Applied Catalysis B: Environmental 156–157 (2014) 72–83 Contents lists available at ScienceDirect Applied Catalysis B: Environmental j ourna l h omepa ge: www.elsevier.com/locate/apcatb Selective catalytic reduction of NO x by hydrogen (H 2 -SCR) on WO x -promoted Ce z Zr 1-z O 2 solids Ari Väliheikki a , Klito C. Petallidou b , Christos M. Kalamaras b , Tanja Kolli a , Mika Huuhtanen a , Teuvo Maunula c , Riitta L. Keiski a , Angelos M. Efstathiou b,∗ a Mass and Heat Transfer Process Laboratory, Department of Process and Environmental Engineering, P.O. Box 4300, FI-90014 University of Oulu, Finland b Department of Chemistry, University of Cyprus P.O. Box 20537, University Campus, 1678 Nicosia, Cyprus c Dinex Ecocat Oy, Product Development, Typpitie 1, FI-90620 Oulu, Finland a r t i c l e i n f o Article history: Received 17 October 2013 Received in revised form 9 February 2014 Accepted 4 March 2014 Available online 13 March 2014 Keywords: Selective catalytic reduction H2-SCR Surface acidity NH3-TPD Tungsten NO-TPD H2/O2-TPSR UV–vis/DRS Nitrogen oxides a b s t r a c t The selective catalytic reduction of NO x by H 2 (H 2 -SCR) under strongly oxidizing conditions (520 ppm NO x /1% H 2 /5% O 2 /10% CO 2 /He; NO:NO 2 –4:1–9:1) in the 150–600 ◦ C range has been studied over 3 wt-% W-promoted CeO 2 –ZrO 2 solids (85 wt-% CeO 2 -15 wt-% ZrO 2 (CeZr), and 17 wt-% CeO 2 –83 wt-% Zr (ZrCe) synthesised by a proprietary method) for the first time. The highest NO x conversion (X NOx = 54%) was obtained on the W-ZrCe (Zr-rich) solid at 300 ◦ C (GHSV of 51,000 h -1 ), whereas N 2 -selectivity was in the 77–92%-range over both W-ZrCe (Zr-rich) and W-CeZr (Ce-rich) catalysts. Significantly higher integral specific rates (R NO , mol NO m -2 min -1 ) were estimated on the W-ZrCe (Zr-rich) catalyst compared to the W-CeZr (Ce-rich) one in the 250–350 ◦ C range. The formation of adsorbed NO x under 0.1% NO/10% O 2 /He gas treatment at 25 ◦ C followed by H 2 /O 2 -TPSR experiments revealed that at least two different kinds of active NO x of low concentration (4–7 mol g -1 ) were formed on both catalysts, whereas other inac- tive (spectator) NO x species formed were of larger concentration (>160 mol g -1 ). UV–vis/DRS studies revealed that deposition of 3 wt-% W on ZrCe (Zr-rich) mixed metal oxide following calcination at 600 ◦ C resulted in the formation of both polymeric WO x and WO 3 clusters, whereas on CeZr (Ce-rich) only the latter phase (W 6+ ) was seen. Large differences in the concentration (mol m -2 ) and strength of surface acid sites between the W-CeZr and W-ZrCe solids were revealed after performing NH 3 -TPD and NH 3 - DRIFTS. These results were found to correlate with the specific H 2 -SCR rate (mol m -2 min) obtained for the two solids. In particular, the surface acid sites on W-ZrCe and W-CeZr solids were found to be 5.96 and 2.76 mol m -2 , respectively, whereas the specific reaction rate was 0.14 and 0.046 mol m -2 min at 300 and 250 ◦ C, at which maximum rates were observed, respectively. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Nitrogen oxide (NO x ) emissions from mobile and stationary sources are major concern due to their effect on human health and the environment. This has led to stringent legislation for reduc- ing NO x emissions during the last decades [1]. Selective catalytic reduction (SCR) has proven to be an effective method for reducing NO x emissions, where different reducing agents such as ammonia (NH 3 ) and urea (OC(NH 2 ) 2 ) [2–6] as well as hydrocarbons (HCs) [7–10] and hydrogen (H 2 ) [11–16] have been applied. The most ∗ Corresponding author. Tel.: +357 22892776; fax: +357 22892801. E-mail addresses: riitta.keiski@oulu.fi (R.L. Keiski), efstath@ucy.ac.cy (A.M. Efstathiou). commonly used reducing agent is ammonia, and the NH 3 -SCR tech- nology has been applied since 1973 [17–21]. However, the today’s great concerns about the increasing emissions of carbon dioxide to the atmosphere, and problems faced by the current NH 3 -SCR technology, such as ammonia slip, ash odor, air-heaters fouling and high running cost, demand the finding of appropriate non carbon- containing reducing agents for the catalytic removal of NO x from combustion exhaust streams, and preferably at low temperatures for reduced operational and investment costs [22,23]. Until the full transition to the hydrogen economy and zero emissions of greenhouse gases are achieved, H 2 -SCR might be considered as a breakthrough NO x control technology in favour of the present NH 3 - SCR, especially applied at industrial sites where H 2 gas is available (e.g. olefin plants, petrochemical plants, oil refineries, hydrogen plants, etc.) [16]. Hydrogen-assisted NH 3 -SCR and urea-SCR are http://dx.doi.org/10.1016/j.apcatb.2014.03.008 0926-3373/© 2014 Elsevier B.V. All rights reserved.