Low-Temperature Water-Gas Shift Reaction over Au Supported on Anatase in the Presence of Copper: EXAFS/XANES Analysis of Gold-Copper Ion Mixtures on TiO2

T. Magadzu, J. H. Yang, J. D. Henao, M. C. Kung, Harold H Kung, M. S. Scurrell

Research output: Contribution to journalArticle

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Abstract

Cu-modified Au/TiO2 (anatase, 200 m2/g) has been prepared by the incipient-wetness technique with introduction of the modifier either before or after Au loading. Such catalysts gave higher catalytic activities for the water-gas shift (WGS) reaction when compared to the unmodified catalysts mainly because of the existence of a synergetic interaction between Cu and Au, as the activities of both Cu/TiO2 and Au/TiO2 are lower than that of the bimetallic system. The WGS activities of both the as-prepared Au/TiO2 and a Au-Cuc/TiO2 catalyst were found to be high and stable. The presence of nitrates on Cuc-Au/TiO2 was found to be detrimental to the activity of Au on TiO2, as a result of the poisoning of Au and enhanced Au agglomeration by NO2 formed during reaction. The activities of Au/TiO2 catalysts modified with Cu-containing acetate counterions were found to decrease during the first 30 min on stream, reaching a constant value of (45 ± 2)%. However, when the poisoning by the acetate anion was eliminated through calcination, the activity increased compared to those of the Au/TiO2 catalysts. An increase in Cu loading was found to lower the activity of Au as a result of the loss of the interaction between the Au and the support interface. The XANES spectra of uncalcined as-prepared catalysts exhibited a pre-edge feature (due to the 1s-to-3d transition) characteristic of Au in a tetrahedral environment. The water-gas-shift-treated sample showed a reduction in the intensity of the white line, indicating a reduction of Au (a partial reduction of Au3+ to Au+/Au0). The XANES spectrum of calcined Cuc-Au/TiO2 (0.44 wt % Cu/3.74 wt % Au) confirmed that Cu exists as ions (Cu+/Cu2+) before and during the WGS reaction. The Au catalyst was not stable and continuously deactivated with time, and the white-line region showed a decrease in peak intensity during use. An increase in gold activity on Cu2+-modified TiO2 was achieved because at least 90% of the gold was reduced to the zero-valent state whereas most of the stabilizing metal (Cu2+) remained in the ionic state. We concluded that the addition of a stabilizing metal in ionic form during the preparation of the Au-Cuc/TiO2 system has marked effects in improving the initial dispersion of the reduced gold and also in helping to maintain a highly dispersed state of gold during use. The atomic ratio of stabilizing metal ions to gold metal plays a crucial role in maximizing the dispersion/stabilizing effects. The formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects can be interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper because of their competion for ion-exchange sites. It is unclear at present how this is achieved. However, EXAFS data analysis of both metals suggests that it is not due to a direct interaction between Au and Cu. In contrast to the results for the WGS reaction, copper was found to show no promotional effect on low-temperature CO oxidation or on preferential CO oxidation (PROX) in excess hydrogen.

Original languageEnglish
Pages (from-to)8812-8823
Number of pages12
JournalJournal of Physical Chemistry C
Volume121
Issue number16
DOIs
Publication statusPublished - Apr 27 2017

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Water gas shift
anatase
Gold
Titanium dioxide
Copper
Ions
gold
catalysts
copper
Catalysts
shift
gases
water
Metals
ions
poisoning
Temperature
Carbon Monoxide
metals
acetates

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Surfaces, Coatings and Films
  • Physical and Theoretical Chemistry

Cite this

Low-Temperature Water-Gas Shift Reaction over Au Supported on Anatase in the Presence of Copper : EXAFS/XANES Analysis of Gold-Copper Ion Mixtures on TiO2. / Magadzu, T.; Yang, J. H.; Henao, J. D.; Kung, M. C.; Kung, Harold H; Scurrell, M. S.

In: Journal of Physical Chemistry C, Vol. 121, No. 16, 27.04.2017, p. 8812-8823.

Research output: Contribution to journalArticle

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abstract = "Cu-modified Au/TiO2 (anatase, 200 m2/g) has been prepared by the incipient-wetness technique with introduction of the modifier either before or after Au loading. Such catalysts gave higher catalytic activities for the water-gas shift (WGS) reaction when compared to the unmodified catalysts mainly because of the existence of a synergetic interaction between Cu and Au, as the activities of both Cu/TiO2 and Au/TiO2 are lower than that of the bimetallic system. The WGS activities of both the as-prepared Au/TiO2 and a Au-Cuc/TiO2 catalyst were found to be high and stable. The presence of nitrates on Cuc-Au/TiO2 was found to be detrimental to the activity of Au on TiO2, as a result of the poisoning of Au and enhanced Au agglomeration by NO2 formed during reaction. The activities of Au/TiO2 catalysts modified with Cu-containing acetate counterions were found to decrease during the first 30 min on stream, reaching a constant value of (45 ± 2){\%}. However, when the poisoning by the acetate anion was eliminated through calcination, the activity increased compared to those of the Au/TiO2 catalysts. An increase in Cu loading was found to lower the activity of Au as a result of the loss of the interaction between the Au and the support interface. The XANES spectra of uncalcined as-prepared catalysts exhibited a pre-edge feature (due to the 1s-to-3d transition) characteristic of Au in a tetrahedral environment. The water-gas-shift-treated sample showed a reduction in the intensity of the white line, indicating a reduction of Au (a partial reduction of Au3+ to Au+/Au0). The XANES spectrum of calcined Cuc-Au/TiO2 (0.44 wt {\%} Cu/3.74 wt {\%} Au) confirmed that Cu exists as ions (Cu+/Cu2+) before and during the WGS reaction. The Au catalyst was not stable and continuously deactivated with time, and the white-line region showed a decrease in peak intensity during use. An increase in gold activity on Cu2+-modified TiO2 was achieved because at least 90{\%} of the gold was reduced to the zero-valent state whereas most of the stabilizing metal (Cu2+) remained in the ionic state. We concluded that the addition of a stabilizing metal in ionic form during the preparation of the Au-Cuc/TiO2 system has marked effects in improving the initial dispersion of the reduced gold and also in helping to maintain a highly dispersed state of gold during use. The atomic ratio of stabilizing metal ions to gold metal plays a crucial role in maximizing the dispersion/stabilizing effects. The formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects can be interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper because of their competion for ion-exchange sites. It is unclear at present how this is achieved. However, EXAFS data analysis of both metals suggests that it is not due to a direct interaction between Au and Cu. In contrast to the results for the WGS reaction, copper was found to show no promotional effect on low-temperature CO oxidation or on preferential CO oxidation (PROX) in excess hydrogen.",
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T1 - Low-Temperature Water-Gas Shift Reaction over Au Supported on Anatase in the Presence of Copper

T2 - EXAFS/XANES Analysis of Gold-Copper Ion Mixtures on TiO2

AU - Magadzu, T.

AU - Yang, J. H.

AU - Henao, J. D.

AU - Kung, M. C.

AU - Kung, Harold H

AU - Scurrell, M. S.

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N2 - Cu-modified Au/TiO2 (anatase, 200 m2/g) has been prepared by the incipient-wetness technique with introduction of the modifier either before or after Au loading. Such catalysts gave higher catalytic activities for the water-gas shift (WGS) reaction when compared to the unmodified catalysts mainly because of the existence of a synergetic interaction between Cu and Au, as the activities of both Cu/TiO2 and Au/TiO2 are lower than that of the bimetallic system. The WGS activities of both the as-prepared Au/TiO2 and a Au-Cuc/TiO2 catalyst were found to be high and stable. The presence of nitrates on Cuc-Au/TiO2 was found to be detrimental to the activity of Au on TiO2, as a result of the poisoning of Au and enhanced Au agglomeration by NO2 formed during reaction. The activities of Au/TiO2 catalysts modified with Cu-containing acetate counterions were found to decrease during the first 30 min on stream, reaching a constant value of (45 ± 2)%. However, when the poisoning by the acetate anion was eliminated through calcination, the activity increased compared to those of the Au/TiO2 catalysts. An increase in Cu loading was found to lower the activity of Au as a result of the loss of the interaction between the Au and the support interface. The XANES spectra of uncalcined as-prepared catalysts exhibited a pre-edge feature (due to the 1s-to-3d transition) characteristic of Au in a tetrahedral environment. The water-gas-shift-treated sample showed a reduction in the intensity of the white line, indicating a reduction of Au (a partial reduction of Au3+ to Au+/Au0). The XANES spectrum of calcined Cuc-Au/TiO2 (0.44 wt % Cu/3.74 wt % Au) confirmed that Cu exists as ions (Cu+/Cu2+) before and during the WGS reaction. The Au catalyst was not stable and continuously deactivated with time, and the white-line region showed a decrease in peak intensity during use. An increase in gold activity on Cu2+-modified TiO2 was achieved because at least 90% of the gold was reduced to the zero-valent state whereas most of the stabilizing metal (Cu2+) remained in the ionic state. We concluded that the addition of a stabilizing metal in ionic form during the preparation of the Au-Cuc/TiO2 system has marked effects in improving the initial dispersion of the reduced gold and also in helping to maintain a highly dispersed state of gold during use. The atomic ratio of stabilizing metal ions to gold metal plays a crucial role in maximizing the dispersion/stabilizing effects. The formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects can be interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper because of their competion for ion-exchange sites. It is unclear at present how this is achieved. However, EXAFS data analysis of both metals suggests that it is not due to a direct interaction between Au and Cu. In contrast to the results for the WGS reaction, copper was found to show no promotional effect on low-temperature CO oxidation or on preferential CO oxidation (PROX) in excess hydrogen.

AB - Cu-modified Au/TiO2 (anatase, 200 m2/g) has been prepared by the incipient-wetness technique with introduction of the modifier either before or after Au loading. Such catalysts gave higher catalytic activities for the water-gas shift (WGS) reaction when compared to the unmodified catalysts mainly because of the existence of a synergetic interaction between Cu and Au, as the activities of both Cu/TiO2 and Au/TiO2 are lower than that of the bimetallic system. The WGS activities of both the as-prepared Au/TiO2 and a Au-Cuc/TiO2 catalyst were found to be high and stable. The presence of nitrates on Cuc-Au/TiO2 was found to be detrimental to the activity of Au on TiO2, as a result of the poisoning of Au and enhanced Au agglomeration by NO2 formed during reaction. The activities of Au/TiO2 catalysts modified with Cu-containing acetate counterions were found to decrease during the first 30 min on stream, reaching a constant value of (45 ± 2)%. However, when the poisoning by the acetate anion was eliminated through calcination, the activity increased compared to those of the Au/TiO2 catalysts. An increase in Cu loading was found to lower the activity of Au as a result of the loss of the interaction between the Au and the support interface. The XANES spectra of uncalcined as-prepared catalysts exhibited a pre-edge feature (due to the 1s-to-3d transition) characteristic of Au in a tetrahedral environment. The water-gas-shift-treated sample showed a reduction in the intensity of the white line, indicating a reduction of Au (a partial reduction of Au3+ to Au+/Au0). The XANES spectrum of calcined Cuc-Au/TiO2 (0.44 wt % Cu/3.74 wt % Au) confirmed that Cu exists as ions (Cu+/Cu2+) before and during the WGS reaction. The Au catalyst was not stable and continuously deactivated with time, and the white-line region showed a decrease in peak intensity during use. An increase in gold activity on Cu2+-modified TiO2 was achieved because at least 90% of the gold was reduced to the zero-valent state whereas most of the stabilizing metal (Cu2+) remained in the ionic state. We concluded that the addition of a stabilizing metal in ionic form during the preparation of the Au-Cuc/TiO2 system has marked effects in improving the initial dispersion of the reduced gold and also in helping to maintain a highly dispersed state of gold during use. The atomic ratio of stabilizing metal ions to gold metal plays a crucial role in maximizing the dispersion/stabilizing effects. The formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects can be interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper because of their competion for ion-exchange sites. It is unclear at present how this is achieved. However, EXAFS data analysis of both metals suggests that it is not due to a direct interaction between Au and Cu. In contrast to the results for the WGS reaction, copper was found to show no promotional effect on low-temperature CO oxidation or on preferential CO oxidation (PROX) in excess hydrogen.

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