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Surface-assisted alkane polymerization: Investigation on structure-reactivity relationship Kewei Sun, Aixi Chen, Meizhuang Liu, Haiming Zhang, Ruomeng Duan, Penghui Ji, Ling Li, Qing Li, Chen Li, Dingyong Zhong, Klaus Müllen, and Lifeng Chi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09097 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Journal of the American Chemical Society

Surface-assisted alkane polymerization: Investigation on structure-reactivity relationship Kewei Sun,1 Aixi Chen,1 Meizhuang liu,2 Haiming Zhang,1* Ruomeng Duan,3 Penghui Ji,1 Ling Li,1 Qing Li,1 Chen Li,3,4 Dingyong Zhong,2 Klaus Müllen4,5 and Lifeng Chi1* 1.

Jiangsu Key Laboratory for Carbon Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials(FUNSOM), Soochow University, Suzhou 215123, P. R. China

2.

School of Physics & State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, 510275 Guangzhou, P. R. China

3.

School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, China

4.

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany

5.

Institute of Physical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz

Keywords: Alkane polymerization, surface reconstruction, low energy electrons, scanning tunneling microscopy

ABSTRACT: Surface-assisted polymerization of alkanes is a remarkable reaction for which the surface-reconstruction of Au(110) is crucial. The surface of (1×2)-Au(110) precovered with molecules can be completely transformed into (1×3)-Au(110) by introducing branched methylidene groups on both sides of the aliphatic chain (18, 19-dimethylidene-hexatriacontane) or locally shifted into (1×3)-Au(110) under exposure to low energy electrons (beam energy from 3.5 eV to 33.6 eV, for alkane dotriacontane). Scanning tunneling microscopy (STM) investigations demonstrate that alkane chains adsorbed on (1×3)-Au(110) are more reactive than on (1×2)-Au(110), presenting a solid experimental proof for structure-reactivity relationships. This difference can be ascribed to the existence of an extra row of gold atoms in the groove of (1×3)-Au(110), providing active sites of Au atoms with lower coordination number. The experimental results are further confirmed by density functional theory (DFT) simulations.

INTRODUCTION Heterogeneous catalysis of organic reactions may comprise different active sites such as the interface between nanoparticles and the oxide support,1 surface atoms in certain oxidation states,2 or spatial 3 arrangement. Obtaining information on the nature of active sites is, thus, fundamentally important for a mechanistic understanding of catalysis and ultimately for a rational design of catalysts. Specifically, this information is relevant to structure-sensitive catalytic processes, such as most gold-catalyzed reactions.4,5 Although gold is chemically inert in most chemical reactions, surface gold atoms and gold nanoparticles supported on metal oxide have been found catalytically active in some cases, e.g., low-temperature CO oxidation5 and self-assembly of chemisorbed thiols.6 While a systematic analysis of a gold-catalyzed reaction is often obstructed by diverse experimental results in real catalyst systems, surface science experiments of well characterized model systems are extremely helpful toward an understanding of the catalytic capacity of gold.4b,7 It has been concluded that gold catalysis in CO oxidation

originates from highly uncoordinated atoms.4b,5f,8 Compared with tremendous efforts toward gold catalysis in CO oxidation, however, heterogeneous gold catalysis of other reactions (e.g., including C-H bond activation) has been rarely dealt with at the molecular level.2c,8,9 We have recently reported the selective C-H bond dissociation on reconstructed Au(110) surfaces, thus, revealing an unexpected catalysis of alkane polymerization by metallic gold under mild conditions (below 200°C).9a The length of produced polyethylene chains is up to hundreds of nanometers, providing a new concept for direct C-C coupling and on-surface synthesis of functional polymers.10,11 However, the real active sites of Au(110) remain unclear as the reconstructed structure of Au(110) has been fully transformed from a (1×2) missing row reconstructed structure (hereafter (1×2)-Au (110)) to a (1×3) missing row reconstructed structure (hereafter (1×3)-Au(110)) after alkane polymerization. The role of (1×3)-Au(110) is ambiguous since one cannot exclude the possibility that the interaction between polymerized alkane and gold provides sufficient energy to reconstruct the surface from

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the (1×2) to the (1×3)-Au(110). Further motivation for studying the active sites of Au(110) in C-H bond activation comes from the selective dehydrogenation of small alkanes, specifically under mild conditions, which is of great industrial importance.2e To understand the role of the surface, a comparison of the n-alkane reactivity between the (1×2) and the (1×3)-Au(110) surfaces is necessary. Since the (1×3) missing row reconstruction is not thermally stable for a clean Au(110) surface, it is technically difficult to obtain a self-assembled n-alkane on the (1×3)-Au(110) surface. Herein, we report a structural control of (1×2)-Au(110) surfaces precovered with alkane molecules in an attempt at establishing a structure-reactivity relationship for alkane polymerization. Prior to the polymerization of alkyl chains the (1×2)-Au(110) surfaces can be either wholly shifted into (1×3)-Au(110) by introducing branched methylidene groups into the aliphatic chain (for the case of 18, 19-dimethylidene-hexatriacontane), or locally shifted from (1×2) to (1×3) at the spot radiated by low energy electrons (for the case of dotriacontane). Scanning tunneling microscopy (STM) investigations reveal that molecules adsorbed on (1×3)-Au(110) are more reactive than those adsorbed on (1×2)-Au(110). Combined with density functional theory (DFT) simulations, we demonstrate that the (1×3) reconstructed structures, which are definitely induced before the polymerization, are more active than the (1×2) reconstructed structure of Au(110) in C-H bond activation. Such an activity of the (1×3) reconstruction must originate from the density of step atoms in the channels. The combination of low energy electrons (LEEs) induced regioselective structural control with coupling reactions of adsorbed molecules opens up a new protocol for the on-surface synthesis of functional polymers.11

shift from (1×2) to (1×3)-Au(110). As depicted in Figure 1b, the STM image reveals a large-scale structural change of the Au(110). The dominant feature of Au(110) is the (1×3) missing row reconstructed structure, except several (1×2) reconstructed rows. Thereby, DMH molecules are adsorbed orderly in the (1×3) grooves in comparison with the random adsorption on the (1×2)-Au(110). As the mild annealing of the substrate is not sufficient to trigger C-H bond dissociation, the aliphatic chains of DMH molecules are chemically unchanged. The existence of unreacted alkane chains is further corroborated by a magnified STM image, as displayed in the inset of panel b. A few of alkane chains are found to be mobile in the (1×3)-Au(110) grooves, showing an enhanced noise level in the image (black arrows).

RESULTS AND DISCUSSION The reconstruction of surface atoms occurs on all the low Miller index facets of gold. As for Au(110), a (1×2) missing row reconstructed structure is thermally stable after a fresh thermal annealing in ultra-high vacuum (UHV). Although the reconstructed structure can be preserved upon the adsorption of organic molecules, it has been reported that physisorbed molecules such as glycine can stimulate a structural change of Au(110), e.g., from (1×2) to (1×3)-Au(110).12 We choose 18, 19-dimethylidene-hexatriacontane (DMH) with two branching points as a precursor to explore the molecule-induced structural shift of Au(110). We reasoned that alkyl chains of DMH would allow us to investigate the alkane polymerization on Au(110) similar to the previous case.9a Shown in Figure 1a is an STM image (77 K) of the (1×2)-Au(110) surface covered by randomly adsorbed DMH molecules. The position of branched methylidene groups is displayed as a bright bump in the middle of each molecule. While the (1×2) reconstructed structure of Au(110) is preserved after the adsorption of DMH at room temperature, a mild annealing at 350 K for 10 hrs (or at 400 K for 10 min) gives rise to a structural

Figure 1 C-H bond activation on (1×3)-Au(110). a) STM image (Vb = -1 V, It = 10 pA, 77 K) of randomly adsorbed DMH on (1×2)-Au(110). The sample was prepared by depositing DMH molecules on freshly annealed Au(110) held at room temperature. b) STM image of DMH molecules adsorbed on (1×3)-Au(110) (Vb = -1 V, It = 25 pA, 77 K). The sample was obtained by annealing as-prepared DMH/(1×2)-Au(110) at 350 K for 10 hrs. c) Polymerized DMH on (1×3)-Au(110) prepared by annealing the DMH/(1×3)-Au(110) sample at 450 K for 30 minutes (Vb = 1 V, It = 100 pA, 77 K). d) Magnified STM image of the area marked in panel c), showing detailed structural information on polymerized DMH chains (Vb = 1 V, It = 200 pA, 77 K).

Further annealing the DMH/(1×3)-Au(110) at a higher substrate temperature (450 K) leads to the surface-assisted polymerization of DMH. Displayed in Figure 1c is an STM image of the sample annealed at 450 K for 30 min. The polymerized structures of DMH are recognizable from a magnified STM image, as depicted in Figure 1d. In polymerized DMH chains several white


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Journal of the American Chemical Society arrows denote the position of branched methylidene groups that can be considered as an internal marker for a better illustration of the polymerized structure. Between two internal markers, a knot, indicated by a red arrow, is frequently observed in the STM image, representing the coupling between the terminal CH3 of one DMH molecule and the nearest CH2 of the neighboring DMH. Such a connection is similar to that obtained in the polymerized structure of n-dotriacontane on Au(110) and named as the terminal-penultimate C-C coupling.9a The substrate temperature (450 K) applied for the polymerization of DMH on the (1×3)-Au(110) is ca. 20 K lower than the typical temperature (470 K) used for n-dotriacontane on the (1×2)-Au(110) with the same instrument (see supporting information, Figure S1). As other parameters, such as heating mode, annealing time, Au(110) substrate and vacuum, are constant in both cases, the lower on-set temperature for the reaction suggests that the C-H activation of alkane chains is more efficient on (1×3)-Au(110). However, considering the difference in precursor molecules and small difference in triggering temperature, a direct comparison of the catalytic activity of (1×2) and (1×3)-Au(110) in the same crystalline facet would be more convincing. In order to directly compare the catalytic difference of (1×2) and (1×3)-Au(110), a local structural control under radiation with low energy electrons (LEEs) employed for the dotriacontane (C32H66) precovered Au (110) substrate under a mild thermal annealing (ca. 400K). Displayed in Figure 2a is a typical STM image obtained at 4.6K on the (1×2)-Au(110) surface covered by ca. 0.8 monolayer (ML) dotriacontane (C32H66). The (1×2) missing row reconstruction is also confirmed by the pattern of low energy electron diffraction (LEED, upper right, Figure 2a). While the self-assembled monolayers (SAMs) of dotriacontane are typically composed of close-packed 4 nm long rods, some longer rods (>16 nm) are occasionally observed on the (1×2)-Au(110) surface (the red arrow marked in Figure 2a) indicating a high mobility of dotriacontane along the [1-10] direction. The mobility of C32H66 at 4.6 K suggests a relatively weak molecule-substrate interaction between a dotriacontane molecule and the (1×2)-Au(110). This is consistent with our previous result that dotriacontane molecules are not visible by STM investigations on the (1×2)-Au(110) (coverage < 1ML) maintained at 77 K.9a Detailed packing structure of adsorbed dotriacontane can be collected from a magnified STM image, as depicted in Figure 2b. The channel of (1×2) reconstructed ridges are highlighted by dotted lines for clarity. It is obvious that one channel must contain two parallel packed molecules. Accordingly, we establish a model of self-assembled dotriacontane on the (1×2)-Au(110) surface, as shown in Figure 2b, lower left. Two dotriacontane molecules are adsorbed in one groove of (1×2)-Au(110) with an intermolecular distance of ca. 0.4 nm which is somewhat shorter than the typical intermolecular distance (ca. 0.48 nm) observed in the SAMs of dotriacontane on Au(111).13 The slight difference in intermolecular distance might be caused by an

unfavorable adsorption geometry of dotriacontane in the groove of Au(110) and could account for its high mobility on (1×2)-Au(110) at 77 K.

Figure 2. Local structural control of dotriacontane precovered Au (110) surfaces via radiation with low energy electrons. (a) STM image (Vb = 0.25 V, It = 15 pA, 4.6 K) of the (1×2)-Au(110) surface covered by ca. 0.8 ML dotriacontane. (b) A close-up image of a domain boundary showing the detailed packing structure of the SAMs (Vb = -0.5 V, It = 50 pA, 4.6 K). The (1×2) reconstruction was preserved after the adsorption of dotriacontane molecules as confirmed by LEED. Both LEED patterns and the structural model of the SAMs on the (1×2)-Au(110) are displayed on the right side of the image. (c) STM image (Vb = -1 V, It = 200 pA, 4.6 K) of the (1×3)-Au(110) prepared by radiation assisted annealing. Inset: LEED patterns recorded after the radiation assisted annealing. (d) Magnified image (Vb = -0.5 V, It = 200 pA, 4.6 K) of a region containing both (1×3) and (1×2) structures. e) High resolution STM images (I: Vb = -1 V, It = 50 pA, 4.6 K, II: Vb = -1 V, It = 10 pA, 4.6 K, III: Vb = -1 V, It = 200 pA, 77 K) of widened alkane molecules obtained by thermal annealing with and without the radiation of LEEs (I: beam energy, 5 eV, II: beam energy, 10 eV, III: beam energy, 0 eV).

To locally change the reconstructed structure of Au (110) a thermal annealing combined with a radiation by low energy electrons (LEEs, 33.6 eV) was applied to the dotriacontane/(1×2)-Au(110) substrate. When a distinct change in LEED pattern along [001] direction was observed during the annealing (e.g., 400 K, 1min), subsequent STM investigations were performed to probe the spatial distribution of the newly formed (1×3) reconstructed structures. Figure 2c displays the STM image acquired in the region where the molecules have been exposed to the low energy electrons during annealing. While a few (1×2) reconstructed structures can still be recognized from the image, the dominant structures of the Au(110) are (1×3) missing row reconstructed structures with typical channel widths of 1.2 nm. A gradual change from the (1×3) to the (1×2) reconstructed structures along the central region radiated by LEEs can be concluded from STM results (see supporting information, Figure S2). This suggests that the



(1×3) reconstructed structures result from the radiation with LEEs in the presence of the molecules. While molecular kinks are recognizable in a magnified STM image (see Figure 2d), the structural change of reconstructed Au(110) can be explained by the widened geometry of dotriacontane molecules. As the annealing temperature (400 K) applied for the structural control is significantly lower than the threshold temperature (470 K) for alkane polymerization, dotriacontane molecules are supposed to be chemically unchanged. However, the existence of molecular kinks in the (1×3) grooves (see Figure 2d) also clearly indicates a structural change of alkane molecules. It is therefore necessary to explore whether the methyl groups of dotriacontane are preserved after the radiation by LEEs. Figure 2e-I, II display STM images of the (1×3)-Au(110) prepared by a mild annealing (400 K) combined with radiation by LEEs at a beam energy of 5 eV (2e-I) and 10 eV (2e-II). The image of Figure 2e-III is occasionally observed in the sample only annealed at higher temperature (470 K) without radiation. Similar molecular kinks are observable in high-resolution STM images of each sample as highlighted by red ellipses, suggesting that the molecular kinks are not directly resulting from radiation. To further clarify this point, we conducted the analogous experiments on Au(111). On the basis of these experimental results, we can safely say that the kinks of dotriacontane cannot originate from the electron-induced dissociation (see point 4 in SI and Figure S3). Considering that the termini of most dotriacontane molecules are recognizable from STM images (Figure 3e-I, II, III, Figure S3), one can conclude that the methyl groups of dotriacontane remain chemically unchanged.

Figure 3 Comparison of the reactivity of dotriacontane on (1×2) and (1×3)-Au(110). (a) STM image (Vb = 1 V, It = 200 pA, 77 K) of completely polymerized dotriacontane in areas of the (1×3)-Au(110). (b) Typical STM image (Vb = -0.5 V, It = 200 pA, 77 K) obtained in a (1×2)-Au(110) region which was located far from the region radiated by LEEs. Only a few molecules have been polymerized (marked by the white arrow). (c) STM image (Vb = -1 V, It = 200 pA, 77 K) obtained near the region radiated by LEEs, showing the coexistence of pristine and polymerized dotriacontane. The asterisks mark the ridge of reconstructed Au(110) for a better view.

When the annealing temperature was lower, e.g., 400 K, than the triggering temperature (ca. 470 K) for alkane polymerization, the reconstruction of the Au(110) from (1×2) to (1×3) only occurred in the area radiated by low energy electrons. This was confirmed by both LEED patterns and STM images. In this way, one can artificially introduce a structural difference on one crystalline facet

and explore the structure-activity relationship on the Au(110) surface. Figure 3a presents the STM image obtained on the (1×3) region of an as-prepared sample after annealing at about 450 K for 30 min. Most alkane molecules are polymerized in the (1×3) grooves which is consistent with previous results for DMH/(1×2)-Au(110) under annealing conditions at 450 K for 30min. Since the substrate temperature (450 K) is slightly lower than the typical temperature (470 K) for alkane polymerization, only few dotriacontane molecules are found to be polymerized in the (1×2) region (see Figure 3b, highlighted by a white arrow). Because the whole sample was annealed homogeneously through a PBN heater, the STM results strongly suggest that alkane molecules adsorbed in the (1×3) region are more reactive than those adsorbed in the (1×2) region. Such a difference in reactivity becomes even more obvious in the transition area from the (1×3) to the (1×2) region. As depicted in Figure 2c, both the (1×3) and the (1×2) reconstructed structures are observable in the relevant STM image. Molecules adsorbed on the (1×2) area can be assigned to the individual dotriacontane molecule. In contrast, molecules in the (1×3) area have much longer chain lengths with randomly distributed dark spots (see the inset of Figure 3c) which is similar to the polymerized C32H66 reported in our previous work. The dark spots originate from the missing gold atoms underneath the polymerized chains.9a The different reactivity of dotriacontane on the (1×2) and the (1×3) areas is thus well documented by STM investigations in the transition area.

Figure 4 Energy diagram for C-H activation of n-hexane on reconstructed Au (110) surface. The calculated activation barrier (2.4 eV) of n-hexane on (1×2)-Au(110) (a) is 1.0eV higher than that of n-hexane on (1×3)-Au(110) (b).

To further validate the difference in reactivity of dotriacontane on the (1×2) and the (1×3)-Au(110) surfaces, DFT calculations on the activation barrier of C-H bond dissociation for n-hexane on the reconstructed Au(110) surface have been performed by using the climbing image nudged elastic band (CI-NEB) method. The initial state (IS) is proposed to be a flatly adsorbed n-hexane on the reconstructed Au(110) surfaces, and the final state (FS) a chemically adsorbed n-hexyl radical in the groove after the C-H bond dissociation of the methyl group. Since the C-H bond dissociation of the methyl group plays a critical role in alkane polymerization, the activation energy from IS to FS should determine the reaction rate of alkane polymerization on Au(110). As depicted in Figure 4, the activation barrier of C-H bond dissociation for n-hexane


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Journal of the American Chemical Society on (1×2)-Au(110) is 2.4 eV (see Figure 4a) which is 1.0 eV higher than that of C-H bond dissociation on (1×3)-Au(110) (1.4 eV, Figure 4b). The lower activation barrier for n-hexane on (1×3)-Au(110) is consistent with the experimental result that C32H66 adsorbed on the (1×3)-Au(110) are more active than molecules adsorbed on the (1×2)-Au(110). By applying molecular modification and LEEs assisted annealing, the reconstruction of molecule-precovered Au(110) appears to be well controlled from naturally-formed (1×2) to (1×3) reconstructed structures. The sample endowed with the structural difference allows one to establish a structural-activity relationship for alkane polymerization. Both STM investigations and DFT simulation indicate that the (1×3)-Au(110) surface is more active in alkane polymerization. This conclusion, however, raises a further question: why is the (1×3)-Au(110) surface more active?

Figure 5 Structural comparison between (1×2) and (1×3) missing row reconstructed Au(110). (a) A side view (in the ( 1 10) plane) of the (1×2)-Au(110). (b) A side view of the (1×3)-Au(110) with an extra gold row (marked by red color) in the grooves. A dotted rectangle along [112] direction marks a close-packed (11 1 ) plane. (c) A top view of the (11 1 ) plane. The atomic size of the first layer has been magnified for a better view.

A structural comparison between the (1×2) and the (1×3)-Au(110) surface helps us to understand the role of the substrate. As illustrated in Figure 5, panel (a) and (b) are structural models of (1×2) and (1×3)-Au(110). Except for an extra gold row (marked by red color, panel b) in the groove of the (1×3)-Au(110), there are no intrinsic differences in chemical properties with respect to the coordination number of gold atoms. DFT simulations on the activation barrier for n-hexane on the normal (1×3)-Au(110) (without the extra gold row in the grooves) also support the conclusion that there are no remarkable differences between the (1×2) and the normal (1×3)-Au(110) (see Figure S4 in the supporting information). Therefore, it is safe to conclude that the difference in reactivity between the (1×2) and the (1×3)-Au(110) surfaces originates from the extra gold atom rows. Since the atoms consisting of the extra row have lower coordination numbers than other atoms on the surface, these atoms (red color) furnish the (1×3)-Au(110) surface with chemically more active sites. In general, gold atoms with lower coordination number are considered as active sites in catalysis of, e.g., CO oxidation.4b,7,8 Recalling that the extra gold rows can always be observed under polymerized alkane chains,9a these atoms can be highlighted as key sites in C-H bond activation. The role of the extra gold row can also be described from another perspective: as marked by the dotted rectangle along the

[112] direction in panel b, the extra gold row makes up a step edge of a close-packed Au(11-1) plane (see Figure 5c). It is well accepted that atoms on step edges are generally more active due to the lower coordination number and can thus provide a higher adsorption energy for molecules.

CONCLUSIONS In summary, we have investigated the polymerization of aliphatic chains (DMH and dotriacontane) on Au(110) surfaces by means of STM, LEED and DFT simulation. Two approaches for the structural control of reconstructed Au(110) have been successfully developed by either introducing branched methylidene groups into an aliphatic chain (for DMH) or applying LEEs-assisted annealing (for dotriacontane). By structural control of the Au(110) precovered with molecules, we have explored the structure-reactivity relationship for alkane polymerization on Au(110) surfaces. Both STM investigations and DFT simulations indicate that molecules adsorbed on the (1×3)-Au(110) substrate are more active than molecules on (1×2)-Au(110). We attribute such a pronounced difference in reactivity to the presence of the extra gold row in the grooves of the (1×3)-Au(110), providing active sites of gold atoms with lower coordination numbers. On the basis of such a deeper understanding, other surface supported polymerizations should be performed in (1×3)-Au(110) grooves with selective C-H bond breaking. Additionally, LEEs induced regioselective structural control of Au(110) and subsequent polymerization of adsorbed molecules can develop into a new technique for the patterning of on-surface synthesized polymers.

ASSOCIATED CONTENT Supporting Information Experimental methods, the polymerization of dotriacontane on Au(110), gradient structural change from (1×3) to (1× 2)-Au(110), DFT calculations, LEEs assisted annealing of Au(110) and Au(111), the synthesis of DMH and Figures S1−S6. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 21527805, 21673154 ， 11374374 and 21790053) and the Ministry of Science and Technology (2017YFA0205002). We also thank the Collaborative Innovation Center of Suzhou Nano Science & Technology, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. K.M. acknowledges support from



the Johannes Gutenberg University via a Gutenberg Research Fellowship.

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