Decarboxylation of Fatty Acids on Anisotropic Au(110) Surfaces - The

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Decarboxylation of Fatty Acids on Anisotropic Au(110) Surfaces Liqin Wu, Zeying Cai, Meizhuang Liu, Wang Ye, Huanxin Ju, Junfa Zhu, and Dingyong Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01847 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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The Journal of Physical Chemistry

Decarboxylation of Fatty Acids on Anisotropic Au(110) Surfaces Liqin Wu1, Zeying Cai1, Meizhuang Liu1, Wang Ye1, Huanxin Ju2, Junfa Zhu2, and Dingyong Zhong1,* 1

School of Physics and State Key Laboratory for Optoelectronic Materials and Technologies, Sun Yat-sen University, 510275 Guangzhou, China

2

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China *E-mail: [email protected]

Abstract Lowering the oxygen content in biofuels is of vital importance since high oxygen level content leads to low stability, low heat value and corruption in engines. Here we report on the thermally activated decarboxylation of fatty acids, raw materials for biofuels production, on an anisotropic Au(110) surface. Due to the one dimensional (1D) geometrical constraint of the surface reconstruction, linear fatty acid molecules (C30H60O2) are decarboxylated and polymerized at their terminal ends at mild temperatures, resulting in the formation of oxygen-free aliphatic hydrocarbons. Different reaction stages of the decarboxylation were monitored by high-resolution scanning tunneling microscopy and X-ray photoemission spectroscopy. Based on density functional theory calculations, a two-step process was proposed for the fatty acid decarboxylation. Our work demonstrates a novel strategy for deoxygenation of fatty acids on a 1D constrained surface as a model catalytic system for producing low oxygen content biofuels.

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Introduction Aliphatic carboxylic acids are important component in biofuels, which are considered as alternative clean and renewable energy sources for economic transportation and ecological manufacture1-6. However, the relatively high oxygen content remaining in biofuels leads to low heat value, low stability and high viscosity7-8. It is a necessity to reduce oxygen content in biofuels, thereby producing high-grade substitutes for traditional fossil fuels. Nowadays, catalytic hydrodeoxygenation (HDO)9-11 is one of the major processes for converting aliphatic acids into oxygen-free biofuels. In this process, additional hydrogen, high temperature or high pressure treatment is required12-14. In order to further decrease the process cost, a more efficient and easy-to-process deoxygenation strategy is sought for. As a powerful technique with atomic resolution, scanning tunneling microscopy (STM) has been widely used to study the physical and chemical processes of molecules adsorbed on a surface. Different surface-supported processes of carboxylic acids, such as self-assembly15-18, conformation switching19, dehydrogenation20 and decarboxylation21 have been studied by means of STM. However, the surface-assisted decarboxylation of aliphatic carboxylic acids, a straightforward pathway to realize deoxygenation, is rarely reported22-24. Here, we report on a comprehensive study on the decarboxylation of aliphatic carboxylic acids. Surface-assisted process of a model aliphatic carboxylic acid molecule (C30H60O2) on an anisotropic Au(110) surface has been investigated with submolecular resolution. It has been found that Au(110) surfaces with missing-row reconstructions exhibit promising effect for selective C−H bond activation, due to the one dimensional (1D) geometric confinement. Linear alkane polymerization have been achieved on Au(110) surfaces through the terminal methyl C−H activation25. Compared with Pt(110), which has similar missing-row reconstruction, the Au(110) surface exhibits relatively weak interactions with dehydrogenated hydrocarbon species, suppressing C−C bond breaking and fragmentation26. Besides linear alkanes, C−H bond activation of oligophenylenes resulting in the preferential formation of linear polyphenyl wires over branched analogous has been demonstrated on Au(110) surfaces27. Here, we investigate thermally activated decarboxylation of fatty acids on Au(110) surfaces at the single-molecular level by means of STM and photoemission spectroscopy. Decarboxylation of aliphatic carboxylic acids occurs at mild temperatures, with the formation of long alkane chains at the surface. Ab initio density-functional theory (DFT) simulations give further insight into the mechanism of decarboxylation.


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Methods The Au(110) single crystal with one side polished was purchased from MaTech GmbH. Sample surface was cleaned by cycles of Argon ions sputtering and vacuum annealing (Sputter: 1.1 kV, 10-20 min. Annealing: 450-500 oC, 8-10 min). The clean Au surface was well examined from STM images. C30H60O2 molecules (Sigma-Aldrich, purity ≥ 98%) were evaporated from a Knudsen cell to the clean Au (110) surface held at RT. STM and XPS experiments were performed in ultrahigh vacuum condition with a base pressure of 1 × 10-11 mbar. The Omicron ultra-high vacuum STM is equipped with a sputter gun, sample heating platform, thermal organic molecules evaporator and a low-temperature scanning chamber. All STM images were obtained under constant current mode using an electrochemically etched tungsten tip at around 78 K. Photoemission spectroscopy experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The detailed description of the ultrahigh vacuum system of this endstation can be found elsewhere28. The C 1s and O 1s XPS spectra were measured with photon energies of 345 eV and 1482.3 eV, and the data analysis was performed using the XPS Peak 41 program with Gaussian functions after subtraction of the Shirley background. Density functional theory calculations were performed using Vienna ab initio simulation package (VASP) with the Perdew-Burke-Ernzerhof exchange-correlation functional. The plane-wave energy cutoff used for all calculations is 400 eV. The van der Waals interactions were considered by using the Becke-Jonson damping DFT-D3 method. The convergence criterion for the forces of all structure relaxations is 0.02 eV/Å. The energy barrier of the phase transition between two surface structures was calculated by using the climbing image nudged elastic band method with six images, and the forces were relaxed to 0.06 eV/ Å.

Results and discussion



Figure 1 | C30H60O2 molecules deposited on Au(110) at room temperature. (a) A clean Au(110) surface with the missing row reconstruction. Brighter dots (indicated by arrow) on the topmost Au atomic rows are impurities. (−1 V, 0.25 nA, 22 nm × 22 nm) (b) C30H60O2 molecules deposited on Au(110). (−2 V, 25 pA, 18 nm × 18 nm) (c) A region with lower coverage, showing the head-to-head configuration of C30H60O2 molecules. Carboxylic groups of C30H60O2 molecules are marked by white dotted rectangles. (2 V, 30 pA, 12 nm × 12 nm) (d) Zoom-in STM image of two molecules with one end (Carboxylic group) looking brighter than the other end (methyl group). (2 V, 30 pA, 4 nm × 4 nm) Fatty acid molecules (C30H60O2) were deposited on the Au(110) surface at room temperature. Figure 1 shows the representative STM images of the clean Au surface and the samples with C30H60O2 deposited at room temperature. As shown in Figure 1a, The clean Au(110) surface exhibits a prominent missing-row reconstruction with parallel atomic grooves (spacing 0.81 nm). Such an anisotropic feature is expected to act as a geometrical constraint on adsorbed molecules and plays an important role on their orientation, diffusion and surface-supported chemical processes. In addition, there are some brighter dots on the topmost Au atomic rows (indicated by arrow), probably originating from impurities such as adsorbed oxygen and segregated carbon atoms. After the deposition of C30H60O2 (115 oC, 1 min), the rod-like molecules with a length of 3.7 nm become visible on the surface, all aligning along the [1−10] direction of the Au(110) surface (Figure 1b to 1d). At one monolayer coverage, the molecules are closely packed on the surface with a distance of ~4.9 Å between the long axes of the neighboring molecules (every five molecules occupying three reconstructive grooves), in accordance with the case of linear alkane


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C32H6625. Figure 1c is the STM image at the region of lower coverage, showing a head-to-head configuration of C30H60O2 molecules. The carboxylic head groups (area indicated with dotted rectangles) exhibit brighter contrast, consistent with previous STM study of carboxylic acids in the literature29-30. A ball-and-stick model of the C30H60O2 molecule is superimposed with the STM image in Figure 1d. Our result indicates that the as-deposited molecules stay intact at the surface with distinct STM feature of the carboxyl group. Furthermore, the reconstruction grooves have a predominant effect on the orientation of the adsorbed molecules.

Figure 2 | Oligomerization of fatty acid molecules at elevated temperatures. (a) C30H60O2 deposited on Au(110) with low coverage (0.15 ML). Molecules are diffusing on the surface with higher residence time surrounding surface impurities (dotted circles). (−1 V, 30 pA, 10 nm × 10 nm) (b) After annealing to 140 oC. Monomers and oligomers are immobilized and aligned in the (1 × 3) reconstructions. (−1.8 V, 50 pA, 14 nm × 14 nm) (c) A dehydrogenated monomer with the –COO− group (rectangle 1 in 2b). (4.0 nm × 1.6 nm) (d) A decarboxylated monomer (rectangle 2 in 2b). (4.0 nm × 1.6 nm) (e) A fully decarboxylated dimer (rectangle 3 in 2b). (7.0 nm × 1.6 nm) (f) A partially decarboxylated dimer with one COO− group left (rectangle 4 in 2b). (7.0 nm × 1.6 nm) The newly formed C−C bonds are shown in blue. Figure 2a shows the STM image of sample with submonolayer coverage (0.15 monolayer). There are fuzzy features and no distinct molecules are observed in the STM image. This result suggests that the molecules are mobile on the surfaces to some extent, similar to the case of linear alkanes25. The mobile feature of the molecules indicates the weak molecule-surface interaction. The brighter regions in the image (dotted circles) indicates relatively longer residence time of the diffusing molecules surrounding the surface defects, probably due to the stronger interactions between the molecules and the surface defects31-32. The as-deposited samples were then annealed to different temperatures and characterized by STM to unveil the thermally activated conversion of fatty acid molecules on the anisotropic Au(110) surface. Significant change was observed on the Au(110) surface after annealing to 140 oC, as shown in Figure 2b. In this stage, most molecules are immobilized and aligned in the newly emerged (1 × 3) reconstructions (see dashed rectangles 1 and 2). Products of dehydrogenation (Figure 2c) and decarboxylation (Figure 2d)



coexist on the surface, both leading to the immobilization of single molecules. The interaction between active COO− and the substrate causes a slight tilting of the COO− head group in the STM image, while the methyl or methylene terminal end exhibits straight and smooth feature. Under 1D constraint, these methylene groups with active C sites can stimulate C−H activation in the adjacent radicals, resulting in linear polymer chains27. Although it has been reported that the dehydrogenation of COOH is a prior process compared with the full dissociation of COOH group22, 33, 35, exclusive dehydrogenation of COOH was not observed in our experiment, probably due to the comparable activation barriers of dehydrogenation and subsequent COO dissociation (see below for DFT calculations). Between two topmost Au atomic rows, there are also dehydrogenated and decarboxylated products located in the deeper (1 × 3) trenches, exhibiting darker and inconspicuous feature (rectangles 5 and 6 in Figure 2b). At the deeper trenches, there are three Au atomic rows missing, two from the topmost atomic layer and one from the layer underlying the topmost layer. Besides the immobilized monomers, early molecular reactions have occurred in Au (110)-(1 × 3) trenches, resulting in oligomers. Dimers and trimers are the major products. On closer examination two types of dimers with a length about 6.5 nm are found, as shown in Figure 2e and 2f (rectangles 3 and 4 in Figure 2b). Dimer in Fig. 2e is a complete decarboxylate with the same terminal feature as the decarboxylated monomers in Fig. 2d. Dimer with a dehydrogenated COO− head group is shown in Fig. 2f. In the middle of the dimer there is a methyl side group originating from the bonding between the decarboxylated terminal and the penultimate carbon in another monomer, which is a common case in alkane polymerization.

Figure 3 | Decarboxylative polymerization of fatty acids after further annealing at 160 oC for 40 minutes. (a) STM image of long polymeric chain alkanes in (1 × 3) reconstructive grooves. (−0.4 V, 0.25 nA, 20 nm × 20 nm) (b) A section of alkane chain with one methyl side group as marked by blue triangle. (−0.01 V, 2.5 nA, 11 nm × 1.5 nm) (c) A section of alkane chain with vicinal methyl side groups (marked by blue triangles) located at the same side of the alkane chain. (−0.4 V, 0.25 nA, 11 nm × 1.5 nm) Further annealing to 160 oC results in the formation of longer polymeric chains (Figure 3). The polymeric chains are adsorbed in the (1 × 3) reconstructive trenches. A periodicity about 0.25 nm is observed for the polymeric chains, corresponding to two CH2 units of polyethylene with a


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zigzag carbon backbone. Due to the mismatch between the periodicities of the polymeric chains and the underlying Au(110) along the [1−10] direction (0.29 nm), a Morié pattern exists on the polymeric chains, resulting in a contrast modulation with a larger periodicity about 1.5-1.7 nm. As shown in Figure 3a, the discontinuous features on the polymeric chains (indicated by arrow) are ascribed to the atomic vacancies in the reconstructive trenches. The existence of polymeric chains was also confirmed by STM manipulation. In our STM manipulation, polymeric chains can be released from the reconstructive trenches, similar with those produced by alkane polymerization in our previous work25. Figures 3b and 3c show the zoomed STM images of sections of the polymeric chains. The feature of methyl side groups can be seen clearly on the polymeric chains: the bright bulges on the edge of the polymers (Figure 3b and 3c, pointed by blue triangles). Such CH3 groups are formed when a penultimate CH2 group is dehydrogenated and involved in the C−C coupling. A careful analysis of their configuration reveals the clue of the formation mechanism of the polymeric products. The branched CH3 groups are in general randomly distributed on the polymers. In some cases, a series of CH3 side groups are located in succession on the edge of the polymeric chains, exhibiting spacing comparable to the molecular length of the fatty acid. A majority (73 %) of the neighboring methyl side groups resulting from the C−C bond formation at the two terminals of the same monomer are located at the same side of the polymeric chain, indicating an odd number of carbon atoms in between. Therefore, we believe decarboxylation is the dominant process in our experiment, instead of the fusion of fragmented alkane chains. Samples annealed above 160 oC show a similar result that the vast majority of molecules have participated in decarboxylative polymerization at this stage and yield long polymeric chains. It has been reported that by using a cooper or silver substrate, metal organic coordination is formed upon annealing instead of C−C coupling. In the case of 2, 6-naphthalenedicarboxylic acid (NDCA) polymerization on Cu(111) and other metal surfaces21, NDCA are assembled on the metal surfaces (Cu and Ag) with initial dehydrogenation of the carboxyl groups at both ends. Further treatment at elevated temperatures leads to polymeric bisnaphthyl−Cu species and eventually poly-naphthalenes. In our work, long hydrocarbon chains are directly formed upon annealing without any organometallic intermediates.



Figure 4 | XPS spectra of the C1s and O1s core-levels at different stages: RT deposition; 140 oC, 160 oC, 180 oC and 200 oC annealing. (a) C1s spectra. The main peak at 284.43 eV and the accompanied shoulder at 283.54 eV are assigned to the C−C/C−H from the molecules beyond the first layer and the first layer molecules, respectively, while the peak at 289.05 eV is assigned to the COOH group. (b) O1s spectra. (c) Oxygen to carbon ratio during the whole reaction process. The decarboxylation process is further confirmed and analyzed by XPS measurements. Figure 4a shows the C1s XPS results for the samples after fatty acid deposition at room temperature and after step-by-step annealing to different temperatures. Molecules are firstly deposited on Au(110) held at RT and then gradually annealed from 140 oC to 200 oC. For the as-deposited sample, the main peak at 284.43 eV is assigned to −CH2 and CH3 component from the C30H60O2 molecules beyond the first layer, while the shoulder with lower binding energy at 283.54 eV is assigned to the molecules from the first layer. In addition, there is a small peak at 289.05 eV originating from the COOH group, in agreement with result reported elsewhere34-35. After annealing to 140 oC, the C1s intensity is significantly weakened due to the thermal desorption of the molecules beyond the first layer. The peak at 283.7 eV is assigned to the molecules from the first layer, while the small shoulder at 284.4 eV is assigned to carbide cluster impurities on the surface. In addition, the peak at 289.05 eV is too weak to be distinguishable. XPS data obtained from the samples after annealing to higher temperatures (160, 180, and 200 oC) show similar results to that after annealing to 140 oC. A straightforward evidence for decarboxylation lies in the XPS spectra of O1s (Figure 4b). There are two peaks: The peak at lower binding energy of 532~532.4 eV is assigned to O=C in the carboxyl group, while the other at 533~533.6 eV is attributed to O−H in the carboxyl group. Similar to the C1s signal, the intensity of the O1s signal undergoes a significant decrease after annealing to 140 oC, due to the desorption of multilayer molecules. Further annealing from 140 oC to 200 oC, the peak intensity gradually drops to a rather low value, which gives an intuitive


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evidence of successive decarboxylation. The trace oxygen content of the samples after annealing to 200 oC may originate from the native contamination of the substrate. The oxygen to carbon ratio at different stages acquired from the XPS results is displayed in Figure 4c. The atomic ratio for the as-deposited sample is normalized to 1/15. There is no significant change for the ratio when annealing the sample to a temperature up to 140 oC, while a sharp decrease takes place after annealing to 160 and 180 oC, indicating intense decarboxylation at such temperatures. Finally, a ratio of 1/37.9 is obtained after annealing to 200 oC.

Figure 5 | Energy diagram of the decarboxylation process from DFT calculations. In the initial state, the intact hexanoic acid molecule is adsorbed in the (1×3) groove of the Au(110) surface with a reference energy of 0.00 eV. (TS1) Transition state 1 where hydrogen atom has detached, 1.36 eV. (IS) Intermediate state with hydrogen migrating for some distance, 0.89 eV. (TS2) COO detaches at transition state 2, 2.10 eV. (FS) Final state with complete decarboxylation, 0.69 eV. We have conducted DFT calculations to gain a deep insight in the decarboxylative process on a Au(110) surface. For simplicity, the C30H60O2 molecule was substituted by a hexanoic acid molecule (C6H12O2). An Au(110)-(1 × 3) containing three atomic layers was built as the substrate. Given the well-recognized deprotonation of carboxylic acids on copper surfaces at RT35 and on silver surfaces upon thermal annealing21, we assume that the interaction between COOH groups and the Au(110) surface is too weak to induce any reaction at RT. The firstly shown evidence of dehydrogenation is the fixation of monomers upon annealing. Hence the first step is to calculate the energy barrier for the dehydrogenation of the carboxyl group. The free energy profile of decarboxylation is shown in Figure 5. In the initial state, an intact hexanoic acid molecule is adsorbed in the (1 × 3) groove of the reconstructed Au(110). The energy barrier for the dehydrogenation step is calculated to be 1.36 eV with the transition state (TS1), in which the hydrogen atom locates on a neighboring bridge site and the deprotonated hexanoic acid molecule almost remains in the same position as before. Upon relaxation, the loosing H atom migrates on the Au(110) surface for some distance until the energy becomes stable, causing an energy drop



from 1.36 eV to 0.89 eV (Intermediate state). In the intermediate state, the hydrogen atom dissociated from the carboxyl group is adsorbed at a neighboring hollow site of the (111) facet. Upon further annealing, the remaining part (COO) of the carboxyl group begins to detach from the chain alkane. There is a second energy climb when COO fully detaches from the molecule, which is effectuated by higher temperature annealing in the experiment. The energy peaks at 2.10 eV (Transition state 2) and finally drops to the final state of 0.69 eV. The de−COO step is endothermic with free energy barrier of 1.21 eV, in good agreement with the energy barrier (1.41 eV) of decarboxylation of TCPB on a Cu(111) surface36. The COO radical moves on the (1 × 3) reconstructive groove and eventually stables itself along (001) direction, perpendicular to the remaining pentane. During the migration of COO, the detached H atom remains on the same hollow site. As annealing hierarchically proceeds, the hexanoic model gets slightly closer to the gold surface (shown in the side view above) manifesting energy stabilization. As mentioned in the work of Senanayake. et al37, the most stable state for a bidentate formate is that both oxygen atoms bond in the Au top sites. Our DF results does not show thorough calculation about the behaviors of loosed OCO but only an explicit schematic for decarboxylation of the carboxylic acid on a Au(110) surface.

Conclusion To conclude, we report on an experimentally observed decarboxylative polymerization of aliphatic acids (C30H60O2) on anisotropic Au(110) surface. STM studies show that the reconstructive 1D atomic grooves and catalytic effect of Au(110) surface provide favorable conditions for linear polymerization. Through stepwise annealing, the H atoms in carboxyl groups detach in the first place followed by desorption of the whole carboxyl group. Polymerization occurs at higher temperature annealing, forming long oxygen-free chain alkanes. High-resolution STM images show details of chain alkanes and polymerization mechanism. XPS measurements intuitively give semi-quantitative analysis of the detachment of carboxyl groups. DFT calculation gives further evidences for the removal of oxygen and atomic behavior of chain alkane through decarboxylation. In our work, we have realized lowering oxygen content in aliphatic acids as raw materials of biofuels in a predictable and selective manner.

Acknowledgement This work was financially supported by NSFC (Project No. 11374374, 11574403) and the computation part of the work was supported by National Supercomputer Center in Guangzhou. We are very grateful to all of the members of NSRL for the help with the XPS experiments.

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Figure 2 | Oligomerization of fatty acid molecules at elevated temperatures. (a) C30H60O2 deposited on Au(110) with low coverage (0.15 ML). Molecules are diffusing on the surface with higher residence time surrounding surface impurities (dotted circles). (-1 V, 30 pA, 10 nm × 10 nm) (b) After annealing to 140 oC. Monomers and oligomers are immobilized and aligned in the (1 × 3) reconstructions. (-1.8 V, 50 pA, 14 nm × 14 nm) (c) A dehydrogenated monomer with the –COO- group (rectangle 1 in 2b). (4.0 nm × 1.6 nm) (d) A decarboxylated monomer (rectangle 2 in 2b). (4.0 nm × 1.6 nm) (e) A fully decarboxylated dimer (rectangle 3 in 2b). (7.0 nm × 1.6 nm) (f) A partially decarboxylated dimer with one COO- group left (rectangle 4 in 2b). (7.0 nm × 1.6 nm) The newly formed C-C bonds are shown in blue. 442x160mm (96 x 96 DPI)


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Figure 3 | Decarboxylative polymerization of fatty acids after further annealing at 160 oC for 40 minutes. (a) STM image of long polymeric chain alkanes in (1 × 3) reconstructive grooves. (-0.4 V, 0.25 nA, 20 nm × 20 nm) (b) A section of alkane chain with one methyl side group as marked by blue triangle. (-0.01 V, 2.5 nA, 11 nm × 1.5 nm) (c) A section of alkane chain with vicinal methyl side groups (marked by blue triangles) located at the same side of the alkane chain. (-0.4 V, 0.25 nA, 11 nm × 1.5 nm) 345x87mm (150 x 150 DPI)



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Decarboxylation of Fatty Acids on Anisotropic Au(110) Surfaces - The

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