Ni-CeO2 is a highly efficient, stable and non-expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO2 at temperatures as low as 300 K, generating CHx and COx species on the surface of the catalyst. Strong metal-support interactions activate Ni for the dissociation of methane. The results of density-functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO2-x (111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CHx or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.

The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized by micro-Raman mapping and time-of-flight secondary ion mass spectrometry. Our investigation shows that during growth at high temperature, the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate. Additionally, the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CH(x), x < 4) trapped in a "nano-chemical vapor deposition" chamber between the first layer and the substrate. On the basis of these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.

Pt nanoparticles grown on fully oxidized and partially reduced CeO x (111) thin films have been studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy to understand the effect of redox properties and nanostructures of ceria supports on the growth of Pt. Deposition of 0.2 ML of Pt on CeO2 at 300 K produces two atomic layer high nanoparticles, while on reduced ceria films Pt favors the growth of smaller particles of one−two layer thick with a larger particle density. With the increase of Pt coverage, Pt particles on CeO2 grow in size while the Pt particle density significantly increases on the reduced ceria. Heating the surface to higher temperatures causes the Pt particle agglomeration, but Pt particles sinter less on the reduced ceria compared to those on the fully oxidized ceria. New particle structures are formed on reduced ceria as a result of heating which are suggested due to the encapsulation of Pt particles by ceria. In addition to the structural changes of the Pt particles, modifications of electronic properties of both ceria and Pt were observed upon Pt deposition as well as after heating. Our combined scanning tunneling microscopy and X-ray photoelectron spectroscopy studies suggest a complex growth behavior of Pt on ceria and a strong interaction between the Pt and the ceria support.

We present an extensive experimental study of the conditions under which Cu forms encapsulated islands under the top surface layers of graphite, as a result of physical vapor deposition of Cu on argon-ionbombarded graphite. When the substrate is held at 800 K during deposition, conditions are optimal for formation of encapsulated multilayer Cu islands. Deposition temperatures below 600 K favor adsorbed Cu clusters, while deposition temperatures above 800 K favor a different type of feature that is probably a singlelayer intercalated Cu island. The multilayer Cu islands are characterized with respect to size and shape, thickness and continuity of the graphitic overlayer, relationship to graphite steps, and stability in air. The experimental techniques are scanning tunneling microscopy and X-ray photoelectron spectroscopy. We also present an extensive study using density functional theory to compare stabilities of a wide variety of configurations of Cu atoms, Cu clusters, and Cu layers on/under the graphite surface. The only configuration that is significantly more stable under the graphite surface than on top of it, is a single Cu atom. This analysis leads us to conclude that formation of encapsulated Cu islands is kinetically driven, rather than thermodynamically driven.

The interaction between Ni and ceria was investigated under ultrahigh vacuum conditions using model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeOx(111) (1.5 < x < 2) thin films grown on Ru(0001). As indicated by X-ray photoelectron spectroscopy studies, metallic Ni is the only species present on the reduced ceria. Ni-0 is the predominate species observed upon deposition of a submonolayer of Ni on CeO2 at 300 K. However, a small amount of Ni is oxidized to Ni2+. A decreased ratio of Ni2+ to Ni-0 was observed with further increase of Ni coverage. Oxidation of Ni on CeO2 can be facilitated by annealing as well as by depositing Ni at 500 K. Scanning tunneling microscopy studies show that Ni forms two-dimensional particles on ceria at room temperature, which suggests a strong Ni-ceria interaction. Upon deposition at 500 K, metallic Ni particles as well as NiO particles can be formed on the reduced and oxidized ceria, respectively. NiO particles exhibit a flatter particle shape than that of Ni particles. Our results can be explained by thermodynamics as well as by previous computational studies.U.S. Department of Energy Office of Fossil Energy (FE); U.S. Department of Energy by Battelle Memorial Institute [DE-AC05-76RL01830

We have studied the reaction of ethanol and water over Ni− CeO 2-x (111) model surfaces to elucidate the mechanistic steps associated with the ethanol steam reforming (ESR) reaction. Our results provide insights about the importance of hydroxyl groups to the ESR reaction over Ni-based catalysts. Systematically, we have investigated the reaction of ethanol on Ni−CeO 2-x (111) at varying Ce 3+ concentrations (CeO 1.8−2.0 ) with absence/presence of water using a combination of soft X-ray photoelectron spectroscopy (sXPS) and temperature-programmed desorption (TPD). Consistent with previous reports, upon annealing, metallic Ni formed on reduced ceria while NiO was the main component on fully oxidized ceria. Ni 0 is the active phase leading to both the C− C and C−H cleavage of ethanol but is also responsible for carbon accumulation or coking. We have identified a Ni 3 C phase that formed prior to the formation of coke. At temperatures above 600 K, the lattice oxygen from ceria and the hydroxyl groups from water interact cooperatively in the removal of coke, likely through a strong metal−support interaction between nickel and ceria that facilitates oxygen transfer. ■ INTRODUCTIONThe steam reforming of ethanol, C 2 H 5 OH + 3H 2 O → 6H 2 + 2CO 2 , is of interest to the chemical industry and fuel cell applications as it provides an alternative route to obtain renewable hydrogen through ethanol (or bioethanol), which can be readily extracted from sources such as biomass. 1,2 Typically, it involves the use of metal/oxide catalysts (i.e., Pt, Rh or Pd supported on CeO x ), and the reaction occurs by a complex series of mechanistic steps which are strongly coupled to a combination of factors, including the structural, chemical and electronic properties of the catalyst. 2,3 It has been postulated that this process requires both the metal and oxide parts of the catalyst to work co-operatively, with the metal component contributing to the C−C and C−H bond scissions of ethanol, and the oxide support promoting the dehydrogenation (or dehydration) reaction as well as the dissociation of H 2 O. 2−6 Catalysts based on nickel have emerged as promising candidates for the ethanol steam reforming reaction and have been reported to exhibit activity and selectivity comparable to that of noble metals such as Rh and Pd. 7,8 However, the drawback of nickel based catalysts is the high propensity for the loss in selectivity (i.e., through methanation), and deactivation which occurs through metal sintering and coke formation. 2,3,8−11 In particular, for coke formation, it could occur through the following side reactions during the steam reforming processes:① Dehydrogenation of the hydrocarbon groups from ethanol: −CH x (ad) → C + xH(ad) ② Boudouard reaction: CO(ad) + CO(ad) → CO 2 (g) + C ③ Polymerization: nC 2 H 4 → polymer → C + nH 2 (g) 2,3 In principle, minimizing the coke deposition could be achieved by perturbing the electronic properties of the metal through interactions with the oxide support, 7,12 by controlling particle size, 13,14...

The effect of nanostructures of ceria associated with oxygen vacancies as well as Ti dopant on the structure of Ni was investigated by adopting model Ni/ceria systems consisting of Ni nanoparticles supported on well-ordered CeO x -(111) (1.5 < x < 2) thin films with/without Ti dopant using scanning tunneling microscopy under ultrahigh vacuum conditions. The surface defects related with oxygen vacancies on the reduced ceria act as the nucleation sites for Ni, which causes the formation of Ni particles with a smaller size and a higher particle density compared to those on CeO 2 at 300 K. Ni experiences a significant particle aggregation on pure ceria surfaces upon heating to higher temperatures, although slightly less Ni sintering was observed on the reduced ceria. Doping ceria with Ti element can help prevent the Ni sintering and stabilize the Ni particles on the surface upon heating, making it an attractive support for practical Ni catalysts with high stability.

We show that 3 metals -Dy, Ru, and Cu -can form multilayer intercalated (encapsulated) islands at the graphite (0001) surface if 2 specific conditions are met: Defects are introduced on the graphite terraces to act as entry portals, and the metal deposition temperature is well above ambient. Focusing on Dy as a prototype, we show that surface encapsulation is much different than bulk intercalation, because the encapsulated metal takes the form of bulk-like rafts of multilayer Dy, rather than the dilute, single-layer structure known for the bulk compound. Carbon-covered metallic rafts even form for relatively unreactive metals (Ru and Cu) which have no known bulk intercalation compound. Defect-mediated, thermally-activated encapsulation of metals at the surface of graphite." Carbon 127 (2018): 305-311.ABSTRACT: We show that 3 metals -Dy, Ru, and Cu -can form multilayer intercalated (encapsulated) islands at the graphite(0001) surface if 2 specific conditions are met: Defects are introduced on the graphite terraces to act as entry portals, and the metal deposition temperature is well above ambient. Focusing on Dy as a prototype, we show that surface encapsulation is much different than bulk intercalation, because the encapsulated metal takes the form of bulk-like rafts of multilayer Dy, rather than the dilute, single-layer structure known for the bulk compound.Carbon-covered metallic rafts even form for relatively unreactive metals (Ru and Cu) which have no known bulk intercalation compound.