The effect of gold on the phase transitions of titania

Debeila, M.A.; Raphulu, Mpfuzeni C.; Mokoena, Emma M.; Авалос, M.; Petranovskii, Vitalii; Coville, Neil J.; Scurrell, Michael S.

doi:10.1016/j.msea.2004.12.047

Cited by 39 publications

(51 citation statements)

References 52 publications

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“…The suggestion has been made by others [28] that extensive work is needed with blank runs using the support alone, possibly in conjunction with other techniques. However, in practice, blank runs themselves may have a limited use, since it is clear that the presence of gold seriously disrupts the surface and the bulk properties of the support [29,30]. Sites present on the gold-free support may be greatly modified by the gold, and this may apply even at relatively large distances from the gold centres.…”

Section: Discussionmentioning

confidence: 99%

Investigation of the active site and the mode of Au/TiO2 catalyst deactivation using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

et al. 2010

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CO is a useful probe in the characterization of surface properties of both metal and metal oxide via adsorption. Adsorption of CO was used to monitor the possible active site of an Au/TiO 2 catalyst for the CO oxidation reaction. CO adsorption on the reduced catalyst results in the band at 2104 cm -1 indicative of Au 0 . During the reaction (in the presence of both CO and O 2 present) the band is shifted to higher wave numbers indicating non-competitive adsorption on the surface of Au species. This study also reveals the relationship between the presence of CO (in the absence of oxygen) and the build-up of surface species such as bicarbonates, formates and carbonate species which decreases the activity of the catalyst. The presence of both the reduced and the cationic species of Au seem to be requirement for the activity of the catalyst.

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Section: Discussionmentioning

confidence: 99%

Investigation of the active site and the mode of Au/TiO2 catalyst deactivation using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

et al. 2010

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“…This would explain the oxygen activation on the support in close proximity to gold particles (see mechanism of Fig. 15.2 ), and that the presence of gold alters the band gap of titania [117] and ceria [118] .…”

Section: Infl Uence Of the Gold Particles On The Oxide Support Propermentioning

confidence: 99%

“…Under the CO : O 2 gas feed, gold also facilitates the reduction of the Fe 2 O 3 support at 353 K [54] . The presence of gold retards phase transformation from anatase to rutile [114] .…”

Section: Infl Uence Of the Gold Particles On The Oxide Support Propermentioning

confidence: 99%

Gold Nanoparticles: Recent Advances in CO Oxidation

Louis

2007

Nanoparticles and Catalysis

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IntroductionThe oxidation of carbon monoxide at around room temperature is the most famous reaction known for gold catalysts. Haruta ' s group discovered in 1987 [1, 2] that gold is a unique catalyst for this reaction when gold metal particles are smaller than 5 nm and supported on oxides. Since then, extensive and intensive fundamental works have been published, and expanding new applications, from air purifi cation (gas masks, gas sensors, indoor air quality control) to hydrogen purifi cation for fuel cells (PROX, preferential selective oxidation of CO in the presence of H 2 ) have been developed.Gold is indeed active in carbon monoxide oxidation at a much lower temperature ( ≤ RT) than any platinum group metal [3] ( Fig. 15.1 ). The difference in reactivity can be due to the fact that Pt, Pd and Rh easily dissociate molecular oxygen at low temperature and bind strongly both atomic oxygen and CO. Tightly bound adsorbates have to overcome sizeable barriers to react, making the reaction rates signifi cant only at rather high temperatures [4] . On the contrary, on gold the reactants are loosely bound, but a higher binding energy of CO on gold nanoparticles than on bulk gold may provide suffi cient concentrations of CO on the surface for the reaction of oxidation to occur with negligible energy barriers. The main problem with this apparently simple reaction is that if CO is known to adsorb on low coordinated surface gold sites, the adsorption and activation of oxygen on gold particles is more puzzling (Section 15.4.3 ). The CO oxidation mechanism is not yet elucidated in spite of the huge number of papers published, and four main mechanisms have been suggested.High activity in CO oxidation when gold particles are smaller than 5 nm, and supported on reducible oxides, such as iron oxide or titania, led Haruta et al. [3, 5 -8] to propose the fi rst mechanism of CO oxidation (Fig. 15.2 ): the reaction takes place at the interface between the gold metal particle and the oxide support, i.e., between CO adsorbed on the gold particles and O 2 activated by the oxide support. Later, based on the assumption that metal cations could be located at the metalsupport interface, Bond and Thompson [9] proposed a mechanism rather similar, 475Nanoparticles and Catalysis. Edited by Didier Astruc

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“…In photovoltaic cells, TiO2 is sensitized with dyes to extend its photosensitivity to longer wavelengths. Alternatively, the light sensitivity of TiO 2 can be shifted into the visible region by narrowing the band gap [3,4]. This is generally done by doping TiO 2 with certain elements [5].…”

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confidence: 99%

Studies of the location of precious metals in nanocrystalline titanium dioxide using XRD and XANES

Rammutla

Savin

Matshaba

et al. 2007

Phys. Status Solidi (c)

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. 64.70.Nd, 81.16.Be Au, Ag, Pt and Pd doped TiO 2 nanocrystals were prepared using sol-gel routes. X-ray Near Edge Structure (XANES) was used to study the location of these precious metal dopants in nanocrystalline TiO 2 . The effects of these dopants on the phase transformation and grain growth were investigated using X-ray diffraction (XRD). Two dopant concentrations i.e. 1 and 5% were studied and similar results were found. Silver and Palladium initially form oxides but annealing at high temperatures converts them to metals. Gold and and silver do not affect the anatase -rutile transformation temperature whereas palladium and platinum do. 1 Introduction Titanium dioxide, TiO 2 , is a very important technological material which is currently being investigated for advanced functional and environmental applications, such as photocatalysis, photovoltaic cells and solid state gas sensing [1,2]. TiO 2 has a large band gap energy and due to this, it absorbs only the ultraviolet (UV) part of solar radiation leading to low conversion efficiency. Various approaches have been employed to extend the absorption edge of TiO2 from the UV into the visible region. In photovoltaic cells, TiO2 is sensitized with dyes to extend its photosensitivity to longer wavelengths. Alternatively, the light sensitivity of TiO 2 can be shifted into the visible region by narrowing the band gap [3,4]. This is generally done by doping TiO 2 with certain elements [5].In the nanocrystalline state, TiO 2 , like other oxides, can dissolve higher concentrations of impurities than its bulk counterparts as they have an increased density of grain boundaries where considerable amounts of dopants can segregate. This dopant segregation has been found in some cases to improve the catalytic activity and electrical properties. In previous computational work [6] we have studied the influence of the dopants (i.e. Pt, Pd, Ag, and Au) on the optical and structural properties of single crystalline anatase TiO 2 using the local spin density approximation (LSDA) within the density functional theory. The results show that doping TiO 2 with these precious metals enhances absorption in the visible range (i.e. above 400 nm) with Ag showing the most enhanced absorption. This offers the possibility of considerable improvement in photovotaic cells.Experimental information is essential for the verification and ultimate improvement of the simulations and EXAFS experiments are unique in providing this kind of information . For example, the EXAFS can determine the oxidation state of the dopant, the detailed local environment of the dopant (nature of the neighbours, bond distances to neighbours, degree of disorder, etc.) and segregation of the dopant.In this contribution we report the XANES and XRD studies of precious metal doped nano-TiO 2.

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The effect of gold on the phase transitions of titania

Cited by 39 publications

References 52 publications

Investigation of the active site and the mode of Au/TiO2 catalyst deactivation using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

Investigation of the active site and the mode of Au/TiO2 catalyst deactivation using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

Gold Nanoparticles: Recent Advances in CO Oxidation

Studies of the location of precious metals in nanocrystalline titanium dioxide using XRD and XANES

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