Materials Chemistry, Materials Synthesis, Heterogeneous Catalysis, Fuel Cell Research, Transition Metal Main Group Clusters (Zintl Ions), NMR Spectroscopy, Crystallography.
Major Recognitions and Honors
Bimetallic Nanoparticle Catalysts. Our heterogeneous catalysis program is focused on selectively preparing and fully characterizing bimetallic NPs in different architectures, i.e. alloy, core-shell, contact aggregate (see drawing below). We subsequently compare and contrast the catalytic activity of the different architectures and establish correlations with physical and spectroscopic measurements (e.g. XPS, Raman, EXAFS) and theoretical models.
Importantly, the compounds must be thoroughly characterized, which requires multiple analytical techniques, including HRTEM, EDX phase mapping, IR-CO probe experiments, micro-Raman imaging, XPS and, through our collaborations, full theoretical mechanistic analysis, EXAFS, Pair Distribution Functional analysis (PDF), and in situ DRIFTs and Raman studies. We perform thorough catalytic temperature programmed reaction studies (TPR) to evaluate the various NPs for use in a variety of energy related processes, such as NOx reduction, arene hydrogenation and preferential oxidation of CO in hydrogen (PROX), to name a few. Highlights include:
• Ru core – Pt shell NPs, (i.e. Ru@Pt NPs) show marked enhancements in PROX activity with low CO concentrations (50% H2, 1000 ppm CO) relative to RuPt alloy and monometallic mixtures of the same composition and loading (Fig. 1). • Rh core – Pt shell NPs, (i.e. Rh@Pt NPs) containing 2 monolayer thick Pt shells display very high selectivity and activity (near 100% selectivity at 80% conversion) for PROX reactions with high CO concentrations (1.0% CO in 50% H2 feeds) relative to the alloy and monometallic NPs. • Cu@Pt and Ru@Pt NP catalysts show extremely high selectivity for NOx reduction to N2 and H2O relative to commercial Pt based catalysts.21
• NiAu alloy NPs have been prepared from a Li-butyl fast reduction method and undergo a sequential physical transition from NiAu alloy Au@Ni core-shell NiO-Au contact aggregates to produce highly stablized CO oxidation catalysts.
This work and our related Fuel Cell research is funded by the DOE, the ONR, Exxon-Mobil and the University of Maryland Energy Research Center.
High Symmetry Naked Metal Clusters. The use of Zintl ions (main group polyanions, such as Pb94- and Sb73-) to make bimetallic clusters and nanoparticles (NPs) have resulted in a new class of cluster materials with unprecedented structures and intriguing potential applications. Examples include the fully characterized M@Pb102- and M@Pb122- ions (Fig. 2a) where M = Ni, Pd, Pt, by X-ray analysis, DFT studies, 207Pb NMR and 195Pt NMR investigations as well as mass spectrometry studies (LDI and ESI). In addition, we showed that the empty and K+ ion-paired clusters Pb102- and Pb122- could exist without transition metal interstitials. We have also synthesized and fully characterized the coupled tin clusters Ni2@Sn174-, Pd2@Sn184- and Pt2@Sn174- through extensive NMR and X-ray studies. The Ni2@Sn174- and Pt2@Sn174- are isoelectronic but have markedly different structures (see Fig. 2b, 1c) and fluxional behavior. The Pt2@Sn174- has a fused capsule-like structure in which all Sn atoms are in fast exchange, even at -50 °C, and couple equally to both Pt atoms. We have prepared and structurally characterized new “intermetalloid” clusters containing polypnictide ions and group 10 transition metals. The new complexes include Ni5Sb174- (Fig. 2d) and Pd7As164-, which do not resemble traditional Zintl clusters nor do they adopt the structures of the intermetallic phases.