Theoretical Investigation Of Ethanol Dehydrogenation Mechanism On Transition Metal Surfaces

Density functional theory （DFT） is a method in quantum mechanics for electronicstructure calcualtions. With the advent of DFT, it has shown a powerful functionality inclarifying mechanism of various surface adsorption and simple reactions. In recent years, thecontinuous deepening of the periodical DFT researchs to the complex reaction greatlypromotes the oxo-dehydrogenation mechanisms on metal surface. As a typical example,dehydrogenation of small organic molecule like ethanol on transition metals is an importantheterogeneous catalytic proeess that are widely studied by experiments. As an ideal hydrogencarrier, ethanol has a broad application prospects in alcohol reforming proeesses and fuel cell,which have profound significance to the development of green energy resource andenvironmental protection. At present, related studies to ethanol dehydrogenation are still at theearly stages, and it is necessary to give a theoretical research to ethanol dehydrogenation ontransition metals. Thus, not only it can expand the application area of DFT, but also deepenpeople’s knowledge of microscopic mechanism of ethanol dehydrogenation and providetheory clues to the designation of new efficient catalyst.Due to the complexity of reaction networks and limination of experimental methods, theknowledges about alcohol dehydrogenation mechanisms are inadequate, especially for thegoverning factor of the C–C bond cleavage and the reaction selectivity. Therefore, in thepresent work, we take ethanol dehydrogenation as the target systems and comprehensivelyinvestigate the complex dehydrogenation reaction networks mainly in the context ofexperiments. On this basis, we put emphasis on the following contents which are related tothe reaction microcosmic mechanism and reaction selective.Micromechanism of ethanol dehydrogenation: From an atomic-level view, reactionselectivity is a comparison result of the rates between different reaction channels. It ismacro-performance of reaction microkinetics. Ethanol dehydrogenation involves differentkinds of bond cleavage like C–H, C–O, O–H, C–C and various adsorbed intermediates. Thereaction path is complex, and the reaction selectivity is closely related to the mirco-reactionprocesses. First, we take ethanol dehydrogenation on Pd（111） as the object to investigate thewhole decomposition mechanism, and the structural features for the adsorbed intermediates.Then we acquire the relevant activation energy of the elementary reaction and further construct the reaction network through the try-and-error method. The calculations indicatethat the most stable adsorption of the involved species tends to follow the gas-phase bondorder rules by bonding with the surface metal atom（s）, wherein C is tetravalent and O isdivalent with the missing H atoms replaced by the metal atoms. The whole reaction pathwaycan be written as: CH₃CH₂OH→CH₃CHOH→CH₃CHO→CH₃CO→CH₂CO→CHCO→CH+CO→CO+H+CH₄+C, the rate limited step is the C–C bond cleavage of CH₃CO.For intermediates going along the decomposition pathway, energy barriers for C–C, C^α–H,and O–H bond scission are gradually decreased, while it is gradually increased for the C–Obond cleavage or does no obviously change for the C^β-H bond cleavage except CH₃CO.Although there are some obviously deviated points corresponding to the C–O path of CHCOand the C–C path of both CH₃CH₂OH and CH₃CHOH, the Br nsted-Evans-Polanyi relationholds roughly for each of the C–C, C–O and C–H paths.Microstructure sensitivity of reaction selectivity: Experiments have shown thatreaction selectivity and activity are largely affected by the surface of metal catalysts. Furthercalculations of ethanol decomposition on Pd（110） indicate that the selectivity is greatlyaffected by the surface structure. The pathways of ethanol decomposition on Pd（110） isCH₃CH₂OH→CH₃CH₂O→CH₃CHO→CH₃CO→CH₃+CO→CO+H+CH₄+C，andthe rate limited step is the initial dehydrogenation of CH₃CH₂OH. The energy barriers of C–Obond cleavage for all the intermedaitews involve high values, which indicate the impossibilityof the minor C–O bond scission pathway. The calculations show that Pd（110） present a goodperformance against the C residue and facicliate the C–C bond cleavage. So it could be usedas the catalyst for the complete oxidation of ethanol. However, the low hydrogenation energybarrier of CH₃could lead to the H loss in the form of CH₄. Therefore special method shouldbe used to Pd（110）, such as surface modification, to improve the dehydrogenationperformance of Pd（110）. The electronic structure and energy decomposition analysis indicatethat the local electronic effect of metals and transition state geometrical effect are indeed thecrucial factor controlling the variations of the activation. For the reactions considered, nosingle BEP relation can be established for all reactions on Pd（111） and Pd（110） surfaces,which indicat that the empirical BEP relationship cannot be universal used on different latticeplane or accurately quantify the reactions.Different material structure sensitivity of reaction selectivity: Different materialstructures are also an important factor to the reaction selectivity and activity. Experimentsfound that ethanol decomposition follows the continuous dehydrogenation mechanism via CH₃CHO intermediate on Pt and Pd, but CH₂CH₂O intermediate on Rh（111）. So weinvestigate the mechanism of ethanol decomposition on Rh（111） to establish the PES, andfind out the critical steps to deeply understand the effect of different metal materials to thereaction mechanism. The calculations show that ethanol decomposition on Rh（111） follow thepathway of CH₃CH₂OH→CH₃CH₂O→CH₂CH₂O→CH₂CHO→CH₂CO→CHCO→CH+CO→CO+C, and the rate limiting step is dehydrogenation of CH₃CH₂O. Forintermediates going along the decomposition pathway, energy barriers for the C^β–H and C–Cbond scission are gradually decreased, while they decrease first and then rise presenting aV-shape for the C^α–H and C–O bond cleavage.Effect of surface modification to the dehydrogenation mechanism: Proper surfacemodification to the Pt surface like alloying is an efficient method to promote the catalysisactivity. Experiments found that PtSn bimetallic catalyst is the most proper for ethanolelectro-oxidation. At present, the mechanism investigation of ethanol oxidation on PtSn alloysurface are scared, especially at the microscopic level. We perform a systemic investigation tothe ethanol oxidation mechanism on Pt₃Sn（111） surface to construct the reaction network, andfurther discuss the ethanol electrooxidation mechanism based on the combining of relevantelectrochemistry experiments. The calculations show that in low temperature and lowpotential, ethanol prefers the successive dehydrogenation to CH₃CO through initial O–H bondscission. However, the dissociation of CH₃CO involves high energy barrier. When thepotential rises, CH₃CO can be oxidated to CH₃COOH by OH. Dehydrogenation ofCH₃COOH to CH₃COO is mildly, but CH₃COO can not be further degradated for its verticalconfiguration. The whole pathway is CH₃CH₂OH→CH₃CH₂O→CH₃CHO→CH₃CO→CH₃COOH→CH₃COO. Although the Pt₃Sn（111） can effectively oxidate ethanol toCH₃COOH, the activity for C–C bond scission is also low. Comparing with investigations ofH₂O dissociation on PtSn alloy, Pt₃Sn（111） surface catalyze the ethanol oxidation mainlythrough the bifunctional mechanism. The analysis to the selective control key steps indicatethat the low C–C bond scission selectivity of CH₃CO is from the weak binding in the TSgeometry, so introduce the low index sureface to alter the TS configurations should increasethe selectivity of C–C bond scission.