Karin Föttinger

How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry

Discussed are the recent experimental and theoretical results on palladium‐based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi‐method and multi‐material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub‐surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a PdC surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub‐surface increases considerably the selectivity of alkyne semi‐hydrogenation over Pd‐based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that PdC hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk PdH. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of PdGa intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively.

The Intermetallic Compound ZnPd and its Role in Methanol Steam Reforming

The rich literature about the intermetallic compound ZnPd as well as several ZnPd near-surface intermetallic phases is reviewed. ZnPd is frequently observed in different catalytic reactions triggering this review in order to collect the knowledge about the compound. The review addresses the chemical and physical properties of the compound and relates these comprehensively to the catalytic properties of ZnPd in methanol steam reforming—an interesting reaction to release hydrogen for a future hydrogen-based energy infrastructure from water/methanol mixtures. The broad scope of the review covers experimental work as well as quantum chemical calculations on a variety of Pd-Zn materials, aiming at covering all relevant literature to derive a sound state-of-the-art picture of the understanding gained so far.

Ga-Pd/Ga2O3 Catalysts: The Role of Gallia Polymorphs, Intermetallic Compounds, and Pretreatment Conditions on Selectivity and Stability in Different Reactions

A series of gallia‐supported Pd‐Ga catalysts that consist of metallic nanoparticles on three porous polymorphs of Ga2O3 (α‐, β‐, and γ‐Ga2O3) were synthesized by a controlled co‐precipitation of Pd and Ga. The effects of formation of Ga‐Pd intermetallic compounds (IMCs) were studied in four catalytic reactions: methanol steam reforming, hydrogenation of acetylene, and methanol synthesis by CO and CO2 hydrogenation reactions. The IMC Pd2Ga forms upon reduction of α‐ and β‐Ga2O3‐supported materials in hydrogen at temperatures of 250 and 310 °C, respectively. At higher temperatures, Ga‐enrichment of the intermetallic particles is observed, leading to formation of Pd5Ga3 before the support itself is reduced at temperatures above 565 °C. In the case of Ga‐Pd/γ‐Ga2O3, no information about the metal particles could be obtained owing to their very small size and high dispersion; however, the catalytic results suggest that the IMC Pd2Ga also forms in this sample. Pd2Ga/gallia samples show a stable selectivity towards ethylene in acetylene hydrogenation (≈75 %), which is higher than for a monometallic Pd reference catalyst. An even higher selectivity of 80 % was observed for Pd5Ga3 supported on α‐Ga2O3. In methanol steam reforming, the Ga‐Pd/Gallia catalysts showed, in contrast to Pd/Al2O3, selectivity towards CO2 of up to 40 %. However, higher selectivities, which have been reported for Pd2Ga in literature, could not be reproduced in this study, which might be a result of particle size effects. The initially higher selectivity of the Pd5Ga3‐containing samples was not stable, suggesting superior catalytic properties for this IMC, but that re‐oxidation of Ga species and formation of Pd2Ga occurs under reaction conditions. In methanol synthesis, CO hydrogenation did not occur, but a considerable methanol yield from a CO2/H2 feed was observed for Pd2Ga/α‐Ga2O3.

Surface modification processes during methane decomposition on Cu-promoted Ni–ZrO2 catalysts

We explored the surface chemistry of methane on Cu-promoted Ni–ZrO2 catalysts and observed a limited stability of the CuNi alloy under relevant reaction conditions.

The oxidation state of copper in bimetallic (Pt–Cu, Pd–Cu) catalysts during water denitration

Catalytic denitration of water with bimetallic systems has emerged as a viable solution for removal of nitrates from drinking water. Despite the progress in process development during the last two decades, only a few studies were performed to determine catalyst structure under working conditions. Herein, we determined the relative population of Cu oxidation states in Pt–Cu and Pd–Cu bimetallic catalysts by in situ high resolution X-ray absorption spectroscopy in combination with principal component analysis. The initial state of the catalyst was a Pt–Cu or Pd–Cu alloy. Segregation of the metal components occurred under reaction conditions especially for a Pt–Cu system. The active oxidation states of copper were metallic and alloy, and their concentration was highly dependent on the amount of hydrogen in the feed. Initial alloy phase of the catalysts ensures close proximity between Cu and the noble metals after segregation, essential to maintain catalysts activity.

Photoluminescence as Complementary Evidence for Short-Range Order in Ionic Silica Nanoparticle Networks.

Recently we published the synthesis of new hybrid materials, ionic silica nanoparticles networks (ISNN), made of silica nanoparticles covalently connected by organic bridging ligands containing imidazolium units owing to a “click-chemistry-like” reaction. Among other techniques small-angle X-ray scattering (SAXS) experiments were carried out to get a better picture of the network extension. It turned out that the short-range order in ISNN materials was strongly influenced by the rigidity of the bridging ligand, while the position of the short-range order peaks confirmed the successful linking of the bridging ligands. The photoluminescence experiments reported in this communication revealed strongly enhanced emission in the hybrid material in comparison with neat imidazolium salts. Moreover the shift of the emission maximum toward longer wavelengths, obtained when varying the aromatic ring content of the bridging ligand, suggested the existence of strong π−π stacking in the hybrid material. Experiments revealed a stronger luminescence in those samples exhibiting the higher extent of short-range order in SAXS.

Infrared spectroscopic investigation of CO adsorption on SBA-15- and KIT-6-supported nickel phosphide hydrotreating catalysts.

The infrared (IR) spectra of CO adsorbed on 10, 20, and 30 wt % nickel phosphide-containing reduced SBA-15 and KIT-6 mesoporous silica-supported catalysts have been studied at 300-473 K. On the catalysts containing a stoichiometric amount of phosphorus with 20 wt % loading, the most intense IR absorption band was observed at 2097-2099 cm(-1), which was assigned to CO terminally bonded to coordinatively unsaturated Ni(delta+) (0 < delta < 1) sites. The frequency of this band was 15 cm(-1), higher than that in the spectrum of a reduced Ni2P/SiO2 catalyst, indicating a modified Ni-P charge distribution. This band shifted to lower wavenumbers, and its intensity decreased, while the relative intensity of another band at 2191-2194 cm(-1) assigned to CO terminally bonded to P increased going to catalytically less active, excess-P-containing SBA-15-supported catalysts. CO also adsorbed as a bridged carbonyl (1910 cm(-1)) and as Ni(CO)4 (2050 cm(-1)) species, and the formation of surface carbonates was also identified. The nature of the surface acidity was studied by temperature-programmed desorption of ammonia (NH3-TPD). Weak and strong acid sites were revealed, and the high excess-P-containing catalyst released the highest amount of ammonia, indicating that a high concentration of strong acidity can be disadvantageous for reaching high hydrotreating catalytic activity. The modified Ni-P charge distribution, the mode of CO adsorption on surface nickel phosphide sites, as well as the acidity can be directly connected to the catalytic activity of these mesoporous silica-supported catalysts.

Colloidally prepared Pt nanowires versus impregnated Pt nanoparticles: comparison of adsorption and reaction properties.

Ligand-capped Pt nanowires, prepared by colloidal synthesis and deposited on a high surface area γ-Al(2)O(3) support, were subjected to surface characterization by electron microscopy and FTIR spectroscopy using CO as a probe molecule. The structural, adsorption, and catalytic reaction properties of the colloidal Pt nanowires were compared to those of conventional, impregnated Pt nanoparticles on the same Al(2)O(3) support. In situ FTIR spectroscopy indicated ligand effects on the CO resonance frequency, irreversible CO-induced surface roughening upon CO adsorption, and a higher resistance of colloidal catalysts toward oxidation (both in oxygen and during CO oxidation), suggesting that the organic ligands might protect the Pt surface. Elevated temperature induced a transformation of Pt nanowires to faceted Pt nanoparticles. The colloidal catalyst was active for hydrodechlorination of trichloroethylene (TCE), but no ligand effect on selectivity was obtained.

PdZn based catalysts: connecting electronic and geometric structure with catalytic performance

In the recent years, the potential of PdZn intermetallic compounds and related compositions for improving and consequently replacing conventionally used catalysts has been explored for a range of diverse processes, such as selective hydrogenation reactions, methanol synthesis and steam reforming. PdZn has similar electronic properties and reactivity as Cu, a widely used metal catalyst, e.g. Cu is industrially applied in the low temperature water gas shift reaction and methanol synthesis. The higher stability of PdZn makes it an attractive alternative for certain applications. This review will give an overview over selected important potential applications and the correlation of the catalytic performance with properties, such as the electronic structure. A broad range of materials from oxide supported nanoparticles to single crystal based model systems is covered.

Different Synthesis Protocols for Co3O4–CeO2 Catalysts—Part 1: Influence on the Morphology on the Nanoscale

Co3O4-modified CeO2 (Co/Ce 1:4) was prepared by a combination of sol–gel processing and solvothermal treatment. The distribution of Co was controlled by means of the synthesis protocol to yield three different morphologies, namely, Co3O4 nanoparticles located on the surface of CeO2 particles, coexistent Co3O4 and CeO2 nanoparticles, or Co oxide structures homogeneously distributed within CeO2. The effect of the different morphologies on the properties of Co3O4–CeO2 was investigated with regard to the crystallite phase(s), particle size, surface area, and catalytic activity for CO oxidation. The material with Co3O4 nanoparticles finely dispersed on the surface of CeO2 particles had the highest catalytic activity.