Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration

博士 === 國立中正大學 === 化學工程所 === 94 === We report in this work a study on the structural properties of Pt surface species on activated-carbon support of Pt/C catalyst during preparation, deactivation and regeneration processes. Pt(NH3)4(NO3)2 was used as a precursor to prepare Pt subnano-particles on act...

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Main Authors: Cheng-Chieh Shih, 施呈杰
Other Authors: Jen -Ray Chang
Format: Others
Language:zh-TW
Published: 2006
Online Access:http://ndltd.ncl.edu.tw/handle/86845113189945306561
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description 博士 === 國立中正大學 === 化學工程所 === 94 === We report in this work a study on the structural properties of Pt surface species on activated-carbon support of Pt/C catalyst during preparation, deactivation and regeneration processes. Pt(NH3)4(NO3)2 was used as a precursor to prepare Pt subnano-particles on activated-carbon (Pt/C). The conversion of Pt(NH3)4(NO3)2 to Pt clusters and the decomposition of oxygen-containing functional groups of the activated-carbon were examined by use of temperature-programmed reduction (TPR) and Fourier transform infrared (FT-IR) spectroscopy . The results showed that the formation of Pt particles and the decomposition of the functional groups started at 140 °C and 450 °C, respectively. The species formed on carbon surface were characterized by X-ray absorption spectroscopy (XAS). Extended X-ray absorption fine structure (EXAFS) spectroscopy results indicated that the sample reduced at 200 °C in hydrogen had an average Pt particle size of about 1.1 nm (average Pt-Pt coordination number of 5.5) and average Pt-Pt bond distance of 0.276 nm. As reduction temperature was increased to 450 °C, there was no significant change in the average bond distance of Pt-Pt whereas the Pt particles grew to about 1.5 nm (with an average Pt-Pt coordination number of 6.7). Treatments before hydrogen reduction, air calcination and nitrogen treatment influenced the structure of Pt/C significantly. In flowing air at 300 °C, PtOx were formed on the surface concomitantly with the oxidative decomposition of Pt(NH3)4(NO3)2. Subsequent hydrogen reduction of the PtOx at 450 °C formed Pt particles of about 0.7 nm (average Pt-Pt coordination number of 3.7). In contrast, in the flowing nitrogen at elevated temperatures, Pt surface species aggregated concomitantly with the removal of water and the decomposition of Pt(NH3)4(NO3)2. This aggregation decreased Pt dispersion. The Pt particle sizes after 450°C hydrogen reduction increased from approximately 1.0 nm to about 2.3 nm as the nitrogen treatment temperatures were elevated from 450 °C to 600 °C. The effect of treatment temperature on particle size was further confirmed by TEM and CO chemisorption. The nature of metal-support interface varies with Pt particle size, which is, in turn, influenced by the treatment conditions. Pt/C of smaller Pt particle size has more Pt atoms in contact with oxygen-containing functional groups. Moreover, based on FT-IR and XAS results, we infer that Pt particles bonded to oxygen-containing functional group have a slightly lower electron density than those on carbon. Wet air oxidation (WAO) process is effective in converting organic pollutants in wastewater to innocuous carbon dioxide. Supported Pt catalysts, particularly Pt/C are effective but unstable. Now a novel catalyst, Pt on TiO2 grafted carbon (Pt/TiO2-C) has been developed for WAO. The catalyst is characterized by high activity and stability in WAO process. By use of transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS) and Fourier transform infrared (FT-IR) spectroscopy, the fresh and aged Pt/C catalysts were characterized and the mechanism of catalyst deactivation was elucidated. The average Pt cluster size increased from about 2.0 to 10 nm after WAO of methanol in water at 220 °C for 200 hrs. These results indicated that Pt/C catalyst was deactivated due to migration and aggregation of the Pt clusters into large Pt particles. The increase in the contents of oxygen-containing groups and carbonate species as characterized by FT-IR spectra in the aged catalyst further suggested that the oxidation on carbon surface decreases the affinity between Pt and carbon support, leading to extensive Pt aggregation. Thus, TiO2 was grafted on to the carbon support to anchor and keep Pt from migration and agglomeration, and obtain a stable Pt/TiO2-C catalyst. The efficacy of TiO2 grafted on activated carbon in stabilizing the Pt/C catalyst was demonstrated in the long-term catalytic performance tests. The evidences of the TiO2 in anchoring and maintaining the Pt clusters in small ensembles were further demonstrated by characterizing the Pt-TiO2 interactions and the morphology of Pt clusters using EXAFS. The metal dispersion for regeneration of Pt on activated carbon has been studied as a function of regeneration conditions. The changes in metal dispersion were followed by CO chemisorption measurements. After treatment for 2 hr, optimum conditions for good dispersion were achieved with oxychlorination at 300℃ and involved mobile platinum complexs in the dispersion process. The surface species responsible for redispersion were identified by EXAFS and TPR. The results showed that redispersion of platinum was associated with the formation of a [Pt(IV)Clx] surface complex. This apparently occurs through a surface transport path, while redispersion in mobile platinum chlorides can occur only by vaper transport.
author2 Jen -Ray Chang
author_facet Jen -Ray Chang
Cheng-Chieh Shih
施呈杰
author Cheng-Chieh Shih
施呈杰
spellingShingle Cheng-Chieh Shih
施呈杰
Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration
author_sort Cheng-Chieh Shih
title Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration
title_short Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration
title_full Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration
title_fullStr Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration
title_full_unstemmed Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration
title_sort structural investigation of pt/c catalysts : preparation, deactivation, and regeneration
publishDate 2006
url http://ndltd.ncl.edu.tw/handle/86845113189945306561
work_keys_str_mv AT chengchiehshih structuralinvestigationofptccatalystspreparationdeactivationandregeneration
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AT chengchiehshih ptcchùméijiégòuyánjiūchùméizhìbèilǎohuàjízàishēng
AT shīchéngjié ptcchùméijiégòuyánjiūchùméizhìbèilǎohuàjízàishēng
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spelling ndltd-TW-094CCU050630072015-10-13T10:45:05Z http://ndltd.ncl.edu.tw/handle/86845113189945306561 Structural Investigation of Pt/C Catalysts : Preparation, Deactivation, and Regeneration Pt/C觸媒結構研究-觸媒製備、老化及再生 Cheng-Chieh Shih 施呈杰 博士 國立中正大學 化學工程所 94 We report in this work a study on the structural properties of Pt surface species on activated-carbon support of Pt/C catalyst during preparation, deactivation and regeneration processes. Pt(NH3)4(NO3)2 was used as a precursor to prepare Pt subnano-particles on activated-carbon (Pt/C). The conversion of Pt(NH3)4(NO3)2 to Pt clusters and the decomposition of oxygen-containing functional groups of the activated-carbon were examined by use of temperature-programmed reduction (TPR) and Fourier transform infrared (FT-IR) spectroscopy . The results showed that the formation of Pt particles and the decomposition of the functional groups started at 140 °C and 450 °C, respectively. The species formed on carbon surface were characterized by X-ray absorption spectroscopy (XAS). Extended X-ray absorption fine structure (EXAFS) spectroscopy results indicated that the sample reduced at 200 °C in hydrogen had an average Pt particle size of about 1.1 nm (average Pt-Pt coordination number of 5.5) and average Pt-Pt bond distance of 0.276 nm. As reduction temperature was increased to 450 °C, there was no significant change in the average bond distance of Pt-Pt whereas the Pt particles grew to about 1.5 nm (with an average Pt-Pt coordination number of 6.7). Treatments before hydrogen reduction, air calcination and nitrogen treatment influenced the structure of Pt/C significantly. In flowing air at 300 °C, PtOx were formed on the surface concomitantly with the oxidative decomposition of Pt(NH3)4(NO3)2. Subsequent hydrogen reduction of the PtOx at 450 °C formed Pt particles of about 0.7 nm (average Pt-Pt coordination number of 3.7). In contrast, in the flowing nitrogen at elevated temperatures, Pt surface species aggregated concomitantly with the removal of water and the decomposition of Pt(NH3)4(NO3)2. This aggregation decreased Pt dispersion. The Pt particle sizes after 450°C hydrogen reduction increased from approximately 1.0 nm to about 2.3 nm as the nitrogen treatment temperatures were elevated from 450 °C to 600 °C. The effect of treatment temperature on particle size was further confirmed by TEM and CO chemisorption. The nature of metal-support interface varies with Pt particle size, which is, in turn, influenced by the treatment conditions. Pt/C of smaller Pt particle size has more Pt atoms in contact with oxygen-containing functional groups. Moreover, based on FT-IR and XAS results, we infer that Pt particles bonded to oxygen-containing functional group have a slightly lower electron density than those on carbon. Wet air oxidation (WAO) process is effective in converting organic pollutants in wastewater to innocuous carbon dioxide. Supported Pt catalysts, particularly Pt/C are effective but unstable. Now a novel catalyst, Pt on TiO2 grafted carbon (Pt/TiO2-C) has been developed for WAO. The catalyst is characterized by high activity and stability in WAO process. By use of transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS) and Fourier transform infrared (FT-IR) spectroscopy, the fresh and aged Pt/C catalysts were characterized and the mechanism of catalyst deactivation was elucidated. The average Pt cluster size increased from about 2.0 to 10 nm after WAO of methanol in water at 220 °C for 200 hrs. These results indicated that Pt/C catalyst was deactivated due to migration and aggregation of the Pt clusters into large Pt particles. The increase in the contents of oxygen-containing groups and carbonate species as characterized by FT-IR spectra in the aged catalyst further suggested that the oxidation on carbon surface decreases the affinity between Pt and carbon support, leading to extensive Pt aggregation. Thus, TiO2 was grafted on to the carbon support to anchor and keep Pt from migration and agglomeration, and obtain a stable Pt/TiO2-C catalyst. The efficacy of TiO2 grafted on activated carbon in stabilizing the Pt/C catalyst was demonstrated in the long-term catalytic performance tests. The evidences of the TiO2 in anchoring and maintaining the Pt clusters in small ensembles were further demonstrated by characterizing the Pt-TiO2 interactions and the morphology of Pt clusters using EXAFS. The metal dispersion for regeneration of Pt on activated carbon has been studied as a function of regeneration conditions. The changes in metal dispersion were followed by CO chemisorption measurements. After treatment for 2 hr, optimum conditions for good dispersion were achieved with oxychlorination at 300℃ and involved mobile platinum complexs in the dispersion process. The surface species responsible for redispersion were identified by EXAFS and TPR. The results showed that redispersion of platinum was associated with the formation of a [Pt(IV)Clx] surface complex. This apparently occurs through a surface transport path, while redispersion in mobile platinum chlorides can occur only by vaper transport. Jen -Ray Chang 張仁瑞 2006 學位論文 ; thesis 150 zh-TW