Fabio B. Passos

Partial oxidation of methane on Pt/Ce–ZrO2 catalysts

The mechanism of partial oxidation of methane was studied on Pt/Al2O3, Pt/ZrO2 and Pt/Ce–ZrO2 catalysts. The reducibility and oxygen transfer capacity were evaluated by temperature programmed reduction (TPR) and oxygen storage capacity (OSC). The effect of the support on the cleaning mechanism of the catalyst surface was investigated by the sequence of CH4/O2 pulses. Moreover, temperature programmed surface reaction (TPSR) measurements were performed to evaluate the reaction mechanism. Pt/Ce–ZrO2 catalysts proved to be more active, stable and selective than Pt/Al2O3 and Pt/ZrO2 catalysts. The results were explained by the higher reducibility and oxygen storage/release capacity of Pt/Ce–ZrO2 catalysts, which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability and activity of the catalysts, as revealed by the CH4/O2 pulses. TPSR experiments showed that the partial oxidation of methane proceeded through a two-step mechanism.

Partial oxidation and CO2 reforming of methane on Pt/Al2O3, Pt/ZrO2, and Pt/Ce–ZrO2 catalysts

Abstract The partial oxidation and CO 2 reforming of methane were studied on Pt/Al 2 O 3 , Pt/ZrO 2 , and Pt/Ce–ZrO 2 catalysts. The reducibility and the oxygen transfer capacity were evaluated by oxygen storage capacity (OSC). The effect of the support on the cleaning mechanism of the catalyst surface was investigated by the sequence of CH 4 /O 2 and CH 4 /CO 2 pulses. The Pt/Ce–ZrO 2 catalyst showed the highest stability on both partial oxidation and CO 2 reforming of methane. The results were explained by the higher reducibility and oxygen storage/release capacity of Pt/Ce–ZrO 2 catalysts, which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalysts, as revealed by the CH 4 /O 2 and CH 4 /CO 2 pulses. For Pt/Al 2 O 3 and Pt/ZrO 2 catalysts, the increase of carbon deposits around or near the metal particle inhibits the CO 2 dissociation on CO 2 reforming of methane. This effect on the CO 2 reforming of methane affects the partial oxidation of methane, which comprehends two steps: combustion of methane and CO 2 and steam reforming of unreacted methane.

Characterization and dehydrogenation activity of Pt/Nb2O5 catalysts

Pt/Nb2O5 catalysts were prepared with different loadings of Pt and characterized by TPR, DRS, H2 and CO chemisorption and H2/O2 titration. Higher metal content catalysts presented an increasing SMSI effect related to a lower adsorption capacity. The Pt blocking by NbOx species causes the formation of new interfacial active sites. After a high temperature reduction, the catalysts were highly selective towards dehydrogenation products and total inactive for hydrogenolysis in the case of n-heptane conversion.

Characterization of Pt-Sn bimetallic catalysts supported on alumina and niobia

Abstract Alumina and niobia Pt-Sn supported bimetallic catalysts were characterized by TPR, hydrogen chemisorption and cyclohexane dehydrogenation. The TPR profiles of niobia supported Pt-Sn catalysts showed the presence of different precursors from the ones obtained on alumina. Furthermore, the hydrogen uptakes indicated that the amount of metallic tin formed in the niobia supported catalysts was higher than in the alumina supported catalysts. After reduction at 773 K, the platinum/niobia catalyst displayed a strong metal-support interaction (SMSI) effect, with the creation of new interfacial active sites. The addition of a small tin content led to a suppression of the SMSI. Increasing the amount of tin, however, induced a marked poisoning of the platinum.

Effect of preparation method on the properties of Nb2O5 promoted platinum catalysts

Abstract Pt/Nb2O5 catalysts were prepared on a reducible support by ion exchange and incipient wetness of different platinum salts. The Pt/Nb2O5/Al2O3 catalysts were prepared with several loading of niobium oxide over alumina, aiming different surface coverages and platinum dispersions. These catalysts were characterized by using different techniques: TPR, DRS, chemisorption of H2, CO, CO2 and pyridine. The catalytic activity was evaluated in the n-heptane conversion. Platinum oxide was the main surface precursor on the Nb2O5 supported catalysts, while a oxychloroplatinum surface complex was the precursor for Pt/Nb2O5/Al2O3 catalysts. These different precursors promote the formation of different platinum particle sizes, being the Pt/Nb2O5/Al2O3 samples more disperse. After high temperature of reduction, the platinum adsorption capacity decrease on all the samples, due to the reduction of Nb2O5, and subsequent blocking of surface platinum atoms. Nb2O5 and Nb2O5/Al2O3 supported catalysts are very selective to olefins in the n-heptane conversion. Furthermore, Bronsted acidic sites are formed on Pt/Nb2O5/Al2O3 catalysts with higher Nb2O5 loading, leading to a better stability of the catalysts.

Characterization of steam-reforming catalysts

The effect of the addition of Mg and Ca to Ni/ a-Al2O3 catalysts was investigatedstudied, aiming to detail the promotion mechanismaddress their role as promoters in the steam reforming reaction. Temperature- programmed reduction and H2 and CO temperature-programmed desorption experiments indicated that Mg interacts with the metallic phase. Mg-promoted catalysts showed a greater difficulty for Ni precursors reduction besides different probe molecules (H2 and CO) adsorbed states. In the conversion of cyclohexane, Mg inhibited the formation of hydrogenolysis products. Nonetheless, the presence of Ca did not influence the metallic phase.

Activity and selectivity of Pt-Sn/Nb2O5 in n-heptane conversion

Abstract The catalytic properties of niobia and alumina supported platinum and platinum-tin catalysts were evaluated at the conversion of n-heptane at 773K. Over a niobia supported platinum catalyst the reaction proceeded via a monofunctional path, with a high selectiviy for oleflns. This was explained by the formation of a SMSI state, leading to the coverage of platinum particles by NbO x and the formation of metal-support interfacial sites. The presence of tin inhibited the formation of a SMSI state, causing a decrease in the olefins/toluene ratio.

Methane Direct Conversion on Mo/ZSM-5 Catalysts Modified by Pd and Ru

The effect of addition of Ru and Pd to Mo/HZSM-5 catalysts used in the dehydroaromatization of methane was investigated. Catalytic tests and temperature-programmed oxidation results showed that Pd-based catalysts were more selective to naphthalene and suffered strong deactivation. The presence of Ru improved the activity and stability, with a decrease in the carbonaceous deposit probably because of a mechanism of protection of the Mo2C surface.

The state of Tin on Pt-Sn/Nb2O5 catalysts

Abstract The effect of tin addition on niobia supported catalysts was studied and compared to the properties of alumina supported bimetallic Pt–Sn catalysts. The catalyst surfaces were probed by methylcyclopentane conversion, showing that both the presence of Sn and the reduction of the support caused a decrease in hydrogenolysis activity, favoring the ring enlargement reaction. The thermodynamics of reduction of these systems, evaluated by following the reduction step (temperature programmed reduction — TPR) with a differential scanning calorimeter (DSC), and irreversible H 2 and CO uptakes, allowed to conclude that a Pt–Sn alloy is formed on niobia supported catalysts.

Quantification of metallic area of high dispersed copper on ZSM-5 catalyst by TPD of H2

Metallic surface of Cu/ZSM-5 catalyst was determined quantitatively with H2 TPD and N2O oxidation methods. Usually well established N2O techniques are the dissociative N2O chemisorption performed as pulse chemisorption or in frontal technique with highly diluted N2O to avoid high heat evolution and bulk oxidation. Here TPR/H2 of the oxygen layer formed under relatively high N2O partial pressure is compared with TPD/H2. The dispersion of Cu0 determined by TPD is very high. The calculated metallic surface copper area was very close to the total exchanged Cu in the sample, which confirms that copper atoms are well dispersed on ZSM-5 sample. These measurements are supported by XPS data, suggesting also a high dispersion of copper ions in the zeolite. The surface metallic copper determined by H2 TPD technique showed that this method avoids sintering, compared to the N2O method, where sintering occurred due to heat evolution during the N2O reaction, causing migration and agglomeration of copper externally the surface of the zeolite.