Potential Routes and Mitigation Strategies for Contamination In

Article Cite This: J. Phys. Chem. C 2018, 122, 24658−24664 pubs.acs.org/JPCC Potential Routes and Mitigation Strategies for Contamination in Interfacial Specific Infrared Spectroelectrochemical Studies Marco Dunwell, Xuan Yang, Yushan Yan,* and Bingjun Xu* Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States *S Supporting Information ABSTRACT: Attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a power- ful tool for probing the electrochemical interface; however, inconsistencies in reported spectroscopic observations persist. In this work, we identify potential sources of contamination in ATR-SEIRAS using CO as a probe molecule by varying the electrolyte purity, counter electrode (CE), and anolyte/ catholyte separator. We show that only a linearly bound CO near 2050−2100 cm−1 is observed in clean experiments, whereas the use of low-purity electrolytes and Pt CEs leads to additional bands near 1800−1950 and 1920−2020 cm−1, respectively, which are commonly assigned to bridge-bonded CO in the previous literature. These observations highlight the extreme sensitivity of ATR-SEIRAS and the importance of avoiding contaminants which lead to additional spectroscopic features. ■ INTRODUCTION difference in potential between the two experiments because of ff 22 The development of in situ and operando interfacial specific the Stark tuning e ect; however, the appearance of the COL infrared spectroscopic techniques such as polarization-modu- band only in NaOH is unexpected, particularly as the intensity fl of the COL band is expected to decrease at lower standard lated infrared re ection absorption spectroscopy (PM-IRRAS) 14,20,23 and attenuated total reflectance surface-enhanced infrared hydrogen electrode potentials. The primary inconsis- absorption spectroscopy (ATR-SEIRAS) represents a signifi- tency between experiments is related to the observation of the cant step forward in probing surface-mediated electrochemical COB band among polycrystalline Au electrodes (AuPC). Several − processes.4 9 With the rise in popularity of coupled infrared reports observe only a single, higher-wavenumber band 14−16,19,21 spectroscopic and electrochemical studies, however, incon- attributed to COL, whereas others report two sistencies in reported spectroscopic observations have arisen. A bands, with the higher- and lower-wavenumber bands assigned 1,12,13,19,20 fl notable example is CO adsorption on Au electrodes observed to COL and COB, respectively. Moreover, con icting during various investigations of small-molecule oxidation (such Downloaded via UNIV OF DELAWARE on November 4, 2018 at 15:18:22 (UTC). reports of either a lone COL band or both COL and COB bands as CO, formate, or methanol) or reduction (such as 1,12,13,15,16,19 14,20 − − can be found in acidic, neutral, and 1 3,10 20 13,15,21 CO2). alkaline electrolytes, so that these discrepancies cannot See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Four primary vibrational modes have been assigned to CO be simply attributed to differences in the electrolyte or adsorbed on Au electrodes during spectroelectrochemical potential range studied. Finally, Sun et al. report two separate studies (Table 1), attributed to linearly bound CO 19 −1 1−3,10−21 1−3,10−13,17−20 bands related to COL. One band from 2110 to 2136 cm (COL), bridge-bound CO (COB), 2,17 resembles features observed in other studies, whereas a lower multibound CO (COM), and weak linearly bound CO band from 2020 to 2045 cm−1 falls between the typical CO (CO ).12,13 The appearance of each of these bands, however, L WL and CO bands commonly observed on Au . This lower- is not consistent throughout the literature, even under similar B PC wavenumber band was attributed to COL adsorbed more experimental conditions. For example, COWL is only observed fi by Chen et al., both in acidic and in alkaline electrolytes.12,13 strongly to speci c sites of the electrode such as grain − CO is observed by Blizanac et al. at 1981 cm 1 on Au(110), boundaries or very near the Si substrate on which the AuPC M film is deposited.16 Despite these numerous examples, the whereas Chang et al. report only a single COL band centered at ∼2100 cm−1 on the same facet.2,11 Similarly, on Au(111) facets origin of the inconsistencies in the existing literature remains poorly understood. Blizanac et al. observe no COL band, but do observe a −1 pronounced COB band at 2042 cm in 0.1 M HClO4, whereas − Rodriguez et al. report two bands at 2000 and ∼1910 cm 1 Received: June 13, 2018 2,17,18 assigned to COL and COB, respectively, in 0.1 M NaOH. Revised: September 22, 2018 The discrepancy in wavenumber can be explained by the Published: October 9, 2018 © 2018 American Chemical Society 24658 DOI: 10.1021/acs.jpcc.8b05634 J. Phys. Chem. C 2018, 122, 24658−24664 The Journal of Physical Chemistry C Article Table 1. Reported CO Adsorption Band Assignments and Peak Positions in cm−1 a b c d e author electrode CE electrolyte COL COWL COB COM 1 Beden Au N/A HClO4 2080 1940 2,3 Blizanac Au(111) N/A HClO4 2042 1954 Au(100) N/A HClO4 2017 Au(110) N/A HClO4 2124 2021 1981 11 − − Chang Au(210) N/A HClO4 2100 2115 1930 1985 Au(210) N/A KOH 1920−1990 10 − Chang Au(210) Au HClO4 2105 2115 − Au(110) Au HClO4 2090 2110 12 Chen Au Pt HClO4 2036 2110 1846 Chen13 Au Pt NaOH 2000 2100 1930 Corrigan21 Au N/A NaOH 1940 14 − Dunwell Au graphite NaHCO3 2050 2100 Kunimatsu15 Au Au NaOH 1940−2000 − Au Au HClO4 1990 2050 16 Miyake Au Au HClO4 2108 Rodriguez17 Au(111) Au NaOH 2000 1900 1830 Rodriguez18 Au(111) Au NaOH + MeOH 2000 1920 19 − Sun Au N/A HClO4 2110 2136 2020−2045 fl − − Au ( ame annealed) N/A HClO4 2110 2136 1925 1975 2020−2045 20 − Wuttig Au Pt NaHCO3 2110 1930 2030 f − this work Au graphite NaHCO3 2075 2104 a b c N/A indicates that the type of CE used was not indicated in the cited work. COL: linearly or atop-bound CO. COWL: weakly bound linear or d e f fi atop CO. COB: bridge-bound CO. COM: multibound or CO adsorbed on hollow sites. Results when using the standard conditions as de ned by experiment 1 in Table 2. In this work, we demonstrate two modes of working water (Banstead Mega-Pure Water Purification System) to electrode (WE) contamination as possible sources of the remove all loose graphite prior to use. Pt wire CEs (99.95%, differences in CO adsorption on Au electrodes and discuss Alfa Aesar) were cleaned by immersing in a 3:1 by volume − mitigation strategies. We show that the lower-wavenumber solution of H2SO4 (95 98%, Sigma-Aldrich) and H2O2 (30%, band commonly associated with COB is likely caused by Sigma-Aldrich) and rinsing in deionized-distilled water prior to electrolyte impurities by monitoring CO adsorption using each use. Experiments were conducted in 0.5 M NaHCO3 ATR-SEIRAS in three electrolytes of different purities: high- solutions of various purities. Low-purity electrolytes were fi purity sodium bicarbonate, which is further puri ed via prepared using NaHCO3 (99.7%, Sigma-Aldrich). Moderate- chelation; the same high-purity electrolyte without additional purity 0.5 M NaHCO electrolytes were prepared by purging fi fi 3 puri cation; and nally using low-purity sodium bicarbonate. 0.25 M Na2CO3 (99.999%, EMD Millipore) overnight with Additionally, we show that the use of Pt counter electrodes high-purity CO2 gas. For the highest purity experiments, these (CEs) causes the appearance of an additional band associated electrolytes were further purified by stirring overnight after the with COL on Pt, which was previously assigned to COB on Au. addition of 5 g of a solid-supported iminodiacetate resin Although all studies presented in this work were conducted (Chelex 100, Sigma-Aldrich) for every 100 mL of electrolyte.25 fi with an Au lm WE in near-neutral pH electrolytes, the general Electrolytes were purged with CO gas (Matheson) for >15 min sources of contamination (the electrolyte and CE) as well as prior to each experiment (solution pH = 8.7) to saturate the the proposed mitigation strategies should be independent of solution with CO and remove the remaining CO2.All both WE structure and composition. The contamination, experiments were conducted under continuous CO purge. however, may manifest differently on different electrodes or ff ATR-SEIRAS Measurements. All ATR-SEIRAS measure- with di erent electrolytes, as the wavenumber and intensity of ments were collected in a three-electrode electrochemical cell vibration bands originating from the contaminant and with a chemically deposited polycrystalline Au film as the WE, adsorbates on the WE may vary, and contaminants may a graphite rod or Pt wire as the CE, and an Ag/AgCl (3.0 M deposit at different rates or potentials on different materials or 24 KCl, BASi) reference electrode (RE) described in our previous at different pH. Avoiding these common routes of 26 work. The REs and CEs were separated by a proton contamination in spectroelectrochemical studies is key to exchange membrane (Nafion 211, Fuel Cell Store) unless developing a proper and widely accepted understanding of otherwise noted. All presented spectra are eight co-added scans adsorbate behavior during electrochemical processes. − with 4 cm 1 resolution and potentials are given on the reversible hydrogen electrode (RHE) scale unless otherwise ■ METHODS AND MATERIALS noted. All potentials given were actively corrected for internal Materials. Au film electrodes for ATR-SEIRAS measure- resistance of the spectroelectrochemicalcellfollowing ments were prepared via a chemical deposition method impedance measurement.

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