Heterogeneous Catalysis âOn Demandâ: Mechanically Controlled

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Heterogeneous Catalysis “On Demand”: Mechanically Controlled Catalytic Activity of a Metal Surface Tomasz Mazur, Slawomir Lach, and Bartosz A. Grzybowski ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15253 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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ACS Applied Materials & Interfaces

Heterogeneous Catalysis “On Demand”: Mechanically Controlled Catalytic Activity of a Metal Surface. Tomasz Mazur†, Slawomir Lach †, Bartosz A. Grzybowski* IBS Center for Soft and Living Matter and the Department of Chemistry, UNIST, Ulsan, South Korea

Supporting Information Placeholder ABSTRACT: Metal surface passivated with a tightlypacked self-assembled monolayer, SAM, can be made catalytically active upon metal’s mechanical deformation. This deformation renders the SAM sparser and exposes additional catalytic sites on the metal’s surface. If the deformation is elastic, return of the metal to the original shape “heals” the SAM and nearly extinguishes catalytic activity. Kelvin Force Microscopy and theoretical considerations both indicate that the catalytic domains “opening up” in the deformed SAM are of nanoscopic dimensions.

Catalysts that can be turned “on” and “off” by external stimuli have become the focus of recent research and are often considered as control elements of futuristic chemical systems/networks, in which reaction sequences could be controlled by the sequences of the stimuli applied1,2,3. To date, “switchable” homogeneous catalysts have been demonstrated based on photoresponsive moieties (azobenzenes4,5,6,7, dihydrophenazines8, bi-cyclopentylidenes9,10), macromolecular complexing agents (e.g., β-cyclodextrins11,12, cavitands13) as well as various metalorganic scaffolds whose conformations and catalytic properties depend on pH14, intramolecular coordination15, ligand binding16, dimerization17, and more (for an excellent recent review, see Blanco et al.18). In heterogeneous catalysis, our group demonstrated a system in which the catalytic activity of metal nanoparticles depends on their aggregation which, in turn, is controlled by photoisomerizable surface ligands19. A separate effort has been devoted to the modulation of catalytic activity by mechanical stimuli. In particular, the group of Sijbesma developed a range of polymers with covalently attached N-heterocyclic carbinebased catalytic moieties and coordinated to, for instance, silver(I)20 or ruthenium(II)21,22 – application of either a mechanical force or ultrasound caused the scission of coordination bonds, thus activating the catalysts towards various types of reactions (e.g., transesterification, ringclosing metathesis, or ring-opening metathesis polymerization). Perhaps not unexpectedly, similar mechanochemical phenomena have not been extended to materials such as metals that are much harder to deform and cannot be easily made catalytic/non-catalytic based by the application of mechanical forces alone.

Figure 1. Figure 1. The principle and realization of mechanically controlled heterogeneous catalysis. (a) A thin platinum rod covered with a tightly-packed SAM of alkane thiolates is catalytically inactive towards hydrogen peroxide present in the surrounding medium. (b) When, however, the rod is slightly bent, the SAM becomes sparser and the Pt-catalyzed decomposition of H2O2 produces oxygen bubbles. The arrow between schemes (a) and (b) means that the activation/exposure of the metal surface is reversible, provided the deformation of the rod is small. (c) The elements of the device used for precise rod bending, including the Thorlabs MTS50-Z8 motorized stage, set of custom designed 3D printed elements, and a glass capillary. (d) The fully assembled setup. (e) The SolidWorks design and (f) the experimental top views of the bending device. The SAM-

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covered Pt wire is pressed (in the images, from right to left) against a vertically positioned glass capillary (visible as a small circle in the middle). (g) Experimental images of the platinum rod covered with the SAM of 1-naphtalenethiol, immersed in the 0.5% H2O2 solution (changed every 20 min, at every bending increment ∆L, to ensure that the concentration of H2O2 remains constant). In the initial state (left), the rod is unreactive. Bending of the rod (middle) exposes catalytic surface and initiates reaction, which manifests itself by the formation of oxygen bubbles. (right) When the compressive force is removed and the rod unbends, catalysis ceases.

Yet, as we demonstrate here, catalytic metals covered with self-assembled monolayers, SAMs23,24, can also serve as rudimentary switchable catalysts. Specifically, we describe a system in which elastic deformation of the metal surface creates additional surface area and effectively makes the SAM supported by this surface sparser (but not “cracked” at the mesoscopic level), allowing access of reagents to the metal’s catalytic surface (Figure 1). Remarkably, when the load is removed and the metal returns to its original shape, the protective SAM “heals” and catalysis nearly ceases. These phenomena are reproducible in the Hookean regime and suggest a new way of controlling heterogeneous catalysis on deformable surfaces that can be stabilized with protective monolayers. The catalytic element in our experiments was a platinum wire 250 µm in diameter (Sigma Aldrich, cat. # 267171). The wire was cut into ~ 2 cm long segments which were then thoroughly cleaned by washing in piranha solution (96% H2SO4:30% H2O2, 3:1, v/v) for at least 30 min, followed by mechanical polishing with an abrasive paper (increasing paper’s gradation stepwise from P600 to P1200) and then with a polishing paste (Buehler, particle size 0.05 µm). Next, the wires were sonicated in H2O (10 min), immersed in diluted (30%) aqua regia solution (HNO3:HCl, 1:3, v/v) for 30 sec, and finally sonicated in H2O and in ethanol for 10 min each (see Figure S1 for images of wires at different stages of polishing/cleaning). Thiol monolayers were prepared according to a previously reported procedure25, by immersing the Pt rod in 16 mM ethanolic solutions (200 proof, SigmaAldrich, degassed for 30 min prior to use) of various thiols for 14 days. Subsequently, the wires were thoroughly rinsed and kept immersed in ethanol. Five thiols were tested: 1hexanethiol, 1-dodecanethiol, 1-octadecanethiol, thiophenol, and 1-naphtalenethiol. Of these, 1-naphtalenethiol was chosen as the SAMs it formed prevented any detectable reactions on unbent wires (cf. below and Figure S2 in the Supplementary Information, SI). Several Pt-catalyzed reactions were also investigated – decomposition of H2O2, decomposition of NaBH4 coupled with 4-nitrophenol reduction, and decomposition of formic acid (cf. SI, Section S4 for details). Of these, simple decomposition of hydrogen peroxide on

platinum, 2 → 2 + ( ) , was chosen as it evolved easily detectable oxygen bubbles off any portions of the wires that were not protected by the SAM. Experiments described in the following are therefore for the system of a Pt wire covered with 1-naphtalenethiol and, unless stated otherwise, immersed in a 0.5% solution of H2O2.

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The key assumption of our work is that it should be possible to control catalytic activity by bending the SAMcovered Pt wire, thus changing the surface area of the metal and “opening up” additional catalytic surface (Figures 1a,b). When the bending is very pronounced and/or uncontrolled, however, the SAM is permanently damaged, and the Pt surface remains exposed and catalytic even when the macroscopic shape of the wire is restored (see Figure S3) – that it, it is possible to switch catalysis “on” but not back “off”. With this in mind, we constructed a mechanical system that controlled the wire’s bending precisely (Figures 1c-f). As key elements, this system comprised a 3D-printed (3Dison Rokit Pro printer; Solidworks software) “wedge” attached to a motorized translational stage (Thorlabs MTS50-Z8) offering 0.05 µm precision of motion. When the stage moved, the wedge pressed and bent the wire against a vertically positioned glass capillary (diameter 1.5 mm); when the stage was retracted, the wire was unbending. The bendingunbending was reversible in the Hookean/elastic regime (up to Lmax ~ 0.8-1.0 mm motion of the translational stage). Figure 1g illustrates the control of the catalytic activity thus achieved – the unbent rod in the left portion of the figure is not evolving any oxygen bubbles, even if kept in the peroxide solution for tens of minutes. Then, the rod is gradually being bent – by moving the translational stage at increments of ∆L = 100 µm – and kept at each position for at least 20 min. Initially, no bubbles appear but the catalysis finally kicks-off at Lcrit = 0.6-0.8 mm. Retraction of the translational stage and unbending of the rod returns the system to the non-catalytic state (or nearly non-catalytic, as some sporadic, small bubbles are observed on some – but not all – wire samples; see below). In order to quantify these effects, we studied reaction kinetics at the minimal bending of the wire, at which oxygen bubbles appear (i.e., at Lcrit). The amount of the evolved oxygen was determined from the sizes of the bubbles, using the Laplace equation, pi = 2γ / R, where pi stands for the pressure inside of the bubble, γ ~7.28·10-2 N/m is surface tension of the air-water interface, and R is bubble’s radius. The semi-logarithmic plot in Figure 2a evidences a linear dependence between the logarithm of the concentration of the peroxide (obtained from the measured concentration of the evolved oxygen via reaction stoichiometry) and time – that is, the reaction is first order in hydrogen peroxide, as indeed, expected26. Next, we confirmed that the catalytic activity is proportional to the surface area that is being exposed during bending. In these experiments, we first established the reaction rate at the displacement Lcrit (cf. above) and gave it a reference value of 100%. We then increased the degree of bending until the limit of the elastic regime. For each deformation, L, we used Finite Element Modelling, FEM (see SI, Section S5), to calculate the newly created surface area A(L). When the reaction rate was plotted against A(L), the dependence was not only linear (as should be expected27) but close to the y = x line (i.e., with slope was close to unity extrapolating to (0,0); Figure 2b) indicating that all the newly created area is available for catalysis.


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ACS Applied Materials & Interfaces when the catalytic activity can be reversibly turned “on” and “off”. The data in Figure 2c is for six different wires, each subject to three consecutive cycles of elastic bending/unbending. As seen, in their initial states all wires were catalytically inactive. After the first bending to Lcrit ~ 0.6 mm, the rows became catalytically active, with the value of the average rate marked in the plot as 100% reference. After the first unbending, the catalysis nearly ceased with average rate dropping to 11%. In fact, three out of the six rods tested were fully reversible, meaning that they showed no catalysis at all – the non-zero rate is due to the fact that the formation and quality of the SAM varies between the Pt-wire samples. Second bending of the wire restored the catalytic activity to roughly the previously observed levels. On the second unbending, however, the catalysis could not be completely switched-off and remained at ~ 50%, signaling some irreversible deterioration of the SAM (and possibly the wire’s deformation becoming plastic rather than elastic). Finally, the third bending caused activity increase but the activity difference between the “straight” and “bent” states decreased. In principle, the switching described above could be ascribed to the changes in the properties of the Pt alone and independent of the SAM. In fact, it is known that catalytic metals under strain can change grain structure (or even electronic structure28), and can become more active. To examine the influence of such effects, we performed a series of control experiments with bare Pt wires. We had to decrease the concentration of H2O2 since at 0.5% (as in the experiments with SAM-covered samples), the large numbers of rapidly forming oxygen bubbles precluded meaningful quantification of differences between straight vs. slightly bent wires.

Figure 2. Rates of the mechanically-controlled decomposition of hydrogen peroxide. (a) The semi-logarithmic concentration-vs-time plot evidences first order kinetics. The data are for the minimal bending of the wire at which evolution of the oxygen bubbles is observed. Error bars correspond to standard deviations from six independent experiments. The reaction rate constant determined from the slope of the best-fitting line is k = 1.23·10-4 s-1. (b) Approximately linear dependence of relative rates of catalysis on the additional surface area created during bending. Surface areas were calculated by Finite Element Modelling (see main text); additional surface area at the deformation at which first bubbles are observed corresponds to 100%. (c) Relative rates of the catalytic activity during wire’s sequential bending and unbending. The value for the first bending is taken as 100%. For the first unbending, the small red markers indicate rate values for individual wires (with three wires being completely reversible and showing no catalytic activity). All error bars are standard deviations from six independent experiments.

With these preliminaries, we examined whether and

Figure 3. Control experiments. (a) Decomposition of hydrogen peroxide on straight vs. bent bare-Pt wires. Concentration of [H2O2] was 0.005% and bending was kept within the same regime as for the



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SAM-covered samples (motion up to Lmax ~ 0.6-0.8 mm). The semilogarithmic concentration-vs-time plot evidences first-order kinetics with the reaction rate constants determined from the slope. Error bars are based on six independent experiments. (b) Decomposition of hydrogen peroxide on similarly bent (Lmax ~ 0.6-0.8 mm) bare-Pt vs. SAM-protected wires. Concentration of [H2O2] was 0.3% and chosen to produce, on similar time scales, some bubbles on the SAMprotected wire while not producing excessive bubbles (obscuring quantification) on bare-Pt samples. Error bars are based on three independent experiments.

Such a quantification was possible when the concentration of H2O2 was lowered by two orders of magnitude – under these conditions29, the rate constant for the peroxide decomposition reaction was k = 4·10-4 s-1 for straight wires vs. k = 7·10-4 s-1 for bent ones (Figure 3a). These rate values are quite similar and cannot account for the drastic none-to-full-catalysis change observed with SAM-protected Pt. We observe that catalytic activity did not decrease when the bent bare-Pt wires were unbent – that is, the bare metal surface cannot “heal” itself like the SAM-covered one. Another important comparison to make is between similarly bent bare-Pt and SAM-protected wires. Since the applied strains are on the order of 1%, one would expect the net catalytic activity of the latter to be, at most, 1% of the bare-Pt control. Indeed, measurements quantified in Figure 3b confirm that the activities differed by slightly more than two orders of magnitude with rate constants k = 3.79·10-3 s-1 for bare-Pt vs. k = 3.62·10-5 s-1 for the SAM-protected samples. With these considerations in mind, one might envision three general scenarios for the deformation-driven “opening

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surface area created over the deforming portion of the wire is only a fraction of a percent (cf. Figure S7) of the original area, and the average increase in thiol-to-thiol distance would thus be no more than ~ 1%. Since the thiols in the tightly-packed SAMs are separated23,24 by ~0.5 nm, this increase would translate into few picometers only, and much too small to open a “gap” into which a molecule of H2O2 could fit. To distinguish between the remaining scenarios (2) and (3), we considered a number of experimental techniques (e.g., ellipsometry, confocal microscopy, atomic force microscopy, see also SI, Section 6), ultimately choosing Kelvin Probe Force Microscopy, KPFM, which is known30.31 to provide high contrast of surface potential between regions of unprotected metal and regions of metal covered with thiolate SAMs. For the ease of fabrication and availability of relevant background studies30,31, we chose an analogous system of 1-naphtalene thiols on gold rather than on platinum. We prepared a deformable substrate by spincoating PMMA (1 min. at 2,000 r.p.m.; Sigma-Aldrich, average MW ~120,000, cat. # 182230) onto 250-µm-thick, market-grade polyethylene substrate, and e-beam depositing 100 nm of Au. When this planar surface was bent, we had only one radius of curvature to consider (as opposed to two radii for a thin wire), which simplified the KPFM analysis. Finally, to delineate bare and SAM-covered regions, we micro-contact-printed an array of thin (6 µm spaced by 8 µm) lines of the SAM. In the unbent state, the KPFM images of lines appeared sharp (Figure 4a). The contrast between the Au and Au/SAM regions was reflected by the distribution of surface potentials featuring two distinct

Figure 4. Kelvin Probe Force Microscopy, KPFM, measurements of the surface potential of Au-on-PMMA (15 mm x 15 mm foil) microcontact-printed with 6 µm wide lines of 1-naphtalenethiol. The images in the left column are for the unbent surface; those in the middle column are for the surface bent by ~ 80°; those in the right column are for the surface returned to its original, unbent shape. (a,c,e) KPFM scans of the SAMs micropatterned – as an array of parallel lines – on Au/PMMA films. (b,d,f) Corresponding histograms of surface potential over regions marked with red boxes in (a,c,e). The KPFM measurements were performed on a Bruker Dimension Icon SPM microscope. PFQNE-AL probes were used - tip radius curvature ≤ 12 nm, spring constant 0.8 N/m and resonant frequency of 300 kHz. Experiments were performed under ambient conditions, with relative humidity c.a. 40% and temperature 22˚C.

up” of the SAM to expose additional catalytic surface: (1) uniform expansion of the monolayer, (2) formation of large (tens of nm), localized cracks, or (3) formation of smaller, more uniformly distributed macromolecular or nanoscopic domains. The uniform expansion (scenario 1) can be eliminated based on theoretical considerations – specifically, the aforementioned FEM calculations indicate that the additional

peaks – the one at lower potentials corresponding to bare Au, and the one at higher potentials corresponding to Au/SAM (Figure 4b). When, however, the surface was bent, the signal over the SAM-covered region moved to lower potentials (Figure 4c), and the two peaks of the potential distribution merged into one peak at “intermediate” potential values (Figure 4d). This means that the SAM got sparser and effectively exposed some underlying gold. We emphasize that


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no distinct cracks were observed (scenario (2)) substantiating instead the formation of uniformly distributed unprotectedmetal domains of dimensions below the ~ 10 nm resolution of the KPFM system (Bruker Dimension Icon operating with a PFQNE-AL tip). Finally, when the surface was allowed to unbend (Figure 4e), the distribution of potential values reverted to bimodal (Figure 4f), though it was not fully reversible as some thiols could have irreversibly migrated to the free-Au regions32 (note: in case of the Pt wire, such net transport is irrelevant since all metal surface is covered with thiols). In sum, the above experiments corroborate “opening up” of nanoscopic domains within the SAM during mechanical deformation of the underlying metal support. At present, we do not know how these domains look at the atomic level, in terms of grain boundaries/defects – obtaining such an understanding would be an interesting albeit possibly very challenging goal for future research. We believe the concept of mechanically controlled heterogeneous catalysis we described here can be extended to other metal/SAM systems (or polymer-covered metals), perhaps in the context of sequential catalysis whereby different catalysts would be activated/deactivated at various times. The major limitation of the current system is that the reversibility of catalysis appears to depend crucially on the quality of the SAM (cf. data in Figure 2b) which, however, could be improved by decreasing surface roughness of the metal support33.

ASSOCIATED CONTENT Supporting Information Additional experimental detail, finite-element analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Correspondence and requests for materials should be addressed to B.A.G. ([email protected]) Author Contributions † These authors contributed equally to this work

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We gratefully acknowledge generous funding from the Institute fo r Basic Science Korea, Project Code IBS-R020-D1.

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Heterogeneous Catalysis âOn Demandâ: Mechanically Controlled

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Heterogeneous Catalysis âOn Demandâ: Mechanically Controlled