Nelson Cardona-Martínez

Relationship between the formation of surface species and catalyst deactivation during the gas-phase photocatalytic oxidation of toluene

Abstract Various partial oxidation products were identified on the surface of TiO2 and an 8% SiO2–TiO2 binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid. The SiO2–TiO2 photocatalyst is more active and appears to deactivate slower than TiO2. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO2. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Bronsted acidity but when SiO2 is cofumed with TiO2, Bronsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it.

Applications of adsorption microcalorimetry to the study of heterogeneous catalysis

Publisher Summary This chapter presents a survey of the theoretical and applied aspects of microcalorimetry to heterogeneous catalysis with particular emphasis on the determination of the acid–base properties of metal oxides and mixed metal oxides. Heterogeneous catalysis involves specific chemical interactions between the surface of a solid and the reacting gas (or liquid phase) molecules. The catalytic cycle is generally composed of adsorption steps, surface reaction processes, and desorption steps. The energetics of these surface chemical events plays an important role in determining the catalytic properties of the surface. Thus, the study of the adsorption of probe and reactive gas molecules onto surfaces is of primary importance in catalysis. The key to the effective utilization of microcalorimetry in heterogeneous catalysis is the judicious choice of gas-phase molecules for study.

Acid strength of silica-alumina and silica studied by microcalorimetric measurements of pyridine adsorption

Abstract Microcalorimetric measurements of the differential heat of pyridine adsorption at 473 K were used to probe the distribution of acid strength on silica and silica-supported alumina catalysts. Depositing alumina on silica increased the acid strength of the catalyst. The acid strength distribution curves for the Al 2 O 3 /SiO 2 sample showed three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Al 2 O 3 /SiO 2 catalyst was found to have both Bronsted and Lewis acidity. Incremental adsorption of pyridine on Al 2 O 3 /SiO 2 indicated that the initial region of constant heat corresponds to strong Lewis acidity while the intermediate region seemed to be Bronsted sites or a combination of weaker Lewis sites and Bronsted sites. The final region observed at higher extents of adsorption showed only H-bonded pyridine which corresponds to adsorption on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed basic molecules. Surface diffusion over the silica support provides an important pathway for the equilibration of pyridine between acid sites on the Al 2 O 3 /SiO 2 sample.

Kinetic simulation of heterogeneous catalytic processes: Ethane hydrogenolysis over supported group VIII metals

A kinetic model for ethane hydrogenolysis over Pt, Pd, Ir, and Co was formulated in terms of essentially two chemical parameters: the strength of bonding between atomic hydrogen and the metal surface and the strength of carbon-metal bonding between hydrocarbon fragments and the surface. These two surface bond strengths were estimated by calorimetric measurements of the heats of H{sub 2} and CO adsorption, combined with bond order conservation calculations. The results of the kinetic simulations suggest that ethane hydrogenolysis over Pt, Pd, Ir and Co takes place through irreversible C-C rupture of C{sub 2}H{sub 4} and C{sub 2}H{sub 3} surface species. Hydrogenation of monocarbon CH{sub y} fragments is kinetically insignificant. Dissociative adsorption of hydrogen is an equilibrated process, while dissociative adsorption of ethane is slow and reversible. Finally, the role of kinetic modeling in the formulation, interpretation, and generalization of experimental research in heterogeneous catalysis is discussed.

Acidity studies of fluid catalytic cracking catalysts by microcalorimetry and infrared spectroscopy

Abstract The acidic properties of a USY-based fluid catalytic cracking catalyst steamed at various severities and amorphous silica-alumina were investigated by microcalorimetry and infrared spectroscopy using pyridine adsorption at 473 K. Microcalorimetric measurements of the differential heat of pyridine adsorption versus adsorbate coverage revealed a heterogeneous acid site distribution for the catalysts. Besides showing the expected progressive decrease in the number of acid sites for pyridine adsorption, our measurements showed that the strength of Bronsted acid sites decreased with increasing severity of steam treatment. Infrared spectra of adsorbed pyridine revealed a significant decrease in the ratio of Bronsted to Lewis acid sites upon steaming. Amorphous silica-alumina had a relatively large number of acid sites of which a large proportion were Bronsted acid sites. However, the strength of these Bronsted sites was lower than that of the mildly steamed USY catalysts. This lower Bronsted acid strength, we believe, is related to lower activity for gas-oil cracking over silica-alumina.

Acid strength of silica-supported oxide catalysts studied by microcalorimetric measurements of pyridine adsorption

Microcalorimetric measurements of the differential heat of pyridine adsorption were used to probe the distribution of acid strength on a series of silica-supported oxide catalysts. Depositing oxides of the following cations onto silica increased the acid strength of the catalyst: Ga{sup 3{plus}}, Zn{sup 2{plus}}, Al{sup 3{plus}}, Fe{sup 3{plus}}, Fe{sup 2{plus}}, Mg{sup 2{plus}}, and Sc{sup 3{plus}}. The acid strength distribution curves for the supported oxide samples showed either two or three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Ga, Al, and Sc samples were found to have both Bronsted and Lewis acidity while the remaining samples showed only Lewis acidity. Incremental adsorption of pyridine indicated that the initial region of highest heat corresponds to strong Lewis acidity while intermediate heats seemed to be due to weaker Lewis acid sites or a combination of Lewis and Bronsted acid sites. The final region of lowest heat was due to H-bonded pyridine on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed pyridine molecules. The initial differential heat of adsorption increases proportionally to the Sanderson electronegativity of the added oxide.

Effect of catalyst deactivation on the acid properties of zeolites used for isobutane/butene alkylation

Coke formation during the isobutane/butene alkylation reaction over zeolites decreases the acidity and acid strength of the catalysts. Microcalorimetric measurements of the differential heat of pyridine adsorption and FTIR spectroscopy of adsorbed pyridine were used to probe the changes in the acid properties caused by the deactivation processes. Specifically, fresh and deactivated commercial acid catalysts such as REY, USY, and Beta zeolite were studied. The adsorption microcalorimetry and FTIR spectroscopy results demonstrated that USY has the strongest acid sites (both Bronsted and Lewis) and the highest concentration of strong sites followed by REY and then by Beta zeolite. This order is the opposite of that observed for the alkylation catalytic performance of these zeolites. In particular, it seems that having a high concentration of strong Lewis sites promotes catalyst deactivation. The deposits formed during deactivation have a strong paraffinic character, but evidence of olefinic species is also observed. The degree of unsaturation of the surface species formed increases from Beta to USY zeolite, implying that the presence of a high concentration of strong Lewis-acid sites promotes the formation of unsaturated compounds. Bronsted sites with intermediate acid strength appear to be the appropriate sites for maintaining good alkylation catalytic performance. The best catalytic performance and the slowest deactivation were achieved with Beta zeolite, followed by REY and USY with low sodium content. Only butene isomerization was observed for USY with high sodium content. For the active catalysts, the alkylation global reaction route dominates initially, but the amount of alkylation products decreases as the catalyst starts deactivating when oligomerization predominates; and, finally, the catalyst loses most of its activity and isomerization is the only reaction observed. The product distribution obtained suggests that, instead of authentic alkylation, the initial prevalent mechanism is polymerization followed by β-scission. Two deactivation models are proposed to explain the deactivation. The direct obstruction of the alkylation active sites by irreversible adsorption of coke or coke precursors and the indirect obstruction of the active sites by pore blocking or pore filling.

Microcalorimetric measurements of basic molecule adsorption on silica and silica-alumina

The differential heats of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and silica were determined microcalorimetrically to investigate how the heat of adsorption for a given site depends on the nature of the basic molecule adsorbed. This provides a test of the utility of the Drago-Wayland theory to heterogeneous reactions. Specifically, the acid/base properties of molecules in solution are described in terms of two parameters, the susceptibility of the species to undergo electrostatic interaction and the susceptibility to form covalent bonds. Accordingly, the calorimetric results of this study were correlated successfully in terms of Drago parameters for each catalyst. These parameters describe well the acidic properties of silica-alumina and silica and may allow the prediction of heats of adsorption for a wide range of basic molecules with known Drago parameters.