Adsorption and Hydrogenation of CO2 on Rh Nanosized Crystals

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Adsorption and Hydrogenation of CO on Rh Nanosized Crystals: Demonstration of the Role of Inter-Facet Oxygen Spillover and Comparative Studies with O, NO and CO 2

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Sten Vinicius Lambeets, Cédric Barroo, Sylwia Owczarek, Luc Jacobs, Eric Genty, Natalia Gilis, Norbert Kruse, and Thierry Visart de Bocarmé J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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The Journal of Physical Chemistry

ADSORPTION AND HYDROGENATION OF CO2 ON RH NANOSIZED CRYSTALS: DEMONSTRATION OF THE ROLE OF INTER-FACET OXYGEN SPILLOVER AND COMPARATIVE STUDIES WITH O2, N2O AND CO

Sten V. LAMBEETS1,*, Cédric BARROO1,2, Sylwia OWCZAREK1,3, Luc JACOBS1, Eric GENTY1, Natalia GILIS1, Norbert KRUSE4, Thierry VISART DE BOCARMÉ1,2,* 1

Chemical Physics of Materials and Catalysis, Université libre de Bruxelles, CP243, 1050

Brussels, Belgium 2

Interdisciplinary Center for Nonlinear Phenomena and Complex Systems (CENOLI),

Université libre de Bruxelles, 1050 Brussels, Belgium 3

Institute of Experimental Physics, University of Wrocław, Wrocław, Poland

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Voiland School of Chemical Engineering and Bioengineering, Washington State University,

Pullman, WA 99164, USA

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ACS Paragon Plus Environment


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ABSTRACT In this work, we investigate the adsorption of carbon dioxide on rhodium (Rh) nanocrystals as well as its catalytic reaction with hydrogen, at the nanoscale, using field ion microscopy (FIM), video-field emission microscopy (FEM) and 1-Dimensional Atom Probe (1DAP). A FEM pattern-and-brightness analysis during the ongoing dissociation process at 700 K provides information on various facet reactivities and how these facets communicate with each other. Our results show CO2 dissociative adsorption to be fastest on {012} facets. Initially dark {113} facets transiently appear bright and we suggest this behavior is due to subsurface oxygen states occupied via spillover from {012} facets. Although local surface reconstructions of individual Rh facets may likewise be encountered, they fail to explain the sequence and time dependence of the observed FEM pattern-and-brightness changes. CO2/H2 co-adsorption studies suggest surface and subsurface oxygen can be reacted off as water. The observations are discussed within the context of the reverse water gas shift reaction. Comparative FEM studies are performed with other O-containing molecules. While the adsorption of N2O and O2 leads to similar FEM pattern-and-brightness changes on an otherwise different time scale than those of CO2, non-dissociative CO adsorption does not produce any noticeable such changes. We conclude that the mechanism of inter-facet communication involving subsurface oxygen states is of general importance in reaction studies with oxygen-containing molecules undergoing surface dissociation.

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I. INTRODUCTION Supported nanoparticles of platinum group metals (PGM), e.g. platinum, rhodium and palladium, are commonly used as catalysts in automobile exhaust treatment to turn toxic gases, such as nitrogen oxides (NOx), carbon monoxide (CO) and unburnt hydrocarbons (CxHy), into harmless N2, H2O and CO2. Similar to methane (CH4) and nitrogen oxides (NOx), carbon dioxide (CO2) is a most problematic greenhouse gas1,2, an issue that is reflected in an increasing number of legislative regulations. However, in any combustion of carbon containing molecules, CO2 production remains unavoidable. Despite many efforts of producing lowconsumption combustion engines or of developing battery or fuel cell-driven vehicles, it is impossible to sustain the constant growth of the automotive market without accepting that CO2 is being formed. Therefore, the CO2 concern needs a multi-path strategy to reduce its impact, which includes the decrease of CO2 emissions, its capture3,4 and its revalorization5. The latter can lead to the formation of useful products, and may then satisfy both environmental and economical demands. The catalytic hydrogenation of CO2 is an example that can lead to the selective formation of methanol or formic acid, which in turn are used in fuel cells, or as building blocks in the production of more complex compounds6–8. To design catalysts with high activity and selectivity, a deep understanding of the catalytic processes is required down to the molecular level. In particular, it is important to characterize the shape of the catalytic nanoparticles, their size distribution, as well as their local composition at different stages of the reaction, so as to provide an understanding of the influence that these parameters have on the catalytic activity. Moreover, a proper catalyst design is needed as long as valuable materials like PGM, including Rh, are used. Despite the high costs of PGM, their superior catalytic properties at low temperature still make them indispensable. Early studies by the Somorjai group9–13 reported CO2 dissociation on a Rh foil and on several Rh single crystal planes above critical CO2 exposures. Contrarily, Weinberg14, on the

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basis of thermodynamic and kinetic arguments available at that time, argued that the CO2 sticking probability at low pressures and room temperature would be by many orders of magnitude too small to observe dissociation using surface analytical techniques like vibrational and thermal desorption spectroscopy (TDS). When analyzing kinetic data for the CO2 hydrogenation over Rh(111), Goodman et al.15 determined a value of 10-11 for the CO2 dissociation probability at 300 K. Using Field Electron Microscopy (FEM), the Nieuwenhuys group16–18 demonstrated CO2 adsorption and dissociation to depend on the crystallography of the surface facets exposed by the Rh field emitter tip. By comparing FEM patterns with TDS spectra, a mechanism was proposed for the dissociative adsorption of CO2 on Rh nanocrystals16,18. In the present work, we use field emission techniques to study the interaction of CO2 and the CO2+H2 reaction occurring at the extremity of a sharp Rh tip sample. We also compare the CO2/Rh and CO2+H2/Rh systems with N2O, O2 and CO adsorption and hydrogenation in order to highlight similarities and differences in the reactivity of these O-containing molecules. The field emission techniques used here comprise video-FEM, but also field ion microscopy (FIM) and 1-dimensional atom-probe (1DAP). FIM allows the direct observation of the sample structure with atomic resolution, whereas video-FEM is used to monitor reaction processes at the nanometer scale19. Using electrical field pulses, the 1-D atom probe20,21 allows identifying the reaction products of CO2 dissociation at the tip surface. Generally, the 3D morphology of a sharp metal tip as used with these field emission techniques mimics the shape of a real catalyst particle much more closely than an extended 2D crystal of one single orientation.

II. EXPERIMENTAL SETUP A complete description of the apparatus in which FEM and FIM experiments were conducted can be found elsewhere22. FIM and FEM are two techniques that characterize the structure of 4


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the surface of a conductive material prepared as a sharp tip with atomic (~ 0.2 nm) and nanometric (~ 2 nm) lateral resolution, respectively. The basic principle of the FIM method consists in the ionization of a gas molecule at the surface of the positively polarized sample. Once formed, the ions are accelerated towards a fluorescent screen. The surface of the tip is imaged with a magnification approximately equal to the ratio of the tip-to-screen distance and the tip radius. The resulting micrograph corresponds, in a first approximation, to the stereographic projection of the tip apex23. Cryogenic temperatures are required to achieve a lateral resolution of 0.2 nm. In the FEM mode, the sample is negatively polarized and electrons emitted from the sample are used to image the surface. According to the Fowler-Nordheim equation24,25, the emission of electrons depends on the work function, which is not homogeneous along the surface, but depends on the type of crystal facet and the possible presence of adsorbates16. For the present study, starting from a high purity wire (99.8 %, Ø ~ 0.127 mm), Rh tips are electrochemically etched in a molten mixture of NaCl and NaNO3 (1:4 w/w) at ~520°C, with a potential of 2 VDC. Before any adsorption or reaction, the samples are characterized by FIM. For this, a cleaning procedure is applied by in-situ cycles of field evaporation, Ne+ ion sputtering and sample annealing to desorb impurities from the surface of the sample26. The sputtering procedure is performed by applying a negative potential to the sample in presence of a pressure of 2.0×10-3 Pa of Ne at 300 K. The potential is then increased so as to reach an emission current of 3-4 µA. These exact conditions are adapted for each sample depending on the importance of the contamination and the presence of surface defects. This treatment is followed by thermal annealing, consisting of heating the sample at ~700 K for 2 hours. This procedure is performed inside the analysis chamber and the evolution of the surface is followed after each step by FIM imaging. Once the sample surface is smooth, with no structural defects appearing, the specimen is heated to the working temperature and imaged in

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the FEM mode. Reactive gases are then introduced in the analysis chamber. The gases of interest for this study are CO2 (purity: 99.996 %), O2 (99.998 %), N2O (99.4 %), CO (99.997 %) and H2 (99.9996 %). The partial pressures were measured by a Bayard−Alpert gauge. The values that we report take into consideration the gas correction factors. In the remainder of the manuscript, zero time corresponds to the time for which we have a response from the pressure gauges. The FEM patterns are recorded during the ongoing interaction with these gases and the brightness signals are extracted for specific surface regions of interest. If temperature and electric field are kept constant, variations of the brightness signals reflect the variations of the local work function. The latter is modified by the presence and the nature of adsorbates. A charge-coupled device Princeton Instruments high-sensibility camera is used to acquire the high-resolution FIM images, while a Panasonic Moonlight high sensitivity night vision camera is used to record the evolution of the brightness signals during the ongoing process. The brightness analysis is performed on digitized video captures with the open source software Tracker 4.8027, which plots the brightness over a defined region as a function of time. With this procedure, FEM can be used to obtain qualitative information on the presence of adsorbates; a quantification is possible using the Fowler-Nordheim equation. In this paper, brightness is expressed in arbitrary units corresponding to the grey levels measured on the detector and subsequently digitized in 8 bits format, thus from 0 to 255. To compare the catalytic behavior on different families of crystallographic orientations, the brightness is probed on different facets and their relative brightness signals are plotted as a function of time. In addition, a direct chemical analysis of the surface during reactive processes is performed with a one-dimensional atom probe (1DAP)28 using field pulses. The method is based on the field desorption-evaporation phenomenon, i.e. the repetitive rupture of the surface species as ions in the presence of a high electric field. The 1DAP is particularly suited for unravelling local kinetic parameters and for studying time-dependent surface processes occurring over only

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a few tens or hundreds of surface atoms28. Field pulses cause field desorption of the adsorbed layer. For kinetic studies, which are performed with high field pulse amplitudes, each pulse will stop the ongoing reaction and remove the entire adsorbed layer to restore a zero-coverage surface for the next reaction cycle. If complete field desorption by pulses is not obtained, the 1DAP analysis still provides a qualitative analysis of the surface composition and variations of the latter with varying reaction time. Ions are accelerated into a flight tube and their flight times are measured by a single ion detector. The time between field pulses can be adjusted between 100 µs and 10 s (corresponding to repetition frequencies between 10 kHz and 0.1 Hz, with a pulse width of ~ 100 ns). For each field pulse repetition frequency, mass spectra are obtained with a typical mass resolution of m/∆m ~ 100 at FWHM29. After introducing reactive gases into the analysis chamber, the Bayard-Alpert gauges are switched off to avoid any contaminations due to gas interaction with hot filaments.

III. RESULTS AND DISCUSSION We start this section of the paper by a careful characterization of a typical Rh sample in FIM. Next, we turn to the adsorption of different gases on Rh using the FEM mode of imaging. We discuss the adsorption of CO2 at suitable temperatures and compare it with that of O2, N2O and CO under similar experimental conditions. The interaction of CO2, O2 and N2O with hydrogen will be described with regard to the surface and desorption processes. The observations will be put into perspective with the oxygen-induced surface reconstruction and the formation of subsurface oxygen O(sub). Eventually, the occurrence of CO2 dissociation over a Rh sample surface will be analyzed using 1D atom probe (1DAP). Table 1 compiles chemical reactions that are to be considered for the present study. Their role in the different reaction scenarios will be examined in the following sections of the paper.

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Table 1. List of chemical reactions of relevance for this study. The symbol “S” corresponds to a surface vacant site, and the symbol “#” corresponds to a subsurface vacant site.

Eq. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Reaction

CO g + S ⇄ CO ads CO ads + S ⇄ COads + Oads CO2 dissociative adsorption CO g + 2 S → CO ads + Oads O g + surface ⇄ O pre∗ 30 O pre + 2 S ⇄ 2 Oads O2 dissociative adsorption O g + 2 S → 2 Oads N Og + S ⇄ N Oads N Oads ⇄ Oads + N g N2O dissociative adsorption N Og + S → N g + Oads CO adsorption COg + S → COads H g + S → H ads H ads + S → 2 H ads H2 dissociative adsorption H g + 2 S → 2 H ads Oads + Hads ⇄ OHads + S OHads + Hads → H Oads + S H2O formation from O(ads) provided by CO2 or O2 or N2O dissociative H Oads → H Og + S adsorption Oads + 2Hads → H Og + 3 S CO desorption COads → COg + S Reverse Water Gas Shift CO g + H g → CO g + H Og O2 Hydrogenation O g + 2H g → 2H O g N2O Hydrogenation N Og + H g → N g + H Og 2 Oads ⇄ O ads + S O ads ⇄ O g + S O2 Recombination 2 Oads → O g + 2 S O(sub) formation Oads+⋕ ⇄ Osub + S COads + Oads ⇄ CO ads + S CO ads ⇄ CO g + S CO2 recombination COads + Oads → CO g + 2 S * O2(pre) denotes the adsorption of an oxygen molecule and its precursor state

3.1 Characterization of the Rh samples Samples used in this study are first characterized with high resolution FIM. A typical clean Rh sample is shown in Figure 1.a. Due to the quasi-hemispherical shape of the sample, facets of different crystallographic orientation are exposed and identified by their Miller indices. The 8


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size of the sample can be estimated by counting the number of atomic layers between two facets of known orientation23. A tip radius of curvature of about 26 nm is obtained this way. Figure 1.b corresponds to the same sample imaged in FEM mode at 60 K. FIM and FEM modes produce micrographs of identical magnification. Despite the lower spatial resolution of FEM, it is still possible to identify the facets by superimposing a FIM image to its corresponding FEM pattern. The (001) pole is defined by its four-fold symmetry in FIM as well as FEM. More open surface structures, such as {012}, appear brighter in FEM whereas the central (001) and {111}, {011} and {113} facets remain dark with this mode of imaging. The sample thus exposes a variety of facets allowing catalytic processes to be followed on different facets at the same time. During FIM and FEM dynamic imaging, an electric field is applied, with typical values of 1-3 V.nm-1 during FEM (see Figure 1-2). While field-induced bond slitting at these field strengths is negligible, reorientation processes with respect to the electric field vector may be encountered31. On the other hand, due to the absence of a permanent dipole moment in the CO2 molecule, only polarization forces come into play. It would be interesting to theoretically investigate if the original D∞h symmetry can be turned into C2v so as to increase the molecule’s propensity to dissociate32, however, such an investigation is beyond the scope of the present paper.

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Figure 1: a) Atomically resolved field ion micrograph of a typical Rh sample showing several poles and facets with different crystallographic orientation. Conditions of acquisition: F=32 V.nm-1, T=60 K, PNe =1.12×10-2 Pa. b) Field electron emission pattern of the same rhodium sample. Conditions of acquisition: F=3.5 V.nm-1, T=60 K (residual pressure < 1.0×10-6 Pa).

3.2 Adsorption of O-containing molecules: CO2, O2, N2O and CO 3.2.1 FEM pattern formation during adsorption processes After FIM characterization, the sample temperature is increased up to 700 K while observing pattern formation in FEM. CO2, or any other gas out of the series of molecules used here, is then slowly introduced into the analysis chamber until variations of the brightness signal are observed. As stated above, the intensity of the brightness signal in FEM depends on the local current density during field emission. For a clean sample, the {012} facets appear brighter in FEM because of the lower work function as compared to other planes33. During CO2 exposure, the brightness of all {012} facets drastically decreases and reaches the background-noise level (decrease from 60 to ~16 units of brightness), as it is depicted in the brightness plot of Figure 2.a. This behavior reflects the dissociative adsorption of CO2 over these facets18, leading to the formation of O(ads) and CO(ads). The presence of O(ads) species increases the work 10


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function34,35 and the brightness signal decreases accordingly. On rhodium surfaces, the variation of the work function in the presence of CO(ads) causes the image to darken but this effect is smaller as compared to the O(ads) case16. Additionally, CO(ads) is expected to undergo desorption at the working temperature of 700 K, besides some dissociation on Rh(012)36. The adsorption of CO2 may be conceived of as the precursor of dissociation as described by Eqs.1-3 of Table 1 (Eq.3 is the combination of Eq.1 and Eq.2)37,38. Figure 2 compiles the brightness evolution upon adsorption of CO2, CO, O2 and N2O in panels a, b, c and d, respectively. Time series of the local brightness of {012} and {113} facets are shown in plots as a function of time. These plots demonstrate that, during CO2 exposure, the brightness of the {012} facets decreases slowly, then rapidly. Interestingly, the fast decrease of brightness is accompanied by a strong brightness increase of the {113} facets. A more detailed analysis of this remarkable FEM pattern transformation in Figure 2a allows four steps to be identified for the CO2 case (Figure 2.a):

Step 1, between 0 and 9 s (Figure 2.a at 0 second): the brightnesses of the four {012} facets surrounding the (001) pole simultaneously decrease. No visible change in the four {113} facet brightnesses is seen in this time range.

Step 2, between 9 and 11 s (Figure 2.a at 9 seconds): the {012} and {113} facets show a similar brightness, forming a “bright circle” around the central (001) pole.

Step 3, between 11 and 15 s (Figure 2.a at 15 seconds): {012} facets reach their lowest level of brightness while that of the {113} facets drastically increases (from 20 to 120 units of brightness).

Step 4, after 15 s, (Figure 2.a at 25 seconds): the brightnesses of the {113} facets decrease and the entire FEM pattern turns dark. Despite the progressive brightness decrease, the fading FEM pattern keeps {113} facets visible.

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We note that the duration of the steps can vary from one experiment to the other, with a maximum of 60 s to reach a full darkening of the FEM image; however, the pattern transformation and brightness evolution remain unchanged.

Figure 2: Brightness plots and FEM snapshots during adsorption of a) CO2, b) CO, c) O2 and d) N2O upon gas exposure at 700 K on a Rh tip sample. CO2, O2 and N2O exposures present similar FEM pattern evolutions, from bright {012} to bright {113} regions. During CO adsorption, the structure of the FEM patterns does not evolve into new ones and the only observation is a monotonous decrease of brightness. We assume that the presence of O(ads)-species is responsible for the transformation in experiments with CO2, O2 and N2O since little, if any, dissociation of the CO molecule is encountered on Rh. Conditions: a) CO2 exposure (conditions of acquisition: F= 1.8 V.nm-1, P = 2.0×10-4 Pa). Red 12


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and black circles on the FEM image correspond respectively to the {012} areas and {113} areas of brightness analysis b) CO exposure (F= 2.0 V.nm-1, Increasing of P between 0 s and 18 s from 3.2×10-4 Pa to 2.0×10-3 Pa). c) O2 exposure (F= 2.0 V.nm-1, P = 1.0×10-5 Pa). d) N2O exposure (F= 2.0 V.nm-1, P

= 1.1×10-4 Pa).

In order to identify the causes leading to the above FEM pattern transformations encountered during CO2 adsorption and reaction, a comparative study was conducted using other oxygen containing gases: O2, N2O and CO. This methodological approach was applied using the same Rh sample in FEM mode, at 700 K and with an electric field of 2.0 V.nm-1. We note that all experiments reported here are remarkably reproducible; either using tip samples having undergone a number of treatments or using freshly prepared ones. As shown in the different brightness plots and respective FEM micrographs, a similar pattern transformation is being observed during N2O and O2 exposure (Figure 2.c and 2.d respectively) while – as mentioned – no such scenario applies to the CO case (Figure 2.b). For a better clarity and considering that all facets of the same crystallographic family evolve in an identical manner with the same time dependence, we present the averaged normalized brightness plots for the four {012} and the four {113} facets during O2 and N2O and CO exposure at pressures up to 1×10-3 Pa. Experiments did not reveal any FEM pattern transformation, except the expected brightness decrease during CO adsorption (see Figure 2.b)39. We note that during O2 exposure at a pressure of 1×10-5 Pa, the intense brightness signal originating from the {012} facets partly overlaps the {113} regions. However, oxygen causes the same FEM pattern transformations, as does CO2 (Figure 2.c). The O2/Rh system thus follows the same four-step process on a time scale which is shorter than that of the CO2/Rh system, although O2 pressure is 10 times lower than that of CO2. For example, the brightness decrease observed for the {012} facets is much faster (in less than 4 s), the “bright circle” (Figure 2.c at 4 seconds) is present for less than 1 s, and, finally, the {113} pattern does not remain bright for more than 2 s before turning dark 13



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(not shown in Figure 2). Regarding the N2O case (at a pressure of 1×10-4 Pa), the experiments show a situation where the {113} FEM patterns appear rather stable once being formed after about 5 s of gas exposure (Figure 2.d). The gas pressure required to provoke FEM pattern changes is two times lower than for the CO2 case, and the first two steps of the transformation process occur slightly faster than for O2 and considerably faster than for CO2. The observed dependence on the gas dose (pressure * time) is in line with the sticking probabilities of the various gases40,41. Accordingly, while the sticking probability of CO2 is low on Rh surfaces12, those of O2 and N2O are much higher (close to 1 for O241 and 0.5 for N2O42). The fast pattern evolution and transformation processes for O2 (on the basis of the gas dose dependence) is reinforced by the fact that two adsorbed oxygen atoms30,43, O(ads), are formed per oxygen molecule while only one such O(ads) is formed from the dissociation of a CO2 or N2O molecule37,38,44. In the latter case, N2 molecules are immediately desorbed (Eqs. 8, 9 in Table 1), while in the case of CO2, the desorption of adsorbed CO is slower due its higher binding energy45,46. Although the causes and nature of the FEM pattern transformations will be further discussed below, it seems at this point that the formation of O(ads), originating from the dissociative adsorption of CO2, N2O and O2 (Table 1 Eq.4-6 and Eq.7-9) is the key to explain our results. Since CO(ads) does not easily dissociate on Rh, no significant FEM pattern transformation is observed (Table 1 Eq.10)36,37,47.

3.2.2 Direct evidence for CO2 dissociative adsorption in the 1-Dimensional Atom Probe In the case of CO2 adsorption, the measurements are performed at 325 K. The probe hole of the 1DAP covers the (115) facet and its vicinals. The number of probed Rh surface atoms equals about one hundred. The (115) facet is located between the (113) plane and the central (001) pole. Periodic pulsed field desorption/evaporation is achieved while continuously exposing the sample to CO2 gas at 4.5×10-3 Pa. The mass spectrum proves that CO2

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dissociation occurs (Figure 3b). At a reaction time of 1 ms between field pulses (corresponding to 1 kHz pulses), CO2+, Rh+, Rh2+ and low amounts of CO+ species are detected. When increasing the reaction time to 10 ms at the same temperature, CO2 dissociation products show up in the spectra and are identified as CO+ and O+. Rhodium oxides, RhO2+ and RhO22+, are also detected. This result demonstrates CO2 dissociation to occur with a characteristic time constant of 1 to 10 ms on Rh (115) at 325 K. The observation that O(ads) is field desorbed/evaporated as Rh-oxide ions is well established from previous PFDMS studies of oxygen adsorption on Rh48,49.

Figure 3. a) Mass spectra obtained by probing the Rh(115) facet during the exposure to CO2 (4.5×103

Pa) at 325 K with a period between pulses of 1 ms (1000 Hz) b) same as a) with 10 ms (100 Hz

pulses). The first spectrum a) shows that after 1 ms of exposure, CO2 adsorption occurs without dissociation, whereas the second mass spectrum b) shows such dissociation to occur within 10 ms of

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CO2 exposure. Indeed, both oxygen-derived species O+ (16 atomic mass units (a.m.u.)), RhO+ (51.5 a.m.u.) and RhO22+ (67.5 a.m.u.) as well as molecular CO+ (28 a.m.u.) confirm this conclusion.

3.3 Hydrogenation of O-containing molecules The occurrence of CO2 dissociative adsorption on Rh calls for hydrogenation studies to be carried out. Pure hydrogen gas is introduced in the analysis chamber after the occurrence of CO2-induced FEM pattern transformation at 700 K and while keeping the CO2 pressure constant at this temperature. Although we vary the temperature from 500 K to 730 K, we focus here on reaction behaviors occurring at 700 K for a H2/CO2 pressure ratio of 1.7. According to Figure 4, an increase in brightness is observed after several seconds which can be attributed to the reaction between hydrogen H(ads) and O(ads) species. Furthermore, after extended times of exposure to H2 gas, the reaction restores the original FEM pattern where only {012} facets are visible (Figure 4). More specifically, once H2 is being introduced into the microscope chamber a drastic increase in brightness of the {113} facets is encountered in less than 4 s, as it can be seen in the two images of Figure 4 between 0 and 9 seconds. Note that the experiments start with a FEM pattern identical to that of the final step in Figure 2. To make the {113} facets visible, the field strength is increased, however. The {113} facets continue to appear bright for a couple of seconds until the FEM pattern changes to another one that is similar to that obtained for the clean sample, before exposure of gases. At that stage, {012} facets are most visible and dominate over all others (Figure 4 between ~17 and 28 s, in the presence of H2). The switch is accompanied by a substantial increase of brightness (from ~50 to ~230 units of brightness between ~12 to ~23 s time span). The observed FEM pattern and brightness changes in the presence of H2 gas demonstrate a reaction between O(ads) and H(ads) occurs. (Table 1 Eq.11-13)30. The reaction is supposed to form water, H2O(ads)30 (Table 1 Eq.14-17), which is known to decrease the work function of clean Rh50. The reversibility of the observed

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phenomenology can also be demonstrated by interrupting the hydrogen supply at 28 s, which results in a FEM pattern turning back to its {113} dominated form.

Figure 4: Brightness plots and FEM micrographs during CO2 hydrogenation at 700 K over a Rh tip sample. CO2 exposures present the same FEM pattern evolution, and the addition of H2 to either gas provokes the switch towards the initial FEM pattern. Conditions of acquisition: steady P = 2.0×104

Pa; P# = 3.4×10-4 Pa addition after 8 s for 20 s until 28 s, F= 2.0 V.nm-1, T= 700 K. Field emission

micrographs illustrate the different stages before, during and after H2 exposure. The different grey regions correspond to the different steps of the process, which are illustrated with corresponding FEM images. The equivalent experiment made with O2 and N2O gases instead of CO2 gas are shown in Figure S1.

The assumption of surface reactions between O(ads) and H(ads) being responsible for our observations is further corroborated by similar experiments with O2+H2 and N2O+H2 systems (see Figure S1). Much like the CO2+H2 case, the FEM micrographs switch from a low level of brightness at an imaging field of 2.0 V.nm-1, in the presence of pure O2 or N2O, to bright {012} areas after addition of H2(g). The transformation process also involves a transient brightness increase-decrease of {113} facets. However, we note again that it is not always straightforward 17



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to deconvolute overlapping {113} and {012} FEM patterns. There is a noticeable difference regarding the initial brightness of the {113} FEM patterns following the sequence: N2O > CO2 > O2, for the same applied electric field and detector screen settings. This difference can be explained by considering the nature of the dissociation products. Accordingly, while N2O dissociates to O(ads) releasing N2(g)44, CO2 leads to O(ads) and CO(ads), which both increase the work function on rhodium surfaces, even though the effect of O(ads) is dominant34,47. Because O2 adsorbs with high sticking probability to form two O(ads) per dissociating molecule, the O(ads) coverage is expected to increase faster than in case of both N2O and CO2 adsorption. For the sake of comparison, the Rh tip sample has also been exposed to CO+H2 gas mixtures (1

Adsorption and Hydrogenation of CO2 on Rh Nanosized Crystals

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