Abas Mohsenzadeh

The Effect of Carbon Monoxide Co-Adsorption on Ni-Catalysed Water Dissociation

The effect of carbon monoxide (CO) co-adsorption on the dissociation of water on the Ni(111) surface has been studied using density functional theory. The structures of the adsorbed water molecule and of the transition state are changed by the presence of the CO molecule. The water O–H bond that is closest to the CO is lengthened compared to the structure in the absence of the CO, and the breaking O–H bond in the transition state structure has a larger imaginary frequency in the presence of CO. In addition, the distances between the Ni surface and H2O reactant and OH and H products decrease in the presence of the CO. The changes in structures and vibrational frequencies lead to a reaction energy that is 0.17 eV less exothermic in the presence of the CO, and an activation barrier that is 0.12 eV larger in the presence of the CO. At 463 K the water dissociation rate constant is an order of magnitude smaller in the presence of the CO. This reveals that far fewer water molecules will dissociate in the presence of CO under reaction conditions that are typical for the water-gas-shift reaction.

A density functional theory study of hydrocarbon combustion and synthesis on Ni surfaces

AbstractCombustion and synthesis of hydrocarbons may occur directly (CH → C + H and CO → C + O) or via a formyl (CHO) intermediate. Density functional theory (DFT) calculations were performed to calculate the activation and reaction energies of these reactions on Ni(111), Ni(110), and Ni(100) surfaces. The results show that the energies are sensitive to the surface structure. The dissociation barrier for methylidyne (CH → C + H: catalytic hydrocarbon combustion) is lower than that for its oxidation reaction (CH + O → CHO) on the Ni(110) and Ni(100) surfaces. However the oxidation barrier is lower than that for dissociation on the Ni(111) surface. The dissociation barrier for methylidyne dissociation decreases in the order Ni(111) > Ni(100) > Ni(110). The barrier of formyl dissociation to CO and H is almost the same on the Ni(111) and Ni(110) surfaces and is lower compared to the Ni(100) surface. The energy barrier for carbon monoxide dissociation (CO → C + O: catalytic hydrocarbon synthesis) is higher than that of for its hydrogenation reaction (CO + H → CHO) on all three surfaces. This means that the hydrogenation to CHO is favored on these nickel surfaces. The energy barrier for both reactions decreases in the order Ni(111) > Ni(100) > Ni(110). The barrier for formyl dissociation to CH + O decreases in the order Ni(100) > Ni(111) > Ni(110). Based on these DFT calculations, the Ni(110) surface shows a better catalytic activity for hydrocarbon combustion compared to the other surfaces, and Ni is a better catalyst for the combustion reaction than for hydrocarbon synthesis, where the reaction rate constants are small. The reactions studied here support the BEP principles with R2 values equal to 0.85 for C-H bond breaking/forming and 0.72 for C-O bond breaking /forming reactions. Graphical AbstractA density functional theory study of hydrocarbon combustion and synthesis on Ni surfaces

A density functional theory study of reactions of relevance to catalytic hydrocarbon synthesis and combustion

Synthesis and combustion of hydrocarbons on a series of metal surfaces (Ag, Au, Al, Cu, Rh, Pt and Pd) were investigated using density functional theory (DFT). The adsorption energies for all species involved in these reactions, as well as the reaction energies and activation barriers on these surfaces, were calculated using the same models and DFT methods. The results were used to test the validity of the Brønsted–Evans–Polanyi (BEP) and transition state scaling (TSS) relationships for these reactions on these metal surfaces. The BEP relationship appears to be a valid indicator for the synthesis reactions with R2 values of 0.83, 0.88 and 0.94 for CO dissociation, CO hydrogenation and formyl (CHO) dissociation to CH + O, respectively. In addition to CH splitting, which has been studied before, the BEP relationship also appears to be valid for the CH oxidation and CHO dissociation to CO + H combustion reactions with R2 values of 0.94, 0.89 and 0.88, respectively. Also, the TSS relationship is excellent with a R2 value of 1 for all synthesis and combustion reactions. The BEP and TSS relationships were subsequently used to estimate the energetics of the synthesis and combustion reactions on Ni, Co and Fe surfaces. The results reveal that the transition state energies estimated by the TSS relationships are in better agreement with data obtained from DFT calculations than the activation energies estimated by the BEP relationships. Therefore, the TSS relationship is preferred when predicting energetics of these reactions on these surfaces.