Initiation of Vacancy-Mediated, Surface Explosion Reactions:Tartaric

Jun 27, 2019 - Surface explosion reactions have highly non-linear reaction kinetics that exhibit auto-acceleration under isothermal conditions. These ...
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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Initiation of Vacancy-Mediated, Surface Explosion Reactions: Tartaric and Aspartic Acid on Cu Surfaces Petro Kondratyuk,† Burcu Karagoz,† Yongju Yun,†,§ and Andrew J. Gellman*,†,‡ †

Department of Chemical Engineering and ‡W.E. Scott Institute for Energy Innovation, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States § Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

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S Supporting Information *

ABSTRACT: Surface explosion reactions have highly nonlinear reaction kinetics that exhibit autoacceleration under isothermal conditions. These can lead to phenomena such as oscillatory surface reaction rates and to highly enantiospecific reactions of chiral adsorbates on chiral surfaces. Tartaric acid (TA) decomposes on Cu surfaces by an explosion mechanism that is propagated by vacancies, empty adsorption sites that self-replicate autocatalytically during TA decomposition. Surface explosion kinetics result from chain-branching steps in which one vacancy decomposes an adsorbate to yield two vacancies. In the absence of vacancies, surface explosions cannot occur; they require some initiation step that creates vacancies. By comparison with the chain-branching explosion step, little is known about the processes that initiate or nucleate surface explosion reactions. Time-resolved XPS measurements during the early stages of explosion initiation of TA/Cu(hkl) reveal a process that involves direct loss of TA from the surface to create the initial vacancies. In the presence of a gas phase flux to the surface, such vacancy nuclei can be repopulated to suppress the onset of explosion. Measurements on 18 different Cu(hkl) surface orientations demonstrate that the kinetics of the initiation process are structure-insensitive. This implies that the highly enantiospecific TA decomposition kinetics observed on chiral Cu(hkl) surfaces must arise from the structure sensitivity of the chain-branching explosion kinetics.

1. INTRODUCTION

position is also probably relevant to the explosive decomposition of Asp on Cu(hkl) surfaces. We focus on the vacancy-mediated explosion mechanism in which an adsorbed reactant decomposes to yield gas phase products but requires the presence of adjacent vacancy (empty adsorption site, *) for decomposition to occur, generically for decomposition of adsorbate A−B

Among the many types of surface reaction mechanisms, there are a number that have the characteristics of autocatalytic explosions, that is, they have rates that increase with the extent of reaction, even under isothermal conditions. These have interesting, nonlinear rate laws that have important implications for surface reaction kinetics. Relevant to the reactions presented herein, the nonlinear kinetics of surface explosions result in highly enantiospecific reactions of chiral compounds adsorbed on naturally chiral surfaces.1−4 For example, the decomposition rates of tartaric acid enantiomer (TA, HO2CCH(OH)CH(OH)CO2H) on Cu(643)R&S surfaces differ by a factor of ∼50 (the convention used to assign handedness to intrinsically chiral surfaces is found in Supporting Information Section S1).1,5,6 Aspartic acid (Asp, HO2CCH(NH2)CH2CO2H) also exhibits highly enantiospecific decomposition kinetics on Cu(643)R&S.3 One of the interesting and poorly understood issues of surface explosion reactions is the nature of the processes that lead to their nucleation or initiation prior to the onset of autocatalytic explosion. The work presented herein presents the first direct observation of the initiation process for a vacancy-mediated surface explosion reaction. This has been detected during the decomposition of TA on 18 different Cu(hkl) surfaces vicinal to the Cu(100) plane. Given the previously observed similarities between the decomposition of TA and Asp on Cu surfaces,7,8 much of what we learn about TA decom© XXXX American Chemical Society

ke

A−B* + * → A (g) + B(g) + 2*

(1)

The corresponding rate law for such a process is given by r=−

dθAB = ke·θAB(1 − θAB) dt

(2a)

where θ AB is the fractional surface coverage of the decomposing adsorbate, and ke is the rate constant for the vacancy-mediated explosion reaction step. During autocatalytic explosion, A−B decomposition at one vacancy yields two vacancies, which can then react with two A−B to yield four vacancies and then eight, sixteen, and so on, resulting in an exponential increase in the vacancy coverage with time (extent of reaction) and an autocatalytic increase in the reaction rate, even under isothermal conditions. This is exactly analogous to the kinetics of a gas phase, radical explosion mechanism in Received: April 25, 2019 Revised: May 30, 2019 Published: June 27, 2019 A

DOI: 10.1021/acs.jpcc.9b03895 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

and kinetics of the vacancy-mediated surface explosion mechanism. One of the obvious short comings of the reaction rate law expressed by eq 2a is that for an initial coverage of θAB(0) = 1, the vacancy coverage is 1 − θAB(0) = 0, and the reaction cannot occur at any temperature. It requires some initiation process to create the initial vacancies. Early descriptions of vacancy-mediated explosion kinetics suggested that the reaction is nucleated at defects on the surface or in the adsorbed monolayer.11,13,38 The nature of these initiation sites is not well defined but they are postulated to act as initial vacancies. The defect coverage has been incorporated into the reaction rate law in the form of a constant, f.

which the radical branching steps are equivalent to the autocatalytic production of vacancies in eq 1.9 In an adsorbed layer with a high coverage of the reactant, θAB ≈ 1, the initial coverage of vacancies is θ* = 1 − θAB ≈ 0. As a result, the rate of the explosion reaction is infinitesimally low at coverages approaching saturation. In order for the reaction to be initiated, there must be a process prior to the onset of the chain-branching step that leads to the initial formation of vacancies in the adsorbed layer. Vacancy-mediated surface reactions were first observed and recognized as surface explosions by Madix et al. in the course of their studies of the thermal decomposition of formic and acetic acid on Ni surfaces.10−13 Since then, surface explosions have been observed during decomposition of a number of adsorbates on well-defined single-crystal surfaces and on dispersed catalysts.7,14−21 A key characteristic of a surface explosion reaction is that the rate accelerates with the extent of reaction, even under isothermal conditions.7,11 Equivalently, during a standard temperature-programmed reaction (TPR) experiment starting with a high adsorbate coverage, decomposition and product desorption occur over extremely narrow temperature ranges. In the case of TA decomposition on Cu(110), starting with a saturated monolayer coverage, θTA(0) ≈ 1, and heating at 1 K/s results in a CO2 product desorption peak at 499 K with an extremely narrow fwhm < 1 K.7 Another common feature of vacancy-mediated surface explosion reactions is that the product desorption peak temperature increases with increasing initial coverage of the adsorbed reactant. As a consequence, the product desorption rates undercut one another at low temperatures. This is indicative of a reaction rate that decreases with increasing θTA. This arises simply from the fact that the initial vacancy coverage, 1 − θTA(0), decreases as θTA(0) increases. These characteristic features of vacancy-mediated decomposition kinetics all arise from the highly nonlinear nature of the reaction rate law (eq 2a−2e). One recently observed consequence of the nonlinear kinetics of the vacancy-mediated explosion mechanism is that they result in highly enantiospecific reaction kinetics for chiral adsorbates on chiral surfaces.1−4,7,8 Chiral single-crystal metal surfaces can be created by preparing surfaces cut along low symmetry directions of the achiral bulk crystal lattice such that there are no bulk mirror planes oriented perpendicular to the surface.6,22,23 There have been a number of observations of enantiospecific reaction kinetics and adsorption equilibria on these surfaces.5,24−34 However, their enantioselectivity is often limited by the fact that the differences between the reaction energetics of enantiomers adsorbed on chiral surfaces are only on the order of a few kJ/mol.24,25,29,30 The nonlinear kinetics of the vacancy-mediated surface explosion mechanism create an opportunity to use autocatalysis as a means of amplifying small differences in enantiospecific rate constants into large differences in enantiospecific reaction rates. Raval et al.21 and Ernst et al.19,20,35,36 first reported that TA and related molecules decompose on Cu(110) via a surface explosion mechanism. Based on those initial observations, Gellman et al. demonstrated that on natural chiral Cu surfaces, one observes high enantiospecific decomposition rates.1−3,7,37 The relative rates of R,R- and S,S-TA decomposition differ by a factor of ∼50 on the Cu(643)R&S surfaces, more than an order of magnitude higher than any previously observed enantiospecific surface reaction kinetics. Understanding the origins of this enantioselectivity requires an understanding of the mechanism

r=−

dθAB = ke·θAB(1 − θAB + f ) dt

(2b)

Numerically, this serves the purpose of eliminating the issue of having r(θAB = 1) = 0, independent of the temperature. Having θAB = 1 initially and f = 0 suggests that the temperature can increase indefinitely without initiating decomposition. An alternative view of the explosion initiation is that it occurs via some chemical process, rather than the presence of some small number of initiation sites.1,7 In this case, the reaction rate law would be written as r=−

dθAB = k i·θAB + ke·θAB(1 − θAB) dt

(2c)

introducing a rate constant for initiation, ki, but obviating the need to invoke the existence of an initiation site. This work discriminates between the existence of defects as initiation sites and the possibility of an initiation step in the reaction. Both TA and Asp are well documented to undergo decomposition on Cu surfaces via a vacancy-mediated explosion mechanism.1,3,7,8,20,21,25,35,37 When adsorbed from the vapor phase onto Cu surfaces at 400 K, both form saturated monolayers of diacids in their singly or doubly deprotonated forms.3,21,39,40 During heating, they undergo explosive decomposition over very narrow temperature ranges (fwhm = 1−5 K) with peak temperatures in the range 480− 520 K depending on the adsorbate and the Cu(hkl) surface orientation. TA decomposes into CO2, CO, H2, and leaves some residual C and O on the surface. Starting as doubly deprotonated aspartate, Asp decomposes stoichiometrically into 2 CO2, H2, and NCCH3. Isotopic labeling has been used to demonstrate the exact sequence of bond cleavage steps in adsorbed Asp.3 In addition, fitting of rate laws to the TA and Asp decomposition kinetics during both constant rate heating and isothermal heating has suggested that the effective rate law is7,8,41 r=−

dθTA = k i·θTA + ke·θTA(1 − θTA )2 dt

(2d)

which implies that decomposition of TA and Asp requires two adjacent vacancies rather than just one. Herein, we provide evidence that in the case of TA decomposition on Cu surfaces, there is an observable chemical initiation process that occurs prior to the onset of explosive decomposition, and we provide insights into the nature of this initiation process. The only existing insight into the explosion initiation process comes from a combined TPR and STM study of TA/ Cu(110).7 In that study, TPR was used to monitor the appearance of CO2 during isothermal decomposition of TA at B

DOI: 10.1021/acs.jpcc.9b03895 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 440 K. Figure 1 shows that with an initial TA coverage of θTA = 1, one waits for ∼300 s at 440 K before observing any sign of

presented herein uses time-resolved X-ray photoemission spectroscopy (XPS) to monitor the initiation process in realtime on 18 different Cu(hkl) surfaces vicinal to the Cu(100) plane and demonstrates that initiation results in stoichiometric loss of TA from the surface.

2. EXPERIMENTAL SECTION 2.1. XPS of TA/Cu(100) Decomposition. Time-resolved XPS was used to monitor the kinetics of TA decomposition on 18 Cu(hkl) surfaces vicinal to the Cu(100) plane. The sample was a 10 mm diameter Cu(100) single-crystal shaped and polished to form a spherical dome with a 21 mm radius of curvature and the Cu(100) plane at its center. The surface of this sample, thus exposes all crystallographic surface orientations within 14° of the Cu[100] direction. To put this in perspective, the step density varies from zero at the center of the crystal to 1.2 nm−1 at a radial distance of 4.5 mm from the Cu(100) pole. Moving azimuthally around the center at a constant radius exposes surfaces with (100) terraces separated by monoatomic steps with all possible step-kink structures based on (111) and (110) microfacets.42 The sample is referred to as a Surface Structure Spread Single Crystal (S4C) using the notation Cu(100) ± 14°−S4C. Surface preparation and analysis was conducted in a ThermoFisher ThetaProbe operating at a base pressure of 10−9 Torr. This apparatus allows heating of the Cu(100) ± 14°−S4C sample, Ar+ ion sputtering, and surface exposure to vapors of TA from a sublimation source. Once prepared, the sample can be analyzed using XPS at any point across its surface and while heating. The Cu(100) ± 14°−S4C surface was cleaned by cycles of Ar+ sputtering followed by annealing at 900 K for 5 min. XPS was used to verify surface cleanliness. S,S-TA was sublimated at 390 K for 10 min and adsorbed on the surface which was held at 400 K. This exposure was sufficient to deposit a saturated monolayer of TA across the surface, and the sample temperature of 400 K ensured that a multilayer of the adsorbed TA did not form. At 400 K, the TA adsorbs in its singly deprotonated monotartrate form (−O2CCH(OH)CH(OH)CO2H) and does not begin to decompose.21,40 The ThetaProbe has a monochromated Al Kα X-ray source that can be focused to generate X-ray spot diameters in the range 30−400 μm. XPS spectra were obtained from discrete 400 μm diameter spots positioned across the Cu(100) ± 14°− S4C surface. The range of surface orientation angles spanned

Figure 1. Isothermal TPRS of TA decomposition to CO2 on Cu(110) at 440 K with accompanying STM images of the surface during the initiation process.5 The initial fractional coverage was θTA = 1. The TPR spectrum reveals a period of ∼300 s during which there is no detectable appearance of CO2. The STM images were obtained by cooling to room temperature. The surface starts in the (4,1;2̅,4) phase at saturation coverage (absolute coverage is 0.278 TA/Cu-atom). The (2,1;4̅,0) phase [also denoted c(4 × 2)] with 0.250 TA/Cu-atom begins to appear after 50 s and is readily apparent after 150 s, well before the detectable onset of CO2 desorption at 300 s. Scale bars are 10 nm. Reprinted with permission from the Journal of Physical Chemistry C 117(15), 7577−7588. Copyright 2013 American Chemical Society.

CO2 desorption arising from TA decomposition. Later, there is an acceleration of the reaction rate, arising from the autocatalytic generation of vacancies in the TA overlayer. The only evidence of anything occurring on the surface during the initiation period spanning t < 300 s comes from STM images taken by intermittently quenching the surface temperature.7 They reveal that the TA overlayer changes structure from a (4,1;2̅,4) overlayer lattice with an absolute local coverage of 0.278 TA/Cu-atom (this is the saturation coverage, θTA = 1) to a (2,1;4̅,0) [also denoted c(4 × 2)] overlayer with 0.250 TA/Cu-atom. Those images imply that prior to the onset of the explosion reaction, some process is reducing the local coverage of TA by ∼10%. The work

Figure 2. XPS signals versus time at 433 K from TA/Cu(100) (taken at P1 on Cu(100) ± 14°−S4C in Figure 3). (A) O 1s and C 1s signals scaled by their relative sensitivity factors and plotted vs time. During initiation at t < 50 min, there is a slow decrease in θTA followed by a rapid decrease during explosive decomposition. (B) O/C stoichiometry during isothermal TPR at 433 K. Prior to explosion, the stoichiometry remains close to that of TA. Data collection time was 30 s/pt for O 1s and 120 s/pt for C 1s. C

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Figure 3. (A) Map of the 18 measurement points across the Cu(100) ± 14°−S4C sample. These are plotted against the polar angle with respect to the [100] direction and the azimuthal angle with respect to the [001] direction. (B) Plots of θTA(t) during isothermal TA decomposition at 433 K on the 18 points indicated on the map. The initial loss of TA by vacancy initiation is weakly surface-structure-dependent, while the explosion kinetics indicated by the slopes on the rapid decay curves are clearly structure-dependent. The TA coverage was obtained from O 1s XPS measurements using data collection for 5 s at each point on the surface.

by the spot was ∼1°. Once the surface had been prepared, O 1s and C 1s XPS spectra at ∼535 and ∼285 eV binding energies, respectively, were obtained as a function of time with the sample held at 433 K. Multiple XP spectra were obtained over periods of ∼120 min in order to monitor the evolution of the coverages of oxygen, θO(t), and carbon, θC(t), during isothermal TA decomposition at 433 K. 2.2. Steady-State Decomposition of Asp/Cu(111). The second type of experiment measured the kinetics of Asp decomposition on a flat Cu(111) surface in an ultrahigh vacuum chamber operating at a base pressure of 2 × 10−10 Torr. Asp was used because it decomposes stoichiometrically to 2 CO2 , NCCH3 , and 2 H2 without depositing contaminants on the surface.41 The sample surface was prepared using Ar+ sputtering while annealing at 900 K for 500 s. Within the chamber, the sample was positioned in front of an Extrel quadrupole mass spectrometer and in line of site with an Asp sublimation source.41 The surface was prepared by first adsorbing a saturated monolayer of Asp, θAsp(0) = 1, with the sample at 400 K. Then, the Cu(111) sample was heated at a constant rate in the presence of a constant flux of Asp, FAsp. During heating, the Extrel mass spectrometer was used to monitor the signal at m/z = 44 arising from CO2 generated by Asp decomposition on the surface.

stoichiometry of the surface species as a function of time. During the induction period, the O/C stoichiometry remains constant at O/C ≅ 1.5, the stoichiometry of TA. This suggests that the initiation process involves direct loss of TA from the surface. One possible initiation mechanism is the disproportionation of two singly deprotonated monotartrate species to yield an adsorbed doubly deprotonated bitartrate and a molecule of TA that desorbs rapidly into the gas phase leaving a vacancy in the overlayer. After the explosion process is complete at 60 min, the stoichiometry of the surface is O/C ≅ 0.5. These data provide the first insight into the chemistry of the initiation process leading to vacancy-mediated explosion of TA/Cu(hkl). Taking advantage of the unique Cu S4C surface available to us, we have examined the time-dependent coverages of TA on 18 different Cu(hkl) surfaces vicinal to the Cu(100) plane. Figure 3A shows a map of the points on the Cu(100) ± 14°− S4C sample at which time-dependent O 1s XP spectra were obtained. The Cu(100) ± 14°−S4C sample has four-fold symmetry about its center, and the data span one quadrant. In other words, they are a representative sampling of the entire set of surfaces exposed by Cu(100) ± 14°−S4C. It is worth noting that in this quadrant, the surfaces within the azimuthal angle range of 0−45° have S-chirality, while the surfaces outside this range have R-chirality.6,22 Figure 3B shows the TA coverages as a function of time, θTA(t), during isothermal decomposition at Tiso = 433 K on the 18 different Cu(hkl) planes. The values of θTA(t) were estimated from the O 1s XP spectra using θTA(t) = (I(t) − I(∞))/(I(0) − I(∞)). These data reveal two things of interest. The first is that the initiation process is observable and seems to occur with roughly the same kinetics on all 18 surfaces. In other words, the initiation kinetics are structure insensitive. The second point of interest is that the explosion process does seem to be structure sensitive in the sense that the slopes of the O 1s loss curves vary significantly among the 18 different surfaces sampled. Of course, the fact that we have already observed highly enantiospecific TA decomposition kinetics on chiral high Miller index surfaces means that the process must be structure-sensitive.1,7 3.2. Flux-Dependent Decomposition of Asp/Cu(111). Asp decomposes on the Cu surface via a vacancy-mediated explosive decomposition path with kinetics similar to those for TA decomposition on Cu (eq 2d).3,25,37,41 One of the key differences is that Asp decomposes stoichiometrically to CO2,

3. RESULTS 3.1. Isothermal Decomposition of TA/Cu(100). During isothermal TPR of TA and Asp on Cu surfaces, there is a long induction period before one observes the detectable appearance of decomposition products (Figure 1).1,3,7,41 Prior STM observations during decomposition of TA/ Cu(110) suggest that there is a process by which the local coverage of TA decreases, presumably resulting in the formation of vacancies.7 Figure 2A reveals direct, time-resolved XPS observations of the TA/Cu(100) decomposition process during isothermal heating at 433 K. For the first 50 min, there is a slow decrease of both the O 1s and the C 1s signals by about 20% of their initial values. The decrease in TA coverage during the induction period is roughly linear in time. This is followed by a rapid loss of TA by explosive autocatalytic decomposition into CO2, CO, and H2.1,7 The explosive decomposition process also results in the deposition of C and O containing fragments that remain on the surface until the experiment is terminated at 100 min. Figure 2B plots the D

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following the explosion peaks are flux-limited; therefore, the CO2 desorption rates at T > 540 K can be calibrated using the areas under the monolayer decomposition peaks to estimate the incident fluxes. The key point is that the temperatures of the explosion peak maxima, Tp, increase with increasing FAsp. This suggests that the initiation process is being retarded by replenishment of Asp from the incident flux. With increasing FAsp, one has to heat to increasing temperatures before the initiation rate exceeds the rate of Asp replenishment from the gas phase. Once the rate of initiation exceeds the rate of replenishment, the explosion process accelerates causing a rapid drop in θAsp which then settles at a low steady-state coverage of θAsp ≈ kaFAsp/ke.

H2, and NCCH3, leaving the surface clean, thereby enabling steady-state Asp decomposition on Cu in an incident flux of Asp, FAsp, from the gas phase.41 The expression for the rate of change of the Asp coverage in the presence of an incident flux from the gas phase is given by −

dθAsp dt

= −kaFAsp(1 − θAsp) + k i·θAsp + ke·θAsp(1 − θAsp)2 (2e)

where the first term describes the adsorption of Asp from the gas phase into empty adsorption sites on the surface. At high initial coverages and before the onset of the explosion, adsorption from the gas phase can balance the loss of Asp due to the initiation process. If the initiation mechanism involves loss of Asp from the Cu surface, as seems to be the case for TA/Cu(100), an incident flux of Asp would replenish the surface. In an isothermal TPR experiment such as that shown in Figure 1 for TA/Cu(110), the replenishment of Asp from the gas phase could maintain θAsp ≈ 1 indefinitely and prevent the surface explosion from being initiated. Alternatively, in a standard TPR experiment in which the surface is heated at a constant rate, an incident flux of Asp would cause the surface explosion to be retarded to higher temperatures than observed in the absence of flux from the gas phase, FAsp = 0. TPR spectra of Asp decomposition on Cu(111) in the presence of a constant flux of Asp from the gas phase demonstrate that the incident flux stabilizes the adsorbed Asp monolayer against explosive decomposition. Figure 4 shows

4. DISCUSSION The primary goal of this work was to provide new insights into the process that nucleates or initiates vacancy-mediated surface explosion reactions, at least in the cases of TA and Asp decomposition on Cu surfaces. Rather than invoking the existence of an intrinsic defect concentration, f, in the surface or the adsorbed monolayer as in eq 2b,11,13,38 we have suggested that explosion initiation be described as an independent chemical process that reduces the initial adsorbate coverage to θ < 1 prior to onset of the explosion (eq 2c).1,7 The data in Figures 2 and 4 provide the first direct observation or detection of this initiation process. 4.1. Initiation of Explosive TA Decomposition on Cu(hkl). Insights into the existence of an initiation process preceding vacancy-mediated explosion reactions was initially obtained from our STM study of the isothermal decomposition of TA/Cu(110).7 The bottom trace of Figure 1 shows the rate of CO2 desorption versus time during the isothermal decomposition of TA/Cu(110). For the first 300 s, there is no detectable CO2 desorption. Next, one observes an accelerating rate of TA decomposition to yield CO2 until ∼500 s, at which point the TA coverage, θTA, is depleted to the point that the rate starts to decrease back to zero. STM images taken during the first 150 s of the isothermal TPR reveal changes in the overlayer structure consistent with a slow decrease of θTA, but with no detectable desorption of CO2. The initial structure with a fractional coverage of θTA = 1 has a (4,1;2̅,4) unit cell and an absolute coverage of 0.278 TA/Cuatom. After 150 s, the overlayer is formed of co-existing domains of the (4,1;2̅,4) phase and a (2,1;4̅,0) phase [also called c(4 × 2)] with a local absolute coverage of 0.250 TA/ Cu-atom. It is worth pointing out that the TA coverage appears to remain fairly uniformly distributed across the surface other than the variations in local coverage between the two coexisting phases. This suggests that as vacancies are generated, they are diffusing rapidly to enable the formation of the lower density overlayer. The first, real-time observations of the loss of TA during explosion initiation have been obtained using XPS during the isothermal decomposition of TA/Cu(100) at 433 K (Figure 2). These show quite clearly that there is a loss of TA from the surface during the ∼50 min period prior to the onset of the explosive decomposition of the overlayer. During this initiation period, the stoichiometry of the overlayer remains O/C ≅ 1.5 indicating stoichiometric loss of TA from the surface. The onset of the explosion occurs once the TA coverage reaches θTA ≈ 0.8. Further insight into the nature of the initiation process comes from measurement of the time-dependent TA coverage

Figure 4. CO2 production from Asp decomposition on Cu(111) during heating at 0.5 K/s in fluxes of FAsp = 0.7, 1.0, 1.4, and 1.7 ML/ min. The peak temperatures for explosive decomposition increase with increasing FAsp. The steady-state rate of CO2 production at T > 530 K increases with increasing FAsp. The temperature-independent CO2 production rates at T > 530 K indicate that the decomposition rate is flux-limited.

four TPR spectra obtained by first saturating the Cu(111) surface with Asp, θAsp = 1, and then heating the surface at 0.5 K/s in the presence of four different incident fluxes of Asp, FAsp = 0.7, 1.0, 1.4, and 1.7 ML/min. Initially, the CO2 desorption rate from Asp decomposition is very low. At ∼510 K, there is an onset of CO2 desorption followed by an explosive CO2 desorption peak, after which the rate falls to a steady state. The areas under the four peaks are very similar and correspond to the amount of CO2 generated by decomposition of one monolayer of Asp. The steady-state decomposition rates E

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continuous flux of Asp from the gas phase, FAsp.41 Prior work has shown that both TA and Asp decompose on Cu(hkl) surfaces via vacancy-mediated explosion mechanisms with similar kinetics and rate law.1,3,7,8,19 One of the key differences is that TA decomposition leaves C and O contamination on the surface (Figure 2), whereas Asp decomposes stoichiometrically to CO2, H2, and NCCH3 leaving the Cu surface clean. During TPR of Asp/Cu(111), the peak CO2 desorption rate occurs at ∼518 K. Figure 4 demonstrates that, in the presence of an impinging flux of Asp, the CO2 desorption peak increases in temperature. This is consistent with an initiation process in which transient Asp vacancies are refilled from the gas phase. At low temperatures, the initiation process results in loss of Asp from the surface at a rate given by −ki·θAsp. This results in the formation of vacancies in the overlayer. Vacancy formation is countered by replenishment of adsorbed Asp from the gas phase at a rate given by kaFAsp(1 − θAsp). The adsorption rate constant, ka, is probably only weakly temperature dependent and the rate of adsorption is flux-limited. During heating, the vacancy formation (initiation) rate eventually exceeds the replenishment rate and the onset of explosive decomposition occurs thereafter. As the incident flux of Asp is increased, the temperature at which the initiation rate exceeds the replenishment rate also increases. This results in the observed increase in the peak explosion temperature with increasing FAsp (Figure 4). These data demonstrate that the initiation process can be reversed by adsorption from the gas phase. The observations for Asp decomposition on Cu(111) are consistent with those of a previous observation of R,R-TA decomposition on Cu(651)S.7 During isothermal heating of R,R-TA/Cu(651)S at 450 K, the peak explosion rate occurred after ∼370 s (similar to TA/Cu(110) in Figure 1). In a second experiment with R,R-TA/Cu(651)S, the temperature was cycled such that after each 100 s at 450 K, the sample was cooled at −1 K/s for 100 s, reheated at 1 K/s for 100 s, and held again at 450 K for 100 s. Under this cycling, the peak explosion rate was reached after the total time spent at 450 K reached 350 s. In other words, the time to reach the peak explosion rate is insensitive to the history of the temperature cycling. This revealed that, in the absence of a gas phase flux that can replenish the surface with TA, the initiation process is irreversible.

across 18 surface orientations vicinal to the Cu(100) plane (Figure 3). These surfaces have step densities as high as ∼1 nm−1 at the greatest radial distance from the Cu(100) pole, that is, structures with (100) terraces that are ∼4 close-packed rows in width separated by monoatomic step edges. The steps are formed by the intersections of (110) and (111) microfacets projecting out of the (100) terraces (Figure 5). Moving

Figure 5. Illustrations of the ideal Cu(510) and Cu(711) surfaces. These are the two achiral planes exposed at the edge of the Cu(100) ± 14°−S4C sample. They have structures based on (100) terraces separated by monoatomic steps edges: (110) steps on Cu(510) and (111) steps on Cu(711). The terraces are ∼1 nm wide. Toward the center of the Cu(100) ± 14°−S4C sample, the terraces become wider until the step density reaches zero. At other azimuthal angles around the perimeter of the Cu(100) ± 14°−S4C, the surfaces have (100) terraces separated by (110) steps with (111) kinks or (111) steps with (110) kinks.

azimuthally around the (100) pole, these 18 surface orientations span the entire range of possible kink densities along the step edges. As revealed in Figure 3, the onset of the explosion reaction always seems to occur at a TA coverage of θTA ≈ 0.8. The kinetics of the explosion step are clearly structure-sensitive in that the slopes of the θTA(t) curves exhibit significant variation among the different surface orientations. 4.2. Asp Decomposition on Cu(111). Our observation of stoichiometric TA loss from the Cu surface during the initiation process suggests that replenishment of TA from the gas phase might suppress the explosion reaction under isothermal conditions or retard its onset to higher temperatures during constant rate heating. This has been demonstrated through steady-state measurements of Asp decomposition on the Cu(111) surface in the presence of a

Figure 6. (A) Plot of χi2 averaged over the 18 points measured on the Cu(100)-S4C and calculated using the solution to eq 3 with m = 1 and n = 1.2−3.2. The minimum at n = 2 indicates that eq 2d yields the optimal fit to the TA decay curves. (B) Calculated solution to eq 3 with (m, n) = (1, 2) (i.e., eq 4) illustrating the shape of the predicted decay curve for TA/Cu(hkl). (C) Map of log10(χ2) vs −log10(ki) and −log10(ke) for fitting of eq 2c to the TA/Cu(100) decay curve measured at P1 on the Cu(100)-S4C. The elongated minimum region indicates that t(1/2) = π /2 k i· ke is predicted by the fit with high accuracy. F

DOI: 10.1021/acs.jpcc.9b03895 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 4.3. TA/Cu(hkl) Decomposition Kinetics. Finally, we address the structure sensitivity of the isothermal TA decomposition kinetics on Cu(hkl) surfaces. The θTA(t) decay curves obtained at Tiso = 433 K (Figure 3B) from the 18 different surfaces vicinal to the Cu(100) plane (Figure 3A) clearly reveal structure sensitivity. Prior studies of the decomposition of TA/Cu(110) and Asp/Cu(100) have suggested that among rate laws of the form r=−

dθ = k i·θ m + ke·θ(1 − θ )n dt

initiation, that is, the coverage at the onset of the explosion. Equation 4 allows us to determine the critical time at which the explosion onset occurs. tc =

(3)

ln(2) 2ke

(7)

5. CONCLUSIONS Isothermal XPS measurements have provided the first insights into the nature and kinetics of the processes that lead to vacancy-mediated surface explosion reactions. In the cases of TA and Asp decomposition on Cu surfaces, initiation involves the loss of adsorbate in its molecular form. This process can be reversed by continuous replenishment of adsorbate from the gas phase. The surface initiation process proposed herein replaces the previously postulated existence of static defects in the surface or adsorbate layer that impose an initial vacancy coverage, even at adsorbate coverage of θ = 1. The kinetics of the initiation process are insensitive to the surface structure. This implies that the origin of the extremely high enantioselectivity observed in the explosive decomposition on TA and Asp on naturally chiral Cu surfaces lies in the structure sensitivity of the explosion rate constant, ke.

(4)

The form for this equation is shown in Figure 6B and clearly reveals the slow initial initiation of vacancies followed by a rapidly accelerating decay of θTA. As illustrated in Figure 6B, eq 4 allows various features of the θ(t) decay curve to be expressed directly in terms of the rate constants ki and ke. For example, the time to reach a coverage of θ = 1/2 is given approximately by π t1/2 = t(1/2) = 2 k i·ke (5)

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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.9b03895.

(see the Supporting Information, Section S3 for proof), and the slope at t1/2 is given by = −ke/8 t1/2

2

+

The simple relationship between t1/2 and ki·ke (eq 5) is particularly useful because t1/2 can be used as a measure of the enantiospecificity of the decomposition kinetics by comparing decomposition of D- and L-TA on the R- or S- enantiomers of L/R D/R D/S L/S D/S a Cu(hkl)R&S surface; tD/R 1/2 − t1/2 = t1/2 − t1/2 ≠ t1/2 − t1/2 = L/S L/R t1/2 − t1/2 . More importantly, the value of t1/2 can be estimated with particularly high accuracy from our data. Figure 6C shows the value of the sum of squared errors, χ2, between the experimental data, θexp(ti) at point P1, and the model values predicted by eq 4, θmdl(ti). The value of log10(χ2) has been plotted versus −log10(ki) and −log10(ke) and shows a clear minimum along a trough that is defined by log10(ki) + log10(ke) = c or equivalently ki·ke = 10c. This behavior of χ2 is also observed for the other 17 surface orientations studied in this work. This means that errors in the individual estimates of ki and ke compensate such that ki·ke can be determined with high accuracy. Equivalently, this implies that fitting of the data will yield accurate estimates of t1/2 and its dependence on the surface structure. Our current understanding of the isothermal TA decomposition kinetics will serve as the basis for analysis of the decay curves, θhkl TA(t), obtained at 169 different points across each of the surfaces of a set of six Cu(hkl)-S4C samples that collectively span the entire stereographic projection. This will yield the most comprehensive understanding of surface structure-sensitive reaction kinetics yet obtained.

The best fit to experimental measurements is found with (m, n) = (1, 2). It is probably the case that fits to the data are not very sensitive to the value of m = 1, because the initiation occurs over a small range of θ = 1.0−0.8. Figure 6A shows the value of ⟨χi2⟩, the average value of the minimum squared error obtained from fitting the 18 TA decay curves (Figure 3B) using the rate law expressed by eq 3 with m = 1 and allowing n to vary between 1.2 and 3.2. The minimum in ⟨χi2⟩ at n = 2 clearly indicates that rate law 2d is well suited to describing the TA decay kinetics across Cu(hkl) surfaces having many different structures. The value of the exponent n = 2 on the explosion term suggests that two adjacent vacancy sites are needed for the branching step in the explosive decomposition of TA and Asp. Different values of ki and ke were found for each of the 18 surfaces indicated in Figure 3. The discrete values associated with each point on the crystal surface are plotted in the Supporting Information, Section S2 (Figure SI2). The ranges for the values of the rate constants are 105·ki = 1 to 6 s−1 and 102·ke = 0.3 to 3 s−1·MLv−2 where MLv is the vacancy coverage. The key point is that ke ≫ ki in all cases. A number of ordinary differential equations of the form given by eq 3 have analytical solutions of the form t = f(θ) that are parametrically dependent on the rate constants ki and ke. The solution to the case with (m, n) = (1, 2) is given by ÄÅ ÉÑ ÉÑ Å Ñ 1 ÄÅÅ ÑÑ k iθ 2 ke ÑÑ ·arctanÅÅÅÅ ke (θ − 1)ÑÑÑÑ + 2 ·lnÅÅÅÅ 2 ki ÅÅÇ k i ÑÑÖ ÅÇ ke(θ − 1) + k i ÑÖÑ t (θ ) = − (ke + k i)

dθ dt

t1/2

(6)

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The maximum rate of decomposition corresponding to the peak desorption temperature observed during isothermal TPR (Figure 1) occurs at a coverage of θTA = 1/3. Given rate eq 2d, one can define a critical TA coverage, θc = 1 − ki/ke, at which the rate of the explosion reaction becomes equal to the rate of

Convention for nomenclature assigning handedness of intrinsically chiral surfaces; ki and ke versus Cu(hkl) surface orientation; and analytical expressions derived from the explosion rate law (PDF)

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DOI: 10.1021/acs.jpcc.9b03895 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C ORCID

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Burcu Karagoz: 0000-0002-1019-051X Yongju Yun: 0000-0001-6497-4128 Andrew J. Gellman: 0000-0001-6618-7427 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the US National Science Foundation under grant number NSF CHE1764252.

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DOI: 10.1021/acs.jpcc.9b03895 J. Phys. Chem. C XXXX, XXX, XXX−XXX