Ag-Cu alloy surfaces in an oxidizing environment: a first-principles study

Recent experiments on model catalysts have shown that Ag-Cu alloys have improved selectivity with respect to pure silver for ethylene epoxidation. In this paper we review our first-principles investigations on the (111) surface of this alloy and present new findings on other low index surfaces. We find that, for every surface orientation, the presence of oxygen leads to copper segregation to the surface. Considering the alloy to be in equilibrium with an oxygen atmosphere and accounting for the effect of temperature and pressure, we compute the surface free energy and study the stability of several surface structures. Investigating the dependence of the surface free energy on the surface composition, we construct the phase diagram of the alloy for every surface orientation. Around the temperature, pressure and composition of interest for practical applications, we find that a limited number of structures can be present, including a thin layer of copper oxide on top of the silver surface and copper-free structures. Different surface orientations show a very similar behavior and in particular a single layer with CuO stoichiometry, significantly distorted when compared to a layer of bulk CuO, has a wide range of stability for all orientations. Our results are consistent with, and help explain, recent experimental measurements

Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics

To help understand the high activity of silver as an oxidation catalyst, e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of methanol to formaldehyde, the interaction and stability of oxygen species at the Ag(111) surface has been studied for a wide range of coverages. Through calculation of the free energy, as obtained from density-functional theory and taking into account the temperature and pressure via the oxygen chemical potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a thin surface-oxide structure is most stable for the temperature and pressure range of ethylene epoxidation and we propose it (and possibly other similar structures) contains the species actuating the catalysis. For higher temperatures, low coverages of chemisorbed oxygen are most stable, which could also play a role in oxidation reactions. For temperatures greater than about 775 K there are no stable oxygen species, except for the possibility of O atoms adsorbed at under-coordinated surface sites Our calculations rule out thicker oxide-like structures, as well as bulk dissolved oxygen and molecular ozone-like species, as playing a role in the oxidation reactions.Comment: 15 pages including 9 figures, Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters : a computational study

Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au13, d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ mol−1. At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ mol−1 relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au38, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and gold–palladium alloys

Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters : a computational study

Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au13, d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ mol−1. At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ mol−1 relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au38, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and gold–palladium alloys

Hydrogen-terminated mesoporous silicon monoliths with huge surface area as alternative Si-based visible light-active photocatalysts

Silicon-based nanostructures and their related composites have drawn tremendous research interests in solar energy storage and conversion. Mesoporous silicon with huge surface area of 400 - 900 m2g-1 developed by electrochemical etching exhibits excellent photocatalytic ability and stablility after 10 cycles in degrading methyl orange under visible light irradiation, owing to the unique mesoporous network, abundant surface hydrides and efficient light harvesting. This work showcases the profound effects of surface area, crystallinity, pore topology on charge igration/recombination and mass transportation. Therein the ordered 1D channel array has outperformed the interconnected 3D porous network by greatly accelerating the mass diffusion and enhancing the accessibility of the active sites on the extensive surfaces

Ag fractals formed on top of a porous TiO2 thin film

This work demonstrates the formation of Ag fractals on top of a Ag:TiO2 thin film. The dendrite-type objects emerged from a homogeneous and highly transparent Ag:TiO2 nanocomposite, via the mechanism of diffusion-limited-aggregation of Ag atoms, during heat-treatment at 500 ºC. A porous TiO2 matrix was also formed during this process, opening a wide range of possible applications, namely in sensing-based ones, together with surface enhanced spectroscopies. Furthermore, fractals incorporate a wide range of shapes and spatial scales, inducing a potentially interesting optical response, over the whole visible range, presumably related with localized surface plasmon modes with very broad spectral distribution.This research was sponsored by FEDER funds through the COMPETE program (Programa Operacional Factores de Competitividade) and by FCT (Fundação para a Ciência e a Tecnologia), under the projects UID/FIS/04650/2013 and PEstOE/MAT/UI0013/2014. A. Ferreira and C. Lopes acknowledge also FCT for grants SFRH/BPD/102402/2014 and SFRH/BD/103373/2014

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

Metal–insulator–semiconductor (MIS) photo‐electrocatalysts offer a pathway to stable and efficient solar water splitting. Initially motivated as a strategy to protect the underlying semiconductor photoabsorber from harsh operating conditions, the thickness of the insulator layer in MIS systems has recently been shown to be a critical design parameter which can be tuned to optimize the photovoltage. This study analyzes the underlying mechanism by which the thickness of the insulator layer impacts the performance of MIS photo‐electrocatalysts. A concrete example of an Ir/HfO2/n‐Si MIS system is investigated for the oxygen evolution reaction. The results of combined experiments and modeling suggest that the insulator thickness affects the photovoltage i) favorably by controlling the flux of charge carriers from the semiconductor to the metal electrocatalyst and ii) adversely by introducing nonidealities such as surface defect states which limit the generated photovoltage. It is important to quantify these different mechanisms and suggest avenues for addressing these nonidealities to enable the rational design of MIS systems that can approach the fundamental photovoltage limits. The analysis described in this contribution as well as the strategy toward optimizing the photovoltage are generalizable to other MIS systems.The competing roles of the insulator layer in metal–insulator–semiconductor water splitting systems are quantitatively explored. Nonidealities such as defects at interfaces significantly limit the photovoltage generated by these systems. Removing these nonidealities and engineering insulators with better selectivity for charge carriers are suggested as critical strategies toward approaching the fundamental photovoltage limits for efficient and stable solar water splitting.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154631/1/aenm201903354_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154631/2/aenm201903354-sup-0001-SuppMat.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154631/3/aenm201903354.pd

Computational Efforts in Support of Advanced Coal Research

The focus in this project was to employ first principles computational methods to study the underlying molecular elementary processes that govern hydrogen diffusion through Pd membranes as well as the elementary processes that govern the CO- and S-poisoning of these membranes. Our computational methodology integrated a multiscale hierarchical modeling approach, wherein a molecular understanding of the interactions between various species is gained from ab-initio quantum chemical Density Functional Theory (DFT) calculations, while a mesoscopic statistical mechanical model like Kinetic Monte Carlo is employed to predict the key macroscopic membrane properties such as permeability. The key developments are: (1) We have coupled systematically the ab initio calculations with Kinetic Monte Carlo (KMC) simulations to model hydrogen diffusion through the Pd based-membranes. The predicted tracer diffusivity of hydrogen atoms through the bulk of Pd lattice from KMC simulations are in excellent agreement with experiments. (2) The KMC simulations of dissociative adsorption of H{sub 2} over Pd(111) surface indicates that for thin membranes (less than 10{micro} thick), the diffusion of hydrogen from surface to the first subsurface layer is rate limiting. (3) Sulfur poisons the Pd surface by altering the electronic structure of the Pd atoms in the vicinity of the S atom. The KMC simulations indicate that increasing sulfur coverage drastically reduces the hydrogen coverage on the Pd surface and hence the driving force for diffusion through the membrane

Development of Sulfur and Carbon Tolerant Reforming Alloy Catalysts Aided by Fundamental Atomistics Insights

Current hydrocarbon reforming catalysts suffer from rapid carbon and sulfur poisoning. Even though there is a tremendous incentive to develop more efficient catalysts, these materials are currently formulated using inefficient trial and error experimental approaches. We have utilized a novel hybrid experimental/theoretical approach, combining quantum Density Functional Theory (DFT) calculations and various state-of-the-art experimental tools, to formulate carbon tolerant reforming catalysts. We have employed DFT calculations to develop molecular insights into the elementary chemical transformations that lead to carbon poisoning of Ni catalysts. Based on the obtained molecular insights, we have identified, using DFT quantum calculation, Sn/Ni alloy as a potential carbon tolerant reforming catalyst. Sn/Ni alloy was synthesized and tested in steam reforming of methane, propane, and isooctane. We demonstrated that the alloy catalyst is carbon-tolerant under nearly stoichiometric steam-to-carbon ratios. Under these conditions, monometallic Ni is rapidly poisoned by sp2 carbon deposits. The research approach is distinguished by a few characteristics: (a) Knowledge-based, bottom-up approach, compared to the traditional trial and error approach, allows for a more efficient and systematic discovery of improved catalysts. (b) The focus is on exploring alloy materials which have been largely unexplored as potential reforming catalysts

Two-step mechanism for low-temperature oxidation of vacancies in graphene

We studied the oxidation of vacancies in graphene by abinitio atomistic thermodynamics to identify the dominant reaction mechanisms. Our calculations show that the low temperature oxidation occurs via a two-step process: Vacancies are initially saturated by stable O groups, such as ether (C-O-C) and carbonyl (C=O). The etching is activated by a second step of additional O2 adsorption at the ether groups, forming larger O groups, such as lactone (C-O-C=O) and anhydride (O=C-O-C=O), that may desorb as CO2 just above room temperature. Our studies show that the partial pressure of oxygen is an important external parameter that affects the mechanisms of oxidation and that allows us to control the extent of etching