I. Carrizosa

Study of the interaction of aliphatic alcohols with TiO2: I. Decomposition of ethanol, 2-propanol, and tert-butanol on anatase

Adsorption of ethanol 2-propanol, and tert-butanol has been studied on a well-characterized anatase sample using volumetric and ir techniques. Differences in the adsorption capacities for the three alcohols have been interpreted as due to the same electronic and steric effects that determine changes in the maximum coordination for the TiIV ions in titanium alkoxides. Decomposition of the adsorbed phase was followed by ir and TPD (temperature-programmed decomposition) methods and was compared with the catalytic dehydration of the respective alcohols. The agreement between the kinetic parameters for both processes indicates that the slowest step for the catalytic reaction corresponds to a monomolecular decomposition of the adsorbed species, as supported by studies of water displacement from the anatase surface by all three alcohols.

NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Abstract Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH3 and NO adsorption over a 15% w/w vanadia/titania catalyst. NH3 is adsorbed as coordinate NH3 and NH4+ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the VO sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced VO groups, resulting in N2, according to a NO+V□→1/2N2+VO reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(VO) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.

Thermal evolution of sol–gel-obtained phosphosilicate solids (SiPO)

Abstract Amorphous phosphosilicate solid with P–O–Si linkages was obtained by a sol–gel process starting from tetraethoxysilane (TEOS) and H 3 PO 4 . The evolution of the structure of as-prepared gel was investigated as a function of the heat-treatment temperatures, by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectra (XPS) studies. XPS reveals that the surface of the particles is poorer in Si than the bulk at low temperatures. Heating the gel allows its crystallisation through the formation of Si–O–P linkages at the expends of the Si–O–Si and P–O–P networks, while an enrichment of silicium at the surface is detected.

In situ DRIFTS study of the SCR reaction of NO with NH3 in the presence of O2 over lanthanide doped V2O5/Al2O3 catalysts

Abstract An in situ diffuse reflectance FTIR spectroscopy (DRIFTS) study of the selective catalytic reduction (SCR) of NO with NH3 in the presence of O2 has been carried out over vanadia/alumina and vanadia/lanthanide-doped alumina catalysts. The SCR reaction data may be interpreted on the basis of an Eley–Rideal type mechanism between gaseous NO and adsorbed ammonia species (NH+4 and/or coordinated NH3). Although both adsorbed ammonia species seems to be reactive, NH+4 groups are more effective in the SCR reaction. Thus, at the temperature of the highest NO conversion (Tmax), ammonia is adsorbed preferentially as NH+4 over the undoped catalyst, meanwhile over lanthanide-doped ones, the adsorption of coordinated NH3 becomes predominant, thus explaining the lower SCR activity of such type of catalysts. At lower temperatures, NO is adsorbed as nitrates and the NH3 adsorption as coordinated NH3 species is enhanced. These two factors drive the activity decrease.

Study of the interaction of aliphatic alcohols with TiO2: III. Formation of alkyl-titanium species during methanol decomposition

The catalytic decomposition of methanol on both anatase and rutile forms of TiO2 with a similar texture has been studied in connection with ir and TPD studies of the adsorbed phase. Coke poisoning readily occurs on rutile, even at 250 °C, giving CH4 as the main gaseous product. However, on anatase, bimolecular dehydration of methanol to ether can be followed in a narrow interval at 350–400 °C, while coke poisoning occurred only at higher temperatures, leading to C2H6 in the gas phase. The formation of TiCH3 species has been characterized by ir spectroscopy and by reaction with ethylene, which produces propylene through a Ziegler-Natta process. The same reaction leads to butenes during dehydration of ethanol through the formation of ethyl-titanium species. An interpretation of the differences in poisoning for the two oxides is given on the basis of the surface coordination of the more exposed TiIV ions.

Study of the interaction of aliphatic alcohols with TiO2: II. On the mechanism of alcohol dehydration on anatase

Abstract Adsorption of primary, secondary and tertiary aliphatic alcohols on TiO2 (anatase) has been studied by volumetric and ir techniques. The results agree with the previously reported coordination model for alcohol adsorption on this oxide, and a relation has been established between the “irreversible adsorptions” and the respective dipole moments of the alcohols. The dehydration mechanism of the above alcohols was elucidated on the basis of both TPD kinetic parameters and the distribution of isomer olefins. A concerted β-E2 elimination has been found with a Hoffmann/Saytzeff product ratio that places TiO2 between acidic and basic catalysts (e.g., Al2O3 and ThO2). Cis/trans selectivity ratios for the 2-olefins have been related to the unsaturation of the coordination sites at the surface. The application of Tafts relation to mechanistic studies is discussed from a critical point of view.

DRIFTS study of adsorbed formate species in the carbon dioxide and hydrogen reaction over rhodium catalysts

Abstract Carbon dioxide methanation has been studied over Rh/Al 2 O 3 and lutetium-promoted Rh/Al 2 O 3 in a catalytic flow reactor. The results obtained have been related to the nature of the adsorbed species under the actual flow reaction conditions, studied in a Fourier-transform infrared spectrometer, operating in the diffuse reflectance mode (DRIFTS). From this, the presence of formate and carbon monoxide species at the catalyst surface during the reaction is proved. In the proposed reaction scheme, adsorbed carbon monoxide species hinder the dissociative chemisorption of hydrogen, thus justifying the lower activity of the rare-earth-promoted sample. Kinetic analysis of the DRIFT spectra, after adsorbing and decomposing formic acid, shows that formate species play a major role in the reverse water-gas shift equilibrium, which determines the rate of carbon dioxide methanation.

Activation of rare earth oxide catalysts

Abstract It has previously been shown that even the heaviest rare earth oxides undergo partial conversion to bulk hydroxycarbonates when exposed to air. However, when rare earth oxides are used as catalysts, activation temperatures lower than those necessary to achieve the complete removal of H 2 O and CO 2 are often reported. To elucidate the actual active phases that operate in catalytic processes involving rare earth oxides, a systematic temperature-programmed desorption study of La 2 O 3 , Sm 2 O 3 and Yb 2 O 3 was carried out. Our results lend support to a particle structure for the activated powder “oxides” consisting of an oxycarbonate nucleus surrounded by an outer oxide layer.

Simultaneous Analysis of Gas Phase and Intermediates in the Hydrogenation of Carbon Oxides by DRIFTS

In this work the experimental details and some examples of applications of diffuse reflectance infrared technique are described. Special interest has been devoted to a correct determination of the temperature of the sample, to gas-phase analysis, and to quantitative aspects in a DRIFTS controlled environmental cell. Applications to the analysis of the gas phase during catalytic hydrogenation and to the obtainment of kinetic parameters for the catalytic decomposition of adsorbates are included.

“In situ” drifts study of adsorbed species in the hydrogenation of carbon oxides

Abstract The hydrogenation of carbon oxides over rhodium supported (alumina, Sm- and Lu-promoted alumina) catalysts has been studied in both, a fixed-bed flow reactor and by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), using a catalytic cell that allows the collection of the vibrational spectra of the adsorbed species in the flow of reactants at the actual reaction conditions. Product distributions and activation energies obtained by these two methods are in fairly good agreement. From the obtained data, a model in which adsorbed formate acts as intermediate in the methanation reaction has been developed, ascribing the differences in activity to the presence of adsorbed carbonyls at the metal sites.