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, M x–y M′ y -MOFs, M x M′ y -MOFs, and M x (ligand-M′ y )-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.
The present study examines the synthesis of unique Cu nanostructured model catalysts and their catalytic activity toward CO2 hydrogenation under moderate temperature and pressure reaction conditions. ...Cu-based nanoparticles (NPs) were synthesized by two chemical deposition methods: (1) 5 nm spherical Cu(OH)2 NPs deposited on highly oriented pyrolytic graphite (HOPG) by exposing the HOPG substrate to a colloidal solution of copper, and (2) photocatalytic reduction of Cu(H2O)62+ onto a high density of 15 nm TiO2 NPs grown on HOPG by physical vapor deposition. This photocatalytic reduction results in the deposition of mixed Cu(OH)2 and Cu2O films, while few-nm sized Cu-based NPs are formed on the TiO2 NPs upon subsequent reduction. The chemistry, structure, and morphology of the resulting samples were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The thermocatalytic activity for the CO2 reduction reaction (CO2RR) under H2 was evaluated with synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and temperature-programmed desorption (TPD) experiments. Several intermediates, including CO2 δ−, HCOO, O–CH3, CO3 2–, CH x , and CO, were observed using AP-XPS. The TiO2 NPs show activity toward the formation of methanol (CH3OH) that occurs mainly through an O–CH3 intermediate. The TiO2 NPs-core–carbon-shell (TiO2@C NPs) shows a clear selectivity toward methane (CH4). The Cu/TiO2 NPs show, however, an activity toward CO, CH4, and CH3OH that depends strongly on the percentage of oxygen present on the Cu NPs surface. This study particularly shows the importance played by the TiO2 NPs for CO2 adsorption and activation and the Cu NPs for H2 and CO2 dissociation. The CO2RR mechanisms are discussed on the basis of the intermediate formation and the surface structure and composition.
We have developed an integrated approach that combines synthesis, X-ray photoelectron spectroscopy (XPS) studies, and theoretical calculations for the investigation of active unsaturated metal sites ...(UMS) in copper-based metal–organic frameworks (MOFs). Specifically, extensive reduction of Cu+2 to Cu+1 at the MOF metal nodes was achieved. Introduction of mixed valence copper sites resulted in significant changes in the valence band structure and an increased density of states near the Fermi edge, thereby altering the electronic properties of the copper-based framework. The development of mixed-valence MOFs also allowed tuning of selective adsorbate binding as a function of the UMS oxidation state. The presented studies could significantly impact the use of MOFs for heterogeneous catalysis and gas purification as well as foreshadow a new avenue for controlling the conductivity of typically insulating MOF materials.
Liquid jet X-ray photoelectron spectroscopy was used to investigate changes in the local electronic structure of acetic acid in the bulk of aqueous solutions induced by solvation effects. These ...effects manifest themselves as shifts in the difference in the carbon 1s binding energy (ΔBE) between the methyl and carboxyl carbons of acetic acid. Furthermore, molecular dynamics simulations, coupled with correlated electronic structure calculations of the first solvation sphere, provide insight into the number of water molecules directly interacting with the carboxyl group that are required to match the ΔBE from the photoelectron spectroscopy experiments. This comparison shows that a single water molecule in the first solvation shell describes the photoelectron ΔBE of acetic acid while at least 20 water molecules are required for the conjugate base, acetate, in aqueous solutions.
The nucleation and growth of Co clusters on vacuum-annealed (reduced) and oxidized TiO2(110) have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ...density function theory (DFT) calculations. On vacuum-annealed TiO2(110), the Co clusters grow as three-dimensional islands at coverages between 0.02 and 0.25 ML, but the cluster heights range from ∼3 to 5 Å, indicating that the clusters are less than three layers high. In addition to the small cluster sizes, the high nucleation density of the Co clusters and lack of preferential nucleation at the step edges demonstrate that diffusion is slow for Co atoms on the TiO2 surface. In contrast, deposition of other metals such as Au, Ni, and Pt on TiO2 results in larger cluster sizes with a smaller number of nucleation sites and preferential nucleation at step edges. XPS experiments show that Co remains in the metallic state, and there is little reduction of the titania surface by Co. A comparison of the metal–titania binding energies calculated by DFT for Co, Au, Ni, and Pt indicates that stronger metal–titania interactions correspond to lower diffusion rates on the surface, as observed by STM. Furthermore, on oxidized TiO2 surfaces, the diffusion rates of all of the metals decrease, resulting in smaller cluster sizes and higher cluster densities compared to the growth on reduced TiO2. DFT calculations confirm that the metal–titania adsorption energies are higher on the oxidized surfaces, and this is consistent with the lower diffusion rates observed experimentally.
CO oxidation has been investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on Pt(111), Re films on Pt(111), and a Pt–Re alloy surface. The Pt–Re alloy surface was prepared ...by annealing Re films on Pt(111) to 1000 K; scanning tunneling microscopy, low energy ion scattering, and X-ray photoelectron spectroscopy studies indicate that this treatment resulted in the diffusion of Re into the Pt(111) surface. Under CO oxidation conditions of 500 mTorr O2/50 mTorr CO, CO remains on the Pt(111) surface at 450 K, whereas CO desorbs from the Pt–Re alloy surface at lower temperatures. Furthermore, the Pt–Re alloy dissociates oxygen more readily than Pt(111) despite the fact that all of the Re atoms are initially in the subsurface region. Mass spectrometer studies show that the Pt–Re alloy, Re film on Pt, and Pt(111) all have similar activities for CO oxidation, with the Pt–Re alloy producing ∼10% more CO2 than Pt(111). The Re film is not stable under CO oxidation conditions at temperatures ≥450 K due to the formation and subsequent sublimation of volatile Re2O7. However, the Pt–Re alloy surface is more resistant to oxidation and therefore also more stable against Re sublimation.
Titania has attracted significant interest due to its broad catalytic applications, many of which involve titania nanoparticles in contact with aqueous electrolyte solutions. Understanding the ...titania nanoparticle/electrolyte interface is critical for the rational development of such systems. Here, we have employed liquid-jet ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the solid/electrolyte interface of 20 nm diameter TiO2 nanoparticles in 0.1 M aqueous nitric acid solution. The Ti 2p line shape and absolute binding energy reflect a fully oxidized stoichiometric titania lattice. Further, by increasing the X-ray excitation energy, the difference in O 1s binding energies between that of liquid water (O 1sliq) and the titania lattice (O 1slat) oxygen was measured as a function of probe depth into the particles. The titania lattice, O 1slat, binding energy decreases by 250 meV when probing from the particle surface into the bulk. This is interpreted as downward band bending at the interface.
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.
Pure and bimetallic Co–Pt clusters were grown on TiO2(110) and studied by scanning tunneling microscopy (STM). Despite the lower mobility of Co atoms on TiO2 compared to Pt, STM experiments ...demonstrate that exclusively bimetallic clusters can be deposited through either order of deposition, provided that deposition of the first metal results in a sufficient density of seed clusters to nucleate all of the metal atoms from the second deposition. Bimetallic clusters of varying compositions were prepared by depositing different Co:Pt ratios at room temperature with a fixed total coverage of 0.25 ML. The average cluster heights decrease with increasing Co fraction since the higher Co coverage results in a greater number of nucleation sites. After heating to 800 K, the pure Pt clusters exhibit the greatest increase in average cluster height; however, both Pt and Co clusters become encapsulated with titania upon annealing to 800 K, which is also the onset temperature for Co desorption. Low energy ion scattering (LEIS) experiments show that for all bimetallic compositions studied, both Co and Pt atoms reside at the cluster surfaces although the surface composition is always richer in Pt than the bulk. Co–Pt clusters with a 55% Co fraction have nearly identical surface compositions, regardless of the order of Co and Pt deposition, and this behavior suggests that diffusion of atoms within the clusters is facile at room temperature. Furthermore, 55% Co–45% Pt clusters exhibit identical activity in terms of CO desorption and methanol reaction for both Co on Pt and Pt on Co. Temperature programmed desorption (TPD) experiments with CO indicate that CO desorbs from both Pt and Co sites on surfaces with high Co fractions, and the desorption temperature of CO decreases with increasing Co fraction. In the reaction of methanol on the pure and bimetallic clusters, CO and H2 are produced as the main gaseous products on all surfaces. On the bimetallic surfaces, the selectivity for methane formation is higher than on either pure Co or Pt, and the bimetallic clusters are less active for C–H bond scission than pure Co or Pt. CO evolution from methanol reaction appears to be desorption-limited, and the surface CO prefers to adsorb at Pt sites over Co sites.
The growth and chemical activity of Re, Pt, and Pt–Re bimetallic clusters supported on TiO2(110) have been studied. Pure Re clusters interact strongly with the titania support, resulting in the ...reduction of the titania surface, and the Re clusters also appear to be partially covered by TiO x at Re coverages as high as 13 ML. Bimetallic clusters can be grown from sequential deposition of Pt and Re in either order at high metal coverages (3.7 ML), where the number of initial nucleation sites is large; in contrast, at lower coverages (0.24 ML), pure Re clusters coexist with Pt–Re clusters for Re deposited on Pt due to the higher nucleation density of Re compared with Pt. The surface composition of the high coverage Pt on Re clusters is ∼100% Pt, but the Re on Pt clusters contain both Pt and Re at the surface after diffusion of some fraction of Re atoms in the bulk. The lower surface free energy of Pt compared to Re makes it thermodynamically favorable for Pt to remain at the surface when Pt is deposited on Re, whereas Re atoms deposited on the Pt clusters will diffuse into the clusters. Isotopic labeling experiments that incorporate 18O into the titania lattice demonstrate that lattice oxygen participates in both CO oxidation on the Pt on Re bimetallic clusters and recombination of carbon and oxygen to form CO on the Re-containing clusters.
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