Density Functional Theory Study of the Partial Oxidation of Methane to Methanol on Au and Pd Surfaces

The partial oxidation of methane to methanol has been a goal of heterogeneous catalysis for many years. Recent experimental investigations have shown how AuPd nanoparticle catalysts can give good selectivity to methanol with only limited total oxidation of CH4 using hydrogen peroxide as an oxidant in aqueous media. Interestingly, the use of colloidal nanoparticles alone, without a support material, leads to efficient use of the oxidant and the possibility of introducing oxygen from O2(g) into the CH3O2H primary product. This observation indicates that a radical mechanism is being initiated by H2O2 but then the oxygen addition step, catalyzed by these nanoparticles, can incorporate O2(ads). In this contribution, we use density functional theory (DFT) to study the elementary steps in the partial oxidation of methane to methanol using H2O2 as a radical initiator and molecular oxygen as an oxidant over the low index surfaces of Pd and Au. We are able to show that pure Pd nanoparticles are prone to oxidation by O2(g), whereas the competitive adsorption of water on Au surfaces limits the availability of O2(ads). Calculations with Au added to Pd or vice versa show that both effects can be alleviated by using mixed metal surfaces. This provides a rationalization of the need to use alloy nanoparticles experimentally, and the insights from these results will aid future catalyst development

Point Group Symmetry Analysis of the Electronic Structure of Bare and Protected Metal Nanocrystals

The electronic structures of a variety of experimentally identified gold and silver nanoclusters from 20 to 246 atoms, either unprotected or protected by several types of ligands, are characterized by using point group specific symmetry analysis. The delocalized electron states around the HOMO-LUMO energy gap, originating from the metal s-electrons in the cluster core, show symmetry characteristics according to the point group that describes best the atomic arrangement of the core. This indicates strong effects of the lattice structure and overall shape of the metal core to the electronic structure, which cannot be captured by the conventional analysis based on identification of spherical angular momentum shells in the superatom model. The symmetry analysis discussed in this paper is free from any restrictions regarding shape or structure of the metal core, and is shown to be superior to the conventional spherical harmonics analysis for any symmetry that is lower than Ih. As an immediate application, we also demonstrate that it is possible to reach considerable savings in computational time by using the symmetry information inside a conventional linear-response calculation for the optical absorption spectrum of the Ag55 cluster anion, without any loss in accuracy of the computed spectrum. Our work demonstrates an efficient way to analyze the electronic structure of non-spherical, but atomically ordered nanocrystals and ligand-protected clusters with nanocrystal metal cores and it can be viewed as the generalization of the superatom model demonstrated for spherical shapes ten years ago (Walter et al., PNAS 2008, 105, 9157)

Structural behaviour of copper chloride catalysts during the chlorination of CO to phosgene

The interaction of CO with an attapulgite-supported Cu(II)Cl2 catalyst has been examined in a micro-reactor arrangement. CO exposure to the dried, as-received catalyst at elevated temperatures leads to the formation of CO2 as the only identifiable product. However, phosgene production can be induced by a catalyst pre-treatment where the supported Cu(II)Cl2 sample is exposed to a diluted stream of chlorine. Subsequent CO exposure at ~ 370°C then leads to phosgene production. In order to investigate the origins of this atypical set of reaction characteristics, a series of x-ray absorption experiments were performed that were supplemented by DFT calculations. XANES measurements establish that at the elevated temperatures connected with phosgene formation, the catalyst is comprised of Cu+ and a small amount of Cu2+. Moreover, the data show that unique to the chlorine pre-treated sample, CO exposure at elevated temperature results in a short-lived oxidation of the copper. On the basis of calculated CO adsorption energies, DFT calculations indicate that a mixed Cu+/Cu2+ catalyst is required to support CO chemisorption

Methane Oxidation to Methanol in Water

Conspectus Methane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C–H activation and prevention of overoxidation to CO2. Indirect methods such as steam reforming produce CO and H2 chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years. The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N2O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H2O2 as an oxidant (effectively O2 + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H2O2 is an expensive oxidant to use in the production of methanol. This observation of activity under mild conditions led to new approaches to utilize O2 as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C–H activation reactions using O2 and H2O2, but the rapid decomposition of H2O2 over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H2O2 consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O2 could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area

Transfer hydrogenation of methyl levulinate with methanol to gamma valerolactone over Cu-ZrO₂:A sustainable approach to liquid fuels

Cu-ZrO2 is demonstrated to be a highly effective catalyst for the transfer hydrogenation of methyl levulinate to γ-valerolactone, using methanol as the hydrogen donor. The emergence of several new strategies for synthesising green methanol, underlines its potential as a sustainable hydrogen source for such transformations. Transfer hydrogenation of methyl levulinate over Cu-ZrO2 was determined to proceed through a two-step ‘hydrogen borrowing’ process. The first step involves methanol dehydrogenation (rate limiting) and the second, levulinate reduction. This proof-of-concept study demonstrates that methanol can be used effectively as a hydrogen source for such transformations when a suitable catalyst is employed

The formation of methanol from glycerol bio-waste over doped ceria based catalysts

A series of ceria-based solid-solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising of Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO2, Pr6O11 and ZrO2. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction and it was found that an increased defect density or reducibility was beneficial. The space time yield of methanol normalised to surface area over CeO2 was found to be 0.052 mmolMeOH m-2 h-1 and over CeZrO2 and CePrO2 this was to 0.029 and 0.076 mmolMeOH m-2 h-1 respectively. The inclusion of Pr reduced the surface area, however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue “Science to enable the circular economy”

Photoactive Ag(I)-based coordination polymer as a potential semiconductor for photocatalytic water splitting and environmental remediation: experimental and theoretical approach

Metal–organic frameworks or metal coordination polymers (CPs) with controlled structure on the micro/nanoscale have attracted intense interest for potential applications in a wide variety of fields, such as energy storage and conversion, chemical and biological sensing, and catalysis. Here, we report a new class of photocatalytic material, Ag(I)-based nano-micro-structured coordination polymer (Ag(I)-CP), which offers performance at a level competitive with known semiconductors in photocatalytic water oxidation and oxidation of organic compounds, such as dye/organic pollutants present in contaminated water. The coordination polymer was synthesized by a wet-chemical route and has been characterized using powder X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy. The Ag(I)-CP has notable semiconducting characteristics and charge transfer ability due to ligand-centered charge transfer in combination with metal-to-ligand charge transfer (Ag–O cluster to ligand center), as established from experimental absorption, luminescence, and photoelectrochemical measurements alongside density functional theory calculations. Notably, Ag(I)-CP exhibits a highly reactive valance band potential +3.40 V vs NHE, composed of hybridized state of O 2p and C 2p through the organic linker and Ag 4d; this acts as an active center for the generation of reactive oxygen species, i.e., hydroxyl radical and h+ under photocatalytic conditions. Consequently, the photogenerated species facilitate effective oxidations of water and organic contaminants such as tartrazine, rhodamine B, and 2,4-dichlorophenol under UV light irradiation. Furthermore, our results suggest that the Ag(I)-CP could be used as a promising material for the development of heterostructure for a variety of photoassisted redox catalysis reactions

Band gap engineering of amine functionalized Ag(I)-based coordination polymers and their plasmonic Ag0 coupled novel visible light driven photo-redox system for selective oxidation of benzyl alcohol

We developed a one-pot synthetic route to design Ag nanoparticles (NPs) coupled mixed ligand Ag(I) coordination polymer (CP), Ag@Ag(I)-CP (40% NH2) for photocatalysis. Initial combined (experimental and DFT) study on mixed ligand CPs demonstrates that a rational substitution of ligand L1: 1,4-benzenedicarboxylate by L2: 2 amino 1,4-benzenedicarboxylate enhances porosity and reduction of energy gap (2.9 eV) due to highest occupied crystal orbital (HOCO; + 2.4 V vs. NHE) suitable for BA oxidation selectively to benzaldehyde (BD) ((E0 BA/BD = + 1.9 V). When Ag NP (~ 6–7 nm) is in-situ encapsulated on CP, formed a coupled structure Ag@Ag(I)-CP (40% NH 2), which offered advantages on BA oxidation (k (O2) = 7.4 × 10-4 min-1; yield: 19.1% BD, and k (persulfate) = 38.7 × 10-4 min-1; yield: 54.1% BD) along with significant stability, reusability and competitiveness than other Ag or precious metal NPs. The new material offers numerous possibilities for applications in oxidative organic transformations reactions. The simple synthetic strategy demonstrated in this work for the coupling of Ag(I) based coordination polymers with metal nanoparticles at the molecular scale for semiconductor like applications under visible light will accelerate extensive research in near future