Elsevier

Journal of Catalysis

Volume 302, June 2013, Pages 58-66
Journal of Catalysis

Photocatalytic synthesis of oxygenated hydrocarbons from diesel fuel for mobile deNOx application

https://doi.org/10.1016/j.jcat.2013.03.003Get rights and content

Highlights

  • Photocatalytic partial oxidation of dodecane to OHC at anhydrous condition is studied.

  • Mixed oxides with <4 nm of TiO2 clusters dispersed on SiO2 matrix is synthesized.

  • Dilution effect of contiguous TiO2 on the selectivity and yield of OHC is confirmed.

  • A new deNOx process for diesel engine is proposed.

Abstract

Photocatalytic partial oxidation of dodecane has been studied as a model reaction of diesel fuel conversion to oxygenated hydrocarbons (OHCs) as an effective nitrogen oxide (NOx) reductant in selective catalytic reduction (SCR) systems. Thus, TiO2-based photocatalysts produced OHCs composed mainly of C1–C6 aldehydes under UV irradiation, and TiO2–SiO2 mixed oxide photocatalysts showed higher selectivity and yield to OHCs than pristine TiO2 by diluting contiguous Ti sites and suppressing complete oxidation to CO2. The effects of reaction variables were studied in detail. A novel scheme of NOx after-treatment system for diesel engine exhaust line was proposed involving the new photocatalytic reaction, where on-board photocatalytic partial oxidation of a small amount of diesel fuel produced OHCs that were supplied to the deNOx system as NOx reductant. Although further improvement is needed in the selectivity for OHCs as well as the system operability, the proposed scheme could be a more environment-friendly option than the reduction by urea, currently considered the most promising technology.

Graphical abstract

On-board photocatalytic partial oxidation of the small amounts of diesel fuel over TiO2–SiO2 mixed oxides produces oxygenated hydrocarbons (mainly C1–C6 aldehydes) as an effective NOx reductant for diesel vehicle exhaust deNOx treatment.

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Introduction

Diesel engine vehicles are the major source of harmful nitrogen oxides (NOx) emission on the road. Especially NOx emission from heavy duty diesel vehicles accounts for almost 30% of total emission despite the number of vehicles is limited to only 2% in California, USA [1]. The fact that all diesel cars must satisfy Bin 5 (NOx  0.07 g/mi) [2] and Euro VI (NOx  80 mg/km) [3] compliancy to be sold in the USA and Europe markets drives diesel car manufactures to develop more efficient NOx after-treatment technology than the past [4].

There have been a huge number of reports concerning lean NOx selective catalytic reduction (SCR) technology categorized by reducing agent such as urea (or ammonia) [5], [6], [7], hydrocarbon (HC) [8], oxygenated hydrocarbons (OHCs) [2], [9], [10], [11], [12, and hydrogen [13]. These reports show highly practical potential of lean NOx SCR technology for application to the exhaust line of real diesel engine vehicle fleet with more than 50% NOx conversion at relatively low temperatures of 200–250 °C, similar to diesel exhaust temperatures under lean conditions. These technologies are proposed on the assumption that those highly effective reductants such as NH3, HCs (short and long chain alkanes and alkenes), and OHCs (alcohols and aldehydes) are sufficiently produced somehow and delivered to the exhaust gas after-treatment convertor filled with a NOx reducing catalyst. In reality though, the reducing gases in the engine exhaust stream are insufficient; NH3 (none), HCs–OHCs (0–600 ppm carbon, varied by driving condition) [14], [15] and H2 (almost none). Thus, additional supply of reducing agents to the NOx SCR convertor is necessary for practical application. Currently, urea is considered the most promising reductant because it generates NH3, the strong reductant of NOx at low temperatures. Yet, ammonia is toxic and carrying a urea tank on board is neither convenient nor safe. Also conversion of urea into ammonia does not reach 100%. Diesel fuel itself can be directly used as a convenient reductant, but it is not reactive enough at the diesel exhaust temperatures under lean conditions [16], [17].

As an alternative technology to supply NOx reductants on board, herein we report photocatalytic partial oxidation of dodecane, a model compound of diesel fuel, into OHCs by using TiO2–SiO2 mixed oxide photocatalysts in a simple continuous flow reaction system that could be installed in the vehicle (Scheme 1 and Fig. S1 of Supporting information, SI). The performance of the photocatalytic reactions that we report here, especially selectivity to OHCs, is still rather low for practical applications. Further, installation of the photocatalytic reactor with a UV lamp inside the vehicle may not be convenient or energy-efficient. But if further improvement is achieved in the reaction selectivity and the system operability, it could be more environment-friendly than carrying the urea tank on board. The system produces OHCs below 100 °C, which are efficient NOx reductants and safer than NH3. By using a very small amount of diesel fuel, the system does not generate a significant fuel penalty (e.g., 6 mol of acetaldehyde can be produced from 1 mol of dodecane). In a scientific point of view, the number of study regarding photocatalytic partial oxidation of organic compound is rather scarce than that of photocatalytic mineralization or total oxidation process [18], [19]. To the best of our knowledge, this work is the first experimental research of photocatalytic partial oxidation of a diesel-like molecule at low temperatures (<100 °C) without water.

Section snippets

Catalyst synthesis

The TiO2–SiO2 mixed oxide was synthesized under typical hydrothermal conditions. (Better be termed “solvothermal” with almost non-aqueous solutions accompanied by heating in a Teflon autoclave reactor.) Thus, titanium (IV) sopropoxide (TTIP, 97%, Sigma–Aldrich), tetraethyl orthosilicate (TEOS, 98%, Acros Organics), tetrabutyl ammonium hydroxide solution (TBAOH, 1 M in methanol, Sigma–Aldrich), 2-propanol (100%, JT baker) were used without further purification. Mixed oxide of Ti/Si = 0.11 was made

Identification of products from photocatalytic partial oxidation of dodecane over TiO2

Since photocatalytic partial oxidation of a diesel-like molecule in the anhydrous flow system has never been studied before, we first tried to understand the reaction behavior depending on operating variables. Thus, at the beginning, we used commercial anatase phase TiO2 (Junsei) as the photocatalyst. Of course, TiO2 is the best known photocatalyst for total oxidation of organic chemicals to carbon dioxide or pollutants mineralization in various environmental applications [21], [22], [23]. Yet,

Conclusion

In the present report, a new reaction has been proposed to supply reagents for on-board NOx reduction in diesel engine exhaust line; photocatalytic partial oxidation of diesel molecules to oxygenated hydrocarbons made mostly of highly reactive C1–C6 aldehydes. To enhance selectivity of desired aldehydes, we made an effort to disperse active TiO2 clusters in SiO2 matrix to dilute the contiguous Ti sites that are responsible for non-selective complete oxidation of dodecane molecules. As a result,

Acknowledgments

We thank for M. Nomura of Photon Factory, KEK, Japan, for XANES analysis. This work was supported by the Hydrogen Energy R&D Center, Korean Centre for Artificial Photosynthesis (NRF-2011-C1AAA0001-2011-0030278), and Basic Science Research Program (No. 2012-017247) funded by the Ministry of Education, Science, and Technology of Korea. It was also supported by the Brain Korea 21 and WCU (R31-30005) Programs.

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