CO Adsorption Enthalpy Research Articles

Correlating the Experimentally Determined CO Adsorption Enthalpy with the Electrochemical CO Reduction Performance on Cu Surfaces.

CO binding energy has been widely employed as a descriptor for effective catalysts in the electrochemical CO2 and CO reduction reactions (CO(2) RR), however, it has yet to be determined experimentally at electrochemical interfaces due to the lack of suitable techniques. In this work, we developed a method to determine the standard adsorption enthalpy of CO on Cu surfaces with quantitative surface enhanced infrared absorption spectroscopy. On dendritic Cu at -0.75 V vs. SHE, the standard adsorption enthalpy, entropy and Gibbs free energy were determined to 1.5±0.5 kJ mol-1 , ≈37.9±13.4 J/(mol K), and ≈-9.8±4.0 kJ mol-1 , respectively. Comparison of the standard adsorption enthalpy of oxide-derived Cu and dendritic Cu, as well as their CORR activities, suggests the presence of stronger binding sites on OD Cu, which could favor multicarbon products. The method developed in this work will help establish the correlation between the CO binding energy and the CO(2) RR activity.

Correlating the Experimentally Determined CO Adsorption Enthalpy with the Electrochemical CO Reduction Performance on Cu Surfaces

AbstractCO binding energy has been widely employed as a descriptor for effective catalysts in the electrochemical CO2 and CO reduction reactions (CO(2)RR), however, it has yet to be determined experimentally at electrochemical interfaces due to the lack of suitable techniques. In this work, we developed a method to determine the standard adsorption enthalpy of CO on Cu surfaces with quantitative surface enhanced infrared absorption spectroscopy. On dendritic Cu at −0.75 V vs. SHE, the standard adsorption enthalpy, entropy and Gibbs free energy were determined to 1.5±0.5 kJ mol−1, ≈37.9±13.4 J/(mol K), and ≈−9.8±4.0 kJ mol−1, respectively. Comparison of the standard adsorption enthalpy of oxide‐derived Cu and dendritic Cu, as well as their CORR activities, suggests the presence of stronger binding sites on OD Cu, which could favor multicarbon products. The method developed in this work will help establish the correlation between the CO binding energy and the CO(2)RR activity.

Electrolyte Competition Controls Surface Binding of CO Intermediates to CO2 Reduction Catalysts

Adsorbed CO is a critical intermediate in the electrocatalytic reduction of CO2 to fuels. The directed design of CO2RR electrocatalysts has centered on strategies to understand and optimize the differences in CO adsorption enthalpy across surfaces. This approach has largely ignored the role of competitive electrolyte adsorption in defining the CO surface population relevant for catalysis. Using in situ infrared spectroelectrochemistry and voltammetry, we uncover the contrasting influence of electrolyte competition on reversible CO binding to Au and Cu catalysts. Although reversible CO binding to Au surfaces is primarily driven by the adsorption processes associated with interfacial water, CO binding to Cu surfaces requires the reductive displacement of adsorbed carbonate anions. The divergent role of electrolyte competition for CO adsorption on Au versus Cu leads to a ∼600 mV difference in the potential region where CO accumulates on the two surfaces. The contrasting CO adsorption stoichiometry on Au and Cu also explains their disparate reactivity: interfacial water adsorption contributes to CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption contributes to CO accumulation on Cu surfaces, allowing for further reduction to hydrocarbons. These studies provide direct insights into how electrolyte constituents fine-tune CO surface populations and, thereby, CO2-to-fuel reactivity.

Controlling the Adsorption Enthalpy of CO2 in Zeolites by Framework Topology and Composition

Zeolites are often investigated as potential adsorbents for CO(2) adsorption and separation. Depending on the zeolite topology and composition (Si/Al ratio and extra-framework cations), the CO(2) adsorption heats at low coverages vary from -20 to -60 kJ mol(-1), and with increasing surface coverage adsorption heats either stay approximately constant or they quickly drop down. Experimental adsorption heats obtained for purely siliceous porous solids and for ion-exchanged zeolites of the structural type MFI, FER, FAU, LTA, TUN, IMF, and -SVR are discussed in light of results of periodic density functional theory calculations corrected for the description of dispersion interactions. Key factors influencing the stability of CO(2) adsorption complexes are identified and discussed at the molecular level. A general model for CO(2) adsorption in zeolites and related materials is proposed and data reported in literature are evaluated with regard to the proposed model.

Combined Theoretical and Experimental Investigation of CO Adsorption on Coordinatively Unsaturated Sites in CuBTC MOF

The adsorption of CO in metal-organic framework CuBTC material is investigated by a combination of theoretical and experimental approaches. The adsorption enthalpy of CO on CuBTC determined experimentally to be -29 kJ mol(-1) at the zero-coverage limit is in very good agreement with the adsorption enthalpy calculated at the combined DFT-ab initio level with the periodic model. Calculations show that polycarbonyl complexes cannot be formed on regular coordinatively unsaturated sites in CuBTC. Experimental IR spectra of the CO probe molecule adsorbed in CuBTC are interpreted based on calculated CO stretching frequencies. Calculations show that long-range interactions are insignificant for the CO/CuBTC system and that this system can be accurately modeled with just a Cu(2)(HCOO)(4) cluster model of the paddle wheel. The reliability of various methods for the description of CO interaction with the Cu(2+) site in CuBTC is discussed based on the experimental results and accurate coupled-cluster calculations. It is shown that standard exchange-correlation functionals do not provide a reliable description of CO interaction with coordinatively unsaturated Cu(2+) sites in CuBTC.

Thermodynamics of Carbon Dioxide Adsorption on the Protonic Zeolite H‐ZSM‐5

Adsorption of carbon dioxide on H-ZSM-5 zeolite (Si:Al=11.5:1) was studied by means of variable-temperature FT-IR spectroscopy, in the temperature range of 310-365 K. The adsorbed CO(2) molecules interact with the zeolite Brønsted-acid OH groups bringing about a characteristic red-shift of the O-H stretching band from 3610 cm(-1) to 3480 cm(-1). Simultaneously, the nu(3) mode of adsorbed CO(2) is observed at 2345 cm(-1). From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm(-1), upon changing temperature (and CO(2) equilibrium pressure), the standard adsorption enthalpy of CO(2) on H-ZSM-5 is DeltaH(0)=-31.2(+/-1) kJ mol(-1) and the corresponding entropy change is DeltaS(0)=-140(+/-10) J mol(-1) K(-1). These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali-metal exchanged, zeolites.

Influence of the preparation method on the catalytic behaviour of PtSn/TiO2 catalysts

PtSn/TiO2 catalysts containing 2wt% Pt and a Pt:Sn atomic ratio of 2:1 and 1:1 were prepared by coimpregnation or successive impregnation method with aqueous solutions of SnCl2·2H2O and H2PtCl6·6H2O of a commercial TiO2 (P25, from Degussa). Both catalyst series, independent of the preparation method, were reduced at 473 and 773K. XPS results show that tin was in an oxidized state after reduction at 473K, and that a fraction was in the metallic state after reduction at 773K. By use of in situ FTIR spectroscopy of adsorbed CO, the presence of bimetallic Pt–Sn phases was assessed after reduction at 773K. Microcalorimetric analysis of CO adsorption enthalpy indicates that reduction at 773K causes the appearance of a more heterogeneous distribution of active sites, as well as a loss in the amount of sites. The catalytic activity for the gas phase hydrogenation of crotonaldehyde was greatly improved when the catalysts were prepared by coimpregnation, at both reduction temperatures. The selectivity toward crotyl alcohol was higher after reduction at 773K and independent of the preparation method, although it increased with the amount of tin, suggesting a promoting effect of tin on this reaction.

Kinetics of the CO+N2O Reaction over Noble Metals: I. Pt/Al2O3

The kinetics of the CO+N2O reaction on Pt/Al2O3 have been studied between 260 and 320°C with partial pressures ranging from 1×10−3 to 8×10−3 atm for N2O and 4×10−3 to 14×10−3 atm for CO. A mechanism has been selected from those suggested in the literature. It involves molecular adsorptions of N2O and CO and a dissociation step of adsorbed N2O on a nearest neighbor vacant site which is assumed to be rate limiting. A rate expression has been derived which led to the estimation of the rate constant of N2O dissociation and of the equilibrium adsorption constants of N2O and CO. Enthalpies of adsorption of N2O and CO, ΔHads,CO and ΔHads,N2O, and the energy of activation for adsorbed N2O dissociation, E, have been estimated in order to model temperature-programmed experiments. A divergence between experimental and calculated conversion vs temperature has been observed mainly at high conversion. Such a discrepancy has been mainly assigned to changes in the adsorption enthalpy of CO and NO with the adsorbate surface coverage. Such an effect has been tentatively quantified.