Madelyn R. Ball

Transition Metal Nitride Core-Noble Metal Shell Nanoparticles as Highly CO Tolerant Catalysts

Core-shell architectures offer an effective way to tune and enhance the properties of noble-metal catalysts. Herein, we demonstrate the synthesis of Pt shell on titanium tungsten nitride core nanoparticles (Pt/TiWN) by high temperature ammonia nitridation of a parent core-shell carbide material (Pt/TiWC). X-ray photoelectron spectroscopy revealed significant core-level shifts for Pt shells supported on TiWN cores, corresponding to increased stabilization of the Pt valence d-states. The modulation of the electronic structure of the Pt shell by the nitride core translated into enhanced CO tolerance during hydrogen electrooxidation in the presence of CO. The ability to control shell coverage and vary the heterometallic composition of the shell and nitride core opens up attractive opportunities to synthesize a broad range of new materials with tunable catalytic properties.

Effect of carbon supports on RhRe bifunctional catalysts for selective hydrogenolysis of tetrahydropyran-2-methanol

Tetrahydropyran-2-methanol undergoes selective C–O–C hydrogenolysis to produce 1,6-hexanediol using a bifunctional RhRe (reducible metal with an oxophilic promoter) catalyst supported on Vulcan XC-72 carbon (VXC) with >90% selectivity. This RhRe/VXC catalyst is stable over 40 h of reaction in a continuous flow fixed bed reactor. The hydrogenolysis activity of RhRe/VXC is two orders-of-magnitude higher than that of RhRe supported on Norit Darco 12X40 activated carbon (NDC). STEM–EDS analysis reveals that, compared to the RhRe/VXC catalyst, the Re and Rh component metals are segregated on the surface of the low activity RhRe/NDC catalyst, suggesting that Rh and Re in close proximity (“bimetallic” particles) are required for an active hydrogenolysis catalyst. Differences in metal distribution on the carbon surfaces are, in turn, linked to the properties of the carbons: NDC has both a higher surface area and surface oxygen content. The low areal density of Rh and Re precursors on the high area NDC and/or interactions of the precursors with its O functional groups may interfere with the formation of the bimetallic species required for an active catalyst.

Microkinetic Analysis and Scaling Relations for Catalyst Design

Microkinetic analysis plays an important role in catalyst design because it provides insight into the fundamental surface chemistry that controls catalyst performance. In this review, we summarize the development of microkinetic models and the inclusion of scaling relationships in these models. We discuss the importance of achieving stoichiometric and thermodynamic consistency in developing microkinetic models. We also outline how analysis of the maximum rates of elementary steps can be used to determine which transition states and adsorbed intermediates are kinetically significant, allowing the derivation of general reaction kinetics rate expressions in terms of changes in binding energies of the relevant transition states and intermediates. Through these analyses, we present how to predict optimal surface coverages and binding energies of adsorbed species, as well as the extent of potential rate improvement for a catalytic system. For systems in which the extent of potential rate improvement is small because of limitations imposed by scaling relations, different approaches, including the addition of promoters and formation of catalysts containing multiple functionalities, can be used to break the scaling relations and obtain further rate enhancement.

Amination of 1-hexanol on bimetallic AuPd/TiO2 catalysts

AuPd/TiO2 catalysts, synthesized using controlled surface reactions, are active for the gas-phase amination of 1-hexanol using ammonia. The bimetallic active sites for these catalysts have been characterized using CO chemisorption and XAS techniques, and the absence of monometallic Pd species in the AuPd catalysts was confirmed using UV-vis and STEM-EDS analysis. The bimetallic catalysts exhibit synergy between Au and Pd, as the rate of hexanol conversion increases from 8.7 μmol ks−1 (μmol total Pd)−1 over Pd/TiO2 to up to 42 μmol ks−1 (μmol total Pd)−1 over AuPd/TiO2 with a Pd/Au atomic ratio of 0.06. The rate of hexanol conversion is also enhanced with respect to Au content, with a 5-fold increase in the total Au-normalized rate from Au/TiO2 to AuPd0.67/TiO2. As Pd is added to Au/TiO2 in increasing quantities, the production rate of primary species (i.e., hexylamine and hexanenitrile) is preferentially increased. The rate of dihexylamine production increases to a lesser extent, while trihexylamine formation remains relatively constant across Pd loadings. Moreover, trihexylamine, which cannot be formed via the condensation of dihexylamine and hexanol, is shown to be produced via the secondary aldimine, N-hexylidene hexylamine. The AuPd bimetallic catalysts also exhibit reduced hydrogenolysis activity compared to monometallic Pd/TiO2.

Correction: Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC

Correction for ‘Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC’ by Carlos A. Carrero et al., Catal. Sci. Technol., 2017, DOI: 10.1039/c7cy01036b.

Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC

A series of supported two- and three-dimensional vanadium oxide surface species on β-SiC with various V coverages are prepared via incipient wetness impregnation and characterized by a variety of ex and in situ techniques. The oxidative dehydrogenation of propane (ODHP) is also used as a probe reaction to complementarily distinguish between two- and three-dimensional VOx surface species. Herein, we show that treating pristine β-SiC with oxygen transforms the existing amorphous SiOxCy surface layer into a more SiO2-type layer, though with a negligible formation of Si–OH sites, which initially were expected to be the anchor sites for VOx species. In its place, the C–OH functional groups identified by X-ray photoelectron spectroscopy (XPS) act as anchor sites for the VOx species during the impregnation process, and are consumed as a function of V coverage. Our experimental observations all corroborate the formation of two- and three-dimensional VOx species on the surface of β-SiC.