Temperature dependence of aqueous-phase phenol adsorption on Pt and Rh

Abstract

Condensed/aqueous phase surface reactions such as electrocatalytic hydrogenation of bio-oil often involve reactant adsorption and displacement of adsorbed solvent molecules. The enthalpy and entropy of these adsorption processes will influence the kinetics of surface reactions in the condensed/aqueous phase. The value of the adsorption entropy will have a significant effect on how the reactant coverages vary as a function of temperature. Here, adsorption isotherms from 10 to 40 °C and van't Hoff plots were constructed to directly extract the adsorption entropy and enthalpy of phenol, a bio-oil model compound, on Pt and Rh in aqueous media. We show that the effective adsorption entropy of phenol on Pt and Rh in aqueous phase is positive, in contrast to the negative entropy expected in gas phase. The positive entropy values in the aqueous phase are consistent with adsorbed water gaining a fraction of the entropy of bulk liquid water upon displacement by adsorbed phenol. Consequently, the phenol surface coverage is less dependent on temperature in the aqueous phase compared to the gas phase. The results here give insight to the way in which temperature impacts reaction rates for aqueous-phase phenol hydrogenation reaction.

Elucidating the Influence of Metal Surface Composition on Organic Adsorbate Binding Using Active Particle Dynamics

“…As shown in Figure , the enthalpy of adsorption for furfural on Pd was 80 ± 1 kJ/mol, approximately 10 kJ/mol stronger compared to adsorption on the Pt surface (71.5 ± 0.1 kJ/mol). Interestingly, these values were significantly less than adsorption energies calculated in vacuum (∼130 and 190 kJ/mol of Pd and Pt, respectively, tabulated in Table S4), consistent with the view that water layers on the catalytic surface dramatically weaken the enthalpic driving force for adsorption . However, both the observed stronger binding on Pd surface compared to Pt and the relatively smaller difference of only ∼10 kJ/mol stand somewhat in contrast to the computationally predicted stronger furfural binding on Pt versus Pd surfaces (Table S5).…”

“…Interestingly, these values were significantly less than adsorption energies calculated in vacuum (∼130 and 190 kJ/ mol of Pd and Pt, respectively, 16 tabulated in Table S4), consistent with the view that water layers on the catalytic surface dramatically weaken the enthalpic driving force for adsorption. 38 However, both the observed stronger binding on Pd surface compared to Pt and the relatively smaller difference of only ∼10 kJ/mol stand somewhat in contrast to the computationally predicted stronger furfural binding on Pt versus Pd surfaces (Table S5). While we note that a difference of only 10 kJ/mol can still induce significant effects in reactivity, if the solvent effect on binding strength in the liquid phase was uniform between metal surfaces, one might expect a similar large difference (60 kJ/mol) in binding strength between the Pd and Pt surfaces in the liquid phase (albeit with lower overall magnitudes of each binding strength).…”

Liquid-Phase Effects on Adsorption Processes in Heterogeneous Catalysis

“…Phenol on Pt(111): Experimental vs Computational. Singh et al 22,24,71 recently also studied adsorption of aqueous phenol at four different temperatures (283, 288, 298, 314 K) to acquire experimental enthalpic and entropic solvation contributions. We also fitted eq 7 to their measured data (see Figures 3 and S10−S12) and Table 3 shows the extracted ΔΔG Ph gas→liq (θ Ph = 0) values at different temperatures.…”

“…Such an understanding becomes more important the larger the solvation effects are and the more the solvation effects vary for the various species and transition states in the catalytic system (changes in trends). Recently, aqueous solvation effects have been measured on the free energy of adsorption for phenol, benzyl alcohol, benzaldehyde, and cyclohexanol over a Pt catalyst at 298 K and for phenol over Pt and Rh catalysts at four temperatures (283, 288, 298, and 314 K). , Importantly, for phenol adsorption on supported Pt catalysts, it has become possible to deconvolute the adsorption isotherm data for the different surface facets, which enabled the measurement of the aqueous solvent effect on the phenol adsorption free energy on the Pt(111) facet, which again facilitates a correlation to computational studies. Experimentally measured aqueous solvation effects are large (>1.0 eV at 298 K) and endergonic for phenol on Pt(111), suggesting that the kinetic properties of Pt nanoparticles for phenol catalysis are a strong function of solvent properties.…”

Electrochemical routes for biomass conversion

“…The papers in this issue highlight the advantages of using electrochemistry to convert biomass-derived compounds. The studies focused on reductive conversions reflect the kinds of organic compounds that are reactive enough for the reactions to occur at low temperature (e.g., aldehydes, phenolic compounds, and aryl ethers) and highlight the high selectivity associated to electrochemical processes [1][2][3]. The results highlight the high sensitivity of electrocatalytic processes to reaction conditions (such as pH and electrolyte), in contrast to many of the reaction condition effects being absent or lessened in thermocatalytic processes [4].…”