SOx storage materials under Lean-Rich cycling conditions--Part II: influence of Pt, H2O, and cycling time.

The role of Pt and the influence of the reaction conditions during lean-rich cycling experiments were studied on a second generation SOx trapping material. The combination of the Generalized 2-D Correlation Analysis, 2-D Sample-Sample Correlation Analysis, and Factor Analysis using the MCR-ALS technique was applied to identify the reactive species. Transient surface sulfate species were formed under oxidative reaction conditions (lean mode) and decomposed under reducing reaction conditions (rich operation mode). The reduction of this species was identified to be the main contribution to the SO2 release observed under dynamic flow conditions. Pt facilitates the formation of sulfates but also catalyzes the reduction of the transient surface sulfate species leading to a higher amount of SO2 released under rich conditions. In the presence of water, this effect was diminished, which was found to be mainly a result of the suppressed formation of surface sulfate species caused by the faster transport of SO2 into the bulk phase of the SOx trapping component (BaCO3). Increasing the time under reducing conditions in the cycles leads to an enhanced reduction of the surface during rich conditions. The presence of water did not influence the bulk type species. It is proposed that for effective SO2 storage materials, strong SOx adsorption sites on the surface, the presence of water, and a short time under reducing conditions are essential.

SO(x) storage materials under lean-rich cycling conditions. Part I: Identification of transient species.

The SO(x) uptake of second generation sulfur trapping materials was studied by in situ IR spectroscopy under lean-rich cycling conditions. The combination of advanced chemometric methods including generalized 2D correlation analysis, 2D sample-sample correlation analysis, and multivariate curve resolution with alternating least squares allowed the detection of the species involved in the storage process. The formation of the bulk sulfate species was always accompanied by the consumption of carbonates. The reduction of a transient surface sulfate species was identified as the key parameter in the storage process under dynamic conditions. Three distinct reaction regimes were differentiated on the industrial materials under SO(x) trapping conditions being imperceptible from conventional spectra.

On the trapping of SOx on CaO-Al2O3-based novel high capacity sorbents.

Calcium-aluminum mixed oxide based materials doped with Na and Mn were explored as sulfur trapping materials. The materials showed a three times higher total storage capacity and a higher time on stream with complete SO2 removal compared to a second generation SOx trapping material which was mesoporous with calcium mainly present in oxidic form. Combining in situ XANES at the S K-edge and IR spectroscopy the key properties of the storage materials and the affiliated storage processes were identified. CaO-Al2O3 acts as the primary support and storage component, while Na+ cations adjust the base strength and enhances the storage capacity. Manganese cations provide the appropriate oxidation capacity in absence and presence of up to 10% water. The transport into the bulk phase, which is markedly influenced by a layer of sorbed water, is the rate-limiting step in presence of Mn cations. In the absence of manganese cations the oxidation step appears controlling the rate. The overall reaction network, identified by in situ IR spectroscopy and the 2D Correlation Analysis, is similar on all materials.

Spectroscopic characterization of cobalt-containing mesoporous materials.

Cobalt-containing mesoporous materials that have been prepared using different procedures have been comparatively characterized by transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge spectroscopy (XANES), and ultraviolet-visible (UV-vis), near-infrared (NIR), and mid-infrared (mid-IR) spectroscopies, and the results provide new insights into the local environment and properties of cobalt in this type of material. TEM/EDS analyses have shown that tetraethyl orthosilicate (TEOS) may be less appropriate as a silicon source during the syntheses of cobalt-containing mesoporous materials, because the distribution of cobalt throughout the framework may become uneven. EXAFS has been determined to be the most suitable method for direct verification of framework incorporation, by identifying silicon as the backscatterer in the second shell. Such a direct verification may not be obtained using UV-vis spectroscopy. From EXAFS analyses, it is also possible to distinguish between surface-bound and framework-incorporated cobalt. There is a good agreement between the results obtained from XANES and UV-vis regarding the coordination symmetry of cobalt in the samples. The presence of cobalt in the silica framework has been determined to create Lewis acid sites, and these acid sites are suggested to be located at tetrahedral cobalt sites at the surface.

Sulfate formation on SOx trapping materials studied by Cu and S K-edge XAFS.

The elementary steps during oxidative chemisorption of SO2 by a novel composite material consisting of highly disordered benzene tri-carboxylate metal organic framework materials with Cu as central cation and BaCl2 as a second component (Ba/Cu-BTC) and by a conventional BaCO3/Al2O3/Pt based material were investigated. EXAFS analysis on the Cu K-edge in Ba/Cu-BTC indicates the opening of the majority of the Cu-Cu pairs present in the parent Cu-BTC. Compared to Cu-BTC, the BaCl2 loaded material has hardly any micropores and has higher disorder, but it has better accessibility of the Cu2+ cations. This results from the partial destruction of the MOF structure by reaction between BaCl2 and the Cu cations. The SO2 uptake in oxidative atmosphere was higher for the Ba/Cu-BTC sample than for the BaCO3/Al2O3/Pt based material. XRD showed that on Ba/Cu-BTC the formation of BaSO4 and CuSO4 occurs in parallel to the destruction of the crystalline structure. With BaCO3/Al2O3/Pt the disappearance of carbonates was accompanied with the formation of Ba- and Al-sulfates. XANES at the S K-edge was used to determine the oxidation states of sulfur and to differentiate between the sulfate species formed. At low temperatures (473 K) BaSO4 was formed preferentially (53 mol% BaSO4, 47 mol% CuSO4), while at higher temperatures (and higher sulfate loading) CuSO4 was the most abundant species (42 mol% BaSO4, 58 mol% CuSO4). In contrast, on the BaCO3/Al2O3/Pt based material the relative concentration of the sulfate species (i.e., BaSO4 and Al2(SO4)3) as function of the temperature remained constant.

In situ S K-edge X-ray absorption spectroscopy for understanding and developing SOx storage catalysts.

In situ S K-edge XANES experiments were carried out on second-generation SO(x)() trapping materials under oxidizing and reducing conditions. The experiments clearly show that the strong release of SO(2) under rich conditions at plug flow conditions is caused by the facilitated reduction of sulfite species on Pt. In the absence of Pt the sulfite species were stable under reducing conditions, while maintaining a similar total SO(2) uptake capacity. Thus, SO(x)() trapping materials without a noble metal are a clearly better option. The enhancing effect on the SO(x)() storage process of water present in the gas mixture is attributed to the formation of a higher sulfate fraction in the samples. The application of the in situ S K-edge XANES technique clearly reveals new information and insights on the behavior of the sulfur in the trapping process compared to that from the ex situ measurements and is therefore essential for designing new SO(x)() trapping materials.