Quantifying Adsorption of Organic Molecules on Platinum in Aqueous

Research Article Cite This: ACS Catal. 2019, 9, 6869−6881

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Quantifying Adsorption of Organic Molecules on Platinum in Aqueous Phase by Hydrogen Site Blocking and in Situ X‑ray Absorption Spectroscopy Nirala Singh,†,‡,§,∥ Udishnu Sanyal,‡ John L. Fulton,‡ Oliver Y. Gutiérrez,‡ Johannes A. Lercher,*,‡,⊥ and Charles T. Campbell*,† †

Department of Chemistry, University of Washington, Seattle, Washington 98105-1700, United States Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Department of Chemical Engineering and ∥Catalysis Science and Technology Institute, University of Michigan, Ann Arbor, Michigan 48109, United States ⊥ Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, 85748 Garching, Germany

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‡

S Supporting Information *

ABSTRACT: The adsorption equilibrium constants of phenol, benzaldehyde, cyclohexanol, and benzyl alcohol on Pt in aqueous phase have been determined via cyclic voltammetry (CV) measuring the capacity to adsorb hydrogen in the presence of organic molecules. The enthalpies of adsorbed phenol on Pt in aqueous phase estimated by this technique were approximately −41 and −21 kJ/mol relative to aqueous phenol for (110)/(100)-like sites and (111) facets, respectively. The enthalpy of phenol aqueous adsorption on (111) (−21 kJ/mol) can be compared to the gas-phase enthalpy of adsorption of phenol on single-crystal Pt(111) from previous work (−200 kJ/mol) to understand the individual factors influencing adsorption in the liquid phase. The results show that adsorbates with very strong intrinsic bonding at a gas/solid interface have markedly reduced adsorption enthalpy due to strong solvation of both the Pt surface and the organic. Using the CV method, the decrease of the heat of adsorption in the sequence benzaldehyde > benzyl alcohol > phenol > cyclohexanol points to a strong interaction of the aromatic ring with the metal surface. Combining CV and X-ray absorption spectroscopy, we show that the inhibition of hydrogen adsorption by organic molecules is a suitable method of determining the coverage of organic reactants and that these molecules have defined Pt−C bonds in the adsorbed state. KEYWORDS: adsorption energy, aqueous-phase hydrogenation, cyclic voltammetry, X-ray absorption spectroscopy, electrocatalysis, phenol, benzaldehyde

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INTRODUCTION

and selective catalysts are required as well as an improved understanding of their catalytic behavior. Here, we report indirect measurements of the adsorption equilibrium constants on Pt in aqueous phase for several oxygenates (phenol, benzaldehyde, cyclohexanol, and benzyl alcohol) using cyclic voltammetry (CV), quantifying the fraction of sites they occupy by measuring the blocking of hydrogen adsorption. Estimating the fraction of hydrogen adsorbed in the presence of organic molecules by CV allows us to determine their equilibrium adsorption constants in the aqueous phase.

Adsorption equilibrium constants and adsorption energies are critical for understanding catalyzed reactions, yet these values are challenging to acquire for small hydrocarbons and oxygenates in aqueous solution.1 However, aqueous-phase reactions such as electrochemical reduction or thermal hydrogenation reactions are important for sustainably and selectively transforming chemicals such as synthesizing fuels or remediating waste.2 This includes many reactions of great current interest with respect to biomass conversion, especially the electrocatalytic hydrogenation of bio-oil streams.3 Active and selective catalysts are required to enable these processes. Certain Pt group metals have been used for hydrogenation of model compounds approximating bio-oil,4,5 but more active © XXXX American Chemical Society

Received: April 6, 2019 Revised: June 3, 2019 Published: June 7, 2019 6869

DOI: 10.1021/acscatal.9b01415 ACS Catal. 2019, 9, 6869−6881

Research Article

ACS Catalysis

minimize contamination, and the reference electrode was measured against a Pt wire in 1 bar H2 in the same electrolyte to calibrate to 0 V vs reversible hydrogen electrode (RHE). After voltammetric cleaning of the Pt wire electrode or the Pt disk electrode, the cyclic voltammograms were measured with 10, 20, and 50 mV/s scan rates to verify the absence of scan rate effects. We show 20 mV/s scan rates for the rest of the work unless explicitly mentioned otherwise. The electrolyte was 100 mM acetic acid (Sigma-Aldrich, 99.995%) and 100 mM sodium acetate (99.999%) (pH 5). The organics were phenol (Sigma-Aldrich, unstabilized, >99%), benzaldehyde (Sigma-Aldrich, >99%), cyclohexanol (Sigma-Aldrich, 99%), cyclohexanone (Sigma, 99.8%), and benzyl alcohol (SigmaAldrich, 99.8%). The amount of underpotentially deposited hydrogen adatoms (Hupd) was calculated by measuring the charge (current × time integral) during deposition and desorption sweeps and subtracting the charge from the baseline capacitance current. The values plotted are the charge from Hupd desorption (which generally were within experimental error, or ∼10%, of the corresponding Hupd charge from adsorption). The effect of organic adsorption on Hupd was shown to be reversible for benzene on Pt.11,17 Although ideally the CV and in situ X-ray absorption spectroscopy measurements would be performed on identical catalysts, the CV isotherm technique would not be reliable on the 5 wt % Pt/C sample, since the Hupd peaks could not be distinguished from the double-layer capacitance background current from the carbon support, which dominates at low metal loadings. One could possibly address this using a higher loading Pt/C (e.g., 30 wt % Pt/C), which has been shown to have quantifiable Hupd peaks that can be used to count available surface sites.18,19 However, the particle sizes would be larger than those used here and so of limited additional value for X-ray adsorption spectroscopy to detect surface changes. X-ray Absorption Spectroscopy. A custom-built electrochemical cell was used for XANES and EXAFS.20 The aspurchased 5 wt % Pt/C catalyst (Sigma-Aldrich) was calcined at 200 °C in air to remove adventitious carbon on the metal surface. Measurements of the Pt nanoparticle size from STEM and H2 chemisorption gave a diameter of approximately 2.5 nm. The catalyst (Pt/C 2.5 nm) was deposited onto a carbon felt (Alfa Aesar) by syringe filtration. In 100 mM acetic acid and 100 mM sodium acetate the electrode was reduced and then run through a cleaning step consisting of 5 min at 1.3 V and then 5 min at −0.8 V vs Ag/AgCl. Following cleaning, the catalyst was exposed to new electrolyte with different concentrations of organics. The electrolyte was purged with argon and degassed before introduction to the electrochemical cell. Between each run the Pt surface was cleaned by this same oxidative cycle followed by a reductive cycle. This catalyst preparation and initial cleaning procedure was proven previously to lead to clean catalyst surfaces as evidenced by the catalytic activity for aqueous-phase catalytic hydrogenation of phenol,21,22 which gave rates per Pt surface atom that agreed with previous literature (using the amount of adsorbed hydrogen to determine the amount of surface Pt). The Pt L3 edge spectra were taken at Sector 20 of the Advanced Photon Source in Argonne National Laboratory. Data analysis was done using Athena software, and EXAFS fitting was done using Artemis.23 Fitting of the EXAFS data was done as discussed in detail in ref 20. In brief, three Pt−Pt shells were used for

The adsorption equilibrium constants are determined by fitting measured coverages of the oxygenates as a function of their aqueous-phase concentrations. Cyclic voltammetry is used to determine the fractional coverages of hydrogen and indirectly of the organic molecules. The current associated with hydrogen underpotential deposition (Hupd) is used to measure this fraction of H sites that is blocked. In this voltage range, we assume only H adsorbs/desorbs (i.e., OH* is not formed). The approach consists of the deposition of submonolayer amounts of hydrogen adatoms (Hupd) on the metal surface at less reducing potentials than required to reduce aqueous H+ to H2(gas), where the amount of charge (current times time) passed during electrochemical deposition or removal is proportional to the amount of hydrogen deposited on the metal or removed from its surface.6−8 The hydrogen chemical potential required to adsorb this monolayer is known based on the applied electrochemical potential and depends on the crystal face of the metal.6,9 Removal occurs at nearly the same potential, indicating a reversible process for Hupd ⇌ H+(aq) + e−(Pt) + * (where * is a free site on Pt) at any given potential within the underpotential deposition region.6−8 During hydrogen underpotential deposition, the presence of organics inhibits the adsorption of hydrogen, as reported for phenol10 and other organics.11−13 The decrease in the charge corresponding to hydrogen adsorption/desorption peaks in CV is generally attributed to adsorbed organic molecules, which block sites that would otherwise adsorb hydrogen.10−12 As the orientation of phenol (adsorbed parallel to the surface) is constant at low coverages14 and the reduction in the current/charge of the hydrogen adsorption/desorption peaks in CV is caused by adsorbed phenol blocking sites, the coverage of phenol at different phenol solution concentrations has been extracted from the difference in charge with and without phenol.10 The same concept has been applied already to other organic molecules on Pt11,12 and other metals.15 We show here that such data can be fitted to a dual-site Temkin adsorption model to derive coverage-dependent adsorption equilibrium constants. We extract from these the enthalpy of adsorption as a function of coverage using estimates of the standard-state entropy of adsorption based on reported experimental trends.16 We also present X-ray absorption spectroscopy (XAS) measurements under aqueous conditions, which directly probe the surface coverages of the hydrogen by Xray absorption near-edge spectroscopy (XANES) and organic species by extended X-ray absorption fine structure (EXAFS based on Pt−C scattering), to corroborate conclusions derived from electrochemical measurements.

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EXPERIMENTAL METHODS Cyclic Voltammetry. A Pt wire (0.5 mm diameter, 99.99%) was first cleaned by a flame to remove adventitious organics and then cleaned with Millipore water. For some measurements, a Pt disk (Pine) was used with consistent results compared to the Pt wire. The Pt disk was cleaned thoroughly with isopropanol and Millipore water. A glass Hcell cleaned with isopropanol to remove organics followed by drying then several series of further rinsing with Millipore water was used for the measurements. A Nafion membrane was used to separate the compartment containing the working electrode from the counter electrode. An Ag/AgCl reference electrode with double-junction protection was used to 6870


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ACS Catalysis

Figure 1. Cyclic voltammograms showing hydrogen underpotential deposition on a Pt wire at room temperature with varying concentrations of (a) phenol (which also shows the results for the Pt disk at 10 and 600 mM phenol), (b) benzaldehyde, and (c) benzyl alcohol at 20 mV/s, pH 5 acetate buffer. (d) Pt wire with cyclohexanol at 50 mV/s, pH 5 acetate buffer, room temperature. Volume of electrolyte is 40 mL, so that even complete adsorption of a monolayer of organics onto the Pt electrode at 1 μM would not significantly deplete the bulk solution’s organic concentration (i.e., by phenol on Pt) and is supported by the higher Pt−C coordination. The adsorption energies determined on Pt for phenol, benzaldehyde, benzyl alcohol, and cyclohexanol can help explain observed activity trends. Both phenol and benzaldehyde hydrogenation are low order at moderate concentrations, and the presence of cyclohexanol does not significantly inhibit phenol hydrogenation due to its much weaker binding to Pt than phenol.

(A3)

θP + θH + θ = 1 (A4) * Using eq A4 to solve for θ* and substituting it into eq A3 gives

KP =

θP C P(1 − θP − θH)

(A5)

which can be rearranged to give θP KPC P = (1 − θH) (1 + C PKP)

(A6)

Thus, at any constant value of θH we can fit the relative coverage of A to the standard first-order Langmuir adsorption/ desorption equation (i.e., the right side of eq A6) to get KP, just as we have done in the main text. The resulting KP value found in this way is really KP(1 − θH), but since θH is low in the way we actually apply this (see below), its value is only slightly changed by the competitive H adsorption. The line shape of the CVs (for a given Pt site type) is almost the same independent of P(aq) concentration (at least on the right half of the CVs, which is the lower H-coverage side). (As noted in the text, the overall CV line shape changes with P(aq) concentration because the Pt(110)- and (100)-like sites are blocked at lower concentrations than the Pt(111)-like sites, but the line shapes of each of the site types is almost constant and only attenuated in intensity with P(aq) addition.) Therefore, one can go to different voltages in the CVs to get the same θH (for that Pt site type) at different P(aq) concentrations. It is true that this line-shape stability may break down on the lefthalf of the CVs (where the H coverage gets high), so our method works best if only used for H coverages found on the right half of the CVs (i.e., low H coverages). This is just as one would expect intuitively, since this is where the H* least affects the adsorption of P*. Equation 1 in the main text is a rearranged version of eq A6, where the constant factor (1 − θH) has been dropped for the reasons outlined above and the two concentration terms (Corganic and θ/(1 − θH)) have been normalized by their standard-state values to convert them to thermodynamic 6877


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ACS Catalysis activities so that the resulting KP value is rigorously connected to the standard-state reaction free energy,16 as in eq 2. Equation 1 can be written in a slightly modified form as follows whenever θ0 is chosen to be 0.5 ij C yz θ aq = jjj P zzz = Keq, θ j C0 z 1−θ k { aq Keq, θ(unitless)

ß

0 0 0 ΔGad,aq,gas, θ ,2 = ΔHad,aq,gas, θ ,2 − T ΔSad,aq,gas,2 gas = −RT ln(Keq,aq, θ ,2) − RT

aq 0 y ij Keq, θ= 0.054(1 − θ ) z zz lnjjjj zz j z θ 0KH (A11) k { where the subscript 2 refers to this new standard-state coverage reference state. To calculate ΔG0ad,aq,gas,θ,2 using eq A11, we will start by assuming that

ij C P yz aq −αθ / RT jj zz K e eq,0 jj C zz ´ÖÖÖÖÖÖÖÖÖÖÖÖÖ Ö≠ÖÖÖÖÖÖÖÖÖÖÖÖÖÖÆ k 0 { aq Keq, θ(unitless)

(A7)

where CP is the concentration of phenol in the aqueous phase. To modify this to another θ0 value just requires multiplying 0 0 Kaq eq,θ by θ /(1 − θ ). If phenol also is in equilibrium with the gas phase, the combined reactions can be written as

0 0 ΔSad,aq,gas ≅ ΔSad,gas no solvent

This assumption of equating the entropy of adsorbed phenol in water (liquid) with the entropy of adsorbed phenol in the absence of liquid (i.e., in gas) is true if the water near the surface has nearly the same entropy independent of whether phenol is there on the surface or not, that is, the interfacial water has similar character whether or not the phenol adlayer is there. It also assumes that we are comparing two states of adsorbed phenol that have essentially the same structure whether or not the water is there above it. This just requires that we must compare to a similar structure in UHV (i.e., in gas) as in the aqueous phase. We can use the correlation of Campbell and Sellers40 to estimate entropy of this adsorbed phenol relative to gaseous phenol as

P(gas) F P(aq) F P(ad, aq)

The equilibrium partial pressure of P (PP) is given by jij PP zyz = K C P j 0z H C0 kP {

(A8)

where KH is Henry’s law constant for P in water and P0 = 1 bar. The net reaction (from gas-phase P to adsorbed P under aqueous solution) is P(gas) F P(ad, aq)

0 0 ΔSad,aq,gas,2 ≅ ΔSad,gas no solvent,2

and its equilibrium constant is defined as gas Keq,aq, θ

=

0 0 = Sad − Sgas

( 1 −θ θ ) /ijj PP yzz j 0z kP {

θ0 1 − θ0

( )

0 0 = (0.70Sgas − 3.3R ) − Sgas 0 = −[0.30Sgas + 3.3R ]

(A9)

The subscript 2 here refers to the different standard-state coverage used there. Using S0gas = 315 J/(mol K) = 37.9 R from ref 60 gives ΔS0ad,aq,gas,2 ≅ ΔS0ad,ga no solvent,2 = −14.7R. However, this value of ΔS0ad,ga no solvent,2 was determined using a standardstate coverage of

Again taking θ0 as 0.5 and substituting eq A8 into eq A7 gives

( )K PP

θ = 1−θ

p0

aq eq, θ

KH

i P y gas = jjj P0 zzzKeq,aq, θ kP {

θ0 =

(A10)

As described in the main text, the equilibrium constant is gas Keq,aq, θ

=e

=

aq Keq, θ / KH

=e

from ref 16. Assuming that the saturation phenol coverage = 1/ 7 of the Pt(111) density,44 this gives saturation phenol coverage = 1 [1.51 × 1015/cm2] = 2.16 × 1014/cm2, so that θ0 =

0 −ΔGad,aq,gas, θ / RT

0 0 −ΔHad,aq,gas, θ / RT ΔSad,aq,gas / R

e

7 1.17 × 1013 / cm 2 2.16 × 1014 / cm 2

(3a)

From our fit to the data, we get Kaq eq,θ, and values of KH for these organics in water are tabulated,36 e.g., 5 × 10−4 bar/M for phenol at 298 K. Rearranging eq 3a gives

= 0.054.

One could correct this entropy to the value of θ0 used in some places here (θ0 = 1/2) by changing ΔS0ad,aq,gas by 2.5R to −17.2R,16 but we prefer to analyze the data at the lower coverage of 0.054. From our fit of the data from phenol on Pt T sites, we obtain the equilibrium constant in aqueous phase and α

0 0 0 ΔGad,aq,gas, θ = ΔHad,aq,gas, θ − T ΔSad,aq,gas aq = −RT ln(Keq, θ / KH)

1.17 × 1013/cm 2 saturation phenol coverage

(3b)

aq Keq, θ= 0 = 38(unitless), with α = 0 kJ/mol

It is convenient to analyze this using the standard-state coverage for which adsorbate entropies have been tabulated, in which case the value of Kaq eq,θ determined in Figure 3 needs to multiplied by θ0/(1 − θ0), where θ0 = 0.054 for phenol (see below). For this value and using the equilibrium constant for phenol for Pt T sites from the fits in Table 1 at this same relative coverage θ = 0.054 of Kaq eq,θ0 = 38, eq 3b becomes

To calculate the equilibrium constant at a coverage of θ0 = 0.054, we use (even though it does not affect the result for the T sites due to α = 0 we include it here for completeness) e−αθ

0

/ RT

J

J

= e−(0 mol )(0.054)/(8.314 mol K )(298K) = 1

so that 6878


Research Article

ACS Catalysis aq Keq, = 38 θ=θ 0

FG02-96ER14630 for support of this work. This research used resources of the Advanced Photon Source Sector 20, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners. We thank and acknowledge Mahalingam Balasubramanian for his help.

Then we can convert the equilibrium constant in the aqueous phase to a different standard coverage, where instead of θ0 = 1/ 2, we use θ0 = 0.054 where the subscript 2 refers to this new standard-state coverage reference state gas Keq,aq, θ= 0.054,2 =

38

(

0.054 1 − 0.054

■

)5 × 10−4

This gives gas 6 Keq,aq, θ= 0.054,2 = 1.3 × 10 = e

0 −ΔHad,aq,gas, θ= 0.054 / RT

e

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0 ΔSad,aq,gas,2 /R

This value of the equilibrium constant can be used to calculate the enthalpy of adsorption of gas-phase phenol onto Pt in the aqueous phase using the entropy of this adsorption process estimated above ÄÅ ÉÑ 0 ÅÅ ÑÑ ΔSad,gas,2 ÅÅ ÑÑ gas 0 ÑÑ ΔHad,aq,gas, θ = 0.054 = −RT ÅÅÅln[Keq,aq, θ = 0.054,2] − ÑÑ ÅÅ R ÑÑÖ ÅÇ = −RT[ln[1.3 × 106] + 14.7] = −RT (28.8) = −71.4

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kJ @298K mol

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01415. CVs at higher concentrations, XANES data before difference, and additional EXAFS (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Nirala Singh: 0000-0003-0389-3927 Udishnu Sanyal: 0000-0002-7935-8691 John L. Fulton: 0000-0001-9361-9803 Oliver Y. Gutiérrez: 0000-0001-9163-4786 Johannes A. Lercher: 0000-0002-2495-1404 Charles T. Campbell: 0000-0002-5024-8210 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.S. was funded by the WRF Innovation Fellowship in Clean Energy Institute. The research described in this paper is part of the Chemical Transformation Initiative at Pacific Northwest National Laboratory (PNNL), conducted under the Laboratory Directed Research and Development Program at PNNL, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. C.T.C. also acknowledges the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division Grant No. DE6879


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Quantifying Adsorption of Organic Molecules on Platinum in Aqueous

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