-ZSM-5 was made by solid-state ion exchange (for details see Ref. [5], for XRD of the sample—Ref. [4]), its Fe content was determined to be 0.4 wt.%. The second SCR component (V2O5–WO3/TiO2, “V–W/Ti”) was prepared by impregnating a 10 wt.% WO3/TiO2 support supplied by Huntsman Pigments and Additives with an aqueous solution of NH4VO3 (0.356 g/L) at pH 9 to obtain 0.5 wt.% of V2O5. After drying at room temperature, the

Hybrids with V2O5–WO3/TiO2

In Fig. 2a, NO conversions measured with hybrids composed of the Ce–Zr oxidation component and Fe-ZSM-5 or V–W/Ti are compared. The Ce–Zr@Fe-ZSM-5 hybrid was already described in our previous report [4], its data and those of its components have been cited from this paper. The NO conversion curve of the Ce–Zr@V–W/Ti hybrid is almost identical with that of the zeolite-containing one up to 700 K. Fig. 2a also presents the NO conversions measured with the V–W/Ti component alone, which is more

Discussion

The results presented here clearly show that the synergistic effects observed do not operate via the formation of NO2. Nitrogen dioxide is a stable molecule which could easily travel between different particles of a bed or from its origin to a downstream place of consumption as it does in commercial urea-SCR facilities. This raises a number of questions: What else mediates the interaction between the hybrid components? Why is NO2 not formed although the oxidation components, in particular those

Conclusions

The performance of hybrid catalysts for the SCR of NO by NH3 consisting of physical mixtures of an oxidation component (Mn2O3, hopcalite, Ce–ZrOx) with an SCR component (Fe-ZSM-5, V2O5–WO3/TiO2) strongly depends on the separation between the components. The drastic synergies described in literature are observed only when the components are thoroughly mixed and pressed into mixed particles. The synergy is strongly though not completely suppressed when particles containing the individual

Acknowledgements

We thank Dr. Stuart Tayler (Cardiff University) for providing hopcalite material from own development (cf. [6]) and Umicore AG & Co. KG for a donation of Ce–Zr mixed oxide. We gratefully acknowledge experimental support by Dr. Thomas Reinecke (XRD). OPT specially thanks the Russian Science Foundation (Grant No. 14-50-00126) for support.

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1

Present address: Oak Ridge National Laboratory, Knoxville, Tennessee, U. S. A.

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