New mixed-oxide catalysts shown as viable substitute for platinum catalysts for diesel exhaust aftertreatment
|NO conversion versus ramp-up and ramp-down temperatures for MnCe-7:1 (●), SmMn2O5 (□), GdSrCeMn7O14.83 (■), and Pt (○). Credit: Science, Wang et al. Click to enlarge.|
Researchers at nano-material catalysts startup Nanostellar and colleagues at the University of Kentucky and Huazhong University of Science and Technology in China have shown that mixed-phase oxide materials based on Mn-mullite (Sm, Gd)Mn2O5 are an efficient substitute for the current commercial platinum (Pt)-based catalysts for the aftertreatment of diesel exhaust.
Under laboratory-simulated diesel exhaust conditions, this mixed-phase oxide material was superior to Pt in terms of cost, thermal durability, and catalytic activity for NO oxidation. The new material is active at temperatures as low as 120 °C with conversion maxima of ~45% higher than that achieved with Pt. A paper on their work is published in the journal Science.
Nanostellar was founded in California in 2004 by scientists from Stanford University and NASA Ames Research Center. Co-founder and Chief Science Officer Dr. Kyeongjae (KJ) Cho, currently professor of materials science and engineering and physics at the University of Texas, Dallas, is a senior author on the Science paper.
Many pollution control and renewable-energy applications require precious metals that are limited—there isn’t enough platinum to supply the millions and millions of automobiles driven in the world. Mullite is not only easier to produce than platinum, but also better at reducing pollution in diesel engines.—Dr. Kyeongjae Cho
|Mullite oxides (Sm, Gd)Mn2O5 have an orthorhombic structure in which Mn3+ and and Mn4+ occupy different crystallographic positions in square pyramidal and octahedral coordination environments, respectively. Wang et al., SI. Click to enlarge.|
Nanostellar uses its proprietary Rational Design Methodology (RDM) to develop advanced catalysts for an array of applications including emissions control, energy efficiency, and the synthesis of chemicals and fuels. RDM incorporates computational models, algorithms and subject matter expertise in nano-materials and advanced synthetic chemistry. Although the company has designed and commercialized a platinum-gold alloy catalyst that is a viable alternative to platinum alone, it had not—until this experiment with mullite mixed oxides—found a catalyst made of materials that are less expensive to produce.
Diesel engines are attractive because of their higher fuel efficiency than gasoline engines. However, their lean exhaust with high oxygen (O2) content requires specialized devices to reduce the engine-generated nitrogen oxide (NOx) pollutants, mostly nitric oxide (NO), to environmentally benign nitrogen (N2). One popular strategy is the use of selective catalytic reduction (SCR) of NOx with ammonia (NH3) through the reaction: 2NH3 + NO + NO2→2N2 + 3H2O. The rate of this reaction is maximized (fast SCR) when the NO:NO2 ratio approaches unity. A lean NOx trap (LNT) adsorbs NOx under lean conditions and reduces NOx to N2 under fuel-rich regeneration cycles. NO oxidation products such as nitrogen dioxide (NO2) have a higher trapping efficiency on LNT materials. In addition to the NOx-based pollutants, diesel engines also generate black soot with polynuclear aromatic content. One process to remove carbon soot is a continuous regenerable trap (CRT), which uses NO2 as an oxidant (2NO2 + C→CO2 + 2NO, NO2 + C→CO + NO). Oxidation of engine-generated NO to NO2 is critical in diesel engine emission control for reducing NOx and carbon soot from the tail-pipe exhaust.
To date, no thermally stable material has matched platinum’s catalytic performance under diesel exhaust temperatures, despite a substantial amount of work that has been undertaken in search for catalysts based on metal oxides. Here, we report a class of hydrothermally stable, mixed-phase oxides based on Mn-mullite, (Sm, Gd)Mn2O5. This catalyst demonstrates activity at temperatures as low as 120 °C and has a ~64% increase in NO oxidation catalytic performance over 2% Pt on γ-Al2O3 at 300 °C. The catalytic behavior is mainly linked to the Mn-Mn dimers on the mullite surface based on characterization methods and quantum-mechanical simulations.—Wang et al.
|Proposed reaction mechanism of NO conversion into NO2 (g) on the stepped mullite (110) surface. TS, transition state. Wang et al.Click to enlarge.|
The team prepared mixed-phase oxide catalysts by a coprecipitation method. To evaluate catalytic performance, they exposed the aged sample Mn7CeSmSrO14.83 (labeled MnCe-7:1), to a reactant gas mixture (450 ppm NO and 10% O2) while ramping the temperature up to 350 °C to test for NO conversion. They found that aged MnCe-7:1 offers improved NO oxidation over the aged 2% Pt on γ-Al2O3 catalyst, and pure SmMn2O5 displays reasonable activity during the ramp-down cycle.
Furthermore, MnCe-7:1 is active at low temperature (120 °C), with a higher NO conversion maximum of ~45% over Pt on the ramp-up cycle. The team concluded that four key factors contribute to the overall catalytic activity of MnCe-7:1:
- Mn dimer sites are the main source of activity on SmMn2O5.
- The addition of Sr increases the surface area, exposing more active sites.
- Ceria adsorbs and dissociates O2 such that O* can diffuse to Mn-Mn dimers at the interface.
- The stabilized spinel Mn3O4 phase results in additional high-temperature (330 ° to 350 °C) catalytic activity.
To determine how MnCe-7:1 performed in simulated diesel exhaust, the team added MnCe-7:1 as a top layer to a commercial-style PtPd diesel oxidation catalyst. Coated cores were aged and subjected to a gaseous mixture of NO, CO, CO2, C3H6, C3H8, n-decane, steam, and O2 in He. The results showed that the additional MnCe-7:1 layer was not detrimental to CO and HC performance and that NO oxidation was enhanced.
The mullite alternative is being commercialized under the trademark name Noxicat. Dr. Cho and his team will also explore other applications for mullite, such as fuel cells.
Dr. Weichao Wang, who earned his PhD in materials science and engineering in 2011 under Dr. Cho’s supervision in Erik Jonsson School of Engineering and Computer Science at UT Dallas, was lead author of this study.
The study was supported by the Texas Advanced Computing Center, Nanostellar and the National Research Foundation of South Korea.
Weichao Wang, Geoffrey McCool, Neeti Kapur, Guang Yuan, Bin Shan, Matt Nguyen, Uschi M. Graham, Burtron H. Davis, Gary Jacobs, Kyeongjae Cho, and Xianghong (Kelly) Hao (2012) Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust. Science 337 (6096), 832-835. doi: 10.1126/science.1225091