The influence of reaction temperature on the chemical structure and surface concentration of active NOx in H2-SCR over Pt/MgO-CeO2:: SSITKA-DRIFTS and transient mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA), transient isothermal, and temperature-programmed surface reaction in H-2 (H-2-TPSR) techniques coupled with online mass spectroscopy (MS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study essential mechanistic and kinetic aspects of the selective catalytic reduction (SCR) of NO with the use of H-2 under strongly oxidizing conditions (H-2-SCR) over a novel Pt/MgO-CeO2 catalyst. The main focus was to study and report for the first time the effects of reaction temperature on the chemical structure and surface concentration of the active NOx intermediate species thereby formed. The information obtained is essential to understanding the volcano-type profile of the catalyst activity versus reaction temperature observed here and also reported previously. In the present work, two active NOx intermediate species identified by SSITKA-DRIFTS were found in the nitrogen-reaction path toward N-2 and N2O formation one, species located in the vicinity of the Pt-CeO2 Support interface region (nitrosyl [NO+] coadsorbed with a nitrate [NO3-] species on an adjacent Ce4+-O2- site pair) and the second located in the vicinity of the Pt-MgO support interface region. The chemical structure of the second kind of active NOx species was found to depend on reaction temperature. In particular, the chemical structure was that of bidentate or monodentate nitrate (NO3-) at T < 200 degrees C and that of chelating nitrite (NO2-) at T > 200 degrees C. The concentration of the active NOx intermediates that lead to N-2 formation was found to be practically independent of reaction temperature (120-300 degrees C) and significantly larger than 1 equivalent monolayer of surface Pt (theta(NOx) = 2.4-2.6). The former result cannot be used to explain the volcano-type behavior of the catalytic activity versus the reaction temperature observed; alternative explanations are explored. The H-spillover process involved in the H-2-SCR mechanism was found to be limited within a support region of about a 4-5 angstrom radius around the Pt nanoparticles (d(pt) = 1.2-1.5 nm). (c) 2008 Elsevier Inc. All rights reserved.