Hydrogen migration at restructuring palladium-silver oxide boundaries dramatically enhances reduction rate of silver oxide.

Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 104 in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium-silver alloy. The palladium-silver interface also dynamically restructures during reduction, resulting in silver-palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials

Identifying the Optimal Pd Ensemble Size in Dilute PdAu Alloy Nanomaterials for Benzaldehyde Hydrogenation

Unraveling metal nuclearity effects is central for active site identification and the development of high-performance heterogeneous catalysts. Herein, a platform of nanostructured palladium (Pd) in gold (Au) dilute alloy nanoparticles supported on raspberry-colloid-templated (RCT) silica was employed to systematically assess the impact of the Pd ensemble size for the low-nuclearity regime in the Au surface layer, from single atoms to clusters, on the catalytic performance in the liquid-phase hydrogenation of benzaldehyde to benzyl alcohol. Combining catalyst evaluation, detailed characterization, and mechanistic studies based on density functional theory, we show that Pd single atoms in the Au surface plane (corresponding to samples with 4 atom % Pd in Au) are virtually inactive in this reaction and benzyl alcohol production is optimal over small Pd clusters (corresponding to samples with 10-12 atom % Pd in Au) due to superior benzaldehyde adsorption and transition state stabilization for the C-H bond formation step. For larger Pd ensembles (samples with ≥10 atom % Pd in Au), C-O bond hydrogenolysis occurs, promoting toluene formation and decreasing the selectivity toward benzyl alcohol, in line with a relatively lowered C-O bond cleavage barrier. Nevertheless, the nanostructured bimetallic Pd13Au87/SiO2-RCT catalyst still outperforms monometallic Pd counterparts in terms of selectivity for benzyl alcohol over toluene at comparable conversion and rate. Furthermore, the stability is improved compared to pure Pd nanoparticles due to inhibited particle agglomeration in the RCT silica matrix

Oksijen türlerinin Au(111) ve Ag(111) model katalizörleri üzerindeki doğası ve O-H, C-H, C-C, N-H bağ aktivasyonundaki rolleri

Cataloged from PDF version of article.Thesis (Ph.D.): Bilkent University, Department of Chemistry, İhsan Doğramacı Bilkent University, 2017.Includes bibliographical references (leaves 92-107).Metal-catalyzed heterogeneous oxidation reactions have high importance for the large-scale production of the commodity chemicals vastly used in the chemical industry. Controlling the selectivity in such processes to increase the product yield and minimize the production of undesired byproducts requires a molecular level understanding of the bond activation mechanisms. Thus, understanding the nature of oxygen species in various bond cleavage processes is critical. In the current work, nature of oxygen species was studied on the planar Au(111) and Ag(111) single crystal model catalyst surfaces via x-ray photoelectron spectroscopy (XPS), temperature programmed desorption/ temperature programmed reaction spectroscopy (TPD/TPRS), low energy electron diffraction (LEED) and infrared reflection absorption spectroscopy (IRAS) techniques under ultra-high vacuum (UHV) conditions. Ozone (O3) was utilized as the oxygen delivery agent providing atomic oxygen to the reacting surface. Various oxygen species were determined on both Au(111) and Ag(111) model catalysts and their role in O-H, C-H, C-C and N-H bond activation was investigated by using probe molecules such as methanol, acetaldehyde and ammonia. Three different oxygen species such as atomic oxygen (Oa), subsurface oxygen (Osub) and surface oxide (Oox) were determined on Au(111) single crystal. Oxygen accumulation on Au(111) surface at 140 K for O<1.0 MLE of oxygen coverage resulted in the surface atomic oxygen (Oa) formation while 2D surface oxide (Oox) started to grow for O>1.0 MLE of oxygen coverage at the same temperature. It was also shown that oxygen atoms dissolved (Osub) into the bulk of the Au(111) single crystal when oxygen was accumulated at 473 K. Atomic oxygen species (Oa) on Au(111) was found to be very active for the cleavage of O-H and C-H bonds in methanol; C-C bond in acetaldehyde; N-H bond in ammonia molecules. Surface oxide (Oox) overlayer was also active for methanol oxidation, however it showed very high selectivity towards CO2. Dissolved oxygen atoms (Osub) revealed almost no activity in methanol oxidation reactions on Au(111). In a similar manner, three different oxygen species were determined on the Ag(111) surface such as surface atomic oxygen (Oa), surface oxide (Oox) and bulk-like oxide (Obulk) species. Disordered atomic oxygen (Oa) and surface oxide (Oox) overlayers prepared at 140 K on Ag(111) for O 0.2 MLE were found to be very active for O-H and CH bond cleavage producing formaldehyde as the dominant product. Increasing oxygen quantity for both oxygen species (0.7 MLE O 1.3 MLE) resulted mostly CO2 formation. Oa ( O < 1.10 MLE) was also found to be highly active in N-H bond cleavage for ammonia and selective to N2 as the dominant product. On the other hand, ordered p(5×1) and c(4×8) surface oxide (Oox) overlayers on Ag(111) prepared 473 K were found to be almost entirely inactive for N-H cleavage. Extreme oxygen exposures on Ag(111) ( O > 1.93 MLE ) at 140 K led to bulk-like silver oxide (Obulk) species with poor N2 selectivity in ammonia oxidation and increasing extent of formation of toxic pollutants such as NO and N2O.by Mustafa Karatok.Ph. D

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

© 2021 American Chemical Society.Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, Oe and nucleophilic, On). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low Oe coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of Oe on Ag(111) revealed that Oe was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH2O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH2O selectivity and methanol conversion, where 93% CH2O selectivity can be achieved for an oxygen surface coverage of θO = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of μ4 for θO = 0.26 ML with a suppression of CH2O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity

Selective Catalytic Ammonia Oxidation to Nitrogen by Atomic Oxygen Species on Ag(111)

Ammonia-selective catalytic oxidation was studied on the planar Ag(111) single-crystal model catalyst surface under ultra-high-vacuum (UHV) conditions. A variety of oxygen species were prepared via ozone decomposition on pristine Ag(111). Surface coverages of oxygen species were quantified by temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy techniques. Exposure of ozone on Ag(111) at 140 K led to a surface atomic oxygen (O_a) overlayer. Low-energy electron diffraction experiments revealed that annealing of this atomic oxygen-covered Ag(111) surface at 473 K in UHV resulted in the formation of ordered oxide surfaces (O_ox) with p(5×1) or c(4×8) surface structures. Ammonia interactions with O/Ag(111) surfaces monitored by temperature-programmed reaction spectroscopy indicated that disordered surface atomic oxygen selectively catalyzed N–H bond cleavage, yielding mostly N₂ along with minor amounts of NO and N₂O. Higher coverage O/Ag(111) surfaces, whose structure was tentatively assigned to a bulklike amorphous silver oxide (O_bulk), showed high selectivity toward N₂O formation (rather than N₂) due to its augmented oxygen density. In contrast, ordered surface oxide overlayers on Ag(111) (where the order was achieved by annealing the oxygen adlayer to 473 K) showed only very limited reactivity toward ammonia. The nature of the adsorbed NH₃ species on a clean Ag(111) surface and its desorption characteristics were also investigated via infrared reflection absorption spectroscopy and TPD techniques. Current findings demonstrate that the Ag(111) surface can selectively oxidize NH₃ to N₂ under well-defined experimental conditions without generating significant quantities of environmentally toxic species such as NO₂, NO, or N₂O

Comparative Analysis of Reactant and Product Adsorption Energies in the Selective Oxidative Coupling of Alcohols to Esters on Au(111)

Gold-based heterogeneous catalysts have attracted significant attention due to their selective partial oxidation capabilities, providing promising alternatives for the traditional industrial homogeneous catalysts. In the current study, the energetics of adsorption/desorption of alcohols (CH3OH/methanol, CH3CH2OH/ethanol, CH3CH2CH2OH/n-propanol) and esters (HCOOCH3/methyl formate, CH3COOCH3/methyl acetate, and CH3COOCH2CH3/ethyl acetate) on a planar Au(111) surface was investigated in conjunction with oxidative coupling reactions by means of temperature programmed desorption (TPD) and dispersion-corrected density functional theory (DFT) calculations. The results reveal a complex interplay between inter-molecular and surface-molecule interactions, both mediated by weak van der Waals forces, which dictates their relative stability on the gold surface. Both experimental and theoretical adsorption/desorption energies of the investigated esters are lower than those of the alcohols from which they originate through oxidative coupling reactions. This result can be interpreted as an important indication in favor of the selectivity of Au surfaces in alcohol oxidative coupling/partial oxidation reactions, allowing facile removal of partial oxidation products immediately after their generation preventing their complete oxidation to higher oxygenates

Tuning reactivity layer-by-layer: formic acid activation on Ag/Pd(111).

The potential for tuning the electronic structure of materials to control reactivity and selectivity in heterogenous catalysis has driven interest in ultrathin metal films which may differ from their bulk form. Herein, a 1-atomic layer Ag film on Pd(111) (Ag/Pd(111)) is demonstrated to have dramatically different reactivity towards formic acid compared to bulk Ag. Formic acid decomposition is of interest as a source of H2 for fuel cell applications and modification of Pd by Ag reduces poisoning by CO and increases the selectivity for H2 formation. Formic acid reacts below room temperature on the 1-atomic layer Ag film, whereas no reaction occurs on pristine bulk Ag. Notably, 2 monolayer films of Ag again become unreactive towards formic acid, indicating a reversion to bulk behavior. A combination of infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) was used to establish that the Ag monolayer is continuous and electronically modified compared to bulk Ag. The work establishes a demonstration of the altered electronic structure of Ag monolayers on Pd(111) and an associated change in reactivity. The effect on reactivity only persists for the first layer, demonstrating the need for precise control of materials to exploit the modification in electronic properties

Photocatalytic Activity of Mesoporous Graphitic Carbon Nitride (mpg-C3N4) Towards Organic Chromophores Under UV and VIS Light Illumination

A template-assisted synthetic method including the thermal polycondensation of guanidine hydrochloride (GndCl) was utilized to synthesize highly-organized mesoporous graphitic carbon nitride (mpg-C3N4) photocatalysts. Comprehensive structural analysis of the mpg-C3N4 materials were performed by XPS, XRD, FT-IR, BET and solid-state NMR spectroscopy. Photocatalytic performance of the mpg-C3N4 materials was studied for the photodegradation of several dyes under visible and UV light illumination as a function of catalyst loading and the structure of mpg-C3N4 depending on the polycondensation temperature. Among all of the formerly reported performances in the literature (including the ones for Degussa P25 commercial benchmark), currently synthesized mpg-C3N4 photocatalysts exhibit a significantly superior visible light-induced photocatalytic activity towards rhodamine B (RhB) dye. Enhanced catalytic efficiency could be mainly attributed to the terminated polycondensation process, high specific surface area, and mesoporous structure with a wide pore size distribution

Hydrogen migration at restructuring palladium-silver oxide boundaries dramatically enhances reduction rate of silver oxide.

Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 104 in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium-silver alloy. The palladium-silver interface also dynamically restructures during reduction, resulting in silver-palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials