Synthesis of Pt3Zn1 and Pt1Zn1 intermetallic nanocatalysts for dehydrogenation of ethane.

Pt3Zn1 and Pt1Zn1 intermetallic nanoparticles supported on SiO2 were synthesized by combining atomic layer deposition (ALD) of ZnO, incipient wetness impregnation (IWI) of Pt, and appropriate hydrogen reduction. The formation of Pt1Zn1 and Pt3Zn1 intermetallic nanoparticles was observed by both X-ray diffraction (XRD) and synchrotron X-ray absorption spectroscopy (XAS). STEM images showed that the 2-3 nm Pt-based intermetallic nanoparticles were uniformly dispersed on a SiO2 support. The relationships between Pt-Zn intermetallic phases and synthesis conditions were established. In situ XAS measurements at Pt L3 and Zn K edges during hydrogen reduction provided a detailed image of surface species evolution. Owing to a combined electronic and geometric effect, Pt1Zn1 exhibited much higher reactivity and stability than Pt3Zn1 and Pt in both the direct dehydrogenation and oxidative dehydrogenation of ethane to ethylene reactions.

Catalytic Applications of Vanadium: A Mechanistic Perspective.

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

High Thermal Stability of La2O3- and CeO2-Stabilized Tetragonal ZrO2.

Catalyst support materials of tetragonal ZrO2, stabilized by either La2O3 (La2O3-ZrO2) or CeO2 (CeO2-ZrO2), were synthesized under hydrothermal conditions at 200 °C with NH4OH or tetramethylammonium hydroxide as the mineralizer. From in situ synchrotron powder X-ray diffraction and small-angle X-ray scattering measurements, the calcined La2O3-ZrO2 and CeO2-ZrO2 supports were nonporous nanocrystallites that exhibited rectangular shapes with a thermal stability of up to 1000 °C in air. These supports had an average size of ∼ 10 nm and a surface area of 59-97 m(2)/g. The catalysts Pt/La2O3-ZrO2 and Pt/CeO2-ZrO2 were prepared by using atomic layer deposition with varying Pt loadings from 6.3 to 12.4 wt %. Monodispersed Pt nanoparticles of ∼ 3 nm were obtained for these catalysts. The incorporation of La2O3 and CeO2 into the t-ZrO2 structure did not affect the nature of the active sites for the Pt/ZrO2 catalysts for the water-gas shift reaction.

Atomic layer deposition overcoating: tuning catalyst selectivity for biomass conversion.

The terraces, edges, and facets of nanoparticles are all active sites for heterogeneous catalysis. These different active sites may cause the formation of various products during the catalytic reaction. Here we report that the step sites of Pd nanoparticles (NPs) can be covered precisely by the atomic layer deposition (ALD) method, whereas the terrace sites remain as active component for the hydrogenation of furfural. Increasing the thickness of the ALD-generated overcoats restricts the adsorption of furfural onto the step sites of Pd NPs and increases the selectivity to furan. Furan selectivities and furfural conversions are linearly correlated for samples with or without an overcoating, though the slopes differ. The ALD technique can tune the selectivity of furfural hydrogenation over Pd NPs and has improved our understanding of the reaction mechanism. The above conclusions are further supported by density functional theory (DFT) calculations.

Capping ligands as selectivity switchers in hydrogenation reactions.

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt(3) NPs can drastically increase selectivity for alkene from 0 to more than 90% with ~99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes.

Stabilization of copper catalysts for liquid-phase reactions by atomic layer deposition.

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2 O3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.

Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane.

Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis. Their distinct catalytic properties are often hypothesized to result from the large fraction of under-coordinated surface atoms. Here, we show that size-preselected Pt(8-10) clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over by-products. Quantum chemical calculations indicate that under-coordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes.

Spectroscopic characterization of alumina-supported bis(allyl)iridium complexes: site-isolation, reactivity, and decomposition studies.

The covalent attachment of tris(allyl)iridium to partially dehydroxylated gamma-alumina is found to proceed via surface hydroxyl group protonation of one allyl ligand to form an immobilized bis(allyl)iridium moiety, (=AlO)Ir(allyl)(2), as characterized by CP-MAS (13)C NMR, inductively coupled plasma-mass spectrometry, and Ir L(3) edge X-ray absorption spectroscopy. Extended X-ray absorption fine-structure (EXAFS) measurements taken on unsupported Ir(allyl)(3) and several associated tertiary phosphine addition complexes suggest that the eta(3)-allyl ligands generally account for an Ir-C coordination number of 2 rather than 3, with an average Ir-C distance of 2.16 A. Using this knowledge, combined EXAFS and X-ray absorption near-edge structure studies reveal that a small amount of Ir(0) is also formed upon reaction of Ir(allyl)(3) with the surface. It was found that the addition of either 2,6-dimethylphenyl isocyanide or carbon monoxide to the supported complex allows spectroscopic identification of the supported bis(allyl)iridium complexes, (=AlO)Ir(allyl)(2)(CNAr) [Ar = 2,6-(CH(3))(2)C(6)H(4)] and (=AlO)Ir(allyl)(2)(CO)(2), respectively. Although samples of the supported bis(allyl)iridium complex are active for the dehydrogenation of cyclohexane to benzene at temperatures between 180 and 220 degrees C, in situ temperature-programmed reaction XAFS and continuous-flow reactor studies suggest that Ir(0) nanoparticles, rather than a well-defined Ir(3+) complex, are responsible for the observed activity.

Catalyst synthesis and evaluation using an integrated atomic layer deposition synthesis-catalysis testing tool.

An integrated atomic layer deposition synthesis-catalysis (I-ALD-CAT) tool was developed. It combines an ALD manifold in-line with a plug-flow reactor system for the synthesis of supported catalytic materials by ALD and immediate evaluation of catalyst reactivity using gas-phase probe reactions. The I-ALD-CAT delivery system consists of 12 different metal ALD precursor channels, 4 oxidizing or reducing agents, and 4 catalytic reaction feeds to either of the two plug-flow reactors. The system can employ reactor pressures and temperatures in the range of 10(-3) to 1 bar and 300-1000 K, respectively. The instrument is also equipped with a gas chromatograph and a mass spectrometer unit for the detection and quantification of volatile species from ALD and catalytic reactions. In this report, we demonstrate the use of the I-ALD-CAT tool for the synthesis of platinum active sites and Al2O3 overcoats, and evaluation of catalyst propylene hydrogenation activity.

In situ EXAFS analysis of the temperature-programmed reduction of Cu-ZSM-5.

High resolution in situ EXAFS during temperature-programmed reduction was performed on Cu-ZSM-5 to elucidate the state of copper under reaction conditions. Improvements in hardware and software allowed rapid acquisition of both XANES and EXAFS data during reduction, in particular, allowing observation of characteristic preedge features from various Cu oxidation states. EXAFS fitting and factor analysis of the normalized XANES edge were performed in an attempt to determine the number and type of Cu species present. The data suggests that initially only Cu(2+) is present in two different locations on the zeolite; both states reduce to Cu(1+) in both H(2) and CO, but under different conditions. Under H(2) conditions, migration of Cu(1+) to Cu(0) clusters is observed at 450 degrees C, while no metallic state is observed during CO reduction.