Interactions of oxygen and hydrogen on Pd(111) surface

Abstract

The coadsorption and interactions of oxygen and hydrogen on Pd(111) was studied by scanning tunneling microscopy and density functional theory calculations. In the absence of hydrogen oxygen forms a (2×2) ordered structure. Coadsorption of hydrogen leads to a structural transformation from the (2×2) to a (√3×√3)R30 o structure. In addition to this transformation, hydrogen enhances the mobility of oxygen. To explain these observations, the interaction of oxygen and hydrogen on Pd(111) was studied within the density functional theory. In agreement with the experiment the calculations find a total energy minimum for the oxygen (2×2) structure. The interaction between H and O atoms was found to be repulsive and short ranged, leading to a compression of the O islands from (2×2) to (√3×√3)R30 o ordered structure at high H coverage. The computed energy barriers for the oxygen diffusion were found to be reduced due to the coadsorption of hydrogen, in agreement with the experimentally observed enhancement of oxygen mobility. The calculations also support the finding that at low temperatures the water formation reaction does not occur on Pd(111).

Solvent molecules form surface redox mediators in situ and cocatalyze O 2 reduction on Pd

“…We examined this possibility by carrying out DFT calculations on a model Pd(111) surface (representative of dominant facets on Pd NPs; fig. S1) with coadsorbed H*-and O*-atom coverages that were consistent with operando extended x-ray absorption fine structure (EXAFS) measurements (see below) and established adlayer structures (supplementary text, sections S3 and S4) (27). Ab initio molecular dynamics simulations were performed with liquid methanol on these covered Pd surfaces to generate the lowest-energy hydrogen-bonding networks over the catalysts and the adsorbates.…”

N2Emission via Intermediate N2O in a Steady-State NO + CO + D2Reaction on Stepped Pd(211) by Angle-Resolved Desorption

“…On the other hand, the reaction of 2H­(a) + O­(a) → H 2 O­(a) on Pd already proceeds at ∼220 K. , The high reactivity of D­(a) is still kept, even in the presence of O­(a), N­(a), CO­(a), and NO­(a), because of the fast diffusion and a small activation barrier for the reaction with O­(a), around 29 kJ/mol . Thus, this reaction can control the coverage of D­(a) and O­(a); i.e., only either Θ O ≫ Θ D or Θ O ≪ Θ D is possible in the steady-state NO + CO + D 2 reaction under a high vacuum.…”

“…Surface species mostly consist of NO(a) and N(a) on Pd(211) at T S = 450−490 K in the presence of a nearly equimolar mixture of NO and CO, although both molecules adsorb efficiently on the clean surface. 75,76 On this surface, NO is partly dissociated at these temperatures → N 2 O(a) is significant, and actually, these adsorbed species are copresent below 420 K. 19,20 On the other hand, the reaction of 2H(a) + O(a) → H 2 O(a) on Pd already proceeds at ∼220 K. 77,78 The high reactivity of D(a) is still kept, even in the presence of O(a), N(a), CO(a), and NO(a), because of the fast diffusion and a small activation barrier for the reaction with O(a), around 29 kJ/mol. 79 .…”

Visualization of Compression and Spillover in a Coadsorbed System: Syngas on Cobalt Nanoparticles

“…7,12,13 The CO and H form compressed, segregated domains on the Pd surface, 7,12À14 and with increasing CO coverage, H is forced to desorb or absorb into subsurface Pd sites, depending on specific conditions. 7,12,13 In related works, Salmeron and coworkers observed the compression of O by H on Pd(111) 15,16 and demonstrated that the strain of this compression is relieved by H dissolution into the bulk. 16 CO and O are also known to form segregated domains on both Pd(111) 17 and Pt(111), 9 and CO was shown to compress O on these surfaces due to its stronger adsorption energy.…”

“…Notably, although most of these investigations have been carried out in ultrahigh vacuum (UHV), several have been corroborated by high-pressure , or macroscale measurements and a few studies have been performed with high-pressure scanning tunneling microscopy (STM), ,, confirming that the effects of segregation have ramifications on reaction kinetics under catalytically relevant conditions. A number of reports have investigated the coadsorption of CO and H on Pd(111), and it has been shown that there are repulsive interactions between the species. ,, The CO and H form compressed, segregated domains on the Pd surface, ,− and with increasing CO coverage, H is forced to desorb or absorb into subsurface Pd sites, depending on specific conditions. ,, In related works, Salmeron and co-workers observed the compression of O by H on Pd(111) , and demonstrated that the strain of this compression is relieved by H dissolution into the bulk …”