PdCu Surfaces Research Articles

DFT Insights into NO Electrochemical Reduction: A Case Study of Pt(211) and Cu(211) Surfaces

Electrochemical reduction of nitrate and nitrite represents a promising technology for wastewater treatment, in which NO reduction largely determines the reaction selectivity. While tremendous efforts have been made, there is a lack of fundamental understanding regarding the selectivity toward value-added products (e.g., NH4+ and NH3OH+) through NO* protonation and toward water purification (e.g., N2O or N2 as the product) through NO*–NO* dimer formations. In this contribution, we employed periodic density functional theory (DFT) calculations to investigate NO electrochemical reduction mechanisms on Pt(211) and Cu(211). We found that both NO* protonation and cis-(NO–NO)* formation are possible on Cu with weak NO bonding, while only NO* protonation is favorable on Pt with strong NO binding. The selective formation of cis-(NO–NO)* on Cu rather than Pt shows that NO* binding has a crucial role in determining the formation of cis-(NO–NO)*. The correlation between the free energy of the cis-(NO–NO)* formation and NO* binding energy, on the Cu as well as on the Ag, Au, and PdCu surfaces at various coverages, shows that the cis-(NO–NO)* formation becomes more favorable with the weakening of NO adsorption and vice versa. Thus, our results indicate that the selectivity of complex NO electrochemical reduction can be described by the simple NO binding energy. This trend-based analysis may ultimately lead to the design of electrocatalysts with the desired performance.

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.

Adsorption, diffusion, and permeation of hydrogen at PdCu surfaces

Ab initio calculations have been utilized to comparatively study the adsorption, dissociation, diffusion, and permeation of hydrogen at FCC (111) and BCC (110) surfaces of Pd50Cu50. It is found that the adsorption of H2 on BCC (110) surface seems thermodynamically more stable with more negative adsorption energy than that on FCC (111) surface, while the dissociation energy of H2 on BCC (110) surface is much bigger. Furthermore, the BCC (110) or FCC (111) surface of Pd50Cu50 possesses much higher hydrogen permeability and lower hydrogen diffusivity than its corresponding Pd50Cu50 bulk. In addition, the BCC (110) surface of Pd50Cu50 should be preferred in actual applications as a result of its excellent hydrogen permeability. The present findings are in nice agreement with experimental results in the literature, and could deepen the comprehension of hydrogen permeation through PdCu membranes.

Probing the effect of Pd coverage towards NH3 decomposition on Cu(1 0 0) surface

Ammonia (NH3) has been considered as a promising alternative for hydrogen storage. In this work, the decomposition mechanism of ammonia was studied on various Pd-doped Cu(1 0 0) surfaces by DFT calculations. It was found that the coverage of Pd atoms on Cu(1 0 0) surface made an evident influence on the interaction between the NHx and substrate. The doped Pd atoms significantly promoted NH3 decomposition kinetically and thermodynamically compared to Cu(1 0 0), confirming the synergistic effect between Pd and Cu. The catalytic activity and selectivity of various PdCu(1 0 0) surfaces for NH3 decomposition were dependent upon the coverage of Pd atoms to some extent.

Origin of Pd-Cu bimetallic effect for synergetic promotion of methanol formation from CO2 hydrogenation

A strong synergetic effect was observed in our previous work on Pd-Cu bimetallic catalysts for CH3OH formation from CO2 hydrogenation when the Pd/(Pd + Cu) atomic ratio lied within 0.25–0.34. In the present study, the importance of Pd-Cu alloy in selective CH3OH promotion was evidenced and correlated with alloy contents quantitatively through X-ray diffraction (XRD), scanning transmission electron spectroscopy with energy-dispersive X-ray spectroscopy (STEM/EDS), and H2-O2 titration and N2O titration. The surface chemical properties of Pd-Cu combinations were characterized by H2-/CO2-temperature-programmed desorption (TPD), diffuse reflectance infrared FT spectroscopy (DRIFTS), and density functional theory (DFT), and experimentally evaluated along with monometallic counterparts. Detailed characterization results reveal a unique shift in adsorption towards weakly-bonded H2 and CO2 on Pd-Cu bimetallic surface which appear to correlate to the CH3OH promotion. DFT calculations on adsorption properties of H2 and CO2 show good agreement with the observation from TPD experiments. DFT study also provides insights into the impact of Pd-Cu combination on the activation and initial hydrogenation of CO2 to formate (HCOO∗) and hydrocarboxyl (COOH∗) intermediates. HCOO∗ formation was found to be kinetically more favored than COOH∗ on monometallic Cu and Pd-Cu surfaces. The lowest barrier for HCOO∗ formation was observed at Pd/(Pd + Cu) atomic ratio of 0.33, around which a good CO2 conversion and high methanol selectivity were achieved experimentally.

Mechanistic Study of Pd–Cu Bimetallic Catalysts for Methanol Synthesis from CO2 Hydrogenation

Density functional theory (DFT) calculations were carried out to explore the adsorptions of reactive species and the reaction mechanisms on Pd–Cu bimetallic catalysts during CO2 hydrogenation to methanol. All the possible preferred adsorption sites, geometries, and adsorption energies of the relative intermediates on pure Cu(111) and three PdCu(111) surfaces were determined, revealing that both the adsorption configuration and corresponding adsorption energy are changed by doping with Pd atoms. The strengthened COOH* adsorption and the greatly weakened OH* adsorption change the rate-limiting step from CO2 hydrogenation forming trans-COOH* on Cu(111), Pd3Cu6(111), and Pd6Cu3(111) surfaces to cis-COOH* decomposition forming CO* and OH* on Pd ML surface. Additionally, the highest activation barriers for the overall reaction pathway are reduced in the following trend: Cu(111) > Pd6Cu3(111) > Pd3Cu6(111) > Pd ML (monolayer). Compared to the reaction on clean Cu(111) surface, the complete reaction pathways for ...

Effects of Adsorbates (CO, COH, and HCO) on the Arrangement of Pd Atoms in PdCu(111)

We investigated the arrangement of Pd atoms in PdCu(111) when CO, COH, and HCO are introduced as adsorbates, by performing density functional theory (DFT) based calculations. We modeled several Pd alloyed Cu(111) surfaces, i.e., PdCu(111), by substituting small numbers of Cu atoms with Pd atoms in the topmost and subsurface layers of Cu(111). The arrangement of Pd atoms in the presence of adsorbates is evaluated by comparing the energy profiles of adsorbate–PdCu configurations with aggregated and nonaggregated surface and subsurface Pd atoms. In clean PdCu(111) surfaces, the Pd atoms prefer the nonaggregated arrangement. In the presence of the adsorbed molecules, however, we found that the Pd atoms will favor the aggregated configuration. CO and HCO adsorption structures are determined by the coordination of Pd atoms in the topmost layer. Their adsorption energies do not depend on the number of Pd atoms in the topmost layer alone but are also influenced by subsurface Pd atoms. On the other hand, COH is al...

The Kinetics of H2–D2 Exchange over Pd, Cu, and PdCu Surfaces

PdCu alloy membranes are promising candidates for hydrogen separation from H2S-containing gas mixtures. Dissociative H2 adsorption and associative desorption are important steps in the process of hydrogen transport across PdCu membranes; however, the kinetics of H2 adsorption and desorption on PdCu surfaces are not well understood. In this work, the kinetics and energetics of H2 adsorption and desorption on Pd, Cu, and PdCu surfaces are investigated by microkinetic analysis of H2–D2 exchange (H2 + D2 → 2HD) over fixed beds of Pd, Cu, Pd70Cu30, and Pd47Cu53 foils at near-ambient pressure and temperatures in the range 300–900 K. The rate of H2–D2 exchange over Cu, which is the least active H2–D2 exchange catalyst used in this study, is limited by the rate of H2 adsorption due to the large activation barrier to dissociative H2 adsorption on Cu (ΔEads⧧ = 0.54 ± 0.06 eV). The Cu content of the PdCu alloys and the crystal structures of the Pd-hydride phases (α- and β-PdH) and of Pd47Cu53 (body-centered cubic an...

Interaction of CO and NO with PdCu(111) Surfaces

A theoretical study of the structural parameters, interaction energies, and bonding mechanism of CO and NO to a Pd center located in two copper-rich bimetallic PdCu(111) surfaces and several coordination positions of the Pd(111) surface is reported. For CO, the bonding nature is predominantly covalent, and the analysis of bonding nature variations through the series is used to interpret the experimentally observed decrease of the CO/PdCu interaction energy with the increase in copper percentage of the alloy, the insensitivity of the C−O stretch frequency to the composition of the central Pd environment, and the linear correlation observed between the CO desorption energy and the X-ray photoelectron spectroscopy (XPS) core-level shift. For NO, the nature of the interaction varies from nearly covalent for pure Pd to a mixture of covalent plus ionic for copper-rich alloys; this parallels the expected growth of the ionic component of the bond with the decrease of the work function in going from Pd to Cu. A co...

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd 0.75Cu 0.25(111) single crystal surface with surface composition of about Pd 0.7Cu 0.3. The main objective was to make a comparison with the previously studied Pd 0.67Ag 0.33(111) (surface composition Pd 0.1Ag 0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10 −9 mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Effect of alloying on the adsorption of CO on palladium; A comparison of the behaviour pf PdAg(111), PdCu(111) and Pd(111) surfaces

Effect of alloying on the adsorption of CO on palladium; A comparison of the behaviour pf PdAg(111), PdCu(111) and Pd(111) surfaces