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Scaling Relations for Adsorption Energies on Doped Molybdenum Phosphide Surfaces Meredith Fields, Charlie Tsai, Leanne D Chen, Frank Abild-Pedersen, Jens K. Norskov, and Karen Chan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03403 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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Scaling Relations for Adsorption Energies on Doped Molybdenum Phosphide Surfaces Meredith Fields†, Charlie Tsai†, Leanne D. Chen†, Frank Abild-Pedersen†‡, Jens K. Nørskov†‡, Karen Chan†‡ † SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States ‡ SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States *Email: [email protected]

Scaling for AH x {A = N, C, O

}

−� Eadsorbate (eV)

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N NH

NH 2

+� EP (eV)

Abstract: Molybdenum Phosphide (MoP), a well-documented catalyst for applications ranging from hydrotreating reactions to electrochemical hydrogen evolution, has yet to be mapped from a more fundamental perspective, particularly in the context of transition metal scaling relations. In this work, we use periodic density functional theory to extend linear scaling arguments to doped MoP surfaces and understand the behavior of the phosphorus active site. The derived linear relationships for hydrogenated C, N and O species on a variety of doped surfaces suggest that phosphorus experiences a shift in preferred bond order depending on the degree of hydrogen substitution on the adsorbate molecule. This shift in phosphorus hybridization, dependent on the bond order of the adsorbate to the surface, can result in selective bond weakening or strengthening of chemically similar species. We discuss how this behavior deviates from transition metal, sulfide, carbide and nitride scaling relations, and discuss potential applications in the context of electrochemical reduction reactions.

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Keywords: density functional theory, scaling relationship, MoP, phosphides, electrocatalysis Introduction: Molybdenum phosphide (MoP), a known catalyst for industrial hydrodenitrogenation and hydrodesulfurization reactions,1 has generated renewed experimental interest due to recent reports of high activity for the electrochemical hydrogen evolution reaction.2-5 These findings have naturally led to empirical comparisons against other Mo-based catalysts like MoS2, a material that has also shown similar activity for hydrotreating1,6 and hydrogen evolution reactions.7 For example, many Mo catalysts have shown selectivity for alcohol production; Mo2O3, 8 MoS2 9 and even Mo2C10 have been explored for syngas conversion, suggesting the possibility of MoP and doped MoP compounds for liquid fuel generation from CO2 or synthesis gas. Given the variety of geometric and electronic differences existing between Mo oxides, carbides, sulfides and phosphides, it is important to decouple the presence of Mo from the intrinsic activity, behavior and reaction mechanism associated with each material. A fundamental understanding of the electronic structure and intermediate binding behavior of MoP and related phosphides is key in determining the catalytic activity for potential applications. Linear scaling relations,11 which relate the energies of adsorbates or transition states to simple thermodynamic descriptors, are an effective tool in large screening studies and have been more recently used to understand reactivity trends and rates for complex reaction networks.12,13 Scaling relations are a quantitative extension of concepts developed through Sabatier analysis and Brønsted-Evans-Polanyi relations,14 in which it can be shown that the adsorption strengths of chemically similar reaction intermediates are linearly related.15 Therefore, if the binding strengths of different intermediates cannot be independently varied, restrictions are placed on the overall thermodynamics of a reaction. While these restrictions impose an upper bound on the catalytic activity of transition metal surfaces, characterizing thermodynamic limitations can also assist in determining the potential of a new catalyst for a desired reaction. For example, density functional theory (DFT) studies on MoS2 predicted high activity for the hydrogen


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evolution reaction6 and, more recently, electrochemical reduction of carbon dioxide.7,16 Theoretical identification of these active doped and undoped MoS2 edge sites were subsequently confirmed experimentally.17 Similar analyses have been used to identify and explain the high selectivity of various catalysts, such as NiGa and platinum alloys, for conversion of syngas to higher alcohols18 and oxygen reduction,19 respectively. Although the scaling properties of transition metal compounds such as sulfides and oxides have been studied in the past,20 there currently exists no systematic study of the binding behavior of MoP in the context of adsorbate scaling relations. The possible multi-valence state of phosphorous also presents new circumstances distinct from the systems that have been studied so far. In this work, we present a DFT study of the binding behavior of carbon, nitrogen, and oxygen hydrogenates on MoP and doped MoP surfaces. We show that doped MoP presents distinct shifts in scaling slopes across chemically similar species that is not observed on sulfides21 and carbides. 22 This shift is characterized by a gradual transition from negative to positive values in slope for increasingly hydrogenated intermediates, which can be rationalized by a shift in the total bond order, or hybridization, of the phosphorous binding site. Analyses of the changes in bond length and charge density are used to further elucidate the origins of this behavior. The implications of this scaling behavior are explored in an example case, CO2 electroreduction. In the case of MoP, the sign of the scaling slope between the adsorbate binding energy and stability of the active site is found to vary with the bond order of the adsorbed intermediates. Hence, a mechanism for differentiating between CO* and CHO* by bond order to the surface is provided through the P-active site. This concept introduces a new possibility, in which the binding environment is no longer strictly defined by the electronic properties of the metallic surface, but influenced by the identity of the reaction intermediates. We discuss how this property may be used to circumvent scaling relations in the search for alternative catalysts. Methods Plane-wave DFT calculations employing ultrasoft pseudopotentials23 were performed using the Quantum ESPRESSO suite,24 as implemented in the Atomic Simulation Environment (ASE) interface.25 The BEEF-vdW exchange-correlation functional26 was


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chosen for its inclusion of van der Waals interactions and accurate description of chemisorption energies. Plane-wave and density cutoffs were set to 500 and 5000 eV, respectively, with a Fermi smearing width of 0.1 eV and (4×4×1) k-point sampling.27 Plane-wave cutoff and k-point convergence were established and verified in previous work. 28 These convergence tests are also provided in the Supporting Information. MoP is known to take a hexagonal-WC structure 29 where Mo atoms are arranged in a hexagonal lattice with trigonal prismatic coordination to P layers both above and below. The (001) surface was selected in previous work from a subset of possible exposed surfaces based on stability analysis using the Bravais-Friedel-Donnay-Harker (BFDH) crystal morphology algorithm as a first approximation. 30,31 Qualitative comparisons were successfully made between theoretical and experimental results using this model P-terminated surface,4 and experimental high resolution TEM and XRD studies have supported assumptions of (001) facet stability.2 The resulting unit cell extended two Mo atoms wide in the x- and y-directions and four layers deep. The top two layers of the MoP surface were allowed to relax until the forces on each atom were less than 0.05 eV/Å, while fixing the remaining layers at the bulk lattice constant. Periodic boundary conditions with dipole corrections32 were invoked, with 16 Å of vacuum set between neighboring slabs in the z direction. The calculated lattice constants of a = 3.24 Å, and c = 3.20 Å are in good agreement with the experimentally determined values of a = 3.23 Å and c = 3.20 Å. 7 The adsorption energy is defined as the energy of an adsorbate-slab system referenced to the clean surface and a collection of stable gas phase molecules. A series of doped MoP catalysts, where ¼ of the surface Mo atoms were substituted with either Ag, Au, Cu, Co, Cr, Ni, Pt, Re, Rh, Nb, Ta, W, or Zr atoms, were studied to understand and explore the potential for fine-tuning the activity of MoP catalysts. Previous studies have shown transition-metal doping to be an effective means of tuning chemical activity33 and understanding the fundamental properties of a class of catalysts.34 In the traditional derivation of scaling relations, simplifying assumptions are made which require limiting the degrees of freedom of a system. Formation energies and relative slab distortion were used as stability criteria, limiting the considered data set. In addition, only adsorbates


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bound to the same binding site were considered in this analysis. We emphasize that scaling relations represent a fundamental way to understand the ideal behavior of a surface, before extrapolating to new layers of complexity associated with non-ideal systems, including geometric distortion and alternate binding orientations. Further calculation details are provided in the Supporting Information. Results The trends in catalytic activity of transition metal surfaces for a particular reaction may be generally mapped by a linear relationship between the adsorption energies of reaction intermediates.11,35 The linear scaling relation derived, in this case, for adsorbed hydrogenates on transition metal surfaces takes the general form: 20,36 Equation 1

= +

Where A is the atomic species (C, N, O, etc.) through which the adsorbate binds to the surface, and EA is the electronic adsorption energy of the central atom. Both the slope and intercept are constants independent of the metal, and they represent the relative reactivity of the adsorbate and describe the binding behavior at the weak interaction limit, respectively.12,37 The slope is given by the following relation Equation 2

=

( − )

where is the maximum number of bonds required to achieve a closed-shell octet for the bound atomic species and the relevant degree of hydrogen substitution on the central atom.12,35 This analysis was derived for low index metal surfaces using the d-band model and based in part on effective medium theory and bond order conservation arguments. It was further extended to nitride, oxide, carbide, and sulfide surfaces, as well as transition metal clusters and surfaces with modified sulfur and selenide sites.20-22


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Figure 1: Top and side view of CHO bound to MoP unit cell and CH3 bound to the extended surface. Due to the lattice spacing of the MoP surface, direct binding with metal atoms is prevented. Phosphorous acts as the active site for all considered adsorbates. P-sites are indicated in orange and Mo atoms in blue.

MoP is a metallic material with alternating metal-P layers packed in a trigonal prismatic lattice. Throughout the bulk, each P-atom is coordinated with three Mo atoms both above and below, leaving a phosphorus layer at the surface with a coordination number of three and a free lone pair available for bonding. All adsorbates interact with the P-atoms when they adsorb, so the P-atom can be considered as the active site as shown in Figure 1. Equation (1) can be written for the adsorbate-P complex, P-AHx, to give the scaling relation between adsorbate binding energy and P-stability: Equation 3

= + Equation 4

= ( − 1) + Equation 5

=

− ( − )

Where reflects the bond order the phosphorus site may accommodate and ( − ) reflects the number of bonds between adsorbate and P-site, i.e. = 4 and


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= 1,2,3 for CHx adsorbates. In the case that there is a shift in bond order of the phosphorus atom upon adsorption, Equation 6

=

− ( − )

where is the bond order of the phosphorus bare P-site and of the

phosphorous bound to adsorbate AHx

Figure 2: Linear Scaling for AHx adsorbates vs Stability of P-site with reported slopes. Table 1: Linear Scaling Relations, True and Predicted Slopes. Predicted γ Values Derived from Equation 4 and 5.

Slope

Intercept

MAE

xP-max

(xmax – x)

CH3

0.55

-0.12

0.06

6

1

0.25

CH2

-0.43

0.80

0.04

4

2

−0.5

CH

-0.70

2.85

0.03

4

3

−0.75

NH2

0.24

-1.15

0.06

6

1

0.25

NH

-0.49

-0.43

0.03

4

2

−0.5

N

-0.65

1.32

0.03

4

3

−0.75

OH

0.32

-0.40

0.06

6

1

0.25

O

-0.23

-0.28

0.04

4

2

−0.5


Predicted γ

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Figure 2 shows the scaling relationships for CHx, NHx and OHx moieties. Linear scaling relationships hold for similar adsorbates that bind to the same site. 11,20,22,37 To isolate the electronic structure effects of the P-site alone, only doped surfaces that experience minimal slab distortion and on which the adsorbate binds directly to the P-atom are considered in this initial analysis (see the SI for details). A fixed xP-max would give a negative slope for Equation 5, but the scaling relations in Figure 2 suggest otherwise. A negative slope was found for adsorbates that are expected to form more than a single bond to the surface (e.g. CH, CH2, N, NH and O).

However, for a series of

adsorbate molecules with increasing hydrogen substitution (e.g. CH vs CH2 vs CH3), the slope gradually shifts from negative to positive, which suggests an increase in xP-max upon adsorption. As the stability of the phosphorous site increases, the binding strength of the adsorbate increases as well. This is in contrast to doped Mo sulfides21 and carbides,22 where no shift in xP-max has been observed and adsorption energies always show a negative slope vs. the stability of the binding site. 38 The varying xmax of phosphorous on MoP parallels the flexible coordination number of phosphorous, as often observed in organometallic chemistry.39 Phosphorous traditionally forms a minimum of three bonds to form a stable octet, but accommodates up to six nearest neighbors in the formation of a compound such as PF6–. Similarly, in metal phosphine complexes, a variety of conformations may be found depending on the identity of the central metal atom and steric influence of the ligand binding group. On surfaces in which P acts as the active site, geometric, steric and electron withdrawing effects may change the preferred hybridization the P-site assumes and the resultant xmax.


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P - M Bondlength (Å)

3.6

3.2

Clean N NH NH2

2.8

2.4

2.0

P - Ads Bondlength (Å)

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1.8

Clean N NH NH2

1.6

1.4

Co Ni Pt Rh Nb Ta W Mo Cr Re

Figure 3: Bond lengths between P and metal site and bond lengths between P and adsorbate. Bondlength plots for CHx and OHx adsorbates in Supporting Information.

The shift in orbital hybridization and xP-max upon adsorption is indicated by changes in bond lengths and charge density. Shorter bond lengths suggest higher bond orders. Figure 3 shows the distance between the active P site and the adsorbate d(P-ads) and the distance between the active P-site and the coordinated metal atom d(P-M). The relative M-P bond length decreases for more reactive metals but is also dependent on the adsorbed species. In general d(P-ads) reflects the expected bond order between the adsorbate and phosphorus, e.g. d(P-NH2) > d(P-NH) > d(P-N) and (xmax-x)NH2 < (xmax-x)NH < (xmax-x)N. For d(M-P), single-bonded adsorbates (CH3, NH2 , etc…) show a decrease in bond length relative to a clean surface, consistent with an increase in bond order between P and the surface and an increase in xP-max upon adsorption. For adsorbates that show a stronger bond to P, the d(M-P) increases while d(P-ads) decreases, consistent with bond order conservation and a lower bond order between P and the surface. These changes in bond length suggest that: 1) for adsorbates with increasingly unsatisfied bond order, the


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phosphorous maintains a lower valence hybridization at the surface; 2) for singly bonded adsorbates, the P-site shifts to a hybridization where the total bond order increases and thus stabilizing the overall system, which is the origin of the positive slope.

A.

B.

Figure 4: Top and side view of charge density differences for CHx adsorbates on tungsten doped MoP. P-sites are indicated in orange, Mo atoms in cyan, C in grey and H in white. The W dopant atom has replaced the left-most Mo site directly below the P-atom to which the adsorbate complex is bound. Dark blue regions and red regions indicate an increase and decrease in electron density upon adsorption with isovalues of +0.008 and -0.008 e Bohr-1, respectively. (Charge density differences for NHx and OHx in Supporting Information)

This picture is supported by the evolution in charge density isosurfaces for the P-adsorbate complexes. The charge density difference is defined with respect to an adsorbate reference so that: ∆ =

!"#$%&!

−

&!

−

!"#$

In these calculations, the slab-adsorbate system is relaxed prior to separation, and a single point charge density calculation is completed for each component (adsorbate with slab, adsorbate) and referenced to charge density of the original clean surface) for comparison. Here, ∆ρ represents the charge transfer between the adsorbate and slab, where the charge density of the gas phase adsorbate,

!"#$ the

&!

is

density of the pristine MoP


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surface, and

!"#$%&!

the density of the adsorbed system. For a P-atom with a higher

valence state, there should be an increase in charge density between the P-atom and metal. Likewise, a P-atom in a lower valence state is expected to show a decrease in charge density around the adjacent metal atoms as a larger fraction of electron density is donated to the adsorbate interaction. In Figure 4, blue isosurfaces represent areas of enriched electron density and red isosurfaces regions of depleted electron density. As expected, for CH3 , a clear increase in density is seen between the P-atom and underlying metal atoms. For CH2, the blue region which sits beneath the P-atom begins to shrink and split, and appears absent in the case of the CH adsorbate. The analysis of bond lengths and charge densities suggest that singly bonded adsorbates induce a shift in hybridization such that xPmax increases relative to the bare surface. We hypothesize that the valence state of a clean surface P atom switches between a tetrahedral like sp3 orientation, and a 6-coordinated sd5 trigonal prismatic orientation reflective of the bulk when interacting with a single bonded adsorbate. If the therefore may vary between 4 and 6, we recover slopes shown in Table 1 for the adsorbates considered, in reasonable agreement with DFT calculated values. Previously, deviations from scaling relations have often been introduced by manipulating the intrinsic properties of the material, such as chemical composition or structure, or by tuning environmental variables through addition of defects or co-adsorbed species. Strategies have included introducing multiple binding sites through alloying or attaching ligand binding groups to alter binding orientation or intermediate adsorption strength to the surface.35,40 This concept of a shift in hybridization of the active site also has potential applications for studying catalysts using thermodynamic scaling relations. While linear scaling prevents decoupled stabilization of chemically similar species, the flexibility of the P-site in MoP and similarly structured phosphides provides a new method for circumventing linear scaling and achieving selective stabilization. In other words, the same active site can provide stronger binding for one adsorbate and weaker binding for another.


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0.0 EAds (eV)

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CH3 CHO HCOO COOH H CO

-0.6

-1.2 -1.4

-1.0 EP (eV)

-0.6

Figure 5: (Left) Single bonded adsorbates plotted versus the stability of the P-active site. All single-bonded adsorbates show positive scaling with the P-active site except for CO (dashed black line). (Right) This trend contributes to deviations from the transition metal CHO vs CO scaling line in CO2 electroreduction screenings.

For example, previous studies on the thermodynamic requirements for electrochemical CO2 reduction on transition-metal catalysts identified the protonation of CO* as the most important step in determining the overall overpotential to reduce CO2 to products beyond CO(g). 35 It should be noted that thermodynamic scaling represents a necessary but insufficient requirement for narrowing the selection of screened catalysts. Before determining the true potential of a catalyst for CO2 reduction, kinetic criterion must also be considered. However, as a preliminary search metric, if the strength of CHO* adsorption can be increased relative to that of CO* it has been assumed that the thermodynamic overpotential will be further reduced. Previous works have relied on lowering overpotential through the presence of additives such as pyridine,41 or through the design of a dual-site mechanism in the case of transition metal sulfides and selenides.34 There, different adsorbates bind onto neighboring sites with distinct scaling properties. Metal phosphide surfaces also break thermodynamic CO* vs CHO* scaling, but through a single-site mechanism in which the same phosphorous site selectively binds CO* more weakly. The slope of the ECHO vs. ECO scaling line is approximately 0.7, deviating from the 0.9 slope of the (211) transition metal surfaces in Figure 5. A variety of carbon based adsorbates predicted to form a single bond to the surface maintain the positive scaling


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against EPstability seen in the case of hydrides. However, CO* deviates with a slope of 0.02, suggesting a bond order to the surface > 1. This effect of bond order may be attributed in part to the selective weakening of the CO bond to the surface with respect to CHO, allowing thermodynamic scaling to be broken. While further work determining relevant equilibrium coverages and kinetic barriers are required for the MoP system, based on thermodynamic criteria alone, MoP provides one conceptual example of how decoupling the binding strengths of chemically similar species through adsorbate bond order can potentially lend useful functionality for a variety of electrochemical or thermochemical reactions. Conclusions In this work, we investigated the electronic structure of MoP and its implications in the field of heterogeneous catalysis. The physical origins of the adsorption properties of MoP fit within DFT derived linear scaling regimes, but introduce a new factor of dependency on the identity of the adsorbate molecule. As all reaction intermediates bind through a phosphorous site, the adsorption properties of the surface are strongly correlated to the hybridization of the P-site. This dependency on hybridization may be tuned through the addition of dopants, which assist in increasing or decreasing the binding energy of various adsorbates to the surface. However, the slope of the derived scaling relations are most strongly influenced by the bond order of any adsorbed intermediate. Reaction intermediates that are stable with a single bond to the surface have a positive correlation with the stability of the P-site. The phosphorous atom adopts a hybridization which differs from that of the clean surface and appears to mimic the valence state of the subsurface phosphorous, stabilizing both the active site and the bound intermediate. This concept is supported through both bond length and charge density analysis for a variety of hydrogen substituted compounds. Simple bond counting arguments, previously used in sulfide and transition metal surfaces, support these trends and give slope predictions of adsorbate vs. P-stability curves in reasonable agreement with calculated values.


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We also demonstrate the possibility of extending this understanding to applied screening studies. From DFT-calculated adsorption energies, trends may be generated that match candidate catalysts most likely to succeed with a reaction of interest. MoP shows the ability to selectively distinguish between chemically similar compounds on the basis of bond order to the surface. The stability of certain adsorbates can be strengthened while others are weakened. This property could be harnessed in the context of reducing the overpotential for electrochemical CO2 reduction, where the CO* to CHO* step needs to be stabilized without leading to surface poisoning by other intermediates. Work to determine new relevant applications for MoP and similarly structured compounds is ongoing. Supporting Information Figure 2 Scaling Data, Figure 3 Bond Length Data, Figure 5 CO2 Intermediate Scaling Data, Gas Phase References, Stability Analysis Critera, Bondlength Plots for CHx and OHx adsorbates, Charge Density Plots for NHx and OHx adsorbates, Stability of P and Metal terminated MoP, Convergence Tests for MoP Surface. Acknowledgements The authors gratefully acknowledge support from the Office of Basic Energy Sciences of the U.S. Department of Energy to the SUNCAT Center for Interface Science and Catalysis.. M.G. Fields acknowledges a graduate fellowship through the National Science Foundation. References: (1)

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