M. Farias | Universidad Nacional Autónoma de México (original) (raw)
Papers by M. Farias
Last years nanosized gold particles attract much attention as a component of industrially perspec... more Last years nanosized gold particles attract much attention as a component of industrially perspective catalysts for some reactions as CO oxidation, NO reduction etc. We studied several systems including gold based on different synthetic zeolites pure or doped with another metals (Fe, Na). It was shown by different techniques (XPS, UV-Vis spectroscopy, TPR) that intrinsic properties of zeolites used and
Surface Science, 2007
Electronic states of gold nanoparticles in mordenite and their transformations under redox treatm... more Electronic states of gold nanoparticles in mordenite and their transformations under redox treatments have been studied by the methods of FTIR spectroscopy of adsorbed CO and diffuse reflectance UV-visible spectroscopy. Different states of ionic and metallic gold were detected in the zeolite channels and on the external surface of the zeolite -Au + and Au 3+ ions, charged clusters Au dþ n , and neutral nanoparticles Au m . Catalytic tests of the samples revealed the existence of two types of active sites of gold in CO oxidation -gold clusters <2 nm (low-temperature activity) and gold nanoparticles (high temperature activity).
Fuel, 2013
ABSTRACT The influence of pretreatments with He, O2 and H2 on the physicochemical and catalytic p... more ABSTRACT The influence of pretreatments with He, O2 and H2 on the physicochemical and catalytic properties of gold supported on hexagonal mesoporous silica (HMS) has been studied by a variety of methods including: XPS, TPR, N2 sorption, UV–visible spectroscopy, FTIR and catalytic testing. The investigations show supported gold to form different states under redox treatments: Au3+ and Au+ ions, neutral and partly charged gold clusters and metal nanoparticles of different sizes. Reducing pretreatments increase the catalytic activity of the samples; while oxidative treatments deactivate the catalyst. Catalytic tests show several regions of the reduced samples with various catalytic behaviours: 20–200 °C, 200–400 °C and >400 °C; this situation is explained by the co-existence of gold active sites of different types. A comparative analysis among spectroscopic and catalytic data allows suggesting Aunδ+ clusters to be responsible for the activity in the low-temperature region (<200 °C); neutral Aun clusters are active in the range 200–400 °C; and gold nanoparticles may catalyze high-temperature CO oxidation. Au3+ ions are inactive in this process.
Chemical Physics, 2007
Au-zeolites with SiO 2 /Al 2 O 3 molar ratios 10 and 206 were prepared by ion exchange of the H-m... more Au-zeolites with SiO 2 /Al 2 O 3 molar ratios 10 and 206 were prepared by ion exchange of the H-mordenites with [Au(NH 3 ) 4 ](NO 3 ) 3 complex solution followed by hydrogen reduction or calcination at different temperatures. The samples were characterized by UV-visible DRS, XPS, XRD, TEM and TPR in order to understand the effect of zeolite acidity and pretreatment conditions on formation of different gold states. Three kinds of gold species were observed in the samples: (1) Au 3+ cations, (2) partly reduced species with the diameter less than 1 nm assigned to charged clusters Au dþ n (with n proposed 68), suggested to be stabilized inside zeolite channels, and (3) Au 0 nanoparticles with diameter varied within interval of 1.5-20 nm. The third type is located on the external surface of zeolite microcrystals. Au dþ n clusters were observed in as-prepared samples just after ion-exchange. Acid site strength did not change significantly amount of the clusters but influenced the electronic state of gold in these clusters and redox properties of gold cations. The contribution of the clusters was not changed significantly with temperature treatment even up to 500°C. Au 0 nanoparticles were formed by reduction of gold ions at temperatures starting from 50 to 200°C depending on the medium of treatment and the molar ratio of the zeolite. The obtained results are important for understanding the effect of multiplicity of catalytically active gold species in CO oxidation.
Catalytic tests of the Au-zeolites pure or doped with Na revealed the existence of two types of a... more Catalytic tests of the Au-zeolites pure or doped with Na revealed the existence of two types of active sites of gold in CO oxidationpartly charged gold clusters Au dþ n (low-temperature activity) and gold nanoparticles Au 0 m (high-temperature activity). The data obtained demonstrates clear direct dependence of gold species reactivity on zeolite acid properties (Si/Al molar ratio and Na addition). Na modified Au-zeolites with different Si/Al molar ratio were found to be activated in CO oxidation in different degrees due to diverse redox state of supported gold species. The revealed effect of activation of one of two active species in CO oxidation ðAu dþ n Þ is promising for improvement of activity and stability of the gold-zeolites catalysts.
Last years nanosized gold particles attract much attention as a component of industrially perspec... more Last years nanosized gold particles attract much attention as a component of industrially perspective catalysts for some reactions as CO oxidation, NO reduction etc. We studied several systems including gold based on different synthetic zeolites pure or doped with another metals (Fe, Na). It was shown by different techniques (XPS, UV-Vis spectroscopy, TPR) that intrinsic properties of zeolites used and
Surface Science, 2007
Electronic states of gold nanoparticles in mordenite and their transformations under redox treatm... more Electronic states of gold nanoparticles in mordenite and their transformations under redox treatments have been studied by the methods of FTIR spectroscopy of adsorbed CO and diffuse reflectance UV-visible spectroscopy. Different states of ionic and metallic gold were detected in the zeolite channels and on the external surface of the zeolite -Au + and Au 3+ ions, charged clusters Au dþ n , and neutral nanoparticles Au m . Catalytic tests of the samples revealed the existence of two types of active sites of gold in CO oxidation -gold clusters <2 nm (low-temperature activity) and gold nanoparticles (high temperature activity).
Fuel, 2013
ABSTRACT The influence of pretreatments with He, O2 and H2 on the physicochemical and catalytic p... more ABSTRACT The influence of pretreatments with He, O2 and H2 on the physicochemical and catalytic properties of gold supported on hexagonal mesoporous silica (HMS) has been studied by a variety of methods including: XPS, TPR, N2 sorption, UV–visible spectroscopy, FTIR and catalytic testing. The investigations show supported gold to form different states under redox treatments: Au3+ and Au+ ions, neutral and partly charged gold clusters and metal nanoparticles of different sizes. Reducing pretreatments increase the catalytic activity of the samples; while oxidative treatments deactivate the catalyst. Catalytic tests show several regions of the reduced samples with various catalytic behaviours: 20–200 °C, 200–400 °C and >400 °C; this situation is explained by the co-existence of gold active sites of different types. A comparative analysis among spectroscopic and catalytic data allows suggesting Aunδ+ clusters to be responsible for the activity in the low-temperature region (<200 °C); neutral Aun clusters are active in the range 200–400 °C; and gold nanoparticles may catalyze high-temperature CO oxidation. Au3+ ions are inactive in this process.
Chemical Physics, 2007
Au-zeolites with SiO 2 /Al 2 O 3 molar ratios 10 and 206 were prepared by ion exchange of the H-m... more Au-zeolites with SiO 2 /Al 2 O 3 molar ratios 10 and 206 were prepared by ion exchange of the H-mordenites with [Au(NH 3 ) 4 ](NO 3 ) 3 complex solution followed by hydrogen reduction or calcination at different temperatures. The samples were characterized by UV-visible DRS, XPS, XRD, TEM and TPR in order to understand the effect of zeolite acidity and pretreatment conditions on formation of different gold states. Three kinds of gold species were observed in the samples: (1) Au 3+ cations, (2) partly reduced species with the diameter less than 1 nm assigned to charged clusters Au dþ n (with n proposed 68), suggested to be stabilized inside zeolite channels, and (3) Au 0 nanoparticles with diameter varied within interval of 1.5-20 nm. The third type is located on the external surface of zeolite microcrystals. Au dþ n clusters were observed in as-prepared samples just after ion-exchange. Acid site strength did not change significantly amount of the clusters but influenced the electronic state of gold in these clusters and redox properties of gold cations. The contribution of the clusters was not changed significantly with temperature treatment even up to 500°C. Au 0 nanoparticles were formed by reduction of gold ions at temperatures starting from 50 to 200°C depending on the medium of treatment and the molar ratio of the zeolite. The obtained results are important for understanding the effect of multiplicity of catalytically active gold species in CO oxidation.
Catalytic tests of the Au-zeolites pure or doped with Na revealed the existence of two types of a... more Catalytic tests of the Au-zeolites pure or doped with Na revealed the existence of two types of active sites of gold in CO oxidationpartly charged gold clusters Au dþ n (low-temperature activity) and gold nanoparticles Au 0 m (high-temperature activity). The data obtained demonstrates clear direct dependence of gold species reactivity on zeolite acid properties (Si/Al molar ratio and Na addition). Na modified Au-zeolites with different Si/Al molar ratio were found to be activated in CO oxidation in different degrees due to diverse redox state of supported gold species. The revealed effect of activation of one of two active species in CO oxidation ðAu dþ n Þ is promising for improvement of activity and stability of the gold-zeolites catalysts.