Elsevier

Applied Catalysis A: General

Volume 469, 17 January 2014, Pages 472-482
Applied Catalysis A: General

Effect of γ-Al2O3 hydrothermal treatment on the formation and properties of platinum sites in Pt/γ-Al2O3 catalysts

https://doi.org/10.1016/j.apcata.2013.10.027Get rights and content

Highlights

  • Increase the fraction of bridging OH groups and concentration of Lewis acid sites.

  • Preferential formation of outer-sphere complexes.

  • Changes in the dispersion and electronic state of supported Pt.

  • Effect of hydrothermal treatment of the support on catalytic properties of Pt/Al2O3.

Abstract

The effect of hydrothermal treatment (HTT) of γ-alumina on the state of its surface functional cover, anchoring of hexachloroplatinate, and properties of platinum sites was investigated. Hydrothermal treatment of γ-Al2O3 was found to increase the fraction of surface bridging OH groups and concentration of Lewis acid sites (LAS) and alter the metal complex–support interaction. Adsorption of hexachloroplatinate on the support surface is accompanied mainly by the formation of outer-sphere complexes, which are characterized by a lower reduction temperature. As a result, dispersion and electronic state of supported platinum are changed. Influence of γ-Al2O3 hydrothermal treatment on the catalytic properties of Pt/Al2O3 in n-hexane transformations and propane dehydrogenation was demonstrated. The revealed effect of γ-Al2O3 hydrothermal treatment on the formation of platinum sites in Pt/Al2O3 catalysts can be of fundamental importance for the steps of supported system synthesis where solid phase is contacting with water and aqueous solutions (impregnation and drying).

Introduction

A unique combination of the surface acid-base properties and pore structure makes γ-alumina a convenient support for metal catalysts, in particular Pt/γ-Al2O3 [1], [2]. Platinum-on-alumina composites are widely used in gasoline reforming and isomerization of normal alkanes C5–C6 [1], [3]. The preparation procedure of Pt/γ-Al2O3 catalytic systems includes several steps: deposition of active metal usually from an aqueous solution of H2[PtCl6], drying, and thermoactivation. Many works [1], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] consider the regularities of active site formation at each synthesis step. The accumulated data provided substantial advances not only in understanding the main transformations of a precursor compound during the synthesis of supported catalysts, but also in the deliberate synthesis of this catalytic system with specified properties by introducing changes into a certain step.

The metal complex–support interaction at the deposition step can affect properties of the final catalyst; the interaction strongly depends not only on the chemical composition of precursor, but also on the nature of surface adsorption sites [4], [5], [6], [7], [8], [9], [13], [14], [15]. The adsorption sites of γ-Al2O3 are represented both by the hydroxyl groups and the Lewis acid sites (LAS). The relative content of certain functional groups on the oxide surface is determined to a great extent by phase homogeneity of the support. Changes in the phase composition of support may occur due to uncontrolled effect of water during the catalyst synthesis and storage. It was found that even at room temperature the action of water on γ-Al2O3 leads to the formation of aluminum trihydroxide phase [16], [17], [18], [19], [20]:γ-Al2O3+3H2O2Al(OH)3

The formation rate and type of Al(OH)3 modifications (gibbsite, bayerite) depend on the temperature, exposure time, and pH of the medium [18], [19], [20].

The most pronounced changes in the phase composition of alumina support are observed under hydrothermal conditions, which can take place both during pretreatment of the support and drying of the catalysts after contacting with aqueous solutions. The hydrothermal treatment (HTT) of γ-Al2O3 at temperatures up to 350 °C leads to boehmite [18], [21], [22], [23], [24], [25], [26]:γ-Al2O3+H2OγAlO(OH)

HTT produces changes in the pore structure of γ-Al2O3 [18], [21], [22], [24], [25], [26]. Efficient control of the temperature and exposure time made HTT applicable to the synthesis of aluminum oxides with variable texture characteristics [21], [22]. Thus, variation of the HTT temperature in the range of 100–350 °C led to alumina with specific surface area of 200 to 70 m2/g and effective pore diameter of 90 to 340 Å [22].

However, data concerning the effect of HTT on the acid-base properties of γ-Al2O3 surface are quite scarce. Recently, IR spectroscopy of adsorbed pyridine was used to demonstrate that HTT of γ-alumina at 200 °С for 6 h decreases the total concentration of LAS from 342 to 42 μmol/g [25]. Qualitative data indicating the effect of HTT on the state of hydroxyl cover of the γ-Al2O3 surface are reported [23], [24]. An IR spectroscopy study [24] revealed that HTT of γ-Al2O3 at 140 °C followed by calcination at 550 °C increases the intensity of absorption bands at 3676 and 3731 cm−1 during the first 2 h of the treatment (these bands were assigned by the authors to the OH stretching vibrations of the bridging group bound to tetrahedrally and octahedrally coordinated aluminum atoms and the terminal group bound to octahedrally coordinated aluminum atom). A more prolonged treatment (6–24 h) decreases the intensity of these absorption bands. Meanwhile, a 24 h HTT produces nearly a twofold increase in the concentration of weak LAS, as shown by IR spectroscopy of adsorbed pyridine.

The goal of the present work was to acquire quantitative data about the effect of γ-Al2O3 interaction with water under hydrothermal conditions on the relative content of various functional groups on the alumina surface. Besides, it was great important to determine the extent to which changes in the composition of functional cover of the support surface could alter its adsorption properties toward H2[PtCl6] and the nature of nascent platinum sites in Pt/γ-Al2O3 catalysts.

Section snippets

Sample preparation

The study was carried out using γ-Al2O3 purchased from Condea Chemie GmbH; its grain size was 0.1–0.25 mm. This aluminum oxide was a high-purity: according to the manufacturer specification sodium and iron content was equal to 0.003 and 0.021 wt%, respectively. HTT was performed in titanium autoclaves. The alumina samples were placed in the autoclaves and flooded with distilled water. The ratio of solid and liquid phases was maintained constant at 1:25 (wt) in all the experiments. The HTT

Effect of HTT on the structural and texture characteristics of γ-Al2O3

HTT of γ-alumina leads to the hydroxide phase of boehmite γ-AlO(OH) according to Eq. (2) (Fig. 1a and b). Fig. 1b displays a powder diffraction pattern of the sample after HTT at 200 °C, which is a maximum temperature used in the study. Under such conditions, the phase transition γ-alumina  boehmite is brought to completion. However, the boehmite fraction in the composition of alumina can readily be controlled by varying the treatment temperature and time. Trends of increasing the content of

Conclusions

Results of the study suggest that the action of water on γ-Al2O3 under hydrothermal conditions (up to 200 °C) produces considerable changes in the texture characteristics and acid-base properties of the alumina surface, and subsequent high-temperature treatment at 550 °C does not provide a complete restoration of the properties of initial support.

According to FTIR spectroscopy data, hydrothermal modification consists in changing the relative content of different types of functional groups on the

Acknowledgments

The authors are grateful to A.B. Arbuzov for the FTIR spectroscopy measurements, N.N. Leont’eva and I.V. Muromtsev for the XRD experiments, and O.V. Maevskaya for the analysis of hexachloroplatinic acid solutions. We also thank A.N. Salanov and E.A. Suprun (Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences) for affording the SEM micrographs. The support of the Russian Foundation for Basic Research (Project No. 09-03-01013-a) is gratefully acknowledged.

References (62)

  • T. Mang et al.

    Appl. Catal., A

    (1993)
  • W.A. Spieker et al.

    Chem. Eng. Sci.

    (2001)
  • W.A. Spieker et al.

    Appl. Catal., A

    (2003)
  • B. Shelimov et al.

    J. Catal.

    (1999)
  • B.N. Shelimov et al.

    J. Mol. Catal. A: Chem.

    (2000)
  • H. Lieske et al.

    J. Catal.

    (1983)
  • G. Lietz et al.

    J. Catal.

    (1983)
  • J.-F. Lambert et al.

    Stud. Surf. Sci. Catal.

    (2000)
  • X. Carrier et al.

    J. Colloid Interface Sci.

    (2007)
  • L. Jun-Cheng et al.

    Appl. Surf. Sci.

    (2006)
  • A. Stanislaus et al.

    J. Mol. Catal. A: Chem.

    (2002)
  • J. Park et al.

    J. Colloid Interface Sci.

    (1995)
  • B.J. Kip et al.

    J. Catal.

    (1987)
  • A.V. Kalinkin et al.

    J. Electron Spectrosc. Relat. Phenom.

    (2010)
  • Z. Nawaz et al.

    J. Ind. Eng. Chem.

    (2010)
  • R. Vidruk et al.

    J. Catal.

    (2011)
  • G. Busca et al.

    J. Catal.

    (1991)
  • C. Morterra et al.

    Catal. Today

    (1996)
  • M. Digne et al.

    J. Catal.

    (2004)
  • A. Zecchina et al.

    J. Catal.

    (1987)
  • J.Z. Shyu et al.

    Appl. Surf. Sci.

    (1988)
  • H. Matsuhashi et al.

    Appl. Catal., A

    (2004)
  • E. Marceau et al.
  • A. Pearson

    Kirk-Othmer Encyclopedia of Chemical Technology

    (2004)
  • L. Lloyd

    Handbook of Industrial Catalysts

    (2011)
  • B. Shelimov et al.

    J. Am. Chem. Soc.

    (1999)
  • O.B. Belskaya

    Chem. Sustainable Dev.

    (2011)
  • O.B. Belskaya et al.

    Smart Nanocompos.

    (2011)
  • E. Marceau et al.
  • D.P. Dobychin

    Dokl. Akad. Nauk. SSSR

    (1955)
  • A.V. Uvarov

    Russ. J. Phys. Chem.

    (1962)
  • Cited by (59)

    • Reduction mechanism of Au(III) species adsorbed on δ-MnO<inf>2</inf>

      2022, Colloids and Surfaces A: Physicochemical and Engineering Aspects
    View all citing articles on Scopus
    View full text