Experimental Microkinetic Approach of De-NO_x by NH₃ on V₂O₅/WO₃/TiO₂ Catalysts. 1. Individual Heats of Adsorption of Adsorbed NH₃ Species on a Sulfate-Free TiO₂ Support Using Adsorption Isobars

Abstract

AIR:RAFFINAGE:INGENIERIE:SURFACES+FGI:CGE:NOG:DBIThe present study is dedicated to the development of experimental procedures allowing the measurement of the individual heats of adsorption of adsorbed NH3 species on a sulfate-free TiO2 solid (P25 from Degussa). This solid has been selected because it is frequently used as support of V2O5- or/and WO3-TiO2 model catalysts for the understanding of the surface processes implicated in the selective catalytic reduction of NOx by NH3 (NH3-SCR). Two original analytical procedures denoted adsorption equilibrium infrared spectroscopy (AEIR) and temperature-programmed adsorption equilibrium (TPAE) (developed in previous works) were applied. These methods are based on (a) the experimental measurement of the change in the adsorption equilibrium coverage of the individual adsorbed species in isobar conditions and (b) the comparison of the experimental data to an adsorption model. It is shown that in the ranges of the ammonia partial pressures and reaction temperatures of the NH3-SCR process, only two adsorbed NH3 species on Lewis sites (Ti+delta) are detected on the solid dehydrated at 673 K. These species noted NH3ads.L1 and NH3ads.L2 are differentiated by their delta(s) NH3 IR bands at 300 K (1149 and 1228 cm(-1), respectively), whereas their delta(as) IR bands are at the same position (1596 cm(-1)). The AEIR and TPAE methods indicate that the heats of adsorption of the NH3ads.L1 and NH3ads-L2 species (noted E-L1(theta) and EL2(theta) accuracy +/- 5 kJ/mol) vary linearly with their respective coverages theta from E-L1(1) = 56 kJ/mol to E-L1(0) = 105 kJ/mol and from E-L2(1) = 105 kJ/mol to E-L2(0) = 160 kJ/mol. These values are compared to (a) isosteric heat of adsorption provided by the Clausius-Clapeyron method and (b) literature data using temperature-programmed desorption, microcalorimetry, and DFT calculations. Forthcoming artides show that the simplicity of the analytical procedures allows studying the impact of the presence of sulfate and VOx/WOy depositions over TiO2, on the nature and heats of adsorption of adsorbed NH3 species

Layered‐Double‐Hydroxide Nanosheets as Efficient Visible‐Light‐Driven Photocatalysts for Dinitrogen Fixation

“…The 2340 and 2362 cm −1 absorption bands are similar in frequency to the values (2330 and 2350 cm −1 ) reported by Domen and co‐workers for N 2 adsorption on layered FeTiO‐based photocatalysts, suggesting that these features are likely due to a chemisorbed N 2 species . The bands at 1661, 1557, and 1448 cm −1 are characteristics for antisymmetric and symmetric deformations of a surface NH 4 + species (the associated NH stretching vibrations in the 3200–3600 cm −1 region will be obscured by the presence of adsorbed water) . O 2 evolution accompanied the photofixation of N 2 over CuCr‐NS, with ≈100 % selectivity to NH 3 being achieved without the formation of N 2 H 4 or H 2 (Figure S8B–D, Supporting Information).…”

Polymeric vanadyl species determine the low-temperature activity of V-based catalysts for the SCR of NO x with NH 3

“…S4A) and DFT calculations (fig. S7, A and B) show that NH 3 is preferentially adsorbed on surface Ti sites ( 31 ). The adsorbed NH 3 is activated by the transfer of an H atom to the vanadyl species and subsequently reacts with NO in the gas phase, resulting in the formation of an intermediate nitrosamide (NH 2 NO, denoted by the IR signals at 1505 cm −1 in fig.…”

A Contribution to the Experimental Microkinetic Approach of Gas/Solid Heterogeneous Catalysis: Measurement of the Individual Heats of Adsorption of Coadsorbed Species by Using the AEIR Method

“…The AEIR method has been applied to different gas/catalyst systems such as: NO on 2.7% Pt/Al 2 O 3 [82]; aromatic hydrocarbons on SiO 2 [70] and NH 3 [18,83,84] and H 2 O [85] on different TiO 2 based solids. For this last application it has been observed that some IR bands provide θ ex = f(T a ) curves which are not consistent with Equations (4) and (5).…”

“…For instance, the reactant CO of the CO/O 2 and CO/H 2 reactions on supported metal particles may lead to the formation of linear, bridged and threefold coordinated CO species which are well characterized by their IR bands in distinct wavenumber ranges [17]. Similarly, for the selective catalytic reduction of NO by NH 3 in excess O 2 (NH 3 -SCR) on x% V 2 O 5 /y% WO 3 /TiO 2 catalysts the adsorption of NH 3 leads to NH 3ads-L and NH 4 + species on the Lewis and Brønsted sites respectively which provide distinct IR bands ( [18] and references therein). The role of each coadsorbed species in the reaction is a key point of microkinetic studies.…”