Elsevier

Catalysis Communications

Volume 11, Issue 15, 25 September 2010, Pages 1238-1243
Catalysis Communications

The effect of phosphorous precursor on the CO oxidation activity of P-modified TiO2 supported Ag catalysts

https://doi.org/10.1016/j.catcom.2010.06.014Get rights and content

Abstract

Nanocrystalline TiO2 and P-modified TiO2 with P/Ti atomic ratio 0.01 were prepared by the solvothermal method and employed as the supports for Ag/TiO2 catalysts for CO oxidation reaction. The incorporation of phosphorus into the TiO2 lattice in the form of Ti–O–P resulted in an increase of both surface area and metal dispersion. The P-modified TiO2 supported Ag catalysts using phosphorus precursor in the form of oxide promoted the weak adsorbed oxygen species and resulted in catalytic activity improvement in CO oxidation. However, the use of phosphorous precursor in the form of phosphate such as H3PO4, (NH4)2HPO4, (C2H5)3PO4 could result in the strongly adsorbed oxygen species and/or the bidentate of phosphate species blocking the active sites instead.

Graphical abstract

Incorporation of phosphorus into the TiO2 lattice in the form of Ti-O-P resulted in an increase of both surface area and metal dispersion. The P-modified TiO2 supported Ag catalysts using phosphorus precursor in the form of oxide promoted the weak adsorbed oxygen species and resulted in catalytic activity improvement in CO oxidation.

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Research highlights

  • The incorporation of phosphorus into the TiO2 lattice in the form of Ti–O–P resulted in an increase of both surface area and metal dispersion.

  • The P-modified TiO2 supported Ag catalysts using phosphorus precursor in the form of oxide promoted the weak adsorbed oxygen species and resulted in catalytic activity improvement in CO oxidation.

  • The use of phosphorous precursor in the form of phosphate could result in the strongly adsorbed oxygen species and/or the bidentate of phosphate species blocking the active sites instead.

Introduction

Titanium dioxide (TiO2) is a reducible metal oxide that has been widely used as a support for noble metal catalysts in CO oxidation because it exhibits a stronger interaction with group VIII noble metals than other metal oxides [1], [2], [3], [4], [5], [6]. The strong metal–support interaction has been shown to play an important role in increasing the metal dispersion, which leads to enhancing the catalytic activity for CO oxidation. Additionally, the promoting effect of the reducible support may result from the creation of second active sites at the metal–support interface [7].

The addition of trace element to TiO2 support is a simple way to modify its physicochemical properties such as crystallite size, crystal defects, surface area, thermal stability, and the interaction between metal and support. Support modification has been shown to result in an improved CO oxidation activity in many catalyst systems. For examples, Yu et al. [8] reported that doping of La in TiO2 during the sol–gel synthesis induced the creation of the second active site on the surface of Au/TiO2 which promoted the fast CO oxidation. Peza-Ledesma et al. [9] reported that the use of SBA-15 modified with 10 wt.% of TiO2 as support materials led to a high dispersion of supported gold catalysts which promoted the catalytic activity toward CO oxidation. Similarly, Hernandez et al. [10] found that the well dispersion of gold catalyst on the cerium-modified silica support provided the formation of small gold particle and the coverage of cerium on the silica support promoted the effectiveness of oxygen mobility, which led to higher catalytic activity in CO oxidation.

Despite a number of studies reporting the support modification effect in CO oxidation activity, the modification of support with a non-metallic modifier has received little attention as compared to the metallic ones. Most of the non-metallic modifier reported in the literature was in the form of anionic species such as nitrate ion [11], [12], sulfate ion [13], [14], and phosphate ion [15]. Both positive and negative effects of the non-metallic modifier on CO oxidation activities have been found. The negative effect of sulfate ion was reported by Ruth et al. [13] in which the Au/TiO2 catalyst was deactivated by the blocking of SO2 at the interface between an Au particle and the TiO2 support. Kim and Woo [14] reported that SO2 treatment increased the adsorption strength between Au and CO which suppressed the migration of absorbed CO on the Au particles to Au–TiO2 interface to form CO2, resulting in a decrease of CO oxidation activity. On the other hand, modification with phosphate ion showed a positive effect on CO oxidation activity. Incorporation of phosphate ion in TiO2 support could prevent sintering of Au particles at high temperature treatment, as a consequence higher CO oxidation activity was obtained [12], [15]. However, an over-loading of phosphate ions may block the active sites instead.

In the present study, the effect of phosphorus precursors on the P-modified TiO2 supported Ag catalysts has been extensively investigated. The catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FT-IR), temperature programmed desorption of oxygen (O2-TPD), and pulse chemisorption. The catalyst activities were evaluated in the CO oxidation in a fixed-bed reactor.

Section snippets

Preparation of TiO2 and P-modified TiO2

The TiO2 nanoparticles were prepared by the solvothermal method according the procedure described in our previous work in Ref [16]. Typically, about 25 g of Titanium (IV) n-butoxide (TNB) was mixed with 100 cm3 of 1,4-butanediol in a test tube. The mixed solution in the test tube was placed in a 300 cm3 autoclave. An addition of 30 cm3 of 1,4-butanediol was added to the gap between the test tube and the autoclave wall. After purging with nitrogen into the autoclave reactor, the system was heated to

Effect of P doping on the physicochemical properties of Ag/TiO2 catalysts

The XRD patterns of the unmodified TiO2 and P-modified TiO2 supported Ag catalysts prepared with different precursors of phosphorus modifier are shown in Fig. 1. The major peak of pure anatase (101) phase TiO2 was observed at 2θ around 25° for all the catalyst samples. Doping of TiO2 with P led to broadening of the XRD peaks as well as a slight shift of the XRD reflections towards higher angles. According to the Bragg's law, an increase of 2θ values indicated a decrease of the distance between

Conclusions

Modification of the TiO2 supports with different phosphorus precursors altered the catalytic behaviors of Ag/TiO2 catalysts in the CO oxidation. The insertion of phosphorus into the TiO2 lattice in the form of Ti–O–P not only increased the metal active sites by increasing the specific surface area of the catalyst and inhibiting the agglomeration of TiO2 crystallites but also altered the strength of O2 adsorption−desorption behavior on the catalyst surface. The P-modified TiO2 supported Ag

Acknowledgments

The financial support from the Thailand Research Fund (TRF) and the Office of Higher Education Commission (CHE), Ministry of Education, Thailand are gratefully acknowledged.

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