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.
Our civilization relies on synthetic polymers for all aspects of modern life; yet, inefficient recycling and extremely slow environmental degradation of plastics are causing increasing concern about ...their widespread use. After a single use, many of these materials are currently treated as waste, underutilizing their inherent chemical and energy value. In this study, energy-rich polyethylene (PE) macromolecules are catalytically transformed into value-added products by hydrogenolysis using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by atomic layer deposition. Pt/SrTiO3 completely converts PE (M n = 8000–158,000 Da) or a single-use plastic bag (M n = 31,000 Da) into high-quality liquid products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H2 and 300 °C under solvent-free conditions for reaction durations up to 96 h. The binding of PE onto the catalyst surface contributes to the number averaged molecular weight (M n) and the narrow polydispersity (Đ) of the final liquid product. Solid-state nuclear magnetic resonance of 13C-enriched PE adsorption studies and density functional theory computations suggest that PE adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concentrations of undercoordinated Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons.
The selective hydrodeoxygenation (HDO) reaction is desirable to convert glycerol into various value-added products by breaking different numbers of C-O bonds while maintaining C-C bonds. Here we ...combine experimental and density functional theory (DFT) results to reveal that the Cu modifier can significantly reduce the oxophilicity of the molybdenum carbide (Mo
C) surface and change the product distribution. The Mo
C surface is active for breaking all C-O bonds to produce propylene. As the Cu coverage increases to 0.5 monolayer (ML), the Cu/Mo
C surface shows activity towards breaking two C-O bonds and forming ally-alcohol and propanal. As the Cu coverage further increases, the Cu/Mo
C surface cleaves one C-O bond to form acetol. DFT calculations reveal that the Mo
C surface, Cu-Mo interface, and Cu surface are distinct sites for the production of propylene, ally-alcohol, and acetol, respectively. This study explores the feasibility of tuning the glycerol HDO selectivity by modifying the surface oxophilicity.
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.
The hydrodeoxygenation (HDO) mechanism of 1,2- and 1,3-propanediols has been investigated over a Cu/Mo2C catalyst using density functional theory to understand the effect of vicinal hydroxyl groups ...on the HDO activity and product selectivity. To correlate the simulation results with experimental data, a microkinetic continuous stirred-tank reactor model was developed, and the activity of these diols was studied under both ultrahigh vacuum (UHV) and near ambient pressure reactor conditions. Our microkinetic model predicted a one order of magnitude higher turnover frequency for the HDO of 1,3-propanediol relative to 1,2-propanediol. Intramolecular H-bonding plays an important role in reducing the activation barrier for the rate-determining O–H bond scission step that occurs via a concerted H-transfer from the neighboring hydroxyl group. Under simulated UHV conditions, the major products are for both diols, the kinetically favored dehydrogenation products; however, a higher selectivity toward the thermodynamically favored HDO products was observed under higher pressure conditions and longer residence times typical for most chemical reactor studies. Our analysis revealed that the HDO of 1,2-propanediol follows a similar mechanistic pathway to glycerol due to the presence of adjacent hydroxyl groups in both molecules; in contrast, the HDO of 1,3-propanediol follows a different HDO mechanism.
The growth, surface composition, and chemical activity of bimetallic Pt−Au clusters on TiO2(110) have been investigated. Scanning tunneling microscopy (STM) experiments demonstrate that the ...deposition of Au on Pt clusters results in the formation of bimetallic Pt−Au clusters due to the seeding of the mobile Au atoms at existing Pt nuclei. The composition of the top surface layer of the clusters was studied by low energy ion scattering (LEIS) for bulk compositions ranging from 25%−87.5% Pt with total metal coverages of 0.25 and 0.50 ML. For both coverages, the cluster surfaces consisted of nearly pure Au at Pt compositions of 50% and lower; however, a mix of Au and Pt atoms were found at the cluster surfaces at higher fractions of deposited Pt. These results are consistent with bulk thermodynamics, which predicts a Pt core−Au shell structure based on the lower surface free energy of Au compared to Pt and the large bulk miscibility gap for the two metals. The adsorption of CO on the Pt−Au clusters at room temperature promotes the diffusion of Pt to the surface of the clusters, and this phenomena is most pronounced for the clusters that are initially pure Au at the surface. Density functional theory calculations demonstrate that it is thermodynamically favorable for Pt to diffuse to the cluster surface in order to bind to CO. In contrast, the extent of CO2 production via sequential adsorption of O2 and CO on the Pt−Au clusters reflects the surface Pt content before adsorption. For CO oxidation, the first step in the reaction is the dissociation of O2 at Pt sites. Since this process requires more than one contiguous Pt site, it is not surprising that O2 dissociation cannot occur on the Pt−Au clusters that are ∼100% Au at the surface before CO exposure, given the low probability for ensembles of Pt sites to form at the surface.
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.
Controlled C–O bond scission is an important step for upgrading glycerol, a major byproduct from the continuously increasing biodiesel production. Transition metal nitride catalysts have been ...identified as promising hydrodeoxygenation (HDO) catalysts, but fundamental understanding regarding the active sites of the catalysts and reaction mechanism remains unclear. This work demonstrates a fundamental surface science study of Mo2N and Cu/Mo2N for the selective HDO reaction of glycerol, using a combination of model surface experiments and first-principles calculations. Temperature-programmed desorption (TPD) experiments showed that clean Mo2N cleaved two or three C–O bonds of glycerol to produce allyl alcohol, propanal, and propylene. The addition of Cu to Mo2N changed the reaction pathway to one C–O bond scission to produce acetol. High-resolution electron energy loss spectroscopy (HREELS) results identified the surface intermediates, showing a facile C–H bond activation on Mo2N. Density functional theory (DFT) calculations revealed that the surface N on Mo2N interacted with the H atoms in glycerol and blocked some Mo sites to enable selective C–O bond scission. This work shows that Mo2N and Cu/Mo2N are active and selective for the controlled C–O bond scission of glycerol and in turn provides insights into the rational catalyst design for selective oxygen removal of relevant biomass-derived oxygenates.
For methanol oxidation reactions, Pt–Re alloy surfaces are found to have better selectivity for CO2 production and less accumulation of surface carbon compared to pure Pt surfaces. The unique ...activity of the Pt–Re surface is attributed to the increased ability of Re to dissociate oxygen compared to Pt and the ability of Re to diffuse gradually to the surface under reaction conditions. In this work, the oxidation of methanol was studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and mass spectrometry on Pt(111), a Pt–Re surface alloy, and a Re film on Pt(111) as well as Pt(111) and Pt–Re alloy surfaces that were preoxidized before reaction. Methanol oxidation conditions consisted of 200 mTorr of O2/100 mTorr of methanol at temperatures ranging from 300 to 550 K. The activities of all of the surfaces studied are similar in that CO2 and H2O are the main oxidation products, along with formaldehyde, which is produced below 450 K. For reaction on Pt(111), there is a change in selectivity that favors CO and H2 over CO2 at 500 K and above. This shift in selectivity is not as pronounced on the Pt–Re alloy surface and is completely absent on the oxidized Pt–Re alloy surfaces and oxidized Re film. AP-XPS results demonstrate that Pt(111) is more susceptible to poisoning by carbonaceous surface species than any of the Re-containing surfaces. Oxygen-induced diffusion of Re to the surface is believed to occur at elevated temperatures under reaction conditions, based on the increase in the Re/Pt ratio upon heating; density function theory (DFT) calculations confirm that there is a thermodynamic driving force for Re atoms to diffuse to the surface in the presence of oxygen. Furthermore, Re diffuses to the surface when the Pt–Re alloy is exposed to O2 at 450 K before methanol oxidation, and consequently this surface has the highest CO2 production at temperatures below that required for Re diffusion during methanol reaction. Although the oxidized Re film also exhibits high selectivity for CO2 production and minimal carbon deposition, this surface is unstable due to the sublimation of Re2O7; in contrast, the Pt–Re alloy is more resistant to Re sublimation since the majority of Re resides in the subsurface region.
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