The development of porous well-defined hybrid materials (e.g., metal-organic frameworks or MOFs) will add a new dimension to a wide number of applications ranging from supercapacitors and electrodes to "smart" membranes and thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In this work, we focused on the fundamental insights responsible for the electronic properties of three distinct classes of bimetallic systems, MM'-MOFs, MM'-MOFs, and M(ligand-M')-MOFs, in which the second metal (M') incorporation occurs through (i) metal (M) replacement in the framework nodes (type I), (ii) metal node extension (type II), and (iii) metal coordination to the organic ligand (type III), respectively. We employed microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures.

Pt–Re clusters supported on titania have shown promise as catalysts for the low temperature water–gas shift reaction. However, the enhanced activity of the bimetallic Pt–Re catalyst versus pure Pt is not well understood. In this work, exclusively bimetallic clusters were grown on TiO2(110) by vapor-deposition of Pt on 2 ML Re clusters and Re on 2 ML Pt clusters. Temperature programmed desorption experiments with CO were used to determine the concentration of Re at the surface, given that CO dissociates on Re but not on Pt. Deposition of 2 ML Pt on 2 ML Re resulted in Re core–Pt shell structures, whereas deposition of low coverages (<0.5 ML) of Re on 2 ML Pt resulted in complete diffusion of Re into the Pt clusters. Both of these Pt on Re bimetallic clusters are thermodynamically favored by the lower surface free energy of Pt compared to Re, and both are also more active than pure Pt clusters in the WGS reaction. Postreaction XPS experiments indicate that Re in the Pt on Re clusters is not oxidized under WGS conditions (130–190 °C). Furthermore, preoxidized Pt–Re clusters exhibit lower activity than both pure Pt and the unoxidized Pt–Re clusters, demonstrating that ReO x does not provide active sites in the WGS reaction. Density functional theory calculations show that CO binds less strongly to the Pt on Re surface alloy compared to pure Pt, and infrared absorption–reflection spectroscopy studies on a Pt–Re surface alloy confirm that the coverage of CO after WGS reaction is lower on the Pt–Re alloy surface. Thus, decreased CO poisoning on Pt–Re could explain the higher WGS activity of the bimetallic clusters.

MoS2 clusters have been grown on a TiO2(110) substrate to provide a model surface for better understanding the adsorbate interactions and chemical activity on titania-supported MoS2 clusters. Scanning tunneling microscopy experiments show that clusters with elongated shapes and flat tops are formed, and the long axes of the clusters have specific orientations with respect to the [001] direction on TiO2(110). In contrast, deposition of Mo in the absence of H2S results in a high density of smaller, round clusters that cover the majority of the surface. The morphologies of the MoS2 clusters do not change after exposure to various gases (D2, CO, O2, H2O, and methanol) in ultrahigh vacuum. However, exposure to higher pressures of O2, H2O, or methanol (10 Torr), as well as exposure to air, causes the clusters to disintegrate as Mo in the clusters becomes oxidized. Temperature-programmed desorption studies with CO on the MoS2 clusters show a distinct desorption peak at 280 K, which is not observed on metallic Mo or titania. Density functional theory calculations suggest that these new adsorption sites for CO are at the edges of the elongated MoS2 clusters, particularly along the (101̅0) edge containing sulfur vacancy sites.

The growth of Sn and Pt–Sn clusters on TiO2(110) has been studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and density functional theory (DFT). At low Sn coverages (0.02 ML), single-layer high clusters of SnO x are formed with a narrow size distribution and uniform spatial distribution. XPS experiments indicate that these clusters consist of oxidized Sn, and the corresponding reduction in the TiO2 substrate is observed. At higher Sn coverages, the surface is still dominated by two-dimensional clusters of SnO x , but larger three-dimensional clusters of metallic Sn also appear. As the Sn coverage is increased, the number of three-dimensional clusters increases, and the ratio of Sn/SnO x increases, suggesting that SnO x and reduced TiO x form at the cluster–support interface. When Pt is deposited on top of the Sn/SnO x clusters, the relatively mobile Pt atoms diffuse across the TiO2 surface and become incorporated into existing Sn/SnO x clusters. Furthermore, the addition of Pt to the Sn/SnO x clusters causes the reduction of SnO x to metallic Sn and the oxidation of Ti3+ to Ti4+; this behavior is attributed to the formation of Pt–Sn alloy clusters, which results in the diffusion of Sn away from the interface with the TiO2 support. In contrast, when Sn is deposited on an equal coverage of Pt clusters, new Sn/SnO x clusters are formed that coexist with Pt–Sn clusters. However, the surfaces of both Pt on Sn and Sn on Pt clusters are Sn-rich due to the lower surface free energy of Sn compared to Pt. DFT calculations demonstrate that M–TiO2 bonding is favored over M–M bonding for M = Sn, unlike for transition metals such as M = Pt, Au, Ni, and Co. Furthermore, the substantial charge transfer from Sn to TiO2 leads to dipole–dipole repulsion of Sn atoms that prevents agglomeration into the larger clusters that are observed for the mid-late transition metals. DFT studies also confirm that the addition of Pt to a Sn cluster results in strong Pt–Sn bond formation and diminished Sn–O interactions.

No abstract