Jacques Saussey

FT-IR study of NO + O2 co-adsorption on H-ZSM-5: re-assignment of the 2133 cm-1 band to NO+ species

Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O2 to NO led to the appearance of bands at 2133 and 977 cm-1. Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of 18O2 did not change the 2133 cm-1 band wavenumber, nor the use of a partly deuteroxylated D–H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm-1 band, generally considered as characterizing NO+2 species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm-1 band is attributed to the Olattice–NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.

Studying the NOx-trap mechanism over a Pt-Rh/Ba/Al2O3 catalyst by operando FT-IR spectroscopy

In situ FTIR spectroscopy coupled with mass spectrometry have been used to study the mechanism of nitrates formation and reduction over a common Pt-Rh/Ba/Al2O3 NOx storage catalyst. Our device consists of a transmission reactor cell (having a very small dead volume) dedicated to the evolution of surface species, and of a mass spectrometer combined with a FTIR micro-cell for gas analysis, allowing time resolved analysis in stationary and transient conditions. After studying the nitration properties of the catalysts under a lean flow, we have investigated its activity and selectivity in nitrate reduction under different reducing agents (CO, H2), using a rich mixture very close to the real composition too. The alternate exposure to rich and lean streams has allowed the identification of some steps in the reduction pathway, active sites, intermediate species and by-products for NOx-trap reaction. In particular we have differentiated the role of the support and the noble metal in the mechanism, as well as of isocyanate adspecies and ammonia among the detected species.

Combined infrared spectroscopy, chemical trapping, and thermoprogrammed desorption studies of methanol adsorption and decomposition on ZnAl2O4 and Cu/ZnAl2O4 catalysts

Abstract It is shown that FT-IR spectroscopy permits discrimination to be made between methoxy (methanol) and formate species adsorbed on ZnAl 2 O 4 and CuZnAl 2 O 4 catalysts. These species were found to be less stable on copper than on ZnAl 2 O 4 . The presence of reduced copper promotes methanol transformation into formates and then into C0 2 : (i) FT-IR results show that copper formate formation from methanol adsorption occurs even at room temperature and that surface oxygen ion participates in its formation; (ii) chemical trapping experiments demonstrate that increasing copper percentage destabilizes formate species, while TPD experiments correlatively indicate an accelerated transformation of formate into CO 2 . Formyl species are detected by chemical trapping only at the end of the reaction and are therefore assumed not to participate in the decomposition reaction.

In situ FT-IR study of NH3 formation during the reduction of NOx with propane on H/Cu-ZSM-5 in excess oxygen

Abstract IR experiments under flow of NO and propane on H/Cu-ZSM-5 evidence at 623 K the appearance of bands at 2248, 2157 and 2047 cm−1 tentatively assigned through the use of 15NO to nitrile, carbonyl (COCu+) and isocyano species respectively. Addition of O2 suggests conversion of isocyano to isocyanato species (2208 cm−1) which by hydrolysis leads to NH3 formation, revealed by IR bands at 3366, 3290, 3192 and 1610 cm−1.

Oxidation state of copper during the reduction of NOx with propane on H-Cu-ZSM-5 in excess oxygen

Abstract NO (reactant) and CO (product) molecules were used as IR probes of the copper oxidation state in H-Cu-ZSM-5 catalysts. CO adsorption is specific to Cu I sites. Its characteristic band at 2158 cm −1 provides quantitative results on integrating its molar extinction-coefficient (e) NO decomposes oxidizing Cu I to Cu II . Propane and oxygen in a special IR reactor cell always yielded chemisorbed CO. Use of e indicates the NO influence on Cu state.

Evidence of a lacunar mechanism for deNOx activity in ceria-based catalysts

Using in situ FTIR spectroscopy and mass spectrometry, it has been demonstrated that deNOx activity in oxide-based catalysts such as ceria can be correlated to the amount of oxygen vacancies present on the surface of the material. These results have been correlated to the total amount of exchanged oxygen for ceria-based compounds.

Reaction mechanisms and kinetics in the n-hexane cracking over zeolites

Abstract The reaction mechanisms of the cracking of n-hexane over MFI zeolites ( Si Al =10–75 ) have been studied at 400°C at low conversions in a microflow reactor. The reaction was found of first order in the partial pressure of n-hexane and second order in the aluminium content of the zeolites. The analysis of the kinetic parameters and selectivities showed that the reaction predominantly proceeded through the protolytic monomolecular mechanism involving the formation then the decomposition of an adsorbed carbonium ion-like species. This decomposition leads to n-alkanes and adsorbed carbenium ion-like species or, most probably, to n-olefins. The linear olefins (mainly butenes) should be the primary products of the cracking of n-hexane and, as such, should be considered as the main source of the subsequent reactions, notably the hydride transfer reactions. The latter reaction was favoured at high conversions, long contact times and high aluminium contents. It is suggested that, in the n-hexane cracking, the acid as well as the conjugated base sites of the zeolite play an important role in the protolysis process and explain the kinetic orders reported here and in the literature data. Two reaction paths have been envisaged: (i) the second order/high temperature (>450°C) mechanism which involved one n-hexane molecule and one zeolite acid site in the rate determining step and; (ii) the third order/low temperature mechanism which involved one n-hexane molecule and two zeolite acid sites in the rate determining step. In the latter case the protolysis step needed the anchimeric assistance of a second active site. In the proposed mechanistic schemes, the gaseous n-alkane molecule was adsorbed on Bronsted acid sites then decomposed with the help of the conjugated base sites.

In situ FT-IR and kinetic study of methanol synthesis from CO2/H2 over ZnAl2O4 and Cu–ZnAl2O4 catalysts

Abstract The kinetics of the CO2/H2 reaction over ZnAl2O4 and Cu–ZnAl2O4 catalysts at 250°C up to 0.3 MPa have been followed by in situ FT-IR spectroscopy. Both methanol and carbon monoxide formation were enhanced in presence of copper. They were also produced by independent routes through different adsorbed species. Formate (type I, I′ and II), methoxy and carbonate species were identified on the support and, in addition, copper formate and copper carbonyl species when copper was present. The hydrogenation of carbonate species to copper formate species was found rate determining in methanol synthesis over the Cu–ZnAl2O4 catalyst whereas type I formate species were shown to be the active intermediate for this reaction over the ZnAl2O4 support. Carbon monoxide resulted from the water gas shift reaction probably through the same species as methanol formed over the Cu–ZnAl2O4 catalyst whereas it seemed to stem from formate species of type II in the case of the ZnAl2O4 support. Type II formate species were shown inactive in presence of copper whereas the methoxy species adsorbed on the support were found inactive in presence and in absence of copper. The comparison of these results with those previously obtained with the CO/H2 mixture showed that the nature and the role of the detected species strongly depended on the reactive atmosphere and on the presence or not of copper in the catalyst composition.

Infrared study of CO2 adsorption on ZnO. Adsorption sites

Adsorption of carbon dioxide on a ZnO Kadox-15 powder has been studied by Fourier transform infrared spectroscopy. The following surface species are formed: bidentate carbonates, polydentate carbonates which appear with time or heating, hydrogenocarbonates and linear CO2 species. Moreover, a band at 1546 cm–1 could correspond to carboxylates reversibly adsorbed at room temperature. Attention is paid to the effect of CO2 addition which splits the νa(OCO) and δ(CO2) bands due to linear species and shifts the bidentate carbonates band from 1595 to 1615 cm–1. Taking account of the band va(O13CO)(in natural abundance), we deduce that the splitting is due to a coupling between two linear species held by the same Zn2+ ion. We propose that such Zn2+ ions that are two-fold coordinate are situated on the edges formed by the (0001)(10text-decoration:overline10) planes.

Evidence by In situ FTIR spectroscopy and isotopic effect of new assignments for isocyanate species vibrations on Ag/Al2O3

Ethyl isocyanate has been introduced on prereduced and preoxidised Ag(2%)/Al2O3 catalysts and on Ag(2%)/SiO2 for comparison at 573 K. Its decomposition on preoxidised silica catalyst gives IR bands at 2294 and 2204 cm−1 whereas on prereduced catalyst vibrations at 2294 and 2243 cm−1 are observed. The 2294 cm−1 feature is attributed to isocyanates on SiO2 while the 2204 and 2243 cm−1 bands are assigned to AgI–NCO and Ag0–NCO vibrations respectively. When alumina replaces silica, we show using isotopic substitutions that the bands observed at 2229 and 2256 cm−1 are characteristic of –NCO groups coordinated on two different sites of alumina.