Feasibility of methyl mercaptane as probe molecule for supported gold nanoparticle surface area determination.

Abstract: Gold nanoparticles supported on TiO2 were probed by adsorption of methyl mercaptane (MM), and the process was quantified gravimetrically. This method allowed discrimination between weakly adsorbed (physisorbed) and strongly bound (chemisorbed) methyl mercaptane. Strong adsorption of MM occured on exposed Au faces, while low-temperature pre-treatment (30 degrees C) completely suppressed adsorption of MM on the TiO2 support. The thus obtained high selectivity of MM adsorption on Au enabled characterization of the gold surface area and the resulting values are comparable with other noble metal systems of similar average particle size. The estimated adsorption stoichiometry indicates that the entire Au surface is probed.

Chemisorption of methyl mercaptane on titania-supported Au nanoparticles: Viability of Au surface area determination.

Well-characterized Au nanoparticles were deposited on commercial TiO(2) (P25, Degussa) and analyzed by means of STEM and thermogravimetry coupled with mass spectrometry (TG-MS). The adsorption was studied on Au/TiO(2) samples with Au loadings in the range of 1.1-9.9wt.% by injecting pulses of CH(3)SH (methyl mercaptane, MM) until no further mass increase could be observed. A prerequisite for determination of the surface area of the deposited gold nanoparticles is the proper discrimination of species adsorbing on the Au nanoparticles and the titania support. The adsorption of methyl mercaptane on the titania support strongly depended on the pretreatment temperature (30-400 degrees C), whereas the adsorption on Au nanoparticles was virtually unaffected by this parameter. A very mild thermal pretreatment was identified as a requirement for avoiding the adsorption of the MM on the titania support. CH(3)SH adsorbed on the support desorbed at lower temperatures (maximal rate of desorption was centered at ca. 150 degrees C) compared to species desorbing from Au nanoparticles (maximum at ca. 200-220 degrees C). Moreover, CH(3)SH adsorbed on Au nanoparticles desorbed in the form of dimethyl sulfide (CH(3))(2)S. Part of MM adsorbed on the gold surface was not desorbed even at high temperatures (above 500 degrees C) and stayed on the surface in the form of relatively stable C(x)H(y)S(z) fragments. This residue could be removed by oxygen pulses resulting in the formation of CO(2), SO(2), and H(2)O. The good discrimination of MM chemisorption on Au nanoparticles and on titania renders the determination of the Au surface area viable. Potential and limitations of the CH(3)SH chemisorption for the surface area determination of Au nanoparticles are discussed.

A versatile in situ spectroscopic cell for fluorescence/transmission EXAFS and X-ray diffraction of heterogeneous catalysts in gas and liquid phase.

A new spectroscopic cell suitable for the analysis of heterogeneous catalysts by fluorescence EXAFS (extended X-ray absorption fine structure), transmission EXAFS and X-ray diffraction during in situ treatments and during catalysis is described. Both gas-phase and liquid-phase reactions can be investigated combined with on-line product analysis performed either by mass spectrometry or infrared spectroscopy. The set-up allows measurements from liquid-nitrogen temperature to 973 K. The catalysts are loaded preferentially as powders, but also as self-supporting wafers. The reaction cell was tested in various case studies demonstrating its flexibility and its wide applicability from model studies at liquid-nitrogen temperature to operando studies under realistic reaction conditions. Examples include structural studies during (i) the reduction of alumina-supported noble metal particles prepared by flame-spray pyrolysis and analysis of alloying in bimetallic noble metal particles (0.1%Pt-0.1%Pd/Al(2)O(3), 0.1%Pt-0.1%Ru/Al(2)O(3), 0.1%Pt-0.1%Rh/Al(2)O(3), 0.1%Au-0.1%Pd/Al(2)O(3)), (ii) reactivation of aged 0.8%Pt-16%BaO-CeO(2) NO(x) storage-reduction catalysts including the NO(x) storage/reduction cycle, and (iii) alcohol oxidation over gold catalysts (0.6%Au-20%CuO-CeO(2)).

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.

New approach to avoid erroneous interpretation of results derived from generalized two-dimensional correlation analysis for applications in catalysis.

Surface characterization and catalysis can significantly benefit from the application of generalized two-dimensional (2D) correlation analysis. This two-dimensional approach allows a better resolution of overlapping peaks, can reveal new features not readily observable in the raw spectra, gives clear evidence for spectral intensities that change as an effect of a perturbation applied to the system, and allows the establishment of time sequences for the changes occurring in different spectral features of interest for determining reaction intermediates and/or mechanisms. The interpretation of the synchronous and asynchronous plots was observed to lead to erroneous time sequences when spectral features change in a non-monotonic way, such as a biphasic or oscillatory behavior, under the influence of a perturbation. We propose a new approach to the 2D correlation analysis to avoid misinterpretation of the results calculated in the asynchronous plot. Progressive correlation analysis (ProCorA) calculates the synchronous plot from the first two spectra of the data matrix and one spectrum is added at every step of the analysis. The sequence of changes can be set up from the progressive evolution of peaks in both the synchronous and asynchronous plots.

Application of the generalized 2D correlation analysis to dynamic near-edge X-ray absorption spectroscopy data.

Application of the generalized 2D correlation analysis to a series of in situ XANES spectra enabled the determination of additional useful information not readily available from the conventional spectra. In addition to the changes in the intensity of the white line and in the pre-edge feature, readily observable in the regular spectra, the generalized 2D correlation analysis clearly evidenced an otherwise imperceptible shift in the main edge energy caused by the gradual reduction of Co(2+) to metallic cobalt. The 2D correlation spectra also allowed the establishment of a time sequence for the changes occurring in the spectral features during hydrogen reduction, which provides valuable information on the reduction mechanism. While the generalized 2D correlation analysis was found to be very useful in obtaining supplementary information from the series of XANES spectra analyzed, interpretation of the correlation intensities should be checked for consistency with the general trend of each spectral feature, as spectral intensities that do not change monotonically may induce changes in the signs of the correlation intensities leading to inaccurately establishing sequences of changes among the spectral features in the series.

X-ray absorption spectroscopic investigation of partially reduced cobalt species in Co-MCM-41 catalysts during synthesis of single-wall carbon nanotubes.

Chemometric tools were employed to analyze the in-situ dynamic X-ray absorption spectroscopy data to probe the state of Co-MCM-41 catalysts during reduction in pure hydrogen and under single-wall carbon nanotube synthesis reaction conditions. The use of the progressive correlation analysis established the sequence in which changes in the spectral features near the Co K edge occurred, and the evolving factor analysis provided evidence for the formation of an intermediate Co(1+) ionic species during reduction of the Co-MCM-41 catalyst in pure hydrogen up to 720 degrees C. This intermediate species preserves the tetrahedral environment in the silica framework and is resistant to complete reduction to the metal in H(2). While the Co(2+) species is resistant to reduction in pure CO, the intermediate Co(1+) species is more reactive in CO most likely forming cobalt carbonyl-like compounds with high mobility in the MCM-41. These mobile species are the precursors of the metallic clusters growing carbon nanotubes. Controlling the rates of each step of this two-stage reduction process is key to controlling the size of the metallic Co clusters formed in Co-MCM-41 catalysts.