Zr(IV) surface chemical state and acid features of sulphated-zirconia samples
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.
References (41)
Appl. Catal. A: General
(1996)- et al.
Mater. Chem. Phys.
(1990) Appl. Catal.
(1990)- et al.
Appl. Catal. A: General
(1996) - et al.
Catal. Today
(1994) Catal. Today
(1992)- et al.
Catal. Today
(1994) - et al.
J. Catal.
(1993) - et al.
J. Catal.
(1994) - et al.
Mater. Chem. Phys.
(1987)
Adv. Catal.
J. Catal.
J. Catal.
Colloids Surf.
J. Catal.
J. Catal.
Appl. Catal. A: General
Colloids Surf.
Mater. Chem. Phys.
Stud. Surf. Sci. Catal.
Cited by (36)
Synthesis of biomass derived levulinate esters on novel sulfated Zr/KIL-2 composite catalysts
2016, Microporous and Mesoporous MaterialsCitation Excerpt :The higher intensity of the Zr 3d5/2 peak detected for SO42−/25ZrKIL-2 is in agreement with the XRD data showing the crystalline ZrO2 formation. The O ls peak centered around 532.0 eV can be attributed to oxygen atoms from silica as well as OH and/or sulfates groups [26,28,34]. A small peak centered at around 530.0 eV in O 1s XP spectrum of SO42−/25ZrKIL-2 is assigned to the oxygen atoms from the zirconia framework.
Ceria supported on sulfated zirconia as a superacid catalyst for selective catalytic reduction of NO with NH<inf>3</inf>
2013, Journal of Colloid and Interface ScienceCitation Excerpt :In the case of CeSZ(0.095), the spectrum of oxygen became much broader, especially the enhanced Ob and Oc. The sulfation treatment could not only introduce an extra component of O in sulfates (enhanced Ob), but also enhance the content of surface chemisorbed water (enhanced Oc) [41,42]. The peak of Zr 3d doublet (Fig. 8III) was observed in the CeZ catalyst with BE values (181.7, 184.1 eV), relative to Zr(IV) species in the oxide phase [42–44].
Activation of Zr-Co-rare earth getter films: An XPS study
2010, Applied Surface ScienceLiquid phase reactions catalyzed by Fe- and Mn-sulphated ZrO<inf>2</inf>
2009, Applied Catalysis A: GeneralCharacterizations of aluminum-promoted sulfated zirconia on mesoporous MCM-41 silica: Butane isomerization
2008, Microporous and Mesoporous MaterialsImpact of temperature ramping rate during calcination on characteristics of nano-ZrO<inf>2</inf> and its catalytic activity for isosynthesis
2008, Journal of Molecular Catalysis A: Chemical