Surface Of Bimetallic Catalysts Research Articles

Dual Lewis Acid-Base Sites Regulate Silver-Copper Bimetallic Oxide Nanowires for Highly Selective Photoreduction of Carbon Dioxide to Methane.

Highly selective photoreduction of CO2 to valuable hydrocarbons is of great importance to achieving a carbon-neutral society. Precisely manipulating the formation of the Metal1 ⋅⋅⋅C=O⋅⋅⋅Metal2 (M1 ⋅⋅⋅C=O⋅⋅⋅M2 ) intermediate on the photocatalyst interface is the most critical step for regulating selectivity, while still a significant challenge. Herein, inspired by the polar electronic structure feature of CO2 molecule, we propose a strategy whereby the Lewis acid-base dual sites confined in a bimetallic catalyst surface are conducive to forming a M1 ⋅⋅⋅C=O⋅⋅⋅M2 intermediate precisely, which can promote selectivity to hydrocarbon formation. Employing the Ag2 Cu2 O3 nanowires with abundant Cu⋅⋅⋅Ag Lewis acid-base dual sites on the preferred exposed {110} surface as a model catalyst, 100 % selectivity toward photoreduction of CO2 into CH4 has been achieved. Subsequent surface-quenching experiments and density functional theory (DFT) calculations verify that the Cu⋅⋅⋅Ag Lewis acid-base dual sites do play a vital role in regulating the M1 ⋅⋅⋅C=O⋅⋅⋅M2 intermediate formation that is considered to be prone to convert CO2 into hydrocarbons. This study reports a highly selective CO2 photocatalyst, which was designed on the basis of a newly proposed theory for precise regulation of reaction intermediates. Our findings will stimulate further research on dual-site catalyst design for CO2 reduction to hydrocarbons.

Dual Lewis Acid‐Base Sites Regulate Silver‐Copper Bimetallic Oxide Nanowires for Highly Selective Photoreduction of Carbon Dioxide to Methane

AbstractHighly selective photoreduction of CO2 to valuable hydrocarbons is of great importance to achieving a carbon‐neutral society. Precisely manipulating the formation of the Metal1⋅⋅⋅C=O⋅⋅⋅Metal2 (M1⋅⋅⋅C=O⋅⋅⋅M2) intermediate on the photocatalyst interface is the most critical step for regulating selectivity, while still a significant challenge. Herein, inspired by the polar electronic structure feature of CO2 molecule, we propose a strategy whereby the Lewis acid‐base dual sites confined in a bimetallic catalyst surface are conducive to forming a M1⋅⋅⋅C=O⋅⋅⋅M2 intermediate precisely, which can promote selectivity to hydrocarbon formation. Employing the Ag2Cu2O3 nanowires with abundant Cu⋅⋅⋅Ag Lewis acid‐base dual sites on the preferred exposed {110} surface as a model catalyst, 100 % selectivity toward photoreduction of CO2 into CH4 has been achieved. Subsequent surface‐quenching experiments and density functional theory (DFT) calculations verify that the Cu⋅⋅⋅Ag Lewis acid‐base dual sites do play a vital role in regulating the M1⋅⋅⋅C=O⋅⋅⋅M2 intermediate formation that is considered to be prone to convert CO2 into hydrocarbons. This study reports a highly selective CO2 photocatalyst, which was designed on the basis of a newly proposed theory for precise regulation of reaction intermediates. Our findings will stimulate further research on dual‐site catalyst design for CO2 reduction to hydrocarbons.

Chemical and Structural Evolution of AgCu Catalysts in Electrochemical CO2 Reduction

Silver-copper (AgCu) bimetallic catalysts hold great potential for electrochemical carbon dioxide reduction reaction (CO2RR), which is a promising way to realize the goal of carbon neutrality. Although a wide variety of AgCu catalysts have been developed so far, it is relatively less explored how these AgCu catalysts evolve during CO2RR. The absence of insights into their stability makes the dynamic catalytic sites elusive and hampers the design of AgCu catalysts in a rational manner. Here, we synthesized intermixed and phase-separated AgCu nanoparticles on carbon paper electrodes and investigated their evolution behavior in CO2RR. Our time-sequential electron microscopy and elemental mapping studies show that Cu possesses high mobility in AgCu under CO2RR conditions, which can leach out from the catalysts by migrating to the bimetallic catalyst surface, detaching from the catalysts, and agglomerating as new particles. Besides, Ag and Cu manifest a trend to phase-separate into Cu-rich and Ag-rich grains, regardless of the starting catalyst structure. The composition of the Cu-rich and Ag-rich grains diverges during the reaction and eventually approaches thermodynamic values, i.e., Ag0.88Cu0.12 and Ag0.05Cu0.95. The separation between Ag and Cu has been observed in the bulk and on the surface of the catalysts, highlighting the importance of AgCu phase boundaries for CO2RR. In addition, an operando high-energy-resolution X-ray absorption spectroscopy study confirms the metallic state of Cu in AgCu as the catalytically active sites during CO2RR. Taken together, this work provides a comprehensive understanding of the chemical and structural evolution behavior of AgCu catalysts in CO2RR.

The catalytic hydrogenation of furfural to 2-methylfuran over the Mg-Al oxides supported Co-Ni bimetallic catalysts

In this paper, an efficient nickel-cobalt bimetallic catalyst was designed and developed for catalytic hydrogenation of furfural, one of the biomass platform products, to 2-methylfuran, a high value-added chemicals. The NiCo-MgAlO catalysts were synthesized by the co-precipitation method and characterized by H2-TPR, XRD, N2 adsorption-desorption analysis, O2-TPO, FAL-TPD, NH3-TPD, TEM and other methods. These results revealed that the Ni/Co ratio, solvent, reaction temperature, pressure, time, and, most importantly, the catalyst reduction conditions had a major impact on this reaction. 100% of furfural conversion and 92.3% of 2-MF selectivity were achieved over the Ni10Co5-MgAlO-3h catalyst with its Ni/Co molar ratio of 2 when employed 2-butanol was the solvent. It could maintain high activity even after using it several times. The bimetallic Ni10Co5-MgAlO catalyst owned homogeneous composition of Ni-Co alloy nanoparticles and more middle-acid sites than the monometallic Ni10-MgAlO catalyst, which might promote C-O bond cleavage and improve 2-MF selectivity due to its oxygen affinity. The high yield of 2-methylfuran from furfural may be due to the synergistic effects of the metal sites and the acid sites on the Ni-Co bimetallic catalyst's surface.

Single Ni-inserted Cu (1 1 1) surface: A DFT study of adsorption and reaction mechanisms of methanol steam reforming

Modeling of a bimetallic catalyst surface and DFT calculations of its adsorption capacity, as well as methanol steam reforming (MSR) pathways, are of great theoretical achievements in the search for efficient hydrogen-production catalysts. Herein, the single Ni-embedded model (NiCu(111)) and single Ni-adsorbed model (A-NiCu(111)) are built, and DFT calculations are carried out to determine different adsorption capabilities. The results show that adding Ni atoms can improve adsorption performance significantly, which is beneficial to the interaction between reactant molecules and catalyst surfaces. The adsorption energies on the A-NiCu(111) surface are much greater than those on the NiCu(111) surface. Further, some key elementary reactions of MSR are carried out to evaluate the catalytic performances and carbon deposition effects of two surfaces. The stepwise dehydrogenation of methanol molecules, the decomposition of water molecules, and other reactions of some intermediates are analyzed in detail. On the A-NiCu(111) surface, the activation energy barriers of stepwise dehydrogenation of methanol molecules and water decomposition are reduced. The NiCu(111) surface shows advantages in inhibiting the surface carbon deposition. This can provide theoretical ideas for the design of high-efficiency Cu-Ni alloy catalysts in the future.

Engineering Heterometallic Bonding in Bimetallic Electrocatalysts Towards Optimized Hydrogen Oxidation and Evolution Reactions

Tuning the type and degree of heterometallic bonding in bimetallic catalysts is crucial to achieving optimal catalytic performance. In this study density functional theory (DFT) and X-ray absorption spectroscopy (XAS) were used to investigate the use of Ru-based bimetallic electrocatalysts for hydrogen oxidation/evolution reactions (HOR/HER) under alkaline conditions. The catalyst's hydrogen absorption and OH adsorption energies can both be tuned by altering the heterometallic bonding in bimetallic systems. DFT calculations were used in conjunction with a RuNi bimetallic system to optimize the hydrogen adsorption energy and weaken the OH adsorption energy. The degree of heterometallic bonding (i.e. the alloying extent) in the Ru2.3Ni1/C catalyst was experimentally confirmed by XAS analysis. Our findings indicate that tuning the hydrogen and OH adsorption energies on the RuNi bimetallic catalyst's surface leads to superior activity and stability in the alkaline HOR/HER compared to as-prepared Ru/C, commercial Pt/C and the reported catalysts. We showed that the oxophilic effect can be manipulated by altering heterometallic bonding and that both the hydrogen and OH adsorption energies are important determinants of the efficiency of low-cost HER/HOR catalysts in alkaline media.

Exploring the Reaction Mechanisms of Furfural Hydrodeoxygenation on a CuNiCu(111) Bimetallic Catalyst Surface from Computation.

In this study, the selectively catalytic hydrodeoxygenation of furfural (F–CHO) to 2-methylfuran (F–CH3) on the CuNiCu(111) bimetallic catalyst surface was systematically investigated based on the periodic density functional theory, including dispersion correction. The formation of furfuryl alcohol (F–CH2OH) involved two steps: the preferred first step was the hydrogenation of the branched C atom, forming the alkoxyl intermediate (F–CHO + H = F–CH2O), and the second step was H addition to the alkoxyl group, resulting in furfuryl alcohol (F–CH2O + H = F–CH2OH), which was the rate-controlling step. In contrast, in the formation of 2-methylfuran, the first step was the dehydroxylation of furfuryl alcohol, resulting in alkyl (F–CH2) and OH (F–CH2OH = F–CH2 + OH) groups, the second step was the hydrogenation of F–CH2 (F–CH2 + OH + H = F–CH3 + OH), and the rate-controlling step was the hydrogenation of OH to H2O (OH + H = H2O). Based on the comparison results of the NiCuCu(111), Cu(111), and CuNiCu(111) surfaces, it was concluded that the catalytic performance of the catalyst was closely related to the adsorption structure of furfural. These results provide a basis for studying the intrinsic activity of NiCu catalysts during the hydrodeoxygenation of refined oxygenated compounds involving biomass-derived oils.

Probing the Local Generation and Diffusion of Active Oxygen Species on a Pd/Au Bimetallic Surface by Tip-Enhanced Raman Spectroscopy.

Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.

Methanol oxidation on AuAg-Zn/MCM-36 – The effect of catalyst components and pretreatment

• ZnO loaded on MCM-36 played a role of structural and electronic promoter. • Alloy was identified after thermal treatment of AuAg-zeolites in Ar and H 2 flows. • Zinc favored the deeper alloying of noble metals in hydrogen reduction conditions. • Zinc on MCM-36 directed the selectivity of Au- and AuAg-catalysts to formaldehyde. • Alloy rich in silver favored the total oxidation of methanol. MCM-36 and Zn/MCM-36 layered zeolites were used as supports for gold and silver. The calcined samples prepared were treated in inert gas or hydrogen flow at 673 K. The effect of gold, silver, zinc and Au-Ag alloy on the surface properties, activity and selectivity of the catalysts in methanol oxidation was studied. The catalysts were fully characterized by XRD, TEM, UV–vis and XPS techniques. The surface acidity of catalysts was measured by pyridine adsorption combined with FTIR studies. The results showed that metallic gold dominated on the surface of calcined catalysts and the amount of negative charge on metal particles was dependent on the chemical composition of the matrix and the active phase. Au-Ag alloy was identified on the surface of bimetallic catalysts after their treatment in argon and hydrogen flow. It was proved that zinc on the surface of MCM-36 promoted the deep alloying of noble metals in strong reducing (hydrogen flow) conditions with the formation of gold-silver alloy rich in silver. The species on the surface of the catalysts, dependent on the activation treatments, determined their activity and selectivity in methanol oxidation. The presence of zinc in the support (Zn/MCM-36) significantly improved the activity of Au-Zn/MCM-36 due to the effect of zinc on the electronic and structural properties of gold. In bimetallic AuAg-samples, the influence of gold-silver alloy on the increase in their activity was dependent on the content of silver in the alloy. Zinc on MCM-36 support increased the selectivity to formaldehyde, whereas the alloy rich in silver favoured the total oxidation of methanol because of the segregation of alloy during the reaction.

Binding maps for the study and prediction of bimetallic catalyst surface reactions: The case of methanol oxidation

AbstractFocusing on the competing pathways of methanol oxidation on platinum and platinum/gold bimetallic catalysts, we explore a novel density functional theory (DFT)‐based approach to the study of reactions on catalyst surfaces. Traditionally, DFT has been used to compute binding energies of products and intermediates as proxies for catalytic activity, and to compute full reaction pathways and their activation energy barriers. Merging the computational simplicity and intuitive clarity of binding energy calculations with the site sensitivity of transition state calculations, we construct maps of the binding energies of relevant atoms and molecules at all sites on a surface. We show that knowledge of the arrangement of strong and weak binding sites on a surface is powerful in rationalizing the ease with which a reaction step proceeds on a given local motif of surface atoms. We highlight the prospects and challenges of this approach toward catalyst screening and prediction.

Synergistic Promotion of NO-CO Reaction Cycle by Gold and Nickel Elucidated using a Well-Defined Model Bimetallic Catalyst Surface

A bimetallic catalyst of Au and Ni significantly increased the catalytic activity of the NO-CO reaction in comparison to monometallic Au and Ni catalysts. Unraveling the roles of Au and Ni atoms in the each of the NO-CO reaction steps occurring on the Au-Ni catalyst surface is crucial to reveal the origin of the increased activity. For this purpose, a well-defined Au/Ni(111) model catalyst was prepared, on which CO and NO adsorption, their coadsorption, NO dissociation, CO2 formation, and N2 formation were investigated using infrared reflection absorption spectroscopy, temperature-programmed desorption/reaction, and density functional theory calculations. In the reaction process, the catalyst surface would be dominantly covered by N and O atoms, which would be removed from the surface by N2 formation and CO2 formation. O atoms preferentially occupy the Ni hollow sites by segregating N atoms to the adsorption sites made up of Au and Ni atoms. Thus, only the N2 formation step was affected by the Au atoms. T...

Nitrate reduction on the surface of bimetallic catalysts supported by nano-crystalline beta-zeolite (NBeta)

A nano-crystalline beta zeolite supported bimetallic catalyst was developed and evaluated for the complete and selective reduction of nitrate to environmentally friendly nitrogen.

Synergistic effects of bimetallic Cu-Fe/SiO2 nanocatalysts in selective hydrogenation of diethyl malonate to 1,3-propanediol

CuxFey/SiO2 catalysts were prepared using urea-assisted sol−gel method. The structure and physicochemical properties of the catalysts were characterized using N2 adsorption–desorption, transmission electron microscopy, H2-temperature-programmed reduction, powder X-ray diffraction, and X-ray photoelectron spectroscopy. Compared with monometallic Cu or Fe catalysts, the bimetallic CuxFey/SiO2 catalysts exhibited enhanced catalytic performance for the selective hydrogenation of diethyl malonate to 1,3-propanediol. The bimetallic catalyst with an optimal Cu/Fe atomic ratio of 2 exhibited the highest activity, which yielded 96.3% conversion to diethyl malonate and 93.3% selectivity to 1,3-propanediol under the optimal reaction conditions. Characterization results revealed that interactions between Cu and Fe contributed to the improvement of diethyl malonate conversion and selectivity to 1,3-propanediol. The X-ray photoelectron spectroscopy results revealed that the addition of appropriate amount of Fe species enhanced the reduction of Cu2+ species, thereby increasing the Cu0 species on the surface of bimetallic catalyst. It led to a better chemisorption capacity of hydrogen and further promoted of the activation of hydrogen molecule. The ethyl acetate temperature-programmed desorption results indicated that the FeOx species provided the additional adsorption sites for substrate molecules, and they activated the CO bond. The improved catalytic performance of bimetallic CuxFey/SiO2 catalyst was mainly attributed to the synergistic effect between Cu0 and FeOx species.

The Effect of Sintering-Redispersion on the Selective Aromatic Yield on Supported Platinum Catalysts

The dispersion of supported platinum-containing catalysts was determined by the pulse chemisorption technique. Two catalyst loadings for the monometallic catalyst and a bimetallic catalyst were investigated: 0.3%Pt/Al 2 O 3 , 0.6%Pt/Al 2 O 3 and 0.3%Pt–0.3%Re/Al 2 O 3 . Results confirm the usually observed trend that dispersions after oxidation are higher than those after reduction. The redispersion profiles peaked at a sintering temperature of 550°C for all catalysts studied. However, the bimetallic catalyst was found to redisperse at all treatment temperatures while the monometallic catalysts sinter and redisperse at various treatment temperatures. The atomic migration model was proposed as a suitable model for the prediction of the particle size distribution (PSD) as it lends itself easily to the prediction of sintering and redispersion phenomena. The computer simulation for the catalyst resulted in the calculation of dispersion generated PSD, for the oxygen sintered catalyst and the reduced catalyst after sintering. The general PSD profiles for both sintering situations show the same pattern of shift to larger particle sizes after specific temperatures. At a temperature transition of 500–550°C, the PSD shifted to lower particle sizes for the 0.3%Pt/Al 2 O 3 and the 0.6%Pt/Al 2 O 3 catalysts. Above 550°C, the PSD shifted to higher particle sizes with larger spreads. On the bimetallic catalyst, the shift of the PSD to the left occurred at two temperature transitions: one at 500–550°C and the other at 750–800°C. The selective aromatic yield was studied on a catalyst sintered at various temperatures and reduced at 500°C. At 500°C, toluene was the sole product of the reforming reaction of n-heptane on 0.3 %Pt/ Al 2 O 3 catalyst; on the bimetallic catalyst surface, toluene as the sole product of reaction occurred a sintering temperature of 800°C. Although the PSD for the sintered PSD on 0.6%Pt/Al 2 O 3 catalyst was quite similar to that of the 0.3%Pt/Al 2 O 3 catalyst, no aromatization products were formed on this catalyst. Aromatization selectivity was attributed to three-atom ensembles of triangular symmetry of small Pt aggregates (Biloen et al ., 1980; Paal, 1980, 2003). If this reaction occurs, it can indicate the presence of facets with (111) symmetry produced during sintering and redispersion.

Interaction between Pd and Ag on the surface of silica

Palladium, silver and palladium–silver catalysts supported on silica were prepared by coimpregnation of support with solution of AgNO 3 and Pd(NO 3) 2. The catalysts were characterized by X-ray powder diffraction (XRD), temperature programmed reduction (TPR), time of flight ion mass spectrometry (ToF-SIMS), chemisorption of carbon monoxide and were tested in the reaction of selective oxidation of glucose to gluconic acid. XRD and TPR studies have shown that an interaction between Pd and Ag on the surface of silica after oxidation at 500 °C and reduction at 260 °C leads to the formation of solid solutions. ToF-SIMS images of the surface of 5% Ag/SiO 2 catalyst after oxidation at 500 °C and reduction at 260 °C show that Ag atoms supported on silica are not distributed homogenously but tend to form regions of enhanced Ag concentration. Positive ions images of the surface of 5% Pd/SiO 2 catalyst also display regions of enhanced concentration of Pd atoms, but they are more homogenously distributed on silica. ToF-SIMS peak intensity ratio 108Pd +/ 107Ag + for bimetallic 5% Pd–5% Ag/SiO 2 catalysts has a lower value than that obtained for physical mixture 5% Pd/SiO 2–5% Ag/SiO 2 which indicates that the surface of bimetallic catalyst is enriched with silver atoms.

Reforming of methane with carbon dioxide over supported bimetallic catalysts containing Ni and noble metal: I. Characterization and activity of SiO 2 supported Ni–Rh catalysts

The activity and stability of silica supported monometallic Ni, Rh and bimetallic Ni–Rh catalysts have been studied towards the carbon dioxide reforming of methane. The catalysts were prepared by incipient wetness impregnation with different contents of Rh and Ni and they were characterized by H 2 chemisorption, TP R H 2 , XRD and FT-IR methods. SiO 2 supported monometallic Ni, Rh and bimetallic Ni–Rh catalysts are comparably good catalysts for carbon dioxide reforming of methane and Rh-rich catalysts are resistant to deactivation and carbon formation. Temperature-programmed hydrogen assisted decomposition of bimetallic Ni–Rh/SiO 2 catalyst precursors leads to formation of Ni–Rh alloys. Segregation of metals leads to the formation of Ni-rich surface alloy. TG-DTA-MS, TPS R H 2 , TPO, TPS R C O 2 , TOC and SEM methods were used in order to characterize the carbonaceous deposits. At least two types of carbon deposit with different reactivity have been detected on the surface of bimetallic catalysts. The results suggest a significant metal–metal interaction in the bimetallic systems.