4. 1 INTRODUCTION
Limited fossil fuel resources have stimulated great research efforts in sustainable production of fuels and chemicals from renewable biomass.1–9 Unsaturated aldehydes such as furfural are among the most promising biomass platform chemicals derived from acidic hydrolysis of hemicellulose. Selective catalytic hydrogenation of unsaturated aldehydes to unsaturated alcohols is a highly desirable reaction due to many applications of unsaturated alcohols in the manufacture of foundry resins, synthetic fiber, farm chemicals, adhesives and fine chemicals.10–18 Unsaturated aldehydes such as furfural have more than one active functional group, which can lead to many possible reaction routes during catalytic hydrogenation; these include total hydrogenation to the alcohol, and other possible side reactions such as decarbonylation and ring-opening reactions to alkenes as shown in Figure 4.1.11,13,17,19–25
Cu-Cr catalysts used in conventional furfural hydrogenation are not ideal due to the toxicity of chromium and the deactivation of the catalyst either through sintering or carbon deposition; thus, the development of Cr-free catalysts is desirable.2,17,26–28 Significant interest has been drawn to achieve well defined bimetallic catalysts for selective hydrogenation of furfural. Different metals such as Pt, Cu, Ni, Pd and Ir supports such as (TiO2, ZrO2, SiO2 and CeO2) have been used to study selective hydrogenation of furfural reaction both in vapor and in liquid phases.10,29–37
Pure Pt is one of the most commonly used metals in hydrogenation reactions. However monometallic Pt tends to favor hydrogenation of C=C bonds over C=O for molecules that have both C=C and C=O bonds.37,38 In order to overcome the unsatisfactory performance of monometallic catalysts, the development of highly selective bimetallic catalysts has been proposed for the production of unsaturated alcohols.
Despite a number of investigations in this field, there is not an agreement on the nature on the catalytic effect improving the selectivity and conversion. Selectivity for furfural alcohol has been significantly improved by addition of Sn to Pt based catalysts to form bimetallic systems.14,21,26,32,37–39 It is believed that the bi-functional nature of the Pt- Sn catalysts allow selective hydrogenation of C=O bonds with high activity. It is well known that Pt is active for hydrogenation40,41 while strong interactions of oxophilic Sn with C=O will lead to preferential hydrogenation.42,43 The TiO2 support will reduce carbon fouling, enhancing the stability of catalysts. The choice of support can also play a vital role for metal-based catalysts improving catalytic activity as a result of electronic and chemical interactions. It is known that high temperature reactions under reducing conditions favor partial coverage of Pt with titanium suboxides (TiO2-x) due to strong metal support
interactions (SMSI). As a result, the carbonyl oxygen atom strongly interacts with coordinatively unsaturated titanium cations of the partially reduced support which act as Lewis acid sites.13,21,34
The origin of the selectivity improvement for selective hydrogenation of unsaturated aldehydes to unsaturated alcohols has been discussed in terms of alloy formation. Studies reported that the intrinsic selectivity and activity of Pt based catalysts can be enhanced by alloying with electropositive elements such as Sn, Fe or Co.44–48 Selective hydrogenation of furfural is sensitive to surface crowding and associated changes in the adsorption geometry. Specifically, changes in furfural adsorption configuration, from tilted to planar, influence product selectivity on Pt(111) single crystals.49
DFT studies have been reported in literature to help understand the mechanism for the furfural hydrogenation on Cu/Pd/SiO2 surfaces and predicts that aldehydes adsorb exclusively in the ŋ1(O) configuration on pure Cu surfaces.31,50 Furfural on Pt(111) is found to bind the surface via the aromatic ring, which facilitates C-C bond scission and ring opening, which leads to nonselective decomposition to CO and H2.29
In this particular study, experiments have been performed to understand the growth, interactions and chemical compositions of model systems prepared by the vapor deposition of Pt-Sn clusters on TiO2(110) under ultrahigh vacuum (UHV) conditions. The nucleation and growth of Pt, Sn and Pt-Sn clusters on TiO2(110) were studied using scanning tunneling microscopy (STM). In addition, these surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS) techniques to investigate electronic interactions and chemical composition. Similar characterization studies were also performed by depositing Sn films on a Pt(111) single crystal. LEIS
experiments were also conducted on Pt-Sn clusters grown on an inert support, highly oriented pyrolytic graphite (HOPG), to compare with the growth on TiO2(110).
4.2 EXPERIMENTAL SECTION
Growth and surface characterization of Pt-Sn clusters were carried out in an ultrahigh vacuum (UHV) chamber that has previously been described in detail elsewhere.51,52 This chamber has a base pressure below 2x10-10 Torr and is equipped with a number of surface analytical techniques, such as scanning tunneling microscopy (STM, Omicron VT-25), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), (Omicron EA 125), and Auger electron spectroscopy (AES)/ low energy electron diffraction (LEED) system (Omicron Spec 3).
A rutile TiO2(110) crystal (Princeton Scientific Corporation, 1 cm x 1 cm x 0.1 cm) mounted on a tantalum back plate was used as the oxide support for the growth of metal clusters. The crystal was cleaned by Ar+ ion sputtering at 1 kV for 20 minutes. This step is immediately followed by annealing to ~1000 K. Heating was achieved via electron bombardment of the positively biased sample, and the temperature of the crystal was monitored by an infrared pyrometer (Heitronics). This treatment results in a TiO2 crystal that is sufficiently conductive for STM, XPS and LEIS experiments.
In addition, a Pt(111) single crystal (99.999%, Princeton Scientific Corporation, 8 mm diameter, 2 mm thickness) was used in which the crystal was mounted by press-fitting two Ta wires into 1.1 mm deep slots cut into the sides of the crystal. The Pt(111) crystal was cleaned by sputtering with 1 kV Ar+ ions for 20 minutes, followed by annealing to
1000 K for 3 minutes by electron bombardment. The temperature of the crystal was monitored using a Heitronics CT 18 pyrometer.
Additionally, a HOPG (SPI supplies, SPI Grade 2, 10 mm x 10 mm x 1 mm) crystal was used for the LEIS experiments, where the crystal was mounted on a standard Omicron Ta back plate using thin Ta foil straps. A clean HOPG surface was achieved by cleaving the surface with adhesive tape in air before each experiment.
Physical vapor deposition was carried out with an Oxford Applied Research (EGCO4) four pocket electron-beam evaporator using a Pt rod (ESPI, 4N, 0.08” x 1”) or Sn pellets (ESPI, 5N purity, 1-3 mm) in a Ta crucible. A bias of +800 V was applied to the surface during deposition in order to avoid deposition of energetic positively charged ions. Metal flux was determined using an independently calibrated quartz crystal microbalance (Inficon, XTM-2). The metal coverage is defined in monolayer (ML) equivalents, which correspond to packing density of Pt(111) ( 1.50 x 1015 atoms/cm2 ) for both Pt and Sn.
STM experiments were carried out to investigate the growth and surface morphology of Pt and Sn clusters and alloys grown on TiO2(110) and Pt(111). STM images were collected at a constant tunneling current of 0.1-0.2 nA, and the sample was biased at +2.3 V with respect to the STM tip for imaging on TiO2, while the crystal was biased at +0.35 V with respect to the tip at ~4.0 nA for Pt(111). STM tips were made by electrochemically etching a 0.38 mm diameter tungsten wire in NaOH.53 Tips were further conditioned by sputtering with argon ions at 3 kV and pulsing to higher voltages up to 10 V.
Low energy ion scattering (LEIS) experiments were performed with 600 eV He+ ions, and acquisition times were adjusted to minimize the damage from He+ during
successive scans. Low energy electron diffraction (LEED) patterns were collected using an 80 eV beam energy.
4.3 RESULTS AND DISCUSSION
Growth Studies of Sn on TiO2 (110)
Previous studies by Chen group have demonstrated growth of a variety of pure and bimetallic clusters (Pt, Re, Au, Ni, Co) on TiO2(110) and characterized particle sizes and size distributions by STM.54–57 Similarly, pure 0.25 ML Sn clusters were grown on TiO2(110), as shown in Figure 4.2, with comparison of similar coverages of pure metals Pt, Re, Ni, Co and Au on TiO2(110). However, the growth and characterization studies of Sn were performed on a different TiO2 single crystal in comparison to all the growth studies of other metals shown in Figure 4.2 and respective cluster height and density measurements in Table 4.1. The admetal-titania bond strength dictates how metals such as Co, Au, Pt, Re and Ni grow on titania. Metals that have stronger admetal O-bonds with TiO2 support tend to form smaller cluster sizes and larger cluster densities due to shorter diffusion lengths. Sn being an oxophilic metal resulted in strong interaction with TiO2, which led to the formation of smaller cluster sizes with larger cluster density compared to the highly mobile metal of Au, which has a low affinity for oxygen.
Characterization of Pt-Sn clusters on TiO2(110)
The ability to prepare metal particles of different coverages is crucial for understanding how metal composition, metal particle sizes and the number of interfacial sites affect the activity. XPS experiments performed on different coverages of Sn clusters deposited on TiO2(110) demonstrated that Sn is partially oxidized by the TiO2. Oxidation of Sn is clearly evident especially for lower coverages of Sn where all the deposited Sn is
in direct contact with the TiO2(110) forming SnOx at the metal-support interface. The second peak at 486.8 eV in the Sn(3d) region shown in Figure 4.3 corresponds to the SnOx
Surface Average height (Å) Cluster density (clusters/cm2) 0.25 ML Au 14.3+4.1 2.39x1012 0.25 ML Ni 10.9+2.7 4.77x1012 0.25 ML Pt 6.2+2.1 1.13x1013 0.25 ML Co 5.2+1.5 1.73x1013 0.25 ML Re 3.2+1.1 2.82x1013 0.25 ML Sn 11.53+1.5 8.58x1012
Table 4.1. Average cluster heights and densities for 0.25 ML of metal deposited on TiO2(110) at room temperature.
Figure 4.2: STM images of 0.25 ML of the following metals deposited at room temperature on TiO2(110): a) Au; b) Ni; c) Pt; d) Co; d) Re and f) Sn. All images are 1000 Å x 1000 Å.
a b c
d e f
a b c
in comparison to the metallic Sn peak at 484.8 eV. Oxidation states of Sn4+ and Sn3+ are hard to distinguish due to very similar binding energies, but it is possible to distinguish Sn4+ and Sn3+ based on their different Auger peak shapes.58 For the higher coverages of Sn, only a fraction of deposited Sn is at the interface with the TiO2(110) surface. As the fraction of metallic Sn increases with Sn coverage, that gives rise to a higher binding energy shoulder peak for SnOx. Furthermore, Figure 4.4 depicts the normalized intensity of Ti(2p) peak, comparing the clean and 2 ML Sn deposited TiO2(110); reduction of Ti4+ is observed as a lower binding energy shoulder upon Sn deposition. Sn is likely to be partially oxidized by the titania, initially forming 2-dimensional islands at low coverages while more 3-
Binding Energy (eV) C P S ( x10 3 ) Metallic Sn 484.8 eV SnO x 486.8 eV
Figure 4.3: X-ray photoelectron spectroscopy data of Sn(3d) region for different coverages of Sn deposited on TiO2(110).
dimensional particles containing metallic Sn form at higher coverages, as has been observed for metals such as Re/TiO2(110).56
Figure 4.5 shows STM images for 2 ML Pt deposited on TiO2(110) followed by 2 ML Sn. Bimetallic clusters have formed as a result of sequential deposition of Sn on top of Pt as the initially deposited metal has provided enough nucleation sites for the second metal growth. Due to the fact that Sn and Pt alloy at all compositions,59 some diffusion of Sn into the Pt particles is expected in this order of deposition. However, the amount of SnOx formed should be small given the limited Sn-titania interface.
No rm ali ze d in te ns it y
Binding Energy (eV)
Figure 4.4: X-ray photoelectron spectroscopy data of Ti(2p) region for clean TiO2(110) (green) and 2 ML Sn deposited on TiO2(110). (110)(red).
Clean TiO2
The compositions of the model Pt-Sn bimetallic surfaces were characterized by LEIS, which probes only the composition of the top monolayer of the surface compared to XPS, which probes the first several layers of the surface. The LEIS data acquired for the two bimetallic surfaces are shown in Figure 4.6 for 2 ML Sn+2 ML Pt and 2 ML Pt+2 ML Sn in Figure 4.6a and 4.6b respectively. Regardless of the orders of metal deposition, both Pt and Sn are present on the surface. In the case of 2 ML Pt deposited on top of Sn, the number of Pt sites is 15% of than on 2 ML Pt/TiO2 based on LEIS data. In the reverse order of deposition, number of Pt sites is 3% of than on 2 ML Pt/TiO2. This can be explained due to the fact that Sn has lower surface free energy (0.51 J/m2) compared to Pt (2.5/m2).60,61 Due to the lower surface free energies, Sn preferentially remains at the surface whereas Pt is more energetically favored to diffuse subsurface.
a b
Figure 4.5: STM images of a) 2 ML Pt, b) 2 ML Pt+2 ML Sn deposited at room temperature on TiO2(110): All images are 1000 Å x 1000 Å.
XPS studies were also carried out on Pt-Sn/TiO2(110) bimetallic surfaces in order to understand metal-metal and metal-support interactions. For the Pt-Sn surfaces where Sn is deposited on top of Pt clusters, the Pt(4f7/2) peak shifts from 70.9 eV to 71.5 eV. This higher binding energy shift in Pt(4f7/2) is attributed to undercoordinated Pt atoms at the surface having a lower binding energy than the fully coordinated Pt atoms in the bulk; therefore, the presence of surface species diminishes the contribution of the lower binding energy peak from undercoordinated Pt, resulting in an overall shift to higher binding energies as shown in Figure 4.7a. A fraction of the Sn is oxidized at the metal-titania interface as evident in Figure 4.7b. Strong binding of Sn to the titania support could inhibit intermixing of the Pt and Sn atoms within the clusters, especially for the surfaces prepared by depositing Pt on the Sn clusters.
Figure 4.6: LEIS data of a) 2 ML Sn+2 ML Pt, b) 2 ML Pt+2 ML Sn deposited at room temperature on TiO2(110).
Growth and Characterization of Sn on Pt(111)
Understanding the Pt-Sn interactions and compositions on the strongly interacting oxide support TiO2 is complicated. Sn being an oxophilic metal resulted in strong binding of Sn to the titania support that could inhibit intermixing of the Pt and Sn atoms within the clusters. Therefore, an understanding of Pt-Sn interactions and compositions on strongly interacting oxide support TiO2 brings extra complexity to isolate metal-metal interactions. The origin of the selectivity improvement Pt-Sn system has also been discussed in terms of alloy formation.62 Therefore, growth and surface composition characterizations were performed by growing Sn on a Pt(111) single crystal. STM image of 1 ML Sn deposited on to Pt(111) at room temperature (Figure 4.8a) and the subsequent annealing of the surface
Figure 4.7: X-ray photoelectron spectroscopy data of a) Pt(4f) region and b) Sn(3d) region for Pt-Sn deposited on TiO2(110).
at 800 K for 1 minute (Figure 4.8b) and 1000 K for 1 minute (Figure 4.8c) clearly demonstrate that Sn is driven subsurface upon annealing. STM images Figure 4.8b and Figure 4.8c demonstrate atomic structures due to the formation of surface alloy. The surface alloy formation was further supported by LEED experiments performed on the same surface, as shown in the obtained LEED pattern in Figure 4.9. Surfaces with more Sn rich Pt2Sn/Pt(111) monolayer alloy film was reported which exhibited a (√3x√3)R30° superstructure with (θ=0.33) annealing around 700 K. The formation of Pt3Sn/Pt(111) surface alloy with (θ=0.25) was reported p(2x2) LEED pattern, which required 1000 K for the diffusion of excess Sn into the Pt bulk according to literature.25,63 In our work, the clean Pt(111) crystal has a primitive (1x1) structure whereas 1 ML Sn annealed at 1000 K gives a mixture of (2x2) (0.25 ML Sn): Red ; √3x√3R30° (0.33 ML Sn): Blue, LEED pattern agrees with structures reported in literature in which Sn/Pt(111) alloys are known to form these two ordered surfaces.25,59,63,64
Both surface alloys are known to exhibit different electronic and geometric properties compared to pure Pt.63 STM experiments reported in literature suggest a
a b c
Figure 4.8: STM images for a) 1 ML Sn at room temperature, b) 1 ML Sn annealed at 800 K for 1 minute, c) 1 ML Sn annealed at 1000 K for 1minute on Pt(111): All images are 1000 Å x 1000 Å.
lowering of local density of states (LDOS) at the Fermi level.48 This behavior is correlated with a downshift of the d-band center and a negative polarization of Pt due to the presence of Sn based on the density functional theory (DFT) calculations on well-defined Pt-Sn alloy surfaces p(2x2) Pt3Sn(111) and (√3x√3)R30° Pt2Sn(111).62
The change in surface composition of 1 ML Sn deposited on Pt(111) was studied using LEIS to understand the differences in surface composition of Pt-Sn as a function of the annealing temperature. Pt single crystal is ideal for this study because there is limited control over the composition of Pt-Sn clusters on TiO2 during annealing treatments; due to strong metal support interactions (SMSI). SMSI occurs, when partially reducible oxides such as TiO2, modify the chemisorption behavior of hydrogen and carbon monoxide on metals from group 8, 9, and 10 at higher temperatures (about 773 K) under reducing environment.21 Furthermore, this interaction was found to modify the activity and selectivity of noble metals in different reactions that involve molecules containing carbonyl bonds.21 Therefore, Pt(111) single crystal provides a better approach to address surface composition studies, particularly on alloy surfaces. The surface composition of 1 ML Sn
Figure 4.9: LEED pattern for a) clean Pt(111), b) 1 ML Sn annealed at 1000 K for 1 minute on Pt(111).
deposited on Pt(111) and the change in surface composition as a function of annealing temperature is depicted in Figure 4.10. The almost complete attenuation of the Pt signal indicates that Sn covers the Pt surface at room temperature. Sn is driven subsurface upon annealing, although all of the surfaces contain both Pt and Sn. Therefore, the surface concentration of each metal was determined based on the attenuation of Pt signal compared to Pt(111), and 25% Sn at 1000 K is consistent with the observed LEED pattern, which suggests a coverage between 0.25 and 0.33 ML.25,63
Surface Composition of Pt-Sn on HOPG
Pt-Re clusters were also grown on HOPG, to de-convolute the effects from the strongly interacting oxide support such as TiO2. The surface composition of the bimetallic clusters was studied using LEIS by depositing 2 ML Pt and 2 ML Sn via both orders of deposition on the freshly cleaved HOPG. LEIS experiments were conducted using a low
Kinetic Energy (eV)
C P S ( x10 3 )
Figure 4.10: LEIS data for change in surface composition of 1 ML Sn on Pt(111) upon 1 minute annealing at different temperatures.
He+ beam current at 25 nA to avoid damaging the surface from He+ ions. Both Pt and Sn are present on the surface regardless of the order of deposition, and the surface free energies dominate leaving Sn, which has the lower surface free energy, at the surface. After annealing the surface at 600 K for 3 minutes, LEIS data was collected to understand the change in surface composition upon annealing. The same procedure was carried out until there was no further change in surface composition. The data shows that the surface has reached its thermodynamically equilibrated surface composition as shown in Figure 4.11. This confirms that, spectra collected after subsequent annealing did not appear to change over 15 minutes. Successive scans further confirm that the surface composition was not greatly affected by the bombardment of the He+ on the surface during the collection of the spectra throughout the LEIS experiments.
Kinetic Energy (eV)
C P S ( x10 3 ) (a) (a)
4.4 CONCLUSIONS
Pt-Sn model surfaces of well-defined size and metal compositions were prepared and surfaces were used to study growth behavior and they were characterized for metal- metal, metal-support interactions. Sn clusters deposited on TiO2(110) are oxidized by lattice oxygen from the support. Pt-Sn clusters prepared by sequential vapor deposition of the metals on TiO2(110) are Sn-rich, regardless of the order of deposition. Metal-metal and metal-support interactions were studied by XPS based on the oxidation state changes and shifts in the Pt(4f), Sn(3d) and Ti(2p) binding energies. Ordered Pt-Sn alloy surfaces were prepared by annealing Sn films on Pt(111), and the alloy structures were in accordance