Differentiation of the C–O and C–C bond scission mechanisms of 1-hexadecanol on Pt(111) and Ru(0001): a first principles analysis

Abstract

C–C and C–O bond scission are important reactions that have been extensively studied experimentally; however, the decomposition mechanism for long-chain alkanols is still not clear. In the present study, density functional theory calculations were performed to study the reaction mechanisms of C–O and C–C cleavage in 1-hexadecanol on Pt and Ru. The adsorption mechanisms and the reaction cycles for 1-hexadecanol (1-C₁₆H₃₄O) decomposition were clarified in this study. The mechanisms include dehydrogenation steps and C–O and C–C cleavage steps. The present calculation results show that the main mechanism of 1-hexadecanol decomposition on Pt(111) is C₁₄H₂₉CH₂CH₂OH → C₁₄H₂₉CH₂CHOH → C₁₄H₂₉CH₂COH → C₁₄H₂₉CH₂C → C₁₄H₂₉CH₂CH → C₁₄H₂₉CH₂CH₂ → C₁₄H₂₉CH₂CH₃, and the C–O bond cleavage mechanism is favored over that of C–C bond cleavage. The major final product is n-hexadecane (C₁₄H₂₉CH₂CH₃), and the C–O cleavage and the formation of the C₁₄H₂₉CH₂CH₂ species are the rate-controlling steps. However, on Ru the mechanism is C₁₄H₂₉CH₂CH₂OH → C₁₄H₂₉CH₂CH₂O → C₁₄H₂₉CH₂CHO → C₁₄H₂₉CH₂CO → C₁₄H₂₉CHCO → C₁₄H₂₉CH + CO → C₁₄H₂₉CH₂ → C₁₄H₂₉CH₃, and the dominant product is n-pentadecane (C₁₄H₂₉CH₃). The rate-controlling steps are the dehydrogenation of C₁₄H₂₉CH₂CO into the C₁₄H₂₉CHCO species and the hydrogenation of C₁₄H₂₉CH₂ into C₁₄H₂₉CH₃ species. The resulting CO is converted into CH₄ according to CO → HCO → H₂CO → CH₂ → CH₃ → CH₄. The rate-limiting step is the formation of HCO. These calculation results clearly show that the C–O bond scission mechanism is favored over the C–C bond scission mechanism on Pt, whereas the opposite situation occurs on Ru; namely, this reaction is strongly metal dependent. The different mechanisms originate from different initial cleavage channels, in which Cα–H scission is preferred for 1-hexadecanol on Pt, whereas O–H scission is the dominant cleavage on Ru. Moreover, it was found that the selectivity and reactivity could be modified by modifying the experimental conditions such as H₂ pressure. Confirming the mechanisms of C–C and C–O cleavage will aid the modification of experimental conditions to obtain high selectivity products. The present calculation results can be extended to other metals, such as Ni, bimetallic alloys, such as Ni/Pt(111), or other alkanol reaction systems.