Can Support Acidity Predict Sub-Nanometer Catalyst Activity Trends

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Can Support Acidity Predict Sub-Nanometer Catalyst Activity Trends? Andrew S Crampton, Marian D Rötzer, Uzi Landman, and Ueli Heiz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01844 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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Can Support Acidity Predict Sub-Nanometer Catalyst Activity Trends? Andrew S. Crampton,†,‡ Marian D. Rötzer,† Uzi Landman,¶ and Ueli Heiz∗,† †Technische Universität München, Lehrstuhl für Physikalische Chemie, Zentralinstitut für Katalyseforschung und Fakultät für Chemie, Lichtenbergstr. 4, 85748 Garching, Germany ‡Current Address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States ¶School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA E-mail: [email protected]

Keywords Platinum Ethylene hydrogenation Clusters Model Catalysis Support Effects SiO2 MgO

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Abstract The effect of metal-oxide support on the activity of ethylene hydrogenation catalyzed by size-selected platinum clusters (8-20 atoms per cluster) is investigated. Sizeselected clusters have been shown to possess unique catalytic properties which change as a function of the precise cluster size. The temperature programmed reaction and isothermal pulsed molecular beam data both demonstrate a size-dependent reactivity on each support. However, predicted trends based on the acidic (SiO2 ) or basic (MgO) properties of the support are more subtle, with the influence of the support also being a function of the cluster size. Only the average hydrogenation activity of all sizes measured, as well as the CO stretching frequency on clean clusters, were observed to follow predicted trends, but atomic level resolution demonstrates a much more complicated picture. Pt13 had the lowest hydrogenation activity of all sizes on SiO2 , while being the most active size on MgO and trends between the same cluster sizes were observed to exhibit opposing behavior between the two supports. This demonstrates the potential of cluster materials for designing catalytic systems, but also the difficulties encountered when formulating general concepts for reactivity in the non-scalable size regime.

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Introduction The interaction of a metal cluster with its support is a fundamental characteristic in heterogeneous catalysis dictated by the physical and chemical properties, size, and dimensionality of the two materials. It plays an essential role in elucidating, and predicting, a supported cluster’s potential interactions with reactive chemical species, catalytic properties, as well as physical/chemical attributes. 1–8 The ability to exploit support interactions with a specific cluster material is an attractive strategy for improving catalyst selectivity and efficiency, and is another tool available to design catalyst systems tailor-made to specific reactions. Size-selected clusters supported on thin metal oxide films under ultra high vacuum (UHV) conditions provide the unique ability to investigate a catalytic system as a function of the precise size of a particle, effectively reducing uncertainties in discriminating between the diverse influences encountered with heterogeneous catalytic systems, such as particle sizedistribution. However, theoretical and experimental studies demonstrate enhanced interaction with the support material, leading to diverse cluster structures and charging effects, which often fail to conform to established trends extracted from larger particle sizes or extended surfaces. 1–5,9–16 This demonstrates that even when precise particle size is known, the unique properties of matter in this non-scalable size regime can lead to counter-intuitive reactivity patterns based on changes in the support material, changes which otherwise have established and predictable effects on larger supported particles (> 1 nm diameter). The inapplicability of predictions make size-selected clusters particularly interesting for catalyst design strategies. 1–4,10–17 In the current study, MgO and SiO2 were chosen as support materials which have different acidity, as this attribute is often used as an indicator for the charging characteristics that may be expected when using thin film supports; indeed, more acidic supports have been found to have a positive effect (that is, favor) hydrogenation rates. 18–24 Ethylene hydrogenation catalyzed by platinum was chosen as the model reaction as it is one of the most studied reactions in surface science, being employed to model catalytic hydrogenation and 3


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hydrocarbon interactions with surfaces. 25–35 The reaction has been shown to be structure insensitive on single crystals and supported metal particles down to sizes of 2 nm, but the origins and mechanisms of this structure insensitivity are still not well understood. 30,31,33,36 Recently, we have demonstrated that small metal clusters (7-40 atoms/particle) supported on MgO(100)/Mo(100) exhibit size-dependent ethylene hydrogenation activity (in contrast to previous work), which we attributed to charge distribution on the clusters upon interaction with the support, and the proclivity to form carbonaceous species at elevated temperatures. 9,37 These types of cluster size effects are the consequence of discrete electronic and geometric properties present in this size-range of matter. Additionally, controlling the support stoichiometry also allows for tuning ethylene hydrogenation activity, as well as deactivation, of Pt13 supported on SiO2 thin films. 38 Other work with respect to the support material has shown that changing thin film thickness can influence reactivity 39 and morphology 40 of gold clusters on MgO, and palladium cluster’s reactivity on Al2 O3 41 , but experimental data on clusters with the same atomicity under identical reaction conditions on two chemically different metal oxide films is scarce. 16 In the following, the question of how support acidity applies to sub-nanometer particle sizes is addressed. To this end ethylene hydrogenation catalyzed by size-selected platinum clusters supported on SiO2 /Pt(111) (referred to as SiO2 ) is compared to our previously published work on the same size clusters on MgO(100)/Mo(100) (referred to as MgO).

Experimental Size-selected platinum clusters were generated by evaporating a Pt metal target (99.95 %, Alfa-Aesar, Germany) with the 2nd harmonic of a Nd:YAG laser (Innolas SpitLight DPSS, Germany) followed by cooling and extraction into vacuum with a delayed helium (Westfalen, 6.0 purity) gas pulse. The cluster beam is then guided with ion optics to a quadrupole bender which extracts the positively charged species, and subsequently through a mass spectrometer

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(Extrel QMA 16000 amu, USA) where a single cluster size can be selected for deposition. The clusters are soft-landed ( 10 min. The MgO film is estimated to be approximately 10 atomic layers thick, lacks any appreciable concentration of defect sites (e.g. F-centers), and has low concentrations of kinks, and step-edges etc. 9 . The TPR experiments were performed by dosing 0.4 H2 (5.0 purity, Air Liquide, Germany) molecules per surface atom followed by 0.4 C2 H4 (3.5 purity, Westfalen, Germany) molecules per surface atom at 100 K using a calibrated molecular beam doser. 43 The crystal was then heated resistively with a temperature ramp of 2 K/s at a distance of ≈1 cm from a quadrupole mass spectrometer (Balzers QMA 430, Liechtenstein) skimmer and the mass of ethane (30 m/z) was recorded. The pulsed molecular beam experiments were performed in a 2 · 10−6 mbar background of deuterium (100 % purity, Westfalen, Germany), with a piezo driven valve pulsing ethylene onto the surface (≈ 1014 molecules per pulse, pulse width = 500 µs). The sample was held in the same position as the TPR experiments. A "quasi steady state" is defined as

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the time regime where ethane production is constant (≈ 100 ms). 9,44–46 By calibrating the mass spectrometer a turnover frequency (TOF) can be determined. IRRAS (Thermo Electron Corp. Nicolet FT-6700, MCTA-TRS external detector) was conducted at 100 K in single reflection mode after dosing 10 Langmuir of CO onto the sample either before or after reaction cycles, see text for designation. 256 scans were averaged with a resolution of 4 cm−1 .

Results

Figure 1: Integration of ethylene hydrogenation temperature programmed reaction curves per Pt atom for clusters supported on MgO and SiO2 . The reactants were dosed in the order: 0.4 molecules/surface atom H2 followed by 0.4 molecules/surface atom C2 H4 at 100 K. Error bars represent a percent error calculated from multiple experiments on one cluster size (Pt10 ). The x-axis gives the number of atoms in each of the platinum clusters used; also included is the result for the reaction catalyzed by an extended Pt(111) surface. As shown in figure 1, platinum clusters supported on MgO and SiO2 exhibit a sizedependent ethylene hydrogenation reactivity from integration of the TPR measurements and normalizing to the number of atoms; see supporting information (SI) for the TPR spectra and ref. [37]). On SiO2 , the most reactive sizes are Pt9 , Pt10 and Pt14 ; Pt13 exhibits a much 6


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lower reactivity, similar to Pt(111). The ethane peak maxiumum is observed to be at a temperature of ≈ 175 K, which is 70 K lower than that observed for Pt(111). 29,47 On MgO, a different reactivity pattern is observed; Pt7−9 are not reactive, whereas Pt10−15 show an onset of reactivity with a peak for Pt13 . Similar to Pt clusters on SiO2 , the clusters on MgO have an ethane peak maximum at 150 K, lower than that of Pt(111). 9,37 Ethylene hydrogenation TOFs at 300 K on size-selected platinum clusters supported on SiO2 and MgO support the previously observed size dependent reactivity (figure 2a and b). On SiO2 (figure 2a), Pt8−10 show an increase in activity with cluster size followed by a decrease for Pt12 and the lowest activity for Pt13 . Pt14 then recovers the activity to above the level of Pt10 and another decrease is observed going to Pt20 . All cluster sizes, except for Pt13 , show an increased activity compared to the Pt(111) single crystal. Heating to 400 K and running the ethylene reaction with hydrogen at the higher temperature, followed by cooling and subsequent measurement of the reactivity at 300 K, shows that the elevated temperature step induces formation of carbonaceous species through dehydrogenation pathways, which has been shown for Pt clusters on MgO to be detrimental to catalytic hydrogenation activity. 9,37 On SiO2 , the platinum clusters deactivate (as a result of the temperature step) in a similar fashion, with the Pt8−12 , Pt15 and Pt20 clusters all exhibiting after cooling to 300 K a TOF similar to that measured Pt(111). Pt13 and Pt14 only deactivate slightly, maintaining their marked difference in activity. Pt clusters supported on MgO (figure 2b) also demonstrate size-dependent activity at 300 K, with Pt12−20 being more active than Pt8−10 and Pt(111), and Pt13 showing the highest ethylene hydrogenation activity in this cluster-size range. Also displayed in figure 2b are the turnover frequencies measured at 300 K, after running the reaction at an elevated temperature of 400 K. Here, all cluster size deactivate subsequent to the temperature step, down to the activity of Pt(111) except for Pt20 which remains more active. Further details regarding the MgO data can be found in our previous work. 9,37

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Figure 2: Turnover frequencies for the ethylene hydrogenation on Pt clusters deposited on MgO (a) and SiO2 (b). Filled circles represent the turnover frequency measured on freshly deposited clusters at 300 K and open triangles are the TOFs at 300 K after the reaction was run at 400 K. Error bars are the standard deviation calculated from multiple experiments on the same cluster size.

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SiO2 MgO

Figure 3: (a) infrared reflection absorption spectra of the CO stretch before (two top curves, black) and after (colored curves) the reaction temperature sequence of 300→400→300 K on the SiO2 supported clusters. The spectra for Pt9 and Pt13 at the top (black) are CO adsorbed on bare clusters before any reaction. (b) plots of the peak position of the CO stretch from (a), illustrating the size-dependent redshift from results obtained for the bare clusters (i.e. CO adsorbed before any reaction studies). Black circles and black squares illustrate the positions of the CO stretch on clusters before the reaction for both SiO2 and MgO, respectively. The open square is the CO position on MgO after the 300→400→300 K reaction sequence, which was the same for all cluster sizes. After performing the reaction cycle including the temperature step to 400 K, IRRAS with CO as a probe molecule was applied to investigate adsorbed carbon species produced 9


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during the course of the catalyzed hydrogenation and dehydrogenation reactions on the SiO2 supported clusters (see refs. [9] and [37] for MgO data). Figure 3a displays the measured spectra before (two top black curves for Pt9 and Pt13 ) and after (bottom curves, colored) reaction; the linear CO stretch peak position for Pt9 and Pt13 are given by black circles in figure 3b. From the peak positions in figure 3b a clear size-dependent redshift (given by the constant, cluster-size-independent results at the top of figure 3b and the size-dependent curve at the bottom of the figure) is observed for all cluster sizes.

Discussion SiO2 Activity vs. MgO The TPR data (figure 1) from the SiO2 supported clusters demonstrate a size-dependent ethylene hydrogenation reactivity. Pt9 , Pt10 and Pt14 are the most reactive sizes, with Pt13 being the least reactive (similar reactivity as Pt(111)). All cluster sizes exhibit a TPR peak maximum which is 70 K lower than Pt(111). The size-dependent reactivity measured for these platinum cluster sizes, combined with the temperature shift (compared to the extended Pt(111) surface), is an unambiguous signature of a structure sensitive reaction. 9,37 The activity measured in figure 2a on the SiO2 supported clusters under isothermal conditions (300 K) with the pulsed valve technique, lends further support to the classification of ethylene hydrogenation as structure sensitive in the sub-nanometer size regime. Pt9 , Pt10 and Pt14 demonstrate increased activity compared to Pt(111), and only Pt13 displays a similar activity to Pt(111). This attenuated activity of Pt13 has also been independently demonstrated on zeolite supported Pt13±2 clusters. 48 This points to either a lower intrinsic activity for Pt13 on SiO2 or dehydrogenation channels are available for ethylene at 300 K poisoning the catalyst. Increasing the reaction temperature leads to opening of dehydrogenation reaction channels, which instigates the irreversible formation of dehydrogenated carbon species on the 10


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cluster catalysts with a subsequent decrease in activity and loss of size-sensitivity. 9,37,38,46 For Ptn /SiO2 , all clusters sizes, except for Pt13 and Pt14 , have lost activity at 300 K after running the reaction at 400 K, and all cluster sizes now have an activity similar to Pt(111) except Pt14 (see figure 2a). Ethylene hydrogenation activity on Pt13 /SiO2 was previously demonstrated to be controllable by changing the SiO2 support stoichiometry. 38 This is a consequence of the local charging of individual cluster atoms caused by the number of Pt-Si or Pt-O bonds which form upon cluster adsorption. This charging property can be tuned by varying the stoichiometry, that is the silicon to oxygen ratio, of the SiO2 support, rather than by altering the d-band center position of the cluster through changing the identity of the metal from which the cluster is made. 38 This local charging influences cluster-adsorbate interactions which affect activation barriers for hydrogenation or dehydrogenation processes. The previous results showed that Pt13 on a stoichiometric SiO2 film dehydrogenates ethylene (e.g. to ethylidyne) already below 300 K, in a similar fashion to Pt(111), which leads to similar activity on both samples. The temperature step to 400 K does not provide enough energy to overcome another dehydrogenation barrier (e.g. ethylidyne dehydrogenation) which would lead to further deactivation, but a silicon rich surface does induce this effect by providing excess electron charge to the clusters and subsequently lowering the activation barrier for dehydrogenation. 38 An oxygen rich SiO2 surface induces the opposite effect on Pt13 . The cluster has enhanced activity at 300 K and does not fully deactivate to the level of Pt(111) after the temperature step to 400 K, due to the charge transfer from the cluster to the support induced by excess Pt-O bonds. 38 Using a similar analysis, Pt14 on stoichiometric SiO2 is expected to exhibit a similar trend: intrinsically lower dehydrogenation barriers, which leads to enhanced activity at 300 K, and resistance to poisoning dehydrogenation pathways up to a temperature of 400 K. The resistance to poisoning at 300 K was also observed and theoretically demonstrated (ref. [9]) for Pt13 on MgO, where dehydrogenation barriers of 1.17 and 1.32 eV were

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calculated, however the cluster was observed to deactivate after the temperature step to 400 K, indicating a unique property of Pt14 /SiO2 . The formation and extent of carbon species formed on the clusters was evidenced using CO adsorption and IRRAS before and after reaction (see figure 3a and b). 46,49–52 A redshift in the CO stretch frequency can be attributed to an increased charge transfer to the 2π ∗ orbital, or an electric field effect from the dipole moment of the co-adsorbed carbon species. While determining which of these effects is operative is not possible from our data, we note that both would necessitate a charge transfer from the carbon species to the metal in order to induce a redshift in the CO stretching frequency. 50,53 On Ptn /MgO, a uniform redshift of 34 cm−1 was observed 9,37 in the CO stretching frequency on Pt10 and Pt13 after running the thermal reaction sequence 300→400→300 K. After the same reaction cycle on SiO2 , the CO stretch on Pt13 is red-shifted by 25 cm−1 but only by 13.5 cm−1 on Pt14 (see figure 3b), which is the smallest of all the cluster sizes measured. Attributing the redshift (observed after the reaction cycle) from the clean CO stretch frequency to the extent of interaction of carbon species with the clusters, i.e. to the magnitude of charge transfer to the metal, then the IR results indicate an enhanced carbon contamination on all cluster sizes compared to Pt14 , being most pronounced for Pt13 . We have previously reported that the aforementioned carbon species formation plays an essential role in attenuating cluster activity, essentially masking any underlying differences which could be imparted by the catalyst and eliminating size-dependent activity. 9 Figure 4 demonstrates this correlation between the measured activity at 300 K and the subsequent deactivation after the temperature increase to 400 K. Similar correlations have been made between x-ray photoelectron binding energy shifts and reactivity for palladium. 12 The smaller red-shift of Pt14 implies an intrinsic resistance to dehydrogenation reaction pathways which lead to carbon species on the cluster and a lowered activity. The cluster size dependency observed in the IR data is an indication of a fundamental difference between the effect of the SiO2 and MgO thin films on the platinum clusters, indicating that dehydrogenation

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activation barriers can be tuned on the same cluster size by changing the support material.

Figure 4: Ethane TOF at 300 K (orange curve) plotted with the CO stretch frequency (dotted black curve) after running the ethylene hydrogenation reaction at the temperature sequence of 300→400→300 K on the SiO2 supported clusters.

Assessing the Applicability of Support Acidity The effects of support acidity or basicity are illustrated by comparing size-selected clusters supported on SiO2 (acidic) and MgO (basic). A detailed analysis of ethylene hydrogenation on MgO-supported size-selected platinum clusters can be found in previous publications. 9,37 The shift of the linear stretching frequency of CO adsorbed on Pt9 and Pt13 (figure 5) supported by either MgO or SiO2 demonstrates an expected consequence of the differing support properties: the CO stretch frequency is 10 cm−1 redshifted on the more basic support (MgO). The redshift is indicative of increased charge donation from the support, which in turn augments the back-bonding charge donation from the metal cluster to the CO 2π ∗ orbital, weakening the CO bond and redshifting the stretch frequency. 18,20,54 The redshift induced by carbon formation also follows the trend of increased electron donation from the more basic support (MgO) facilitating dehydrogenation pathways. On SiO2 the redshift varies from 13.5 cm−1 (Pt14 ) to 25 cm−1 (Pt13 ), both of which are smaller than the 34 cm−1 observed for Pt10 and Pt13 on MgO. The larger redshift from MgO supported 13


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clusters can be attributed to enhanced irreversible carbon formation due to lowered dehydrogenation barriers, a direct consequence of increased charge donation from the support. This effect of the support on the electronic structure of a supported particle has been previously observed to impact catalytic activity, with decreased electron density on the metal particle leading to increased hydrogenation rates. 22–24 The data presented here demonstrate an interesting pattern. From the TPR studies (figure 1) the reactivity observed from Pt8 and Pt9 on SiO2 , and absence thereof on MgO, indicate that the electron withdrawing effect of SiO2 can be beneficial for hydrogenation reactivity. However, the lower reactivity of Pt13 departs from this trend, indicating that other factors must be taken into consideration. Figure 6 depicts the ethane TOF for size-selected Pt clusters on MgO and SiO2 . In their respective colors, orange (SiO2 ) and blue (MgO), the average activity of all the cluster sizes is depicted as a colored bar spanning the x-axis. The width of the bar is the average error from the measurements. Although, the SiO2 support exhibits an overall slightly higher average TOF, as could be expected based on the acidity of the support, when considerations of the precise cluster sizes (number of Pt atoms) and substrate effects are included, trends observed for certain specific sizes in the size-range of clusters studied here exhibit some unexpected behavior, with that of Pt13 found to be different from the projected one. The discrepancy between the predicted effects based upon acid/base properties of the support and the CO IR spectra, and the actual measured catalytic activity, indicates that particle support interactions in the sub-nanometer regime may bring about behavior that not always obeys trends deduced on the basis of observations made for an ensemble of particle sizes. The case of Pt13 illustrates an extreme example, as the CO stretch is in accordance with expected support interactions, i.e. the more acidic support leads to a higher stretching frequency, but this does not lead to a higher ethane TOF, as expected from previous studies of hydrogenation reactions on acidic/basic supports. 22–24 The relative trends between individual sizes may also demonstrate opposing behavior. Thus Pt12 is more active than Pt10 on MgO, but on SiO2 the opposite is observed; similar opposing behavior has also been observed

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between Pt13 and Pt14 , and Pt15 and Pt20 . The expected pattern based upon ensemble results and chemical intuition would be simply an enhanced activity on the SiO2 supported clusters (i.e. the MgO curve would be uniformly shifted to higher activity), and changes between sizes would be the same. Again, the data presented here demonstrate that such conclusions are not uniformly applicable in the subnanometer regime. The different trends between cluster sizes point to specific interactions between each individual cluster size and each support. While this makes for elucidating catalytic properties of clusters tedious, discovering a cluster size which behaves contradictory to expected trends in a given environment is particularly interesting for designing multifunctional catalyst systems.

Figure 5: Infrared reflection absorption spectra of CO adsorbed at 100 K on freshly deposited Pt9 and Pt13 supported on MgO (blue) and SiO2 (orange). Isolating the cluster property which leads to the differences observed between the two supports is demanding, and more experimental data is required before definitive explanations can be given. Nevertheless, there are some plausible models that may address certain trends that emerge when comparing the behavior involving different supports. The particle-support interaction is not limited to purely electronic/charging effects, but 15


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Figure 6: Turnover frequencies at 300 K for the ethylene hydrogenation on Pt clusters deposited on MgO(100) (blue) and SiO2 (orange). The colored bars spanning the x-axis represent the average TOF of the clusters on each support, with the width equaling the average error. also influences cluster structure, particularly in the nanoscale regime where clusters are known to show a high degree of fluxionality, and close-in-energy isomers, especially when adsorbed on a metal-oxide support. 2,4,8,9 Such effects are particularly interesting when comparing the behavior of the same metal catalysts (platinum clusters in this case) on two metal oxide supports of different chemical nature (crystalline MgO(100) and amorphous SiO2 , in our study), which may be expected to give rise to different cluster morphologies. Indeed, the structures found for Pt13 in our previous work illustrates this effect. 9,38,55 Different morphologies lead to different binding sites and local charging which can influence the clusters’ activity, especially with reactants that have multiple possible pathways on a surface, such as unsaturated hydrocarbons. Additionally, the amorphous structure of the SiO2 film provides surface heterogeneities, which increase the complexity of interactions occurring between the clusters and the support. In contrast, the single crystalline surface of the MgO thin film leads to an ideal, relatively homogeneous surface structure, minimizing the variability of cluster-support interactions. Indications for this effect can be seen in figure 5, where the 16


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full-width half-maximum of the CO band is ≈ 30 cm−1 on SiO2 but only ≈ 20 cm−1 on MgO, pointing to more homogeneous cluster structures and adsorption sites on the latter. It is interesting to note that the the CO stretch frequency was found to follow closely the trend expected from the literature on support effects, leading us to inquire about the reasons underlying the observed behavior of the ethylene hydrogenation reaction which we found to proceed along lines that differ from the expected ones. As a tentative explanation we observe that the CO stretch is probing an ensemble of single surface atomic adsorption sites, whereas ethylene hydrogenation requires unique multi-atom surface configurations to facilitate a bimolecular reaction. Our previous theoretical study on Ptn /MgO demonstrated that there are a limited number of sites which can provide an auspicious bonding environment for ethylene and dissociated hydrogen to react compared to the total number of atoms, which supports this concept of the fundamental differences between the CO IR experiment and ethylene hydrogenation trends observed on the two supports. This is further supported by the observed increase in average hydrogenation activity for SiO2 supported clusters compared to MgO, where the average activity over all cluster sizes conforms to the expected support effect, but single cluster sizes introduce effects which can only be resolved on an atomic scale. We remark here that in the case of ethylene hydrogenation catalyzed by Ni nanparticles supported on MgO we have also found differing trends between those found through CO IR measurements and ethylene hydrogenation data. For that system, complete deactivation of the clusters was not accompanied by a complete loss of CO stretch signal. In contrast, no CO stretch was observed on Pd after complete deactivation, and a strongly attenuated signal was observed for Pt with activity still present. 46 In the case of Ni, only the bridge bonded CO stretch signal disappeared after complete deactivation. These examples indicate that the linear CO stretch frequency is not necessarily a universal indicator of cluster reactivity. The copious effects which could be applied to rationalize the difference in activity observed for platinum clusters adsorbed on different supports (MgO and SiO2 ), demonstrates the need for further experimental work on size-selected clusters. Clusters have the potential

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to be highly selective and efficient catalysts, presenting a diverse toolbox for the design of catalyst systems for specific reactions, with the choice of support surface providing an added dimension to the multitude of potential applications.

Conclusion Size-dependent ethylene hydrogenation activity of platinum clusters supported by an amorphous SiO2 thin film using both temperature programmed and isothermal techniques was demonstrated and compared to previously reported results for the same cluster sizes on an MgO(100) thin film. On SiO2 , size-dependent activity was observed, with Pt13 being the least active cluster size, and Pt14 showing the highest activity, as well as a resistance to poisoning through ethylene dehydrogenation pathways. This was corroborated by infrared spectroscopy, where the carbon induced redshift in the CO stretch was observed to be smallest for Pt14 . IRRAS of CO adsorbed on clean cluster catalysts demonstrated a redshift on the more basic support (MgO), in concordance with previous studies pertaining to the effects which support acidity has on CO adsorption. However, substantial differences in ethylene hydrogenation reactivity trends, most notably for Pt13 , were observed for the same cluster sizes on the SiO2 and MgO supports, illustrating that for clusters in the sub-nanoscale size range, cluster-support interactions entail a deeper degree of complexity and intricacy than the fundamental charging arguments employed for particle size ensembles larger than 1 nm. The structural diversity and fluxionality of clusters of the same size, but on different support materials, predicted by theoretical investigations, appear to underlie the departure we observed in the behavior of cluster materials from established adsorption and reaction trends based on support acidity. In this context we reiterate that the different trends found from the CO-stretch IR measurements and the ethylene hydrogenation/dehydrogenation data are likely to originate from the local nature of CO bonding (that is, CO adsorption probing

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a single atomic site, particularly for the linearly bonded CO which we observed), whereas ethylene hydrogenation involves multiple adsorption sites and thus shows sensitivity to the surface morphology and atomic arrangement. We know of no other study where the catalytic properties of size-selected clusters have been shown to behave so differently under identical reaction conditions, with only the metal oxide support being changed. This serves to highlight the breadth and depth of insights gained through experimental and theoretical investigations of size-selected clusters (particularly in the sub-nanometer size regime). Indeed, explorations of these systems offer increased fundamental understanding beyond studies applied to systems characterized by particle sizedistributions, and/or larger particle sizes.

Supporting Information Ethylene hydrogenation TPR spectra for the platinum clusters supported on SiO2 (figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work has been supported by the Deutsche Forschungsgemeinschaft through the grant He 3454/23-1. A.S.C. acknowledges support by a Feodor-Lynen Fellowship from the Alexander von Humboldt Foundation. We also acknowledge support of Dr. Florian F. Schweinberger at the beginning of this project. The work of U.L. was supported by the Air Force Office of Scientific Research under Award No. FA9550-15-1-0519.

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TOC Graphic Change Support Acidity

Support Effects?

MgO

Average Rate

Ensemble

e

MgO

SiO2

Cluster Size

e

Cluster Rate

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SiO2

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Can Support Acidity Predict Sub-Nanometer Catalyst Activity Trends

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