The Mo/Al2O3, Mo/CeO2, Mo/SnO2, Ce-Mo/Al2O3 and Sn-Mo/Al2O3 samples prepared by impregnation and co-precipitation methods showed either different Mo dispersity or catalytic activity towards CO adsorption and CH4 transformation. These catalysts with high Mo loadings were prepared at low pH taking into account the calcination temperature (600°C) and time as well as the solution pH, the isoelectric point and surface area of the solid support.
Two types of surface molybdena species have been identified. These are the surface bound MoO42- polymeric species due to the concomitant strong interaction revealed between Mo and support and free MoO3 crystallites that validated by XRD and DRIFT results. Accordingly, high Mo loadings (>15 wt% on Al2O3) are necessary to obtain considerable amounts of free MoO3 favorable for keeping the active metal in a higher dispersion state. Free MoO3
crystallites are more easily reducible than MoO42- molybdates with tetrahedral configuration strongly bound to Al2O3 (Mo-O-Al bonds) in the form of Al2(MoO4)3.
The Mo/Al2O3 material had the highest specific surface area (117.6 m2/g) and rather presented mesopores of regular distribution. However, this material showed the highest thermal stability while the Mo/CeO2, Mo/SnO2, Ce-Mo/Al2O3 samples undergo structural modifications above 700°C. This may be due to high oxygen storage and release of ceria and tin resulting in lattice defects and thus enhancing the mobility of metal ions and mutual interactions between them.
One may indicate that the introduction of cerium promotes aggregation of the Mo particles (particularly as a support) probably due to charge effects and/or to the strong basic property possessed by CeO2 as further emphasized by means of DRIFT (Mo–O–Ce linkages) and XRD (decreased crystallites size of MoO3 species and different molecular formulae between Mo and Ce). This led to the increase of polymerized surface Mo species besides the formation of coupled O=Mo=O bonds (bands at 995 and 1035 cm-1) indicative of polymeric MoO3.
In the meanwhile, doping Mo/Al2O3 with SnO2 leads to surface structure definitely different from that of Mo/SnO2. In the case of Sn-Mo/Al2O3, MoO3 crystals completely disappeared and transformed into MoO2 with the presence of SnO, whereas Mo/SnO2 was formed only by the two phases of MoO3 and SnO2 with high crystallinity but in both catalysts no linkages were observed between Mo and Sn ions after calcination at 600°C and molybdate species strongly affect the growth of SnO2 crystals. The major point that should be outlined is that the characteristic of Mo-Sn system is the result of the preparation method employed.
On the other hand, the thermal behaviour of Mo/SnO2 showed the dissolution of molybdena ions in the SnO2 crystals above 750°C. Thereby its high activity revealed in CH4 conversion might be associated with the former. It is reasonable to note that the marked dissolution was observed in all thermal treatments of Mo/SnO2 above 750°C (TG-DTA, CO adsorption and CH4 reaction after H2 reduction at 800°C) with somewhat different extents.
One may notice that the reduction of the catalysts improves the surface reactivity leading to the presence of small amounts of metallic Mo after reduction at 700°C. Moreover, further reduction up to 800°C enhanced the adsorption of CO so as to forming various types of carbonates, therefore as the reduction continues more coordinatively unsaturated (CUS) sites will be produced and thus resulting in more CO adsorption sites.
CO chemisorption at 100°C on Mo/Al2O3 reduced at different temperature mainly occurred on small MoO3 crystallites with only one band appearing at 2197 cm-1 corresponding to octahedral alumina sites (Aloct+3–CO) implying that Al2(MoO4)3 is difficult reducible.
Furthermore, CO chemisorption at 100°C on Mo/Al2O3 leads to the formation of formates, carboxylates and carbonates. On the other hand, CO chemisorption on Mo/Al2O3, Mo/CeO2, Mo/SnO2, Ce-Mo/Al2O3 and Sn-Mo/Al2O3 catalysts reduced at 800°C involves oxygen in the catalysts such oxygen could be present in Mo=O, Mo–O–Mo, Ce–O–Mo, Ce–O–Ce and Sn–O–Sn associates to form carbonates. It is reasonable to suggest that the formation of carbonate species involves reduced catalysts with hydroxyl groups and oxygen vacancies implying the existence of reactive lattice oxygen that may be due to the weakening of the covalence of the metal-oxygen lattice bonds and/or the enhancement of the mobility of lattice oxygen sites. Within this context, intimate coupling of Mo with Ce and Sn ions of different oxidation states has great facilities for electron exchange interactions. Thus, the electron-mobile environment necessitated by redox reactions is established that has a great share in enhancing the CO adsorption and therefore, leading to apparently high coverage of carbonate species on the catalysts reduced at 800°C.
Concerning the CH4 decomposition on Mo/Al2O3 the results showed that molybdena oxide could activate methane and oxidize it into surface formates and carbon monoxide but methane conversion is low. However, for CH4 decomposition on Mo/CeO2 and Ce-Mo/Al2O3 the results permit to infer that the beneficial effects occurred either because the cooperation between Ce and Mo interfacial active sites generated with higher activity or because the oxidative properties of CeO2 increased the dissociation of CH4 resulting in the liberation of the apparently high coverage of carbonates besides CO and CO2, and as a result the methane conversion increased mainly on Mo/CeO2.
For CH4 decomposition on Sn-Mo/Al2O3 the CH4 conversion is low with the formation of carbonate species while Mo/SnO2 revealed the highest CH4 activity (high conversion into CO2) and selectivity leading to the formation of formaldehyde intermediate and to almost total CH4 oxidation too. This relevant activity and selectivity is presumably due to the dissolution of molybdena ions in the SnO2 crystals resulting in more active sites.
Comparatively, the presence of CeO2 and SnO2 contributes to the rapid activation of CH4, thereby accelerating the carbon gasification reaction to produce CO2 decreasing the coke could be provoked by carbon deposition that has shown to intervene effectively with somewhat different rates throughout CH4 decomposition. Accordingly, the following order of the catalysts can be affirmable in accordance with the increase of CH4 conversion:
Mo/SnO2 > Mo/CeO2 > Ce-Mo/Al2O3 > Sn-Mo/Al2O3 > Mo/Al2O3
Finally, the present results suggest that Mo/CeO2 and Mo/SnO2 reduced at 800°C have the most likely active species for CO adsorption and CH4 decomposition (especially Mo/SnO2) due to highly dispersed MoO3 species besides Ce3+/Ce4+ and Sn2+/Sn4+ redox couples that have high capacity towards oxygen. These species were responsible for their catalytic activity revealed in CH4 oxidation.
Indeed, these results in correlation with the literature suggested that the higher dispersion of MoO3 on a highly reducible support leads to more active and selective catalysts by optimizing the interaction with the support throughout the preparation procedure.
Furthermore, it is inferred that there is an approximately linear correlation either between the increase of the extent of reduction up to 800°C and the increasing integrated absorbance of CO adsorbed on the catalysts or between the amount of converted methane and the sum of the amount of CO and CO2 gases plus the amount of carbon stored on the catalysts.
ACKNOWLEDGMENTS
I wish to express my greatest thanks to my supervisor Professor Ákos Rédey for enabling this research to be feasible and for his friendly guidance and his incredible assistance.
I would like to express my sincere appreciation to the following persons whose support made this research work possible:
Dr. József Kovács and Dr. Tibor Egyházy for their extraordinary help in making some measurements.
Prof. Monica Caldararu from the Institute of Physical Chemistry “ilie murgulescu” of the Romanian Academy of Sciences and Dr. Roman Dula from the Institute of Catalysis and Surface Chemistry in Krakow for their knowledge and cooperation, guidance and help in the Electron Spin Resonance (ESR) measurements.
Prof. János Kristóf for a very good course in infrared spectroscopy.
Prof. Dénes Kalló, Prof. Pál Tétényi and Dr. Jenő Hancsók for their advices in preparing the manuscript.
Dr. Tatiana Yuzhakova, Pál Bui and all my colleagues at the Institutional Department of Environmental Engineering and Chemical Technology for their friendship and encouragement received from them throughout my research work.
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THESES
The main results of the dissertation are summarized in the following thesis points:
1. Taking into account the calcination temperature and time as well as the solution pH, the isoelectric point and the surface area of the solid support high Mo loadings (>15 wt% on Al2O3) are necessary to obtain considerable amounts of free MoO3 crystallites favorable for keeping the active metal in a higher dispersion state. Thus, MoO3
clusters are more easily reducible than MoO42- molybdates with tetrahedral configuration strongly bound to Al2O3 (Mo-O-Al bonds) in the form of Al2(MoO4)3
and the higher activity of Mo/Al2O3 may be associated with the former.
2. For high Mo loadings obtained two types of molybdena species were the predominant surface species. These are the surface bound MoO42- polymeric species due to the concomitant strong interaction revealed between Mo and support and free MoO3
crystallites that validated by XRD and DRIFT results.
3. The Mo/Al2O3 material showed the highest thermal stability up to 900°C while the Mo/CeO2, Mo/SnO2, Ce-Mo/Al2O3 samples undergo morphological and structural modifications above 700°C resulting in lattice defects, thus enhancing the mobility of metal ions and the possibility of interactions between them.
4. The introduction of cerium promotes aggregation of the Mo particles (particularly as a support) probably due to charge effects and/or to the strong basic property possessed by CeO2 so as to forming different molecular formulae. This led to the increase of polymerized surface Mo species besides the formation of coupled O=Mo=O bonds indicative of polymeric MoO3 as further emphasized by means of DRIFT and XRD.
5. Concerning the use of SnO2, the major point that should be outlined:
a) The characteristic of Mo-Sn system is the result of the preparation method adopted.
However, doping Mo/Al2O3 with SnO2 leads to surface structure definitely different from that of Mo/SnO2. Accordingly, when using SnO2 as promoter the MoO3 crystals completely disappeared and transformed into MoO2 with the presence of SnO whereas Mo/SnO2 was formed only by MoO3 and SnO2 oxides. Meanwhile, in both cases, no linkages were observed between Mo and Sn ions after calcination at 600°C and molybdate species strongly affect the growth of SnO2 crystals.
b) The thermal behaviour of Mo/SnO2 showed the dissolution of molybdena ions in the SnO2 crystals above 750°C with somewhat different extents, thereby resulting in more active sites and thus leading to a high catalytic activity of Mo/SnO2 catalyst.
6. The hydrogen reduction of the catalysts improves the surface reactivity generating oxygen vacancies and coordinatively unsaturated sites (CUS) leading to the presence of small amounts of metallic Mo after reduction at 700°C. Moreover, further reduction up to 800°C enhanced their activity towards CO adsorption and CH4 dissociation.
7. The peculiarities of in situ DRIFT studies of CO adsorption and CH4 transformation on the catalysts have been achieved under reaction conditions. Thus, the noticed gain in the intensity for the bands in conjunction with various types of carbonate species has been observed upon CO adsorption at 100°C and CH4 decomposition at 700°C.
This involves reduced catalysts containing coordinatively unsaturated sites (CUS) with hydroxyl groups and oxygen vacancies so as to forming various carbonate species implying the existence of the reactive lattice oxygen in the catalysts such oxygen could be present in Mo=O, Mo–O–Mo, Ce–O–Mo, Ce–O–Ce and Sn–O–Sn entities.
8. The Mo/CeO2 and Mo/SnO2 catalysts reduced at 800°C have the most likely active species for CO adsorption and CH4 dissociation. The highly dispersed MoO3 species besides Ce3+/Ce4+ and Sn2+/Sn4+ redox couples that have high capacity towards oxygen
8. The Mo/CeO2 and Mo/SnO2 catalysts reduced at 800°C have the most likely active species for CO adsorption and CH4 dissociation. The highly dispersed MoO3 species besides Ce3+/Ce4+ and Sn2+/Sn4+ redox couples that have high capacity towards oxygen