Monoethanolamine Adsorption on TiO₂(110): Bonding, Structure, and Implications for Use as a Model Solid-Supported CO₂ Capture Material

We have studied the adsorption of monoethanolamine (MEA, HO­(CH₂)₂NH₂), a well-known CO₂ capture molecule, on the rutile TiO₂(110) surface using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy measurements indicate that MEA adsorbs with the oxygen atom of the hydroxyl group and the nitrogen atom of the amine group bonded to adjacent 5-fold coordinated Ti-sites (Ti­(5f)) in the Ti-troughs, leading to a saturation coverage of 0.5 ML at room temperature. Density functional theory calculations confirm that this adsorption configuration is the most stable one with an adsorption energy of 2.33 eV per MEA molecule. The bonding of MEA to TiO₂(110) is dominated by local donor–acceptor bonds between the oxygen and nitrogen atoms of the MEA molecule and surface Ti­(5f) sites. Hydrogen bonds between adjacent MEA molecules stabilize the adsorption structure at saturation coverage. The implications of this bonding configuration for the use of MEA/TiO₂(110) as a model CO₂ capture material will be discussed

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 CHx (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

<i>In Situ</i> Formation of FeRh Nanoalloys for Oxygenate Synthesis

Early and late transition metals are often combined as a strategy to tune the selectivity of catalysts for the conversion of syngas (CO/H₂) to C₂₊ oxygenates, such as ethanol. Here we show how the use of a highly reducible Fe₂O₃ support for Rh leads to the <i>in situ</i> formation of supported FeRh nanoalloy catalysts that exhibit high selectivity for ethanol synthesis. <i>In situ</i> characterizations by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) reveal the coexistence of iron oxide, iron carbide, metallic iron, and FeRh alloy phases depending on reaction conditions and Rh loading. Structural analysis coupled with catalytic testing indicates that oxygenate formation is correlated to the presence of FeRh alloys, while the iron oxide and carbide phases lead mainly to hydrocarbons. The formation of nanoalloys by <i>in situ</i> reduction of a metal oxide support under working conditions represents a simple approach for the preparation bimetallic catalysts with enhanced catalytic properties

Enhanced Stability of Pt-Cu Single-Atom Alloy Catalysts: In Situ Characterization of the Pt/Cu(111) Surface in an Ambient Pressure of CO

The interaction between a catalyst and reactants often induces changes in the surface structure and composition of the catalyst, which, in turn, affect its reactivity. Therefore, it is important to study such changes using in situ techniques under well-controlled conditions. We have used ambient pressure X-ray photoelectron spectroscopy to study the surface stability of a Pt/Cu(111) single-atom alloy in an ambient pressure of CO. By directly probing the Pt atoms, we found that CO causes a slight surface segregation of Pt atoms at room temperature. In addition, while the Pt/Cu(111) surface demonstrates poor thermal stability in ultrahigh vacuum conditions, where surface Pt starts to diffuse to the subsurface layer above 400 K, the presence of adsorbed CO enhances the thermal stability of surface Pt atoms. However, we also found that temperatures above 450 K cause restructuring of the subsurface layer, which consequently strengthens the CO binding to the surface Pt sites, likely because of the presence of neighboring subsurface Pt atoms

Low-Temperature Conversion of Methane to Methanol on CeO_<i>x</i>/Cu₂O Catalysts: Water Controlled Activation of the C–H Bond

An inverse CeO₂/Cu₂O/Cu­(111) catalyst is able to activate methane at room temperature producing C, CH_<i>x</i> fragments and CO_<i>x</i> species on the oxide surface. The addition of water to the system leads to a drastic change in the selectivity of methane activation yielding only adsorbed CH_<i>x</i> fragments. At a temperature of 450 K, in the presence of water, a CH₄ → CH₃OH catalytic transformation occurs with a high selectivity. OH groups formed by the dissociation of water saturate the catalyst surface, removing sites that could decompose CH_<i>x</i> fragments, and generating centers on which methane can directly interact to yield methanol

Understanding the Surface Structure and Catalytic Activity of SnO_<i>x</i>/Au(111) Inverse Catalysts for CO₂ and H₂ Activation

Carbon dioxide hydrogenation is a promising approach for the reduction of greenhouse gas pollution via the production of fuels and high-value chemicals utilizing C1 chemistry. In this process, the activation of nonpolar molecules, CO2 and H2, at mild conditions is challenging. Herein, we report a well-defined inverse SnOx/Au­(111) catalyst that shows the ability to activate both CO2 and H2 at room temperature. Scanning tunneling microscopy (STM) and ambient pressure X-ray photoemission spectroscopy (AP-XPS) are combined to understand the surface structure, growth mode, chemical state, and activity of SnOx/Au­(111) surfaces. Nanostructures of SnOx at the sub-monolayer level were prepared by depositing Sn on Au(111) followed by O2 oxidation. For the as-prepared SnOx/Au­(111), two-dimensionally formed SnOx thin films on a Au(111) substrate were observed with STM of two different moieties, discernible based on their height: clusters (∼0.4 Å) and nanoparticles (NPs, 1–2.5 Å), which are assigned to Sn–Au alloys and SnOx, respectively, in corroboration with XPS analysis. Furthermore, SnOx/Au­(111) was annealed under UHV to test its thermal stability. Upon annealing at 400–600 K, a disappearance of SnOx NPs and reappearance of highly dispersed Sn clusters were clearly noticeable from the STM and XPS results, identifying the thermal decomposition of SnOx and subsequent formation of Sn–Au alloys on the surface due to the recombination of Sn clusters with Au. We investigated the reactivity of the SnOx/Au­(111) surfaces toward CH4, CO2, and H2. The SnOx/Au­(111) surfaces have excellent CO2 and H2 activation abilities even at room temperature with negligible reactivity for methane activation. Our AP-XPS results show that H2 can be activated on the SnOx NPs by the reduction to Sn. For CO2, the activation and further dissociation are identified by a reoxidation of Sn with newly formed Sn–O bonds and the formation of surface carbon. Therefore, we propose that SnOx is a potential catalyst or additive to achieve CO2 hydrogenation under mild conditions

Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO₂ Electroreduction

Metallic silver (Ag) is known as an efficient electrocatalyst for the conversion of carbon dioxide (CO₂) to carbon monoxide (CO) in aqueous or nonaqueous electrolytes. However, polycrystalline silver electrocatalysts require significant overpotentials in order to achieve high selectivity toward CO₂ reduction, as compared to the side reaction of hydrogen evolution. Here we report a high-surface-area Ag nanocoral catalyst, fabricated by an oxidation–reduction method in the presence of chloride anions in an aqueous medium, for the electro-reduction of CO₂ to CO with a current efficiency of 95% at the low overpotential of 0.37 V and the current density of 2 mA cm^–2. A lower limit of TOF of 0.4 s^–1 and TON > 8.8 × 10⁴ (over 72 h) was estimated for the Ag nanocoral catalyst at an overpotential of 0.49 V. The Ag nanocoral catalyst demonstrated a 32-fold enhancement in surface-area-normalized activity, at an overpotential of 0.49 V, as compared to Ag foil. We found that, in addition to the effect on nanomorphology, the adsorbed chloride anions play a critical role in the observed enhanced activity and selectivity of the Ag nanocoral electrocatalyst toward CO₂ reduction. Synchrotron X-ray photoelectron spectroscopy (XPS) studies along with a series of control experiments suggest that the chloride anions, remaining adsorbed on the catalyst surface under electrocatalytic conditions, can effectively inhibit the side reaction of hydrogen evolution and enhance the catalytic performance for CO₂ reduction

Reduction of Nano-Cu₂O: Crystallite Size Dependent and the Effect of Nano-Ceria Support

Copper­(I) oxide (Cu₂O) is an effective catalyst in the CO oxidation reaction. While high surface to volume ratio in nanoparticles will increase their catalytic efficiency, it posts a stability problem. Here we study the stability of nano-cuprite against reduction as a function of its crystallite size and upon interaction with a nano-ceria support. A systematic analysis of isothermal reduction of a series size of monodispersed Cu₂O nanocrystals (±7%) with time-resolved X-ray diffraction (TR-XRD) provides the time-resolved phase fraction of Cu₂O and the time when reduction product of Cu (fcc) first appears. The initial phase fraction of nano-Cu₂O is less than one with the balance attributed to an amorphous CuO shell. Since no peaks of crystalline CuO (monoclinic) were observed, a core–shell structure with an amorphous CuO shell is proposed. From the analysis, Cu²⁺ content in corresponding to shell increases from 0 to 33% as Cu₂O decreases to 8 nm from the bulk. Based on the reduction profiles, a time size reduction (TSR) diagram is constructed for the observed Cu₂O phase behavior during reduction. The incorporation onto a nano-CeO₂ support (7 nm) significantly stabilizes our nano-Cu₂O in a reducing atmosphere. The oxygen supply propensity in terms of oxygen nonstoichiometry of CeO_2–<i>y</i> is shown to be lower when a larger crystallite size CeO₂ (20 nm) support is used. The larger oxygen capacity in smaller nano-CeO₂ support is analyzed and explained by the “Madelung model” with size-dependent bulk modulus of nano-ceria

Tuning the Placement of Pt “Single Atoms” on a Mixed CeO₂–TiO₂ Support

Defect sites on the oxide supports can be used to anchor and activate “single-atom” catalysts (SACs). By engineering the anchoring sites for supporting SACs, one can alter their electronic and atomic structures which, in turn, define their activity, selectivity, and stability for catalytic reactions. To create and tune unique sites for Pt SACs on CeO2 support, in this work, we synthesized a system consisting of CeO2 decorated on TiO2 nano-oxides for supporting the Pt SACs and investigated the effect of Pt weight loading. A combination of multiple structural characterization methods including diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) was employed to characterize the distribution of charge states of single atoms and evaluate the heterogeneity of their binding sites. We have found that the placement of Pt atoms can be tuned on a mixed oxide surface by changing the weight loading of Pt

Implementation of New TPD Analysis Techniques in the Evaluation of Second Order Desorption Kinetics of Cyanogen from Cu(001)

The interactions of cyanide species with a copper (001) surface were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Adsorbed cyanide species (CN(a)) undergo recombinative desorption evolving molecular cyanogen (C2N2). As the adsorbed CN species charge upon adsorption, mutually repulsive dipolar interactions lead to a marked desorption energy reduction with increasing CN(a) coverages. Two new TPD analysis approaches were developed, which used only accurately discernible observables and which do not assume constant desorption energies, Ed, and pre-exponential values, ν. These two approaches demonstrated a linear variation of Ed with instantaneous coverage. The first approach involved an analysis of the variations of desorption peak asymmetry with initial CN coverages. The second quantitative approach utilized only temperatures and intensities of TPD peaks, together with deduced surface coverages at the peak maxima, also as a function of initial surface coverages. Parameters derived from the latter approach were utilized as initial inputs for a comprehensive curve fit analysis technique. Excellent fits for all experimental desorption curves were produced in simulations. The curve fit analysis confirms that the activation energy of desorption of 170−180 kJ/mol at low coverage decreases by up to 14−15 kJ/mol at CN saturation