383593 First-Principles Investigation of the Mechanism and the Active Site for Hydrodeoxygenation over Ru/TiO2
Using periodic density functional theory (DFT) we have explored the hydrodeoxygenation (HDO) pathways of acetaldehyde and phenol, two surrogate molecules for some of the over 400 different oxygenates from biomass fast pyrolysis , over supported Ru/TiO2 catalysts. The main functional groups represented by acetaldehyde and phenol are aldehydes, ketones, and aromatic alcohols. These groups are abundant output products of fast pyrolysis, and catalytic conversion of these products to oxygen free compounds is of significant importance for bio-oil upgrade. There are three major HDO pathways: (1) hydrogenation (HYD), (2) direct deoxygenation (DDO), and (3) decarbonylation (DCN). Among these, DDO is the most desired pathway because it requires less hydrogen than HYD, and in contrast to DCN, the number of carbon atoms during HDO remains constant. In a systematic study of HDO pathways for acetaldehyde and phenol we have calculated the thermodynamic and kinetic parameters for a large number of possible elementary surface reactions with specific focus on product selectivity, i.e. the preference for C-H, C-O, and C-C bond scission reactions at each reaction intermediate.
In experimental studies Ru/TiO2 has shown promising results in terms of selectivity and catalytic activity for HDO of different molecules (e.g. furfuran, acetone, phenol) [2-4]. To represent the supported Ru/TiO2 catalysts we tested various active site models including metallic Ru(0001), pure rutile metal-oxide surfaces [RuO2(110), TiO2(110)], modified metal-oxide catalysts [1.0 ML RuO2/TiO2(110), Ru1/TiO2(110)] and a 10-atom cluster of Ru supported on TiO2(110) [Ru10/TiO2(110)]. Our results show that the metallic Ru(0001) surface is not suitable for DDO. It preferentially catalyzes HYD for phenol, resulting in the fully hydrogenated products cyclohexane or cyclohexanone, or DCN for acetaldehyde. The dominant DCN pathway for acetaldehyde can be described as: adsorption (CH3CHO) → αC-H bond breaking (CH3CO*) → ßC-H bond breaking (CH2CO*) → C-C bond breaking (CH2*). At high hydrogen pressure CH2* may leave the catalyst surface as CH4, while at low hydrogen pressure stepwise dehydrogenation leads to CH* and eventually surface carbon (C*), causing catalyst deactivation due to coke formation. HYD and DDO may occur on Ru(0001) and the formation of ethane or ethylene is feasible, but the removal of surface oxygen requires significant energy barriers (Ea > 1.7 eV) and limits the overall reaction. In turn, the accumulation of surface oxygen can lead to high oxygen coverages, partial oxidation or even the formation of a surface oxide.
Oxygen vacancy sites are expected to be important for HDO catalysis on metal-oxide surfaces. Hence, we initially derived a constrained thermodynamic phase diagram for the rutile TiO2(110), Ru1TiO2(110), 1.0 ML RuO2/TiO2(110), and RuO2(110) surfaces. Under typical HDO conditions (T > 500 K, P(H2) > 200 bar) these surfaces are partially reduced and exhibit bridging hydroxyl groups (HObr). Using this reduced state as reference surface we investigated vacancy formation pathways on each metal-oxide. Upon further hydrogen adsorption the surface hydroxyl groups can be removed as water along three different reaction pathways: (1) the common disproportionation pathway HObr + HObr → H2Obr + Obr, and two pathways involving molecular hydrogen adsorption at the coordinatively unsaturated Ti site (Ticus) (2) Ticus-H2 + Obr → Ticus-H + HObr → Ticus + H2Obr, and (3) Ticus-H2 + HObr → Ticus-H + H2Obr. Among these pathways, reaction (3) occurs with the lowest energy barrier on Ru1TiO2, 1.0 ML RuO2/TiO2, and RuO2. Due to weak interactions between molecular hydrogen and the Ticus site of TiO2, reactions (2) and (3) are not availableand reaction (1) is favored instead . In all cases, the desorption of the resulting H2Obr forms the surface oxygen vacancy. This active vacancy site provides the preferred adsorption site for acetaldehyde and phenol, and plays an important role in the preferential activation of C-O scission over C-C scission reactions. Hence, the DDO pathway occurs most likely on the considered metal-oxides surfaces and follows the sequence of elementary steps given by: adsorption on surface vacancy (CH3CHO*-s) → isomerization (CH3CHO*-d) → αC-O bond breaking (CH3CH*) → ßC-H bond breaking (CH2CH*) → αC-H bond formation (CH2CH2(g)), leading to the desired product ethylene. In addition to the DDO pathway, metal-oxide surfaces can also catalyze the HYD pathway resulting in the intermediate formation of ethanol, which can subsequently be dehydrated to ethane.
The highest selectivity for ethylene is predicted on the TiO2(110) surface; however, the dissociation of hydrogen molecules and the formation of HObr groups, the precursor to the formation of a required oxygen vacancy, are kinetically limited. The presence of Ru clusters on TiO2 may facilitate hydrogen delivery from the gas-phase to TiO2(110) through hydrogen spillover, and we investigated this possibility using a Ru10 cluster model supported on TiO2(110). DFT results suggest that spillover of H atoms originating from the Ru cluster is kinetically hindered (Ea ≈ 1 eV), but the Ru/TiO2 interface is very active for hydrogen dissociation (Ea ≈ 0.1 eV) and enables the formation of oxygen vacancy sites as required for C-O bond breaking reactions during HDO on TiO2. The interface also provides sites with unique bifunctional properties: for example, the C-O bond in phenol adsorbed in a vacancy site at the interface can be selectively broken in a concerted step where hydrogen is provided from the Ru10 cluster and the eliminated OH group heals the bridging oxygen vacancy site on TiO2(110): C6H5ObrH + H-Ru10 → C6H6(g) + HObr + Ru10.
In conclusion, detailed DFT calculations suggest that DCN is preferred over DDO and HYD on the metallic Ru(0001) surface, leading to surface carbon or methane, while on the investigated metal-oxide surfaces DDO and HYD are more likely than DCN. Oxygen vacancy formation on TiO2 is limited by hydrogen delivery but can be facilitated at the Ru/TiO2 interface. These vacancy sites selectively catalyze the DDO pathway of acetaldehyde and phenol, leading to the desired products, ethylene and benzene, respectively.
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