Monitoring the states of vanadium oxide during the transformation of TiO2 anatase-to-rutile under reactive environments: H2 reduction and oxidative dehydrogenation of ethane

doi:10.1016/j.catcom.2009.08.002

Catalysis Communications

Volume 11, Issue 1, 10 October 2009, Pages 15-19

https://doi.org/10.1016/j.catcom.2009.08.002 Get rights and content

Abstract

The influence of vanadium oxide on the TiO₂ (anatase) to rutile transformation for supported VO_x/TiO₂ catalysts under different environments (oxidizing, reducing and ethane oxidative dehydrogenation (ODH)) was investigated. The supported VO_x/TiO₂ catalysts were synthesized by incipient wetness impregnation and characterized with XPS surface analysis, Raman spectroscopy and temperature-programmed oxidation and reduction. The XPS surface analysis and Raman spectroscopy demonstrate that the supported vanadium oxide phase is initially present as surface VO_x species on the titania (anatase) support below monolayer coverage (∼9 V/nm²) and that crystalline V₂O₅ nanoparticles are also present above monolayer coverage. The TiO₂ anatase-to-rutile transformation is found to depend both on the gaseous environment and presence of vanadia. The anatase-to-rutile transformation is accelerated by the presence of vanadium oxide and the incorporation of V⁺⁴ cations into the TiO₂ (rutile) lattice. The incorporation of the V⁺⁴ cations decreases the catalytic activity since the active sites are removed from the surface into the bulk, which is not accessible to the reactant gases. Although the titania anatase-to-rutile transformation readily occurs in oxidizing environments (O₂ and ODH reaction), the solid-state transformation does not take place under reducing environments (H₂) since large V³⁺ ions form prior to the transition temperature and cannot enter the titania lattice.

Introduction

Vanadium oxides supported on titania have been extensively studied and used due to their high catalytic activity and selectivity in many chemical reactions, especially in the o-xylene oxidation and in the selective catalytic reduction of NO_x by NH₃ [1], [2]. The supported VO_x/TiO₂ catalysts consist of surface VO_x species that exhibit high activity for alkane oxidative dehydrogenation, but low values of selectivity to alkenes have been obtained [3], [4], [5], [6], [7], [8]. The catalytic performance of supported vanadium oxide catalyst depends on the specific oxide support [9], surface coverage and additive promoters that determine the vanadium oxidation state and molecular structure. Among different oxide supports, titania is of great interest since it imparts high activity per vanadium site [10]. The anatase polymorph of TiO₂ is typically the TiO₂ phase employed in industrial processes for titania-supported vanadium oxide catalysts [11], [12]. Although, the activity per vanadium site and its molecular structure on titania is independent of the titania polymorph. The supported V₂O₅/TiO₂ (anatase) catalyst deactivates by transforming the anatase-to-rutile, with the latter phase incorporating V⁺⁴ cations in the rutile structure [13], [14], [15].

In this study, we examine the relationship between surface vanadium oxide supported on titania (anatase) and its catalytic performance for ethane ODH. The influence of H₂ reduction, ethane ODH and temperature upon the structures and catalytic activity were investigated with in situ Raman spectroscopy.

Section snippets

Experimental

The TiO₂ (anatase), Degussa P-25, was used as catalysts support. The oxide catalysts were prepared by the incipient wetness impregnation technique employing V-isopropoxide in isopropanol [9] to obtain catalysts from 1 wt.% to 8 wt.% of V₂O₅. The catalysts are referred to as “xVTi”, where “x” represents the weight percent of V₂O₅ on TiO₂. Surface vanadium loading (V atom/nm²) was calculated with respect to the BET area of the titania support.

The BET surface areas of the samples were determined

Structural characterization

The physicochemical characteristics of the catalysts are listed in Table 1. The BET surface area decreases slightly with vanadium oxide loading, which is primarily due to the added mass of the vanadia. The XPS V/Ti atomic ratio increases linearly with vanadium oxide loading up to 6VTi and then levels off. This suggests a high dispersion degree of vanadium oxide species up to this 6% vanadium oxide/titania loading.

Temperature-programmed reduction (H₂-TPR)

The H₂-TPR profiles of the supported VO_x/TiO₂ catalysts are presented in Fig. 1.

Discussion

The anatase-to-rutile transition, happens above 900 °C for vanadium-free anatase, vanadium enables this transition at 600 °C. Surface vanadium oxide species trigger the titania anatase-to-rutile transformation in oxidizing atmosphere (e.g., air) due to substitution of Ti⁴⁺ ions by similarly sized V⁴⁺ ions. Such a process appears facilitated during ethane ODH since surface vanadium sites undergo a redox cycle this is consistent with the formation of V⁴⁺ species during reaction. The presence of V⁴⁺

Conclusions

Titania-supported vanadia catalysts consist of surface VO_x species highly dispersed. Surface vanadium oxide species strongly facilitate the titania anatase-to-rutile transformation, which depend on temperature and vanadium loading. Ethane oxidative dehydrogenation reaction conditions trigger such a transformation since partially reduced V⁴⁺ ions form and readily diffuse into titania support. More reduced V³⁺ species are not capable to promote the titania anatase-to-rutile transformation, since

Acknowledgments

This research was funded by Spanish Ministry of Science and Innovation, Project CTQ-2008-02461/PPQ. M.V.M.-H. acknowledges the Ramon y Cajal program by the Ministry of Science and Innovation for financial support. The authors are indebted to Prof. I.E. Wachs and Dr. X. Gao for preparing and supplying the titania-supported samples.

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Monitoring the states of vanadium oxide during the transformation of TiO2 anatase-to-rutile under reactive environments: H2 reduction and oxidative dehydrogenation of ethane

Abstract

Introduction

Section snippets

Experimental

Structural characterization

Temperature-programmed reduction (H2-TPR)

Discussion

Conclusions

Acknowledgments

Appl. Catal. A: Gen.

Appl. Catal.

Stud. Surf. Sci. Catal.

J. Mol. Catal. A: Chem.

J. Catal.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal.

Appl. Catal. A: Gen.

J. Solid State Chem.

Stud. Surf. Sci. Catal.

Catal. Today

J. Catal.

Monitoring the states of vanadium oxide during the transformation of TiO₂ anatase-to-rutile under reactive environments: H₂ reduction and oxidative dehydrogenation of ethane

Temperature-programmed reduction (H₂-TPR)