Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano-Islands.

CeO2-supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2-Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

Highly loaded bimetallic iron-cobalt catalysts for hydrogen release from ammonia.

Ammonia is a storage molecule for hydrogen, which can be released by catalytic decomposition. Inexpensive iron catalysts suffer from a low activity due to a too strong iron-nitrogen binding energy compared to more active metals such as ruthenium. Here, we show that this limitation can be overcome by combining iron with cobalt resulting in a Fe-Co bimetallic catalyst. Theoretical calculations confirm a lower metal-nitrogen binding energy for the bimetallic catalyst resulting in higher activity. Operando spectroscopy reveals that the role of cobalt in the bimetallic catalyst is to suppress the bulk-nitridation of iron and to stabilize this active state. Such catalysts are obtained from Mg(Fe,Co)2O4 spinel pre-catalysts with variable Fe:Co ratios by facile co-precipitation, calcination and reduction. The resulting Fe-Co/MgO catalysts, characterized by an extraordinary high metal loading reaching 74 wt.%, combine the advantages of a ruthenium-like electronic structure with a bulk catalyst-like microstructure typical for base metal catalysts.

Efficient noble metal promoted bimetallic cobalt catalysts in the selective synthesis of acetaldehyde dimethyl acetal.

Herein we report the one-pot cobalt catalysed synthesis of the dimethylacetal of acetaldehyde from synthesis gas and methanol. The product can be used as a fuel additive either as it is or after transacetalisation with long-chain alcohols. The product is obtained at moderate temperatures in good selectivities and high CO-conversions. A variation of the promotor metal (Au, Pt, Pd, and Ru) and of the support (γ-Al2O3 and CeO2) in the catalyst was conducted, which showed a great impact of both the support and promotor on the activity and structure of the catalyst. Furthermore, a specific variation of temperatures and pressure for the most active catalyst and a model catalyst was conducted giving an interesting insight into ongoing processes.

Shape-Dependent CO2 Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO.

The hydrogenation of CO2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.

Unlocking synergy in bimetallic catalysts by core-shell design.

Extending the toolbox from mono- to bimetallic catalysts is key in realizing efficient chemical processes1. Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition1-3, often resulting in a compromise between the catalytic properties of the two metals4-6. Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single-crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell-thickness-dependent catalytic activity, indicating that not only the nature of the surface but also several subsurface layers play a crucial role in the catalytic performance, and rationalize this finding using density functional theory calculations. Our results open up an alternative avenue for the structural design of bimetallic catalysts.

Supported Intermetallic PdZn Nanoparticles as Bifunctional Catalysts for the Direct Synthesis of Dimethyl Ether from CO-Rich Synthesis Gas.

The single-step syngas-to-dimethyl ether (STD) process entails economic and technical advantages over the current industrial two-step process. Pd/ZnO-based catalysts have recently emerged as interesting alternatives to currently used Cu/ZnO/Al2 O3 catalysts, but the nature of the active site(s), the reaction mechanism, and the role of Pd and ZnO in the solid catalyst are not well established. Now, Zn-stabilized Pd colloids with a size of 2 nm served as the key building blocks for the methanol active component in bifunctional Pd/ZnO-γ-Al2 O3 catalysts. The catalysts were characterized by combining high-pressure operando X-ray absorption spectroscopy and DFT calculations. The enhanced stability, longevity, and high dimethyl ether selectivity observed makes Pd/ZnO-γ-Al2 O3 an effective alternative system for the STD process compared to Cu/ZnO/γ-Al2 O3 .

Formation Mechanism of the First Carbon-Carbon Bond and the First Olefin in the Methanol Conversion into Hydrocarbons.

The elementary reactions leading to the formation of the first carbon-carbon bond during early stages of the zeolite-catalyzed methanol conversion into hydrocarbons were identified by combining kinetics, spectroscopy, and DFT calculations. The first intermediates containing a C-C bond are acetic acid and methyl acetate, which are formed through carbonylation of methanol or dimethyl ether even in presence of water. A series of acid-catalyzed reactions including acetylation, decarboxylation, aldol condensation, and cracking convert those intermediates into a mixture of surface bounded hydrocarbons, the hydrocarbon pool, as well as into the first olefin leaving the catalyst. This carbonylation based mechanism has an energy barrier of 80 kJ mol(-1) for the formation of the first C-C bond, in line with a broad range of experiments, and significantly lower than the barriers associated with earlier proposed mechanisms.

Role of Amine Functionality for CO2 Chemisorption on Silica.

The mechanism of CO2 adsorption on primary, secondary, and bibasic aminosilanes synthetically functionalized in porous SiO2 was qualitatively and quantitatively investigated by a combination of IR spectroscopy, thermogravimetry, and quantum mechanical modeling. The mode of CO2 adsorption depends particularly on the nature of the amine group and the spacing between the aminosilanes. Primary amines bonded CO2 preferentially through the formation of intermolecular ammonium carbamates, whereas CO2 was predominantly stabilized as carbamic acid, when interacting with secondary amines. Ammonium carbamate formation requires the transfer of the carbamic acid proton to a second primary amine group to form the ammonium ion and hence two (primary) amine groups are required to bind one CO2 molecule. The higher base strength of secondary amines enables the stabilization of carbamic acid, which is thereby hindered to interact further with nearby amine functions, because their association with Si-OH groups (either protonation or hydrogen bonding) does not allow further stabilization of carbamic acid as carbamate. Steric hindrance of the formation of intermolecular ammonium carbamates leads to higher uptake capacities for secondary amines functionalized in porous SiO2 at higher amine densities. In aminosilanes possessing a primary and a secondary amine group, the secondary amine group tends to be protonated by Si-OH groups and therefore does not substantially interact with CO2.

Photoswitching in nanoporous, crystalline solids: an experimental and theoretical study for azobenzene linkers incorporated in MOFs.

In this article, we use the popular photoswitchable molecule, azobenzene, to demonstrate that the embedding in a nanoporous, crystalline solid enables a precise understanding of light-induced, reversible molecular motion. We investigate two similar azobenzene-containing, pillared-layer metal-organic frameworks (MOFs): Cu2(AzoBPDC)2(BiPy) and Cu2(NDC)2(AzoBiPy). Experimental results from UV-vis spectroscopy and molecular uptake experiments as well as theoretical results based on density-functional theory (DFT) show that in the Cu2(AzoBPDC)2(BiPy) MOF structure, the azobenzene side groups undergo photoisomerization when irradiated with UV or visible light. In a very similar MOF structure, Cu2(NDC)2(AzoBiPy), the experimental studies show an unexpected absence of photoisomerization. The DFT calculations reveal that in both MOFs the initial and final states of the photoswitching process (the trans and the cis conformation) have similar energies, which strongly suggests that the reason for the effective blocking of photoswitching in the AzoBiPy-based MOFs must be related to the switching process itself. More detailed calculations show that in Cu2(NDC)2(AzoBiPy) a naphthalene linker from the molecular framework blocks the photoisomerization trajectory which leads from the trans to the cis conformation. For Cu2(AzoBPDC)2(BiPy), as a result of the different geometry, such a steric hindrance is absent.

Postsynthetic Metal and Ligand Exchange in MFU-4l: A Screening Approach toward Functional Metal-Organic Frameworks Comprising Single-Site Active Centers.

The isomorphous partial substitution of Zn(2+) ions in the secondary building unit (SBU) of MFU-4l leads to frameworks with the general formula [M(x)Zn(5-x)Cl4(BTDD)3], in which x≈2, M = Mn(II), Fe(II), Co(II), Ni(II), or Cu(II), and BTDD = bis(1,2,3-triazolato-[4,5-b],[4',5'-i])dibenzo-[1,4]-dioxin. Subsequent exchange of chloride ligands by nitrite, nitrate, triflate, azide, isocyanate, formate, acetate, or fluoride leads to a variety of MFU-4l derivatives, which have been characterized by using XRPD, EDX, IR, UV/Vis-NIR, TGA, and gas sorption measurements. Several MFU-4l derivatives show high catalytic activity in a liquid-phase oxidation of ethylbenzene to acetophenone with air under mild conditions, among which Co- and Cu derivatives with chloride side-ligands are the most active catalysts. Upon thermal treatment, several side-ligands can be transformed selectively into reactive intermediates without destroying the framework. Thus, at 300 °C, Co(II)-azide units in the SBU of Co-MFU-4l are converted into Co(II)-isocyanate under continuous CO gas flow, involving the formation of a nitrene intermediate. The reaction of Cu(II)-fluoride units with H2 at 240 °C leads to Cu(I) and proceeds through the heterolytic cleavage of the H2 molecule.

Scorpionate-type coordination in MFU-4l metal-organic frameworks: small-molecule binding and activation upon the thermally activated formation of open metal sites.

Postsynthetic metal and ligand exchange is a versatile approach towards functionalized MFU-4l frameworks. Upon thermal treatment of MFU-4l formates, coordinatively strongly unsaturated metal centers, such as zinc(II) hydride or copper(I) species, are generated selectively. Cu(I)-MFU-4l prepared in this way was stable under ambient conditions and showed fully reversible chemisorption of small molecules, such as O2, N2, and H2, with corresponding isosteric heats of adsorption of 53, 42, and 32 kJ mol(-1), respectively, as determined by gas-sorption measurements and confirmed by DFT calculations. Moreover, Cu(I)-MFU-4l formed stable complexes with C2H4 and CO. These complexes were characterized by FTIR spectroscopy. The demonstrated hydride transfer to electrophiles and strong binding of small gas molecules suggests these novel, yet robust, metal-organic frameworks with open metal sites as promising catalytic materials comprising earth-abundant metal elements.

Adsorption structure determination of a large polyaromatic trithiolate on Cu(111): combination of LEED-I(V) and DFT-vdW.

The adsorption geometry of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) on Cu(111) is determined with high precision using two independent methods, experimentally by quantitative low energy electron diffraction (LEED-I(V)) and theoretically by dispersion corrected density functional theory (DFT-vdW). Structural refinement using both methods consistently results in similar adsorption sites and geometries. Thereby a level of confidence is reached that allows deduction of subtle structural details such as molecular deformations or relaxations of copper substrate atoms.

Control of intermolecular bonds by deposition rates at room temperature: hydrogen bonds versus metal coordination in trinitrile monolayers.

Self-assembled monolayers of 1,3,5-tris(4'-biphenyl-4"-carbonitrile)benzene, a large functional trinitrile molecule, on the (111) surfaces of copper and silver under ultrahigh vacuum conditions were studied by scanning tunneling microscopy and low-energy electron diffraction. A densely packed hydrogen-bonded polymorph was equally observed on both surfaces. Additionally, deposition onto Cu(111) yielded a well-ordered metal-coordinated porous polymorph that coexisted with the hydrogen-bonded structure. The required coordination centers were supplied by the adatom gas of the Cu(111) surface. On Ag(111), however, the well-ordered metal-coordinated network was not observed. Differences between the adatom reactivities on copper and silver and the resulting bond strengths of the respective coordination bonds are held responsible for this substrate dependence. By utilizing ultralow deposition rates, we demonstrate that on Cu(111) the adatom kinetics plays a decisive role in the expression of intermolecular bonds and hence structure selection.

Reversible gas-phase redox processes catalyzed by Co-exchanged MFU-4l(arge).

Postsynthetic metal ion exchange in a benzotriazolate-based MFU-4l(arge) framework leads to a Co(II)-containing framework with open metal sites showing reversible gas-phase oxidation properties.

Determination of CO, H2O and H2 coverage by XANES and EXAFS on Pt and Au during water gas shift reaction.

The turn-over-rate (TOR) for the water gas shift (WGS) reaction at 200 degrees C, 7% CO, 9% CO(2), 22% H(2)O, 37% H(2) and balance Ar, of 1.4 nm Au/Al(2)O(3) is approximately 20 times higher than that of 1.6 nm Pt/Al(2)O(3). Operando EXAFS experiments at both the Au and Pt L(3) edges reveal that under reaction conditions, the catalysts are fully metallic. In the absence of adsorbates, the metal-metal bond distances of Pt and Au catalysts are 0.07 A and 0.13 A smaller than those of bulk Pt and Au foils, respectively. Adsorption of H(2) or CO on the Pt catalysts leads to significantly longer Pt-Pt bond distances; while there is little change in Au-Au bond distance with adsorbates. Adsorption of CO, H(2) and H(2)O leads to changes in the XANES spectra that can be used to determine the surface coverage of each adsorbate under reaction conditions. During WGS, the coverage of CO, H(2)O, and H(2) are obtained by the linear combination fitting of the difference XANES, or DeltaXANES, spectra. Pt catalysts adsorb CO, H(2), and H(2)O more strongly than the Au, in agreement with the lower CO reaction order and higher reaction temperatures.