Mechanistic Insights into Photocatalyzed Hydrogen Desorption from

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Mechanistic Insights into Photocatalyzed Hydrogen Desorption from Palladium Surfaces Assisted by Localized Surface Plasmon Resonances Vincent A. Spata, and Emily A. Carter ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00352 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Mechanistic Insights into Photocatalyzed Hydrogen Desorption from Palladium Surfaces Assisted by Localized Surface Plasmon Resonances Vincent A. Spataa and Emily A. Carterb,ǂ a

Department of Mechanical and Aerospace Engineering, and bSchool of Engineering and Applied Science,

Princeton University, Princeton, NJ, 08544-5263, United States

ǂ [email protected]

ABSTRACT: Nanoparticles synthesized from plasmonic metals can absorb low-energy light, producing an oscillation/excitation of their valence electron density that can be utilized in chemical conversions. For example, heterogeneous photocatalysis can be achieved within heterometallic antenna-reactor complexes (HMARCs), by coupling a reactive center at which a chemical reaction occurs to a plasmonic nanoparticle that acts as a light-absorbing antenna. For example, HMARCs composed of aluminum antennae and palladium (Pd) reactive centers have been demonstrated recently to catalyze selective hydrogenation of acetylene to ethylene. Here, we explore within a theoretical framework the rate-limiting step of hydrogen photodesorption from a Pd surface – crucial to achieving partial rather than full hydrogenation of acetylene – to understand the mechanism behind the photodesorption process within the HMARC assembly. To properly describe electronic excited states of the metal-molecule system, we employ embedded complete active space self-consistent field and n-electron valence state perturbation theory to second order within density functional embedding theory. The results of these calculations reveal that the photodesorption mechanism does not create a frequently invoked transient negative ion species but instead enhances population of available excited-state, low-barrier pathways that exhibit negligible charge-transfer character.

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KEYWORDS: composite nanostructures, heterometallic antenna-reactor complex, localized surface plasmon resonance, transition metal catalysts, acetylene hydrogenation, hydrogen dissociation, heterogeneous catalysis Current chemical production is frequently very energy intensive, requiring high temperatures and pressures to carry out chemical reactions, as well as relying heavily on the use of fossil fuels to achieve the necessary conditions for favorable reactions. Novel synthetic approaches utilizing renewable energy sources to drive these chemical reactions are being sought, not solely because of the need to reduce financial costs of chemical production but also because of the rising need to alleviate environmental impacts. One promising strategy exploits properties of plasmonic metal nanoparticles to perform a novel form of heterogeneous photocatalysis. 1, 2 As in photoelectrocatalysis at semiconductor electrodes, light energy is absorbed and converted to electronic energy, which in turn helps catalyze chemical reactions. However, the mechanisms for energy conversion in these two types of materials are very different, as semiconductors directly form electron-hole pairs whereas metal nanoparticles first create plasmons. Localized surface plasmon resonances (LSPRs) occur via absorption of low-energy light, often within the visible region. LSPRs manifest as a collective oscillation of a metal’s valence electrons, which then may decay by exciting electron-hole pairs. The electron-hole pairs can further decay via radiative recombination or they may transfer energy or charge to induce otherwise inaccessible reactions. The electron-hole pair may also decay through the generation of hot carriers 3 when charge screening is fast, ultimately releasing heat via inelastic electron-phonon scattering.

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Several energetically inhibited

chemical reactions can occur via LSPR utilization. Examples include activation of oxygen (O2), epoxidation of ethylene,

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oxidation of carbon monoxide (CO) and ammonia on silver (Ag),

5

4

and

hydrogen (H2) dissociation and desorption on gold (Au) 6, 7 and aluminum (Al). 8 However, the list of plasmonic metals that are strong light absorbers at the nanoscale consists only of a few elements (Au, Ag, copper (Cu), and Al), limiting the chemical reactions that can be directly photocatalyzed. The numbers and types of accessible chemical reactions can be expanded further via coupling catalytically reactive

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metals to plasmonic metals in alloyed or composite (“hybridized”) nanostructures or assemblies. Increased reaction yields under mild conditions have been reported for several hybridized systems, including palladium-Au (Pd-Au) nanoparticles,

9, 10

platinum-Ag (Pt-Ag) nanoparticles,

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Au-Cu

nanoparticles, 12 Pt-capped Au nanorods, 13 and Al/Pd nanoparticle complexes. 14 Furthermore, devices have been tested that utilize Au combined with Pt and other materials in photoelectrodes capable of photocatalyzed water splitting. 15 - 17 Heterometallic antenna-reactor complexes (HMARCs) offer the most elemental flexibility of the above mixtures. HMARCs utilize the property of near-field enhancement of the photon flux surrounding the plasmonic metal nanoparticle to facilitate catalysis on an adjacent reactive center. 14, 18, 19 The device consists of a plasmonic metal such as Al, which directly absorbs/scatters light, and a reactive metal, e.g., Pd, where catalysis occurs.

In the case of Al/Pd HMARCs, the reaction tested was the selective

hydrogenation of acetylene to ethylene. This reaction is important in industry for purifying ethylene streams prior to polymerization to polyethylene; it is a fundamental, selective hydrogenation reaction that serves as a model for other such reactions. 20 Pd is an effective hydrogenation catalyst, as dissociative adsorption of H2 on Pd has a negligible barrier 21 and is exothermic with a heat of adsorption of ~ -0.91 eV. 22 These characteristics are also a disadvantage, as surfaces of Pd easily saturate with hydrogen (H) atoms, preventing other molecules from binding and increasing the probability of over-hydrogenation. A full monolayer of H atoms forms readily on Pd(111) at only 50 K. 23 Adsorbed H atoms on Pd can also dissolve into the bulk and form hydrides, which can cause additional problems for hydrogenation, such as a loss of selectivity to ethylene. 24 Prior work showed that desorption of H2 on Pd is the rate-limiting step in selective hydrogenation of acetylene to ethylene by Al/Pd HMARCs, which exhibits a selectivity of up to 40:1 conversion of acetylene to ethylene compared to ethane. 14 The current work elucidates the mechanism by which HMARCs achieve H2 photodesorption from Pd surfaces, within embedded correlated wavefunction (ECW) theory.

25 - 27

We obtain accurate

predictions by utilizing an embedded complete active space self-consistent field (CASSCF) approach 28 3 ACS Paragon Plus Environment

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and the n-electron valence state perturbation theory to second order 31 - 33 (NEVPT2) in conjunction

with density functional embedding theory (DFET).

25, 27

ECW theory has the correct physical

underpinnings to accurately describe charge-transfer (CT), excited states, conical intersections (CIs), and bond formation/breaking processes not properly treated by DFT. 26 The method allows us to accurately evaluate reaction energetics by incorporating both static and dynamic electron correlations via CW theory, which are critical to capturing the physics and chemistry of transition-metal surface interactions with gas molecules and adsorbates. It also enables us to examine the energy and character of excited states, in order to predict photophysical and photochemical behaviors with a high level of accuracy. The method successfully characterizes adsorbate-metal interactions, including H2 dissociation on Au(111), 34, 35

O2 dissociation on Al(111), 36, 37 CO adsorption on Cu(111), 38 H2 dissociation on Al(111), 39 and N2

dissociation on iron- and molybdenum-doped Au(111). 40, 41 It is most often assumed that CT is involved in photocatalysis on plasmonic metals, either via direct population of CT states or by indirect formation of a transient negative ion (TNI) via transfer of hot electrons from the plasmon to the adsorbate. 42, 43 Illumination of non-plasmonic transition metals (Pd, Pt, rhodium (Rh), and iridium (Ir)) also can enhance catalytic rates, with further increases achieved by increasing the light intensity. 44 These measurements suggest that excited states intrinsic to the transitionmetal-surface/molecule complex are enhancing the catalysis. The current work reveals that the photoinitiated mechanism for H2 desorption from Pd indeed involves direct population of hybridized adsorbate/metal exciton states enabled by the Al nanoparticle LSPR’s localized near-field enhancement. Characterization of the excited states involved in the reaction pathway further supports this non-CT mechanism, as CT is not apparent in the lower-energy excited states, with no evidence for formation of a TNI species. RESULTS AND DISCUSSION NEVPT2 ground-state energy path. The ground-state energetics of H2 on Pd were studied previously

14

using periodic projector-augmented-wave (PAW)-DFT calculations 4 ACS Paragon Plus Environment

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with the Perdew–

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Burke–Ernzerhof (PBE) exchange-correlation (XC) functional, 46 applying D3 dispersion corrections. 47, 48

The structures in Figure 1 taken from Ref. 14 are used for single-point calculations at the embedded

NEVPT2 level of theory with input wavefunctions obtained from embedded CASSCF theory. Further details of the calculations to obtain the structures are available in Ref. 14. The reported NEVPT2 energies are obtained using a converged active space composed of 12 electrons in 12 orbitals (12e, 12o), expressed within the cc-pVDZ-PP ECP28MDF Stuttgart basis set and effective core potential for Pd, 49 and the aug-cc-pVDZ basis set for H. 50 All calculations are performed on an embedded 10-atom cluster serving as a model of Pd(111) (Figure 1). Further details regarding calculations determining convergence of basis-set and active-space sizes are provided in sections 1 and 2 of the Supplementary Information (SI). This theoretical formalism will be referred to as “emb-NEVPT2” hereafter, or else with reference to a particular basis set, such as emb-NEVPT2/pVDZ (for an embedded NEVPT2 calculation using the ccpVDZ-PP/aug-cc-pVDZ basis set). The same notation will be used to describe the embedded CASSCF calculations. At the DFT-PBE-D3 level of theory, the “fcc-fcc,” “p-hcp-fcc,” “top,” and “away” structures (Figure 1) are minima along the H2 desorption pathway (Figure 2). The predicted dissociative adsorption energy for H2 on Pd(111) is -1.25 eV. The lowest-energy structure is fcc-fcc, which has both H atoms in fcc hollow-sites, fully chemisorbed on the Pd surface. The other chemisorbed minimum, p-hcp-fcc, with two H atoms adsorbed on fcc and hcp hollow-sites, is slightly higher in energy than fcc-fcc, by ~0.2 eV. The energy barrier for the H atoms to recombine and reach the physisorbed, top structure from p-hcp-fcc is 0.88 eV, with this transition rate-limiting. The away structure models gas-phase H2 5 Å away from the Pd(111) surface. The necessary energy to overcome the physisorption barrier to form gas-phase H2 is predicted to be 0.35 eV. At the emb-NEVPT2 level of theory, the dissociative adsorption energy for H2 on Pd(111) is significantly less exothermic than at the DFT level (-0.85 eV; Figure 2), and much closer to the measured value of -0.91 eV.

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The relative energies of the DFT-calculated structures along the H2 desorption 5 ACS Paragon Plus Environment

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pathway are mostly destabilized by emb-NEVPT2, with the sole exception of the top structure and a structure adjacent to it along the reaction coordinate (RC). The lowest-energy structure is still fcc-fcc, with a 0.82 eV barrier to reach the “int-1” structure, which is an intermediate along the DFT-PBE-D3predicted minimum energy path and a possible new local minimum predicted by emb-NEVPT2. Similarly the “int-2” structure is a possible new local maximum, which lies ~0.4 eV higher in energy than the top, physisorbed minimum, according to emb-NEVPT2. Because it is prohibitively expensive to verify with emb-NEVPT2 whether these intermediates are indeed critical points, we only state that they are possibly so. The overall barrier is 0.85 eV for the H atoms to recombine to form the physisorbed, top geometry. To go from the physisorbed structure to form gas-phase H2 requires 0.41 eV. The ground-state barriers from ECW theory thus are similar to those from DFT-PBE-D3, while the overall thermodynamics (i.e., the heat of adsorption) improves considerably by using ECW.

Figure 1. Structures along the minimum-energy, ground-state H2 desorption pathway obtained from periodic DFT-PBE-D3 calculations (Ref. 14).


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As evident from Figure 2, the p-hcp-fcc structure is much less stable within emb-NEVPT2 compared to DFT. Also, although p-hcp-fcc is not a transition state, emb-NEVPT2 predicts that the structure actually may be near a CI, as evidenced by the close approach of excited states in Figure 3 (vida infra). DFT approximations tend to overestimate the extent of charge delocalization while tending to underestimate barrier heights of chemical reactions.

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In p-hcp-fcc, the hcp H atom is nearby an

additional (compared to the fcc site) Pd atom in the second layer, increasing the interaction between Pd and the hcp H atom for this geometry. Bader charge analysis 52, 53 on the embedded cluster ground-state density reveals a larger negative charge on the hcp H atom for p-hcp-fcc when treated at the ECW level: 0.28 e, according to emb-CASSCF, versus -0.05 e, according to periodic DFT-PBE. Given that this comparison involves changes in both the model (embedded cluster vs. periodic slab) and the level of theory (CASSCF with a Gaussian-type orbital (GTO) basis vs. DFT-PBE with a planewave (PW) basis), it is important to determine whether this change in charge distribution is due to the former or the latter. By performing additional emb-DFT-PBE calculations using GTO and PW basis sets, we ascertain the effect of changing the model/basis set while keeping the level of theory (DFT-PBE) constant. We find that the charge changes negligibly going from periodic DFT-PBE (-0.05 e) to emb-DFT-PBE using PW (0.04 e) or GTO (-0.08 e) basis sets. Thus, the origin of the significant increase in charge is not the model or basis set change but is the improved level of theory (emb-CASSCF). Charges on the H atoms in phcp-fcc calculated with various methodologies are included in section 3 of the SI. The additional interaction between the more highly charged hcp H atom and the nearby second-layer Pd atom is probably the cause of the predicted destabilization of p-hcp-fcc at the ECW level of theory. The overall electron distribution is more delocalized at the DFT-PBE level, which probably reduces the repulsion with the second-layer Pd atom, therefore artificially decreasing the ground-state energy of the p-hcp-fcc structure. The mechanism of H2 dissociative adsorption on Pd(111) has been suggested to occur by dynamical steering and trapping of H2 as it fully dissociates in an almost barrierless fashion. 21 Both levels of theory considered here (DFT and ECW) predict a physisorbed precursor to dissociative adsorption,


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with a small activation barrier to reach fully dissociated, chemisorbed H atoms. This activation barrier along the dissociative adsorption pathway is only 0.04 eV at the DFT-PBE-D3 level, consistent with earlier DFT predictions, 54 but is an order of magnitude larger at the emb-NEVPT2 level, 0.40 eV. The latter is in apparent conflict with experiment. However, the measured negligible barrier in fact is probably the effective overall barrier. If a pre-equilibrium exists between gaseous H2 and the physisorbed molecular precursor to dissociative adsorption, and the latter does not have time to thermalize, then it may retain the kinetic energy necessary to overcome the local barrier of 0.4 eV. Then measurements would merely probe the height of the barrier above the energy of gaseous H2 instead of the local barrier from the bottom of the physisorption well. Our emb-NEVPT2 results indeed predict a negligible overall barrier, leading to fast and efficient dissociation. We therefore suggest an additional aspect to the mechanism, namely that the large difference in the masses of H and Pd atoms suppresses energy transfer to surface phonons, thereby allowing the molecule to retain sufficient kinetic energy to surpass the barrier at structure

int-2,

leading

to


complete

dissociation.

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Figure 2. H2 desorption from Pd(111) ground-state energy curves described at the DFT-PBE-D3 and emb-NEVPT2/pVDZ levels of theory. Energy barriers for desorption for each method are illustrated with red vertical arrows. Dot colors on each curve refer to the color bar on the right giving the distance between the two H atoms along the reaction coordinate (RC) in Ångstroms. The RC is defined according to Eqns. S1 and S2 in section 4 of the SI. The experimental dissociative adsorption energy for H2 on Pd is taken from Ref. 22. H2 desorption from Pd(111) in the ground state also can be represented as a multiple-step mechanism. The chemisorbed H atoms first must reorganize before recombining to form the physisorbed (top) structure. We are unable to determine precisely the height of the barrier for transition between fccfcc and p-hcp-fcc at the emb-NEVPT2 level because it is too expensive to further sample the multidimensional potential energy surface (PES). Based on the emb-NEVPT2 energies of the structures available from the DFT-optimized RC, we instead consider the transition from chemisorbed (fcc-fcc) to physisorbed (int-2) to be rate limiting with an energy barrier of 0.85 eV.

Invoking microscopic

reversibility, the same argument applies with respect to the mismatch in masses (H vs. Pd) allowing for no thermalization as the H atoms pass over the top minimum. Thus, the subsequent energy barrier of 0.41 eV to desorb physisorbed H2 into the gas phase will not be rate limiting. Excited-State Pathways. To understand photochemistry we must obtain accurate descriptions of excited-state character and energetics. Multi-reference methods such as emb-NEVPT2 are advantageous for describing the energetics of closely spaced excited states. Figure 3 displays excited-state potential energy curves along the H2 desorption RC, derived from 14-state-averaged emb-NEVPT2 calculations. Tests of excited-state energy convergence with respect to the number of states averaged are given in section 5 of the SI; excitation energies are converged to within ~0.1 eV when 14 states are averaged instead of 10.


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Figure 3.

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Excited-state activation/deactivation pathways calculated from 14-state-averaged emb-

NEVPT2/pVDZ. Vertical red arrows represent excitations/de-excitations. Curly red arrows represent thermal activation/deactivation processes.

For reference, the energy of the peak plasmon resonance in

Al/Pd HMARCs is at ~2.47 eV. The peak plasmon resonance in Al/Pd HMARCs occurs at ~2.47 eV.

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Initial population of

excited states can occur from the fcc-fcc structure (Figure 3), with an energy of ~1 eV to populate a band of hybridized hydrogen/metal excited states. At the fcc-fcc minimum, the energies of the two bands of states (five at lower energies, nine at higher energies) are well separated, leading to relatively longer-lived excited states. Some of the barriers in the excited states are only ~0.13 eV to reach p-hcp-fcc compared to a ground-state energy barrier of 0.82 eV, allowing progression along the RC over the transition state. De-excitation may occur nonradiatively, with the reaction then proceeding forward to the emb-NEVPT2predicted possible minimum on the ground state PES (int-1). Admittedly, decay to the ground state might occur quickly because of a continuum of Pd states not represented in Figure 3. However, given the small


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amount of thermal energy required to overcome the barrier, we expect the system will reach the p-hcp-fcc CI before decaying to Int-1. A second excitation then may occur to the higher-lying states, with an excitation energy of ~1.4 eV, with subsequent excited-state barriers reduced to only ~0.3 eV to reach the top structure. At those top-like geometries where H2 is located further from the metal surface, decreased hybridization between hydrogen and metal orbitals raises the relative excited-state energies of these states. Nonradiative decay back to the lower-lying manifold of states instead allows population of the physisorbed precursor ground state. From this point along the RC, because of the decreased H-Pd hybridization, any excited-state character most likely will just fully populate the d-orbitals on Pd as H2 begins to separate from the surface.

The energy required to desorb in the ground state from the

physisorbed top site is only 0.33 eV. Additional energy to overcome the 0.33 eV barrier may come from local heating caused by deactivation of excited metal states. The Pd d-band is not entirely filled in the extended metal (unlike the atom), producing a continuum of states that could facilitate fast deactivation processes, as alluded to above. The fractionally occupied d orbitals can interact strongly with the H 1s orbitals, which is why Pd binds hydrogen so well. The continuum of Pd states that can interact with H states thus increases the number of closely lying hybridized excited states.

The close spacing of states spurs ultrafast nonradiative decay through

nonadiabatic processes, 55 with deactivation through CIs 56 where two or three states can cross within the bands of H/Pd excited states. Our calculations do not fully capture the continuum of excited states along the RC because there are more interacting d-orbitals within the system than are described within the necessarily finite active space of our multi-reference calculations. Active spaces for the fcc-fcc, p-hcp-fcc, and top geometries are displayed in section 2 of the SI. Due to inclusion of the H orbitals in the active space, all of the states represented do capture some hydrogen character (whether it be hybridized H/Pd or molecular H2). We can only speculate on the rate of decay within the continuum of Pd metal states because we are unable to perform, e.g., potential energy surface hopping simulations. The transition between states 11 ACS Paragon Plus Environment

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depends on the nuclear velocity and the nonadiabatic coupling vectors that depend on the wavefunction overlap between the electronic states. To have good overlap, they should exhibit similar character. The bands closest to the ground state are excited states of predominantly metal-hydrogen bonding character, according to the CASSCF occupation numbers. This assignment is also evidenced in Crystal Orbital Hamilton Population (COHP) analysis in our prior published work utilizing DFT. 14 According to DFT and COHP analysis, the edge of the anti-bonding states isn’t evident until at least ~1 eV above the Fermi level (which is a lower bound due to errors in the exchange-correlation functional), implying the antibonding metal-hydrogen states lie in the higher-energy excited-state band. The overlap of these states with the continuum of Pd metal states will not be as strong as their overlap with each other. The separation of the bands of states illustrated in Figure 3 thus supports a prolonged excited state lifetime. Our finite active space calculations only describe intraband (d-d) excitations and not interband (dsp) excitations.

The change in angular momentum intrinsic to interband excitations renders them

quantum mechanically allowed whereas the intraband transitions are forbidden. This fact, coupled with higher excitation energies that increase oscillator strength, should produce more efficient light absorption for the interband transitions. Yet it is the surface Pd d-orbitals that interact most strongly with the H orbitals and evolve along the RC to desorb H2, so it is these orbitals that are needed – and therefore included - in the active space to represent the character of the states we want to probe. Possibility of direct light absorption and resonance energy transfer.

The mechanism

responsible for the observed photochemical reaction is currently under debate. It is predominantly believed that plasmon-induced photochemistry is caused by LSPR-generated charge injection into unoccupied adsorbate states.

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According to Linic and coworkers, electron injection from the metal

catalyst into the orbitals of the adsorbate can be due to either direct light absorption processes that directly populate CT states, or indirect light absorption processes that inject an excited electron from a Fermi-like distribution of hot carriers within the low-energy edge of the conduction band into low-lying orbitals on the adsorbate.

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Our system is different from other plasmonic photocatalytic systems, as Pd is a poor 12 ACS Paragon Plus Environment

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plasmonic metal; 57 Al is the stronger plasmonic metal in the HMARC. Pd instead is the reactive center, so the photophysics/photochemistry may differ from systems where the reaction occurs directly on the surface of a plasmonic metal. Yet it is important to test for the existence of these mechanisms in our system as well. The electric field enhancement between the Al/Pd nanoparticles also will intensify the photon density in both the near- and far-field of the HMARC, resulting in enhanced absorption on the Pd catalytic reactors. 14 If efficient transitions to excited states from direct absorption do exist, then our calculations indicate that the energy barriers in the excited states are much lower in energy than the barriers along the ground-state pathway. This would allow photodesorption to occur through transient population of even low-lying excited states (relative excitation energies around or just above 1 eV) in a multi-step process to form gas-phase H2. In the weak-field regime, the oscillator strength is proportional to the intensity of absorption. 58 We therefore calculated oscillator strengths for the 13 lowest-lying excited states along the RC to further assess the probability for absorption. Figure 4 displays simulated absorption spectra along the reaction pathway as a collection of 45 transitions, from the lower band of five states to the nine higher-lying excited states, up to S13, all singletto-singlet transitions. Each spectrum is constructed individually as a sum of Gaussians using embNEVPT2 excitation energies and emb-CASSCF transition dipole moments to derive the oscillator strengths, which give the height of each Gaussian, centered at each excitation energy. The full-width-athalf-maximum of each Gaussian is 0.05 eV. By summing the functions, we can analyze the collective degeneracies of states due to the contributions from additional states in the excited-state bands and the effect on the probability of absorption.


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Figure 4. Calculated absorption spectra for important structures along the desorption pathway, based on oscillator strengths. Absorption is plotted as a sum of Gaussian functions with amplitudes based on oscillator strength for 45 singlet-to-singlet transitions per structure between S0-S4 to S5-S13 using energies from emb-NEVPT2/pVDZ and transition dipole moments calculated using emb-CASSCF/pVDZ.

Our calculated absorption spectra exhibit low oscillator strengths across the reaction pathway (Figure 4). The largest value predicted is 0.0044 for the fcc-fcc structure, which is a stronger absorber than the other geometries due to the well-separated two bands of states and the near-degeneracies within the bands of states (Figure 3). The Al/Pd HMARC might compensate for the low oscillator strengths because absorption at Pd increases via the near-field enhancement due to light scattering originating from the plasmonic excitation in the Al antenna. 14 Finite-difference time-domain (FDTD) calculations predict


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that the local electric field enhancement at hotspots on Pd surfaces reaches values up to a ~40-fold magnification. 14 These calculations were performed for a single Pd island on Al; the effect was predicted to be enhanced greatly by clusters of HMARCs. For example, FDTD calculations predict the electric field enhancement to reach values of more than 106 for two Ag nanocubes separated at the tips by 1 nm on α–Al2O3.

4

Near-field enhancement also has been observed in HMARCs composed of Pt reactive

centers and Ag antennae by the co-localization of plasmonic Ag nanoparticles. 11 The electric field enhancement is a prefactor to the radiation density in the expression for the absorption probability. 59 If we assume enhancement at hotspots and that the light intensity on the Pd island due to scattering is magnified locally by 40 times, and if we consider absorption to be local, then light absorption by the fcc-fcc structure could have a probability as high as 0.17. This would result in reasonably efficient absorption at the chemisorbed minimum and absorption will be even more efficient in clusters of HMARCs. Note that the predicted oscillator strengths increased when averaging over fewer states, indicating that the transitions discussed here could be brighter than what is presented in Figure 4 (see section 5 of the SI for further details). Another possible mechanism may enhance reaction rates on Pd nanoclusters in HMARCs. Fluorescence resonance energy transfer (FRET) rates between a donor and acceptor can be enhanced via localized near-field enhancement from adjacent plasmonic nanoparticles. 60, 61 Furthermore, FRET has been suggested to facilitate chemical reactions on the surfaces of plasmonic nanoparticles with rates dependent on the localized near-field enhancement.39,

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This mechanism may also apply to the Al/Pd

HMARCs. In Förster theory, the rate of FRET depends directly on the projection of the transition dipole moment between the donor and acceptor, the spectral overlap, and a 1/R6 dependence of the energy transfer rate on the distance between the donor and acceptor. This mechanism may occur more often at locations on the Pd nanocluster closest in vicinity to the Al nanodisk, but could be limited by decreased near-field enhancement at these locations because the predicted enhancement is roughly half that of hotspots on the outer edges of the Pd island. FRET will be less efficient than absorption at the edges, due 15 ACS Paragon Plus Environment

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to the 1/R6 distance dependence of FRET rates. This leads us to believe that direct absorption is primarily responsible for the photocatalytic mechanism for the current HMARC case. The direct absorption mechanism usually suggests a linear photon intensity dependence, where a single excitation event results in photochemical product. 43 Because of the measured superlinear laser power dependence for HD formation on Al/Pd HMARCs,

14

photodesorption most likely occurs via

multiple excitations. Also, as noted earlier, the continuum of states inherent at the Pd(111) surface may enable fast, nonradiative decay, requiring multiple excitations to proceed forward along the RC. Thus, the reaction probably evolves along the RC in a stepwise photocatalytic mechanism, using the energy from multiple absorption/or energy transfer events. Support for direct absorption of hybridized adsorbate/metal states and enhanced absorption due to the local-field enhancement in HMARCs has been observed in other reactions as well. Christopher and coworkers demonstrated that CO on Pt nanoparticles can undergo direct absorption due to hybridized metal/adsorbate states, which can alter the selectivity of chemical reactions.

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Specifically, they

compared the oxidation of hydrogen versus CO. CO/Pt hybridized states were found to have higher absorption efficiency than H/Pt hybridized states, leading to enhanced selectivity for CO oxidation compared to water production. Their results offer another example of chemical control based on the inherent photophysical properties of metal/adsorbate states. Role of charge transfer in the overall process. To test for the existence of CT states, we performed Bader charge analysis on the excited-state emb-CASSCF densities and took the differences in charge summed over the two H atoms between the excited states, Sx, and the ground state, S0 (Figure 5). This metric allows us to ascertain if a direct CT state is accessed. Figure 5 shows that the maximum charge transferred from the ground state to any excited state is 0.06 e, a negligible amount. If there are CT states responsible for the mechanism, then they must exist at excited-state energies higher than ~1.5 eV, an approximate upper bound to our range of excited-state energies.


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Figure 5. Bader charge (excited minus ground state) differences, summed over both H atoms, for each state obtained from emb-CASSCF/pVDZ ground- and excited-state densities. The structures along the desorption pathway are presented from left to right as chemisorbed H atoms to gas-phase H2. Note the vertical scale is not the same in each panel. If a TNI were to form instead, we would expect to see evidence for it by examining evolution of charge in each state along the RC (Figure 6). Figure 6 clearly demonstrates that no TNI forms along the desorption pathway. The chemisorbed H atoms are partially negatively charged to begin with (~ -0.25 e on each H) and upon desorption this charge only decreases in every state examined.


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Figure 6. Summed Bader charge for both H atoms for each state obtained from emb-CASSCF/pVDZ ground- and excited-state densities. The structures along the desorption pathway are presented from left to right as chemisorbed H atoms to gas-phase H2. As mentioned above, earlier work presented a COHP analysis for H2 on Pd (111).

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COHP

analysis is a method for projecting the bonding and anti-bonding character between different atomic orbitals onto the density of states (DOS). The initial conclusion drawn from COHP analysis of the ground-state DFT-PBE DOS was that the low-lying, anti-bonding character between H and Pd orbitals allows for charge injection from hot carriers to catalyze the reaction. These types of misconceptions based on the DOS can be avoided by analyzing excited state energies and wavefunctions, and illustrates the necessity for explicit calculations of many-electron excited states rather than relying on qualitative features of the unoccupied one-electron energy levels from a ground-state DFT calculation. The antibonding orbitals for the chemisorbed geometries contribute to the DFT-PBE DOS at energies of ~ +1 eV and greater. By contrast, the emb-CASSCF anti-bonding orbitals have very small occupations (see the 18 ACS Paragon Plus Environment

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circled orbitals in Figures S1-S3). In the chemisorbed minimum, fcc-fcc, the population of higher-lying states with a very small amount of anti-bonding, hybridized Pd/H character will occur from direct absorption/excitation or FRET processes.

The population of these states then will lead to the

recombination of atomic H to form the physisorbed, molecular H2 on the Pd surface. The physisorbed geometry is higher in energy in part due to the increased anti-bonding character between the H atoms and the surface. There is only a ~0.4 eV barrier to fully desorb, which will allow for desorption of molecular H2 in a second step through thermal heating, as noted earlier. The smaller particle size of 10 nm will increase the amount of electron-electron scattering, 63 thus suppressing generation of high-energy electrons that could be used for charge injection based on the indirect mechanism. The fast thermalization times (~300 fs in 10 nm Ag nanoparticles) 64 compared to slow interfacial electron-transfer times with molecules (>10-100 ps) further supports a direct absorption mechanism for plasmonic photocatalysis. 63 Pd is a poor plasmonic metal but is able to absorb light. 14 Increased light absorption has been shown to produce increased catalytic rates for Pd, Pt, Ir, and Rh. 44 Our results suggest that the energies of CT states lie higher than ~1.5 eV, indicating that the direct CT mechanism for H2/Pd desorption is also not active. Instead, we suggest that direct light absorption populates hybridized H/Pd excited states, providing enough energy to overcome smaller excited-state barriers compared to the larger barriers existing on the ground state PES. In addition, the enhanced electric field on the Pd nanoparticles in the HMARC can induce multiple excitations, reconfiguring the H atoms on the surface (e.g., the transition from fcc-fcc to p-hcp-fcc) before recombining to form the topsite, physisorbed precursor and then desorbing from the surface. CONCLUSIONS Ground- and excited-state PESs for the interaction of H2 with Pd(111) were calculated using ECW methods to analyze their energy and electronic character, as well as to elucidate mechanisms for photocatalysis in HMARCs consisting of Al and Pd. Analysis of the excited-state character provides no evidence for formation of CT states or of a TNI, either by direct excited-state population or indirectly 19 ACS Paragon Plus Environment

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through an evolution of charge on the H atoms along the RC. The experimental quantum yield for HD desorption on Pd (111) is superlinear, consistent with our results that point to multiple photon absorptions to photodesorb H2 instead of a single absorption event. The near-field enhancement, at hotspots on the Pd reactor caused by the Al antenna, can lead to two different possible mechanisms. Our results suggest that the mechanism for photocatalysis in HMARCs involves a combination of direct absorption and less frequent but potentially competing FRET processes, which populate hybridized H/Pd excited states with sufficient energy to overcome smaller reaction barriers in the excited states. When direct absorption occurs, the reaction selectivity will depend on the photophysical and photochemical properties of the interacting adsorbate and metal surface. If specificity for products is dependent on the photophysical properties, then surfaces could be tailored to enhance light absorption or rates of formation of low-lying CT states to alter the resulting photochemistry. It may be possible, via alloying Pd, to achieve even greater efficiency of H2 desorption and thereby improve the selectivity for partial hydrogenation reactions even further. METHODS ECW theory first uses periodic DFT to obtain a mean-field description of the extended metal surface, with the electronic structure subsequently refined in a region of interest, typically a small portion of the metal (a cluster) and an adsorbate. The adsorbate-cluster complex then is treated with higher levels of theory (CASSCF, NEVPT2, etc.) in the presence of an embedding potential that describes the effect of the surroundings on the embedded region. The ECW calculations utilize the DFET formalism 25, 27 to obtain the embedding potential for a bare Pd (111) slab. The embedding potential, optimized at the DFT-PBE level, describes the interaction between the bare metal cluster and the extended metal surface. This potential term is added to the electronic Hamiltonian for the cluster calculations to account for interaction with the periodic environment:


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( + ) = , ,

(2)

where is the electronic Hamiltonian, is the embedding potential, is the wavefunction describing the cluster, and , is the embedded cluster energy. The overall energy expression, , replaces the DFT description of the cluster, , , with a correlated wavefunction description, , .

= − , + , ,

(3)

where is the DFT-PBE energy of the periodic system.

The model system used to obtain the embedding potential is a bare Pd slab, constructed as a 5×5 supercell four layers thick, for a total of 100 atoms and a vacuum thickness of ~15 Å. DFET calculations employed the PBE generalized gradient approximation XC functional 46 within an in-house embedding potential optimization code

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interfaced to the Vienna Ab Initio Simulation Package (VASP) version

5.3.5. 66 - 68 The calculations utilized a PW basis kinetic energy cutoff of 600 eV, a smearing width of 0.09 eV, and k–point meshes of 3×3×1 and a single Γ point for the environment and cluster, respectively. The ECW methods used here include CASSCF, NEVPT2, and CAS with second order perturbation theory (CASPT2) 69, 70 within the MOLPRO software package version 2012.1. 71 was added to the zeroth-order, one-electron Hamiltonian using the matrix manipulation feature in MOLPRO. Our open-source, standalone code

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sets up the embedding integral matrices in the primitive Gaussian

basis, in order to include them in the Hamiltonian of the embedded cluster calculations. Dunning’s aug-cc-pVDZ and cc-pVTZ 50 basis sets for H and the Stuttgart cc-pVDZ-PP and ccpVTZ-PP 49 basis sets for Pd were tested. 28 core electrons of each Pd atom were treated implicitly within the ECP28MDF effective core potential. 49 Benchmarking results (see Table S1 in section 1 of the SI) demonstrated that double-ζ basis sets are sufficiently accurate for the present analysis of mechanisms. Initial calculations were performed utilizing an active space of 10 electrons in 10 orbitals (10e,10o), while our final, reported calculations utilized a (12e,12o) active space consisting of two H 1s orbitals and 10 Pd d orbitals (see section 2 of the SI). The optimized orbitals obtained with emb-CASSCF were used as 21 ACS Paragon Plus Environment

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input for subsequent emb-NEVPT2 and emb-CASPT2 calculations. After comparing the ground-state energies between the two methods, subsequent ground- and excited-state calculations employed embedded NEVPT2 theory due to improved accuracy in the H2 dissociative adsorption energy compared to CASPT2 (the latter yielded a 0.61 eV more negative adsorption energy, far off from experiment). Excited-state calculations utilized an active space of (12e,12o), with the emb-CASSCF wavefunctions averaged over 14 states before dynamical correlation effects were included for each state using embNEVPT2. The ground-state emb-NEVPT2 energies exhibit significant basis set superposition error (BSSE), which we corrected via standard counterpoise (CP) corrections.

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As an example, the CP-corrected

ground-state energies using a (10e,10o) active space are given in Table S1 of section 1 of the SI. The final PESs therefore were constructed from the CP-corrected emb-NEVPT2 (12e,12o) ground-state and the 14-state-averaged excitation energies from emb-NEVPT2 (12e,12o). emb-CASSCF densities and transition dipole moments were analyzed to assess physical properties. The oscillator strengths were calculated using emb-NEVPT2 energies and emb-CASSCF transition dipole moments according to:

, = ( − ) | | + ! ! + |" | #,

(4)

where − is the energy difference between states, and , , and " represent the x, y, and z components of the transition dipole moment, respectively. Embedded DFT calculations for the cluster were performed with both GTO and PW basis sets using the PBE XC functional. BSSE corrections were necessary for the GTO DFT calculations, as well; however, differences between the CP-corrected GTO DFT and PW DFT results were very small (Table S3 in section 7 of the SI). Because the DFT term in the ECW energies calculated with PW DFT does not need to be corrected for BSSE, the results presented in the main text simply utilize PW DFT calculations performed on the embedded cluster to evaluate , in Eq. 3.

Bader charges 52, 53 of the embedded cluster were derived from emb-CASSCF/pVDZ densities for the ground and each excited state at select geometries, and for the ground-state densities of the p-hcp-fcc


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geometry using both GTO and PW basis sets in the emb-DFT-PBE formalism. The grid utilized for embCASSCF and GTO emb-DFT-PBE density evaluations was 200×200×400 grid points with a spacing of 0.1 bohr. Bader analysis also was performed on the 3x3 lateral unit cell, seven-layer-thick periodic slab used for the original periodic DFT calculations, in order to compare the charges in the p-hcp-fcc structure calculated from PW-DFT-PBE to the emb-CASSCF values. These periodic DFT calculations employed a 600 eV PW basis kinetic energy cutoff, a 7x7x1 k-point mesh for the 3x3 periodic cell and a single Γ point in the case of PW emb-DFT-PBE, and a smearing width of 0.09 eV. The grid utilized for the VASP calculations was 100×100×420 with a spacing of 0.151 bohr. Additional software utilized for the work included VESTA version 3.2.1 74 to visualize densities and the calculated embedding potential, Jmol 75 and Molden 76, 77 to render and view the orbitals from the multi-reference calculations, and MATLAB 78 for data analysis. Acknowledgements V. A. S. gratefully acknowledges current and former group members, respectively Dr. John Mark Martirez and Dr. Caroline Krauter, for their assistance and guidance with the calculations and for useful discussions. V. A. S. also thanks Dr. Martirez, Dr. Qi Ou, Dr. Johannes M. Dieterich and Ms. Nari L. Baughman for their editing help. E. A. C. acknowledges financial support from the Air Force Office of Scientific Research via the Department of Defense Multidisciplinary University Research Initiative, under Award FA9550-15-1-0022. The High Performance Computing Modernization Program (HPCMP) of the U.S. Department of Defense and Princeton University’s Terascale Infrastructure for Groundbreaking Research in Engineering and Science (TIGRESS) provided the computational resources. Supporting Information Available: Additional information is available which includes a comparison of results with different basis sets, convergence and images of the active spaces, Bader charge analysis with various methodologies, definition of the reaction coordinate, state-averaging convergence tests for excited states calculations, BSSE analysis, and comparison of PW-DFT versus GTO-DFT for the ECW energies. This material is available free of charge via the Internet at http://pubs.acs.org. 23 ACS Paragon Plus Environment

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Mechanistic Insights into Photocatalyzed Hydrogen Desorption from

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