The Surface Reactions of Ethanol over UO2(100) Thin Film · 1 The Surface Reactions of Ethanol over...

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The Surface Reactions of Ethanol over UO2(100) Thin Film

Journal: The Journal of Physical Chemistry

Manuscript ID jp-2015-08577d.R1

Manuscript Type: Article

Date Submitted by the Author: n/a

Complete List of Authors: Senanayake, Sanjaya; Brookhaven National Laboratory, Chemistry Mudiyanselage, Kumudu; Brookhaven National Laboratory, Chemistry Burrell, Anthony; Argonne National Laboratory, CSE Electrochemical Energy Storage Sadowski, Jerzy; Brookhaven National Laboratory, Center for Functional Nanomaterials Idriss, Hicham; University of Aberdeen, Scotland, UK, Chemistry

ACS Paragon Plus Environment

The Journal of Physical Chemistry BNL-111755-2016-JA

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The Surface Reactions of Ethanol over UO2 (100) Thin Film

S. D. Senanayake1*, K. Mudiyanselage1, A. K. Burrell2, J. T. Sadowski3, and H. Idriss4*

1The Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 2CSE Electrochemical Energy Storage Department, Argonne National Laboratory, Argonne,

Illinois 60439 3Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York

11973 4Fundamental Catalysis, Centre for Research and Innovation (CRI), SABIC, KAUST, Saudi

Arabia

*Corresponding authors Emails: [email protected]; [email protected]

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Abstract

The study of the reactions of oxygenates on well-defined oxide surfaces is important for

the fundamental understanding of heterogeneous chemical pathways that are influenced by

atomic geometry, electronic structure and chemical composition. In this work, an ordered

uranium oxide thin film surface terminated in the (100) orientation is prepared on a LaAlO3

substrate and studied for its reactivity with a C-2 oxygenate, ethanol (CH3CH2OH). With the use

of synchrotron X-ray photoelectron spectroscopy (XPS), we have probed the adsorption and

desorption processes observed in the valence band, C1s, O1s and U4f to investigate the bonding

mode, surface composition, electronic structure and probable chemical changes to the

stoichiometric-UO2(100) [smooth-UO2(100)] and Ar+-sputtered UO2(100) [rough-UO2(100)]

surfaces. Unlike UO2(111) single crystal and UO2 thin film, Ar-ion sputtering of this UO2(100)

did not result in noticeable reduction of U cations. The ethanol molecule has C-C, C-H, C-O and

O-H bonds, and readily donates the hydroxyl H while interacting strongly with the UO2 surfaces.

Upon ethanol adsorption (saturation occurred at 0.5 ML), only ethoxy (CH3CH2O-) species is

formed on smooth-UO2(100) whereas initially formed ethoxy species are partially oxidized to

surface acetate (CH3COO-) on the Ar+-sputtered UO2(100) surface. All ethoxy and acetate

species are removed from the surface between 600 and 700 K.

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1. Introduction

Metal oxides are important materials in catalytic applications due to several key

properties including: (1) ability to disperse and support metals, (2) activate metal nanoparticles,

(3) offer active perimeter / interfacial sites (metal – metal oxide), (4) act as a vital reservoir of

oxygen and (5) offer sites for adsorption and conversion of reactants. Though the use of oxides

as supports and as vital components in the catalytic pathways has been realized, there remain

unanswered questions about many fundamental properties behind their catalytic activity and

influence on reaction selectivity. Surface science studies of well-defined binary oxides provide

important insights to key aspects of the reaction pathway, the role of the last few layers of the

material 1-5 and the nature of interactions between reactants and surface atoms.6

Uranium oxides (UxOy) represent one of the richest catalytic metal-oxide systems due to

the presence of the 5f orbitals, giving unique chemical properties not shared by early transition or

lanthanum metal oxides7-10 and shielding of the nucleus making redox reactions less energy

demanding. Within the uranium oxide system, UO2 shares some common surface reactions with

ceria and titania, yet it is oxidizable (UO2+x), not only reducible (UO2-x) – because U cations

accommodate higher oxidation states than +4. This leads to a complex set of stable polymorphs

of UxOy, some of which have large amounts of interstitial oxygen anions; their effect on surface

reactions is largely unstudied. The most stable and energetically favored surfaces of UO2 are the

(111), (110) and (100)-orientated ones. Several groups have previously studied in great detail the

surface reactions of the UO2(111) single crystal as well as that of thin film and polycrystalline

UO2. Most of the work is summarized previously.4 Some early work was also done on the

UO2(100) surface by Madey group6, 11, mainly to extract structural, not chemical information. In

this work, we focus on the (100) surface where it was possible to make a well-defined film on

top of LaAlO3(100)12. UO2(100) is a polar surface that has considerably more open geometry

than the (111) and (110) surfaces, with zig-zagging arrangement of O atoms and an identifiable

reconstruction that creates trenches of 111 facets that exist between the O rows.2 The

coordination of atoms on the 100 surface is very different to the other surfaces and offers a

distinct opportunity to probe the chemical sensitivity of this ordered last layer towards reaction

selectivity.

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Thus, our motivation is to link the surface of this f-metal oxide, to its reactivity towards

simple probes such as ethanol. Such heterogeneous probes have been used extensively over

many metal oxides3, 5, 13 including UO2. Ethanol (CH3CH2OH) is an important C-2 oxygenate

that has a distinct C-C bond and an acidic H (-OH) to donate to surface oxygen anions. Ethanol

typically adsorbs molecularly below room temperature4-5, 13-14, followed by de-protonation of the

acidic H for chemisorption (as ethoxy) at room temperature and above. The dissociative

adsorption can be presented as follows.

CH3CH2OH (g) � CH3CH2OH (a) [1]

CH3CH2OH (a) + O (l) � CH3CH2O (a) + OH (a) [2]

Subsequent scission of the C-C or C-O bond of the ethoxy species requires activation with

temperature and can lead to several surface pathways involving dehydrogenation, dehydration

reforming15 and carbon-carbon bond formation reactions. These reactions have been

summarized previously.14

The electronic core level structure of the uranium oxide system is complex due to the presence of

many satellite structures and because, as mentioned above, deviation from stoichiometry is the

norm rather than the exception. Yet, the U4f lines of the U4+ cations have been studied by many

authors16,17 and are relatively well understood. The U4f7/2 of U4+ cations in UO2(111) single

crystal is found to be at 380.0 eV with a splitting of 10.9 to 11.0 eV. Similar observations are

seen for a thin film of UO2.17-18 The (shake up) satellites are seen at 6.9-7.0 eV above each main

4f peak. Their sharpness indicates the closeness to stoichiometry. The presence of U cations in

lower oxidation than +4 can only be encountered in a reducing environment and disappears fast

even in ultra-high vacuum (UHV) conditions within hours. These reduced states can be seen as a

shoulder at the low binding energy side of the U4f7/2 and U4f5/2 peaks. Yet, because of deviation

from stoichiometry, an overall shift to higher binding energy (due to excess electrons left upon

oxygen removal making UO2 an n type semiconductor) is commonly seen making the exact

energy position prone to errors because of the need for recalibration. On the other hand, excess

oxygen atoms, while keeping the fluorite structure intact up to UO2.25, shift the XPS main peaks

of the U4f to lower binding energy because of the resulting p-type nature of the semiconductor.

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In this case, the satellite structures of the U4+ decrease in intensity and the presence of U5+

cations may result in broadening of the main U4f peak and the appearance of satellites at about 8

eV above each main peak.19-20

In this work we start by presenting the as prepared stoichiometric UO2(100) surface as

studied by low energy electron diffraction (LEED) then by XPS (U4f and O1s) as a function of

the take-off angle with respect to the analyzer to gauge for the degree of stoichiometry. This is

followed by the study of ethanol adsorption at room temperature and with respect to annealing to

higher temperatures on both stoichiometric and Ar+-sputtered UO2(100) surfaces.

2. Experimental methods

UO2 films were prepared onto LaAlO3 (100) surface using a polymer assisted deposition

(PAD) technique as described elsewhere.12, 21 These surfaces were characterized previously using

TEM, XPS and other tools.12

Soft X-ray photoelectron spectroscopy (sXPS) experiments were conducted at beamline

U12a of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory.

This beam-line and its UHV end-station have been described in detail previously.1 Sample

cleanliness and adsorbate characterization data were collected using soft X-ray synchrotron

radiation. The C 1s was excited at 400 eV with 0.3 eV resolution; O1s and U4f were excited at

600 eV at 0.3eV resolution. The binding energies were calibrated relative to the Cu (sample

mount) Fermi energy.

Selected area low-energy electron diffraction (µ-LEED) was collected on a commercial

Elmitec LEEM III instrument located at U5UA of the NSLS. Electrons from a LaB6 single

crystal were incident through 2µm aperture onto the UO2 films and diffracted electrons were

collected using a multi-channel plate (MCP) detector of the LEEM system.

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3. Results and Discussion

3.1 Characterization of UO2 (100) films grown on LaAlO3

The surface of the UO2 (100) is given in Figure 1a. The alternating uranium and oxygen

atoms of the bulk termination results in a polar surface, which is inherently unstable. The

investigation of this surface using ion scattering spectroscopy (ISS) and LEED showed that the

surface oxygen atoms were arranged in a zigzag manner along the (100) direction, the result of a

periodic lattice distortion.22 This structure has 50% oxygen atoms vacancies in order to stabilize

the surface by removing the dipole moment. Scanning tunneling microscopy (STM) study has

shown a reconstruction of the surface consisting of a half monolayer of oxygen, with bridge

bonded oxygen along the (110) and (ī10) directions.2 Microscopic (111) facets are formed on

the surface between the ridges of oxygen atoms and the combined effects of these

reconstructions stabilize the surface.

This UO2(100) thin film was characterized previously by X-ray diffraction, high

resolution transmission electron microscopy (HRTEM) and angle resolved valance band

photoemission12. First, we have re-characterized the UO2(100) film grown on LaAlO3 using

LEED and XPS before investigating its reactions. Figure 1b shows the LEED images of the UO2

thin film on LaAlO3 collected at 60 and 35 eV energies. These LEED patterns indicate the

presence of a (100) surface, with a well ordered arrangement of the last few layers of atoms.

Detailed core level spectroscopy was collected. These include U4f, O1s and the valence band at

different energies and take off angles. The U4f region of the XP spectrum shows peaks at

~390.5 and ~ 379.5 eV for 4f5/2 and 4f7/2, respectively, as indicated in Figure 2a. The FWHM of

the 4f5/2 and 4f7/2 peaks are 1.90 and 1.75 eV, respectively. Another structure is seen at 7.0 eV

above each main peak. The binding energy position, the difference in the splitting of the main

peaks of 11.0 eV, together with the presence of these satellites (7 eV above each line) is an

indication of U4+ cations. It is however possible that higher oxidation state with a small

contribution exists. As seen in the figure, it is possible to put a pair of peaks at about 4 eV above

each main peak. Recently, we have studied by core level spectroscopy the oxidation/reduction of

U cations in CeU oxides.19 The presence of U5+ cations with their satellites at ca. 4 and 8 eV

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above each line was found to be a clear intermediate between the fresh sample containing U6+

and the Ar ions sputtered surface mainly containing U4+. In the present case the 8 eV

contribution would be masked by the large contribution of the U4+ satellites (7.0 eV above each

main peak) yet the 4 eV satellite can be seen.19

Figure 2b shows the valence region of the UO2(100) film, where peaks corresponding to

the U5f, U6d-O2p, U6p3/2, O2s, and U6p1/2 orbital bands at binding energy = 1.0 , 4.5 , 17.2 ,

22.4 and 27.5 eV, respectively. One way of qualitatively gauging the degree of contribution of U

cations in +4 oxidation overall is to look at the U5f and O2p line shape. On UO2(111) single

crystal and UO2 thin film the U5f is seen to be far more accentuated than that of the O2p. This is

not the case here where the U5f while intense is not as intense as expected when compared to

that of UO2(111) single crystal. In order to further probe into this we have collected U4f and

O1s lines as a function of surface orientation, as shown in Figure 3. Two sets of data were

collected at 260 and 350 K. The one at 260 K is to see if water in the background can affect the

results. Figure 3a presents the O1s lines at 260 K at different angles with respect to the analyzer.

The O1s line shows a hydroxyl peak with about 25% contribution of the total signal. Given that

the signal of the O1s is coming from about three layers then this 25% most likely represents full

surface hydroxylation. Surface hydroxylation has two effects. First, to heal any possible oxygen

defects and second to change the surface dipole. Healing of the surface oxygen defects is a

complex phenomenon that was studied by us and others previously.23-27 It has two pathways

depending on the extent of defects. When oxygen defects have high surface concentration then

one molecule of water may react with the surface oxygen defects and the two electrons of the

oxygen vacancies are transferred to the two hydrogen atoms of water making one hydrogen

molecule. When the number of surface oxygen vacancies is small, then partial dissociation

occurs that results into two surface hydroxyls and one hydrogen molecule too, but with higher

activation energy. O1s data collection at 350 K indicated no angular variations of the signal with

respect to the analyzer. This is most likely because at this temperature surface hydroxyls would

have recombined to give water and desorbed. Figure 4a presents the peak area ratios of the O1s

attributed to surface hydroxyls to that of lattice oxygen as a function of the angular orientation to

the surface normal at 260 K. In order to see the effect of these on the U cations, similar U4f

spectra were collected. Figure 4b presents the complete U4f line while Figure 3c presents that of

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the U4f7/2 at similar orientation with respect to the analyzer. From Figure 4b one can extract the

contribution of U4+ cations with respect to the overall U cations by taking the satellites at about 4

eV as a reference for U5+ cations (neglecting the high binding satellite as that of U4f7/2 is

masked). Figure 4c shows that indeed the sum of the U4f7/2,5/2 and the U4+ satellites over the U5+

pair of satellite decreases with increasing surface information. This indicates that a fraction of

the surface U cations is more positively charged than the ideal U4+ cations in the near surface.

This can also be seen by the width of the U4f lines. Figure 3d presents the FWHM of the U4f7/2

as a function of angle from the surface normal. It is noted that the FWHM increases slightly

from the surface to the near surface as evidence of less homogenous charge on the metal cations.

We have investigated the interactions of this stoichiometric UO2(100) film with ethanol as well

as the Ar+-sputtered UO2(100) film as described in the following sections.

3.2 Adsorption of ethanol on UO2 (100)

Figure 5 shows the C1s, O1s and U4f XP spectra following the adsorption of ethanol

(CH3CH2OH) on the stoichiometric UO2(100) film at 285 K as a function of exposure. At low

exposures, the C 1s spectrum has two unresolved features at 285.5 and 286.5 eV whereas at

higher exposures these two features partially resolved into two peaks. These two peaks at 285.5

and 286.5 eV are assigned to the -CH3 and –CH2O- groups, respectively, of the ethoxy

(CH3CH2O) species indicating that ethanol dissociatively adsorb on UO2(100) forming ethoxy

species at 285 K .3, 5, 28-29 The O1s spectrum, after exposure of UO2(100) to ethanol, had two

features at 530.0 and 531.5 eV due to the lattice oxygen, Ol, of the UO2(100) film and the

oxygen atom in CH3CH2O(a) species derived from ethanol, respectively. With increasing ethanol

exposure, the intensity of the peak at 531.5 eV increases gradually and concomitantly the

intensity of the peak for lattice oxygen of UO2(100) decreases. The corresponding XP spectra

for U4f region show slight decrease in intensity of 4f peaks with increasing ethanol exposures as

shown in Figure 5c. Ethanol coverage on the UO2(100) film and IB/IB0 (IB0 and IB are peak

intensities of 4f7/2 feature before and after exposure to ethanol) as a function of exposures are

shown in Figure 5d. From the U4f data one can estimate the surface coverage knowing the size

of ethoxy (about 3.4 Å) and the escape depth for photoelectron with KE = 200 eV (13.5 Å). This

was found to equate ca. 0.5 ML. This data shows that the intensity of 4f7/2 peak gradually

decreases as the ethoxy coverage increases. Annealing of this ethoxy layer to higher

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temperatures leads to the gradual decrease of intensities of features for ethoxy species and

eventually all the peaks for ethoxy disappear at 700 K due to the complete decomposition (data

not shown).

Even though many different forms of uranium oxides [UO2(111) thin film grown on top

of a Mo substrate, UO2(111) single crystal and UO2 powder] can be reduced by either reduction

with H2, annealing or Ar+-sputtering as reported previously4, our UO2(100) was not reduced by

Ar+ sputtering. This might be due to either the considerable surface reconstruction or to the

method the thin film was grown, on top of the metal oxide, LaAl2O4. In all cases despite many

attempts to reduce the surface with Ar+-sputtering the U4f lines were still similar to those of the

stoichiometric surface. In this study we have investigated the interaction of ethanol on both

surfaces, the stoichiometric (smooth) and the one prepared by Ar+-sputtering (rough). Figure 6

shows the comparison of C1s XP spectra obtained after exposing stoichiometric UO2(100) and

Ar+-sputtered UO2(100) film to ethanol at 285 and 260 K, respectively. On the smooth

UO2(100) surface, two peaks appear at 285.5 and 286.5 eV for -CH3 and –CH2O- groups,

respectively, of the ethoxy as described in the previous section and shown in Figure 5. After

exposure of Ar+-sputtered UO2(100) to ethanol at 260 K, C1s spectrum shows three peaks at

285.5, 286.5 and 290.0 eV. The peaks at 285.5 and 286.5 eV are assigned to -CH3 and –CH2O-

groups of ethoxy species, similar to the peaks observed on smooth UO2(100). The higher

binding energy peak at 290.0 eV, which is not observed on smooth UO2(100), is assigned to –

COO- group of acetate (CH3COO)30 as confirmed by the XPS peak observed for acetate formed

from the acetic acid (CH3COOH) on this film (data not shown; manuscript in preparation). The

C1s peak for -CH3 group of acetate overlaps with that of –CH3 group of ethoxy.

Figure 7 shows the C1s and O1s spectra obtained after exposing the Ar+-sputtered

UO2(100) film to ethanol at 260 K and subsequent annealing to the specified temperatures. After

exposure of Ar+-sputtered UO2(100) to ethanol at 260 K, C1s spectrum shows three peaks at

285.5, 286.5 and 290.0 eV, which are assigned to -CH3 and –CH2O- groups of ethoxy and –

COO- group of acetate, respectively, as described above. The C1s peak for -CH3 group of acetate

overlaps with that of –CH3 group of ethoxy. These results indicate that ethanol adsorbs

dissociatively on rough-UO2(100) forming ethoxy species part of which further react with the

surface to make acetates at 260 K. Most likely, acetates are formed due to the presence of more

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active defect sites on the sputtered film. Both ethoxy and acetate decompose completely after

annealing to 700 K as shown in Figure 7 indicating their similar thermal stabilities. A new

feature observed at ~ 284.4 eV after annealing to 700 K is likely due to the residual CxHy

species formed on the surface during the decomposition of ethoxy and acetates. The O1s XP

spectrum obtained after the exposure of sputtered-UO2(100) to ethanol at 260 K has two peaks at

529.8 and 531.4 eV for the lattice Ol and the O in the ethoxy and acetate, respectively.

Annealing of this co-adsorbed ethoxy and acetate layer to higher temperatures leads to the

increase in the intensity of the peak for lattice O and simultaneous decrease of the intensity of the

peak for O of adsorbate species. Figure 7c shows the increase in intensity of the U4f region

during annealing of co-adsorbed ethoxy and acetate layer from 260 to 700 K. Figure 7d shows

the changes in peak intensities of C1s of CH3 group of ethoxy and acetate species and O1s of

lattice Ol as a function of annealing temperature. These results show that the intensity of the O1s

peak of lattice Ol increase upon heating due to the decomposition of ethoxy and acetate species

as shown by the decrease of the intensity C1s peak for the –CH3 group. The maximum

desorption rate is at ~ 700 K as plotted from the change in the slope (dC/dT; where C is the XPS

C1s signal of the CH3 group) versus temperature as shown by the inset in Figure 7d. By analogy

with the acetic acid TPD on the UO2(111) single crystal where most of acetates react to give

ketene at 610K, dC/dT is a measure of the maximum desorption/reaction where a large fraction

of the lattice oxygen is freed and about ½ of acetates have been removed.31

4. Conclusions

UO2(100) grown on LaAlO3 (100) has similar surface electronic characteristics to

UO2(111) single crystal yet is harder to reduce since the annealed surface still contains a small

fraction of U cations with oxidation state higher than +4. On this surface, ethanol adsorbs

dissociatively to form ethoxy with a saturate surface coverage of 0.5 ML at 285 K. The Ar ions

sputtered surface did not show U cations with lower oxidation state than +4 (as was the case for

UO2(111) single crystal and UO2 thin film). On this sputtered UO2(111), ethanol also adsorbs

dissociatively forming ethoxy species yet part of which further react with the surface to make

acetates, even at 260 K. Both ethoxy and acetate species decompose/react upon annealing to ~

700 K which is similar to their corresponding reaction on UO2(111). The difference in the

surface properties (the presence of satellites structures that can be related to U5+) and reaction

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(oxidation of a fraction of ethoxy on the Ar ion sputtered surface) can be linked to the surface

atomic structure of the UO2(100) surface.

Acknowledgment

The research carried out at Brookhaven National Laboratory, was supported by the U.S.

Department of Energy, Office of Science and Office of Basic Energy Sciences under contract

No. DE-SC0012704. This work used resources of the National Synchrotron Light Source

(NSLS) and Center for Functional Nanomaterials (CFN) which are DOE Office of Science User

Facilities.

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20. Al-Shankiti, I.; Al-Otaibi, F.; Al-Salik, Y.; Idriss, H., Solar Thermal Hydrogen Production

from Water over Modified CeO2 Materials. Top Catal 2013, 56, 1129-1138.

21. Burrell, A. K.; McCleskey, T. M.; Jia, Q. X., Polymer Assisted Deposition. Chem.

Commun. 2008, 1271-1277.

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22. Taylor, T. N.; Ellis, W. P., Distorted Surface Oxygen Structure on UO2(100). Surf. Sci.

1981, 107, 249-262.

23. Senanayake, S. D.; Waterhouse, G. I. N.; Chan, A. S. Y.; Madey, T. E.; Mullins, D. R.;

Idriss, H., The Reactions of Water Vapour on the Surfaces of Stoichiometric and Reduced

Uranium Dioxide: A High Resolution XPS Study. Catal. Today 2007, 120, 151-157.

24. Senanayake, S. D.; Rousseau, R.; Colegrave, D.; Idriss, H., The Reaction of Water on

Polycrystalline UO2: Pathways to Surface and Bulk Oxidation. J. Nucl. Mater. 2005, 342, 179-

187.

25. Senanayake, S. D.; Idriss, H., Water Reactions over Stoichiometric and Reduced UO2(111)

Single Crystal Surfaces. Surf. Sci. 2004, 563, 135-144.

26. Shamir, N.; Tiferet, E.; Zalkind, S.; Mintz, M. H., Interactions of Water Vapor with

Polycrystalline Uranium Surfaces. Surf. Sci. 2006, 600, 657-664.

27. Tiferet, E.; Zalkind, S.; Mintz, M. H.; Jacob, I.; Shamir, N., Interactions of Water Vapor

with Polycrystalline Uranium Surfaces – The Low Temperature Regime. Surf. Sci. 2007, 601,

936-940.

28. Farfan-Arribas, E.; Madix, R. J., Role of Defects in the Adsorption of Aliphatic Alcohols on

the TiO2(110) Surface. J. Phys. Chem. B 2002, 106, 10680-10692.

29. Jayaweera, P. M.; Quah, E. L.; Idriss, H., Photoreaction of Ethanol on TiO2(110) Single-

Crystal Surface. J. Phys. Chem. C 2007, 111, 1764-1769.

30. Senanayake, S. D.; Gordon, W. O.; Overbury, S. H.; Mullins, D. R., Adsorption and

Reaction of Acetone over CeOx(111) Thin Films. J. Phys. Chem. C 2009, 113, 6208-6214.

31. Chong, S. V.; Idriss, H., Reactions of Acetic Acid on UO2(111) Single Crystal Surfaces. J.

Vac. Sci. Technol., A 2000, 18, 1900-1905.

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Figure Captions

Figure 1. (a) Representation of the UO2 (100) surface, showing bulk termination in the (100)

plane. Small red circles represent uranium, large yellow circles represent oxygen. (b) LEED

patterns of the UO2(100) films grown on LaAlO3, taken at 60eV.

Figure 2. (a) XPS spectrum in the U4f region (U4f7/2, U4f5/2, S7/2, S5/2) and (b) valence band

spectrum of UO2(100) film grown on LaAlO3 (U5f, U6d, U6p3/2, U6p1/2, O2p, O2s).

Figure 3. XPS O1s and U4f binding energy at different collection angles at 260 and 350 K

(a) XPS O1s of surface at 260 K at the given takeoff angles. (b) XPS O1s of freshly cleaned

surface at 350 K at the given takeoff angles. (c) XPS U4f of freshly cleaned surface at 350 K at

the given takeoff angles. (d) Plot of the FWHM of the XPS U4f (in c) as a function of the takeoff

angle. The inset is a schematic representation of hydroxylated and clean UO2 surface.

Figure 4. (a) The peak area ratios of the O1s attributed to surface hydroxyls to that of lattice

oxygen as a function of the angular orientation to the surface normal for the 260 K

measurements. (b) U4f XPS spectra collected at different collection angles 350 K. (c) The sum

of the U4f7/2,5/2 and the U4+ satellites over the U5+ pair of satellites.

Figure 5. (a) C1s and (b) O1s (c) U4f XPS spectra following the adsorption of ethanol on the

UO2(100) films at 285 K as a function of exposure (0-1.65L), (d) ethanol coverage on the

UO2(100) film and IB/IB0 (IB0 and IB are peak intensities of 4f7/2 feature before and after

exposure to ethanol) as a function of ethanol exposure.

Figure 6. C1s XP spectra following the saturation adsorption of ethanol on stoichiometric

UO2(100) and Ar+-sputtered UO2(100) films at 285 and 260 K, respectively.

Figure 7. (a) C 1s and (b) O 1s (c) U 4f XPS spectra obtained after exposure of Ar+-sputtered

UO2(100) to ethanol at 260 K and subsequent annealing to the specified temperatures. (Ar+-

sputtering conditions: PAr ~3.8x10-5 Torr, 3 KeV, 30 min, 250 K). (d) Changes in intensities of C

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1s of CH3 group in ethoxy and acetate species and O 1s of lattice O after annealing of ethoxy and

acetate co-adsorbed surface from 260 to 800 K.

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Figure 1.

(b)

(a)

60eV

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405 40 2 39 9 3 96 393 390 387 384 381 37 8 3 75

Intensity (Arbitrary units)

B in d in g E ne rg y (eV )

U4f (hv=600eV) 4f7/2

4f5/2

S7/2S5/2

35 30 25 20 15 10 5 0


B inding Energy (eV)

VB (hv=150eV) U5f

U6d-O2p

U6p3/2O2sU6p1/2

Figure 2.

(b) (a)

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536 534 532 530 528

35o

45o

55o

65o


Binding Energy (eV)

O1s 260 K

Angle Resolved XPS of UO2(100)

382 381 380 379 378 377

65o

55o

45o

35o

25o


Binding Energy (eV)

U4f 4f7/2350 K

Analyzer

θ

surface hydroxyls

536 534 532 530 528

55o

45o

35o

25o


Binding Energy (eV)

O1s 350 K

70 60 50 40 30 201.78

1.80

1.82

1.84

1.86

1.88

1.90

FWHM (eV)

theta (degree)

FWHM f(θ)

≂0.1 eV

OOU

U UUOO

HH

U U

i

ii

iii

iv

Figure 3.

(a)

(b)

(c)

(d)

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Figure 4.

35 40 45 50 55 60 65

0.1

0.2

0.3

0.4

Intensity[O1s (hydroxyl O/ lattice O

l)]

Collection Angle (�)

405 400 395 390 385 380

65o

55o

45o

35o

25o


Binding Energy (eV)

U4f

S 7/2

S5/2

4f 7/2

4f 5/2

350 K

30 35 40 45 50 55 60 65 70

20

22

24

26

28

30

32

(U 4f 7/2+U 4f 5/2 + U 4+ satellites)/ (U 5+ satellite)

Collection Angle (°)

(a)

(b)

(c)

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Ethanol / UO2(100)

294 292 290 288 286 284 282 280


Binding Energy (eV)

C1s Θ=1.65L

1.05L

0.65L

0.35L

0.2L

0.05L

0L

C-O

C-CH3

536 534 532 530 528


Binding Energy (eV)

Θ=1.65L

1.05L

0.65L

0.35L

0.2L

0.05L

0L

CH3CH2-O

U-OO1s

0.34nm

8

8.4

8.8

9.2

9.6

10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 0.5 1 1.5 2

λ = 13.5 Åa = 3.4 ÅKE = 200 eV C

overage

Exposure, Langmuir

IB /IBo

U4f

405 400 395 390 385 380


Binding Energy (eV)

Θ=1.65L

1.05L

0.65L

0.35L

0.2L

0.05L

0L

4f7/2

S7/2S5/2

4f5/2

600 eV

Figure 5.

(a)

(b)

(c)

(d)

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292 288 284 280

-COO

C-O

Ar+ sputttered

UO2(100)


Binding Energy (eV)

Stoichiometric

UO2(100)

C-CH3

Figure 6.

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O1s

534 532 530 528

260 K 350 K 500 K 600 K 700 K 800 K


Binding Energy (eV)

-C-O

O (l)

405 400 395 390 385 380 375

260 K 350 K 500 K 600 K 700 K


Binding Energy (eV)

U4f

Effect of temperature

CH3CH2O(a)

CH2COO(a)

250 450 650 850

dT

dC

Ed = 150 kJ/mol

T (K)

3.4

3.9

4.4

4.9

0.5

1.5

2.5

3.5

200 300 400 500 600 700 800 900

O (l)

-CH3

Temperature (K)

Peak height (arb. Units)

(b)

(c)

(d)

290 285 280

260 K 350 K 500 K 600 K 700 K 800 K


Binding Energy (eV)

C1s C-CH3C-O

Ar+ sputttered

UO2(100)

-COO(a)

Figure 7.

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TOC

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