CATALYTIC MEASURES FOR THE ABATEMENT OF NITROGEN OXIDES IN THE ATMOSPHERE

MAKOTO MISONO

Department of Applied Chemistry, Graduate School of Engineering,

The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

1. Introduction

Nitrogen oxides (NO, NO2, and N2O) are pollutants that cause several serious environmental problems such as acid rain, photochemical smog, global warming, and depletion of ozone layer as well as direct hazardous effects on human bodies.

About three quarters of the entire NOx emission are from human activities.  It is remarkable that the major measures to abate NOx are catalytic processes, selective catalytic reduction (SCR) by ammonia for stationary sources and three way catalyst (TWC) for the exhausts of gasoline-engined automobiles.

(SCR)      4NH3 + 4NO + O2 ----> 4N2 + 6H2O

(TWC)      NO + CO, CmHn ----> N2, CO2, H2O

It is also notable in Japan that, in spite of the nearly complete employment of the two processes, the concentration of NO2 in the atmosphere is not decreasing, in contrast to the sharp decrease attained for SO2.  This is mainly due to the rapid increase in the total number of automobiles and to the insufficient control of the emission from diesel and gas engines of automobiles and co-generation systems.  In the latter cases, TWC does not function because of the presence of excess oxygen, and ammonia required for SCR cannot be used for mobile sources and in urban districts.  Exhausts from the lean-burn engine which is desirable for fuel efficiency also contains excess oxygen.

This is the reason why we need a new catalytic technology the reduce the concentration of nitrogen oxides in the atmosphere.

2. Evolution of HC-SCR and bifunctional catalysts

In the 1970's, the reduction of NO by hydrocarbons in the presence of excess oxygen (HC-SCR) was abandoned, because it was much less selective than SCR by ammonia.  However, since Cu zeolites, particularly ZSM-5 and mordenite, were found fairly selective in the late 80's (patents) to early 90's, HC-SCR has collected much attention.

Although various kinds of catalysts have been reported, there are still high barriers before commercialization.  Zeolite-based catalysts suffer from the poor stability under hydrothermal conditions, and non-zeolite catalysts show only low activities in the lower temperature range.

We have been studying the catalyst design for HC-SCR based on the concept of bi(multi)functional catalysis, in order to overcome the above difficulties.  We consider the mechanism of SCR by propene over Ce-ZSM-5 to be simply as below [1], although the mechanism is actually very complex.

O2          C3H6                  O2, NOX

NO -----> NO2 -----> [R-NO2, R-ONO] -----> N2

Ce                                    Ce

Here, Ce functions in the initial step of NO oxidation and the last step of oxidative decomposition of organic intermediates.  If so, the two steps may be accelerated separately by different catalysts. The usefulness of this idea was demonstrated by the fact that the combination of Mn2O3 or CeO2 with Ce-ZSM-5 exhibited much improved performance [2].

The concept of bifunctional catalysts was further demonstrated for the steam resistant HC-SCR catalysts. We found to our surprise that the mechanical mixture of Mn2O3 and Sn-ZSM-5 showed much higher activity in the entire temperature range in the presence of steam than its absence [3].  The important point was that the activity of Mn2O3 for NO oxidation was little affected by the presence of steam, while the wasteful combustion of propene was suppressed by steam.  This is the mechanism of the rate enhancement in the high temperature region. At low temperatures, polymerization of propene and nitrogen oxides seems to be surpressed.

It was reported recently that the combination of Mn2O3 and Au/Al2O3 showed excellent performance even in the presence of steam [4].

In steam    on Mn2O3                on Ce-ZSM-5

NO ------> NO2 --------> NO2 --□□

□------> N2, CO2, H2O

C3H6 --×---> CO2                   □

□-----------------------> C3H6 □□□

Another example where the concept of bifunctional catalysis is useful was the reduction of NO by methane catalyzed by Pd-loaded ZSM-5 [5].  Although Pd-HZSM-5 and Pd-NaZSM-5 catalyzed similarly the oxidation of methane, only the former reduced NO to N2.  This was explained reasonably by the bifunctional mechanism, that is, Pd for the activation of activation to  CHx and acidic proton for the reaction between NO2 and CHx.

CH4 ---------> CHx ---□□

Pd                 □-------> N2, CO2, H2O

NO  ---------> NO2 -□--□   H+

3. Direct decomposition and absorption/decomposition of nitrogen oxides

Direct decomposition is the ideal process to return nitrogen oxides back to the air,

2NO -----> N2 + O2    and    2N2O ------> 2N2 + O2

We studied these two reactions using well-defined perovskites, La2-XB'xCuO4, of which the valency of Cu was controlled from 1.6 to 2.3. Although the catalytic activites were not very high, informative results were obtained.  Catalytic activity for both reactions increased with an increase in the average oxidation number (AON) of Cu.  The activity was particularly high when AON exceeded two [6]. The average oxygen vacancy had little correlation with the activity.  Kinetic and TPD studies revealed that the desorption of oxygen formed by the decomposition of nitrogen oxides was not rate-determining at the steady state, but the dissociation of the N-O bond is most probably the slow step.  The competitive adsorption of O2 is very influential and retards the reactions.  The steady-state reaction proceeds on few surface sites which are not covered by oxygen species.  We proposed a redox mechanism between Cu2+ and Cu3+.

Absorption/decomposition of NO [7] can be carried out fairly efficiently by the combination of YBa2CuOx (YBC) which reversibly absorbs a large amount of NO [8] and Ce-ZSM-5 which reduces NOx in the presence of oxygen and hydrocarbons.

on YBC                  NO + O2 --------> NO2-, NO2-

on Ce-ZSM-5            NO2-, NO3- + CmHn ---------> N2