Hydrogen Adsorption on Nanosized Platinum and Dynamics of

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Hydrogen Adsorption on Nano-Sized Platinum and Dynamics of Spillover onto Alumina and Titania Clelia Spreafico, Waiz Karim, Yasin Ekinci, Jeroen Anton van Bokhoven, and Joost VandeVondele J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03733 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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The Journal of Physical Chemistry

Hydrogen Adsorption on Nano-sized Platinum and Dynamics of Spillover onto Alumina and Titania Clelia Spreafico,∗,† Waiz Karim,‡,¶,§ Yasin Ekinci,¶ Jeroen A. van Bokhoven,‡,§ and Joost VandeVondele† †Nanoscale Simulations, Department of Materials, ETH Zurich, Wolfgang-Pauli-Str. 27, CH-8093 Zurich, Switzerland ‡Institute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1-5/10,CH-8093 Zurich, Switzerland ¶Laboratory for Micro and Nanotechnology, Paul Scherrer Institute, Villigen PSI, 5232, Switzerland §Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, Villigen PSI, 5232, Switzerland E-mail: [email protected]

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Abstract An extended theoretical investigation of the electronic and interface properties of titania and alumina with and without supported platinum nano-particles is presented and compared to recent experimental data with the aim to understand the mechanism of hydrogen activation, adsorption and spillover. Thirteen-atom platinum particles on titania adopt a distinct different structure than on alumina, which results in distinct hydrogen coverages. Upon hydrogen adsorption, titania is reduced with creation of Ti(III) electronic trap states, strongly interacting with the surface adsorbed protons. The combined Ti(III)/proton migration rate is slower than the one of single surface protons and it is not influenced by the presence of coadsorbates, such as water molecules. Hydrogen is instead heterolytically split on defect sites on alumina with the formation of a surface proton and a hydride moiety, bound to the particularly reactive surface tri-coordinated aluminum site. The electronic structure of alumina is only marginally altered, without formation of defect states. The mobility of the hydride moiety is limited, in particular in the presence of coadsorbed water molecules, that compete for the adsorption sites. Modelling of hydrogen spillover from a platinum cluster to the metal oxide demonstrates that the spillover rate depends on the hydrogen partial pressure and on the thickness of the oxide acceptor layer. Kinetic Monte Carlo results confirm that hydrogen spreading on titania leads to an homogeneous coverage, while on alumina hydrogen can only be found up to a few nm from the platinum cluster because of kinetic competition between diffusion and desorption.

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1

Introduction

Metal oxides are at the core of a variety of applications in the petroleum, chemical and environmental industries, including catalysis, gas sensors and energy storage and conversion. 1–5 Such materials must be thermally and chemically stable and they are sometimes used in combination with other oxides to improve their properties. 6 For more specialized applications other aspects have to be considered, such as the surface and charge transport properties and the presence of intrinsic defects and doping species. As an example, modification of the titania band structure with a dopant or sensitizing species is fundamental to improve its light utilization efficiency for solar-based photocatalysis. 7 In particular, ad hoc reduced titania (or black titania) is a promising candidate for solar based catalysis, thanks to its improved optical absorption of visible and infrared radiation. 8 Alumina and titania are widely employed as substrate for heterogeneous catalytic reactions, which are of great importance in the petrochemistry, chemical synthesis and energy fields. 9 Some examples are the treatment of automotive gas exhaust, 10,11 the conversion of crude oil into high octane liquid products 12 and the production of hydrogen from liquid fuels. 13,14 A phenomenon that is often involved in explaining catalytic reactions over supported catalysts is hydrogen spillover. 15–19 The hydrogen molecule is at first dissociatively adsorbed on a metal catalyst and then single H atoms spillover onto and migrate over the oxide and, sometimes, carbon support. Despite the fact that the spillover process has been thoroughly investigated since its first observation, 20 there are open questions. In particular, it is generally accepted that spillover occurs on reducible supports, such as titania, ceria, MoO3 and WO3 , while the behavior on nonreducible materials, such as alumina and silica is widely debated. 21–24 The first observation of hydrogen spillover was the evidence of reduction of WO3 when Pt/WO3 was exposed to hydrogen - the platinum catalyst here is needed to dissociate the hydrogen molecule to hydrogen atoms, which are otherwise not produced over pure WO3 . It is understood that, after dissociative chemisorption on platinum, hydrogen atoms move 3



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onto the oxide support where they transfer as a combination of protons and electrons. The electrons reduce the cations of the metal oxide, while the protons bind to the surface oxygen anions with a net migration of hydrogen atoms. Experimental proof of this phenomenon are the detection of trapped electrons in titania produced by atomic hydrogen delivered from gold particles in an Au/TiO2 catalysts using IR spectroscopy 25 and the use of in-situ diffraction to show the lattice distortion of ceria when a Ce4+ cation is reduced to Ce3+ by effect of spillover. 26 While spillover can totally or partially reduce a reducible metal oxide particle that is in direct contact with a metal particle, it is assumed that the spilt over hydrogen will also reduce other metal oxide particles that are not in direct contact with the metal catalyst. However, no clear evidence of this had been shown, since such well-defined model systems with clearly separate catalyst functions have not been studied before, though many attempts have been made. 17,23,24,27 The spillover of hydrogen as coupled electron-proton pairs cannot take place on nonreducible supports. In this case, the detection of hydrogen atoms on the nonreducible metal oxide support could be considered to be a proof of spillover, as claimed in the observation of hydrogen atoms on alumina with a supported palladium catalyst using electron paramagnetic resonance (EPR) spectroscopy. 27 However, the study could not guarantee that the observed EPR effect was solely due to surface adsorbed hydrogen, and it did not constitute a definitive proof of spillover hydrogen on alumina. On the other hand, many theoretical predictions and experimental studies agree that hydrogen spillover is unfavourable and energetically unlikely on nonreducible supports. 28,29 An example is the temperature-programmed reduction (TPR) study of a mixture of Pt/d, zeolite and iron oxide, which proved that the reduction of iron oxide does not occur via spillover, but thanks to the migration of platinum oxide. 30 The migration or surface contamination has often been proposed as a possible explanation for the observation of spillover over nonreducible supports. Despite the frequent observation of hydrogen spillover in hydrogenation reactions on such supports, such as the enhancement of carbon dioxide methanation when a silica-supported platinum catalyst is added, 17 a molec-

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ular level description is lacking. The debate about the existence of hydrogen spillover is to a large extent due to lack of model systems that can separate the catalytic functions and guarantee no migration of the metal, movement of reactants to the metal site, and contamination of the catalysts. An uncontaminated model system, incorporating the metal catalyst and the reacting species on the same support at precise spatial distances is needed and this has to be probed in a manner to visualize the spillover effect. The combination of state-of-the-art nanofabrication 31 and spatially resolved X-ray photoemission electron microscopy is a powerful tool to elucidate chemical reactions at the single-particle level. 32,33 We recently employed this experimental approach to study in depth the role of support in the phenomenon of hydrogen spillover. 21 This experiment, which involved preparation of precise model systems using electron beam lithography followed by single-particle X-ray absorption spectro-microscopy, compared the performance of titania, a reducible support, and alumina, a nonreducible support. The surfaces of the two oxides were decorated with platinum particles, from which hydrogen can spill over, and iron oxide particles, a probe material that can be reduced provided that hydrogen atoms are available. Several pairs of platinum and iron oxide particles were placed on the same substrate at increasing and well-defined distances, ranging from 0 (overlapping pair) to 45 nm, with accuracy within one nanometer, and the spillover effect was assessed in all the pairs simultaneously by monitoring the X-ray absorption spectra of the iron oxide particles. The noble metal plays a fundamental role in the process and the reduction of remote species is possible via hydrogen spillover. The nature of the oxide support influences the availability of reducing species away from the platinum particle and, for the first time, the efficiency and spatial extent of the spillover effect on titania and alumina supports was quantified. A distance-dependent behavior was observed on the alumina: the further the distance between the platinum catalyst and the iron oxide particle, the lower is the degree of reduction of the iron oxide. The spillover effect was limited to very short distances, due to a hydrogen concentration gradient. In contrast, in the case of titania, hydrogen spillover

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was highly efficient and uniform over large distances reducing the iron oxide particles equally irrespective of the distance from platinum. The restricted spillover on alumina is mediated by three-coordinated aluminum centers which also interact with water 34 and DFT revealed that this interaction gives rise to hydrogen desorption rate which is faster or comparable to the surface mobility causing the gradient. Here we provide a quantitative description of the mobility of hydrogen on the tiania and alumina surfaces, providing more detail of the surface reactions on the oxide, with ab initio and Kinetic Monte Carlo simulations carried out on model systems specifically designed to match the conditions of the experiment. The hydrogen adsorption, mobility and spillover properties of the two materials were computed by means of density functional theory (DFT). The behavior of titania was investigated by adopting as model the most stable (101) surface of the anatase crystal phase. The clean surface has the characteristic sawtooth profile, that exposes, along the [010] direction, alternating rows of fivefold and sixfold coordinated titanium cations (Ti5c and Ti6c ), as well as twofold and threefold coordinated oxide anions 35 (O2c and O3c ). In general, the interface structure is not perturbed by the interaction with the adsorbates that, in turn, can retain their molecular form or dissociate. 1 The Bohemitebased crystal structure of γ-alumina proposed by experimental and theoretical studies 36,37 was adopted in our model system. The crystal structure of alumina consists of aluminum ions in octahedral (75%) and tetrahedral (25%) coordination, that have a determining role for the material reactivity. In fact, even if the (001) surface is the most stable one, the (100) surface is the one exposing aluminum sites with an undercoordinated tetrahedric geometry (Al3c ), which are highly reactive. 38 Multiple techniques, such as the nudged elastic band, ab initio thermodynamics and kinetic Monte Carlo method were employed to assess activation energies, surface coverage and the time evolution of the two systems, identifying similarities and differences in the behavior and reactivity towards hydrogen.

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2

Computational Methods

First principles DFT calculations were carried out with the CP2K package, 39 based on the hybrid Gaussian and plane wave (GPW) scheme. 40,41 A plane wave density cutoff of 800 Ry and periodic boundary conditions have been adopted. GGA calculations were performed with the Perdew-Burke-Enzerhof 42 (PBE) exchange correlation functional, while hybrid calculations have been carried out with the truncated PBE0 (trPBE0) functional. 43 A cutoff radius of 4.5 Å, sufficient to ensure converged electronic properties has been chosen for all calculations with the trPBE0 functional. 44,45 The auxiliary density matrix method (ADMM), which relies on an auxiliary basis for Hartree-Fock exchange, 46 was employed to speed up hybrid calculations. Dispersion interactions were included by means of an empirical analytical potential, using the Grimme D3 method, 47 within a range of 15 Å. Standard coefficients and scaling factors for PBE and PBE0 functionals were adopted. The primary Gaussian basis set adopted in all calculations is of double ζ quality, in combination with GTH pseudopotentials. 48 Validation of the computational setup was carried out for both TiO2 and Al2 O3 , by comparison with experimental lattice parameters and bond lengths. An example of the CP2K input file, including all the relevant simulation parameters, is included in Supplementary Information S9. Differences between computed and experimental parameters were generally less than 1%. Activation energies for ions and charge migration were computed with the climbing image nudged elastic band (ci-NEB) method. 49,50 Density of states analysis was carried out with a resolution of 0.03 eV. In the modelled slabs, the interface under study is built to be perpendicular to the z axis, facing a ∼20 Å vacuum layer, while the x and y directions are fully periodic. In the rotated set of coordinates, parameters of the unit cell adopted for building the different slabs are: x = 10.227Å; y = 3.782Å; z = 3.513Å for TiO2 and x = 8.068Å; y = 8.413Å; z = 5.587Å for γ-Al2 O3 . Slabs are identified by a three digit code, corresponding to the repetitions of the unit cell along the x,y and z directions: as an example, the anatase 2 × 6 × 7 slab notation corresponds to a 20.454 × 22.692 × 24.591Å3 slab, while the alumina 7



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2 × 2 × 3 notation corresponds to a 16.14×16.82 × 16.74Å3 slab.

3 3.1

Results and Discussion Thermodynamics of hydrogen adsorption

The adsorption energy of a hydrogen molecule on the oxide surface can be computed as:

E(ads,H2 ) = E(2H@oxide) − E(H2 ,vacuum) − E(non

covered oxide)

With E(2H@oxide) = energy of two hydrogen atoms adsorbed on the oxide surface, E(H2 ,vacuum) = energy of an hydrogen molecule in vacuum and E(non

covered oxide)

= energy of the oxide

surface, without adsorbates. The adsorption energy of a single H atom on the oxide can then be computed as: 1 E(ads,H) = E(ads,H2 ) 2 Different slab dimensions, relative positions of the H atoms and spin multiplicities were considered. Figure 1 and the Supplementary Information Tables (Table S1, Table S2) summarize the results.

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Neighboring Sites Separated Sites

T iO2 anatase (101)

γ − Al2 O3 (100)

γ − Al2 O3 (001)

-0.51 -0.60

-1.26 -0.57

-0.09* +1.95

* Dispersive Interaction

(a)

(b)

(c)

(d)

(e)

(f)

Figure 1: H2 adsorption energies (eV) and corresponding geometries (top view) on the T iO2 anatase (101), γ − Al2 O3 (100) and (001) surfaces. (a): 2H@TiO2 (101), neighboring sites. (b): 2H@Al2 O3 (100), neighboring sites. (c): H2 @Al2 O3 (001), dispersive interaction. (d): 2H@TiO2 (101), distant sites. (e): 2H@Al2 O3 (100), distant sites . (f): 2H@Al2 O3 (001), distant sites Similar to the case of lithium, 45,51,52 upon hydrogen adsorption the titanium dioxide slab is reduced with formation of two protons, bound to surface O2c sites and transfer of the two electrons to neighboring Ti5c sites. The computed adsorption energy of a single hydrogen molecule on anatase (101) is 0.51 eV, with a slight dependence on the distance between the two protons, in agreement with theoretical results from previous investigations. 53,54 On titania, charge localization depends on the level of theory adopted. PBE calculations result in a delocalized charge in the slab bulk. On the other hand, PBE0 calculations, that in general better reproduce the oxide electronic structure, 44 lead to localized charges, whose stability with respect to the delocalized electrons depends on the percentage of exact Hartree-Fock 9



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exchange (%hfx) in the functional. However, it is clear that in both cases, system size plays a role: a larger oxide slab (i.e. with lower hydrogen coverage) will reduce the overlap of polarons/delocalized electrons and yield more stable adsorption. 44 On the contrary, smaller system sizes (i.e. with higher adsorbates coverage) result in lower adsorption energies. Computation of the differential adsorption energy for subsequent adsorption of multiple hydrogen molecules on the oxide, suggests that the limit concentration for hydrogen exothermic adsorption is between 1.3 and 1.7 molecules per nm2 (Supplementary Information Table S3). The stability of hydrogen adsorption on γ-alumina dependens heavily on the surface exposed. Figure 1 shows that the hydrogen adsorption is not feasible on the thermodynamically most stable (001) crystal surface, apart from weak physisorption (2: 2H, surface adsorbed. (b) Hydrogen adsorption, alumina. Red: clean surface. Green: intermediate water coverage (30% free adsorption sites). Blue: full water coverage. R.C.=-2.5: H2 , detached. R.C.>2: 2H, surface adsorbed. For both oxides hydrogen adsorption is dissociative and the Reaction Coordinate (R.C.) is defined as the difference between the H-H bond length and the distance of the center of mass (CM) of the hydrogen molecule to the oxide surface: R.C. = d(H − H) − d(H2CM − surf ). Increasing R.C. indicates progressive approach to the surface and dissociation of the hydrogen molecule. Full data for the energy profiles are available in supplementary information S14. In most experimental systems, water is a common surface adsorbate which can be accounted for in a realistic simulation setup. Figure 3 shows the results for hydrogen adsorption

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in presence and absence of water on the oxide surface. On anatase (101), water is molecularly adsorbed on the surface Ti5c centers, not influencing the hydrogen adsorption sites. The under-coordinated Al3c surface sites are highly reactive towards dissociative adsorption of water molecules, with formation of Al3c -OH and O2c -H bonds. Hydrogen is at first coordinated to the Al3c center, and then a single proton is transferred to a neighboring O2c site. This process is energetically favorable, in agreement with previous studies. 34 The energy barrier for this process is lower than the one for homolytic splitting on anatase, and, on the clean (100) slab, occurs with a relatively low activation energy (0.65 eV). The preliminary coordination step is also evident along the reaction profile, as the energy of the approaching molecule is lowered with respect to the gas phase by 0.19 eV. If the alumina surface is fully covered in water, instead, hydrogen adsorption implies an energy loss, and the activation barrier for the process is 1.02 eV. In addition, the presence of surface water also hinders the formation of the preliminary coordinated state, as the energy of the hydrogen molecule is not lowered upon approaching the oxide slab. Intermediate situations, in which the water coverage is not uniform, are also possible. In this case, the activation energy for hydrogen adsorption is influenced by the relative position of the approaching hydrogen molecule and surface adsorbates and by the availability of free sites.

3.3

Hydrogen Mobility at Metal Oxide Surface

When hydrogen dissociatively adsorbes at the anatase (101) interface, two surface protons and two polaronic Ti(III) centers are created. The two species are strongly interacting with a stabilization energy between 0.37 and 0.45 eV, as demonstrated by computing the total energies of hydrogenated systems with isolated species and different electron localization positions (Supplementary Information Table S4). When this value is compared to the surface stabilization of electrons in absence of nearby protons (0.02-0.06 eV) or even in presence of Li+ cations (0.1 eV 45 ), it is evident that interface electron trapping is much more pronounced in the presence of surface protons than with the Li+ /electron pair. 13



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Due to the rather strong interaction of electrons and protons, significant mobility variations are expected if compared to the migration of isolated species. In the absence of neighboring species, the mobility of excess electrons at the anatase (101) interface can be assumed to be similar to bulk mobility, with an activation energy of 0.23 eV. 45 On the other hand, isolated top-surface protons diffuse preferentially along the [010] crystal direction, between neighboring O2c sites, with an activation energy of 0.6 eV. 53–55 Our calculations, carried out at both the PBE and PBE0 level, confirm a similar activation energy value (0.64 eV) for lone protons, which is however drastically reduced in the presence of coadsorbed species that can act as a mediating bridge. One example of such mediation is water acting as a bridging molecule for proton transfer, with an activation energy of 0.15 eV.

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(a)

(b)

(c)

(d)

Figure 4: Activation energy profiles for charge carrier mobility on anatase(101) and γ-Al2 O3 (100). (a): coupled e− /H+ migration on the clean anatase (101) slab (red) and in presence of water mediation (blue). (b): hydride migration on γ-Al2 O3 (100) on the clean (red) and water covered (blue) oxide slab. (c) and (d): top view of the titania and alumina slabs, including water coverage and migration direction of the charge carrier. Silver: Ti. Red: O (slab). Cyan: Al. Yellow:charge carrier. Blue/Green: O and H, water molecules. Full data for the energy profiles available in supplementary information S14. On the other hand, when both species are present on the anatase surface, the two charges (e− /H+ ) migrate concurrently along the (010) direction, with an activation barrier of 0.73 eV. The combined migration profile of the two species was obtained applying the NEB method

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between the initial and final proton and electron position The latter can be controlled by exploiting the distortion of the anatase lattice induced by the charge localization. 44,45 The profile shows a first peak, corresponding to proton transfer between the two O2c centers, immediately followed by an essentially barrierless electron transfer. Other possible hydirgen migration paths (e.g. initial electron migration followed by proton, or single species migration in presence the other kept fixed, migration in other lattice directions) are associated with higher activation energy barriers (>3eV) and thus are much less likely. A similar argument is valid when the barrier for the migration of the other possible reducing defects in titanium oxide, oxygen vacancies, is considered: the barier for vacancy migration in a direction parallel to the surface plane, thus enabling effective long-range migration of the defect, is at least 1.35 eV, 56 so that a vacancy-based reduction mechanism is possible, but less likely. Contrary to what happens for proton migration, the presence of coadsorbed molecules, that could act as bridging mediator, has no significant effect on the combined charge migration (Figure 4). This behavior is due to the strong interaction between the surface adsorbed protons and the electrons localized on the titanium cations (formally Ti(III) centers) that makes charge separation energetically unfavorable. In addition, the distance between surface protons and bridging molecules that could facilitate the transfer is much larger in the vicinity of Ti(III) than of Ti(IV), due to the weaker Tisurf -OH2 O interaction and the lattice distortion caused by the electron localization. Migration along the other surface direction [-111] is also possible, even if less likely, with an activation energy of 1.69 eV. Hydrogen adsorption on the alumina surface results in the formation of Al3c -H and O2c -H bonds. There is no net electron transfer to the oxide and the reducing species is confined at the (100) interface, so that the only possible charge transfer route will proceed with hydride migration between Al3c surface sites. Activation barriers for hydride migration between Al3c sites were computed also in this case for the clean and hydrated alumina (100) surface (Figure 4). On the clean surface, the lowest energy hydride transfer between Al3c sites proceeds through an intermediate adsorption step on a Al5c site, with a net transfer along the [010] 16


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direction and an overall energy barrier of 1.32 eV. The presence of a neighboring coadsorbed proton stabilizes the initial Al3c adsorption site by 0.7 eV, so that the overall activation energy for hydride movement, starting from the most stable hydrogen adsorption position, is around 1.93 eV. As opposite to the titania case, migration towards neighboring Al3c sites along the other possible surface direction [100] is hindered by a significantly higher activation energy barrier of 3.65 eV. This is also due to the fact that the distance between the Al3c sites along the [100] direction has to be covered in a single step with no possibility of intermediate adsorption or coordination steps. As water adsorption on alumina is energetically favorable, it is highly probable to have coadsorbed water molecules, occupying neighboring aluminium sites. In general, neighboring protons deriving from coadsorbed water molecules on Al5c sites coordinate and facilitate the hydride transfer between the Al3c sites along the [010] direction, with an activation energy of 1.23 eV (Figure 4). Investigating the mobility of the reducing species on the alumina surface is quite complex, due to the multiplicity of possible adsorption sites for both water and hydrogen. Also in this case, however, the formation of oxygen vacancies in the oxide is associated with a much higher energy barrier than for hydride migration and is therefore not considered, in agreement with previous literature. 34 A complete summary of all the investigated activation energies for hydride transfer is reported in the Supplementary Information Figure S1 and Figure S2. Supplementary Information Figure S3 presnts a view of the full oxide slabs considered in the simulation, including water coveage. According to the random walk model for diffusion, the macroscopic surface diffusion coefficient (D) of a species on a two-dimensional surface relates to the microscopic jump frequency (ν) by the following expression: 1 D = a2 ν z with a = jump distance (spacing between adsorption sites), ν= microscopic jump frequency and z= number of neighboring sites where the atom can hop to. 57 Given the large difference in activation energy depending on the direction of migration, diffusion is not isotropic. Diffusion 17



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coefficients for the most favorable direction can be obtained from a one dimensional model. For one dimensional diffusion, i.e. when the atom can either hop to the left or to the right neighboring site, as is the case for TiO2 and γ-Al2 O3 the value of z is 2. The microscopic jump frequency depends on the computed activation energy for the diffusion process (Eact ): ν = ν0 exp(−

Eact ) kT

with ν0 = attempt frequency to overcome the activation energy barrier. Results for the experimental temperature (300 K) are listed in Table 1.

Table 1: Computed attempt frequencies, activation energy barriers and diffusion coefficients for hydrogen diffusion on titania and alumina. Path

ν0 , Jump Frequency Hz

Eact , Activation Energy eV

a, Jump Distance Å

D, Diffusion coefficient cm2 s−1

e− /H+ @TiO2 H− @Al2 O3

2.13·1014 1.48·1013

0.73 1.32

3.73 4.41

8.1 ·10−14 6.7 ·10−25

Despite the various possible migration paths and energy barriers, it is clear that the mobility of the hydride species on alumina is much more limited when compared to the combined H+ /e− migration on the anatase interface, with an almost double activation energy barrier. At room temperature, the difference in the activation energy barriers translates into a substantial difference in the diffusion coefficient, so that on titania the diffusion proces is almost ten orders of magnitude faster.

3.4

Hydrogen Spillover

By comparing hydrogen adsorption and migration on titania and alumina it is clear that, even if the clean, dehydrated alumina surface is reactive towards hydrogen splitting, the surface mobility of the reducing species is limited. On the other hand, anatase is more easily reduced and demonstrates higher charge mobility. Direct hydrogen adsorption from the gas 18


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phase is however characterized by a rather high activation energy barrier, so that alternative reduction routes have to be considered. Metal clusters at the surface of the oxide can dissociate hydrogen, which can then spillover onto the support. This section investigates the spillover mechanism in more detail for the two oxide surfaces to assess the kinetics of the process and determine the parameters that influence it. A 13-atom platinum cluster (Pt13 ), often used to represent a highly dispersed platinum catalyst, 58,59 was employed to model the surface metal cluster. Thirteen also corresponds to the smallest number of atoms which is necessary to build a cuboctahedron, often considered a relevant particle morphology. Adsorption of the cluster on the oxide was modelled by placing the cluster on the surface with three different possible geometries (cuboctahedron and amorphous 2- and 3-layered structures) and then letting the system evolve with a molecular dynamics (MD) simulation for approximately 5 picoseconds (10’000 steps) at 700 K. Cluster geometry samples were obtained at every 1000st MD step, and subsequently geometry optimized. The most stable geometries and a schematic representation of the structure search method are included in the Supplementary Information Figure S8. The lowest energy cluster geometry was selected as a reasonable approximation for the metal geometry. The final morphology of the platinum cluster is quite different for the two oxides: on titania the metal has the amorphous bilayered structure, while on alumina the cluster forms a single layer and spreads more widely on the surface (Supplementary Information Figure S8). 3.4.1

Prediction of Hydrogen Surface Coverage on Platinum

Investigation of the effects of hydrogen concentration confirms that the dimension and coverage of the oxide slab and metal cluster are crucial: as an example, increasing the anatase slab thickness from 10.5 to 24.6 Å inverts the relative stability of the Pt-H and TiO2 -H solutions, yielding more favorable hydrogen adsorption on the oxide than on the metal cluster. Results presented are obtained with an energy converged slab dimension of

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20.454 × 22.692 × 24.591 Å3 (anatase, 267 slab) and 16.14×16.82 × 16.74Å3 (γ-alumina, 223 slab). At a given temperature and pressure (T, p), the most stable surface composition and geometry is the one that minimizes the system free energy γ(T, p), computed according to the equation: γ(T, p) =

1 support,H2 support,clean [E − Etot − NH µH (T, p)] A tot

With A = system surface area, Etot = total energy of the system and µi (T, p) = chemical potential of the species i. Details and computed parameters of the thermodynamics calculation are listed in the Supplementary Information Figure S4 and Table S5.

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(a)

(b)

(c)

(d)

Figure 5: Hydrogen coverage for (a) Pt13 on anatase (101) and (b) γ-Al2 O3 . The average hydrogen coverage is indicated with colored lines as a percentage, but those are indicative only, because of the large multitude of possible hydrogen and platinum configurations that can be present. The experimental conditions correspond to an estimated hydrogen coverage around 150% on the metal cluster. (c) and (d) system free energy as a function of hydrogen loading for Pt13 on anatase (101) and γ-Al2 O3 (T=300K, pH2 =10−5 mbar). Figure 5 shows the hydrogen coverage on platinum at experimental conditions of 300 K and 10−5 mbar pH2 , which relate to our previous experiment. 21 The most favorable coverage is around 150%, i.e. around 18-20 H atoms on the Pt13 cluster. Higher coverage is difficult to achieve unless a higher partial pressure of the gas is adopted, due to the increasingly weaker interaction of the adsorbed molecules with the metal cluster surface. The average free energy of hydrogen adsorption decreases linearly with increasing hydrogen coverage of the platinum 21



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cluster and is similar for both supports at the experimental conditions: -0.49 eV (titania support) and -0.65 eV (γ-alumina). However, as shown in the lower panels of Figure 5, the free energy curve for the platinum cluster loading displays a shallow minimum on the titania support, probably due to the amorphous and layered geometry of the metal cluster, that can support a rather large number of hydrogen atoms without significant alteration of the overall surface free energy. The formation of a multi-layered noble metal cluster on the oxide surface is indicative of weak metal-oxide interactions and was also reported for platinum on γ-alumina (110) 58 and for ruthenium on titania and zirconia. 60 In this case, the typical hydrogen coverage of the metal cluster is around 1.5 hydrogen per metal atoms but the hydrogen/metal ratio can be as high as 3 and still be energetically favorable. The range of hydrogen loading on the alumina supported platinum cluster is instead smaller, as the metal atoms have a rather limited mobility, due to the stronger metal-support interaction with the oxide surface below. 61 3.4.2

Hydrogen Spillover Kinetics

Two possible spillover mechanisms were investigated. Mechanism 1: hydrogen splitting on the platinum cluster and subsequent transfer of a single atom onto the oxide, and mechanism 2: one-step hydrogen splitting at the cluster edge with transfer of a single hydrogen atom to the oxide. For the same cluster size and hydrogen coverage, the activation energies for transferring a hydrogen atom to the oxide surface with the two mechanisms are comparable (e.g. 1.02 eV vs. 0.98 eV for a platinum cluster covered with 10 hydrogen atoms on anatase (101)), as shown in Supplementary Information Figure S5. Given the very similar activation energies of the two mechanisms, a dominant one can not be clearly determined. The occurrence of mechanism 2 over mechanism 1 will most probably depend on the availability and accessibility of edge platinum sites, and therefore on the dimension and shape of the metal cluster. Another factor that influences the activation energy for oxide reduction is the hydrogen loading on the metal cluster. Higher hydrogen loading reduces the accessibility

22


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of edge platinum sites for the occurrence of mechanism 2 and, at the same time, it might reduce the platinum-hydrogen interaction, favoring hydrogen transfer from the cluster to the oxide (mechanism 1). On anatase, the activation barrier for hydrogen spillover decreases from 1.02 eV (10 hydrogen atoms on the cluster) to 0.45 eV (18 hydrogen atoms, as expected at the experimental conditions) and finally becomes negligible for 24 hydrogen atoms on the cluster (Figure 6). In addition, at higher coverage, more hydrogen atoms will be located at the metal cluster edge sites, in favorable position for transfer to the oxide surface, and, at the same time, limiting the available sites for mechanism 2 to occur. The situation is similar for the case of alumina, even if, due to the more spread shape of the platinum cluster, the abundance of edge sites will favour a mechanism of type 2. The activation energy barrier for hydrogen spillover on alumina is higher than on titania (i.e. 1.32 eV), but can be anyway overcome at room temperature. In this case, however, the spillover and diffusion rates for hydrogen on alumina will be very similar (Supplementary Information Table S7). A kinetic Monte Carlo study, described below, has been carried out to determine the influence of such rates on the final oxide hydrogen coverage. The partial pressure of hydrogen in the system and the cluster deposition technique are therefore crucial factors influencing the shape and hydrogen coverage of the metal nanoparticle, and thus the kinetics of the spillover process.

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Figure 6: Energy profile for hydrogen spillover from a Pt13 cluster on titania. R.C.=0: H2 , gas phase. R.C.=1: H2 @Pt13 . R.C.=2: 2H@Pt13 . R.C.=4: H@TiO2 . Red: 100%Pt coverage. Yellow: 150%Pt coverage. Green: 200%Pt coverage. The activation energy for the spillover step (R.C. = 3) onto alumina is 1.32 eV. To evaluate whether the oxide slab is reduced during spillover from the platinum cluster, a density of states analysis was performed. The total density of states was analyzed for the three main stages of the spillover process, according to mechanism 1: i) a gas phase hydrogen molecule not interacting with the hydrogenated platinum cluster, ii) the additional hydrogen molecule adsorbed and split on the metal cluster and iii) hydrogen atom spilled over to the oxide slab. A comparison of the electronic structure for cases i) and ii) does not reveal significant alterations of the oxide energy bands, confirming that the first hydrogen adsorption and splitting step on the metal cluster does not affect the support oxide. With the transfer of two hydrogen atoms from the platinum cluster to the anatase slab, an occupied T id state appears in the anatase conduction band, while no such features appear for the γ-alumina case (Supplementary Information Figure S6). This is analogous to what happens in the case of hydrogen adsorption on the non-functionalized metal oxide (Figure 2) and to what is reported for other metal clusters/reducible oxide combinations, such as ruthenium on titania and zirconia. 60 It also confirms that hydrogen transfer from the platinum cluster results in effective titania reduction, with electrons being injected into the oxide valence 24


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band. Results from Mulliken charge analysis (Supplementary Information Table S6) are in agreement: the charge difference on titanium sites before and after hydrogen transfer from the platinum cluster is negative, confirming that electrons are transferred to the oxide.

Figure 7: Average hydrogen occupation on γ-alumina (100) as a function of the distance from a platinum cluster source, along the most favorable migration direction. The distribution was obtained with a Kinetic Monte Carlo scheme, including hydrogen spillover, desorption and diffusion rates for an intermediate water coverage on alumina (30%free hydrogen adsorption sites). With the kinetic rates computed in this work, including rates for diffusion, adsorption, desorption and spillover, a comprehensive picture of the hydrogen population on the surface can be obtained. In particular, certain steps, such as adsorption on platinum and transfer from the metal to the oxide surface are fast, while others, such as diffusion and desorption, are slower. It is the competition between surface diffusion and desorption that will, at intermediate timescales, determine the surface coverage. It is intuitive that, for titania, the rapid diffusion, compared to the much slower desorption, will allow for a rapid and almost homogeneous spreading of hydrogen on the surface. On the other hand, the diffusion and desorption rates for alumina are much more comparable (i.e. 5.52×102 for diffusion 25



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vs. 2.61 for desorption) and, depending on the water coverage, they compete. It is then conceivable that hydrogen can be found on the surface only near the spillover source. To quantify this, a simple kinetic Monte Carlo scheme has been adopted. 62,63 In this model, the rates for diffusion, spillover and desorption have been taken explicitly into account, while the very fast adsorption on the metal cluster is treated implicitly. The pressure dependence is introduced in the calculation via the rate of spillover, as discussed previously in this paragraph. A table with all employed rate constants is available in the supplementary material (Supplementary Information Table S7). For titania, the results are unsurprising and a homogenous two-dimensional coverage of the model slab is reached within millisecond timescales. On the other hand, diffusion on alumina is strictly one dimensional, at least for the perfect crystal lattice employed in our model. For a 60% water coverage of the alumina surface, which is consistent with experimental data and theoretical estimates at 10−10 mbar water partial presure, the hydrogen diffusion rate is maximum. The results with this setting thus provide an upper bound for the spread of hydrogen on this surface and are illustrated in Fig. 7. They indicate that hydrogen might be observed within a radius of 12-15nm from the platinum source. This distribution is reached within seconds and remains stationary afterwards. This computed distance is in good agreement with the range observed in our previous experimental study. 21 At different water coverages, this range reduces to at most 3 nm from the spillover source (2 diffusion steps on the oxide surface).

4

Conclusion

This work presents a detailed investigation of the electronic and interface properties of titania and alumina carried out by means of ab initio density functional theory calculations. Both oxides are adopted as support in heterogeneous catalysis, but their behavior towards reducing molecules, such as hydrogen, is contrasting. Titania has a limited reactivity towards direct hydrogenation, but is easily reduced via spillover from a noble metal cluster,

26


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with formation of electronic trap states. In this case the chosen oxide size is relevant, 44,45,55 as the distortion caused by the localized electron centers can extend for several lattice constants. Modulation of the reaction conditions, such as temperature and gas partial pressure, hinder or facilitate the spillover process and can be of use for controlling the oxide reduction. Direct hydrogenation of the reactive surface of alumina is instead possible, but only on specific sources exposing undercoordinated Al(3c) sites. However, no reduction of the oxide is achieved and the process results in the formation of an hydride anion, confined at the interface Al(3c) -H bond, without distortion of the oxide lattice, so that the chosen system size has little influence on the hydrogenation process. The presence of coadsorbates, water in particular, that could compete for the surface adsorption sites, is instead crucial, making the system thermal treatment and vacuum conditions critical in determining the reactivity. Depending on the water coverage on alumina, the activation enegy for charge mobility is ∼ 1.5 eV, almost double than that on titania (0.7 eV), explaining another aspect in the different performance of the materials. As a result of the balance between desorption and surface diffusion, Kinetic Monte Carlo simulations confirm that, on alumina, hydrogen can be found, in favorable conditions, up to 12-15 nm away from the metal cluster, while spreading is homogeneous on titania. Future work could aim at taking into account combinations of multiple defect sites and other coadsorbates, as well as the possible effects of grain boundaries on the species migration.

5

Supporting Information

Table S1: H2 adsorption energies computed with different functionals on titania, with different slab sizes, adsorption positions and spin multiplicities. Table S2: most relevant H2 adsorption positions and energies on alumina. Table S3: total, average and differential adsorption energies for multiple hydrogen molecules on titania. Table S4: electron/proton interaction energies on titania. Figure S1: activation energies for hydride migration on alu-

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mina. Figure S2: activation energies for other possible charge carriers migration mechanisms on titania and alumina. Figure S3: oxide slabs employed in the NEB simulations, including water coverage. Figure S4: Details of the ab initio thermodynamics calculations. Table S5: computed vibrational, rotational and ideal gas entropy contributions to the chemical potential of H2 . Figure S5: two posible hydrogen spillover mechanisms to the metal oxide. Figure S6: density of State analysis for hydrogen adsorption on a metal oxide-supported Platinum cluster and subsequent hydrogen spillover. Table S6: charge variation of atomic species upon spillover of an hydrogen atom from a Pt cluster to a metal oxide surface. Table S7: details, schematic and rate list included in the KMC simulation. Figure S7: full NEB profiles for the data reported in the NEB figures of the main text. Figure S8: most stable geometries for a 13-atom Pt cluster adsorbed on the oxdie surfaces and scheme of the structure sampling method. Figure S 9: Example of CP2K input file,

6

Acknowledgements

J.V. acknowledges financial support by the European Union FP7 in the form of an ERC Starting Grant under Contract No. 277910. Calculations have been enabled by a grant from the Swiss National Supercomputer Center (CSCS) under Project ID ch5. W.K. acknowledges the PSI Research Commission for the financial support through the CROSS program.

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Hydrogen Adsorption on Nanosized Platinum and Dynamics of

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