Elsevier

Applied Surface Science

Volume 136, Issue 3, November 1998, Pages 213-220
Applied Surface Science

Zr(IV) surface chemical state and acid features of sulphated-zirconia samples

https://doi.org/10.1016/S0169-4332(98)00335-3Get rights and content

Abstract

ZrO2–SO4 powders have been prepared by following either hydrothermal or sol–gel precursor preparative routes and adopting different sulphation procedures. All the samples have been calcined at the same temperature (470°C) and for the same time length (5 h), and have been characterised for phase composition (XRD) and surface area (BET). Determinations of the actual acidity pKa and density of the oxide surface sites have been obtained by a revised Hammett–Bertolacini technique. XPS analyses of the samples have been obtained by elaboration of the zirconium 3d, oxygen 1s and sulphur 2p spectral regions. XPS data are discussed in respect to the conditions of the powders preparation and cross-compared with acidity characterisations. The outcome of fitting procedures of the Zr 3d doublet of the ZrO2–SO4 powders are presented and commented in relation to the possible stoichiometry of the oxide active surface sites.

Introduction

In recent years solid super acid catalysts have been synthesised by strong coordination of sulphate material on the surface of metal oxides (Fe, Ti, Zr, Hf, Sn, Si). Powders of these oxide materials are active catalysts for skeletal isomerization of light paraffins, acylation reactions and in heterogeneous processes which are generally catalysed by strong acids 1, 2.

Among other oxides, ZrO2–SO4 powders have been the object of extensive research both applied and fundamental 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Many parameters have been observed to affect the nature and strength of the active sites on the material and consequently its catalytic performance: the ZrO2 preparation procedure and prefiring temperature, the conditions of sulphate doping, the degree of sulphur coverage and the temperature of calcination of the sulphate-treated precursor [16]. Simple correlations are entangled by the close interplay of the different parameters with one another and contradictory data are often reported in the literature. Much of the debate concerns the actual stoichiometry and coordination of the surface species as these reflect the acid–super acid character of the samples. Results on the nature of the acidity are mostly based on IR spectra of adsorbed molecules (mainly pyridine and carbon monoxide) and diverge principally regarding the Lewis or Bröensted character of the surface sites 3, 9, 17, 18, 19.

Direct characterisations of the actual state of ZrO2–SO4 samples by UHV spectroscopies are not numerous 14, 20, 21; also in this case some conflicting conclusions are reported concerning, for example, the actual location of sulphur containing species, either merely at the surface or also in the bulk of the material 14, 21. Recently, Sayari and Dicko [22]and Dicko et al. [23]have discussed, mainly on the basis of XPS data, the actual redox conditions of Pt species in Pt-sulphated zirconia. Milburn et al. [24]have reported a correlation between the catalytic activity and XPS data of Pt–SO4–ZrO2 catalysts. The authors relate the oxygen 1s peak area with the surface sulphate concentration and this latter, in turn, with the catalytic performance of the oxide.

In this paper XPS data of ZrO2–SO4 samples based on the elaboration of the O 1s peak together with the Zr 3d and S 2p regions are reported. The powders have been obtained by different procedures (different precursor preparative routes and sulphate doping) but were all calcined at the same firing temperature to ensure comparable hydration conditions. XPS data are cross-compared with acidity characterisations obtained by a modification of the Hammett–Bertolacini technique 25, 26, 27and discussed with the aim to single out some general feature not affected by the specific route of the catalyst preparation.

Section snippets

Experimental

All the chemicals were of reagent grade purity and were used without further purification; doubly distilled water passed through a Milli-Q apparatus was used to prepare solutions and suspensions.

Acidity features

The acidity features obtained by characterisation of the different ZrO2–SO4 powders, with the H–B technique, are reported in Fig. 1. The figure reports the density of the surface sites relative to the given pKa value.

As a first point, before considering the specific trends shown by the various samples, the values of the total site densities obtained experimentally will be commented. The data range between 1.5 and 6.5 μmol/m2. The maximum value of site density (6.5 μmol/m2, i.e., 3.9 site/nm2,

Conclusions

ZrO2–SO4 samples obtained by both hydrothermal (HT) and sol–gel (SG) routes and following different sulphation procedures have been submitted to bulk and surface characterisations. The acidity features, obtained by a modification of the H–B method show that SG samples are generally more acid than the HT samples. The origin of this effect is related to the (lack of) structure–crystallinity of the zirconia precursor. O 1s, S 2p and Zr 3d spectral regions have been investigated by XPS for all

Acknowledgements

Financial support from MURST (40 and 60% Research Funds) is gratefully acknowledged.

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