Our recent studies have focused on:
- Selective catalytic reduction (SCR) of NOx with lower hydrocarbons: Dual-catalyst after-treatment system for NOx reduction under lean-conditions.
- Sulfur tolerance and hydrothermal stability of the dual-catalyst bed.
- Wash-coat development for lean-burn engine-exhaust after-treatment.
- Simultaneous reduction of NOx and SOx.
- Complete oxidation of VOCs.
In the United States, catalysis is an essential technology not only for the production of most chemicals, but also for the treatment of waste and exhaust emissions. Nitrogen oxides (NOx) and sulfur dioxide (SO2) are the primary causes of acid rain. To meet the increased use of fossil fuels coupled with environmental regulations which are becoming increasingly stringent, our research strives to develop next generation catalysts able to effectively reduce pollutant levels from various emission streams.
Nitrogen oxides (NO, NO2, N2O) contribute to several environmental hazards including global warming, smog, ground level ozone formation, and acid rain. NOx is primarily produced during the high temperature combustion of fossil fuels. Reciprocating engines, both mobile and stationary, are contributors to NOx emissions. Emission reduction is possible through modification of combustion parameters, but reducing NOx emissions to acceptable levels will require an effective after-treatment technology. Current catalytic NOx reduction control technologies include three-way catalysts and ammonia-based selective catalytic reduction. While these methods are highly effective for current combustion technologies, they are unsuitable for the next generation of high efficiency lean-burn engines.
We have developed a catalytic after-treatment system for natural-gas fueled stationary emission sources engines to reduce the emissions of nitrogen oxides (NOx), unburned hydrocarbons (CH4, C2H6, and C3H8) and carbon monoxide under lean burn conditions. The catalytic after-treatment system is a dual bed consisting of a physical mixture of a reduction catalyst (palladium supported on sulfated zirconia) and an oxidation catalyst (cobalt oxide on ceria). This dual catalyst system has three functions:
(i) NO oxidation to NO2
(ii) NO2 reduction to N2
(iii) oxidation of unutilized hydrocarbons and CO to CO2.
In this scheme, NO gets oxidized to NO2 over the oxidation catalyst (CoOx /CeO2). This step is important because NO2 has a stronger oxidizing potential when compared to NO and hence gets reduced more easily. The oxidation catalyst also oxidizes unburned hydrocarbons and CO. On the reduction catalyst (Pd/SZ), the NO2 gets reduced to N2. Such a catalytic system which utilizes methane present in the exhaust stream offers several advantages considering that the emissions of air-pollutant greenhouse gases are controlled in a single unit without a need of injecting and handling an external reducing agent such as ammonia.
Present work is devoted to the development of a catalytically active washcoat for monolith cores and demonstrating the hydrothermal stability and sulfur tolerance of the dual-catalyst bed. From an application point of view, the dual-catalyst system needs to be loaded onto a monolith core so that operational issues associated with fixed bed reactors such as excessive pressure drop and temperature fluctuations can be resolved. However, if the washcoat adheres poorly to the monolith core, the after-treatment unit will suffer from the irreversible loss of the catalytically active phase of the core. In our recent studies, we have aimed to improve the adhesive properties at the molecular level. For this purpose, several binders have been incorporated in situ to the sol-gel medium of Pd/SZ prior to the gelation during synthesis. Samples prepared by this novel approach have shown superior differences in terms of catalytic performance than the samples prepared by the conventional method. Addition of binder before gelation to the sol-gel medium also resulted in changes of textural and structural properties of binder-free samples, and adhesive properties of the washcoat as shown by a wide variety of techniques. The hydrothermal stability and the sulfur tolerance of the dual-catalyst system have been demonstrated by time-on-stream reaction experiments up to 40 hours in the presence of 10% water vapor and 5 ppm SO2 in the feed.