G. Cerrato

Lewis and Brønsted acidity at the surface of sulfate-doped ZrO2 catalysts

Both Bronsted (protonic) and Lewis (aprotic) acidic centres can exist at the surface of an homo-geneous family of sulfated zirconia catalysts, provided sulfate groups (either isolated or, better, polynuclear units) exist on the regular patches of few low-index crystal planes representing the top termination of the scale-like ZrO2 crystallites. The Bronsted/Lewis acidity ratio depends then on (at least) two variables: the surface hydration/dehydration degree, and the highest temperature reached by the system because Bronsted acidity is induced only by the thermally most labile fraction of surface sulfates.

Surface characterization of yttria-stabilized tetragonal ZrO2 Part 1. Structural, morphological, and surface hydration features

Abstract The characterization of three microcystalline Y2O3-stabilized tetragonal zirconia (t-ZrO2) preparations was carried out by BET, XRD, HRTEM, and FTIR spectroscopy, and revealed that morphological and surface features of t-ZrO2 specimens depend, much more than in the case of the monoclinic modification (m-ZrO2), on the preparative history of the material. Preparations fired at T ⩽ 1000 K are mostly made up of coin-like particles terminating with the same crystal plane and present only one surface OH species, but after firing at higher temperatures crystal shape and morphology of the various preparations start changing appreciably and differing from one another. The morphological changes are also monitored by changes in the IR spectrum of the surface OH layer, which becomes quite similar to that of the microcrystalline m-ZrO2 phase. After firing at T ⩾ 1173 K, some of the t-ZrO2 preparations will generate to various extents (depending on the Y2O3 content and on the preparative procedure) an amorphous phase coating the particles. This modification can be monitored by appreciable changes in the IR spectrum of the surface OH layer.

Surface acidity of metal oxides. Combined microcalorimetric and IR-spectroscopic studies of variously dehydrated systems

Abstract The systematic use of CO adsorption at room temperature as a probe for (strong) Lewis acidity at the surface of a number of non-d/d0 metal oxides of interest in catalysis is illustrated. The advantages of the combined use of adsorption microcalorimetry and IR spectroscopy in giving an exhaustive picture of the distribution of acid surface sites are stressed. The influence of the chemical nature (TiO2, ZrO2, HfO2 and Al2O3) and of the structure (monoclinic/tetragonal for ZrO2 and γ-/δ,θ- phase for Al2O3) of the metal oxide, as well as the dehydration degree of the surface and the presence of anionic (sulfates for zirconia) or cationic (ceria for alumina) surface dopants were considered. The blue shift of the stretching frequency of adsorbed CO and the adsorption enthalpy were measured. In the case of the group IV metal oxides the two parameters were found to be correlated, whereas in the case of pure Al2O3 no correlation was found. The peculiar behavior of alumina was interpreted on the basis of different processes occurring at the CO/Al2O3 interface. An endothermic reversible reconstruction of the surface was supposed to occur upon adsorption, leading to exceptionally low heat values. The presence of small amounts of cations, other than Al3+, in the alumina matrix seemed to inhibit this effect.

Isomerization ofn-butane on sulfated zirconia: Evidence for the dominant role of Lewis acidity on the catalytic activity

The catalytic activity of a ZrO2/SO4 catalyst in the isomerization ofn-butane at 423 K is reversibly suppressed by addition of CO. IR analysis of the adsorption of CO indicates that the only σ-coordination of CO onto coordinatively unsaturated surface Zr4+ cations occurs in the 300–473 K interval.

Infrared study of some surface properties of boehmite (γ-AlO2H)

Two specimens of microcrystalline boehmite (γ-AlO2H) with rather different particle size and crystallinity have been examined by FTIR spectroscopy, and the surface behaviour has been found to depend very little (if at all) on the degree of crystallinity. The vacuum thermal activation of the material occurs in two steps: the first step involves the reversible elimination of coordinated water molecules (with no change in surface structure and/or coordination), whereas the second involves surface dehydroxylation together with a substantial and nonreversible structural/coordinative modification of the surface. Both processes occur at temperatures lower than that of the phase transition γ-AlO2H →γ-Al2O3, i.e. on a material still unmodified in its bulk features. IR investigation of the adsorption of CO, CO2 and pyridine yields adequate information on the surface properties of both the unmodified and the surface-modified material. At the surface of the ‘virgin’ boehmite there are already some coordinatively unsaturated Al atoms with (quasi-) tetrahedral coordination, and a larger number of coordinatively unsaturated Al atoms with octahedral coordination. The latter centres are active towards CO2 adsorption (yielding various types of carbonate-like species and an end-on linear CO2 complex) and towards pyridine, whereas they are inactive towards CO at 300 K and partly active at 78 K. The probe molecules used can monitor, though with a different degree of promptness, the early structural/coordinative transformations occurring, upon vacuum activation, at the surface of boehmite unmodified in its bulk features.

On the role of the calcination step in the preparation of active (superacid) sulfated zirconia catalysts

In the preparation of active SO4-ZrO2 catalysts, several steps involving various chemical and/or physical processes are necessary. In particular it has been reported that, after sulfation of amorphous Zr hydrates, a calcination atTcalc>773 K is needed to guarantee the crystallization of ZiO2 in the tetragonal phase. By the use of a stabilized tetragonal ZrO2, it is here demonstrated that a calcination atTcalc>773 K is indeed necessary for all SO4-ZrO2 systems, and that its actual role is the selective elimination of sulfates from highly energetic crystallographic defects. The calcination step atTcalc>773 K so creates the conditions for the formation of strong Lewis acid centres, that are necessary in the catalytic process, and the presence of which is here monitored spectroscopically by the reversible adsorption of carbon monoxide.

Platinum promoted zirconia-sulfate catalysts: one-pot preparation, physical properties and catalytic activity

A new innovative methodology for the single-stage preparation of ZrO2-SO4 and Pt/ZrO2-SO4 catalysts is reported, based on the sol-gel technique. Catalysts are characterized by analysis, XRPD, BET and IR methods and are tested in the isomerization of butane.

Catalytic behavior and nature of active sites in copper-on-zirconia catalysts for the decomposition of N2O

Abstract Copper-on-zirconia catalysts prepared by a sol-gel technique were found to have an activity in the decomposition of N 2 O comparable to that of Cu-ZSM-5, but a higher stability at high N 2 O concentrations. Zirconia promotes the activity of supported copper species and plays a catalytic role itself, but the tetragonal crystalline form of zirconia is more effective in the promotion than the monoclinic form. Characterization of the samples by ESR, IR (CO chemisorption) and oxygen thermodesorption suggests that during dehydroxylation of zirconia at temperatures in the 400–500°C range Cu 1 species form which are stable against consecutive reoxidation, when the zirconia surface is not rehydroxylated. It is suggested that the good behavior of Cu-ZrO 2 catalysts in N 2 O decomposition derives from the stabilization of Cu 1 species by the support. Water in the feed inhibits the surface reactivity due to the modification of the redox properties of copper ions as well as competition with N 2 O for chemisorption on copper ions.

On the strength of Lewis- and Brønsted-acid sites at the surface of sulfated zirconia catalysts

The surface acidity (of both Lewis and Bronsted type) of an anion-free t-ZrO 2 , stabilized in the tetragonal form by forming a solid solution with ca. 3 mol% Y 2 O 3 , of the corresponding sulfated t-ZrO 2 (SZ) system, and of another SZ t-ZrO 2 system prepared through a conventional method have been compared; the two SZ systems turned out to be essentially equivalent. By FTIR and microcalorimetry it was observed that the presence of sulfate modifies to a very limited extent both the concentration an strength of the types of strong (aprotic) Lewis sites that can be revealed by CO adsorption at 300 K. In absolute terms, the acidity of these Lewis sites turns out to be definitely lower than that of typical acidic oxides (e.g., catalytic aluminas). IR spectroscopic data indicate that surface sulfates induce in SZ systems the presence of (protonic) Bronsted acidity, otherwise absent in (anion-free) t-ZrO 2 . On SZ, Bronsted-acid centres are present over the whole vacuum-activation temperature range explored (300–823 K), though in decreasing amounts with increasing temperature. The acid strength of Bronsted sites is confirmed to be definitely lower than that of typical protonic oxide systems (such as, for instance, H-exchanged zeolites). As the Lewis-acid sites present on SZ systems are not particularly strong, and the Bronsted-acid sites present on SZ systems in catalytic conditions are not particularly strong (nor very abundant), it is concluded that the unique catalytic properties of SZ systems, sometimes termed superacid catalysts, must have another origin, possibly connected with the simultaneous presence at the surface of SZ of acid sites of both types.

Sr-containing hydroxyapatite: morphologies of HA crystals and bioactivity on osteoblast cells

A series of Sr-substituted hydroxyapatites (HA), of general formula Ca(10-x)Srx(PO4)6(OH)2, where x=2 and 4, were synthesized by solid state methods and characterized extensively. The reactivity of these materials in cell culture medium was evaluated, and the behavior towards MG-63 osteoblast cells (in terms of cytotoxicity and proliferation assays) was studied. Future in vivo studies will give further insights into the behavior of the materials. A paper by Lagergren et al. (1975), concerning Sr-substituted HA prepared by a solid state method, reports that the presence of Sr in the apatite composition strongly influences the apatite diffraction patterns. Zeglinsky et al. (2012) investigated Sr-substituted HA by ab initio methods and Rietveld analyses and reported changes in the HA unit cell volume and shape due to the Sr addition. To further clarify the role played by the addition of Sr on the physico-chemical properties of these materials we prepared Sr-substituted HA compositions by a solid state method, using different reagents, thermal treatments and a multi-technique approach. Our results indicated that the introduction of Sr at the levels considered here does influence the structure of HA. There is also evidence of a decrease in the crystallinity degree of the materials upon Sr addition. The introduction of increasing amounts of Sr into the HA composition causes a decrease in the specific surface area and an enrichment of Sr-apatite phase at the surface of the samples. Bioactivity tests show that the presence of Sr causes changes in particle size and/or morphology during soaking in MEM solution; on the contrary the morphology of pure HA does not change after 14 days of reaction. The presence of Sr, as Sr-substituted HA and SrCl2, in cultures of human MG-63 osteoblasts did not produce any cytotoxic effect. In fact, Sr-substituted HA increased the proliferation of osteoblast cells and enhanced cell differentiation: Sr in HA has a positive effect on MG-63 cells. In contrast, Sr ions alone, at the concentrations released by Sr-HA (1.21-3.24 ppm), influenced neither cell proliferation nor differentiation. Thus the positive effects of Sr in Sr-HA materials are probably due to the co-action of other ions such as Ca and P.