Article Cite This: J. Phys. Chem. C 2019, 123, 16194−16209
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Preparation of Size- and Composition-Controlled PtnSnx/SiO2 (n = 4, 7, 24) Bimetallic Model Catalysts with Atomic Layer Deposition Timothy J. Gorey,† Borna Zandkarimi,‡ Guangjing Li,† Eric T. Baxter,† Anastassia N. Alexandrova,*,‡,§ and Scott L. Anderson*,† †
Chemistry Department, University of Utah, 315 S. 1400 E., Salt Lake City, Utah 84112, USA Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA § California NanoSystems Institute, Los Angeles, California 90095, USA Downloaded via BUFFALO STATE on August 2, 2019 at 04:11:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: Model catalysts prepared by mass-selected cluster deposition are useful in fundamental studies, but for bimetallic clusters, a precise selection of both size and composition can be difficult except for very small clusters. We discuss an alternative method for preparing bimetallic PtSn model catalysts, in which size-selected Pt4, Pt7, and Pt24 clusters, deposited on SiO2 and then hydrogenated, are used to seed selective addition of Sn by a self-limiting reaction with SnCl4. Catalysts are characterized using Xray photoelectron spectroscopy, ion scattering spectroscopy, and CO temperatureprogrammed desorption, and structures are calculated by plane-wave density functional theory. It is found that Sn deposition saturates at a relatively low SnCl4 exposure, indicating that the reaction is self-limiting, and that nonselective Sn deposition on the SiO2 support is ∼40 times less efficient than Sn deposition on hydrogenated Pt cluster sites. The resulting PtnSnx stoichiometry is near 1:1 for Pt4 and Pt7, clusters, whereas for Pt24 clusters, x is ∼7.5. Nearly, all of the Cl in the adsorbed SnCl4 desorb as HCl during the room-temperature Sn deposition process, and the balance can be desorbed by mild heating.
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INTRODUCTION
For bimetallic combinations where the mass of one metal is a near-integral multiple of the other mass, and one or both have significant natural isotope distributions, clean selection by mass may be impossible. In PtnSnx, for example, 194Pt, 195Pt, 196 Pt, and 198Pt, and 116Sn, 117Sn, 118Sn, 119Sn, 120Sn, 122Sn, and 124 Sn all have a significant natural abundance. As a result, the mass distributions for various PtnSnx begin to overlap significantly with increasing cluster size, such that a clean selection of size and composition becomes impossible. In this example, for masses near the nominal mass of Pt10, every mass number has significant contributions from Pt10, Pt2Sn13, Pt7Sn5, Pt5Sn8, and Pt4Sn10, i.e., mass selection even at unit mass resolution would result in a diverse collection of cluster sizes and composition that presumably would have quite different chemistry and other properties. Traditional synthesis strategies for supported PtSn nanoparticles, such as chemical impregnation,28−31 have been widely used for the production of >2 nm PtSn particles in supported catalysts and electrocatalysts. Some chemical reduction methods were modified to gain control over catalyst shape. For example, Liu et al. developed a microwave-assisted polyol synthesis of a PtSn methanol electro-oxidation catalyst, which showed well-ordered and monodispersed face-centered
Understanding the factors responsible for catalyst properties such as activity, selectivity, and stability is important in catalyst design, and size-selected model catalysts are a useful tool in mechanistic studies.1−14 For example, the ability to precisely vary the size, while keeping all other properties constant, allows the observation of size-dependent correlations with properties such as catalyst electronic properties,15,16 cluster proximity,17 or the physical size of the binding sites available.18 Such welldefined model catalysts are also relatively tractable for detailed theoretical modeling, allowing theory and experiment to study identical systems.4,19−22 Bimetallic catalysts provide additional opportunities for catalyst optimization, therefore it is desirable to extend the size-selected model catalyst approach to bimetallic clusters. Bimetallic clusters can be produced using laser vaporization or sputtering/gas-aggregation cluster sources that use either alloy or dual targets.23−27 For small clusters, direct mass selection of the exact size and composition is possible, however, clean selection by mass becomes more difficult with increasing cluster size. The intensity for a given size cluster is spread over an increasing number of MnNx combinations as size increases, and the mass resolution required to select a single MnNx also increases, further reducing the signal. Low signal matters, because adventitious contamination increases with deposition time. © 2019 American Chemical Society
Received: March 22, 2019 Revised: May 30, 2019 Published: June 3, 2019 16194
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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The Journal of Physical Chemistry C cubic structures of 2−7 nm catalyst particles.32 Relatively narrow size and composition distributions have been reported for PtSn nanocatalysts. For sub-nano model catalysts for fundamental mechanistic studies, more precise, ideally atomic level control over the size and stoichiometry is desirable. Atomic layer deposition (ALD) has been used to stabilize and alter the catalytic behavior of supported catalysts by oxide overcoating33−35 and also to form bimetallic supported nanoparticles by adding a second metal to nanoparticles formed by conventional methods.36−38 The advantage of ALD is that it is a self-limiting deposition process, limited by the number of reactive sites in the surface layer.39,40 Previously, ALD deposition of SnO2 has been done by iteratively dosing H2O and tetrakis(dimethylamino)tin(IV) vapors.41 This method has been used to produce PtSn catalyst nanoparticles by first growing a SnO2 ALD layer, then evaporating a Pt layer, followed by heating under reducing conditions. The thickness of the Pt and SnO2 layers determined the particle size and stoichiometry and allowed control between 5 and 58 nm particles.42 Here, we demonstrate a different approach, in which sizeselected, sub-nanometer Pt clusters are first deposited on a planar SiO2 support using a mass-selected cluster beam, then used to seed selective Sn deposition by an ALD-like process. Results are presented for Ptn (n = 4, 7, 24) seed clusters, i.e., clusters with very different expected levels of Pt−Pt coordination, and for several different Sn deposition protocols, all done in high or ultrahigh vacuum. The resulting samples were characterized by X-ray photoelectron spectroscopy, He+ ion scattering, and CO adsorption/desorption. Plane-wave density functional theory (PW-DFT) was used to model the resulting cluster structures.
with an area-selective lens that allows XPS, UPS, and ISS to be done with 1.1 mm diameter analysis areas, smaller than the ∼2 mm cluster spot. The other instrument has a larger antechamber equipped for sputtering, annealing, gas exposures, and mass spectrometric gas analysis, with a separate load lock chamber attached, allowing sample exchange without venting the antechamber (base pressure 2100 K are required. The type C junction potential is quite small at cryogenic temperatures, which reduces the temperature accuracy below ∼200 K. Compounding the problem, the clip/heater wire arrangement used to mount the SiO2 substrates breaks occasionally, rebuilding the sample holder may slightly change the thermal conductivity between the sample and the backing plate, where temperature is measured. The only significant effect from the perspective of the experiments here is that the temperature used to dose CO for TPD experiments (180 K) probably varied slightly from sample to sample. This mainly affects the amount of CO that binds to the SiO2 substrate and has no effect on binding sites associated with the clusters. Because the clusters are our primary interest, we have not attempted to correct for the small variations in CO coverage on SiO2.
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EXPERIMENTS Experiments were done on two different ultrahigh vacuum (UHV) cluster deposition instruments which have been described in detail elsewhere.12,43,44 The instruments are equipped with nearly identical cluster deposition beamlines that include a laser vaporization cluster source, ion guides for transport through differential pumping stages, and a quadrupole mass filter used for cluster mass selection. Both instruments have “main” UHV sections (base pressure < 1.5 × 10−10 Torr) for cluster deposition and analysis by X-ray and UV photoelectron spectroscopy (XPS, UPS) and low-energy He+ ion scattering spectroscopy (ISS). In addition, both instruments have differentially pumped mass spectrometers with both continuous and pulsed gas dosers, for temperatureprogrammed desorption/reaction (TPD/R) experiments. Each instrument also has one or more attached antechambers that can be used for sample exchange and for processes such as annealing in O2 or Sn deposition by atomic layer deposition (ALD), thus avoiding the exposure of the main UHV section to the reactants involved. The main differences between the instruments are the capabilities of the antechambers and the way that samples are mounted. Most experiments were done on an instrument where the sample remains attached to the main manipulator throughout the experiments, with a small antechamber serving both as a chamber for annealing in O2 and Sn ALD and as a load lock allowing samples to be exchanged. Because the antechamber is used for daily sample exchange and preparation, its base pressure is typically 10−8 Torr. The hemispherical energy analyzer on this instrument is equipped 16195
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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The Journal of Physical Chemistry C Ptn+ were generated by pulsed laser vaporization of a rastering Pt target into a pulsed helium flow, followed by supersonic expansion into the vacuum. Cationic clusters were collected by a radio frequency quadrupole ion guide, the beam was bent by 20° to reject neutrals, then guided through several stages of differential pumping, before being mass selected by a quadrupole mass filter. Here, we have used Pt4, Pt7, and Pt24 clusters. The mass-selected Ptn+ were guided into the UHV system, where they were deposited on an SiO2 substrate through a 2 mm diameter exposure mask positioned ∼1 mm from the surface. The deposition energy of the Pt clusters was calibrated by retarding potential analysis of the beam on the sample and set to ∼1 eV/atom. The deposition was monitored by integrating the neutralization current, and all samples here contained 1.5 × 1014 Pt atoms/cm2 (∼10% of a close-packed Pt monolayer), deposited as Pt4, Pt7, or Pt24. An ALD-like process was used to deposit Sn, and this was done in the antechamber to minimize the exposure of the main UHV chamber to the ALD reactants. In the preferred protocol, referred to as “H2/SnCl4/H2”, the Ptn clusters were exposed to H2 at 1.5 × 10−5 Torr ion gauge pressure for 10 min. Exposure was done through a dosing tube that ends ∼1 cm from the sample surface, increasing the flux at the surface by a factor of ∼3, giving an estimated exposure of ∼21 000 L, corresponding to ∼1020 H2 collisions/cm2. We have previously shown that H2 exposure to Ptn/SiO2 leads to hydrogen chemisorption on the surface of the Pt clusters, presumably as H atoms.12 The hydrogenated sample was then exposed, using a 2nd dosing tube, to SnCl4 for 2 min at 2.0 × 10−7 Torr gauge pressure and 300 K sample temperature. The ionization gauge sensitivity correction for SnCl4 was estimated to be ∼3, assuming (α/IE)1/2 scaling,48 using experimental N2 and SnCl4 ionization energies (IE)49 and polarizabilities (α) calculated at the Hartree−Fock level. The SnCl4 exposure to the sample was ∼24 L or ∼3 × 1015 SnCl4 collisions/cm2 (∼2 collisions per surface atom). During the SnCl4 exposure, the antechamber background gas was monitored, and a large increase in the mass 36 (H35Cl+) signal was observed. This HCl presumably forms by the reaction of SnCl4 with H-covered surfaces in the antechamber at 300 K, leading to HCl desorption. To try to remove as much Cl as possible, the samples were exposed to another ∼21 000 L of H2 at 300 K after the SnCl2 exposure, leading to additional HCl production, as detected by mass analysis of the antechamber background. To test if the ∼24 L SnCl4 exposure was sufficient to saturate Sn binding to the clusters, a few experiments were done with double the SnCl4 exposure, resulting in no additional Sn deposition on the Pt clusters and only a slight increase in nonselective Sn deposition on the SiO2 support. To explore the importance of hydrogen preadsorption for Sn deposition, we also examined a “H2/SnCl4” process, in which the initial H2 saturation step was omitted, and we also tested the effects of simply exposing the sample to SnCl4 without any deliberate H2 exposure at all. It should be noted, however, that would have been significant adventitious H2 exposure to all of the samples when they were moved to the antechambers which use pumps with relatively poor compression for H2. For example, 10−9 Torr of adventitious H2 for 10 min (during the sample transfer and the SnCl4 exposure) amounts to 0.6 L exposure, corresponding to ∼9 × 1014 H2 collisions/cm2. After the completion of the gas exposures, samples were transferred back to the main UHV chamber, cooled to 180 K, and then probed in situ using XPS, ISS, and TPD/R. For XPS,
the Pt 4f, Sn 3d, Si 2p, and O 1s XPS were measured using either Al Kα or Mg Kα sources. Spectra collected using Al Kα vs Mg Kα X-ray lines were put on a common intensity scale by using subshell photoionization cross-sections from Yeh and Lindau,46 and any day-to-day variation in X-ray intensity was corrected by normalizing to the Si 2p intensity from the support. Select experiments also looked at Cl 2p XPS. The binding energy scale was checked by verifying that the O 1s from silica and Si 2p from the Si substrate were at the correct energy, and none of the samples showed any evidence of significant charging. The Pt 4f/Si 2p intensity ratio was used to verify the desired Ptn cluster coverage, and the Pt 4f/Sn 3d and Sn 3d/Cl 2p ratios were used to quantify the Pt/Sn and Sn/Cl stoichiometries. Survey spectra were taken occasionally to check for contaminants, but none were observed. ISS was used to probe the composition of the surface layer, providing some insight into the cluster morphology and adsorbate binding geometries. A 0.3 μA beam of 1 keV He+ beam was directed onto the samples at 45°, and scattered He+ was detected along the surface normal. To avoid beam damage effects on other measurements, ISS was always done either at the end of experiment sequences or on separately prepared samples. To examine the effect of various Sn ALD protocols on the number and energetics of accessible Pt sites, CO TPD was used, comparing the CO desorption from as-prepared Ptn/SiO2 and Ptn/SiO2 with various H2 and SnCl4 treatments. Samples were cooled to 180 K and exposed to 10 L of 13CO, which is roughly double the saturation dose. The 180 K dose temperature was chosen to minimize CO adsorption on the SiO2 support. The sample was then positioned 0.5 mm away from the 2.5 mm diameter aperture in a skimmer cone, allowing gas desorbing from the sample to pass into a differentially pumped chamber housing a quadrupole mass spectrometer. The sample temperature was ramped at 3 K/s to 800 K. During heating, D2, H2O, 12CO, 13CO, H35Cl, 12CO2, and 13CO2 were monitored, cycling through each mass for 50 ms/cycle. In some experiments, we also looked for 120Sn+, which could come from the desorption of SnClx or Sn evaporation, and for 35Cl2+, however, no significant signals were observed at either mass. After the completion of each TPD experiment, the sample was moved away from the aperture, and the mass spectrometer sensitivity was calibrated by leaking in 2.0 × 10−8 Torr of either 13CO or Ar, thus creating a well-defined flux of molecules entering the mass spectrometer ionizer through a skimmer cone aperture. This calibration information was used to put the results for CO and HCl desorption on an absolute basis as follows. The mass spectrometer ionization and transmission efficiency for a particular ion mass should be independent of whether the molecules desorb from a surface or effuse from the gas phase, except for the fact that the velocity distributions for desorbing molecules will vary with the sample temperature. In principle, an electron impact ionizer of the type used in these experiments is a number density, rather than flux detector, because slower molecules have a higher probability of detection. In practice, because the ionizer is inside the skimmer cone, molecules tend to collide with ionizer and cone surfaces, resulting in a more complicated situation where the residence time and velocity distributions are not simply related to either the surface or gas temperatures. We previously compared calibrations obtained by the effusive flux method used here to calibration by desorption of well-defined (2 × 2)16196
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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The Journal of Physical Chemistry C
Figure 1. Sn 3d and Pt 4f XP spectra obtained after H2/SnCl4/H2 treatment of Pt4/SiO2, Pt7/SiO2, and Pt24/SiO2 (red). For comparison, results are also shown for H2/SnCl4/H2-treated SiO2 (black), and for Pt4 and Pt7, the effect of simply exposing the samples to SnCl4 (blue). Vertical dashed lines give literature values for the Pt 4f7/2 and Sn 3d5/2 binding energies for the bulk metals and for bulk tin halides as well as the measured Pt 4f7/2 binding energies for as-deposited Ptn/SiO2.
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COMPUTATIONAL Plane-wave density functional theory calculations of Pt4Sn3/ SiO2 were performed using the Vienna ab initio simulation package52−55 using projector-augmented wave potentials56 and the Perdew−Burke−Ernzerhof57 functional. For the relaxation calculations presented in this paper, large kinetic energy cutoffs of 400.0 eV and convergence criteria of 10−5 (10−6) eV for geometric (electronic) relaxations were employed. The relaxation was done by keeping the bottom half of the slab fixed. Also, Gaussian smearing with the sigma value of 0.1 eV was used. For the frequency calculation, the finite displacement method was used with the convergence criteria of 10−8 eV for electronic calculations. Dipole correction was also employed and calculated only parallel to the direction of the third lattice vector. The SiO2 slab obtained from the bulk cubic unit cell of amorphized cristobalite was previously optimized58 at the B3LYP/6-31G(d,p)59−62 level of theory. The cell parameters used in this study are a = 12.4 Å, b = 13.1 Å, c = 32.0 Å, α = 90°, β = 90°, and γ = 88° including the vacuum gap of 10 Å. Only Γ-point sampling was used to get the energy since the super cell is quite large meaning that the volume of the
CO layers from Pd single crystals and obtained very similar results.50,51 Therefore, we do not correct the calibration for possible temperature effects. For CO TPD intensities, CO effusion is used for calibration. For H35Cl, we used Ar instead, to avoid large exposures of the UHV system to HCl. The Ar+ and H35Cl+ masses are similar, as should be the transmission efficiencies. Relative ionization gauge sensitivity factors were used to correct for the difference in HCl and Ar ionization cross-sections. Integrated intensities measured during TPD experiments are also corrected for the measurement duty factor for each mass. The major uncertainty in this calibration approach is that the skimmer cone aperture (2.5 mm) is bigger than the 2 mm cluster spot, and it is not clear how the detection efficiency varies with the distance from the aperture center due to possible angular distribution effects. Because of this and other issues (e.g., velocity distributions), we estimate the absolute uncertainty in the calibrated desorption signals to be ∼±50%, with relative uncertainty in comparing signals from experiment to experiment of ∼±15%. 16197
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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Pt4 and Pt7 samples, compared to Pt24. Finally, Figure 1 shows that deliberate H2 exposure significantly increases the amount of Sn adsorption, compared to the effect of only adventitious H2 exposure, which is insufficient to saturate the samples. That hydrogen preadsorption enhances, rather than blocks Sn binding, suggests that impinging SnCl4 reacts with adsorbed H, presumably generating HCl. This conclusion is supported by the observation of a significant HCl+ signal in the mass spectrum of the background gasses during the SnCl4 exposure. To extract quantitative information about the stoichiometries of the samples, the spectra were fit to Gaussian− Lorentzian line shapes and integrated. To put the surface coverages on an absolute basis, we take advantage of several facts. The Pt coverage is accurately known from the deposition process, and Pt, Sn, and Cl are present only on the surface of the silica support. Furthermore, the Pt, Sn, and Cl coverages are low, thus to a good approximation, it should be possible to ignore attenuation of Pt, Sn, and Cl photoelectrons.43 In that case, the Sn/Pt XPS intensity ratio (ISn/IPt) is related to the Sn/Pt stoichiometry (XSn/Pt) by the ratio of the Sn 3d and Pt 4f photoemission cross-sections: ISn/IPt = (σSn/σPt)XSn/Pt. For this analysis, we used the subshell photoionization crosssections (σ) of Yeh and Lindau.46 For the Pt-free SiO2 samples, the Sn concentration can still be estimated using this approach, by simply referencing the Sn intensity to that in one of the samples containing Ptn, thus allowing the estimation of the Sn concentration per unit area of the surface. The extracted stoichiometry information is presented in Table 1.
reciprocal space would be small. The cut-off energy of 0.4 eV was used to choose the thermodynamically accessible isomers at relevant temperatures. Note that to produce the initial cluster geometries on the surface, we use our in-house code, PGOPT, which automatically generates these structures based on the bond length distribution algorithm.63 Then, each structure was optimized using DFT calculation, and duplicates were filtered out thereafter. In this study, 200 unique structures were generated and optimized to find the putative global minimum. More detailed discussion about obtaining the vibrational partition functions and corrected Boltzmann population can be found here.63 Finally, Bader charge analysis64−67 was used to obtain partial charges on each atom in the cluster.
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RESULTS AND DISCUSSION We focus on three aspects of the PtnSnx/SiO2 model catalysts: selectivity of SnCl4 adsorption on Ptn sites, removal of Cl and H adsorbates to produce PtnSnx clusters, and the effects of Sn alloying on the available binding sites associated with the clusters. Experiments have been done for Pt4, Pt7, and Pt24 seed clusters to examine the effects of cluster size on the stoichiometry of the resulting PtnSnx. The theory was done only for the computationally tractable case of Pt4 seed clusters. Selectivity of Adsorption in Association with Ptn Sites. Figure 1 shows the Sn 3d and Pt 4f XPS measured after applying the 300 K H2/SnCl4/H2 treatment to Ptn/SiO2 (n = 4, 7, 24). For comparison, data are also shown for a Ptfree SiO2 sample after 300 K H2/SnCl4/H2 treatment and for Ptn/SiO2 (n = 4, 7, 24) samples that were exposed to just a 300 K SnCl4 dose, without deliberate H2 exposure. The Pt 4f7/2 binding energies measured for the as-deposited Ptn/SiO2 samples (no gas exposure) are indicated by black vertical dashed lines, as are literature values for Pt 4f7/2 in bulk Pt68 and for Sn 3d5/2 in bulk Sn and Sn halides.69 All doses were performed at 300 K sample temperature, and the samples were not heated, so that the intensities reflect the total amount of Sn adsorbed at 300 K. Several points are clear even in the raw spectra. The Pt intensity is constant from sample to sample, as might be expected, given that the total number of Pt atoms deposited on each sample was identical. This result is actually quite important in the analysis below, because it shows that the fraction of the deposited Pt in the central 1.1 mm of the ∼2 mm cluster spot is identical for all samples, i.e., the spot profiles are independent of the cluster size. Note that the Pt 4f binding energies for the as-deposited clusters (black dotted lines) are all well above the binding energy for bulk Pt, and that the binding energy increases with the decreasing cluster size. As shown previously for Ptn/SiO2,12 these binding energy shifts are unrelated to the cluster oxidation state. Instead, they are an inherent effect of the cluster size, resulting from less stabilization of the photoemission final state for small clusters on an insulating support. Note that after the full H2/SnCl4/H2 treatment, the Pt binding energies shift to lower energy for Pt4 and Pt7 but not for Pt24. Both the Pt and Sn binding energies remain strongly dependent on the Pt seed cluster size. The amount of Sn present is substantially higher for the Ptn/ SiO2 samples, compared to Pt-free SiO2. That this happens, even though the Ptn coverage is only 0.1 ML equivalent, implies strongly preferential binding of SnCl4 to the clusters. It is also clear that there is substantially more Sn present for the
Table 1. Ptn Surface Density from Deposition and Sn Coverage as Determined from XPS sample
Ptn per 10 nm2
Sn per 10 nm2
Sn per cluster
SiO2 Pt4 Pt7 Pt24
none 4 2.2 0.64
3.5 16.7 17.4 8.4
no clusters 3.3 6.3 7.5
It is important to note that, as mentioned above, doubling the SnCl4 exposure results in no additional Sn deposition on the clusters, i.e., the Sn deposition is driven to saturation. This result is important, because it shows that the reaction leading to Sn deposition is self-limiting, presumably by the availability of binding sites on the size-selected hydrogenated Ptn clusters and the bulkiness of the SnCl4. For a self-limiting reaction driven to saturation, there should be little if any cluster-tocluster variation in the number of Sn atoms delivered to each cluster, apart from possible small variations due to different cluster geometries on the surface. Thus, both the size of the Pt seed and the number of Sn atoms added are controlled by this method. As shown in Figure 1, some Sn is deposited even for Pt-free SiO2, corresponding to ∼3.5 Sn atoms per 10 nm2 area on the support (3.5 × 1013/cm2), presumably due to SnClx adsorption at defects in the silica film. For reference, the number of Si and O atoms per 10 nm2 can be estimated from the bulk density of amorphous silica to be ∼235. The number of Sn adsorbed per 10 nm2 increases substantially when Ptn are present, particularly for the smaller clusters. The Pt/Sn stoichiometry can be estimated within two limits. If we assume that the presence of deposited Ptn does not change the number of Sn adsorbed at silica defect sites, then the stoichiometries are Pt4Sn3.3, Pt7Sn6.3, and Pt24Sn7.5. This assumption is appropriate 16198
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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alumina found that single-layer isomers dominate for Pt7, accounting for the high Pt ISS intensity, but that Pt8 is dominated by prismatic, multilayer isomers where only a fraction of the Pt atoms are ISS accessible. Evidently, this transition to multilayer isomers occurs at a smaller cluster size for the SiO2 support. Figure 3 compares ISS spectra for Pt4/SiO2 samples asdeposited (A) and with H2/SnCl4/H2 treatment (B). For
if the deposited Ptn do not block Sn adsorption at silica defect sites, and if Sn adsorbed at silica defect sites does not migrate to Ptn. The other limit is to assume that all of the Sn for Ptcontaining samples are bound to the clusters, either asdeposited, or when the samples are subsequently heated to desorb HCl. In that limit, the stoichiometries would be Pt4Sn4.4, Pt7Sn7.9, and Pt24Sn13.1. For the smaller clusters, the stoichiometries are similar in the two limits, and with Pt/Sn close to 1:1. For Pt24, the difference between the two limits is larger, but in either limit, the smaller amount of adsorbed Sn results in Sn/Pt ratios that are substantially lower than those for the smaller clusters. This lower amount of Sn presumably explains why the Pt 4f binding energy did not shift significantly upon H2/SnCl4/H2 treatment (Figure 1). The origin of the lower number of Sn adsorption sites is likely related to the sizedependence of the Ptn cluster geometry. We previously showed using grazing incidence small-angle X-ray scattering that Pt24/ SiO2 deposited under identical conditions has a sharp scatterer size distribution close to what would be expected for Pt24 with a hemispherical shape.12 Smaller clusters did not give enough scattering signal to extract structural information, however, morphology can also be inferred from ISS measurements, as shown in Figure 2, which gives the raw ISS spectra for the
Figure 2. Raw ISS spectra of (black) Pt4/SiO2; (red) Pt7/SiO2; (blue) Pt24/SiO2. Counts are plotted as a function of the scattered kinetic energy (Ek) to incident kinetic energy (Eko) ratio. The Pt fraction represents the fraction of total scattering counts in the Pt peak.
three cluster sizes, as-deposited. As a result of shadowing and blocking, low He+ ion survival probability (ISP) during scattering, the three ISS peaks primarily derive from He+ scattering from O, Si, or Pt atoms in the top-most sample layer.45,51,70,71 Multiple or 2nd layer scattering tends to contribute to the broad background that appears at low E/ E0. For samples containing the same total number of Pt atoms, as is the case here, the Pt peak intensity would be similar if cluster morphology was size-independent. Instead, the Pt peak intensities (inset table) decrease with the increasing size, indicating a transition to more compact, multilayer structures in which only a fraction of the Pt atoms are in the ISSaccessible surface layer. Fewer exposed Pt sites presumably would also result in less SnCl4 adsorption, as observed. It is interesting that for Ptn/SiO2, there is already a significant decrease in Pt ISS intensity at Pt7. This is in contrast to what was observed for Ptn/alumina16 where Pt ISS intensity is constant from Pt2 to Pt7, then drops abruptly for Pt8 and larger clusters. A DFT study of the distribution of Ptn isomers on
Figure 3. Raw ISS spectra for as-prepared Pt4/SiO2 (A); H2/SnCl4/ H2−Pt4/SiO2 (B); and SiO2 (C).
comparison, the spectrum for H2/SnCl4/H2-treated SiO2 is shown in frame C. The spectra have peaks corresponding to He+ scattering from O, Si, Cl, Sn, and Pt atoms in the surface layer. Because different elements have different scattering cross-sections and ISPs, peak intensities are not directly proportional to surface layer concentrations, however, changes in the peak intensities are directly related to changes in the surface layer. For example, the 300 K H2/SnCl4/H2 treatment has several effects. The Pt peak is attenuated by ∼90%, indicating that the Pt clusters are covered by some 16199
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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Figure 4. ISS sputter series before and after heating to 800 K to remove H and Cl. (A) and (C) are for (H2/SnCl4/H2)−Pt4/SiO2, (B) and (D) are for (H2/SnCl4/H2)−Pt24/SiO2. Pt signal is shown with blue circles, Sn signal with hollow red circles, Cl signal with green diamonds. Insets are the raw ISS spectra collected at the beginning of each series. The blue stars in the two top insets show the Pt intensity for as-deposited Pt4/SiO2 and Pt24/SiO2.
For unheated, H2/SnCl4/H2-treated Pt4/SiO2 (Figure 4A), it can be seen that Sn intensity is higher than the Pt intensity at the beginning of the experiments, but the Pt signal increases during the initial ∼150 μA·s of He+ exposure, indicating that additional Pt atoms are being exposed by sputtering of overlying H, Cl, and Sn, at a rate faster than Pt loss by sputtering. This observation is consistent with the ∼90% attenuation Pt ISS intensity caused by the H2/SnCl4/H2 treatment, relative to the intensity for adsorbate-free Pt4 (Figure 2). At ∼150 μA·s, the Pt intensity begins to decline as the rate of Pt sputter loss exceeds the rate at which additional Pt is exposed. At this point, the Pt intensity is still well below that for as-deposited Pt4, indicating that there is still significant adsorbate coverage. The Cl ISS intensities simply decrease monotonically with He+ exposure, as would be expected if they are present only in the surface layer. The Sn intensity increases slightly for very low He+ exposures, but after that it also decreases monotonically. This pattern suggests that the sample has Pt largely covered by adsorbed Sn, Cl, and H, whereas the Sn is more likely covered, presumably by Cl. For unheated, H2/SnCl4/H2-treated Pt24/SiO2 (Figure 4B), the results are similar. Again, the Cl intensity declines monotonically, whereas the Pt and Sn intensities initially increase slightly before beginning a monotonic decline. The initial Sn intensity is lower than Pt4, as might be expected from the lower amount of Sn deposited on the Pt24. The initial Pt intensities are simlar for the Pt4 and Pt24 samples, but note that the Pt signal for the adsorbate-free, as-deposited samples is much lower for Pt24 and Pt4, as discussed above.
combinations of adsorbed Sn, Cl, and H adsorbates, which attenuate He+ scattering from the underlying Pt. In contrast, the Si and O intensities are essentially unaffected by H2/ SnCl4/H2 exposure, indicating that the adsorbate coverage on the silica support is small. New peaks are observed for Sn and Cl, indicating that both are adsorbed in the surface layer. The Cl peak is just in the energy range where the multiplescattering background increases, and to help indicate the expected variation of background with energy, we fit this energy range of the Cl-free spectrum in Figure 3A and plot the fit as a smooth curve in Figure 3B,C. Figure 3C shows analogous results for Pt-free SiO2 after H2/SnCl4/H2 exposure. As expected from the XPS results, there is a small Sn peak and possibly a slight increase in the intensity in the range expected for Cl, but clearly, there is less Sn and Cl in the surface layer, compared to the sample with Pt4. Additional insight into the surface morphology can be obtained by observing changes in ISS signals as the surface is slowly sputtered by the He+ beam, as shown in the top frames of Figure 4 for Pt4/SiO2 and Pt24/SiO2 samples examined immediately after 300 K H2/SnCl4/H2 treatment and in the bottom frames for analogous samples measured after heating to 800 K at 3 K/s. Data for Pt7/SiO2 is quite similar to that for Pt4, which is not surprising given that the Pt/Sn stoichiometry for both is similar. The insets show the raw ISS measured at the beginning of each sputtering experiment, and the blue stars represent the Pt intensity observed for the as-deposited (i.e., adsorbate-free) Ptn/SiO2 samples. 16200
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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The Journal of Physical Chemistry C Removal of Chlorine. Although there is clearly significant HCl desorption during the 300 K H2/SnCl4/H2 treatment, the ISS results clearly show that some Cl remains on the surface. Given the large final H2 exposure, the samples almost certainly also have significant adsorbed hydrogen, not detectable by either ISS or XPS. Several experiments were done to examine the removal of Cl (and H) by heating the samples. The effects of heating on ISS can be seen by comparing the top and bottom rows of Figure 4, which compares ISS raw spectra and exposure series data for Pt4 and Pt24 samples given the 300 K H2/SnCl4/H2 treatment, with analogous data for samples flashed to 800 K at 3 K/s after the H2/SnCl4/H2 treatment. As shown in the inset spectra, the small Cl peaks observed in the unheated samples (frames A and B) disappear after heating (C and D). The multiple-scattering background in the energy range of the Cl peak makes it difficult to estimate the decrease in Cl intensity quantitatively, but as in Figure 3, we have indicated the estimated background level in each of the raw spectra. Clearly, most, if not all, of the Cl is removed from the surface area by 800 K heating. XPS, which is sensitive to the top few nanometers of the sample, was also used to examine the effects of heating on the Cl concentration. The key result, summarized in Figure S1, is that the Cl 2p XPS signal is extremely weak after the H2/ SnCl4/H2 treatment, even before the sample is heated. Given that ISS clearly shows some Cl in the surface layer, the Cl XPS was analyzed as described in the text accompanying Figure S1, resulting in an estimated Cl coverage for H2/SnCl4/H2-treated Pt4/SiO2 corresponding to only ∼5 Cl atoms/10 nm2. This can be compared to the Sn coverage of ∼16.7 Sn/10 nm2 (Table 1). This much Sn (as SnCl4) would have brought ∼67 Cl atoms/10 nm2 to the surface, thus the weakness of the Cl 2p XPS intensity implies that less than ∼10% of the expected Cl is present after the 300 K H2/SnCl4/H2 treatment, the rest having presumably desorbed as HCl. Furthermore, the weak Cl XPS signal disappears entirely after the sample is heated (Figure S1), i.e., like the ISS results in Figure 4, the Cl XPS results also imply that no Cl remains on the samples after heating, within the sensitivity limits of the two measurements. To probe the temperature dependence of Cl loss and accompanying changes in the number and energetics of binding sites on the clusters, a series of TPD experiments was performed, monitoring desorption of HCl, Cl2, and CO, which was used as a probe of exposed metal sites. Each sample was prepared by Ptn deposition followed by one of the H2 and SnCl4 exposure protocols at 300 K, then cooled to 180 K and exposed to 10 L of 13CO to saturate any binding sites stable at that temperature. The samples were then heated at 3 K/s, while monitoring desorption of 13CO, H35Cl, and 35Cl2. To look for the desorption of Sn or of volatile Sn compounds, some experiments also monitored 120Sn+, however, no signal was seen. Figure 5 shows H35Cl desorption during the first TPD run for each sample and for comparison, the HCl desorption from H2/SnCl4/H2-treated SiO2 is also plotted in each panel. No HCl was observed in any subsequent TPD runs. Essentially identical HCl desorption results were observed in a few experiments in which the 180 K CO exposure was omitted, i.e., exposure to CO has no significant effect on the surface coverage of the adsorbates that lead to HCl or on the kinetics desorption during TPD. Mass 70 signal (35Cl2+) was also monitored, however, no significant signal was observed, i.e., the main Cl-removal mechanism is HCl loss.
Figure 5. HCl thermal desorption comparing Ptn and H2/SnCl4/H2treated Ptn (red). Also shown in each frame is the HCl desorption from H2/SnCl4/H2-treated SiO2 (black).
First consider the HCl desorption observed for H2/SnCl4/ H2-treated SiO2, shown in all three of the frames. Desorption starts at 300 K, plateaus between 450 and 600 K, and then increases again at higher temperatures. The integrated HCl signal observed in this temperature window corresponds to only ∼1.3 × 1013 HCl/cm2 or 1.3 HCl/10 nm2. This corresponds to less than 10% of the ∼14 Cl/10 nm2 that would have accompanied the ∼3.5 Sn/10 nm2 that are observed (Figure 1, Table 1) binding to the treated SiO2 surface. The Cl 2p XPS signal on this surface was far too weak to measure, as might be expected from the observation that Sn adsorbs more efficiently when Ptn are present (Table 1). The fact that the HCl signal continued to increase up to 800 K might be taken as evidence that 800 K was not hot enough to desorb all of the Cl; however, in our TPD arrangement, it is not uncommon to see increased background at high sample temperatures due to desorption from the sample holder surfaces that are warmed by the conduction from the hot sample. Normally, this background is small compared to the desorption signal; however, in this case, the actual HCl desorption is quite weak. Furthermore, the sample holder and support structure are at cryogenic temperature during the 300 K H2/SnCl4/H2 treatments, and this apparently results in the 16201
DOI: 10.1021/acs.jpcc.9b02745 J. Phys. Chem. C 2019, 123, 16194−16209
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The Journal of Physical Chemistry C condensation of material that evolves HCl when warmed. Figure S2 and the accompanying text discuss an experiment, which demonstrates that most, if not all, of the apparent HCl desorption signal at high sample temperatures is simply background. For the Ptn/SiO2 samples, the HCl signals are larger than those for SiO2, as might be expected since XPS and ISS both show that substantially more SnCl4 adsorbs when Pt is present. The “extra” HCl desorption is largely in the ∼300−650 K range. Again, the high HCl background at high sample temperatures makes it difficult to determine the amount of HCl actually desorbing from the samples; however, we can estimate an upper limit by simply including all of the HCl in the 300−650 K range. This amounts to ∼2.4, ∼2.5, and ∼1.7 HCl/10 nm2 for the three Pt4, Pt7, and Pt24, in each case, only a small fraction of the ∼66.8, 68.8, and 33.6 Cl/10 nm2 upper limits from the measured Sn coverages assuming a 4Cl:1Sn maximum (Table 1). It should be noted that no HCl desorption is observed during a second CO TPD run nor is any HCl desorption seen if the heated sample is cooled, exposed to additional hydrogen, and then heated to 800 K. Summary of Results Pertaining to PtnSnx Preparation. The data regarding the amount of Sn and Cl delivered to and remaining on the surface can be summarized as follows. SnCl4 has a strong propensity to bind at Pt cluster sites. For example, 16.7 Sn bind/10 nm2 with Pt4 is present at ∼10% coverage, compared to ∼3.5/10 nm2 on SiO2, implying that binding is ∼40 times more efficient on Pt sites, compared to SiO2. H2 preadsorption certainly enhances Sn binding. Indeed, because the clusters always have significant adventitious H2 exposure, we cannot exclude the possibility that hydrogen preadsorption on the Pt clusters is necessary for SnCl4 binding. HCl(g) is generated during the 300 K H2/SnCl4/H2 treatment, and Cl 2p XPS shows that
