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
The present study concerns an experimental microkinetic approach of the photocatalytic oxidation (PCO) of isopropyl alcohol (IPA) into acetone on a pure anatase TiO2 solid according to a procedure previously developed. Mainly, the kinetic parameters of each surface elementary step of a plausible kinetic model of PCO of IPA are experimentally determined: natures and amounts of the adsorbed species and rate constants (preexponential factor and activation energy). The kinetics parameters are obtained by using experiments in the transient regime with either a FTIR or a mass spectrometer as a detector. The deep oxidation (CO2 and H2O formation) of low concentrations of organic pollutants in air is one of the interests of the PCO. For IPA, literature data strongly suggest that acetone is the single route to CO2 and H2O and this explains that the present study is dedicated to the elementary steps involving gaseous and adsorbed C3H(x)O species. The microkinetic study shows that strongly adsorbed IPA species (two species denoted nd-IPA(sads) and d-IPA(sads) due to non- and dissociative chemisorption of IPA, respectively) are involved in the PCO of IPA. A strong competitive chemisorption between IPA(sads) and a strongly adsorbed acetone species controls the high selectivity in acetone of the PCO at a high coverage of the surface by IPA(sads). The kinetic parameters of the elementary steps determined in the present study are used in part 2 to provide a modeling of macroscopic kinetic data such as the turnover frequency (TOF in s(-1)) of the PCO using IPA/O2 gas mixtures.
The present article is dedicated
to the impacts of sulfatation and/or V
x
O
y
deposition on TiO2 supports
on the individual heats of adsorption of adsorbed NH3 species.
The S and V loadings are similar to those of TiO2 based
commercial catalysts for the selective catalytic reduction of NO
x
by NH3: NH3–SCR.
Two original experimental procedures are used, namely, adsorption
equilibrium infrared spectroscopy (AEIR) and temperature-programmed
adsorption equilibrium (TPAE). They have been developed in previous
works and adapted in Part 1 for the species formed by the adsorption
of NH3 on a sulfate free TiO2 support (P25 from
Degussa). In agreement with the literature, Raman and FTIR spectroscopies
show that the impregnation of the sulfate-free and sulfated TiO2 supports by the vanadium precursor leads to well-dispersed
V
x
O
y
species,
which are involved in the adsorption of NH3 in the temperature T
a range of 300–673 K and for adsorption P
a < 0.5 kPa. Sulfate and V
x
O
y
groups favor the amount of NH4
+ species without modifying their heats of adsorption
as compared to the TiO2 support: they are not detected
for temperatures of interest for NH3–SCR reaction.
In these experimental conditions and whatever the solids, two main
adsorbed NH3 species on Lewis sites (denoted NH3ads‑L1 and NH3ads‑L2) are present, characterized by two
δs IR bands below 1300 cm–1 and
a common δas IR band at ≈1600 cm–1. By comparison with TiO2 P25; it is shown that sulfate
groups have strong impacts neither on the proportion, x
1 and x
2, of the two adsorbed
species nor on their heats of adsorption (E
L1(θ) and E
L2(θ) with θ
the coverage of the NH3ads‑L1 and NH3ads‑L2 species: x
1 = 0.65, x
2 = 0.35, E
L1(1) = 56 kJ/mol, E
L1(0) = 102 kJ/mol, E
L2(1) = 110 kJ/mol, E
L2(0) = 140 kJ/mol.
The deposition of V-containing species on the sulfate-free and sulfated
TiO2 surfaces leads to similar conclusions: in the experimental
conditions (T
a and P
a) of the NH3–SCR, two adsorbed NH3 species on Lewis sites are detected with comparable proportions
and heats of adsorption to those observed on the TiO2 supports.
The heats of adsorption of two linear CO species adsorbed on the Au degrees particles (denoted L(Au degrees)) and on the Ti(+delta) sites (denoted L(Ti+delta)) of a 1% Au/TiO(2) catalyst are determined as the function of their respective coverage by using the AEIR procedure (adsorption equilibrium infrared spectroscopy) previously developed. Mainly, the evolutions of the IR band area of each adsorbed species (2184 cm(-1) for L(Ti+delta) and at 2110 cm(-1) for L(Au degrees)) as a function of the adsorption temperature T(a), at a constant CO adsorption pressure P(CO), provide the evolutions of the coverages theta(LTi+delta) and theta(LAu degrees ) of each adsorbed CO species with T(a) in isobar conditions that give the individual heats of adsorption. It is shown that they linearly vary from 74 to 47 kJ/mol for L(Au degrees ) and from 50 to 40 kJ/mol for L(Ti+delta) at coverages 0 and 1, respectively. These values are consistent with literature data on model Au particles and TiO(2). In particular, it is shown that the mathematical formalism supporting the AEIR procedure can be applied to literature data on Au-containing solids (single crystals and model particles).
The adsorption of CO on a 4.7% Cu/Al2O3 catalyst either reduced or oxidized is studied using the FTIR spectra recorded at adsorption temperatures Ta in the range 298-740 K and with two constant partial pressures Pa (10 3 and 10 4 Pa). On the reduced solid and at Ta = 300 K a single IR band is detected at 2092 cm -1 ascribed to a linear CO species (denoted by L0) on Cu particles. The FTIR spectra lead to the determination of the evolution of the coverage of the L0 species with Ta for the two CO pressures. In the temperature range 380 K > Ta > 480 K, the curves = f(Ta) for the two CO pressures are in very good agreement with Temkin's adsorption model. Moreover, it is shown that, in the range 300-600 K, the curves are in agreement with an adsorption model considering (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the coverage in the range 0-1. This leads to the determination of the heat of adsorption E , at various coverages from E0 = 82 kJ/mol at = 0 to E1 = 57 kJ/mol at = 1. It is shown that these values are similar to those obtained using the Clausius-Clapeyron equation. On the oxidized Cu/Al2O3 catalyst, an IR band is detected at 2120 cm -1 at 300 K. Its intensity increases either with time on stream in 1% CO/He or with increase in Ta. This IR band is ascribed to a linear CO species (denoted by L1) on Cu + sites and the increase in its intensity is assigned to the reduction of the copper oxide surface. The maximum in the superficial concentration of the Cu + sites is obtained after reduction in 1% CO/He at 473 K. This allows the determination of the heats of adsorption of L1 species at various coverages which linearly vary with from E'0 = 115 kJ/mol at = 0 to E'1 = 58 kJ/mol at = 1. The two L species are present simultaneously on the surface for an incomplete reduction of the solid, and it is shown that this does not significantly affect the heats of adsorption of the two adsorbed species. Moreover, the aging of the reduced catalyst has no effect on the heats of adsorption of the L0 and L1 species but leads to the detection of a new IR band at 2003 cm -1 ascribed to a bridged CO species on Cu sites. The heat of adsorption of this species is strongly higher than those of the two L species varying with the coverage from 130 kJ/mol at = 0 to 78 kJ/mol at = 1. Finally, it is shown that the heats of adsorption of the L0 and L1 species are in accord with the literature data on the stability of the two adsorbed species in the course of a desorption at room temperature.
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