G. Ramis

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

Abstract The adsorption and transformation of ammonia over V 2 O 5 , V 2 O 5 /TiO 2 , V 2 O 5 -WO 3 /TiO 2 and CuO/TiO 2 systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH 2 amide species or to its dimeric form N 2 H 4 , hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N 2 − and azide anions N 3 − are further observed over CuO/TiO 2 . The comparison of the infrared spectra of the species arising from both NH 3 and N 2 H 4 adsorbed over CuO/TiO 2 strongly suggest that N 2 H 4 is an intermediate in NH 3 oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH 2 species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V 2 O 5 -based systems, but not over CuO/TiO 2 , in spite of the high SCR and SCO activity of this catalyst. Consequently Bronsted acidity is not necessary for the SCR activity.

Low-temperature CO2 adsorption on metal oxides: spectroscopic characterization of some weakly adsorbed species

Abstract Carbon dioxide adsorption at temperatures between 173 K and 273 K on metal oxides such as MgO, Fe2O3, TiO2, ZrO2 and Al2O3 produces two types of weakly adsorbed species. Linearly coordinated CO2 is characterized by a shift up of the v3 vibrational mode and by the activation in IR and the slight shift down of the v1; 2v2 Fermi resonance doublet. Bent CO2 species are characterized by a couple of strong bands in the regions 1900-1650 cm−1 and 1320-1150 cm−1, respectively. Weaker bands are also evident below 1000 cm−1 on oxides that allow the inspection of this region. Species thought to be side-on bonded on cationic sites are observed on all surfaces, while species thought to be C-bonded CO2− are also observed on TiO2 and ZrO2. Bent adsorbed CO2 is a likely intermediate in the formation of surface carbonate and bicarbonate species.

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

Abstract The physico-chemical characteristics and the reactivity of sub-monolayer V2O5-WO3/TiO2 deNOx catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO2 surface. The presence of tungsten oxide stabilizes the surface VIV and modifies the redox properties of V2O5/TiO2 samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V2O5-WO3/TiO2 catalysts exhibits higher activity than the binary V2O5/TiO2 and WO3/TiO2 reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.

FOURIER TRANSFORM IR STUDY OF NOX ADSORPTION ON A CUZSM-5 DENOX CATALYST

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO2, NH3, H2O). NO2 adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu2+NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N2O3 → N2O4 → NO2 → NO3. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO2, N2, O2, and are only slightly affected by H2O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NOx into highly active surface nitrates.

Surface characterization of amorphous alumina and its crystallization products

Abstract A nonconventional method, based on the thermal decomposition of aluminum nitrate in the presence of a suitable reducing agent, was applied to the preparation of amorphous alumina with high surface area. Modified aluminas were obtained in a further step by hydration and crystallization of the amorphous form. The characteristics of all these materials were investigated by XRD, DTA, and FT-IR spectroscopy. The presence of surface acidic sites was examined with the aid of pyridine as an adsorbed probe molecule. The catalytic properties of the materials were tested using as a probe reaction the dehydration of methanol to dimethyl ether at temperatures between 450 and 560 K. The results of catalytic tests and surface characterization of the materials are discussed in terms of three different Al 3+ cationic sites.

Characterization and reactivity of MoO3/SiO2 catalysts in the selective catalytic oxidation of ammonia to N2

Abstract Silica-supported MoO3 catalysts with MoO3 loadings up to 21%xa0w/w were prepared, characterized and tested in the selective catalytic oxidation (SCO) of ammonia to N2 under dilute conditions. It is found that the catalysts are active and selective in the reaction, and that the catalytic performance increases on increasing the Mo loading. Crystalline MoO3, detected over the silica support, is supposed to be the active species in the reaction. The reactivity of the catalysts is depressed by water addition to the feed at low temperatures and is enhanced by the presence of selected promoters, like Bi and Pb.

Spectroscopic characterisation of MoO3/TiO2 deNOx-SCR catalysts: Redox and coordination properties

MoO3/TiO2 n catalysts prepared by impregnation of anatase with ammonium heptamolybdate have been investigated with n the aim of understanding their redox chemistry and the coordination state of surface active species. n Adsorption of ammonia has been followed by FTIR and by EPR on thermally treated, partially reduced samples. n FTIR has shown that surface isolated molybdenylic species (with one short MoO bond) are present at the n surface which can coordinate an NH3 molecule in their coordination sphere. The effect of thermovacuum treatments n at increasing temperature is the formation of Mo(V) centres whose amounts depend on the total Mo loading. n Several families of Mo(V) have been identified by EPR, differing in their coordinative environment. n Reoxidation by O2 of the reduced samples follows a unique pathway involving formation of partially reduced n Ti3+ ions in the matrix. This pathway is different from that followed by silica-supported MoO3 systems n and the difference n has been explained n in terms of the semiconducting nature of titanium dioxide.

Surface structure and reactivity of molybdena–titania catalysts prepared by different methods

MoO3–TiO2 samples have been prepared from aqueous solutions of ammonium heptamolybdate and titania (by conventional impregnation and equilibrium adsorption) and by mechanically mixing both oxides (with and without further hydrothermal treatment). The samples have been characterized by X-ray diffraction, specific surface area assessment, Fourier-transform infrared (FTIR) and ultraviolet–visible diffuse refectance (UV–VIS DR) spectroscopies and temperature-programmed reduction (TPR). Surface properties have been studied following the adsorption of probe molecules by FTIR spectroscopy. Dispersed phases are formed preferentially in samples prepared by equilibrium adsorption and in physical mixtures submitted to hydrothermal treatment; in all other cases crystalline MoO3 is mainly formed. Dispersed phases are more easily reduced. They have been identified as surface molybdenyl species bonded to Ti cations through bridging oxygens, and their overall coordination depends on the degree of surface hydration. These species predominate quantitatively on the catalyst surfaces and have been found to be strong Lewis and Bronsted acids. They are also more easily reduced than bulk MoO3 under TRP conditions, and very active as oxidizing agents for methanol.

FT-IR spectra of ethylene molecularly adsorbed on metal oxides

Abstract The FT-IR spectra of ethylene molecularly adsorbed on some oxidic surfaces at low temperature are presented and discussed both in terms of vibrational frequencies and of band intensities. A hydrogen-bonding interaction is observed between ethylene and surfaces having active hydroxy groups, like amorphous silica and HY zeolite. Strong adsorption is observed on some transition metal oxide surfaces (TiO 2 , ZrO 2 , Fe 2 O 3 , ZnO) where interaction is thought to occur on coordinatively unsaturated cationic centers. In both these cases the bands that are only Raman active in the ghas-phase molecule, become activated, some bands (namely CH stretchings and Cue5fbC stretching) are shifted toward lower frequencies while some others (CH 2 out-of-plane bending) are shifted toward higher frequencies. Band intensities reveal a strong increase on the protonic charge on the ethylene hydrogens, evidencing that electron flows from the ethylene molecule to teh surface sites, the C 2 H 4 moiety retaining its planar structure. On rock-salt structure metal oxides (MgO, CaO and NiO) the interaction is weaker, and is thought to occur on surface oxide anions. When incompletely reduced centers like Cu(I) are exposed, interaction dominated by backbonding from the surface to π★-type antibonding orbitals of ethylene occurs, like on typical ethylene-metal homogeneous complexes.

On the mechanisms of light alkane catalytic oxidation and oxy-dehydrogenation: an FT-IR study of the n-butane conversion over MgCr2O4 and a Mg-vanadate catalyst

The interaction of n-butane, 1-butene, 1,3-butadiene and of C4 oxygenates on the surface of oxidized Mg-chromite and Mg-vanadate catalysts has been studied by FT-IR spectroscopy. The results compare well with the catalytic behavior of these materials one of which is a combustion catalyst (MgCr2O4) and the other is a rather selective oxy-dehydrogenation catalyst (Mg-vanadate). A mechanism for these reactions is proposed.