TiO2: A Charge-Sensitive Reaction

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Ethylene Hydrogenation over Pt/TiO2: A Charge-Sensitive Reaction G. T. Kasun Kalhara Gunasooriya, Edmund G. Seebauer, and Mark Saeys ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02906 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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Ethylene Hydrogenation over Pt/TiO2: A Charge-Sensitive Reaction G.T. Kasun Kalhara Gunasooriya,† Edmund G. Seebauer,*,§ Mark Saeys*,† †

Laboratory for Chemical Technology, Ghent University, Technologiepark 914, 9052 Gent, Belgium

§

Department of Chemical and Biomolecular Engineering, University of Illinois - Urbana Champaign, 114 Roger Adams Laboratory, 600 S. Mathews Ave, Urbana, IL 61801, USA

ABSTRACT: Controlled charge transfer between a support and small metal particles provides unique opportunities to tune the activity of supported metal catalysts, as first proposed by Schwab. By controlling the thickness of polycrystalline anatase TiO2 films, the TiO2 carrier concentration can be manipulated by an order of magnitude. When 1-nm Pt particles are deposited on these TiO2 films, the variation in the charge transfer between the TiO2 support and the Pt particles is found to dramatically increase the ethylene hydrogenation activity. The sensitivity of ethylene hydrogenation to charge transfer was anticipated from the large effect of the Pt charge on the ethylene and ethylidyne adsorption energy, e.g., compared to CO and H. Our results demonstrate that the controllable Schwab effect provides a powerful tool to tune catalytic activity. An even larger effect can be expected for supported sub-nm clusters, and for the selectivity of hydrogenation reactions. KEYWORDS: Schwab effect, metal-support interactions, charge transfer, band engineering, Hydrogenation, TiO2, Pt Controlled charge transfer from a semiconducting support to supported metal nano-particles provides a powerful tool to tune the electronic properties and hence the activity and selectivity of supported catalysts, as first proposed by Schwab.1-4 Such metal-support interactions have recently been the subject of several studies.5-8 In homogeneous catalysis, the effect of ligands on the activity and selectivity is well-established,9 but the extension to heterogeneous catalysis has proven to be challenging. Indeed, control over the amount of charge transfer, and separation of effects due to charge transfer from effects due to alloying of the metal particle with dopants, covering of the metal particles with a thin oxide film, and changes in the shape of the supported particles is challenging.8, 10-11 In the 1960s, Schwab performed a series of landmark experiments in which the electronic influence of a support on the overlying active catalytic material was systematically investigated.1-4 Later, Akubuiro et al.12-13 illustrated that the CO chemisorption capacity and the CO oxidation activity of Pt supported on TiO2 can be influenced by doping the TiO2 support with higher valence cations such as Ta5+, Sb5+ and W6+. Recently, Shi et al.6 illustrated that charge transfer in Pt/TiO2 can be controlled by introducing oxygen vacancies or by doping with fluorine, resulting in a variation in the methanol oxidation activity of these catalysts. Serna and Gates14-15 demonstrated that the acidic or basic nature of the support can be used to reverse the selectivity between ethylene hydrogenation and dimerization over well-defined Rh4 clusters. By preparing a series of anatase TiO2 films with a range of carrier concentrations, we demonstrated the effect of the carrier concentration on the CO oxidation activity of 1-nm Pt particles.7 Increasing the carrier concentration and hence the charge on the Pt particles increases the CO oxidation rate under excess CO conditions and

reduces the rate under excess O2 conditions. Both effects could be attributed to the variation in the CO adsorption energy with Pt surface charge. Key distinguishing features of this work were the direct measurement of the carrier concentration in the TiO2 film, a prerequisite for efforts to link reactivity to the electronic chemical potential, EF, and the ability to manipulate the carrier concentration in the absence of foreign dopants that could exert their own independent effects on the activity. Using resonant photoemission spectroscopy and X-ray photoelectron spectroscopy, Lykhach et al.8 recently quantified the amount of charge transfer for a model Pt/CeOx system. A maximum charge transfer of 0.1 electrons per Pt atom was reported for 1 nm Pt particles, and the amount of charge transfer per Pt atom rapidly decreased for larger particles. In their work, the amount of charge transfer was changed by manipulating the Pt particle size, rather than by manipulating the electronic properties of the CeOx semiconductor support. Useful control of the Schwab effect however requires electronic characterization and control of the support semiconductor’s electronic properties. Recent advances in semiconductor defect engineering, driven in part by applications in photocatalysis and semiconductor manufacturing,16-17 provide opportunities to control and tune the carrier concentration in oxide films and hence the amount of charge transfer to supported metal particles. An alternative approach to tune the charge on the surface of metal particles was introduced by the group of Vayenas.18-20 In the electrochemical promotion of catalysis (EPOC) approach, potentials up to 2 V are applied between the catalyst and a second electrode to drive O2− species from the support to the metal particle via reverse spillover. The O− species together with their image charge create a double layer at the metal-gas interface and polarize the catalyst surface, which affects both the

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 is the dielectric constant of semiconductor (F/m), Nd is the carrier concentration (m-3), Φm and Φs are the metal and the semiconductor work functions (eV), respectively. For Pt/TiO2 with a TiO2 carrier concentration of 1018 cm-3,

Figure 1. Energy band diagram of a metal and n-type semiconductor system with a metal work function (Φm) higher than that of the semiconductor (Φs) prior to contact and at equilibrium after contact. To reach equilibrium, electrons flow from the semiconductor to the metal. EF,m Fermi energy of the metal, EF,s Fermi energy of the semiconductor, EV energy of the valence band maximum, EC energy of the conduction band minimum, Evac vacuum energy, χs electron affinity of the semiconductor, ΦSB Schottky barrier, Vbi builtin contact potential.

chemisorption behavior and the activity. In this way, the group of Vayenas illustrated controllable changes in the activity for the oxidation of methanol, ethanol, ethylene, propane and for the water-gas-shift reaction.18-20 The sensitivity of a reaction to the amount of charge transfer depends on the influence of the surface charge of the metal particles on the stability of the kinetically relevant intermediates and transition states. Because the CO adsorption energy results from a balance between donation from the 5σ orbital and strong back-donation to the 2π* orbitals,21-22 the CO adsorption energy was found to be rather insensitive to the Pt surface charge. Indeed, a 12fold increase in the carrier concentration of the TiO2 support, and a corresponding change in the charge transfer from TiO2 to the 1-nm Pt particles, only increased the CO oxidation rate by 70% under excess CO conditions and reduced the CO oxidation activity by only 30% under excess O2 conditions.7 For ethylene adsorption, however, donation is expected to be dominant, and ethylene hydrogenation is therefore expected to be more sensitive to changes in carrier concentration and charge transfer. When a metal is brought in to contact with a semiconductor support, the electrochemical potential becomes uniform throughout the system (Figure 1) and free electrons transfer between the metal and semiconductor until potentials are equalized.23 If the metal workfunction (Φm) is higher than the workfunction of the semiconductor (Φs), electrons will flow from the semiconductor to the metal. At equilibrium, a Helmholtz double layer is established at the metal/semiconductor interface, introducing a depletion layer of several nm and band bending in the semiconductor (Figure 1), and a Schottky barrier at the metal-semiconductor interface. For a simple 1D model system (Figure 1), the amount of charge transfer is given by:    ∅ ∅ 

(1)

where, Q is the charge transferred per unit semiconductor surface area (C/m2), q is the electron charge (1.6 x 10-19 C),

Figure 2. (a) Variation of the n-type carrier concentration for anatase TiO2 with film thickness. Four TiO2 samples were prepared and the carrier concentration Nd was determined by capacitance–voltage (C–V) measurements in a 25 specially designed Schottky diode structure. (b) HAADF2 STEM image for 0.178 ± 0.005 μg Pt/cm sputtered on a Cu TEM grid. The Pt particle size distribution (insert), with an average of 0.90 ± 0.05 nm, is obtained by considering 25 particles. 2

a charge transfer of 0.06 e/nm is calculated. For a Pt thickness of 0.5 nm, this corresponds to a value of 0.002 electrons/Pt atom. In more realistic 2D models, the amount of charge transferred to supported particles can be several times larger, but similar trends are obtained.24 For example, assuming a fully developed TiO2 depletion layer as in the 1D model and 1 nm semi-spherical Pt particles with a typical particle density of 0.3 particles/nm2 on the TiO2 support (i.e., a particle-particle spacing of about 2 nm), a charge transfer of 0.01 electrons/Pt atom is obtained. The catalyst activity is controlled by the charge of the Pt atoms at the free surface of the supported particles. The charge distribution at the free surface of the supported particles however depends on the particle shape, the adsorbates, the electronic structure of the sub-nm particles and the field at the metal-semiconductor interface. The charge distribution at the free particle surface hence cannot be modeled quantitatively. The charge build-up in the metal, the charge accumulation layer, is mostly concentrated at the metal-semiconductor interface due to the presence of a positively charged depletion layer in the semiconductor (Figure 1). Strong screening of charges in metals with a Thomas-Fermi screening length of only 0.5 Å25-26 moreover concentrates the extra electrons in the first few Å of the interface, as illustrated in Figure 1. The extra electrons transferred from the semiconductor to the metal particles hence do not accumulate at the free surface of the metal particle as is the case for an unsupported metal particle in the gas phase (see Figure S2 in the Supporting Information). Charge transfer and the resulting Schwab effect will hence be most pronounced for particles in the sub-nm range, as observed by Lykhach et al.,8 and rapidly fall off with particle thickness. In principle, interface states at the Pt/TiO2 interface could absorb part of the charge and screen charge transfer. However, TiO2 is an ionic semiconductor with little or no Fermi level pin-

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ACS Catalysis carrier concentration Nd increases by an order of magnitude. An advantage of using the film thickness to control the carrier concentration is that the introduction of extrinsic dopants is avoided. Extrinsic dopants could partially segregate to the TiO2 surface and alter the catalytic activity, in particular at elevated temperatures. To avoid modification of the surface of the TiO2 thin films and to create pure Pt particles, Pt was deposited by direct current (DC) magnetron sputtering at room temperature on the four TiO2 samples placed symmetrically with respect 2 to the sputtering target. About 0.178 ± 0.005 μg Pt/cm or approximately 0.37 ML was sputtered on each support. To characterize the Pt particle size, Pt was also sputtered on a Formvar-coated TEM grid. The STEM image (Figure 2b) displays quite uniform Pt particles with a size of 0.90 ± 0.05 nm. Charge transfer from the TiO2 support to the Pt particles was evidenced by the shift in the Ti 2p3/2 and O 1s XPS peaks from 458.4 to 459.0 eV and from 529.6 to 530.3 eV, respectively, after Pt deposition, indicating charge depletion near the surface of the TiO2 film.7

Figure 3. Variation in the C2H4 hydrogenation rate as measured by the m/e = 30 ion current of the mass spectrometer as a function of (a) the limiting C2H4 partial pressure (PC2H4) for excess H2 conditions (PH2 = 6666 Pa) and (b) the limiting H2 partial pressure (PH2) for excess C2H4 conditions (PC2H4 = 6666 Pa). For excess H2 conditions, the average reaction order in C2H4 is -0.39, while for excess C2H4 conditions, the average reaction order in H2 is 1.20. Using these reaction orders, effective rate coefficients for the different catalyst samples were determined (shown in the legend) for both excess H (k ) and excess C H (k ) conditions.

ning at the metal-TiO2 interface, which suggests that screening effects should be minimal.23

Polycrystalline anatase TiO2 thin films of varying thickness were synthesized by atomic layer deposition (ALD).27-28 The carrier concentration was determined by capacitance-voltage (C-V) measurements in a specially designed Schottky diode structure.29 The anatase TiO2 carrier concentration varies inversely with the film thickness (Figure 2a) due to a reduction in the concentration of electrically active grain boundaries when the material becomes thicker.27-28 These grain boundaries serve as aggregation sites for native donor defects, such as oxygen vacancies and titanium interstitials, and result in increased n-type conductivity.27 Samples with four different thicknesses (93, 145, 194 and 246 nm) were prepared, and the carrier concentration decreased by a factor 6 from about 8.1 x 1017 cm-3 to 1.3 x 1017 cm-3 when the thickness increased from 93 nm to 246 nm. Using equation (1), the transferred charge Q increases by factor 3 when the TiO2

Ethylene hydrogenation kinetics were measured in a custom-built low-pressure batch reactor by monitoring the variation of the C2H4 ion current (m/e = 30) with a quadrupole mass spectrometer (Pfeiffer Vacuum PrismaPlus). A reaction temperature of 303 K was selected based on the sensitivity of the mass spectrometer. For each reaction run, Ar was first fed to the batch reactor up to 133 Pa to minimize day-to-day transitions in the base pressure. The reaction studies were carried out for a range of limiting C2H4 partial pressures (PC2H4) from 133 to 666 Pa in excess H2 (6666 Pa), and for a range of limiting H2 partial pressures (PH2) from 133 to 666 Pa in excess C2H4 (6666 Pa). Control experiments with Pt-free TiO2 samples showed a two orders of magnitude lower reaction rate. The absence of mass transfer limitations in our reaction system was confirmed by measuring the activation energy (see Figure S1 in the Supporting Information). The measured activation energy of 39 ± 2 kJ/mol near 300 K and for excess H2 (pH2 = 6666 Pa, pC2H4 = 666 Pa) conditions is in good agreement with activation energies of 36 to 45 kJ/mol reported for comparable conditions.30-32 Using the Pt particle size of 0.9 nm (Figure 2b), a turn-over frequency (TOF) of 1.2 s-1 is estimated for a C2H4 pressure of 666 Pa and a H2 pressure of 6666 Pa at 303 K. Our TOF is somewhat lower than the value of 17 s-1 reported by Zaera and Somorjai31 at 300 K over Pt(111), for a C2H4 pressure of 1333 Pa and a H2 pressure of 5332 Pa, but close to the value of 2.4 s-1 obtained more recently by Zaera et al.33 at 300 K and for a C2H4 pressure of 266 Pa and a H2 pressure of 1333 Pa. Initial ethylene hydrogenation TOFs for the four samples with different carrier concentrations are plotted in Figures 3a and 3b as a function of the partial pressure of the limiting reactant. The measured rate clearly depends on the TiO2 carrier concentration under both conditions, demonstrating that ethylene hydrogenation is indeed a charge-sensitive reaction. Both for excess H2 conditions (Figure 3a) and for excess C2H4 conditions (Figure 3b), the reaction rate increases with the TiO2 carrier concentra-

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Figure 4. Change in DFT-vdW-DF adsorption energies for πbound C2H4* (▲ ), C2H3* (●), CO* (♦) and H* (■) for low coverage on Pt82 cluster as a function of the charge of the surface Pt atoms. The variation in the surface charge results from changes in the number of electrons in the Pt82 cluster. The neutral cluster is indicated by a dotted line and corresponds to a negative Bader surface charge of 0.005 electrons/Pt atom. The π-bound C2H4*, C2H3* and CO* adsorption configurations are shown in the insert. In all calculations, a more negative charge on the surface Pt atoms weakens adsorption while less negatively charged surface Pt atoms strengthen the adsorption of the adsorbates considered here. Moreover, πbound C H adsorption is much more sensitive to charge

tion. For excess C2H4 conditions, the average H2 reaction order is 1.20 ± 0.14 and the effective reaction rate coefficient for each sample is shown in the insert. Under these conditions, the effective reaction rate coefficient increases by a factor of 3 when the carrier concentration increases by a factor of 6. Under excess C2H4 conditions, the Pt surface is likely poisoned by strongly bound ethylidyne.31-32, 3435 Note that the variation in the effective rate coefficient reflects changes in the elementary rate coefficients, in the adsorption enthalpies and in the surface coverages.36 For excess H2 conditions, the measured TOF is much less sensitive to the carrier concentration. Under these conditions, the average C2H4 reaction order is negative, -0.39 ± 0.12, demonstrating that C2H4 adsorption is kinetically relevant and weaker C2H4 adsorption will increase the effective reaction rate coefficient.36 The corresponding effective reaction rate coefficient under these conditions indeed increases by 50% when the carrier concentration increases by a factor of 6. The measured reaction orders for C2H4 and H2 are in agreement with literature data, and suggest that even under excess H2 conditions, the surface remains partially poisoned by strongly-adsorbed ethylene-derived species.31-32, 34-35 Decreasing the stability of ethylene or ethylene-derived ethylidyne is hence expected to increase the reaction rate for both sets of experiments. To analyze the effect of charge transfer on the catalyst activity, we computed the adsorption energies of π-bound ethylene, of site-blocking ethylidyne, of hydrogen, and of CO on the (111) facet of a model Pt82 particle as a function of the Pt surface charge (Figure 4, see Table S1 and S2 in

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the Supporting Information) using DFT-vdW-DF.37-38 The Pt82 particle was placed inside a 28x28x28 Å3 box, and charge transfer was modeled by adding (removing) electrons to (from) the Pt cluster. π-bound ethylene has been reported to be the active species during ethylene hydrogenation.31-32, 34-35 For the neutral cluster, the π-bound ethylene adsorption energy, –55 kJ/mol, the C2H3 adsorption energy, –572 kJ/mol, and the H adsorption energy, –266 kJ/mol, agree with low-coverage experimental values for Pt(111) and with previous theoretical values.39-41 The charge on the central surface Pt atom is slightly negative (0.005 excess electrons) for the neutral cluster, as expected from the jellium model. Addition of one electron to the Pt82 cluster increases the charge of the central surface Pt atoms by 0.008 electrons. The calculations show that the adsorption energies of both π-bound ethylene and ethylidyne are very sensitive to the surface charge and a 0.008 electrons/Pt atom increase in the surface charge (more negatively charged surface Pt atoms) reduces the adsorption energies by 7 and 6 kJ/mol, while a 0.008 electrons/Pt atom decrease in the surface charge (less negatively charged) increases the adsorption energies by 9 and 6 kJ/mol, respectively. In comparison, it is important to note that the CO adsorption energy is much less sensitive to the Pt charge (Figure 4) than either π-bound ethylene or ethylidyne, and the H adsorption energy is nearly unaffected by changes in the surface charge, as anticipated from the very different balance between donation and back-donation. Ethylene hydrogenation is hence expected to be more charge-sensitive than CO oxidation, in agreement with our kinetic measurements. In conclusion, the Schwab effect proposes that tuning the electronic properties of the semiconducting support can control the activity of supported metal catalysts. Developments in defect engineering have made it possible to gradually change the TiO2 anatase carrier concentration by an order of magnitude by controlling the TiO2 film thickness, a range that can be significantly expanded by more advanced defect engineering techniques. When 1nm Pt particles are deposited on a series of anatase TiO2 films with different carrier concentrations, changes in the support carrier concentration affect the charge of the Pt particles. The sensitivity of a reaction to the Schwab effect depends on the sensitivity of the kinetically relevant intermediates to the surface charge. DFT calculations demonstrate that charge injection into the Pt particles decreases both the C2H4 and C2H3 adsorption energy, but has a minor effect on the H adsorption energy and a small effect on the CO adsorption energy. Ethylene hydrogenation is hence expected to be a more charge-sensitive reaction than CO oxidation. Indeed, experimentally a 6-fold increase in the TiO2 carrier concentration is found to increase the effective rate coefficient for ethylene hydrogenation by a factor 3 under excess C2H4 conditions while a 12-fold increase in the TiO2 carrier concentration only increases the CO oxidation rate is only multiplied by 1.7.7 Increasing the n-type carrier concentration even further for this Pt/TiO2 system, ideally making the TiO2 degenerative n-type, would maximize the Schwab effect. By ex-

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tending equation (1) and the related calculations to this limit (see Supporting Information), a maximum charge transfer of 0.07 e/Pt is obtained for 1-nm Pt particles. This charge transfer is expected to increase the hydrogenation rate further by an order of magnitude. It should be noted that the charge-sensitivity of a reaction can be either positive (charge transfer increases the rate) or negative,16 and depends on the kinetic regime.7 While a support fabricated by polycrystalline ALD is hardly typical for industrial catalysis, carrier concentration measurement and manipulation are currently being developed for more realistic porous oxide support materials.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of experimental and computational methods, Arrhenius plot for C2H4 hydrogenation rates under excess H2 and limiting C2H4 conditions, low coverage change in adsorption energies and molecular coordinates for π-bound C2H4*, C2H3*, CO* and H* on a Pt82 particle as a function of the charge of the central surface Pt atoms and charge transfer calculation for degenerate n-type Pt/TiO2.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank D. Eitan Barlaz for providing the TiO2 thin films, Dr Michel Bosman for the HAADF-STEM images, Dr Daniel Ong Sze Wei, Dr Chee Kok Poh and Dr Chen Luwei for the discussions, and are grateful to the Institute of Chemical and Engineering Sciences (A*STAR), Singapore for the use of their facilities and equipment. The computational resources (Stevin Supercomputer Infrastructure) and services used in this work were provided by Ghent University. G. T. Kasun Kalhara Gunasooriya was supported by an Odysseus grant from the Research Foundation-Flanders.

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