Context: Indoor air treatment by catalysis-adsorption.

1. Investigation of NO and NO2 Adsorption Mechanisms on TiO2 at Room Temperature: Influence of Concentration L. SIVACHANDIRAN1,2,3 , F. THEVENET*1,2, P. GRAVEJAT1,2, A.ROUSSEAU3 1 Université Lille Nord-de-France, F-59000, Lille, France 2 Ecole des Mines de Douai, Dpt Chimie & Environnement, 941 rue Bourseul, F-59500 Douai, France 3 Laboratoire de Physique des Plasmas, Ecole Polytechnique, UPMC, Université Paris Sud 11, CNRS, F-91120 Palaiseau, France e-mail : frederic.thevenet@mines-douai.fr Experimental / Analytical Setup • Context: Indoor air treatment by catalysis-adsorption. • In heterogeneous catalysis assisted air treatment techniques (Photocatalysis, NTP technology, thermal catalysis) adsorption of VOCs on catalyst surface is key step1. • In these processes, catalyst deactivation is often reported owing to the storage of COX (CO and CO2) and NOX (NO and NO2) species on catalysts surface2. • NOx are produced by thermal plant, combustion engine and NTP techniques3,4. 50 ppb 50 ppb Detection Limits : FTIR FT-IR – – 10m 10m path path Cell Cell 700 ppb NO NO2 NO and NO2 adsorption on TiO2 at 296 K under dark condition. VENT N2O • In literature5. For adsorption, 11-55 ppm of NO2, and 10 ppm of NO are separately sent at the total flow rate of 1 L.min-1 (diluted with N2 and/or air) to the TiO2 coated packed bed reactor. Downstream analytical characterization is performed by gas phase FT-IR spectroscopy. Objectives : Investigation of NO2 adsorption mechanisms and storage capacity. Demonstrating the influence of NO2 concentration on catalyst aging. 2. NO2 adsorption and desorption. 1. NO adsorption. At 296 K, under dry and dark condition, 10 ppm of NO and 3.5 ppm of tracer N2O are sent to the reactor inlet. Ads: NO2 =23 ppm, N2O = 4.5 ppm TPD: Under N2, heated to 700 K at the rate of 1.1 K.s-1. (1) • No delay between NO and N2O breakthrough curves : NO does not adsorb on TiO2 at 296 K. • Thus, NO would not compete with VOCs for adsorption sites on TiO2. • Delay between NO2 and N2O breakthrough curves : NO2 adsorbs on TiO2, and produces NO in the gas phase. • Amount of NO2 adsorbed : 8.3 ± 0.3 µmol/m2 • Amount of NO produced 2.7 ± 0.3 µmol/m2 • NO2 desorbs in two different peaks: 1st peak at 450 K, 2nd peak above 520 K. • The total amount of desorbed NO2 is 5.6 ± 0.1 µmol/m2 (1st peak = 0.2 ± 0.31µmol/m2 ) 4. NO2 adsorption mechanism. • In this study: the condition of surface coverage. The ratios between consumed NO2, desorbed NO2 during TPD and produced NO during adsorption. In literature, 2:1:15. In this study, 3:2:1. 3. Influence of NO2 concentrations. • NO2 (ppm) Vs NO formation • NO is produced irrespectively of the NO2 inlet concentrations. • NO production mainly depends on the amount of consumed NO2, i.e. 3 ± 0.2 µmol/m2. There is a threshold limit to produce NO in the gas phase. • NO2 inlet concentration affects the surface coverage kinetics. 5. Conclusions and perspectives. • Three adsorbed NO2 molecules on TiO2 produce two NO3- on TiO2 surface and evolve one NO in the gas phase. • NO2 may induce significant surface aging. • NO2 may significantly influence the VOCs adsorption and mineralization on TiO2 surface. • The strongly adsorbed NO3- species could affect the modes of VOCs adsorption and the corresponding decomposition/ adsorbed species mineralization products. • The identification of adsorbed species is necessary to support the proposed mechanism. • Validation of proposed mechanism. • The proposed mechanism is valid for the investigated NO2 concentrations. • For higher NO2 concentration, amount of consumed NO2 decreases owing to self poisoning of TiO2 surface by NO3- produced in Eq (4). • For lower concentration, the amount of NO2 consumption depends on the diffusion of NO2- and NO3- species produced in Eq (2) & (3). • A. Mattsson, L. Osterlund, J.Phys. Chem. C, 114 (2010) 14121–14132. • H. Wang, Z. Wu, W. Zhao, B. Guan, Chemosphere, 66 (2007) 185–190. • R. Alvarez, M. Weilenmann, J.Y. Favez, Atmos. Environ, 42 (2008) 4699–4707. • H.H. Kim, Plasma Process and Polymers, 1 (2004) 91–110. • 5. J. Haubrich, R.G. Quiller, L. Benz, Z. Liu, C.M. Friend, Langmuir 26 (2010) 2445–2451.