Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal–Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking

Unveiling Direct Electrochemical Oxidation of Methane at the Ceria/Gas Interface

Solid oxide fuel cells (SOFCs) stand out in sustainable energy systems for their unique ability to efficiently utilize hydrocarbon fuels, particularly those from carbon-neutral sources. CeO2-δ (ceria) based oxides embedded in SOFCs are recognized for their critical role in managing hydrocarbon activation and carbon coking. However, even for the simplest hydrocarbon molecule, CH4, the mechanism of electrochemical oxidation at the ceria/gas interface is not well understood and the capability of ceria to electrochemically oxidize methane remains a topic of debate. This lack of clarity stems from the intricate design of standard metal/oxide composite electrodes and the complex nature of electrode reactions involving multiple chemical and electrochemical steps. This study presents a Sm-doped ceria thin-film model cell that selectively monitors CH4 direct-electro-oxidation on the ceria surface. Using impedance spectroscopy, operando X-ray photoelectron spectroscopy, and density functional theory, it is unveiled that ceria surfaces facilitate C─H bond cleavage and that H2O formation is key in determining the overall reaction rate at the electrode. These insights effectively address the longstanding debate regarding the direct utilization of CH4 in SOFCs. Moreover, these findings pave the way for an optimized electrode design strategy, essential for developing high-performance, environmentally sustainable fuel cells.

From Methane to Methanol: Pd-iC-CeO2 Catalysts Engineered for High Selectivity via Mechanochemical Synthesis

In the pursuit of selective conversion of methane directly to methanol in the liquid-phase, a common challenge is the concurrent formation of undesirable liquid oxygenates or combustion byproducts. However, we demonstrate that monometallic Pd-CeO2 catalysts, modified by carbon, created by a simple mechanochemical synthesis method exhibit 100% selectivity toward methanol at 75 °C, using hydrogen peroxide as oxidizing agent. The solvent free synthesis yields a distinctive Pd-iC-CeO2 interface, where interfacial carbon (iC) modulates metal-oxide interactions and facilitates tandem methane activation and peroxide decomposition, thus resulting in an exclusive methanol selectivity of 100% with a yield of 117 μmol/gcat at 75 °C. Notably, solvent interactions of H2O2 (aq) were found to be critical for methanol selectivity through a density functional theory (DFT)-simulated Eley-Rideal-like mechanism. This mechanism uniquely enables the direct conversion of methane into methanol via a solid-liquid-gas process.

Electronic and Structural Properties of Thin Iron Oxide Films on CeO2

Modification of CeO2 (ceria) with 3d transition metals, particularly iron, has been proven to significantly enhance its catalytic efficiency in oxidation or combustion reactions. Although this phenomenon is widely reported, the nature of the iron-ceria interaction responsible for this improvement remains debated. To address this issue, we prepared well-defined model FeOx/CeO2(111) catalytic systems and studied their structure and interfacial electronic properties using photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction, coupled with density functional theory (DFT) calculations. Our results show that under ultrahigh vacuum conditions, Fe deposition leads to the formation of small FeOx clusters on the ceria surface. Subsequent annealing results in the growth of large amorphous FeOx particles and a 2D FeOx layer. Annealing in an oxygen-rich atmosphere further oxidizes iron up to the Fe3+ state and improves the crystallinity of both the 2D layer and the 3D particles. Our DFT calculations indicate that the 2D FeOx layer interacts strongly with the ceria surface, exhibiting structural corrugations and transferred electrons between Fe2+/Fe3+ and Ce4+/Ce3+ redox pairs. The novel 2D FeOx/CeO2(111) phase may explain the enhancement of the catalytic properties of CeO2 by iron. Moreover, the corrugated 2D FeOx layer can serve as a template for the ordered nucleation of other catalytically active metals, in which the redox properties of the 2D FeOx/CeO2(111) system are exploited to modulate the charge of the supported metals.

High-entropy-perovskite subnanowires for photoelectrocatalytic coupling of methane to acetic acid

The incorporation of multiple immiscible metals in high-entropy oxides can create the unconventional coordination environment of catalytic active sites, while the high formation temperature of high-entropy oxides results in bulk materials with low specific surface areas. Here we develop the high-entropy LaMnO3-type perovskite-polyoxometalate subnanowire heterostructures with periodically aligned high-entropy LaMnO3 oxides and polyoxometalate under a significantly reduced temperature of 100 oC, which is much lower than the temperature required by state-of-the-art calcination methods for synthesizing high-entropy oxides. The high-entropy LaMnO3-polyoxometalate subnanowires exhibit excellent catalytic activity for the photoelectrochemical coupling of methane into acetic acid under mild conditions (1 bar, 25 oC), with a high productivity (up to 4.45 mmol g‒1cat h‒1) and selectivity ( > 99%). Due to the electron delocalization at the subnanometer scale, the contiguous active sites of high-entropy LaMnO3 and polyoxometalate in the heterostructure can efficiently activate C - H bonds and stabilize the resulted *COOH intermediates, which benefits the in situ coupling of *CH3 and *COOH into acetic acid.

Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

Water-participated mild oxidation of ethane to acetaldehyde

The direct conversion of low alkane such as ethane into high-value-added chemicals has remained a great challenge since the development of natural gas utilization. Herein, we achieve an efficient one-step conversion of ethane to C2 oxygenates on a Rh1/AC-SNI catalyst under a mild condition, which delivers a turnover frequency as high as 158.5 h-1. 18O isotope-GC-MS shows that the formation of ethanol and acetaldehyde follows two distinct pathways, where oxygen and water directly participate in the formation of ethanol and acetaldehyde, respectively. In situ formed intermediate species of oxygen radicals, hydroxyl radicals, vinyl groups, and ethyl groups are captured by laser desorption ionization/time of flight mass spectrometer. Density functional theory calculation shows that the activation barrier of the rate-determining step for acetaldehyde formation is much lower than that of ethanol, leading to the higher selectivity of acetaldehyde in all the products.

Product Peroxidation Inhibition in Methane Photooxidation into Methanol

Methane photooxidation into methanol offers a practical approach for the generation of high-value chemicals and the efficient storage of solar energy. However, the propensity for C─H bonds in the desired products to cleave more easily than those in methane molecules results in a continuous dehydrogenation process, inevitably leading to methanol peroxidation. Consequently, inhibiting methanol peroxidation is perceived as one of the most formidable challenges in the field of direct conversion of methane to methanol. This review offers a thorough overview of the typical mechanisms involved radical mechanism and active site mechanism and the regulatory methods employed to inhibit product peroxidation in methane photooxidation. Additionally, several perspectives on the future research direction of this crucial field are proposed.

Comprehensive Density Functional and Kinetic Monte Carlo Study of CO2 Hydrogenation on a Well-Defined Ni/CeO2 Model Catalyst: Role of Eley-Rideal Reactions

A detailed multiscale study of the mechanism of CO2 hydrogenation on a well-defined Ni/CeO2 model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley-Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni4 cluster supported on CeO2 (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO2 methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni4 cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This effect is even more evident for the simulations explicitly including Eley-Rideal steps. The kMC simulations reveal that CO is formed via the dissociative pathway of the reverse water-gas shift reaction, while methane is formed via a CO2 → CO → HCO → CH → CH2 → CH3 → CH4 mechanism. Overall, our results show the importance of including the Eley-Rideal reactions and point to small Ni clusters supported on the CeO2 (111) surface as potential good catalysts for high selective CO2 methanation under mild conditions, while very active and selective toward CO formation at higher temperatures.

Role of H2O in Catalytic Conversion of C1 Molecules

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

Theoretical Perspective of Promoting Direct Methane-to-Methanol Conversion at Complex Metal Oxide-Metal Interfaces

Direct methane conversion to methanol has been considered as an effective and economic way to address greenhouse effects and the current high demand for methanol in industry. However, the process has long been challenging due to lack of viable catalysts to compromise the activation of methane that typically occurs at high temperatures and retaining of produced methanol that requires mild conditions. This Perspective demonstrates an effective strategy to promote direct methane to methanol conversion by engineering the active sites and chemical environments at complex metal oxide - copper oxide - copper interfaces. Such effort strongly depends on extensive theoretical studies by combining density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations to provide in-depth understanding of reaction mechanism and active sites, which build a strong basis to enable the identification of design principles and advance the catalyst optimization for selective CH₄-to-CH₃OH conversion.

Direct Catalytic Oxidation of Low-Concentration Methane to Methanol in One Step on Ni-Promoted BiOCl Catalysts

The direct oxidation of low-concentration methane (CH₄) to methanol (CH₃OH) is often regarded as the "holy grail". However, it still is very difficult and challenging to oxidize methane to methanol in one step. In this work, we present a new approach to directly oxidize CH₄ to generate CH₃OH in one step by doping non-noble metal Ni sites on bismuth oxychloride (BiOCl) equipped with high oxygen vacancies. Thereinto, the conversion rate of CH₃OH can reach 39.07 μmol/(g_cat·h) under 420 °C and flow conditions on the basis of O₂ and H₂O. The crystal morphology structure, physicochemical properties, metal dispersion, and surface adsorption capacity of Ni-BiOCl were explored, and the positive effect on the oxygen vacancy of the catalyst was proved, thus improving the catalytic performance. Furthermore, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was also performed to study the surface adsorption and reaction process of methane to methanol in one step. Results demonstrate that the key to keep good activity lies in the oxygen vacancies of unsaturated Bi atoms, which can adsorb and active CH₄ and to produce methyl groups and adsorbing hydroxyl groups in methane oxidation process. This study broadens the application of oxygen-deficient catalysts in the catalytic conversion of CH₄ to CH₃OH in one step, which provides a new perspective on the role of oxygen vacancies in improving the catalytic performance of methane oxidation.

Role of oxygen vacancy in spinel (FeCoNiCrMn)3O4 high entropy oxides prepared via two different methods for the selective CH bond oxidation of p-chlorotoluene

The selective CH bond oxidation of aromatic hydrocarbon is an interesting but challenging task, it is desirable to develop efficient heterogeneous non-noble metal catalyst for this reaction. Herein, two kinds of spinel (FeCoNiCrMn)₃O₄ high entropy oxides were fabricated via two different methods (i.e., c-FeCoNiCrMn, prepared by a co-precipitation method, and m-FeCoNiCrMn, prepared by physically mixing method). Different from traditional environmentally-unfriendly Co/Mn/Br system, the prepared catalysts were employed for the selective CH bond oxidation of p-chlorotoluene to p-chlorobenzaldehyde in a green approach. Compared to m-FeCoNiCrMn, c-FeCoNiCrMn have smaller particles size and larger specific surface area, which were related to the enhanced catalytic activity. More importantly, characterization results disclosed that abundant oxygen vacancies were formed over c-FeCoNiCrMn. Such a result facilitated the adsorption of p-chlorotoluene on the catalyst surface and promoted the formation of *ClPhCH₂O intermediate as well as the desired p-chlorobenzaldehyde, as revealed by DFT (Density functional theory) calculations. Besides, scavenger tests and EPR (Electron paramagnetic resonance) results indicated that hydroxyl radical derived from H₂O₂ homolysis was the main active oxidative species for this reaction. This work revealed the role of oxygen vacancy in spinel high entropy oxide and also demonstrated its promising application for the selective CH bond oxidation in an environmentally-benign approach.

Cu and Si co-doping on TiO2 nanosheets to modulate reactive oxygen species for efficient photocatalytic methane conversion

In this study, we successfully construct Cu and Si co-doped ultrathin TiO₂ nanosheets. As confirmed by comprehensive characterizations, Cu and Si co-doping can rationally tailor the electronic structure of TiO₂ to maneuver reactive oxygen species for effective photocatalytic methane conversion. In addition, this co-doping greatly enhances the utilization efficiency of photogenerated charges. Furthermore, it is revealed that Cu and Si co-doping can significantly boost the adsorption and activation of methane on TiO₂ nanosheets. As a result, the optimized catalyst achieves a C₂H₆ production rate of 33.8 μmol g^-1 h^-1 with a selectivity of 88.4%. This work provides insights into nanocatalyst design toward efficient photocatalytic methane conversion into value-added compounds.

Highly Active and Stable Ni/La-Doped Ceria Material for Catalytic CO2 Reduction by Reverse Water-Gas Shift Reaction

The design of an active, effective, and economically viable catalyst for CO₂ conversion into value-added products is crucial in the fight against global warming and energy demand. We have developed very efficient catalysts for reverse water-gas shift (rWGS) reaction. Specific conditions of the synthesis by combustion allow the obtention of macroporous materials based on nanosized Ni particles supported on a mixed oxide of high purity and crystallinity. Here, we show that Ni/La-doped CeO₂ catalysts─with the "right" Ni and La proportions─have an unprecedented catalytic performance per unit mass of catalyst for the rWGS reaction as the first step toward CO₂ valorization. Correlations between physicochemical properties and catalytic activity, obtained using a combination of different techniques such as X-ray and neutron powder diffraction, Raman spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, electron microscopy, and catalytic testing, point out to optimum values for the Ni loading and the La proportion. Density functional theory calculations of elementary steps of the reaction on model Ni/ceria catalysts aid toward the microscopic understanding of the nature of the active sites. This finding offers a fundamental basis for developing economical catalysts that can be effectively used for CO₂ reduction with hydrogen. A catalyst based on Ni_0.07/(Ce_0.9La_0.1O_x)_0.93 shows a CO production of 58 × 10^-5 mol_CO·g_cat^-1·s^-1 (700 °C, H₂/CO₂ = 2; selectivity to CO > 99.5), being stable for 100 h under continuous reaction.

Catalytic hydrogenolysis of plastic to liquid hydrocarbons over a nickel-based catalyst

The catalytic hydrogenolysis of a typical model compound of mulching film waste, polyethylene, was investigated as a potential way to improve economic efficiency of mulching film recycling. Nickel-based heterogeneous catalysts are proposed for polyethylene hydrogenolysis to produce liquid hydrocarbons. Among catalysts supported on various carriers, Ni/SiO₂ catalyst shows the highest activity which may due to the interactions between nickel and silica with the formation of nickel phyllosilicate. As high as 81.18% total gasoline and diesel range hydrocarbon was obtained from the polyethylene hydrogenolysis at relatively mild condition of 280 °C, and 3 MPa cold hydrogen pressure. The result is comparable to what have been reported in previous studies using noble metal catalysts. The gasoline and diesel range hydrocarbon are n-alkanes with a distribution at a range of C₄-C₂₂. The gas products are primarily CH₄ along with a small amount of C₂H₆ and C₃H₈. High yield of CH₄ as much as 9.68% was observed for the cleavage of molecule occurs along the alkane chain.

Recent Insights into Cu-Based Catalytic Sites for the Direct Conversion of Methane to Methanol

Direct conversion of methane to methanol is an effective and practical process to improve the efficiency of natural gas utilization. Copper (Cu)-based catalysts have attracted great research attention, due to their unique ability to selectively catalyze the partial oxidation of methane to methanol at relatively low temperatures. In recent decades, many different catalysts have been studied to achieve a high conversion of methane to methanol, including the Cu-based enzymes, Cu-zeolites, Cu-MOFs (metal-organic frameworks) and Cu-oxides. In this mini review, we will detail the obtained evidence on the exact state of the active Cu sites on these various catalysts, which have arisen from the most recently developed techniques and the results of DFT calculations. We aim to establish the structure–performance relationship in terms of the properties of these materials and their catalytic functionalities, and also discuss the unresolved questions in the direct conversion of methane to methanol reactions. Finally, we hope to offer some suggestions and strategies for guiding the practical applications regarding the catalyst design and engineering for a high methanol yield in the methane oxidation reaction.

Platinum-Catalysed Selective Aerobic Oxidation of Methane to Formaldehyde in the Presence of Liquid Water

The aerobic, selective oxidation of methane to C1 -oxygenates remains a challenge, due to the more facile, consecutive oxidation of formed products to CO2 . Here, we report on the aerobic selective oxidation of methane under continuous flow conditions, over platinum-based catalysts yielding formaldehyde with a high selectivity (reaching 90 % for Pt/TiO2 and 65 % over Pt/Al2 O3 ) upon co-feeding water. The presence of liquid water under reaction conditions increases the activity strongly attaining a methane conversion of 1-3 % over Pt/TiO2 . Density-functional theory (DFT) calculations show that the preferential formation of formaldehyde is linked to the stability of the di-σ-hydroxy-methoxy species on platinum, the preferred carbon-containing species on Pt(111) at a high chemical potential of water. Our findings provide novel insights into the reaction pathway for the Pt-catalysed, aerobic selective oxidation of CH4 .

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

Direct conversion of methane (CH₄) to C_1-2 liquid oxygenates is a captivating approach to lock carbons in transportable value-added chemicals, while reducing global warming. Existing approaches utilizing the transformation of CH₄ to liquid fuel via tandemized steam methane reforming and the Fischer-Tropsch synthesis are energy and capital intensive. Chemocatalytic partial oxidation of methane remains challenging due to the negligible electron affinity, poor C-H bond polarizability, and high activation energy barrier. Transition-metal and stoichiometric catalysts utilizing harsh oxidants and reaction conditions perform poorly with randomized product distribution. Paradoxically, the catalysts which are active enough to break C-H also promote overoxidation, resulting in CO₂ generation and reduced carbon balance. Developing catalysts which can break C-H bonds of methane to selectively make useful chemicals at mild conditions is vital to commercialization. Single atom catalysts (SACs) with specifically coordinated metal centers on active support have displayed intrigued reactivity and selectivity for methane oxidation. SACs can significantly reduce the activation energy due to induced electrostatic polarization of the C-H bond to facilitate the accelerated reaction rate at the low reaction temperature. The distinct metal-support interaction can stabilize the intermediate and prevent the overoxidation of the reaction products. The present review accounts for recent progress in the field of SACs for the selective oxidation of CH₄ to C_1-2 oxygenates. The chemical nature of catalytic sites, effects of metal-support interaction, and stabilization of intermediate species on catalysts to minimize overoxidation are thoroughly discussed with a forward-looking perspective to improve the catalytic performance.

Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C-H and O-H Bonds over NiCu/CeO2 Catalysts

The efficient activation of methane and the simultaneous water dissociation are crucial in many catalytic reactions on oxide-supported transition metal catalysts. On very low-loaded Ni/CeO₂ surfaces, methane easily fully decomposes, CH₄ → C + 4H, and water dissociates, H₂O→ OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH₄, O₂), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of C-H bonds in CH_x (x = 1-3) species on Ni/CeO₂ surfaces can be manipulated by adding Cu, forming bimetallic NiCu clusters, whereas the ease for cleavage of O-H bonds in water is not affected by ensemble effects, as obtained from density functional theory-based calculations. CH₄ activation occurs only on Ni sites, and H₂O activation occurs on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni₃Cu₁/CeO₂ model catalyst predicts a higher selectivity and a lower activation barrier for methanol production, compared with that for Ni₄/CeO₂. These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions.

High-performance photocatalytic nonoxidative conversion of methane to ethane and hydrogen by heteroatoms-engineered TiO2

Nonoxidative coupling of methane (NOCM) is a highly important process to simultaneously produce multicarbons and hydrogen. Although oxide-based photocatalysis opens opportunities for NOCM at mild condition, it suffers from unsatisfying selectivity and durability, due to overoxidation of CH4 with lattice oxygen. Here, we propose a heteroatom engineering strategy for highly active, selective and durable photocatalytic NOCM. Demonstrated by commonly used TiO2 photocatalyst, construction of Pd-O4 in surface reduces contribution of O sites to valence band, overcoming the limitations. In contrast to state of the art, 94.3% selectivity is achieved for C2H6 production at 0.91 mmol g-1 h-1 along with stoichiometric H2 production, approaching the level of thermocatalysis at relatively mild condition. As a benchmark, apparent quantum efficiency reaches 3.05% at 350 nm. Further elemental doping can elevate durability over 24 h by stabilizing lattice oxygen. This work provides new insights for high-performance photocatalytic NOCM by atomic engineering.

Structure and reactivity of model CeO2surfaces

As a key component in many industrial heterogeneous catalysts, the surface structure and reactivity of ceria, CeO₂, has attracted a lot of attention. In this topical review we discuss some of the approaches taken to form a deeper understanding of the surface physics and chemistry of this important and interesting material. In particular, we focus on the preparation of ultrathin ceria films, nanostructures and supported metal nanoparticles. Cutting-edge microscopic and spectroscopic experimental techniques are highlighted which can probe the behaviour of oxygen species and atomic defects on these model surfaces.

Partial Methane Oxidation in Fuel Cell-Type Reactors for Co-Generation of Energy and Chemicals: A Short Review

The conversion of methane into chemicals is of interest to achieve a decarbonized future. Fuel cells are electrochemical devices commonly used to obtain electrical energy but can be utilized either for chemicals’ production or both energy and chemicals cogeneration. In this work, the partial oxidation of methane in fuel cells for electricity generation and valuable chemicals production at the same time is reviewed. For this purpose, we compile different types of methane-fed fuel cells, both low- and high-temperature fuel cells. Despite the fact that few studies have been conducted on this subject, promising results are driving the development of fuel cells that use methane as a fuel source for the cogeneration of power and valuable chemicals.

Characterization of Surface Species during Benzene Hydroxylation over a NiO-Ceria-Zirconia Catalyst

NiO/ceria-zirconia (CZ) is a promising catalyst for the selective oxidation of benzene, as the Lewis-acidic NiO clusters can activate C-H bonds and the redox-active CZ support can activate O₂ and supply active oxygen species for the reaction. In this study, we used transmission in situ infrared (IR) spectroscopy to examine surface species formed from benzene, water, oxygen, phenol, and catechol on a NiO/CZ catalyst. The formation of surface species from benzene and phenol was compared at different temperatures in the range of 50-200 °C in the presence and absence of water vapor. We also examined the role of the NiO clusters and the CZ support during benzene activation by comparing the surface species formed on NiO-CZ with those formed on a Ni-free CZ support and on a NiO/SiO₂ catalyst. The spectrum of surface species from dosing benzene at 180 °C provides evidence for C-H bond activation. Specifically, the observation of C-O stretching vibrations indicates the formation of phenolate species. Introduction of water enhances these IR signals and introduces several additional peaks, indicating that a variety of different surface species are formed. These results show that NiO/CZ could catalyze direct conversion of benzene to phenol.

A DFT Study of Boron Nitride-confined Nickel Single Atoms for the Oxidation of Methane to Methanol

Direct oxidation of methane to methanol (DMTM) remains an economically tantalizing but fundamentally challenging goal because of the highly stable C-H bonds. By using density functional theory calculations, we investigated...

Binary Au-Cu Reaction Sites Decorated ZnO for Selective Methane Oxidation to C1 Oxygenates with Nearly 100% Selectivity at Room Temperature

Direct and efficient oxidation of methane to methanol and the related liquid oxygenates provides a promising pathway for sustainable chemical industry, while still remaining an ongoing challenge owing to the dilemma between methane activation and overoxidation. Here, ZnO with highly dispersed dual Au and Cu species as cocatalysts enables efficient and selective photocatalytic conversion of methane to methanol and one-carbon (C1) oxygenates using O₂ as the oxidant operated at ambient temperature. The optimized AuCu-ZnO photocatalyst achieves up to 11225 μmol·g^-1·h^-1 of primary products (CH₃OH and CH₃OOH) and HCHO with a nearly 100% selectivity, resulting in a 14.1% apparent quantum yield at 365 nm, much higher than the previous best photocatalysts reported for methane conversion to oxygenates. In situ EPR and XPS disclose that Cu species serve as photoinduced electron mediators to promote O₂ activation to ^•OOH, and simultaneously that Au is an efficient hole acceptor to enhance H₂O oxidation to ^•OH, thus synergistically promoting charge separation and methane transformation. This work highlights the significances of co-modification with suitable dual cocatalysts on simultaneous regulation of activity and selectivity.

Local Environment Determined Reactant Adsorption Configuration for Enhanced Electrocatalytic Acetone Hydrogenation to Propane

We demonstrate a widely applicable method to alter the adsorption configuration of multi-carbon containing reactants by no catalyst engineering but simply adjusting the local reaction environment of the catalyst surface. Using electrocatalytic acetone to propane hydrogenation (APH) as a model reaction and common commercial Pt/Pt-based materials as catalysts, we found local H⁺ concentration can significantly influence the adsorption mode of acetone reactant, for example, in vertical or flat mode, and target product selectivity. Electrocatalytic measurement combined with in situ spectroscopic characterizations reveals that the vertically adsorbed acetone is favorable for propane production while the flatly adsorbed mode suppresses the reaction. DFT calculations indicate that the H coverage on catalyst surface plays a decisive role in the adsorption configuration of acetone. The increased local acidity can facilitate the adsorption configuration of acetone from flat to vertical mode and suppress the competing hydrogen evaluation reaction, which consequently enhances the APH selectivity.

Selective Methane Oxidation to Methanol on ZnO/Cu2O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/Cu₂O/Cu(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambient-pressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu₂O/Cu(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO-Cu₂O step sites are active centers for methanol synthesis when exposed to CH₄ and O₂ due to an effective O-O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH₄ → CH₃OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu₂O/Cu(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2surfaces

To study the dependence of the relative stability of surface (V^A) and subsurface (V^B) oxygen vacancies with the crystal facet of CeO₂, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce³⁺polarons, are facet- and concentration-dependent. At the higher vacancy concentration, theV^Ais more stable than theV^Bat the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both theV^Aand theV^Bhave similar stability. The stability of theV^Avacancies, compared to that of theV^B, is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce³⁺-Ce³⁺interactions.

Surface characterization and methane activation on SnOx/Cu2O/Cu(111) inverse oxide/metal catalysts

To activate methane at low or medium temperatures is a difficult task and a pre-requisite for the conversion of this light alkane into high value chemicals. Herein, we report the preparation and characterizations of novel SnO_x/Cu₂O/Cu(111) interfaces that enable low-temperature methane activation. Scanning tunneling microscopy identified small, well-dispersed SnO_x nanoclusters on the Cu₂O/Cu(111) substrate with an average size of 8 Å, and such morphology was sustained up to 450 K in UHV annealing. Ambient pressure X-ray photoelectron spectroscopy showed that hydrocarbon species (CH_x groups), the product of methane activation, were formed on SnO_x/Cu₂O/Cu(111) at a temperature as low as 300 K. An essential role of the SnO_x-Cu₂O interface was evinced by the SnO_x coverage dependence. Systems with a small amount of tin oxide, 0.1-0.2 ML coverage, produced the highest concentration of adsorbed CH_x groups. Calculations based on density functional theory showed a drastic reduction in the activation barrier for C-H bond cleavage when going from Cu₂O/Cu(111) to SnO_x/Cu₂O/Cu(111). On the supported SnO_x, the dissociation of methane was highly exothermic (ΔE∼-35 kcal mol^-1) and the calculated barrier for activation (∼20 kcal mol^-1) could be overcome at 300-500 K, target temperatures for the conversion of methane to high value chemicals.

Nature of the Active Sites on Ni/CeO2 Catalysts for Methane Conversions

Effective catalysts for the direct conversion of methane to methanol and for methane’s dry reforming to syngas are Holy Grails of catalysis research toward clean energy technologies. It has recently been discovered that Ni at low loadings on CeO₂(111) is very active for both of these reactions. Revealing the nature of the active sites in such systems is paramount to a rational design of improved catalysts. Here, we correlate experimental measurements on the CeO₂(111) surface to show that the most active sites are cationic Ni atoms in clusters at step edges, with a small size wherein they have the highest Ni chemical potential. We clarify the reasons for this observation using density functional theory calculations. Focusing on the activation barrier for C–H bond cleavage during the dissociative adsorption of CH₄ as an example, we show that the size and morphology of the supported Ni nanoparticles together with strong Ni-support bonding and charge transfer at the step edge are key to the high catalytic activity. We anticipate that this knowledge will inspire the development of more efficient catalysts for these reactions.

Efficient and selective photocatalytic CH4 conversion to CH3OH with O2 by controlling overoxidation on TiO2

The conversion of photocatalytic methane into methanol in high yield with selectivity remains a huge challenge due to unavoidable overoxidation. Here, the photocatalytic oxidation of CH₄ into CH₃OH by O₂ is carried out on Ag-decorated facet-dominated TiO₂. The {001}-dominated TiO₂ shows a durable CH₃OH yield of 4.8 mmol g⁻¹ h⁻¹ and a selectivity of approximately 80%, which represent much higher values than those reported in recent studies and are better than those obtained for {101}-dominated TiO₂. Operando Fourier transform infrared spectroscopy, electron spin resonance, and nuclear magnetic resonance techniques are used to comprehensively clarify the underlying mechanism. The straightforward generation of oxygen vacancies on {001} by photoinduced holes plays a key role in avoiding the formation of •CH₃ and •OH, which are the main factors leading to overoxidation and are generally formed on the {101} facet. The generation of oxygen vacancies on {001} results in distinct intermediates and reaction pathways (oxygen vacancy → Ti–O₂^• → Ti–OO–Ti and Ti–(OO) → Ti–O^• pairs), thus achieving high selectivity and yield for CH₄ photooxidation into CH₃OH.

The photocatalytic conversion of CH₄ into CH₃OH with high activity and selectivity must avoid product overoxidation. Here, authors minimize overoxidation by using a (001)-dominated TiO₂ nanosheet to circumvent CH₄ overoxidation intermediates plus reaction pathways that occur on (101) facets.

Harnessing of Diluted Methane Emissions by Direct Partial Oxidation of Methane to Methanol over Cu/Mordenite

The upgrading of diluted methane emissions into valuable products can be accomplished at low temperatures (200 °C) by the direct partial oxidation of methanol over copper-exchanged zeolite catalysts. The reaction has been studied in a continuous fixed-bed reactor loaded with a Cu–mordenite catalyst, according to a three-step cyclic process: adsorption of methane, desorption of methanol, and reactivation of the catalyst. The purpose of the work is the use of methane emissions as feedstocks, which is challenging due to their low methane concentration and the presence of oxygen. Methane concentration had a marked influence on methane adsorption and methanol production (decreased from 164 μmol/g Cu for pure methane to 19 μmol/g Cu for 5% methane). The presence of oxygen, even in low concentrations (2.5%), reduced methane adsorption drastically. However, methanol production was only affected slightly (average decrease of 9%), concluding that methane adsorbed on the active centers yielding methanol is not influenced by oxygen.

Reaction Pathway for Coke-Free Methane Steam Reforming on a Ni/CeO2 Catalyst: Active Sites and the Role of Metal-Support Interactions

Methane steam reforming (MSR) plays a key role in the production of syngas and hydrogen from natural gas. The increasing interest in the use of hydrogen for fuel cell applications demands development of catalysts with high activity at reduced operating temperatures. Ni-based catalysts are promising systems because of their high activity and low cost, but coke formation generally poses a severe problem. Studies of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) indicate that CH₄/H₂O gas mixtures react with Ni/CeO₂(111) surfaces to form OH, CH_x, and CH_xO at 300 K. All of these species are easy to form and desorb at temperatures below 700 K when the rate of the MSR process is accelerated. Density functional theory (DFT) modeling of the reaction over ceria-supported small Ni nanoparticles predicts relatively low activation barriers between 0.3 and 0.7 eV for complete dehydrogenation of methane to carbon and the barrierless activation of water at interfacial Ni sites. Hydroxyls resulting from water activation allow for CO formation via a COH intermediate with a barrier of about 0.9 eV, which is much lower than that through a pathway involving lattice oxygen from ceria. Neither methane nor water activation is a rate-determining step, and the OH-assisted CO formation through the COH intermediate constitutes a low-barrier pathway that prevents carbon accumulation. The interactions between Ni and the ceria support and the low metal loading are crucial for the reaction to proceed in a coke-free and efficient way. These results pave the way for further advances in the design of stable and highly active Ni-based catalysts for hydrogen production.