Theoretical Investigations on Complex Dehydrogenation Reaction Networks at Platinum Metal Surfaces
|Course||Physical and chemical|
|Keywords||mechanisms of complex dehydrogenation reaction network density functional theory ethanol electro-oxidation formic acid electro-oxidation transition metal Pt reaction selectivity surface structure sensitivity periodic continuum solvation model water solvation|
With the advent of Density Functional Theory (DFT), it has shown a powerful functionality in clarifying mechanisms of simple surface reactions, such as H2 dissociation, NO or CO reduction/oxidation on transition metal surfaces and so on. However, exploring mechanisms of complex dehydrogenation reaction networks at transition metals with DFT is an emerging area in the recent years, and very limited research work has been reported. As a typical example, oxidation of small organic molecules like methanol, ethanol and formic acid on transition metals is widely involved in many important heterogeneous catalytic processes, because they are of great interest in alcohol reforming processes and fuel cell applications as ideal hydrogen carrier, which is important in relieving the current energy crisis and environmental pressures. At present, experimental studies on the alcohol reforming processes and direct alcohol/acid fuel cells are at the early stages in clarifying reaction mechanisms and screening proper catalysts. It is therefore of both theoretical and practical interest in investigating mechanisms of alcohol and formic acid dehydrogenation on transition metal surfaces applying with DFT calculations, which not only can broaden the DFT functionality in dealing with more complex reaction network on metal surfaces, but also can help to understand the atomic-level reaction mechanisms and provide important clues for choosing new efficient metal catalysts.Due to the complexity of reaction networks and limitation of experimental methods, there is inadequate knowledge about mechanisms of alcohol reforming for hydrogen production and fuel cell reactions, especially for the pattern of C-C bond breaking and the key factors in dictating reaction selectivity. Therefore, in the present work, we take the dehydrogenation of ethanol and formic acid (in the presence of water) as model systems and comprehensively investigate the complex dehydrogenation reaction networks on platinum surfaces within DFT frameworks mainly in the context of electro-oxidation experiments. On this basis, we put emphasis on the following contents which are related to reaction selectivity.Selectivity determined by unique reaction pathways:From an atomic-level view, reaction selectivity is a comparison result of the rates between different reaction channels. It is macro-performance of reaction microkinetics. Apparently, reaction selectivity is closely related to the micro-reaction processes. It has been reported that in ethanol electro-oxidation, ethanol is mainly partially oxidized into acetaldehyde and acetic acid instead of being fully oxidized and the highest selectivity to CH3CHO is observed to be slightly affected by electric potential. This undesired selectivity severely limits the practical energy efficiency of direct ethanol fuel cells (DEFCs). The traditionally proposed stepwise dehydrogenation mechanism can not rationalize the high selectivity of CH3CHO, and no answer for the issue that selectivity to CH3CHO is always higher than that of its oxidation product CH3COOH. By extensive density functional theory calculations on the distinct reaction channels from ethanol to acetaldehyde and acetic acid on Pt(111), the rate constants of the different reaction pathways were compared and effect of the coadsorbates were considered. We demonstrated that ethanol is partially oxidized into CH3CHO via a unique concerted dehydrogenation path which is rarely occurred in surface reactions. Such a concerted path is shown to be largely affected by the reactants’structures and its existence can well explain the observed high selectivity to acetaldehyde.Structure sensitivity of reaction selectivity:How to selectively activate chemical bond towards making desired product must rank the top concern in chemistry. Research on reaction selectivity is the key point in the mechanistic studies. Amounts of experiments have shown that reaction selectivity and activity is largely affected by the surface structure of metal catalysts. For example, in ethanol electro-oxidation with traditional Pt catalysts, the observed main products are CH3CHO and CH3COOH and the complete oxidation product CO2 is very low; addition of Sn to Pt can accelerate the production of C2 products (species containing 2-C atoms) while the selectivity to CO2 is little affected; on the stepped surfaces like Pt(557) and Pt(335), it was found that the selectivity to acetic acid decreased with the increasing surface steps; when the electrode catalysts composed of high density of (100) terraces, the selectivity to CO2 was greatly enhanced. All these experimental facts indicate that selectivity of ethanol oxidation on platinum is surface structure sensitive.Therefore, in the present work, we performed an extensive investigations on the whole reaction network of the complete oxidation of ethanol at different Pt surfaces, including Pt(111), Pt(211) and Pt(100). Two critical steps in dictating the selectivity of ethanol oxidation were clarified, namely the initial dehydrogenation of ethanol and the oxidation of the acetyl (CH3CO) intermediate. The former mainly determines the selectivity to CH3CHO and the latter determines the selectivity to CH3COOH and CO2. These two selectivity-determining steps have distinct behaviors on differently coordinated Pt surfaces. On Pt(111) surface, ethanol is mainly oxidized into CH3CHO and CH3COOH, while on Pt(211) and Pt(100), ethanol mainly proceeds consecutive C-H bond breaking and finally via C-C bond splitting into C1 species. By detailed electronic structure analysis and barrier decomposition, we clarified the physical origin of the surface structure-sensitivity of the reaction selectivity:i)Δd-PDOS results show that transition state of the a-dehydrogenation has strong bonding interaction with surface Pt atom which can account for the surface structure-sensitivity of the a-dehydrogenation barrier; ii) on low coordinated Pt(211) and Pt(100), hydroxyl (OH) bonds at bridge sites too strongly to oxidize other species like acetyl, which suppresses the formation of acetic acid. Besides, we found that a linear Bronsted-Evans-Polanyi (BEP) relation holds for different bond-breaking reactions across different surfaces, which is only related to the type of bond (say, C-C, C-H etc.) but not related to the surface structures. Applied with this BEP relationship, it can qualitatively predict the general pattern in C-C bond dissociation reactions, such as reaction sites and the reaction precursor.Relationships of selectivity with reactant adsorption configurations and solvation effect:water solvation has important influences on reaction activity and selectivity. It has been a long-standing challenge for theorists to investigate the solvation effect on the heterogeneous catalytic processes within DFT framework, especially simulations at water/metal interfaces. As a typical example, formic acid shows distinct degradation behaviors under the ultra-high vacuum conditions and electro-oxidation environment. This indicates that the aqueous environment plays an important role in formic acid oxidation. However, there is still no consensus about the reaction mechanism of formic acid electro-oxidation under water solution at the present, and the role of the intermediate formate (HCOO), whether an active intermediate or a spectator species, is still open to discussion. Combined DFT calculations with a discrete-continuum solvation model, we extensively studied the adsorption and reaction behaviors of formic acid on both clean Pt(111)/H2O interface and formate covered Pt(111)/H2O interface. We found that during the adsorption processes in which formic acid transfers from bulk phase to Pt/H2O interfaces, it cost lots of solvation energiesΔEsol, which have important influences on the adsorption configurations of formic acid at the metal/water interfaces. It was concluded that i) formic acid is directly oxidized into CO2 with the reactive precursor of CH-down configured formic acid, and water plays a key role in the direct oxidation pathway; ii) the presence of formate promotes the adsorption of reactive CH-down configuration on Pt sites, which is beneficial to the direct oxidation of formic acid. The physical reason is that the adsorbed formate results in rearrangement of the H-bonding network of water at the water/metal interface, which then leads to reduction of the solvation energy cost during the adsorption of CH-down configuration.Relationships between selectivity and surface modification:Modification of Pt with other species and secondary metal atoms is considered to be an efficient way to improve the catalytic activity of electrodes in electro-chemistry. It was generally reported that modification of Sb on Pt (Sb/Pt) hindered the dissociative adsorption of formic acid (leading to CO formation) and promoted the direct oxidation of formic acid into CO2. While some one proposed that the function of Sb on Pt can be attributed to the steric effect which blocks the surface active sites for CO formation, it is still elusive for the high reactivity of formic acid on Sb/Pt electrodes. As we reported, the selectivity of formic acid direct oxidation to CO2 is greatly related to the adsorption configurations. Therefore, we then compared the adsorption behaviors of formic acid (in the presence of water) on clean Pt(111) and various Sb-covered Pt(111). It was found that as the Sb coverage achieves 0.375 ML and forms a 2D island structure on Pt(111), the CH-down configuration becomes the dominant species on the surface, which will benefit the direct oxidation of formic acid into CO2. According to Bader charge analysis, Sb adlayer donates some electron to Pt surface and leads to the formation of a Sbδ+-Ptδ- dipole normal to the surface. It is such an electrostatic dipole that enhances the adsorption energy of CH-down configuration on Pt surfaces and thus the reactivity of formic acid direct oxidation. Our mechanisms get well supported by experimental facts.Besides, considering the limitation of the traditional transition state (TS) searching method in dealing with complex dehydrogenation reaction networks, we developed a new TS searching method. This new method inherits the advantages of Quasi-Newton Broyden optimization in the searching processes with a significant speed-up in convergence. It allowed us to study explicitly the whole reaction network of complex dehydrogenation systems in reduced computational time. Besides, we built a periodic continuum solvation model to investigate the solvation effect on reactions at metal/water interfaces. We applied a discrete-continuum method to simulate the solvation shells of reaction systems, namely, we surrounded the reaction centre with up to six discrete water molecules as the core solvation shells and simulated the rest of solvation shells with a reported dielectric model function in a continuum solvation model.