(435b) 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 [1], 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 (E_a > 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 (HO^br). 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 HO^br + HO^br → H₂O^br + O^br, and two pathways involving molecular hydrogen adsorption at the coordinatively unsaturated Ti site (Ti^cus)  (2) Ti^cus-H₂ + O^br → Ti^cus-H + HO^br → Ti^cus + H₂O^br, and (3) Ti^cus-H₂ + HO^br → Ti^cus-H + H₂O^br. 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 Ti^cus site of TiO₂, reactions (2) and (3) are not availableand reaction (1) is favored instead [5]. In all cases, the desorption of the resulting H₂O^br 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 (CH₂CH_2(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 HO^br 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 (E_a ≈ 1 eV), but the Ru/TiO₂ interface is very active for hydrogen dissociation (E_a ≈ 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): C₆H₅O^brH  + H-Ru₁₀  → C₆H_6(g) + HO^br + 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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