High performance diesel oxidation catalysts using ultra-low Pt loading on titania nanowire array integrated cordierite honeycombs

doi:10.1016/j.cattod.2017.11.019

Catalysis Today

Volume 320, 15 January 2019, Pages 2-10

https://doi.org/10.1016/j.cattod.2017.11.019 Get rights and content

Highlights

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Development of Pt diesel oxidation catalysts using of nano-structured support, ultrafine Pt, and promoter added upstream.
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Uniform coating of mesoporous rutile titania nano-array on cordierite honeycomb monolith using solvothermal synthesis.
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Ultrafine Pt particles (0.95 ± 0.24 nm) with a loading of 1.1 g/ft³ was dispersed using atomic layer deposition.
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CO and hydrocarbon oxidation activity of ultrafine Pt/TiO₂ nano-array in the simulated exhaust can be promoted by adding H₂.

Abstract

High performance of an ultra-low Pt loading diesel oxidation catalyst can be achieved by using a combination of novel nano-array structured support, precise control of ultrafine active Pt particles, and an addition of H₂ as a promoter into the exhausts. Highly stable mesoporous rutile TiO₂ nano-array was uniformly grown on three-dimensional (3-D) cordierite honeycomb monoliths using a solvothermal synthesis. Atomic layer deposition was employed for precise dispersion of ultrafine Pt particles (0.95 ± 0.24 nm) on TiO₂ nano-array with a Pt loading of 1.1 g/ft³. Despite low Pt loading, the Pt/TiO₂ nano-array catalyst shows impressive low-temperature oxidation reactivity, with the conversion of CO and total hydrocarbon (THC) reaching 50% at 224 and 285 °C, respectively, in the clean diesel combustion (CDC) simulated exhaust conditions. The excellent activity is attributed to the unique nano-array structure that promotes gas-solid interaction and ultra-small Pt particle dispersion that increase surface Pt atoms. We also demonstrate that addition of more H₂ into the exhaust can lower light-off temperature for CO and THC by up to ∼60 °C and ∼30 °C, respectively.

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 (NO_x), and lean NO_x trap (LNT) or NO_x 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], Mn_xCo_3-xO₄ [6], Cu-Co-CeO_x [7] etc.), and La-based perovskites (e.g. La_1-xSr_xCoO₃, La_0.9Sr_0.1MnO₃ [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 Mn_xCo_3-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.

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High performance diesel oxidation catalysts using ultra-low Pt loading on titania nanowire array integrated cordierite honeycombs

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

TiO2 nanowire array (nano-array) growth on honeycomb cordierite monolith

Morphology and microstructure

Conclusions

Acknowledgments

Appl. Catal. B: Environ.

Catal. Today

Appl. Catal. B: Environ.

Appl. Catal. B: Environ.

Nano Energy

Chem. Eng. Sci.

Catal. Today

Appl. Catal. B: Environ.

Appl. Catal. B: Environ.

Prog. Org. Coat.

Catal. Today

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

J. Catal.

Appl. Catal. A: Gen.

Appl. Catal. B: Environ.

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

TiO₂ nanowire array (nano-array) growth on honeycomb cordierite monolith

Strontium-doped perovskites rival platinum catalysts for treating NO_x in simulated diesel exhaust