Chapter 11 Heterogeneous catalysis on solids

Adsorption of polar probe molecules on plasma-oxidised high-strength carbon fibres

Characterisation of clays and aluminium pillared clays by adsorption of probe molecules

In this paper we investigate the adsorption of various probe molecules in order to characterize the porous structure of a series of pillared interlayered clays (PILC). To that aim, volumetric and microcalorimetric adsorption experiments were performed on various Zr PILC samples using nitrogen, toluene, and mesitylene as probe molecules. For one of the samples, neutron scattering experiments were also performed using toluene as adsorbate. Various structural models are proposed and tested by means of a comprehensive computer simulation study, using both geometric and percolation analysis in combination with Grand Canonical Monte Carlo simulations in order to model the volumetric and microcalorimetric isotherms. On the basis of this analysis, we propose a series of structural models that aim at accounting for the adsorption experimental behavior, and make possible a microscopic interpretation of the role played by the different interactions and steric effects in the adsorption processes in these rather complex disordered microporous systems.

Metal oxides are gaining increasing interest in electrocatalysis research as alternative to commonly used high surface area carbon supports for electrocatalyst nanoparticles. On the one hand, metal oxide supports can offer improved corrosion resistance in electrocatalytic processes occurring at high electrode potentials where carbon supports suffer from severe oxidation. On the other hand, intrinsic catalytic properties of the supported catalyst nanoparticles can be influenced by interactions between catalyst and metal oxide support. Thus, metal oxides open a wide field for optimization of supported electrocatalysts similar to the situation in heterogeneous catalysis. However, in electrocatalysis applications, electronic conductivity of the support material is necessary, a requirement which is difficult to be met by metal oxides. Whereas few metal oxides possess intrinsic metallic conductivity, a wider selection of semiconducting materials is achieved by generating defects, such as dopant atoms or oxygen vacancies, inside the lattice of an intrinsically isolating metal oxide. Our research on semiconducting Sb-doped SnO2 (ATO) support for Pt nanoparticles in the context of oxygen reduction reaction (ORR) electrocatalysis revealed a fundamental descriptor for the choice of suitable metal oxide support materials [1]: Well-known from the theory of the semiconductor–electrolyte interface, the metal oxide flat-band potential separates the potential range where the oxide surface is enriched with mobile charge carriers from the potential range of charge carrier depletion leading to a drastically reduced support conductivity at certain electrode potentials. This potential-dependent in situ conductivity switching can be seen as direct electrochemical analog of a field-effect transistor. On the one hand, this electrochemical transistor effect can limit metal oxide support conductivity and, therefore, electrocatalyst performance if the conductivity switching potential, i.e. the metal oxide flat-band potential, overlaps with the potential range of application. On the other hand, as shown for Pt/ATO ORR catalyst, the electrochemical transistor switching of ATO conductivity has a highly beneficial effect on the stability of supported Pt nanoparticles by limiting electrochemical dissolution and Ostwald ripening at high electrode potentials. Therefore, the electrochemical transistor effect of metal oxide supports offers a way for the development of ORR catalysts combining high performance with strongly improved stability. Acknowledgement This work was supported by CCEM Switzerland and Umicore AG & Co KG within the project DuraCat. The authors thank the BESSY II synchrotron at Helmholtz-Zentrum Berlin, Germany, for providing beamtime at the 7T MPW SAXS beamline.

Characterization of a delaminated clay and pillared clays by adsorption of probe molecules

Adsorption of CO probe molecules on metal catalysts is widely used to characterize the surface reactivity and morphology of these nanomaterials by assigning measured C-O vibrational frequencies to particular surface sites. Density-functional calculations of the corresponding CO adsorption complexes provide key complementary data for such characterization. However, even for the adequate structural models, the calculated frequencies do not quantitatively match the experimental values due to approximations in conventional generalized-gradient exchange-correlation functionals. We proposed a frequency-dependent scaling of the density-functional C-O frequencies for adsorption on different sites of nanostructured Pd catalysts, enabling quantitative agreement with the reference experimental values. Then, we computationally studied coverage-dependent bridge CO adsorption on edge sites of Pd nanoparticles, which revealed the energetic feasibility of the full CO occupation of these sites. Due to the static and dynamic CO-CO interactions, the calculated C-O stretching frequency grows by as much as 100 cm-1 from the singleton CO adsorbed value with the number of coadsorbates at the neighboring bridge-edge sites. The saturation frequency approaches 1990 cm-1, quantitatively matching the value experimentally observed for moderately large Pd particles. Using our frequency scaling, such particles are estimated to be at least 3 nm large.

Applications of Adsorption Microcalorimetry to the Study of Heterogeneous Catalysis

Theory-Guided Machine Learning Finds Geometric Structure-Property Relationships for Chemisorption on Subsurface Alloys

The presence of both acidic and basic adsorption sites on the surface of hydroxyapatite [${\mathrm{Ca}}_{10}{({\mathrm{PO}}_{4})}_{6}{(\mathrm{OH})}_{2}$; HAP] is an interesting property for catalytic applications. Here, we report a density functional theory investigation of the adsorption properties of $\mathrm{CO}, {\mathrm{CO}}_{2}, {\mathrm{C}}_{2}{\mathrm{H}}_{2}, {\mathrm{CH}}_{4}, {\mathrm{H}}_{2}, {\mathrm{H}}_{2}\mathrm{O}, {\mathrm{NH}}_{3}, {\mathrm{SO}}_{2}$, and ${\mathrm{BCl}}_{3}$ on the $\mathrm{HAP}(0001)$ surface. All probe molecules have a lower energy when they are adsorbed in the region between the most exposed ${\mathrm{Ca}}^{2+}$ ion (electron acceptor) and a neighboring $\mathrm{PO}{{}_{4}}^{3\ensuremath{-}}$ group, where the $\mathrm{O}$ atoms (electron donor) contribute to the stabilization of the adsorbed molecule. By evaluating the redistribution of the electron density and the change of the atomic charges, the ${\mathrm{Ca}}^{2+}$ and $\mathrm{PO}{{}_{4}}^{3\ensuremath{-}}$ sites were identified as Lewis acidic and Lewis basic adsorption sites, respectively, which indicates that simultaneous acid-base interactions occur upon adsorption of all studied probe molecules. All adsorbates interact with the surface via atoms of opposing charges, which enhances the ionic character of molecular bonds by increasing the distinction between cationic and anionic charges within the molecule. Furthermore, molecules with greater ionic character show stronger interaction with the substrate and greater geometric deformation. Although most adsorbed molecules ($\mathrm{CO}, {\mathrm{CO}}_{2}, {\mathrm{C}}_{2}{\mathrm{H}}_{2}, {\mathrm{CH}}_{4}, {\mathrm{H}}_{2}, {\mathrm{H}}_{2}\mathrm{O}$, and ${\mathrm{NH}}_{3}$) do not show substantial net charge transfer, polarization effects due to the redistribution of charge are observed upon adsorption of all probe molecules. The change in the work function increases linearly with the total change in the surface dipole moment for ${\mathrm{H}}_{2}\mathrm{O}, {\mathrm{NH}}_{3}, {\mathrm{SO}}_{2}$, and ${\mathrm{BCl}}_{3}$, while for the remaining systems, the magnitude of the work function change remains more uniform. By identifying the type of interaction between each probe molecule and the $\mathrm{HAP}(0001)$ surface, the present study contributes to the understanding of the acid-base properties of the $\mathrm{HAP}(0001)$ surface, which we elaborated in a short discussion based on the individual bond orders for the acidic and basic sites.

A new generation of zirconia supported metal oxide catalysts for converting low grade renewable feedstocks to biodiesel

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTCombination Catalysts Consisting of a Homogeneous Catalyst Tethered to a Silica-Supported Palladium Heterogeneous Catalyst: Arene HydrogenationHanrong Gao and Robert J. AngeliciView Author Information Department of Chemistry and Ames Laboratory Iowa State University, Ames, Iowa 50011 Cite this: J. Am. Chem. Soc. 1997, 119, 29, 6937–6938Publication Date (Web):July 23, 1997Publication History Received31 March 1997Published online23 July 1997Published inissue 1 July 1997https://pubs.acs.org/doi/10.1021/ja9710058https://doi.org/10.1021/ja9710058rapid-communicationACS PublicationsCopyright © 1997 American Chemical SocietyRequest reuse permissionsArticle Views1298Altmetric-Citations71LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Aromatic compounds,Catalysts,Hydrocarbons,Hydrogenation,Metals Get e-Alerts

In this thesis a study is focused on the synthesis of organic compounds of interest in Fine Chemistry and biomass derivatives by design of more sustainable chemical processes and the use of heterogeneous catalysts with the optimal properties for each reaction.\nIn the first approach, it has been carried out the synthesis of nitriles, compounds of great application in the chemical and pharmaceutical industry, by the dehydration of aldoximes in the presence of different heterogeneous catalysts with both acidic and basic properties. Catalytic studies showed that the best results in terms of nitrile yield and selectivity are obtained with catalysts having Lewis acid centers. Among them, the iron MOFs (MIL-100 (Fe) and Fe (BTC)) proved to be the most suitable catalysts for this reaction obtaining excellent yields and selectivity towards nitriles. By means of a post synthesis treatment with ammonium fluoride it was possible to increase the catalytic activity of MIL-100 (Fe) increasing its BET area and pore volume. Studies using IR spectroscopy and XPS analysis led to the conclusion that the catalytic activity of MIL-100 (Fe) is related to the Fe species of the crystalline lattice. It has been shown that MIL-100 (Fe) -NH4F is stable and reusable in several consecutive reaction cycles without loss of activity and has been successfully applied to the synthesis of a wide variety of nitriles.\nOn the other hand, the synthesis of nitriles from aldoximes was carried out in the presence of several metal oxides, being cerium oxide the most active catalyst. The acid-base properties of the metal oxides were studied by the adsorption of probe molecules on their surface analyzed by IR spectroscopy. Thus, a relationship between the acid-base properties of the oxides with their catalytic activity was established, being the oxides with the strongest basic centers such as CeO2 and MgO the most active catalysts. Based on the IR studies of the aldoxime dehydration reaction on the surface of MgO and CeO2 in situ, a mechanism of reaction was proposed. Excellent yields were obtained for aromatic, aliphatic and cyclic nitriles using cerium oxide as the catalyst. In addition, CeO2 proved to be a stable and reusable catalyst being possible to be reused during four consecutive cycles without loss in its catalytic activity. The study was expanded to obtain different amides and esters with pharmacological properties from aldoximes by one-pot processes using the nanocrystalline cerium oxide as a catalyst.\nFinally, the work is focused on obtaining products derived from biomass, specifically on the synthesis of DFF and furylidenpropanenitrile derivatives with potential application as monomers. The DFF was obtained by carrying out the oxidation of 5-HMF in the presence of several MOFs as heterogeneous catalysts. It was shown that using the MIL-100 (Fe)-NH4F/TEMPO/NaNO2 as catalyst system for the oxidation of 5-HMF it is possible to obtain 100% yield and 100 % selectivity to DFF. In addition, this catalytic system was used for the oxidation of different primary and secondary alcohols obtaining good yields to the corresponding carbonyl compounds. Finally, in a second step the synthesis of furylidenepropanenitrile derivatives was carried out by Knoevenagel condensation between previously obtained DFF and active methylene compounds (malononitrile and ethyl cyanoacetate) obtaining excellent yields to the desired products.\n¿

Chapter 11 - Catalysis By Supported Metal Oxides

Methanol oxidation over metal oxide catalysts is industrially important for the production of formaldehyde, but knowledge about the intrinsic catalysis taking place is often obscured by a lack of knowledge as to the number of active sites present on the catalyst surface. In the present study, the number of surface sites active in methanol oxidation has been determined over a wide range of supported metal oxide catalysts using quantitative methanol chemisorption and in-situ infrared titration techniques performed at an experimentally optimized temperature of 110 °C. It was found that a steric limitation of about 0.3 methoxylated surface species (e.g., strongly Lewis-bound CH3OHads and dissociatively adsorbed −OCH3,ads, which are the reactive surface intermediates in methanol oxidation) exists per active deposited metal oxide metal atom across all supported metal oxides. Hence, the use of methanol chemisorption for counting active surface sites is more realistic than other site-counting methods for the kine...

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation