Highly efficient and simultaneously catalytic removal of PM and NOx from diesel engines with 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts
Graphical abstract
The novel SCRPF combining catalyst path is an efficient and economical way to purify PM and NOx from diesel engines. 3DOM Ce0.8Mn0.1Zr0.1O2 catalyst possesses unique macroporous structure, abundant oxygen, low-temperature reducibility and proper acid site. It gives the surprising highly catalytic activity for the simultaneous removal of PM and NOx.
Introduction
Diesel engines operating at lean-burning atmosphere show characteristics of high fuel efficiency, excellent durability and low CO2 emission that are welcomed by the vehicle manufacturers (Di Sarli and Di Benedetto, 2015, Li et al., 2010, Wang et al., 2015); hence, these engines are universally used in certain light and heavy trucks, steamship and private cars. Despite its attractive advantages, the main air pollutants emitted by the combustion in the diesel engines are mainly comprised of soot particulates (PM) and nitrogen oxides (NOx), the hazardous effects of which on both human being and environmental system are now well-known. For example, PM are easy to absorb and accumulate in the human body through the respiratory system, which can cause serious respiratory illness and cancer (Choi and Lee, 2014, Gálvez et al., 2013). NOx have been regarded as the major sources of the formation of acid rain, photochemical smog, ozone depletion and greenhouse effects (Kang and Choi, 2016, Twigg, 2011, Xu et al., 2012, Zhang et al., 2014). For decades, the development and application of the after-treatment catalytic technology for the control of the main air pollutants emitted by diesel engines has been dramatically welcomed due to the need to conform to increasingly rigorous emission standards.
Typically the abatement of PM and NOx is controlled by separate after-treatment components (Watling et al., 2012). The high-efficiency technology for controlling PM emission is focused on the technique of diesel particulate filters (DPF) combined with a diesel oxidation catalyst, which can trap a great number of PM from the combustion chamber that are then oxidized by the noble metal Pt catalyst to regenerate the DPF. For NOx reduction, a technique based on selective catalytic reduction (SCR) is widely used with ammonia or urea as the reductant, and is one of the most promising NO removal technologies (Metkar et al., 2013). The combination of a SCR system and catalyzed DPF traps in a conventional emission control system is considered to be an efficient after-treatment technique for PM and NOx elimination. Nevertheless, this type of technology suffers from some drawbacks such as the increase in the overall package volume, mass and cost. To simplify the exhaust after-treatment system and obtain a high efficiency, we propose a new catalytic purification technology for achieving the simultaneous high-performance abatement of PM and NOx. A novel SCRPF catalytic purification approach that can replace the otherwise separate catalytic DPF and SCR steps (i.e., SCRPF), is desirable for use in a single catalytic converter. This approach is quite cost-effective because it lowers the pressure drop and reduces the system volume and mass.
Mostly importantly, a high-efficiency catalyst is the crucial core of the SCRPF technology. It can solve the problems of the lower conversion of NOx to N2 in the lean-burning atmosphere and the high PM combustion temperature that directly affect the conversion efficiency. Among the catalysts investigated to date, some catalysts exhibit high performance for the removal of PM or NOx, such as metal oxides (Pisarello et al., 2002), mixed oxides (Atribak et al., 2008), hydrotalcites (Wang et al., 2014), and alkali oxides catalyst (Raj et al., 2011) and noble metal catalyst (Diehl et al., 2015, Duan et al., 2015, Yashnik et al., 2016). Nevertheless, these conventional catalysts exhibit the drawback of small pore sizes. PM cannot enter the inner region of these catalysts with a high sufficient surface area, and only the material in the outer surface can participate in PM combustion. Three-dimensionally ordered macroporous (3DOM) materials could solve this problem due to their inter-connected macrospore structure that offers more channels. PM can be easily transferred and diffuse on the inner pores, and can make full use of the active sites of the catalyst. Thus, the efficiency of the catalyst is strongly influenced by the contact between the PM and the catalyst (Peng et al., 2015), dramatically improving the catalytic performances. Moreover, the low conversion of NOx reduction is an important problem. Generally, this is less than 50% (Fino et al., 2003, Fino et al., 2006, Kureti et al., 2003, Shangguan et al., 1997). Thus, the introduction of the extra reductant-NH3 into reactant gas is an efficient solution for promoting the NO conversion. Additionally, the adsorption and activation of NH3 is also important for NOx reduction.
Combining the textural properties and the intrinsic features of PM and NOx, 3DOM Ce0.8X0.1Zr0.1O2 (X = Mn, Co, Ni) mixed-oxide catalysts was employed as the monolithic catalyzer to remove PM and NOx from the diesel engine exhaust simultaneously. More importantly, the inexpensive catalyst may be highly welcomed for practical applications.
Section snippets
Catalyst preparation
All starting chemicals were purchased from Sigma Aldrich and used without further purification. The monodispersed polymethyl methacrylate (PMMA) microspheres with the average diameter of 300 nm were prepared following a previously reported procedure (Xu et al., 2011, Zhang et al., 2011). Three-dimensionally ordered macroporous (3DOM) Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) mixed-oxide catalysts were synthesized by carboxy-modified colloidal crystal templating (CMCCT). The template synthesis was
De-NOx performance of SCRPF catalysts
Fig. 1(a) shows the temperature range of 100% NO conversion over 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts under loose contact condition. For Ce0.8Zr0.2O2 sample, high NO conversion can be detected in the range of 390–498 °C. It can be observed that the partial substitution of Ce with transition metal elements could indeed influence the SCR activity of the Ce0.8Zr0.2O2 catalyst. Under identical operating conditions, among all 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts, Ce0.8Mn0.1Zr0.1O2
Influence of the structure on the performance of 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts
The reaction of PM combustion is a typical heterogeneous catalysis reaction that occurs at the three-phase boundary among a solid catalyst, a solid reactant (PM), and gaseous reactants (O2, NO). Thus, a vital issue strongly influencing the catalytic activity of a catalyst for the solid-solid reaction is the contact conditions between PM and the catalyst. As revealed by many studies, the catalytic performance of catalysts for PM combustion is very excellent under tight condition between PM and
Conclusions
The new SCRPF combining catalyst technology is an efficient and economical approach for purification of PM and NOx from diesel engine exhaust. For the simultaneous removal of PM and NO, a series of highly ordered monolithic 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts are successfully prepared by a colloidal crystal template method. The diffusion and combustion reaction of PM and the reduction of NOx are process intensified by the unique 3DOM structure and the doping metal ions.
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3DOM Ce0.8M0.1
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (U1662103, 21673290 and 21376261) and the 863 Program (2015AA034603).
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