Effect of periodic lean/rich switch on methane conversion over a Ce–Zr promoted Pd-Rh/Al2O3 catalyst in the exhausts of natural gas vehicles

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Abstract

The behavior of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst for the abatement of methane from the exhausts of natural gas vehicles (NGVs) is studied in presence of large amounts of water under both stationary conditions and by periodically switching from lean to rich feed. Under stationary conditions with both stoichiometric (λ = 1.00) and lean (λ = 1.02) feed catalyst deactivation is observed after prolonged exposure to the reaction mixture. Periodic rich pulses in a constant lean feed gas result in the stabilization of catalytic performances.

A higher methane conversion than those obtained with stoichiometric and lean feed mixtures is observed under rich conditions, during an experiment carried out by performing lean pulses (λ = 1.02) in a constant rich feed gas (λ = 0.98). The analysis of reactants conversion and products distribution suggests that different chemistries are involved under lean and rich conditions. Only reactions of complete oxidation of H2, CO, CH4 and NO occur under excess of oxygen, whereas under rich conditions NO reduction, CH4 steam reforming and water gas shift also occur.

The effect of symmetric oscillation of the exhausts composition around stoichiometry is also addressed by periodically switching from slightly rich to slightly lean composition with different oscillation amplitudes (Δλ = ±0.01, ±0.02 and ±0.03). Higher and more stable methane conversion performances are obtained than those observed under constant λ operations. The presence of a more active PdO/Pd0 state is suggested to explain the enhancement of catalytic performances.

Highlights

► Rich pulses in a constant lean feed gas stabilize catalytic performances. ► Symmetric oscillations around λ = 1.00 stabilize and enhance catalytic performances. ► Higher CH4 conversions are obtained under rich than under lean conditions. ► The presence of O2 in the rich phase plays a key role in CH4 conversion enhancement. ► The activity enhancement is likely ascribable to a more active mixed PdO/Pd0 state.

Introduction

Natural gas vehicles (NGVs) have been introduced in the market since a long time. However, at present the low number of filling stations and the problems related to the storage of gaseous fuels limit their diffusion. Nevertheless, the use of compressed natural gas (CNG) in automotive applications is of wide interest primarily for the need to diversify energy sources and for the large worldwide resources of natural gas. Besides, NGVs represent a “green” alternative to gasoline and diesel engines because of the very low sulfur content and the reduced NOx and particulate emissions when operating under lean conditions. Furthermore CH4 (which is the main component of NG exhausts) has the highest hydrogen content, which allows achieving a reduction of CO2 per MJ emissions with respect to other hydrocarbon fuels. However, the abatement of unburned methane, which constitutes about 90% by volume of NG, still remains a demanding challenge in view of the 100 mg/km limit introduced in EU5/EU6 regulations for total hydrocarbon (THC), since the methane molecule is very stable and requires high temperatures to be oxidized.

Stoichiometric and lean burn combustion systems are commonly used in NGV, working respectively with a stoichiometric mixture of fuel and air and with an excess of oxygen [1]. The lean-burn technology guarantees an efficient fuel use and a reduction of CH4 emissions in comparison with stoichiometric engines. However it requires complex aftertreatment systems usually composed of a first catalyst for the oxidation of carbon containing compounds followed by a second catalyst for NOx reduction. On the contrary, the aftertreatment systems of the exhausts from stoichiometric engines are simpler, relying predominately on three-way catalysts (TWCs) technology adapted from gasoline applications. In both lean and stoichiometric systems noble metals are employed as active components, among which Pd is predominantly used due to its high activity in CH4 oxidation [2], [3]. It is well known in the literature that the oxidation state of palladium plays a major role on CH4 oxidation activity of Pd catalysts [4]. Accordingly periodically oscillating lean/rich conditions, which are intrinsic in operation of TWCs and can be forced in operation strategies of lean systems [5], may seriously affect the emission abatement performances. Although a large number of papers have been devoted to the study of TWCs under periodic operation, there is no agreement between the results: both improvements [6], [7], [8], [9], [10] and negative effect [11], [12], [13] in hydrocarbon conversion were found when working under oscillating conditions. However most of them refer to the exhausts of gasoline vehicles, largely represented by hydrocarbons of high molecular weight, which are relatively easy to oxidize, whereas only a few studies are devoted to periodic operation in the exhausts of NGVs [10].

In this work the activity of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst was studied under stationary stoichiometric and lean conditions in presence of large amounts of water. The effect of periodic rich pulses in a constant lean gas mixture and periodic lean pulses in a constant rich gas mixture was also investigated. Symmetric oscillations of gas composition around stoichiometry were also performed by periodically switching the feed from slightly rich to slightly lean compositions with different cycle periods and oscillation amplitudes.

Section snippets

Catalyst

A commercial Pd-Rh based catalyst developed by Ecocat in the form of a 400 CPSI/6 mils washcoated ceramic honeycomb containing 7.1 g/l of Pd:Rh (39:1 of loading) has been used in this study. The washcoat material content referred to the geometric area of the catalyst is 50 g/m2. The catalyst coating consists of layered washcoat mainly containing stabilized Al2O3. In addition, Ce–Zr mixed oxide (Zr/Ce = 3.5) were used to improve the oxygen storage capacity (OSC) of the catalyst and transition metal

Testing under stationary lean and stoichiometric conditions

The methane combustion activity of the commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst provided by Ecocat was measured in a nearly isothermal experiment performed at stationary λ = 1.00 and Toven = 450 °C; this corresponds to a measured temperature just after the monolith entrance of about 470 °C, in view of the exothermicity of the reactions. CH4 conversion is plotted as a function of time in Fig. 3. A decrease in CH4 conversion is observed, passing from 52 to 43% after 30 min. Furthermore, after

Conclusions

The performances of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst have been studied under both stationary and periodically switching lean/rich conditions.

Results show that operations under stationary lambda conditions lead to progressive decrease of CH4 conversion. Activity can be recovered by performing rich pulses in a stationary lean feed gas mixture.

Higher and more stable average methane conversions than those obtained under constant λ (lean and stoichiometric) operations are reached

Acknowledgment

This study has been performed within the INGAS-project, financially supported by the European Commission FP7 Programme (Proj. no. 218447), which is gratefully acknowledged.

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