Pd-supported on sulfated monoclinic zirconia for the reduction of NO 2 with methane under lean conditions
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Catalysis Letters Vol. 111, Nos. 1–2, October 2006 ( 2006) DOI: 10.1007/s10562-006-0125-8
Pd-supported on sulfated monoclinic zirconia for the reduction of NO2 with methane under lean conditions Erik M. Holmgreen, Matthew M. Yung, and Umit S. Ozkan* Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH, 43210, USA
Received 7 June 2006; accepted 5 August 2006
NO2 reduction with methane under lean conditions is studied as a part of a two-stage scheme where NO is first oxidized to NO2 over an oxidation catalyst and NO2 is subsequently reduced to N2 over a reduction catalyst. Pd supported on a sulfated monoclinic zirconia support was observed to have good activity for the reduction of NO2 with CH4 under lean conditions, giving N2 yields over 60% over a broad temperature range below 450 C. Sulfate groups are shown to be thermally stable below the calcination temperature. KEY WORDS: NO2 reduction; methane; sulfated zirconia; monoclinic.
1. Introduction Nitrogen oxides (NOx) represent a significant environmental pollutant, contributing to the formation of acid rain, smog, and ground level ozone. As such they have received much regulatory and research attention. Hydrocarbons in particular have been studied as potential NOx reducing agents, with methane being a focus due to its relative low cost and widespread availability [1–5]. A significant challenge to the use of methane as a reducing agent is the presence of excess oxygen (lean conditions) in many combustion exhaust streams, as under lean conditions methane tends to preferentially combust with oxygen [6,7]. We have previously demonstrated a series of Pd-based catalysts were active for the reduction of NO with CH4, although activity was highly dependant on the oxidation state of Pd [8–10]. Addition of Gd to this system was shown to improve O2 tolerance and maintain activity under lean conditions [11]. On H-ZSM-5 supported Pd, Nishizaka and Misono [12,13] discovered a link between high N2 selectivity in excess O2 and acidity of the support material. Loughran and Resasco [14] also observed activity over other acidic supports, including sulfated zirconia. Further work by Resasco et al. showed that acidic supports served to stabilize Pd as active 2+ ions, while over non-acidic supports PdO clusters were formed and were responsible for CH4 combustion [15]. Sulfated zirconia was further shown to maintain activity as well or better than zeolitic supports in the presence of H2O and SO2 [16–18]. *To whom correspondence should be addressed. E-mail: [email protected]
The most common preparation method of sulfated zirconia is exposure of ZrOH to either sulfuric acid [19] or ammonium sulfate [20]. Upon calcination, the tetragonal zirconia crystal phase is preferentially formed, and it is believed to be stabilized by the presence of sulfate groups [21]. However, the tetragonal phase is thermodynamically stable only at high temperatures. Under common reaction conditions zirconia transitions to the monoclinic phase, which has been shown to be le
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