Zhenwei Wu

Pd–In intermetallic alloy nanoparticles: highly selective ethane dehydrogenation catalysts

Silica supported Pd and Pd–In catalysts with different In:Pd atomic ratios and similar particle size (∼2 nm) were tested for ethane dehydrogenation at 600 °C. For a monometallic Pd catalyst, at 15% conversion, the dehydrogenation selectivity and initial turnover rate (TOR, per surface Pd site) were 53% and 0.03 s−1, respectively. Addition of In to Pd increased the dehydrogenation selectivity to near 100% and the initial TOR to 0.26 s−1. Carbon monoxide IR, in situ synchrotron XAS and XRD analysis showed that for Pd–In catalysts with increasing In loading, different bimetallic structures were formed: at low In loading a fraction of the nanoparticle surface was transformed into PdIn intermetallic compound (IMC, also known as intermetallic alloy) with a cubic CsCl structure; at higher In loading, a Pd-core/PdIn-shell structure was formed and at high In loading the nanoparticles were pure PdIn IMC. While a Pd metal surface binds CO predominantly in a bridge fashion, the PdIn IMC predominantly binds CO linearly. Formation of the PdIn IMC structure on the catalyst surface geometrically isolates the Pd catalytic sites by non-catalytic, metallic In neighbors, which is suggested to be responsible for the high olefin selectivity. Concomitant electronic effect due to Pd–In bond formation likely leads to the increase in TOR. Though multiple IMC structures with different atomic ratios are possible for the Pd–In binary system, only a cubic PdIn IMC with CsCl structure was observed, implying a kinetically controlled solid state IMC formation mechanism.

High-Performance Transition Metal Phosphide Alloy Catalyst for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is a pivotal process in many energy conversion and storage techniques, such as water splitting, regenerative fuel cells, and rechargeable metal-air batteries. The synthesis of stable, efficient, non-noble metal-based electrocatalysts for OER has been a long-standing challenge. In this work, a facile and scalable method to synthesize hollow and conductive iron-cobalt phosphide (Fe-Co-P) alloy nanostructures using an Fe-Co metal organic complex as a precursor is described. The Fe-Co-P alloy exhibits excellent OER activity with a specific current density of 10 mA/cm2 being achieved at an overpotential as low as 252 mV. The current density at 1.5 V (vs reversible hydrogen electrode) of the Fe-Co-P catalyst is 30.7 mA/cm2, which is more than 3 orders of magnitude greater than that obtained with state-of-the-art Fe-Co oxide catalysts. Our mechanistic experiments and theoretical analysis suggest that the electrochemical-induced high-valent iron stabilizes the cobalt in a low-valent state, leading to the simultaneous enhancement of activity and stability of the OER catalyst.

Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange

Molecular catalysts offer tremendous advantages for stereoselective polymerization because their activity and selectivity can be optimized and understood mechanistically using the familiar tools of organometallic chemistry. Yet, this exquisite control over selectivity comes at an operational price that is generally not justifiable for the large-scale manufacture of polyfolefins. In this report, we identify Co-MFU-4l, prepared by cation exchange in a metal-organic framework, as a solid catalyst for the polymerization of 1,3-butadiene with high stereoselectivity (>99% 1,4-cis). To our knowledge, this is the highest stereoselectivity achieved with a heterogeneous catalyst for this transformation. The polymers low polydispersity (PDI ≈ 2) and the catalysts ready recovery and low leaching indicate that our material is a structurally resilient single-site heterogeneous catalyst. Further characterization of Co-MFU-4l by X-ray absorption spectroscopy provided evidence for discrete, tris-pyrazolylborate-like coordination of Co(II). With this information, we identify a soluble cobalt complex that mimics the structure and reactivity of Co-MFU-4l, thus providing a well-defined platform for studying the catalytic mechanism in the solution phase. This work underscores the capacity for small molecule-like tunability and mechanistic tractability available to transition metal catalysis in metal-organic frameworks.

Stabilized Vanadium Catalyst for Olefin Polymerization by Site Isolation in a Metal-Organic Framework

Vanadium catalysts offer unique selectivity in olefin polymerization, yet are underutilized industrially owing to their poor stability and productivity. Reported here is the immobilization of vanadium by cation exchange in MFU-4l, thus providing a metal-organic framework (MOF) with vanadium in a molecule-like coordination environment. This material forms a single-site heterogeneous catalyst with methylaluminoxane and provides polyethylene with low polydispersity (PDI≈3) and the highest activity (up to 148 000 h-1 ) reported for a MOF-based polymerization catalyst. Furthermore, polyethylene is obtained as a free-flowing powder as desired industrially. Finally, the catalyst shows good structural integrity and retains polymerization activity for over 24 hours, both promising attributes for the commercialization of vanadium-based polyolefins.

Reactive metal–support interactions at moderate temperature in two-dimensional niobium-carbide-supported platinum catalysts

The reactive metal–support interaction (RMSI) offers electronic, geometric and compositional effects that can be used to tune catalytic active sites. Generally, supports other than oxides are disregarded as candidates for RMSI. Here, we report an example of non-oxide-based RMSI between platinum and Nb2CTx MXene—a recently developed, two-dimensional metal carbide. The surface functional groups of the two-dimensional carbide can be reduced, and a Pt–Nb surface alloy is formed at a moderate temperature (350 °C). Such an alloy exhibits weaker CO adsorption than monometallic platinum. Water-gas shift reaction kinetics reveals that the RMSI stabilizes the nanoparticles and creates alloy–MXene interfaces with higher H2O activation ability compared with a non-reducible support or a bulk niobium carbide. This RMSI between platinum and the niobium MXene support can be extended to other members of the MXene family and opens new avenues for the facile design and manipulation of functional bimetallic catalysts.Reactive metal–support interactions can tune the activity of heterogeneous catalysts, but have mainly been reported for oxide supports. Now, the metal–support interaction of platinum with MXenes at moderate temperature is reported, using the water-gas shift reaction as an example to showcase the properties of a representative catalyst.

Changes in Catalytic and Adsorptive Properties of 2 nm Pt3Mn Nanoparticles by Subsurface Atoms

Supported multimetallic nanoparticles (NPs) are widely used in industrial catalytic processes, where the relation between surface structure and function is well-known. However, the effect of subsurface layers on such catalysts remains mostly unstudied. Here, we demonstrate a clear subsurface effect on supported 2 nm core-shell NPs with atomically precise and high temperature stable Pt3Mn intermetallic surface measured by in situ synchrotron X-ray Diffraction, difference X-ray Absorption Spectroscopy, and Energy Dispersive X-ray Spectroscopy. The NPs with a Pt3Mn subsurface have 98% selectivity to C-H over C-C bond activation during propane dehydrogenation at 550 °C compared with 82% for core-shell NPs with a Pt subsurface. The difference is correlated with significant reduction in the heats of reactant adsorption due to the Pt3Mn intermetallic subsurface as discerned by theory as well as experiment. The findings of this work highlight the importance of subsurface for supported NP catalysts, which can be tuned via controlled intermetallic formation. Such approach is generally applicable to modifying multimetallic NPs, adding another dimension to the tunability of their catalytic performance.

CCDC 1579914: Experimental Crystal Structure Determination

Related Article: Romain J.-C. Dubey, Robert J. Comito, Zhenwei Wu, Guanghui Zhang, Adam J. Rieth, Christopher H. Hendon, Jeffrey T. Miller, and Mircea Dincă|2017|J.Am.Chem.Soc.|139|12664|doi:10.1021/jacs.7b06841

Synthesis and Testing of Supported Pt-Cu Solid Solution Nanoparticle Catalysts for Propane Dehydrogenation

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are demonstrated here. The catalyst forms a substitutional solid solution structure, with a small and uniform particle size around 2 nm. This is realized by careful control over the impregnation, calcination, and reduction steps during catalyst preparation and is identified by advanced in situ synchrotron techniques. The catalyst propane dehydrogenation performance continuously improves with increasing Cu:Pt atomic ratio.

Effect of Cu content on the bimetallic Pt–Cu catalysts for propane dehydrogenation

Abstract Silica supported, 2 nm Pt and Pt–Cu catalysts with different Cu:Pt atomic ratios and similar size were evaluated for propane dehydrogenation at 550 °C. Monometallic Pt showed low propylene selectivity of 61% at 20% conversion and a TOR of 0.06 s−1. For the Pt–Cu catalysts, the dehydrogenation selectivity and TOR continuously increased with increasing Cu level in the nanoparticle, to eventually 96% selective at 20% conversion with a TOR of 0.98 s−1 for a catalyst with a Cu:Pt atomic ratio of 7.3. Synchrotron in situ X-ray diffraction and X-ray absorption spectroscopy analysis showed that Pt–Cu catalysts with increasing Cu loading formed solid solution type bimetallic structures. For example, a Pt–Cu catalyst with Cu:Pt atomic ratio of 7.3 formed solid solution containing 87% Cu. In this catalyst, the Pt active sites were geometrically isolated by the inactive metallic Cu, which was suggested to be responsible for high selectivity to propane dehydrogenation. The Cu neighbors surrounding the Pt also likely modified the energy level of Pt 5d orbitals and contribute to a TOR about 16 times higher than that of monometallic Pt.