<i>In Situ</i> Studies of Carbon Monoxide Oxidation on Platinum and Platinum–Rhenium Alloy Surfaces

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

Understanding the Growth, Chemical Activity, and Cluster–Support Interactions for Pt–Re Bimetallic Clusters on TiO₂(110)

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_<i>x</i> 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

Nucleation, Growth, and Adsorbate-Induced Changes in Composition for Co–Au Bimetallic Clusters on TiO₂

The nucleation, growth, and CO-induced changes in composition for Co–Au bimetallic clusters deposited on TiO₂(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. STM experiments show that the mobility of Co atoms on TiO₂(110) is significantly lower than of Au atoms; for equivalent or lower coverages of Co, the number of clusters is higher and the average cluster height is smaller than for Au deposition. Consequently, bimetallic clusters are formed by first depositing the less mobile Co atoms, followed by the addition of the more mobile Au atoms. Furthermore, the reverse deposition of Au followed by Co results in clusters of pure Co coexisting with clusters that are Au-rich. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and an increase of ∼3–5 Å in average height for all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are 80–100% Au for bulk Au fractions greater than 50%, but Co and Au coexist at the surfaces when there are not enough Au atoms available to completely cover the surfaces of the clusters. After heating to 800 K, pure Co clusters become partially encapsulated by titania, and for bimetallic clusters, the Co is selectively encapsulated at the cluster surface. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface, but the extent of this diffusion is less than what is observed in the Ni–Au and Pt–Au systems. Density functional theory calculations confirm that for a 50% Co/50% Au bimetallic structure: the surface is predominantly Au in the absence of CO; CO induces diffusion of Co to the cluster surface; and this CO-induced diffusion is less extensive on Co–Au than on the Ni–Au and Pt–Au surfaces

<i>In Situ</i> Ambient Pressure X‑ray Photoelectron Spectroscopy Studies of Methanol Oxidation on Pt(111) and Pt–Re Alloys

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

Growth of Uniquely Small Tin Clusters on Highly Oriented Pyrolytic Graphite

Sn clusters have been grown on highly oriented pyrolytic graphite (HOPG) surfaces and investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. At low Sn coverages ranging from 0.02 to 0.25 ML, Sn grows as small clusters that nucleate uniformly on the terraces. This behavior is in contrast with the growth of transition metals such as Pd, Pt, and Re on HOPG, given that these metals form large clusters with preferential nucleation for Pd and Pt at the favored low-coordination step edges. XPS experiments show no evidence of Sn–HOPG interactions, and the activation energy barrier for diffusion calculated for Sn on HOPG (0.06 eV) is lower or comparable to those of Pd, Pt, and Re (0.04, 0.22, and 0.61 eV, respectively), indicating that the growth of the Sn clusters is not kinetically limited by diffusion on the surface. DFT calculations of the binding energy/atom as a function of cluster size demonstrate that the energies of the Sn clusters on HOPG are similar to those of Sn atoms in the bulk for Sn clusters larger than 10 atoms, whereas the Pt, Pd, and Re clusters on HOPG have energies that are 1–2 eV higher than in the bulk. Thus, there is no thermodynamic driving force for Sn atoms to form clusters larger than 10 atoms on HOPG, unlike for Pd, Pt, and Re atoms, which minimize their energy by aggregating into larger, more bulk-like clusters. In addition, annealing the Sn/HOPG clusters to 800 and 950 K does not increase the cluster size, but instead removes the larger clusters, while Sn deposition at 810 K induces the appearance of protrusions that are believed to be from subsurface Sn. DFT studies indicate that it is energetically favorable for a Sn atom to exist in the subsurface layer only when it is located at a subsurface vacancy

Active Sites in Copper-Based Metal–Organic Frameworks: Understanding Substrate Dynamics, Redox Processes, and Valence-Band Structure

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

Electronic Properties of Bimetallic Metal–Organic Frameworks (MOFs): Tailoring the Density of Electronic States through MOF Modularity

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_{<i>x</i>–<i>y</i>}M′_<i>y</i>-MOFs, M_<i>x</i>M′_<i>y</i>-MOFs, and M_<i>x</i>(ligand-M′_<i>y</i>)-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