Application of the concepts of electrochemical promotion and carrier doping in designing novel DE-NOx catalysts for lean-burn engines

• Research unit: EU RFP
• Project number: 97.0411

Key words (English)

Bipolar reactors; Electrochemical promotion; Lean-burn catalysts; NO reduction; Propylene; Rhodium

Alternative project number (English)

EU project number: BRPR-CT97-0460

Research programs (English)

EU-programme: 4. Frame Research Programme - 2.1 Industrial and materials technologies

Short description (English)

See abstract

Partners and International Organizations (English)

University of Patras (EL), University of Cambridge (UK), University of Messina (I), University of Thessaloniki (EL), Johnson Matthey (UK), Volkswagen ag (D)

Abstract (English)

The objective of this project was to develop innovative catalysts for NOx reduction to N2 in the presence of oxygen for auto exhaust aftertreatment. Although efficient catalytic devices exist today for reducing NOx for stoichiometric operated gasoline engines, no such devices are available for lean-burn and diesel engines. The need for such catalysts is great, considering that lean-burn engines offer substantial, up to 15%, enhancement in fuel efficiency and that nitrogen oxides released to the environment cause severe environmental damage related to smog formation and acid rain. The development of novel catalytic materials was based on the concepts of non-faradaic electrochemical modification of catalytic activity (NEMCA) and dopant-induced metal-support interactions (DIMSI). These phenomena are used to alter the surface potential and work function of catalytic surfaces what, in turn, alters significantly their chemisorptive and catalytic parameters in the desired direction. Our activity (EPFL partner) was focused on application of the NEMCA concept in designing novel de-NOx catalysts.

For the first time, different bipolar configurations for practical applications have been realized using YSZ (yttria-stabilized zirconia) solid electrolyte: Rh/YSZ ring reactor of negligible current bypass (bipolar configuration of 1st generation) and monolithic YSZ reactor with multiple channels (bipolar configuration of 2nd generation). They are directly applicable for NEMCA studies in continuous plug flow reactors. A theoretical model has been developed for the design of these new configurations. The model allows estimation of current bypass from simple steady state current-potential measurements. Its predictive force was experimentally confirmed. It provides an excellent tool in the design and scale-up of highly dispersed bipolar systems. The feasibility of the electrochemical promotion by bipolar polarization was demonstrated for the reduction of NO by C3H6 in presence of O2. Indirect bipolar polarization, close to real applications, induces an enhanced productivity since promotion is obtained on both the anodic and the cathodic part of the catalyst. In the bipolar configuration of 1st generation (Rh film in an YSZ ring reactor), a 100% increase in NO conversion (15% for pen circuit) was achieved under indirect bipolar polarization. In the bipolar configuration of 2nd generation (RuO2 film catalyst in a monolithic YSZ reactor with multiple channels) up to 50% increase in a model test reaction was achieved at a high open circuit conversion (35%). The results demonstrate the feasibility of electrochemical promotion in bipolar configuration and the practical possibility to bridge the gap between single film electrodes and practical disperse/monolith catalysts.

Electrochemical polarization of Rh/YSZ catalysts with a near-zero consumption of electric power was shown to considerably decrease the light-off temperature for the reduction of NO by propylene under lean-burn conditions. Since rhodium catalysts are very sensitive to deactivation via oxidation of surface sites, a temporary shift towards more oxidative feed compositions or a short temperature jump may cause a dramatic drop in catalyst activity. At moderate working temperatures (up to 300°C) the spontaneous recovery of such a loss in activity is very slow but it may be strongly accelerated by electrochemical polarization of the Rh/YSZ catalyst film. The NEMCA effect obtained over a deactivated Rh catalyst is composed of a reversible and an irreversible part. While the 'reversible promotion' is due to steady state accumulation of electrochemically generated active species at the gas exposed catalyst surface, the 'irreversible promotion' which persists after current interruption is due to the progressive reduction of oxidized surface sites assisted by positive current application via weakening the Rh-O bond. Once the lost activity is recovered, the irreversible effect vanishes. As a practical consequence, electrochemical control of the Rh catalyst allows faster recovery of lost activity and/or lowering the operating temperature with invariant catalytic performance.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 97.0411