Quantifying Adsorption of Organic Molecules on Platinum in Aqueous Phase by Hydrogen Site Blocking and in Situ X-ray Absorption Spectroscopy

Abstract

The adsorption equilibrium constants of phenol, benzaldehyde, cyclohexanol, and benzyl alcohol on Pt in aqueous phase have been determined via cyclic voltammetry (CV) measuring the capacity to adsorb hydrogen in the presence of organic molecules. The enthalpies of adsorbed phenol on Pt in aqueous phase estimated by this technique were approximately −41 and −21 kJ/mol relative to aqueous phenol for (110)/(100)-like sites and ( 111) facets, respectively. The enthalpy of phenol aqueous adsorption on (111) (−21 kJ/mol) can be compared to the gas-phase enthalpy of adsorption of phenol on single-crystal Pt(111) from previous work (−200 kJ/mol) to understand the individual factors influencing adsorption in the liquid phase. The results show that adsorbates with very strong intrinsic bonding at a gas/solid interface have markedly reduced adsorption enthalpy due to strong solvation of both the Pt surface and the organic. Using the CV method, the decrease of the heat of adsorption in the sequence benzaldehyde > benzyl alcohol > phenol > cyclohexanol points to a strong interaction of the aromatic ring with the metal surface. Combining CV and X-ray absorption spectroscopy, we show that the inhibition of hydrogen adsorption by organic molecules is a suitable method of determining the coverage of organic reactants and that these molecules have defined Pt−C bonds in the adsorbed state.

Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase

“…We conclude that the presence of phenol and benzaldehyde adsorbing on Pd, decreased the concentration of accessible Pd available to transfer electrons to H + . This is in perfect agreement with the effect of the presence of organic compounds on the underpotential generation of adsorbed H on Pt [32–34] …”

“…The rate expression reflects the dependence on the product of the two coverages that appears to reach a maximum at around 50 mM of phenol (Figure 1, for details see the SI). Using CV, the standard free energies of adsorption of benzaldehyde and phenol on the Pd/C catalyst were determined [34] . Together with the ECH rates, we used them to fit the kinetic model of Equation (3) as described in the supporting information.…”

Solvation Free Energies and Adsorption Energies at the Metal/Water Interface from Hybrid Quantum-Mechanical/Molecular Mechanics Simulations

“…71 The experimental standard solvation free energies of water, phenol and benzene in water the Pt(111)/water interface. 68,72 To investigate the coverage dependence and the preferred adsorption mode, we consider four different coverages (from 1/7 to 1/36 monolayer (ML),…”

Dependency of solvation effects on metal identity in surface reactions

“…he widespread use of solvents in applications varying from pharmaceutical [1][2][3][4][5][6] to electrochemistry [7][8][9][10] and catalysis [11][12][13][14][15][16] has given rise to an extensive range of studies aimed at understanding and predicting the role of solvents. The concept that a solvent can alter the performance of a catalyst, including its rate, selectivity, and stability, is well known; yet predicting a specific solvation effect remains a challenge [17][18][19][20][21][22][23][24] . While impressive progress has been made in understanding the effects of solvents in homogenous catalysis [25][26][27][28][29] , for heterogeneously catalyzed processes that benefit from easier separation of the catalyst relative to homogeneously catalyzed processes 30 , the role of solvents is hardly understood and only rarely studied.…”