Article Cite This: J. Phys. Chem. C 2019, 123, 17925−17932
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Kinetics Analysis of Multichannel Hydrogen Reactions on PlasmonicBased Au−GdC Thin-Film Nanocomposites Laila Banu,† Radislav A. Potyrailo,‡ and Michael A. Carpenter*,† †
College of Nanoscale Engineering and Technology Innovation, SUNY Polytechnic Institute, 257 Fuller Road, Albany, New York 12203, United States ‡ General Electric Global Research Center, Niskayuna, New York 12309, United States
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S Supporting Information *
ABSTRACT: The reaction kinetics of hydrogen on gold nanoparticles (AuNPs) embedded in gadolinium-doped ceria has been studied in an oxygen-free environment through the time-dependent analysis of the localized surface plasmon resonance (LSPR) peak of AuNPs. The change in the LSPR peak position over time during gas exposure is extracted from the absorbance spectra and is used as the sensing signal. The signal transduction is proposed to be a combination of charge exchange from hydrogen dissociation on the AuNP surface to form the intermediate Au−hydride species and atomic hydrogen spillover to the GdC matrix. The reaction kinetics study carried out at 500 °C for 5 to 20% H2 in N2 showed a linear relation between the rate of reaction and P1/2 H2 , indicating that H2 dissociative adsorption is the rate-limiting step for the initial sensing response. A subsequent spillover-type mechanism induces a secondary red shift of the plasmon peak position. From the Arrhenius analysis, the activation energy determined for hydrogen dissociative adsorption on AuNP is 0.14 eV. An apparent negative activation energy is observed for the likely spillover channel, which is determined to be due to the competing kinetics of the two reaction pathways.
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INTRODUCTION
detector electronics, and some others, all of which is an active area of research.1,13−16 Carpenter and co-workers have investigated several metal oxides as host materials for Au nanoparticles that include YSZ, TiO2, and CeO2 for plasmonic-based gas sensing applications at high temperatures.17−19 Joy et al.11 through sensing experiments at high temperature in combination with statistical analysis have shown that Au−CeO2 can be a potential candidate for H2, CO, and NO2 sensing with excellent sensitivity and selectivity. The driving force for a change in the sensing signal for plasmonic sensors includes charge exchange on the Au surface as well as a change in the dielectric environment surrounding the Au particles, both of which can be optimized by modification of morphology and material choices. A very recent study involving H2 sensing on Au−CeO2 validated the aforementioned reaction mechanism using XPS, kinetics analysis, and Arrhenius analysis.20 Cerium in CeO2 is known to participate in redox reactions in the presence of oxidizing and reducing agents by reversibly switching between Ce3+ and Ce4+ states. This property makes CeO2 an excellent candidate in the field of gas sensing, catalysis, SOFCs, and solar cells.21 Properties of CeO2 can be further tuned by the introduction of aliovalent cations to improve its thermal stability and oxygen exchange and reducibility (storage capability) as well as catalytic properties
Current and emerging demand for high-quality gas sensors includes industrial process control and safety, environmental monitoring and protection, emission control, transportation, food and medical packaging, and many others.1−8 Existing analytical techniques such as gas chromatography, mass spectrometry, ion mobility spectrometry, and optical spectroscopy methods offer highly selective detection of target gases while at the expense of relatively high-power consumption, cost, and size. The use of these techniques is inevitable because existing sensors have several prominent limitations including poor gas selectivity, insufficient stability, cross-sensitivity, sensor drift, and sensor recovery.1 Nanocomposite thin film based optical gas sensors are under development for applications including monitoring of emission gases from different units such as turbines and jet engines and monitoring of processes such as the oxygen-free fuel inlet stream of solid oxide fuel cells (SOFCs) where temperatures can range from 200 to 400 °C9 or higher.10 The benefits of using plasmonicbased optical sensors are high-temperature compatibility, minimum electromagnetic interference, potential for in situ measurements over a relatively broad range of gas concentrations, and no requirements of electrical contacts, which reduces the possibility of system failure. Furthermore, opticalbased methods have the potential for using a multivariable approach for the sensitive and selective detection of target gases.11,12 Potential limitations of the optical sensors may include their fouling, instabilities of the light source and © 2019 American Chemical Society
Received: December 20, 2018 Revised: June 3, 2019 Published: June 28, 2019 17925
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Figure 1. (a) ESEM micrograph of an uncapped Au−GdC thin film with AuNPs. (b) UV−vis absorbance spectra of large-particle GdC sample showing the absorption peak of AuNP at 564 nm.
such as activity and selectivity.22,23 Gd3+, La3+, Y3+, Eu3+/2+, Ca2+, Sr2+, and Sm3+ are a few examples of metallic cation dopants within the ceria lattice and serve as modifiers of its corresponding chemical and structural properties. Among these cations, Gd3+ and Sm3+ doping facilitates the highest conductivity compared to other ceria dopants due to the smallest association enthalpy between the dopant cation and the oxygen vacancy in the fluorite lattice.23,24 Temperatureprogrammed reduction (TPR) and thermogravimetry experiments showed that 20 to 25% gadolinium doping increases the reducibility as well as oxygen storage capacity of doped ceria at 800 °C.23 Dopant concentration, as well as the approximate matching of ionic radii of the dopant cation and metal oxide, can optimize the ionic conductivity of the host oxide by minimizing the vacancy associated enthalpy of the doped oxide.25 Application of doped ceria as an electrolyte or anode in the reducing environment of SOFCs has been investigated by several groups due to its moderate to high temperature stability.26−28 In the present work, initial optical-based H2 detection measurements using gold nanoparticles (AuNPs) embedded in gadolinium-doped ceria (GdC) are used for the detection of H2 from 0 to 20% by volume in an oxygen-free environment using N2 as a carrier gas. The oxygen-free exposures are targeted not only to serve as baseline measurements but also present a challenge for the metal oxide based material set due to the absence of oxygen anions within the rich H 2 environment. Of particular interest in this study are not only the initial detection characteristics but also the analysis of the rich kinetics and multistep reduction reaction mechanism that is observed and characterized for the first time for the Au− GdC system within the oxygen-free H2 environment. In the literature, H2 dissociation on the gold surface was investigated for Au nanoparticles with different metal oxide supports such as CeO2,19 TiO2,29,30 ZnO,31 and SiO2.32 These studies reported that interfacial charge transfer between Au nanoparticles and the support oxide material creates active sites and the charged AuNP at the interface facilitates H2 activation and dissociation.29,31 From the TPR profile of the Gd2O3-doped Pt/CeO2 system, an enhanced interaction at the metal−metal oxide interface is also suggested due to Gd2O3 doping, which promotes the surface reduction process, compared to pure ceria.33 The focus of the present work is to investigate the sensing mechanism of H2 on the Au−GdC nanocomposite film through kinetics and Arrhenius analysis and moreover to determine the role of Au nanoparticles and the oxide matrix in
the observed H2 response. This fundamental study is needed for future gas-sensor development and optimization for applications including the inlet stream of the SOFC where gas sensing in an oxygen-free environment is necessary and is used to maintain the right compositions of CO and H2 for optimum performance of the unit.7,20,34,35
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EXPERIMENTAL METHODS Nanocomposites of Au nanoparticles embedded in gadolinium-doped cerium oxide thin films were fabricated by a layerby-layer technique using physical vapor deposition (PVD). Metal oxide (GdC) films were deposited utilizing a customdesigned PVD chamber equipped with a radiofrequency (RF) magnetron sputtering gun. A 20% gadolinium-doped ceria target was used for deposition (Kurt Lasker, 99.9% purity). GdC thin films were deposited on quartz substrates at an operating pressure of 15 mtorr and RF power of 200 W in the presence of argon at room temperature. The planar layer-bylayer sample was made in three steps. In the first step, a 70 nm thick GdC base layer was deposited on a precleaned quartz substrate followed by annealing at 1000 °C for 1 h in argon to stabilize the film. A 3 nm gold (Au) film was then deposited using electron beam evaporation onto the GdC base layer and annealed in argon at 500 °C for 1 min to dewet the Au film and obtain Au nanoparticles (AuNPs) with an average diameter of 17 ± 3 nm. Gold nanoparticles formed after dewetting are shown in Figure 1 along with the corresponding UV−vis absorption spectra. A 50 nm GdC capping layer was then deposited and annealed at 800 °C for 3 h in Ar. Along with the Au−GdC sensor, a GdC reference film was also deposited. The high-temperature optical absorbance experimental bench used for the sensing tests is the same as that used in a previous study.11 During gas sensing experiments, the temperature of the flow tube containing the sample was raised to the desired temperature and then allowed to stabilize in nitrogen for 5 h. Computer-controlled mass flow controllers ensured a total gas flow of 1000 sccm for each gas exposure cycle. Hydrogen sensing experiments were performed at 500 °C and 1 atm for 5, 10, 15, and 20% H2 concentrations in the presence of N2 carrier gas. Gas exposure was carried out in an oxygen-free, nitrogen background environment, and data were collected every 30 s. The kinetics analysis was carried out for 10% H2 at 450, 500, and 550 °C in a nitrogen background. The absorption spectra for the kinetics studies were collected every 2 s to capture the initial rate of change in absorption spectra 17926
DOI: 10.1021/acs.jpcc.8b12267 J. Phys. Chem. C 2019, 123, 17925−17932
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The Journal of Physical Chemistry C immediately after H2 gas introduction. Gas exposures were repeated six times at each temperature. For all experiments, the H2 gas exposure time was set to 1 h followed by 1.5 h in pure N2 to recover the sample. The change in LSPR peak positions during H2 gas exposure is recorded as a function of time, and the LSPR peak position information is extracted using peak fitting routines. Representative absorption spectra are shown in Figure 2 for H2 gas on and off conditions, which are used to extract the
ω=
Noe 2 (εb + 2εm)meεo
(1)
From the above equation, the LSPR peak position change during gas exposure depends on both the electron density of the metal nanoparticle and the dielectric constant of the surrounding matrix. For gas sensing experiments, a change in the LSPR peak position over time extracted from the absorbance spectra is used as the sensing signal. Figure 3a shows the changes in peak positions for 5, 10, 15, and 20% H2 gas exposure on the Au−GdC film at 500 °C and 1 atm. A calibration curve, which summarizes the sensing responses for 5 to 20% H2, is derived from data shown in Figure 3a. The data points on the calibration curve are determined by subtracting 20 averaged plasmon responses for gas on and gas off regions at each H2 concentration. The slope of the calibration curve is a measure of the sensor sensitivity. The calibration curve in Figure 3b shows the response for each gas concentration, and the sensitivity of the Au−GdC sensor for H2 gas determined from the slope was −0.96 nm/%H2. The transients observed in Figure 3a are related to the dynamic response of the system to the H2 exposures and the corresponding multistep reaction mechanism that impact the plasmon peak position. The transient characteristics of the plasmon response coupled with the increase in error bars at concentrations above 10% in the calibration curve lend the initial data set not suitable for sensing purposes but rather as a rich data set for mechanistic analysis. Specifically, during the hydrogen gas exposure, there are several subsequent reaction pathways that might contribute to the sensing signal as well as to the overall plasmon response. These reaction pathways include (1) interaction between hydrogen and Au via dissociative adsorption as noted from Figure 3, (2) atomic hydrogen spillover, and (3) interaction between hydrogen and the metal oxide. A comprehensive study of the H2 activation mechanism will provide a better understanding of the sensing reaction mechanism as well as the roles played by the AuNPs and the oxide support. Hydrogen Reaction with Gold. Interaction of hydrogen with supported and unsupported gold surfaces at room temperature as well as high temperatures has been investigated by several groups.38−41 Even though bulk Au remains inert in contact with hydrogen, low coordinated edges and corner atoms on gold nanoparticles (AuNPs) promote H2 dissociation at 298−373 K.42 An FTIR study reported H2 dissociation at room temperature on the AuNP surface within a Au−TiO2
Figure 2. LSPR absorption spectra acquired during H2 gas on (red line) and H2 gas off (blue line) cycles at 500 °C. The plasmon peak blue shifted for 20% H2 gas exposure at 500 °C.
LSPR peak position information. A reversible blue shift in absorption spectra is observed due to 20% H2 gas exposure compared to absorption spectra collected in N2 at 500 °C. XRD was carried out on two samples, one that was not exposed to H2 and a second after high-temperature H2 exposure to further investigate the reversibility of the interaction. The respective overlapping spectra are shown in the Supporting Information, and are a further indication of the reversibility of the H2 reactions on the Au−GdC system.
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RESULTS AND DISCUSSION The LSPR peak position for the sensing experiment is characterized using the Drude model36,37 shown in eq 1, where ω is the surface plasmon resonance frequency, No is the free electron density of the metal nanoparticles, e is the elementary charge, εb is the interband transition term, εm is the dielectric constant of the metal oxide matrix, me is the electron mass, and εo is the permittivity of free space.
Figure 3. (a) Sensor responses as a function of time for 5, 10, 15, and 20% H2 gas exposures in N2 background at 500 °C on Au−GdC system. (b) Calibration curve calculated by averaging 20 values in the gas on and gas off regions. 17927
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Figure 4. (a) Enlarged view of linear shift in signal immediately after H2 exposure at 500 °C. The inset shows the linear fit of the data points in the red box. (b) Partial pressure of hydrogen raised to the power of one-half vs rate of peak position change.
Figure 5. (a) Plasmon response from 10% H2 exposure on GdC (70 nm)−AuNPs−GdC (50 nm) at 450, 500, and 550 °C with six repeats. (b) A zoom-in of the response at 450 °C shows both the blue shift and red shift in the plasmon response.
from ex situ ellipsometry measurements noted in the literature. Specifically, upon reduction of ceria thin films with hydrogen, the refractive index is observed to increase and proportionally the dielectric constant increases as well.46,47 Similarly, increases in the refractive index upon reducing conditions have been observed for both TiO2 and YSZ using in situ ellipsometric techniques.44,18 In situ ellipsometric studies of 1% H2 in N2 reacting with 120 nm thick GdC films deposited on silicon were attempted in the present study with a 70° measurement angle and the use of an Instec temperature cell; however, these studies were inconclusive. Further studies are planned that will incorporate an ellipsometric study of GdC films as well as detailed ellipsometry of Au−GdC films as a function of temperature and H2 gas exposure. These future studies will decouple the role of the Au nanoparticles and the GdC on the H2 reaction dynamics and provide a conclusive study of this reaction and its effects on the GdC optical properties. While the initial in situ ellipsometric analysis was inconclusive, the kinetics of the H2 dissociative adsorption blue shifting channel and the subsequent red shifting channel is detailed below. From previous studies of H2 reaction kinetics on various Au−metal oxide systems, it was established that the dissociation of H2 to atomic hydrogen on the Au surface is the rate-limiting step and the rate of reaction is proportional to hydrogen partial pressure raised to the one-half power.17,30,40
catalyst and on Au clusters within Au−CeO2 and Au−ZrO2 along with the formation of Au−H and Au−OH species43 Au δ+ + H 2 → Au − H (gold hydride formation)
(2)
This dissociation of hydrogen and subsequent metastable Au− H formation will contribute to a blue shift in the plasmon absorbance spectra due to the increase in free electron density of gold via electron density donation from the H species to Au as noted from the Drude equation. Studies involving plasmonbased H2 gas sensing using supported AuNPs also stated that the gold nanoparticle induced H2 dissociation on the Au surface followed by the Au−H formation in an oxygen-free environment.17,20 Hydrogen Spillover. Diffusion of atomic hydrogen from the metal surface sites onto the supporting metal oxide surface is another common surface phenomenon that might take place during H2 exposure to the metal−metal oxide system. Atomic hydrogen spillover was demonstrated by several groups for TiO2-supported Au,30,43,44 SiO2-supported AuNPs,32 and ZrO2-supported Pt.45 From FTIR studies, Manzoli et al.43 reported that atomic H spillover to the TiO2 surface at room temperature is strongly dependent on contact time as well as the type of support oxide. The spilled-over atomic H gets incorporated into the oxide support by the formation of OH species.32 This OH species is formed by reduction of Ce4+ to Ce3+, which should contribute to the red shift in the plasmon peak by increasing polarizability as well as the dielectric constant of the oxide matrix as noted from the Drude equation. Support for the likely change in dielectric constant is noted
1 H 2(g) + S ↔ H(ad) 2 17928
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Figure 6. Arrhenius plots for (a) gold-induced hydrogen adsorption dissociation reaction. (b) Red shifting channel. Error bars are also included for Figure 6b but are too small to be visible beyond the data points.
r ∝ PH1/2 2
universal gas constant. From the ln(k) versus 1/T plot, the slope (−Ea/R) is determined, which provides the activation energy Ea. Figure 6 shows the Arrhenius plot for both the hydrogen dissociation reaction and the red shifting reaction channel. The linearity of both plots at the three temperatures is noted with the linear fit matching well with respect to the error bars determined for the respective rates. The calculated activation energies, as determined from the slope, are 0.14 and −1.39 eV, respectively. The activation energies for gold-induced hydrogen dissociation determined for Au−YSZ and Au−CeO2 are 0.17 ± 0.02 and 0.18 eV, respectively.17,20 Comparing the activation energies reported in the literature and determined in this work for the dissociative adsorption of H2 on the Au surface, it can be inferred that the activation energy is seemingly independent of the metal oxide support chemistry. The apparent negative activation energy can be compared to that determined by Panayotov and Yates,30 who reported the activation energy for hydrogen spillover on TiO2-supported Au as 0.52 ± 0.02 eV. However, with the present study’s activation energy value for the red shifting channel being apparently negative, this clearly warrants further discussion. While enzyme catalysis, interstellar chemistry, and atmospheric science are examples of a few different fields that have highlighted reactions with confirmed negative activation energies,48 they should be studied with care as these are not typically observed. Joshi et al.49 also reported an apparent negative activation energy for the selective catalytic reduction (SCR) of NOx with NH3 through studies at temperatures ranging from 200 and 500 °C. In this study, it was noted that, since the activation energy of the standard SCR reaction is lower than that of NH3 desorption, at higher temperatures, the reaction rate of SCR is outpaced by the NH3 desorption reaction. This produced an apparent negative activation energy for the overall reaction, which was determined to be an artifact of the competing kinetics of the various steps within the reaction mechanism. The apparent negative activation energy in the present study can be explained through inspection of the various reaction paths. These are highlighted in the reaction schemes noted below and include formation of the Au−hydride intermediates followed by the secondary GdC red shifting channel. The corresponding competing reaction kinetics among these species as equilibrium is approached leads to the observed transient kinetics. The Au−hydride formation reaction and red shifting channel reaction might be expressed in the following scheme where k1 and k−1 are the rate coefficients for Au−
(4)
In the above expression, S is the available surface sites for adsorption and r is the rate of change in the sensing signal. The initial response is considered here, so it is assumed that the reverse reaction is negligible and the number of adsorption sites remains constant. For these plasmonic studies, the initial rate of change of the plasmon peak position as a function of H2 concentration is monitored to confirm the half-order dependence for the initial H2 reaction process. The slope of the most linear portion of the peak position versus time plot (Figure 3a) gives the initial rate of reaction, and representative data is shown in Figure 4a. To determine this rate of change, data fitting was performed for each concentration of hydrogen and all three repeats. The initial rate of reaction determined from all three repeats is averaged to get the final rate of reaction at each gas concentration. The rate of reaction was then plotted versus the partial pressure of hydrogen (PH2) raised to the power of one-half to verify the half-order reaction kinetics. Figure 4b shows a linear relationship between the rate of signal change and the hydrogen partial pressure of one-half order, which is in agreement with hydrogen dissociative adsorption on Au−metal oxide systems reported in the literature. Arrhenius Analysis. For completion of the kinetics analysis, Arrhenius analysis was used as has been done for previous studies.17,20 The activation energies for the H2 dissociative adsorption reaction and the subsequent red shifting reaction channel were both determined from data sets arising from six repeated exposures at three different temperatures of 450, 500, and 550 °C as shown in Figure 5, as these two channels result in the observed blue and red shifts. Here, the Au−H formation reaction is favored at higher temperature, whereas the red shifting channel is promoted at a lower temperature as can be seen from the large red shifts observed at 450 °C. The Arrhenius analysis was performed for both the blue shift region (H2 dissociative adsorption) and red shift region of the plasmon response shown. Since activation energy is not a function of gas concentration,17 the activation energy was determined for 10% H2 concentration in nitrogen background gas according to the following Arrhenius expression i −E yi 1 y ln(k) = jjj a zzzjjj zzz + ln(A) k R {k T {
(5)
where k is the rate constant defined as Δpeak position/Δtime, T is temperature, Ea is the activation energy, and R is the 17929
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dependence. Next steps will be focused on a detailed in situ ellipsometric study on the H2−GdC and Au−GdC reactions as a function of temperature, sensor-optimization activities that will include determination and optimization of the limit of detection of the sensor, and reduction of sensor response and recovery times through materials optimization with subsequent evaluation of gas cross-sensitivity. We will evaluate effects of sensor morphology on the dynamics of gas-sensor response and will determine such optimal sensor morphology. It is expected that the sensing material will have some crosssensitivity to other gases (such as CO, NO2, and O2) as was illustrated with other metal oxides.9,11,19 However, we will implement our recently demonstrated multivariable bioinspired gas-response principles to mitigate this problem.50
hydride formation in forward and reverse reactions, respectively, and k2 is the rate coefficient for the red shifting species formation. k1
k2
Au + H 2 HooI Au − H → GdC k −1
(6)
According to this proposed reaction pathway, an apparent negative activation energy is possible when at the lower temperatures in this study k1 > k2 > k−1, which allows after the buildup of Au−H a transient formation of the red shifting species, thus inducing a modest and secondary red shift of the plasmon band. However, at higher temperatures, the secondary red shift is less prominent, which implies a stronger temperature dependence of the Au−H k1 reaction channel realized as the dominant blue shift of the plasmon band. Therefore, the interplay of the dominant k1 and secondary k2 reaction kinetics leads to the apparent negative activation energy for the red shifting species, which is proposed to be an experimental artifact due to the competing reaction channels. This implies that the activation energy for the GdC-related red shifting reaction channel is 0 < Ea < 0.14 eV, which is significantly less than the H-spillover activation energy for the Au−TiO2 system as noted by Panayotov and Yates.30 Therefore, while H2 dissociative adsorption does not appear to have a strong dependence on the metal oxide chemistry, given the values previously determined for YSZ, ceria, and GdC, it appears that the secondary red shifting reaction channel, not unexpectedly, will likely have a strong dependence on the metal oxide chemistry. Given that GdC has been extensively explored for H2 storage and catalysis related reactions, it is apparent that the H-related secondary reaction activation energy as noted in the present study is a prominent factor in its potential usage, and future work should be focused on further characterizing this value and using it as a design parameter for development of metal oxide chemistries for improved functionalities.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b12267.
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XRD of Au−GdC (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Michael A. Carpenter: 0000-0002-9259-1551 Author Contributions
The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Funding
U.S. Department of Energy, National Energy Technology Laboratory Cooperative Agreement DE-FE0027918. Notes
The authors declare no competing financial interest.
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CONCLUSIONS
ACKNOWLEDGMENTS This work has been supported in part by the U.S. Department of Energy, National Energy Technology Laboratory Cooperative Agreement DE-FE0027918. The findings and conclusions in this review should not be interpreted to represent any determination or policy of the U.S. Department of Energy. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or agency thereof.
Detailed work is carried out to comprehend the reaction kinetics for the Au−GdC nanocomposite at temperatures of 450, 500, and 550 °C. The optical sensing mechanism for H2 detection on Au−GdC includes the following: (1) change in effective electron density in AuNPs by the dissociative adsorption reaction of H2 on the Au nanoparticle surface remaining there as Au−hydride or in a secondary reaction stage, (2) possible spillover to the oxide surface, and (3) the secondary reaction channel inducing the subsequent red shift in the plasmon band. The proposed reaction pathways contribute to both the blue shift and red shift in the LSPR peak position as observed in this study. Half-order reaction kinetics is observed upon H2 exposure to the Au−GdC film at 500 °C in a nitrogen environment, which also implies that the transduction process is dominated by the dissociative adsorption of H2 on the Au surface. The activation energy calculated for hydrogen dissociative adsorption was 0.14 eV, while the secondary reaction channel displayed an apparent negative activation energy due to the competing reaction channels. Given the temperature dependence of the reactions, it is surmised that the secondary reaction has an activation energy that is less than 0.14 eV, and given the dominance of the dissociative adsorption channel as a function of temperature, the secondary channel has a weaker temperature
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ABBREVIATIONS LSPR, localized surface plasmon resonance; GdC, gadoliniumdoped ceria; AuNP, gold nanoparticle; SOFC, solid oxide fuel cell; TPR, temperature-programmed reduction 17930
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