Isotopic and Chemical Titration of Acid Sites in Tungsten Oxide Domains Supported on Zirconia

Abstract

The acid and surface properties of WO x -ZrO 2 solid acid catalysts were examined as a function of WO x surface density. Surface density strongly influences the size and electronic properties of WO x domains and the rate of acid-catalyzed reactions on these materials. The adsorption and isotopic exchange of acid and redox probe molecules (NH 3 , pyridine, H 2 , and O 2 ) were used to measure the density and type of active sites in the reducing environments required to achieve high isomerization reaction rates. In this manner, the role of H 2 and of surface density on the formation of Bronsted or Lewis acid sites was determined by combining infrared spectroscopy and desorption measurements using NH 3 titrants. At submonolayer WO x coverages, the density of Bronsted acid sites increased with increasing WO x surface density up to ∼0.20 NH 3 /W atom, indicating their presence at the interface between ZrO 2 and polytunsgtate domains. Above monolayer WO x coverages, Bronsted acid site densities (per W atom) decreased markedly because of the formation of WO 3 clusters. The addition of H 2 during NH 3 adsorption or desorption increased only slightly the ratio of Bronsted to Lewis acid sites and did not change the total adsorption uptake. H 2 chemisorption uptakes at typical reaction temperature (523 K) showed that the number of H atoms adsorbed on WO x -ZrO 2 was negligible at low WO x surface densities (<4 W/nm 2 ). Hydrogen uptakes increased as reducible polytungstate domains formed with increasing surface density, reaching a maximum value of 0.063 H/W at 7.35 W/nm 2 . H 2 uptakes decreased at higher surface densities because of reduction processes that lead to oxygen removal from WO x domains, as shown by O 2 chemisorption uptakes on these samples. Oxygen-deficient WO 3-x species are unable to stabilize adsorbed hydrogen by delocalizing electron density to form H δ+ . The density of these minority H δ+ Bronsted acid sites formed during exposure to H 2 , and the rate of o-xylene isomerization on WO x -ZrO 2 , shows a similar dependence on WO x surface density. These findings suggest that temporary acid sites formed by H 2 dissociation and adsorption on basic oxygens in WO x domains lead to very active Bronsted acid sites, which catalyze o-xylene isomerization with turnover rates much higher than on H-ZSM5. Isotopic exchange of surface OH groups with D 2 , after H 2 pretreatments that lead to chemisorbed hydrogen, confirmed the low density of O-H species formed from H 2 on WO x -ZrO 2 (0.017 H atoms/W atom at 7.8 W/nm 2 ).

Structure and function of oxide nanostructures: catalytic consequences of size and composition

“…2 In this latter region, a second absorption edge gradually emerged at 2.6 eV, which corresponds to the band gap for crystalline WO 3 . The intensity of this second edge feature increased monotonically with increasing WO x surface density, as expected from the increasing abundance of bulk WO 3 with increasing surface density.…”

“…1,3,5 These changes can be attributed to electronic modifications in less direct ways than for confined quantum structures in electronic devices. The size and local structure of inorganic domains useful as catalysts are seldom detectable by methods that probe long-range order, such as X-ray diffraction, or even by high-resolution transmission electron microscopy.…”

“…Yet, as we move from crystalline oxides to oligomers and ultimately to isolated monomers, we have found, almost without exception, marked consequences for the surface chemistry and the catalytic function of metal oxides. [1][2][3][4][5][9][10][11][12] The synthesis of these small catalytic structures presents significant challenges, but emerging methods have exploited the anchoring of oxo-ions on hydroxylated surfaces and the thermodynamic tendency of oxides to spread on other oxide surfaces 13 to form dispersed oxide domains. The structural assessment of these domains is even more challenging, especially when we aim to probe accessibility, atomic connectivity, and local structure, the features relevant for catalytic function.…”

“…Phys., 2008, 10, 5331-5343 | 5335 inaccessible hydrogen atoms. As a result, correlations between oxylene isomerization rates and pre-edge intensities become inaccurate for samples containing bulk-like three-dimensional WO 3 clusters. 3 The substantial absence of a bulk region in twodimensional polytungstates allows protons to remain accessible at external surfaces and leads to the unique ability of these polytungstate species to catalyze reactions requiring Brønsted acid sites.…”

Aerogel and xerogel WO3/ZrO2 samples for fine chemicals production

“…6. For both AZW and XZW systems a band at $1000 cm À1 can be observed: it can be assigned to the symmetrical stretching vibration of W = O bonds of octahedrically coordinated species present at the ZrO 2 surface [36]. This band is located at higher frequencies after activation at 400°C and this shift in its spectral position can be attributed to an increase in the strength of interaction between tungsten and zirconia, most likely due to the dehydration effect connected to the thermal treatment.…”