High performance diesel oxidation catalysts using ultra-low Pt loading on titania nanowire array integrated cordierite honeycombs
Graphical abstract
Introduction
Diesel oxidation catalyst (DOC) is one of the most important components in the aftertreatment system of diesel-powered vehicles [1]. DOC is used for catalytically converting CO and unburnt hydrocarbons (HCs) into CO2 and H2O [1], [2]. Additionally, DOC can oxidize NO to NO2 to help downstream functionalities such as passive diesel particulate filter (DPF) regeneration, selective catalytic reduction of nitrogen oxides (NOx), and lean NOx trap (LNT) or NOx storage and reduction (NSR) [3]. DOCs rely on active platinum group metals (PGM) such as Pt and Pd supported on high surface area supports, usually Al2O3, zeolite, or ZrO2 and other additives. To ensure high oxidation performance and thermal stability, high loading of Pt or Pt-blend from 10 g/ft3 to 100 g/ft3 is required, depending on targeted emissions, duty cycle and sulfur content of the fuel [1]. However, Pt and noble metals are rare and expensive, thus the demand for efficient low PGM or even PGM-free catalysts is growing. Several metal oxides or mix-metal oxides (e.g. MnO2 [4], Co3O4 [5], MnxCo3-xO4 [6], Cu-Co-CeOx [7] etc.), and La-based perovskites (e.g. La1-xSrxCoO3, La0.9Sr0.1MnO3 [3]) have shown promises as alternative oxidative catalysts for PGMs. However, they lack either low-temperature activity (especially for HC oxidation) or thermal stability, thus necessitating additional loadings of PGM, at least 4.75 g/ft3, for practical applications [8], [9]. Further reduction of PGM usages will require better control of support’s structure and geometrical configuration to improve gas-solid interaction and to enhance promotional interaction with PGM active sites. Additionally, increasing PGM dispersion by decreasing their particle sizes could boost the catalytic oxidation activity due to the cluster size effect or structure sensitivity [10], [11], thereby improving atomic utilization efficiency [12]. Another effective approach has been demonstrated by utilization of promoters (e.g. H2) added upstream to improve oxidation reactivity of CO, hydrocarbons oxidation, and NO2 of different DOCs, especially for low PGM catalysts [13], [14], [15], [16], [17].
When first introduced, the automobile oxidation catalysts employed supports as pellets in a canister or a packed bed arrangement [1]. However, the pellets disintegrated due to vibration and inter-particle collision in the mobile environment, thus decreasing catalytic performance and even damaging downstream functional devices [1]. Current DOC relies on supports that are ‘washcoated’ on honeycomb monoliths to ensure high gas-solid interaction and to immobile catalysts in high exhaust flow. However, current washcoat technology is still facing several challenges including (1) under-utilized washcoat material in the extreme conditions of gas exhaust, (2) lack of control over uniformity and nano/micro-structure of the support, thus compromising material utilization efficiency, and (3) low adherence of the washcoat to the monolith, which can cause disintegration of the washcoat in erosive exhaust flow and automotive vibration conditions [18], [19], [20], [21], [22]. We have recently developed a novel configuration of catalysts employing nano-array structures (nanowire, nanorod, nanosheet, nanotube etc.) of various metal oxides (ZnO, CeO2, brookite TiO2, Co3O4, and MnxCo3-xO4) rooted on monolithic substrates for oxidative catalysts or for supports of active noble metal catalysts [6], [19], [23]. These nano-array-based catalysts have demonstrated high catalyst utilization efficiency and excellent thermal/mechanical robustness.
Herein, we report the development of a highly active DOC using ultra-low Pt loading of only 1.1 g/ft3, based on rutile TiO2 nano-array rooted on cordierite honeycomb monoliths. Mesoporous rutile TiO2 nano-array with a very high surface area (72 m2/g including substrate) was uniformly grown on cordierite honeycomb monolith using a solvothermal synthesis. Atomic layer deposition (ALD) method was employed for the precise control over loading of ultrafine Pt particles (0.95 ± 0.24 nm) on the TiO2 nano-array. The activity of Pt/TiO2 nano-array catalyst was evaluated under clean-diesel combustion (CDC) simulated exhaust conditions following the USDRIVE’s protocol [24], demonstrating an impressive performance with the conversion of CO and total hydrocarbon (THC) reaching 50% at temperatures as low as 224 and 285 °C, respectively. We also demonstrate the feasibility of further lowering light-off temperature by the addition of H2 to the exhaust. With the addition of 2400 ppm H2 (total 2500 ppm H2), T50 of CO and THC reduces significantly (23–31 °C) to 201 and 254 °C, respectively.
Section snippets
TiO2 nanowire array (nano-array) growth on honeycomb cordierite monolith
TiO2 nano-array was grown on honeycomb cordierite monolith via solvothermal synthesis. Prior to the growth of the TiO2 nano-array, the substrates were seeded by soaking overnight in a TiO2 polymeric sol, prepared by a sol-gel process [25], followed by calcination at 500 °C for 2 h. Seeded substrates (size of up to 8 cm × 8 cm × 5 cm) were placed on inert supports (∼2 cm height) on a 1 L Teflon-lined autoclave. The reaction solution contains 60 mL titanium (IV) n-butoxide (99%, ACROS Organics), 50 mL 37% HCl
Morphology and microstructure
SEM and TEM were employed to investigate the morphological features of the TiO2 nanowire arrays grown on the cordierite honeycomb monolithic substrate (core). As displayed in Fig. 1a, b, the TiO2 nanowire arrays consist of numerous vertically aligned nanowires with a length of ∼2 μm and diameter of ∼50–100 nm. Higher magnified SEM image (Fig. 1c) and TEM image (Fig. 1d) reveals that each nanowire is a bundle of 10–20 smaller nanowires with an average diameter of 9.1 ± 2.3 nm. The large pores formed
Conclusions
In conclusion, we have demonstrated that low light-off temperature of CO and hydrocarbons on ultra-low Pt loading can be achieved by using a combination of novel nano-array structured support, precise control of ultrafine active Pt particles synthesis, and the addition of H2 as a promoter into the exhausts. High surface area rutile TiO2 nano-array was uniformly grown on cordierite honeycomb monoliths via non-polar solvent/hydrophilic substrate interaction. Ultrafine Pt particles are dispersed
Acknowledgments
The authors are grateful for the financial support from the US Department of Energy (Award # DE-EE0006854) and the US National Science Foundation (Award # CBET-1344792). This research was in part carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory (BNL), which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.
References (51)
- et al.
Low temperature propane oxidation over Co3O4 based nano-array catalysts: Ni dopant effect, reaction mechanism and structural stability
Appl. Catal. B: Environ.
(2016) - et al.
CO and H2 oxidation on a platinum monolith diesel oxidation catalyst
Catal. Today
(2006) - et al.
Enhancing the low temperature oxidation performance over a Pt and a Pt–Pd diesel oxidation catalyst
Appl. Catal. B: Environ.
(2014) - et al.
Evaluation of H2 effect on NO oxidation over a diesel oxidation catalyst
Appl. Catal. B: Environ.
(2015) - et al.
Robust 3-D configurated metal oxide nano-array based monolithic catalysts with ultrahigh materials usage efficiency and catalytic performance tunability
Nano Energy
(2013) - et al.
Monolithic catalysts as efficient three-phase reactors
Chem. Eng. Sci.
(2001) - et al.
Catalytic converters: state of the art and perspectives
Catal. Today
(1999) - et al.
Improvement of catalytic activity and sulfur-resistance of Ag/TiO2–Al2O3 for NO reduction with propene under lean burn conditions
Appl. Catal. B: Environ.
(2008) - et al.
Dispersion, reduction and catalytic performance of CuO supported on ZrO2-doped TiO2 for NO removal by CO
Appl. Catal. B: Environ.
(2011) - et al.
Surface defects in drying paint films
Prog. Org. Coat.
(1980)
Effects of pore assembly architecture on catalyst particle tortuosity and reaction effectiveness
Catal. Today
Mass transport and catalytic activity in hierarchical/non-hierarchical and internal/external nanostructures: a novel comparison using 3D simulation
Appl. Catal. A: Gen.
The promotion of carbon monoxide oxidation by hydrogen on supported platinum catalyst
Appl. Catal. A: Gen.
Experimental and kinetic study of NO oxidation on model Pt catalysts
J. Catal.
Modeling the simultaneous oxidation of CO and H2 on Pt – promoting effect of H2 on the CO-light-off
Appl. Catal. A: Gen.
Inverse hysteresis during the NO oxidation on Pt under lean conditions
Appl. Catal. B: Environ.
Platinum oxide formation and reduction during NO oxidation on a diesel oxidation catalyst – experimental results
Appl. Catal. B: Environ.
Diesel oxidation catalysts
Catal. Rev.
Automobile Exhaust Control, Ullmann’s Encyclopedia of Industrial Chemistry
Strontium-doped perovskites rival platinum catalysts for treating NOx in simulated diesel exhaust
Science
Manganese oxide nanoarray-based monolithic catalysts: tunable morphology and high efficiency for CO oxidation
ACS Appl. Mater. Interfaces
Scalable integration of highly uniform MnxCo3-xO4 nano-sheet array onto ceramic monolithic substrates for low temperature propane oxidation
Chem. Cat. Chem.
Low-temperature CO oxidation over a ternary oxide catalyst with high resistance to hydrocarbon inhibition
Angew. Chem. Int. Ed.
Development of advanced ultra-low PGM DOC for BS VI DOC+CDPF+SCR system
SAE Int. J. Mater. Manuf.
Development of Ultra-Low Synergized PGM as Diesel Oxidation Catalyst for Heavy-Duty Applications
Cited by (28)
A landscape review on biodiesel combustion strategies to reduce emission
2023, Energy ReportsState of the art developments in oxidation performance and deactivation of diesel oxidation catalyst (DOC)
2023, Catalysis CommunicationsEffect of catalyzed diesel particulate filter and its catalyst loading on emission characteristics of a non-road diesel engine
2023, Journal of Environmental Sciences (China)Citation Excerpt :This was attributed to the lower exhaust temperature at the two conditions, which was not conducive to the oxidation effect of the after-treatment (Kumar et al., 2021). With the increase in the engine load, the exhaust temperature increased, which improved the oxidation activity of the after-treatment (Hoang et al., 2019); then, the CO conversion rates increased. Under idle and 10%-load conditions, the CO conversion rates were less than 7%, and under other conditions, the DOC decreased the CO emissions by 41.0%-75.9%.
The development of diesel oxidation catalysts and the effect of sulfur dioxide on catalysts of metal-based diesel oxidation catalysts: A review
2022, Fuel Processing TechnologyCitation Excerpt :The supports of DOCs are usually made of porous ceramics or porous metals, coated with a layer of oxides and active metals (the main active components are noble metals such as Pt and Pd with rare earth metals) [30]. DOC is installed in the engine exhaust pipeline, which is the first step in the whole aftertreatment process [31]. Through oxidation, it oxidizes the organic parts of CO, HC, and PM in engine exhaust emissions to form harmless water and CO2, which is emitted to the atmosphere [32,33].
Transition-metal doped titanate nanowire photocatalysts boosted by selective ion-exchange induced defect engineering
2022, Applied Surface Science