Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Article
Strongly Modified Scaling of CO Hydrogenation in Metal Supported TiO Nanostripes Robert Sandberg, Martin H. Hansen, Jens K. Norskov, Frank Abild-Pedersen, and Michal Bajdich ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03327 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Strongly Modified Scaling of CO Hydrogenation in Metal Supported TiO Nanostripes Robert B. Sandberg1, Martin H. Hansen2, Jens K. Nørskov1,2,3, Frank Abild-Pedersen2, and Michal Bajdich2* 1SUNCAT
Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA 2SUNCAT
Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA 3Department
of Physics, Technical University of Denmark, DK-2800 Kgs, Lyngby, Denmark
*
[email protected] Abstract The boundary between a metal-oxide and its metal support (metal-oxide|support) provides an intriguing structural interface for heterogeneous catalysis. The hydrogenation of CO is a reaction step believed to be rate limiting in electro-chemical CO2 reduction. Density functional theory (DFT) calculations were performed to study this reaction step for a class of catalytic material: metal supported TiO nanostripe. The most stable adsorption sites were identified for all metal supports which showed a striking difference in adsorbate geometry between the strong and weak binding metal supports. The modified CO hydrogenation scaling shows a significant strengthening over (111) and (211) transition metal surfaces. Such enhancement can be attributed to a combination of geometrical effects and metal-oxide|support electronic interactions. A correlation analysis was performed to identify the key features needed to accurately predict *CO and *CHO adsorption energies on the TiO nanostripes and to further validate our physical analysis of the results. This structural motif seems to be a promising avenue to explore scaling modification in other metal-oxide materials and reactions. Keywords: nanostripe, ultra-thin overlayers, CO2 reduction, CO hydrogenation, scaling relations, metal-oxides, computational catalysis, DFT INTRODUCTION Simple transition metals (TM) are widely used in catalysis with several decades of experimental and theoretical studies.1,2 Metal-oxide materials have historically been viewed only as inert supports for the active metal catalyst. More recently, the role of the metal-oxide as an active participant in the reactions has been realized for a number of thermochemical3–5, electrochemical6,7, and thermoelectrochemical processes.8,9 The interactions at the socalled triple-phase boundary of gas molecules at the metaloxide|metal interface are less studied due to a much larger combinatorial space and a model for the catalysis at the interface which is still in its infancy.10,11 In this paper, we offer a systematic analysis of the effects associated with the triple-phase boundary using CO hydrogenation as a prototype reaction. The metal-oxide|metal interface is represented by an ultra-thin TiO metal-oxide stripe supported on close-packed (111) transition metal surfaces. The structure of ultra-thin supported metal-oxides have been investigated extensively both experimentally and theoretically.12–14
The hydrogenation of CO is an important first step in many catalytic reactions – it is considered a likely rate-limiting step in electrochemical CO2 reduction7,15, in the production of methanol and higher alcohols from syngas16 and in the generation of higher hydrocarbons in the Fischer-Tropsch synthesis reaction.17–19 CO hydrogenation is an important step on most transition metal catalysts when converting either CO2 or CO to C1 products such as CH4.15 In electrochemical CO2 reduction, most transition metal catalysts have an uphill *CO to *CHO step, making this step thermodynamically unfavorable. It has been shown that modifications to this scaling relation have a direct consequence on the predicted rate of CH4.20 CO oxidation has been studied extensively, including studies of FeO films/nanoislands on metal supports, similar to the system investigated here. A thin FeO film on Pt(111) exhibited an enhanced CO oxidation activity compared to clean Pt(111) and nm-thick Fe3O4 films through the formation of an oxygen-rich FeOx. Higher CO pressures (up to 60mbar) greatly increased CO2 production.21 The first direct experimental evidence for an interfacial CO oxidation mechanism was identified. Surface hydroxyls were prepared
ACS Paragon Plus Environment
ACS Catalysis on FeO(111) and were found to react readily with CO* on Pt(111) to form CO2 at low temperatures.22 At low pressures up to 10-3 mbar, the FeO(111) film was inert towards CO oxidation and restructured with increasing O2 pressure. DFT calculations showed CO adsorption had a lower barrier on the FeO(111)/Pt(111) system compared to Pt(111). The proposed mechanism would occur via formation of an oxygen vacancy, and this formation energy would likely be a key factor in CO oxidation activity in this system.23 CO oxidation activity is enhanced with the introduction of an FeO thin film/nanoisland on a metal support. This enhancement was due to a combination of stronger binding of *CO and binding of different adsorbates on the FeO film and metal support to form CO2. Scaling relationships, based on bond order conservation principles, have become one of the preferred tools in computational catalysis enabling a reduction of a high dimensional reaction space, thus allowing for fast screening of materials based on a few simple descriptors. These simple correlations enforce limitations on the maximum rate of the reaction and possible strategies to circumvent these have been proposed.24–28 Three-dimensional catalysts, catalysts with promoters, and catalysts with different classes of materials in close proximity, such as metal|metal-oxide interfaces, provide a strategy for modifying scaling relations.29 Our model TiO nanostripe system for CO hydrogenation entails both the modification of scaling properties relative to TM (111) and (211) surfaces20,25 and the fundamental challenge of identifying the descriptor space of such systems. Recently, a number of metal supported nanostripes (nanoribbons) have been investigated, such as supported MgO30 and FeO/Pt(111).31 In these systems, both structural and electronic effects play a significant role. Polarity, rumpling, reconstruction, and hydroxylation are some structural effects while metal-oxide/support charge transfer, work function changes, and edge metallization are some electronic effects. The strong metal-oxide|support interactions (SMSI) in these systems can affect the overall stability of the catalyst, as seen in the systematic investigation of rock-salt (111) ultra-thin overlayers by the Nørskov group.32 Consequently, the nanoribbons of rock-salt (111) ultra-thin overlayers have two distinct types, the nonpolar A(111) and polar Z(111) stripes as extensively analyzed in the works of Goniakowski, Noguera et al.30,33,34 The polar Z(111) is typically the most stable of the two terminations, both freestanding and when hydroxylated. The differences between the two terminations are discussed later in this manuscript. Our initial result for rock-salt (111) ultra-thin overlayers identified TiO as a stable non-magnetic oxide with a high degree of metal-oxide|support interaction.32 Other recent studies have identified a variety of stable TiOx overlayers, including rock-salt TiO on Pd and Rh supports affecting methane activation35 and CO hydrogenation chemistries36, respectively. TiO is a prototype 3d-transition metal-oxide with high reactivity; hence, the effects on CO hydrogenation scaling are likely to be large.
In this work, we present a detailed DFT study of the modified scaling between *CO and the first intermediate in the CO hydrogenation, *CHO, on metal supported rock-salt TiO nanostripes. First, we constructed TiO overlayers on 7 different face centered cubic (fcc) metal supports: Ag, Au and Cu, Ir, Pd, Pt, Rh. The model overlayers were cut to form the nanostripes tested. All adsorption sites were tested on all metal supports. In the presence of water, edge hydroxylation was investigated. A modified *CHO vs *CO scaling showed the effect of the nanostripes. Geometrical effects and electronic effects were investigated to explain the origin of the significant changes in scaling. Lastly, a correlation analysis was performed to identify key features that could be used to accurately predict *CO and *CHO adsorption energies on the nanostripe. COMPUTATIONAL METHODOLOGY Density Functional Theory (DFT) calculations were performed to determine the structure of the metal supported TiO nanostripes and the subsequent adsorption energies of species such as *CO and *CHO. Geometry optimizations were converged until force components became less than 0.05 eV/Å. The BEEF-vdW exchange-correlation functional37 was used as the standard functional to describe surface reactivity. DFT+U corrections were not explored in this work as our previous work on ultrathin supported oxide films showed an agreement between observed trends with PBE and PBE+U with respect to energies, magnetic moments, and charge transfer in non-magnetic oxides.32 Ultrasoft pseudopotentials were used with the Quantum Espresso code38 within the Atomic Simulation Environment (ASE).39 A 500eV plane-wave cutoff and 5000eV density cutoff were used. A k-point grid with 3 k-points was used for all calculations, either (3,1,1) or (1,3,1), depending on the orientation of the stripe. To calculate free energies, a normalmode analysis was performed and the harmonic oscillator approximation was used. Zero-point energies, entropic contributions, and heat capacities were calculated based on the vibrations.40 The free energy corrections to the adsorption energies of *CO, *OH, and *H were 0.50, 0.30, and 0.32 eV, respectively. For further details please refer to the Supporting Information (SI). The computational data are also available to view online.43 RESULTS TiO overlayers on fcc(111) supports 3.5
√3x√3
2x2
Ag Au Cu Ir Pd Pt Rh
3.0 2.5
E (eV)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 11
2.0 1.5 1.0 0.5 0.0 2.4
2.6
2.8
3.0
M M Distance ( )
ACS Paragon Plus Environment
3.2
3.4
3.6
Page 3 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Figure 1. Energy of a free-standing TiO overlayer as a function of hexagonal lattice constant. Dashed lines show the corresponding metal-metal distances in (111) surfaces of fcc support metals used in this study. The two minima at 3.10Å and 3.33Å correspond to the original rocksalt and stretched TiO geometries, shown as insets.
A free-standing infinite TiO overlayer was constructed in a 1×1 unit cell with a single Ti and O atom. The effect of strain was tested by expanding or contracting the unit cell to determine the proper number density of Ti and O atoms, a different ratio for each Moiré pattern.10 Figure 1 shows the energy of this free-standing overlayer versus the metal-metal distance of the TiO lattice. The nonmagnetic solution was found to be the most stable. The first minimum at 3.10Å corresponds to an oxide structure with the oxygens above the plane of the Ti atoms, while the second minimum at 3.33Å corresponds to a structure with the oxygens in-plane. The second minimum is disregarded as the geometry is no longer a rock salt structure. The minimum energy structure at 3.10Å was cut to construct overlayers corresponding to a commensurate (2×2)/(2×2) structure. Another structure was used to construct overlayers corresponding to a (√3×√3)/(2×2) structure, where the coverage of overlayer atoms is ~0.75ML.32 The lattice mismatch between the TiO lattice and the underlying support resulted in two types of polar nanostripes. Both structures were tested for all 7 metal supports. Table 1 shows the higher lattice constant metals (Ag, Au) preferentially form the (2×2) overlayer while the other metals (Cu, Ir, Pd, Pt, Rh) prefer the (√3×√3) structure. All lattice mismatches are ~5% or less, attainable strains in experiment.
were constructed in an orthogonal unit cell with at least 10Å between periodic images to eliminate self-interaction. In addition, a stripe width of 3 TiO units was chosen to test all possible sites on the Ti-edge, O-edge, and the TiO unit in between. Employing these constraints yielded a minimum metal support of 2×6×3 for the (2×2) structure and 6×4×3 for the (√3×√3). Structures of Z(111) stripes with Au and Pt supports are shown in Figures 2 and 3, respectively. Classification of all possible adsorption sites Table 2. Adsorption energies for the most stable binding sites of *CO and *CHO on TiO/fcc(111) for all metals at the Ti-edge, including the support site for comparison. Metal
Au Ag Pt Cu Ir Pd Rh
a
E(CO) [eV] E(CHO) [eV] Ti-edge Support Ti-edge Support bridge top1 top2 bridge top1 top2 -1.21 -1.00 -0.92 0.24 -1.77 N/A N/A 0.12 -1.48 -1.21 -0.99 -0.08 -1.94 N/A N/A 0.41 -1.61 -1.66 -1.42 -1.39 -1.80 -2.20 -1.90 -1.14 -0.56 -1.12 -1.60 N/A -1.94 N/A N/A 0.16 -2.21 -1.90 -2.40 N/A -2.20 -2.46 N/A -1.38 -1.73 -1.58 -1.71 -1.46 -1.96 -1.91 N/A -0.87 -1.63 N/A -2.08 N/A N/A N/A N/A -0.88
O-edge
Basal plane
relative to bulk TiO. Lattice mismatches for the (2×2)/(2×2) structure and the (√3×√3)/(2×2) structure are also shown for each support metal. The more stable structure for each support is bolded.
Ag Au Cu Ir Pd Pt Rh
ΔEads [eV/TiO] 0.22 -0.26 -0.11 -1.37 -1.04 -1.51 -1.25
Lattice Mismatch Lattice Mismatch (22) [%] (√3√3) [%] -3.8 -3.7 -19.0 -12.6 -9.9 -9.2 -13.5
-10.1 -10.2 3.1 -2.4 -4.8 -5.4 -1.7
*CHO
Support
Table 1. Adsorption energy of the stripe on each support metal,
Metal
b
*CO
bridge Ti-edge
top2 top1
bridge
Figure 2. Most stable binding configurations of (a) *CO and (b) *CHO on TiO/Au(111). The other weak binding metal (Ag) has similar geometries. The unit cell of the 2×3 TiO nanostripe on top of 2×6 Au(111) is shown with a dashed black line. Site 1 is the strongest binding and support, basal plane, and O-edge sites are shown for comparison.
In Figures 2 and 3, the classification of all possible adsorption sites for relevant reaction intermediates in CO2 Structure of Metal-supported TiO Nanostripes reduction (*CO and *CHO) are shown. Depending on the nature and adsorbate orientation on the adsorption sites, The overlayer structures were cut to create the metalenergies can vary dramatically. Ag and Au, in the (2×2) unit supported TiO nanostripes, in which two types of nanostripes cell structure, show the same trends in binding configuration can be formed, depending on the cut. One cut results in a on the stripe so we limit our discussion to the results on Au. polar stripe, with a Ti-metal terminated-edge (Ti-edge) and Figure 2 shows the structure of the Au supported TiO an Oxygen terminated-edge (O-edge), denoted as Z(111) nanostripe. All sites were tested on the Ti-edge, on the O(stripe structure shown in figures 2 and 3). The second cut edge, on the stripe basal plane, and on the metal support. results in a nonpolar stripe (structure not shown) that Since only one stable configuration was chosen for the basal contains Ti and O atoms on both edges and is denoted plane, O-edge, and support, these labels refer to both the 30 A(111). Figure S1 in the supporting information shows that region of the stripe and the binding configuration. The Tithe Z(111) stripe has a more favorable formation energy than edge was always found to be the most active region. The Othe A(111) for most supports so in the following we chose edge and the basal plane of the stripe bind *CO more strongly the Z(111) nanostripe for all 7 supports. To reduce the than the Au support but overall the Ti-edge is significantly computational cost we constrained the unit cells to the more active. The most stable binding configurations for *CO smallest possible stripes with the least strain. The stripes ACS Paragon Plus Environment
ACS Catalysis and *CHO are through two adjacent Ti atoms on the edge without any coordination to the support metal. In Table 2 the adsorption energies for each binding site for *CO and *CHO are shown on the Ti-edge along with the support site for comparison. The complete table (Table S2) can be found in the SI. There is a significant enhancement in bond strength for both *CO (1.45eV) and *CHO (1.89eV) along the Tiedge compared to the support site, as seen clearly in the scaling of *CHO vs *CO in Figure 6.
carbon end through one support metal atom. Table 2 shows the adsorption energies for the most stable binding sites for *CO and *CHO on the Ti-edge of the stripe with the support site for comparison. The complete table (Table S2) can be found in the SI. Similarly to the results on Au, there is a significant improvement for *CHO (1.06eV) along the Tiedge vs the support site; however, there is only a minimal improvement for *CO (0.05eV). The *CHO vs. *CO scaling lines in Figure 6 reflect these results.
a
Structure of Nanostripes
b
*CO
*CHO
Support
O-edge
top1
bridge top2
Ti-edge
top1
top2
bridge
Figure 3. Most stable binding configurations of (a) *CO and (b)
Clean
*CHO on TiO/Pt(111). The other strong binding metals (Cu, Ir, Pd, Rh) have similar geometries. The unit cell of the 3×3 TiO nanostripe on top of 4×6 Pt(111) is shown with a dashed black line. Site 1 is the strongest binding and support, basal plane, and O-edge sites are shown for comparison.
Clean
θ= 2/3
θ= 2/3
-0.4 Ag Au Cu Ir Pd Pt Rh
-0.6 -0.8 -1.0 -1.2 -1.4
-1.2
with arrows to indicate the sites where *H and *OH bind upon hydroxylation. The hydroxylated structure on the right is with θH = θOH = 1. *H
*CO
*H,*OH
*OH
-0.5
θ=1/3
-0.2
Hydroxylated
Figure 4. The clean TiO/Au(111) stripe is shown on the left, along
Formation Free Energy (eV)
0.0
TiO
*OH
b
Hydroxylated
Metal-supported
*H
Cu, Ir, Pd, Pt, and Rh, all with the (√3×√3) structure, show the same trends in binding configuration on the stripe so we discuss the results on Pt. Figure 3 shows the structure of the Pt supported TiO nanostripe. In all images, Pt is colored dark gray to differentiate from Ti. All sites were tested on the Tiedge, O-edge, the stripe basal plane, and the metal support. Since only one stable configuration was chosen for the basal plane, O-edge, and support, these labels refer to both the region of the stripe and the binding configuration. No stable configuration for *CO was found on the O-edge of TiO/Pt(111), as the adsorbate migrated to the support site. The Ti-edge was always found to be the most active region. The most stable binding configurations for *CO and *CHO bind the oxygen end through one or two Ti atoms and the
a
Hydroxylated
Figure 4 shows the clean TiO/Au(111) nanostripe on the left. When the system is introduced to water, the structure becomes hydroxylated, binding *H to the O-edge and *OH to the Ti-edge of the stripe, similar to the case of MgO.30 In addition to a more favorable formation energy for the Z(111) stripe, Figure S1 also shows a more favorable formation energy for the Z(111) hydroxylated stripe. Therefore, all of the clean and hydroxylated calculations are of the Z-type.
Basal plane
Formation Energy (eV)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 11
-1.0
-0.8 H2O
-0.6
-0.4
-0.2
(eV)
0.0
-1.0
-1.5
-2.0 -2.0
-1.5
-1.0
-0.5
0.0
0.5
URHE (V)
Figure 5. The formation energy of clean and hydroxylated stripes as a function of (a) the chemical potential of water and (b) the RHE potential. The legend in panel (a) corresponds to both panels.
To accurately calculate adsorption energies, it is important to of CO hydrogenation, we need to consider the effect of water know surface coverages of key intermediates. In the context under both a) thermochemical and b) electrochemical ACS Paragon Plus Environment
Page 5 of 11
conditions. We therefore calculated binding energies of *H and *OH for various single-adsorbate and mixed coverages. Figure 5a) shows the formation energy of the stripes as a function of water chemical potential. The left end of the figure represents water-poor conditions, where the clean stripe is present. The right end represents water-rich conditions, where the fully hydroxylated stripe is present. For some of the stripes, varying degrees of hydroxylation were observed to be the most stable, denoted by the coverages. The fully hydroxylated stripe is represented by the steepest line for each support metal, shown furthest to the right. The stripe is expected to be at least partially hydroxylated for all support metals for any water chemical potential greater than -1.1 eV. As a result, CO hydrogenation scaling calculations were done for both the clean and hydroxylated stripes. Figure 5b) shows the formation free energy of the stripes, relative to the clean stripe, as a function of the Reversible Hydrogen Electrode (RHE) potential. Free energies were calculated by adding free energy corrections to the adsorbates as listed in the Computational Methodology section. Therefore, the clean stripe for all support metals is at a binding free energy of 0 eV (not shown) and is not expected to be the dominant surface coverage at any potential. For each support metal, the left leg represents a full coverage of *H on the O-edge, the right leg represents a full
0.5 0.0
coverage of *OH on the Ti-edge, and in between constitutes a full coverage of both *H and *OH. *H and *OH have an equal and opposite potential dependence, so the full coverage of both *H and *OH has no potential dependence, indicated by the horizontal line. Dashed lines represent the binding free energy of CO on a fully *OH covered Ti-edge, with one site on the Ti-edge available for *CO. CO2/CO reduction operating potentials lie in the regime of -1 V to 0.5 V (RHE). In this regime, *CO outcompetes *OH on the Ti-edge. Although we expect a low *OH coverage on the Ti-edge and *H coverage on the O-edge, *CO will likely cover most of the stripe, leading to *CO poisoning.41 Since there are only 2 or 3 Ti-edge and O-edge sites on these small stripes, a limited number of *CO, *OH, and *H coverages were explored. However, we expect the results to be applicable for a large range of coverages due to the significant difference in binding energies between *CO and all other surface species considered. Scaling for CO Hydrogenation in TiO nanostripes Transition metal surfaces in this study have an uphill *CO to *CHO step, making this step thermodynamically unfavorable. Stabilization of *CHO relative to *CO would result in a much more favorable thermodynamic pathway, thus likely to lead to higher rates. Here we attempt to modify the existing fcc(111) TM scaling with the introduction of the TiO nanostripe.
fcc(111) fcc(211) Nanostripe-edge Hydroxylated-NS-edge
Ag Cu Au
Ir Pd Rh
-1.0
Pt
-1.5
Ir
-2.0
Rh
Rh
-2.5
Pt Rh
Au
Cu Ag
Pt Pd Pd Cu Ag Pt
Au
ECHO = 0.78(±0.13)ECO + 0.35 ECHO = 0.94(±0.12)ECO + 0.41 ECHO = 0.64(±0.20)ECO 0.97 ECHO = 0.83(±0.10)ECO 0.39
Ir
-3.0 -2.5
Au
Pd
Ir
-3.0
Ag
Cu
-0.5
ΔECHO (eV)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ΔECO (eV) Figure 6. Calculated *CHO versus *CO scaling for clean (red line) and hydroxylated (green line) TiO/fcc(111) nanostripes as compared to known scaling for (111) (blue line) and (211) (black line) transition metals. The corresponding structures and energies are also available to view online.43
Figure 6 shows the calculated scaling of CO hydrogenation compared to scaling on (111) and (211) TMs. All values can for metal-supported TiO stripes and hydroxylated stripes be found in Table S3 in the SI. There will likely be a mixed ACS Paragon Plus Environment
ACS Catalysis coverage of *OH and *CO on the Ti-edge, which effects the calculated *CO binding energy. The clean stripes represent a best-case scenario while the hydroxylated stripes represent a worst-case scenario for the given cell size, with an *OH coverage of 0.67 ML along the edge. The true *OH coverage likely lies between these coverages and we therefore expect the scaling line under reaction conditions to be in between the red and green scaling lines. Overall, there is a large improvement as the scaling line moves down from the TMs to the stripes. Although the scaling for the hydroxylated stripes yields a less stable *CHO compared to the clean stripes, this is still a large improvement compared to the (111) and (211) TMs.
Top
*CO
Top
Side
*CHO
Side
Au
Pt Figure 7. Top and side views of *CO and *CHO on a TiO nanostripe with Au and Pt supports.
Our data can be separated into two groups: weak binding (Ag, Au, Cu) and strong binding (Ir, Pd, Pt, Rh) support metals. The weak binding metals move significantly both down and to the left as *CO and *CHO are heavily stabilized relative to the support metal sites. This is due to the binding of *CO and *CHO through two adjacent Ti atoms and absence of coordination to the support metal. The strong binding metals move down significantly, but only slightly to the left as *CHO is heavily stabilized relative to the support metal sites while *CO is not. This is due to the binding of *CO and *CHO through one or two Ti atoms and one underlying support metal atom. *CO already binds strongly on the support so there is not much improvement; however, *CHO binds weakly on the metal support and shows significant improvement on the Ti-edge of the stripe. The data points shift down and to the left of this figure moving from fcc (111) to the nanostripe-edge (NSedge). However, the scaling lines, which are more reliable than a single data point, show similar slopes of 0.78 and 0.64, respectively. Additionally, uncertainties in the slopes for the scaling lines are 0.13 and 0.20, respectively, implying that within error, the two scaling lines have the same slope with an approximately constant offset of ~1.3eV. This is possibly an inherent additional stabilization of *CHO on the NS-edge and a constant offset that depends on the choice of metaloxide.
The triple-phase boundary explored here is traditionally in the form of a metal nanoparticle supported on a metal oxide. At this conventional interface, electronic and geometric perturbations to the energetics are known to occur.43–45 In our inverse system, we expect to observe similar effects as the interface remains independent of the orientation of the system. In addition, scaling relations have been shown to be modified at the traditional metal-on-oxide interface.46 Geometrical Effects. Adsorbate geometry is known to have a significant impact on calculated adsorption energies. Figure 7 shows the top and side views of the most stable *CO and *CHO binding configurations on one weak binding (Au) and one strong binding (Pt) support metal. There is a significant geometrical difference between stripes with metal supports that bind *CO very weakly (Ag, Au) and those that bind stronger (Cu, Ir, Pd, Pt, Rh). Ag and Au bind *CO and *CHO through only the stripe and not the metal support; however, there is still likely some weaker long-range interaction of the adsorbate with the support. For the stronger binding metals, adsorbates are coordinated to both the TiO stripe and the metal support. This introduces a geometrical variable that affects binding energies and the observed *CO to *CHO scaling. a
0.6
The fcc (111) and (211) metals are not efficient in producing carbon-containing fuels and chemicals from CO2 or CO due in large part to a *CO to *CHO step that is uphill. All of the TiO metal supports seem promising, as *CO to *CHO becomes a downhill process with the exception of Rh, which is thermoneutral. Since the thermodynamics are so favorable, it is a fair assumption that the barrier for this process will be lowered relative to TM catalysts, given BEP scaling.42 Assuming a lower barrier, we would expect a high rate for this elementary step on all supports. Ir and Rh bind both *CO and *CHO too strongly, so we expect these surfaces may become *CO and *CHO poisoned, likely causing a later step to become rate limiting. However, without calculated barriers and microkinetic modeling it is difficult to predict the best support material for this process which is subject of our future work. DISCUSSION
Q/QML
0.4 0.2 0.0 0.2
Ag Au Cu Ir Pd Pt Rh
M
O
M
O
M
O
M
O
M
O
M
O
0.4
b
0.6
O
0.4
Q/QML
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 11
0.2 0.0 0.2 0.4
Figure 8. Bader charges, relative to the infinite metal-oxide monolayer, on the metal (M) and oxygen (O) atoms, averaged across each row of the stripe for the (a) clean and (b) hydroxylated structures.
Charge as Indicator of metal-oxide|support interaction. Noguera previously examined the charges at Z(111)-type nanostripes and found that the introduction of a metal support significantly reduced the charges at the polar edges, relative to the ribbon center.30 Therefore, we explore the ACS Paragon Plus Environment
The charge at the Ti-edge can be explained in part due to the work function of the support metal. Comparing the coinage metals, Ag and Cu have similar work functions (~4.5) while Au has a larger work function (>5). This results in a greater degree of charge transfer with the TiO stripe, an indicator of strong metal-oxide|support interactions. Since Au’s Ti-edge has more unsaturated bonds, we would expect Au to bind adsorbates stronger than Ag or Cu; however, upon inspection of scaling, we observe that Au binds *CO and *CHO weaker than Ag and Cu. Geometrical effects and metaloxide|support interactions therefore both influence adsorption energies and the combination of both effects gives rise to the improved scaling of the metal-supported TiO stripes compared to fcc (111) and (211) metals.
a
1.0 CO CHO
0.8
0.6
2
Bader charges of the Ti and O atoms across the nanostripe for the clean and hydroxylated stripes defined relative to monolayer of TiO (Q/QML) to investigate electronic effects in the system. Figure 8 shows a positive Q/QML charge at the Ti-edge and a negative Q/QML charge at the O-edge, pointing to the existence of unsaturated bonds throughout the polar stripe, similar to the findings of Noguera.30 Since all of the observed geometries of the adsorbates coordinate to the TiO stripe, these unsaturated bonds contribute to such strong binding of *CO, *CHO, and *OH on the Ti-edge.
0.4
0.2
0
b
EC
EOs
ECOs
ECs ECHOs EOHs
EOH WFs
qTi
EO
Descriptor -1.0 ECO = 0.58* EC + 0.13* ECOs + intercept
-1.2
Au
2
R = 0.93, MAE = 0.009 ECHO = 0.49* EC - 0.14* ECOs - 0.38*WFs + intercept
-1.4
2
R = 0.79, MAE = 0.014
Ag Cu
-1.6
Actual (eV)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
R
Page 7 of 11
Pd Au
Pt
-1.8
Cu Pd -2.0
Rh
Ag
Rh
Pt
-2.2
Ir
-2.4 -2.6
CO CHO
Ir -2.6
-2.4
-2.2
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
Prediction (eV)
Figure 9. (a) R2 values showing the linear correlation between each descriptor with energies of *CO and *CHO adsorbed on the Ti-edge. (b) Parity plot for predicted and actual *CO and *CHO adsorption energies.
Correlation Analysis. A correlation analysis was done to identify which descriptors (mostly binding energies) influence the CO hydrogenation scaling we observed. Ten descriptors were investigated to find the best linear fit of the adsorption energies of *CO and *CHO, ΔECO and ΔECHO respectively, on the Ti-edge. Fig. 9a) shows the squared Pearson correlation coefficients for each descriptor with each of the adsorption energies. This shows that the adsorption energy of carbon on the Ti-edge, ΔEC, scales most strongly with both adsorption energies, while the *O adsorption energy on the metal support, ΔEOs, also scales well with both targets. A correlation matrix was constructed for the descriptors (see SI Fig. S2), which identified a strong linear correlation between ΔEOs and ΔECOs and between the support metal work functions, WFs, and the charge transfer, qTi. To construct predictive linear regression models for the binding energies of *CO and *CHO, we chose a subset of the ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 11
descriptors separately for each target and avoided descriptors that were highly correlated with one another. Fig. 9b) shows the parity plot for prediction of the *CO and *CHO binding energies, detailing the equation of the linear fit, the square of the Pearson correlation coefficient, and the Mean Absolute Error (MAE). Overall, there is quite good agreement between the predicted values from the regression and the actual DFT values, using only 2 or 3 descriptors that are computationally cheaper to calculate than both ΔECO and ΔECHO on the stripe. The descriptors C* and O* have no orientation dependence, while *CO and *CHO require several calculations to test all configurations of the adsorbates. The WF was tabulated from experiment and did not require an additional calculation.
The authors would like to acknowledge the funding from BES-DOE SUNCAT FWP and the use of the computer time allocation for the “Transition metal-oxide and metal surfaces: applications and reactivity trends in catalysis” at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
(1)
Nørskov, J. K.; Studt, F.; Abild-Pedersen, F.; Bligaard, T. Fundamental Concepts In Heterogeneous Catalysis; 2014.
CONCLUSIONS
(2)
Hammer, B.; Norskov, J. K. Why Gold Is The Noblest Of All The Metals. Nature 1995, 376, 238– 240.
(3)
Royer, S.; Duprez, D. Catalytic Oxidation Of Carbon Monoxide Over Transition Metal Oxides. ChemCatChem 2011, 3, 24–65.
(4)
Yang, N.; Yoo, J. S.; Schumann, J.; Bothra, P.; Singh, J. A.; Valle, E.; Abild-Pedersen, F.; Nørskov, J. K.; Bent, S. F. Rh-MnO Interface Sites Formed By Atomic Layer Deposition Promote Syngas Conversion To Higher Oxygenates. ACS Catal. 2017, 7, 5746–5757.
(5)
Liu, J.; Tao, R.; Guo, Z.; Regalbuto, J. R.; Marshall, C. L.; Klie, R. F.; Miller, J. T.; Meyer, R. J. Selective Adsorption Of Manganese Onto Rhodium For Optimized Mn/Rh/SiO 2 Alcohol Synthesis Catalysts. ChemCatChem 2013, 5, 3665–3672.
(6)
Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. A Review Of Catalysts For The Electroreduction Of Carbon Dioxide To Produce Low-Carbon Fuels. Chem. Soc. Rev. 2014, 43, 631–675.
(7)
Kirk, C.; Chen, L. D.; Siahrostami, S.; Karamad, M.; Bajdich, M.; Voss, J.; Nørskov, J. K.; Chan, K. Theoretical Investigations Of The Electrochemical Reduction Of CO On Single Metal Atoms Embedded In Graphene. ACS Cent. Sci. 2017, 3, 1286–1293.
(8)
Graves, C.; Ebbesen, S. D.; Mogensen, M. CoElectrolysis Of CO2 And H2O In Solid Oxide Cells: Performance And Durability. Solid State Ionics 2011, 192, 398–403.
(9)
Skafte, Theis L.; Guan, Zixuan; Machala, Michael L.; Gopal, Chirranjeevi B.; Monti, Matteo; Martinez, Lev; Zhang, Liming; Stamate, Eugen; Sanna, Simone; Crumlin, Ethan; Bluhm, Hendrik; Garcia-Melchor, Max; Bajdich, M; Chueh, William C.; Graves, C. Establishing The Roles Of Oxidized Carbon Intermediates For Suppressing Carbon Deposition During CO2 Solid-Oxide Electrolysis.
In this work, we have computationally explored the metaloxide|metal interface with metal-supported TiO nanostripes and investigated their effect on CO hydrogenation. Two different types of nanostripes, (2×2) and (√3×√3), were constructed for the weak (Ag, Au) and strong (Cu, Ir, Pd, Pt, Rh) binding metal supports, respectively. For all supports, the Ti-edge of the stripe was found to be the most active region. The weak interacting supports were found to not coordinate to *CO and *CHO, only binding to one or two Ti atoms. The strong interacting supports bind *CO and *CHO coordinated to both Ti atoms and the underlying support metal atoms. In the presence of water, the stripes were found to readily hydroxylate, binding *OH on the Ti-edge and *H on the O-edge. *CO was found to adsorb on the Ti-edge in the presence of these species. The metal-supported stripes showed a great improvement in *CO to *CHO scaling compared to (111) and (211) TMs. The coinage metals (Ag, Au, Cu) bind *CO relatively weakly and therefore have a dramatically stronger *CO binding energy on the stripe, while the strong binding metals show less of an enhancement. However, all support metals show a significant increase in *CHO binding, both for the clean and hydroxylated stripes, due to the special coordination to the Ti-edge. The origins of this enhancement have a geometrical component inherent to the binding configuration and an electronic component due to unsaturated bonds at the metal edge, given by the charges. This study shows a structural motif at the metal-oxide|metal interface that has significantly modified CO hydrogenation scaling and could be tested to improve other heterogeneous catalytic processes, such as N2 reduction or the oxygen evolution and reduction reactions. In addition to tuning the metal support, we can perform a similar analysis for other metal-oxide stripes to understand the trends. Since the oxygen end of both *CO and *CHO binds to the metal atom in the metal-oxide (Ti in this case), the oxophilicity of the metal in the metal-oxide is likely an important descriptor for the observed binding energies and ultimately, reaction rates.
REFERENCES
SUPPORTING INFORMATION Additional tables and figures, including complete tables with energetics, referred to in the main text. ACKNOWLEGMENTS (10)
Zeng, Z.; Chang, K.-C.; Kubal, J.; Markovic, N. M.; Greeley, J. Stabilization Of Ultrathin ACS Paragon Plus Environment
Page 9 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis (Hydroxy)Oxide Films On Transition Metal Substrates For Electrochemical Energy Conversion. Nat. Energy 2017, 2, 17070.
(11)
Zeng, Z.; Hansen, M. H.; Greeley, J. P.; Rossmeisl, J.; Björketun, M. E. Ab Initio Thermodynamic Modeling Of Electrified Metal-Oxide Interfaces: Consistent Treatment Of Electronic And Ionic Chemical Potentials. J. Phys. Chem. C 2014, 118, 22663–22671.
(12)
Freund, H.; Goodman, D. W. Ultrathin Oxide Films. In Handbook of Heterogeneous Catalysis; Wiley, 2008.
(13)
(14)
(15)
Freund, H.-J.; Pacchioni, G. Oxide Ultra-Thin Films On Metals: New Materials For The Design Of Supported Metal Catalysts. Chem. Soc. Rev. 2008, 37, 2224. Shaikhutdinov, S.; Freund, H.-J. Ultrathin Oxide Films On Metal Supports: Structure-Reactivity Relations. Annu. Rev. Phys. Chem. 2012, 63, 619– 633. Peterson, A. a.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. How Copper Catalyzes The Electroreduction Of Carbon Dioxide Into Hydrocarbon Fuels. Energy Environ. Sci. 2010, 3, 1311.
Direct Evidence For The Interfacial Oxidation Of {CO} With Hydroxyls Catalyzed By Pt/Oxide Nanocatalysts. 2009, 131, 16366–16367. (23)
Sun, Y. N.; Giordano, L.; Goniakowski, J.; Lewandowski, M.; Qin, Z. H.; Noguera, C.; Shaikhutdinov, S.; Pacchioni, G.; Freund, H. J. The Interplay Between Structure And CO Oxidation Catalysis On MetalSupported Ultrathin Oxide Films. Angew. Chemie - Int. Ed. 2010, 49, 4418– 4421.
(24)
Abild-Pedersen, F. Computational Catalyst Screening: Scaling, Bond-Order And Catalysis. Catal. Today 2016, 272, 6–13.
(25)
Peterson, A. a.; Nørskov, J. K. Activity Descriptors For CO2 Electroreduction To Methane On Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3, 251–258.
(26)
Michalsky, R.; Zhang, Y. J.; Medford, A. J.; Peterson, A. A. Departures From The Adsorption Energy Scaling Relations For Metal Carbide Catalysts. J. Phys. Chem. C 2014, 118, 13026– 13034.
(27)
Khorshidi, A.; Violet, J.; Hashemi, J.; Peterson, A. A. How Strain Can Break The Scaling Relations Of Catalysis. Nat. Catal. 2018, 1, 263–268.
(28)
Montoya, J. H.; Tsai, C.; Vojvodic, A.; Nørskov, J. K. The Challenge Of Electrochemical Ammonia Synthesis: A New Perspective On The Role Of Nitrogen Scaling Relations. ChemSusChem 2015, 8, 2180–2186.
(29)
Greeley, J. Theoretical Heterogeneous Catalysis: Scaling Relationships And Computational Catalyst Design. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 605–635.
(16)
Schumann, J.; Medford, A. J.; Yoo, J. S.; Zhao, Z.J.; Bothra, P.; Cao, A.; Studt, F.; Abild-Pedersen, F.; Norskov, J. K. Selectivity Of Synthesis Gas Conversion To C 2+ Oxygenates On Fcc(111) Transition Metal Surfaces. ACS Catal. 2018, No. 111, 3447–3453.
(17)
Schulz, H.; Riedel, T.; Schaub, G. Fischer-Tropsch Principles Of Co-Hydrogenation On Iron Catalysts. Top. Catal. 2005, 32, 117–124.
(18)
Visconti, C. G.; Martinelli, M.; Falbo, L.; Fratalocchi, L.; Lietti, L. CO2hydrogenation To Hydrocarbons Over Co And Fe-Based FischerTropsch Catalysts. Catal. Today 2016, 277, 161– 170.
(30)
Goniakowski, J.; Giordano, L.; Noguera, C. Polarity Compensation In Low-Dimensional Oxide Nanostructures: The Case Of Metal-Supported MgO Nanoribbons. Phys. Rev. B - Condens. Matter Mater. Phys. 2013, 87, 1–10.
(19)
Grabow, L. C.; Mavrikakis, M. Mechanism Of Methanol Synthesis On Cu Through CO2 And CO Hydrogenation. ACS Catal. 2011, 1, 365–384.
(31)
(20)
Liu, X.; Xiao, J.; Peng, H.; Hong, X.; Chan, K.; Nørskov, J. K. Understanding Trends In Electrochemical Carbon Dioxide Reduction Rates. Nat. Commun. 2017, 8, 15438.
Chen, Z.; Mao, Y.; Chen, J.; Wang, H.; Li, Y.; Hu, P. Understanding The Dual Active Sites Of The FeO/Pt(111) Interface And Reaction Kinetics: Density Functional Theory Study On Methanol Oxidation To Formaldehyde. ACS Catal. 2017, 7, 4281–4290.
(32)
Sun, Y. N.; Qin, Z. H.; Lewandowski, M.; Carrasco, E.; Sterrer, M.; Shaikhutdinov, S.; Freund, H. J. Monolayer Iron Oxide Film On Platinum Promotes Low Temperature CO Oxidation. J. Catal. 2009, 266, 359–368.
Plessow, P. N.; Bajdich, M.; Greene, J.; Vojvodic, A.; Abild-Pedersen, F. Trends In The Thermodynamic Stability Of Ultrathin Supported Oxide Films. J. Phys. Chem. C 2016, 120, 10351– 10360.
(33)
(21)
(22)
Goniakowski, J.; Finocchi, F.; Noguera, C. Polarity Of Oxide Surfaces And Nanostructures. Reports Xu, L.; Ma, Y.; Zhang, Y.; Jiang, Z.; Huang, W. Prog. Phys. 2008, 71, 016501. ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(34)
Noguera, C.; Goniakowski, J. Polarity In Oxide Nano-Objects. Chem. Rev. 2013, 113, 4073–4105.
(35)
Zhang, S.; Plessow, P. N.; Willis, J. J.; Dai, S.; Xu, M.; Graham, G. W.; Cargnello, M.; Abild-Pedersen, F.; Pan, X. Dynamical Observation And Detailed Description Of Catalysts Under Strong MetalSupport Interaction. Nano Lett. 2016, 16, 4528– 4534.
Page 10 of 11 Bifunctional Metal/Oxide Interfaces: A Case Study Of Au Nanoparticles On Doped MgO Substrates. Angew. Chemie Int. Ed. 2018.
(43)
The computational data is available online at https://www.catalysishub.org/publications/SandbergStrongly2018
(36)
Levin, M. E.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. The Enhancement Of CO Hydrogenation On Rhodium By TiOxoverlayers. J. Catal. 1987, 106, 401–409.
(37)
Wellendorff, J.; Lundgaard, K. T.; Møgelhøj, A.; Petzold, V.; Landis, D. D.; Nørskov, J. K.; Bligaard, T.; Jacobsen, K. W. Density Functionals For Surface Science: Exchange-Correlation Model Development With Bayesian Error Estimation. Phys. Rev. B - Condens. Matter Mater. Phys. 2012, 85, 32–34.
(38)
Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: A Modular And Open-Source Software Project For Quantum Simulations Of Materials. J. Phys. Condens. Matter 2009, 21.
(39)
Bahn, S. R.; Jacobsen, K. W. An Object-Oriented Scripting Interface To A Legacy Electronic Structure Code. Comput. Sci. Eng. 2002, 4, 56–66.
(40)
Cramer, C. J. Essentials Of Computational Chemistry Theories And Models; 2004; Vol. 42.
(41)
Grunes, J.; Zhu, J.; Yang, M.; Somorjai, G. A. CO Poisoning Of Ethylene Hydrogenation Over Pt Catalysts: A Comparison Of Pt(111) Single Crystal And Pt Nanoparticle Activities. Catal. Letters 2003, 86, 157–161.
(42)
Bligaard, T.; Nørskov, J. K.; Dahl, S.; Matthiesen, J.; Christensen, C. H.; Sehested, J. The BrønstedEvans-Polanyi Relation And The Volcano Curve In Heterogeneous Catalysis. J. Catal. 2004, 224, 206– 217.
(43)
Molina, L. M.; Hammer, B. Active Role Of Oxide Support During CO Oxidation At Au/MgO. Phys. Rev. Lett. 2003, 90, 4.
(44)
Duan, Z.; Henkelman, G. CO Oxidation At The Au/TiO 2 Boundary: The Role Of The Au/Ti5c Site. ACS Catal. 2015, 5, 1589–1595.
(45)
Mehta, P.; Greeley, J.; Delgass, W. N.; Schneider, W. F. Adsorption Energy Correlations At The Metal-Support Boundary. ACS Catal. 2017, 7, 4707–4715.
(46)
Choksi, T.; Majumdar, P.; Greeley, J. Electrostatic Origins Of Linear Scaling Relationships At ACS Paragon Plus Environment
CHO*
Page 11 of 11 ACS Catalysis 1 2 3 4 5 6
1eV ) 111
fcc(
e -edg tripe Paragon sACS
o Nan
Plus Environment CO*
