PdCu Surface Research Articles

Formation mechanism of CO2 in the production of allyl acetate from propylene on PdCu(111) surface: A DFT study

Allyl acetate is an important organic chemical raw material and chemical intermediate. The gas-phase synthesis of allyl acetate from propylene is currently the most widely used process. So far, no researchers have reported the formation mechanism of CO2, the by-product that accounts for the most in the reaction process, which hinders the optimization of the catalyst and process. In this study, the combination of Density Functional Theory (DFT) and Kinetic Monte Carlo (kMC) was used to systematically study the formation mechanism of CO2 from propylene to allyl acetate over PdCu(111). The possible formation reaction network of CO2 in the reaction process was constructed. The calculation results showed that species are more easily adsorbed near the surface Pd atoms, and the adsorption sites of Pd-Cu bridge sites and fcc sites on PdCu(111) surface are more active. From the perspective of energy barrier, the acetic acid path tends to generate CO2 through chain scission and then dehydrogenation, while the propylene path produces CO2 through first dehydrogenation and then oxidative chain scission. Considering comprehensively, the two most likely paths to generate CO2 from propylene and acetic acid on PdCu(111) are:① CH3COOH→CH3COO→CH3(+CO2)→CH2→CH→C→CO→CO2,②CH3CHCH2→CH2CHCH2→CH2CHCH→CH2CHC→CHCHC→CHCC→CHCCO→CHC(+CO)→CHCO→CO(+CH) →CO2.

Experimental and in situ DRIFTs studies on confined metallic copper stabilized Pd species for enhanced CO2 reduction to formate

Understanding the basic principle of Pd-based bimetallic catalysts design is essential in CO2 hydrogenation to formate. In this work, Cu partially replaced the bivalence position in MgAl hydrotalcite structure was developed for achieving the highly confined Cu species, whose interaction with Pd species was systematically studied and compared with the traditional co-impregnation or stepwise impregnation method. The stable and maximum formate formation rate of 12.8 mmol/h/gmetal was obtained on Pd0.4 @CuMgAlOx catalyst under 100 °C, 4.0 MPa, H2/CO2 ratio of 3, being about double of the sum of formate production rate over the monometallic Cu and Pd catalysts. Strong synergetic interaction between Pd and the confined Cu species was observed with Cu/Pd atomic ratios of 14–181. Extensive characterization using TEM, XPS, XANES, coupled with kinetic study and in situ DRIFTs analysis revealed that the superior performance of Pd0.4 @CuMgAlOx was attributed to the confined PdCu nanoparticles with electron-rich PdCu surface.

Surface segregation of PdM (M=Cu, Ag, Au) alloys and its implication to acetylene hydrogenation, DFT-based Monte Carlo simulations

The surface composition and atomic arrangement of heterogeneous catalysts determine the surface chemistry and are necessary information for understanding surface catalytic reactions. Compared with monometallic catalysts, alloy catalysts are more complicated because of possible surface segregation or adsorption-induced segregation which may change under different reaction atmosphere. Construction of reasonable slab models to study reactions on alloy surfaces is of importance. Here we proposed a density functional theory based Monte Carlo method (DFT-MC) and used it to investigate the surface composition and atomic arrangement of the (111) surfaces of 1:1 PdAg, PdAu and PdCu alloys in vacuum and under the acetylene partial hydrogenation conditions modeled by co-adsorption of H and acetylene and of H and ethylene. Our DFT-MC simulations indicate that Au and Ag segregate on the PdAu(111) and PdAg(111) surfaces while Pd is enriched on the PdCu(111) surface under vacuum conditions. The surface Pd contents of PdAg and PdAu tend to increase under partial hydrogenation conditions. Analyses reveal that the co-adsorption energies on the ideal (bulk-cut) surfaces correlate linearly with the values on the segregated surfaces and there is also a linear relationship for the d-band centers between the ideal and segregated surfaces. The present study provides a way to construct more realistic models for studying reactions on alloy surfaces.

CH4 dissociation on the Pd/Cu(111) surface alloy: A DFT study

AbstractThe periodic four-layered model of the pure Cu(111) surface has been considered, and the effect of doping with palladium on CH4dissociation has been investigated. The most stable adsorption geometries of CHxspecies (x= 1–4) and H atom on the PdCu(111) and pure Cu(111) surfaces have been obtained. Their computed adsorption energy results on the pure Cu(111) surface have been compared with the previously reported studies. Then, transition state geometries of CH4dehydrogenation steps on both surfaces were calculated by the climbing image nudged elastic band method. Finally, the relative energy diagram for CH4complete dehydrogenation has been represented. The results show that the PdCu(111) surface is more favorable than the Cu(111) surface in terms of the activation energies. The addition of Pd atoms to the Cu(111) surface significantly improves the catalytic activity. This knowledge can enable an efficient catalyst design at a lower cost using different strategies.

Boosting Oxygen and Peroxide Reduction Reactions on PdCu Intermetallic Cubes

AbstractPalladium‐based nanocatalysts have the potential to replace platinum‐based catalysts for fuel‐cell reactions in alkaline electrolytes, especially PdCu intermetallic nanoparticles with high electrochemical activity and stability. However, unlike the synthetic methods for obtaining the nanoparticles, the effect of PdCu shape on the performance is relatively less well studied. Here, we demonstrate the facet dependence of PdCu intermetallics on the oxygen reduction reaction (ORR) and peroxide reduction, and reveal that the {100} dominant PdCu cubes have a much higher ORR mass activity and specific activity than spheres at 0.9 V vs. RHE, which is four and five times that of commercial Pd/C and Pt/C catalysts, respectively, and show only a 31.7 % decay after 30 000 cycles in the stability test. Moreover, cubic PdCu nanoparticles show higher peroxide electroreduction activity than Pd cubes and PdCu spheres. Density functional theory (DFT) calculation reveals that the huge difference originates from the reduction in oxygen adsorption energy and energy barrier of peroxide decomposition on the ordered {100} PdCu surface. Given the relationship between the shape and electrochemical performance, this study will contribute to further research on electrocatalytic improvements of catalysts in alkaline environments.

Theoretical investigation on the effect of doped Pd on the Cu(1 1 1) surface for formic acid oxidation: Competing formation of CO2 and CO

DFT calculations are employed to study the formic acid oxidation mechanism on the Pd-Cu(1 1 1) surface and explore the effect of the doped-Pd atoms on the catalytic selectivity. Compared with that on Cu(1 1 1), it is indicated that the doped-Pd atoms alter the adsorption intensity of intermediates with the d-band center change of the bimetallic surface. About formic acid oxidation, the C-H bond cleavage in HCOOH is prior to the H-O and C-O bond cleavage. The competing formation of CO2 and CO is identified on the Pd-Cu(1 1 1) surface. The doped Pd atoms change the reaction pathways and enhance the selectivity of CO2.

Hydrogen flux of BCC and FCC PdCuAg membranes

First-principles calculations have been used to study the effects of Ag addition on adsorption and dissociation of H2 on BCC and FCC PdCu surfaces as well as hydrogen diffusion and recombinative hydrogen desorption through the PdCu membranes. It is found that the Ag addition makes it energetically difficult for the adsorption of H2 on PdCu surfaces and hydrogen diffusion through PdCu, while could help the recombinative desorption of H atoms from both BCC (110) and FCC (111) surfaces of PdCu. Moreover, substitution of Ag for Pd or Cu would impede or improve the dissociation of H2 on PdCu surface. Calculations also reveal that the overall hydrogen flux of BCC Pd8Cu8 (Pd8Cu7Ag) membranes is determined by the recombinative desorption and diffusion when the membrane thickness is smaller and bigger than 10.31 (4.73) μm, respectively. In addition, hydrogen diffusion is the dominant step of hydrogen permeation of FCC PdCu and PdCuAg as well as BCC Pd7Cu8Ag membranes. The present results not only agree well with experimental observations in the literature, but also deepen the understanding of the effect of Ag alloying on hydrogen permeation through PdCu membranes.

The cost-effective Cu-based catalysts for the efficient removal of acetylene from ethylene: The effects of Cu valence state, surface structure and surface alloying on the selectivity and activity

Cost-effective Cu-based catalysts with high activity and selectivity towards C2H4 formation are vital for the efficient removal of C2H2 from C2H4 in industry. A comprehensive understanding about the effects of Cu-based catalyst properties, including the valence state, surface structure and surface alloying, on the selectivity and activity towards C2H4 formation is essential for developing any novel Cu-based catalysts. In this work, C2H2 hydrogenation to C2H4 over various Cu-based catalysts, including Cu(0), Cu(I), Cu(II), and alloyed MCu(M = Ni, Pd, Pt, Au), were systematically investigated. The results indicated that the activity and selectivity of Cu catalysts with different oxidation states towards C2H4 formation are very different. Cu(I) led to the highest selectivity, while Cu(0) led to the highest activity. Moreover, the catalytic performances of Cu(I) and Cu(0) catalysts are closely related to their surface structures, especially their defective and stable (111) surfaces. For the most stable Cu(0) catalyst in reducing atmosphere, the activity and selectivity towards C2H4 formation on M-doped Cu(111) (M = Ni, Pd, Pt, Au) surface are affected by the surface d-band center, PdCu(111) surface with d-band center at the middle site has the best activity and selectivity towards C2H4 formation compared to the individual Cu(111) and Pd(111) surfaces. Thus, the activity of Cu-based catalysts and the selectivity of C2H2 hydrogenation towards C2H4 formation are determined by the valence state, surface structure, and surface d-band center of Cu catalysts. The catalytic activity and selectivity of all active surfaces can be predicted by analyzing the catalyst surface characteristics. The results are considerably beneficial to the development of new generation of Cu-based catalysts in the selective hydrogenation of C2H2.

Mechanistic Understanding of Alloy Effect and Water Promotion for Pd-Cu Bimetallic Catalysts in CO2 Hydrogenation to Methanol

Density functional theory (DFT) calculations on Pd-Cu bimetallic catalysts reveal that the stepped PdCu(111) surface with coordinatively unsaturated Pd atoms exposed on the top is superior for CO2 and H2 activation and for CO2 hydrogenation to methanol in comparison to the flat Cu-rich PdCu3(111) surface. The energetically preferred path for CO2 to CH3OH over PdCu(111) proceeds through CO2* → HCOO* → HCOOH* → H2COOH* → CH2O* → CH3O* → CH3OH*. CO formation from CO2 via a reverse water-gas shift (RWGS) proceeds more quickly than CH3OH formation in terms of kinetic calculations, in line with experimental observation. A small amount of water, which is produced in situ from both RWGS and CH3OH formation, can accelerate CO2 conversion to methanol by reducing the kinetic barriers for O–H bond formation steps and enhancing the TOF. Water participation in the reaction alters the rate-limiting step according to the degree of rate control (DRC) analysis. In comparison to CO2, CO hydrogenation to methanol on PdCu(111...

The interaction of CO with PdCu hydrogen separation membranes: An operando infrared spectroscopy study

Abstract Pd and PdCu alloy membranes are promising candidates for separating hydrogen from mixed gas streams due in part to their infinite selectivity to hydrogen separation. However, other gases such as CO can inhibit hydrogen transport across Pd-based membranes. In this work, the mechanism by which CO inhibits hydrogen transport across a 25 μm-thick Pd47Cu53 (mol%) membrane is investigated by operando infrared-reflection absorption spectroscopy (IRAS) in the 373–533 K temperature range. In the absence of hydrogen, CO adsorbs on three different sites on the PdCu surface: (1) bridging between contiguous Pd atoms, (2) on top of isolated Pd atoms surrounded by Cu atoms, and (3) on top of oxidized Cu atoms. CO induces agglomeration of isolated Pd atoms on the PdCu surface, which is driven by the higher stability of CO adsorbed on bridging sites between Pd atoms than on isolated Pd atoms surrounded by Cu atoms. The rate of hydrogen permeation across the PdCu alloy membrane decreases with increasing CO concentration in the feed gas, and the poisoning effect of CO is more severe at lower temperatures. CO inhibits hydrogen transport across the membrane by adsorbing only on Pd sites on the PdCu surface and blocking H2 dissociation on these sites. Due to the weaker interaction of CO with PdCu alloy surfaces than with Pd, the PdCu alloy is more resistant to CO poisoning than pure Pd.

Theoretical study of NH3 decomposition on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces: A comparison with clean Pd (1 1 1) and Cu (1 1 1)

The adsorption and successive dehydrogenation mechanisms of NH3 on Pd-Cu (111) and Cu-Pd (111) surfaces (the Pd atoms substitution of the first and second layers of Cu (111) surfaces) have been systematically investigated by density functional theory (DFT) method with a periodic slab model. All possible adsorption configurations of relevant intermediates on Pd-Cu (111) and Cu-Pd (111) surfaces are identified. It is revealed that the adsorption configurations and corresponding adsorption energies of adsorbates are slightly changed on Pd-Cu (111) and Cu-Pd (111) surfaces. The adsorption energies of NHx(x=0–3) species exhibit the following trend: NH3<NH2<NH<N. Then, the minimum energy path for the complete dehydrogenation of NH3 into adsorbed N and H is identified to explore the dehydrogenation mechanisms on different surfaces. The highest energy barrier and reaction energy on Pd-Cu (111) surface are greatly reduced to 1.56 and 0.99eV, implying that the complete dehydrogenation of NH3 on Pd-Cu (111) surface is favorable both kinetically and thermodynamically, namely, the doped-Pd atoms in the first layer are the reaction active center. Compared to that on clean Pd (111) and Cu (111) surfaces, it is found that the synergistic effect exits in different layers of catalyst surfaces. The calculated results show that the layer-substituted Pd atoms on the surface of Cu catalysts exhibit a better catalytic activity for NH3 dehydrogenation compared to the clean Cu (111) surface.

Analysis of H2S Tolerance of Pd-Cu Alloy Hydrogen Separation Membranes

The presence of a limited amount of H2S in H2-rich feed adversely affects the Pd-Cu membrane permeation performance due to the sulphidization of the membrane surface. A theoretical model was proposed to predict the S-tolerant performance of the Pd-Cu membranes in presence of H2S under the industrial water-gas-shift (WGS) reaction conditions. The ideas of surface coverage and competitive adsorption thermodynamics of H2S and H2 on Pd-Cu surface were introduced in the model. The surface sulphidization of the Pd-Cu membranes mainly depended on the pressure ratio of H2S to H2, temperature and S-adsorbed surface coverage, i.e., the occurrence of sulphidization on the surface was not directly related with the bulk compositions and structures [body centered cubic and face centered cubic (bcc or fcc)] of Pd-Cu alloy membranes because of the surface segregation phenomena. The resulting equilibrium equations for the H2S adsorption/sulphidization reactions were solved to calculate the pressure ratio of H2S to H2 over a wide range of temperatures. A validation of the model was performed through a comparison between lots of literature data and the model calculations over a rather broad range of operating conditions. An extremely good agreement was obtained in the different cases, and thus, the model can serve to guide the development of S-resistant Pd alloy membrane materials for hydrogen separation.

Catalytic and Electrocatalytic Oxidation of Ethanol over Palladium-Based Nanoalloy Catalysts

The control of the nanoscale composition and structure of alloy catalysts plays an important role in heterogeneous catalysis. This paper describes novel findings of an investigation for Pd-based nanoalloy catalysts (PdCo and PdCu) for ethanol oxidation reaction (EOR) in gas phase and alkaline electrolyte. Although the PdCo catalyst exhibits a mass activity similar to Pd, the PdCu catalyst is shown to display a much higher mass activity than Pd for the electrocatalytic EOR in alkaline electrolyte. This finding is consistent with the finding on the surface enrichment of Pd on the alloyed PdCu surface, in contrast to the surface enrichment of Co in the alloyed PdCo surface. The viability of C-C bond cleavage was also probed for the PdCu catalysts in both gas-phase and electrolyte-phase EOR. In the gas-phase reaction, although the catalytic conversion rate for CO2 product is higher over Pd than PdCu, the nanoalloy PdCu catalyst appears to suppress the formation of acetic acid, which is a significant portion of the product in the case of pure Pd catalyst. In the alkaline electrolyte, CO2 was detected from the gas phase above the electrolyte upon acid treatment following the electrolysis, along with traces of aldehyde and acetic acid. An analysis of the electrochemical properties indicates that the oxophilicity of the base metal alloyed with Pd, in addition to the surface enrichment of metals, may have played an important role in the observed difference of the catalytic and electrocatalytic activities. In comparison with Pd alloyed with Co, the results for Pd alloyed with Cu showed a more significant positive shift of the reduction potential of the oxygenated Pd species on the surface. These findings have important implications for further fine-tuning of the Pd nanoalloys in terms of base metal composition toward highly active and selective catalysts for EOR.

H2dissociation on individual Pd atoms deposited on Cu(111)

We present a Molecular Dynamics (MD) study based on Density Functional Theory (DFT) calculations for H(2) interacting with a Pd-Cu(111) surface alloy for low Pd coverages, Θ(Pd). Our results show, in line with recent experimental data, that single isolated Pd atoms evaporated on Cu(111) significantly increase the reactivity of the otherwise inert pure Cu surface. On top of substitutional Pd atoms in the Pd-Cu(111) surface alloy, the activation energy barrier for H(2) dissociation is smaller than the lowest one found on Cu(111) by a factor of two: 0.25 eV vs. 0.46 eV. Also in agreement with experiments, our DFT-MD calculations show that a large fraction of the dissociating H atoms efficiently spillover from Pd (i.e. the active sites), thanks to their extra kinetic energy due to the ~0.50 eV chemisorption exothermicity. Still, our DFT-MD calculations predict a dissociative sticking probability for low energy H(2) molecules that is much smaller than the estimated value from scanning tunneling microscopy experiments. Thus, further theoretical and experimental investigations are required for a complete understanding of H(2) dissociation on low-Θ(Pd) Pd-Cu(111) surface alloys.

Interaction of CO with PdCu surface alloys supported on Ru(0001)

Interaction of CO with PdCu surface alloys supported on Ru(0001)

Interaction of CO with PdCu surface alloys supported on Ru(0 0 0 1)

The adsorption and desorption of CO on monolayer PdCu surface alloys on a Ru(0 0 0 1) substrate and, for comparison, on Pd/Ru(0 0 0 1), and Cu/Ru(0 0 0 1) monolayer films, have been studied by temperature programmed desorption and infrared reflection absorption spectroscopy, aiming at a detailed understanding of the different effects controlling the chemical properties of bimetallic surfaces, geometric ensemble effects, electronic ligand effects and strain induced effects. The results show that all of these effects are present in these surfaces, acting in a cooperative, synergetic way. Strain effects lead to a destabilization of the Pd–CO and a stabilization of the Cu–CO interaction as compared to CO adsorption on the respective bulk substrates. Geometric ensemble effects are imposed by the highly disperse distribution of the Cu and Pd atoms, reducing the number of otherwise favorable Pd 3 sites, and electronic ligand effects lead to an increasing stability of the Pd–CO bond with increasing Cu content.

Theoretical analysis of clock-reconstructed PdCu surface alloy

Ab initio density-functional techniques (DFT's) have been used to investigate the structural properties of PdCu surface alloys for which a clock reconstruction with p4g symmetry has been reported. Our calculations not only allow to discriminate between the various models proposed on the basis of the experimental data, they also elucidate the physical mechanism leading to the clock-rotated p(2 X 2)-p4g surface structure. It is shown that free-standing square monolayers are unstable-not only relative to the most dense hexagonal lattice, but also relative to a square-triangle lattice characteristic of a p4g symmetry. This transformation is even barrier-less or limited by a low energetic barrier. Whether this transformation will also occur in a surface layer depends on the coupling of the adlayer to the substrate-this explains why the reconstruction occurs only in the topmost layer of a bilayer PdCu alloy, but not in a single alloy layer. Our work demonstrates that DFT calculations provide a reliable tool for predicting surface reconstructions.

Trichloroethene Dechlorination Reactions on the PdCu(110) Alloy Surface: A Periodical Density Functional Theory Study of the Mechanism

In the present work the dissociation of the trichloroethene on the PdCu(110) surface has been investigated by applying ab initio periodic density functional theory. The reaction steps, intermediates and transition states of this reaction have been identified. Two different mechanisms have been studied, starting from the most stable adsorption modes of trichloroethene (di-σ structures). Regarding the reaction path, all intermediate steps are exothermic. In addition, all activation energies are relatively small (<40 kJ mol−1). Therefore, this reaction is shown to be kinetically and thermodynamically favorable on this surface. From the intermediate steps of both mechanisms, it was possible to attain additional information about dechlorination reactions, for the other chloroethene molecules (dichloro and chloro). There is good evidence that the intermediates and transition states for the dechlorination reaction of these molecules have a similar configuration on the PdCu surface. A detailed study of the transition states indicated that the activation energy of the dissociation step increases with the rebonding energy of chlorine on the surface, which is a common fragment to all different transition states. This also confirms that these transition state are “early” ones. A simple kinetic model has been constructed with the elementary step rate constants evaluated from the calculations. The reactional system has been shown to be very complex. Most of all intermediates are present on the surface simultaneously, in accordance with experimental studies.

Chemisorption of Trichloroethene on the PdCu Alloy (110) Surface: A Periodical Density Functional Study

In the present work different adsorption modes of trichloroethene on the PdCu alloy have been investigated systematically. With application of ab initio periodic density functional theory, some insights about these adsorption modes have been revealed. The two different (110) terminations of the Cu3Pd alloy have been employed as models for the surface region of the Cu50Pd50 alloy because of Cu segregation. They are based on the regular phase of the face-centered cubic (fcc) structure. The first model shows a mixed (Pd/Cu =1) top layer, whereas the other one is Cu-terminated. The analysis of the position of the center of the d-band projected on the Pd and Cu atoms of both surfaces indicated that the Pd atom in the alloy has similar reactivity to the pure metal surface, whereas the opposite trend has been found for the Cu atoms. This has also been confirmed by the adsorption energy, calculated for the distinct modes. Trichlorethene prefers to interact with the Pd atoms on the mixed PdCu surface. Both di-σ an...

Theoretical Study of the Interaction of Molecular Hydrogen with PdCu(111) Bimetallic Surfaces

A density functional cluster model approach has been applied to the study of the interaction of molecular hydrogen with two different cluster models of the PdCu(111) surface corresponding to a different formal alloy composition both having a single Pd atom in the surface. Despite the similar surface morphology of the two bimetallic clusters, they exhibit a rather different reactivity toward molecular hydrogen. The coordination of the surface Pd atom to other Pd atoms in the second layer appears to be necessary for this atom be able to trap and dissociate molecular hydrogen with a very low energy cost, thus being a potential active site for catalysis. This important result points out that electronic, or ligand, effects do also play an important role in the activity of the PdCu(111) surface sites toward molecular hydrogen.