ORIGINAL PAPER

Water–Gas Shift and CO Methanation Reactionsover Ni–CeO2(111) Catalysts

Sanjaya D. Senanayake • Jaime Evans •

Stefano Agnoli • Laura Barrio • Tsung-Liang Chen •

Jan Hrbek • Jose A. Rodriguez

Published online: 25 January 2011

� Springer Science+Business Media, LLC 2011

Abstract X-ray and ultraviolet photoelectron spectros-

copies were used to study the interaction of Ni atoms with

CeO2(111) surfaces. Upon adsorption on CeO2(111) at

300 K, nickel remains in a metallic state. Heating to ele-

vated temperatures (500–800 K) leads to partial reduction

of the ceria substrate with the formation of Ni2? species

that exists as NiO and/or Ce1-xNixO2-y. Interactions of

nickel with the oxide substrate significantly reduce the

density of occupied Ni 3d states near the Fermi level. The

results of core-level photoemission and near-edge X-ray

absorption fine structure point to weakly bound CO species

on CeO2(111) which are clearly distinguishable from the

formation of chemisorbed carbonates. In the presence of

Ni, a stronger interaction is observed with chemisorption of

CO on the admetal. When the Ni is in contact with Ce?3

cations, CO dissociates on the surface at 300 K forming

NiCx compounds that may be involved in the formation of

CH4 at higher temperatures. At medium and large Ni

coverages ([0.3 ML), the Ni/CeO2(111) surfaces are able

to catalyze the production of methane from CO and H2,

with an activity slightly higher than that of Ni(100) or

Ni(111). On the other hand, at small coverages of Ni (\0.3

ML), the Ni/CeO2(111) surfaces exhibit a very low activity

for CO methanation but are very good catalysts for the

water–gas shift reaction.

Keywords Nickel � Ceria � Carbon monoxide �CO methanation � Water � Water–gas shift reaction

1 Introduction

Nickel-based catalyst are often used in industrial processes

that involve the hydrogenation of olefins, the hydrotreating

and hydrogenolysis of hydrocarbons, the production of

hydrogen through the steam reforming of hydrocarbons or

the water–gas shift (WGS) reaction, and CO methanation

[1–4]. Oxide supports such as alumina, silica, zirconia,

ceria, and titania have been used to disperse the nickel

particles. The activity and selectivity of the supported

nickel catalysts are strongly dependent on the amount of

metal employed, the size of the dispersed metal particles,

metal-support interactions and the composition of the

support [1, 2]. A fundamental understanding of these

parameters is necessary for a rational design of nickel-

based catalysts [1–5]. In this article, we investigate the

WGS and CO methanation reactions over Ni–CeO2(111)

catalysts. X-ray and ultraviolet photoelectron spectrosco-

pies (XPS and UPS) are used to study the interaction of Ni

atoms with CeO2(111) surfaces. The chemisorption of CO

on the Ni–CeO2(111) systems is studied with synchrotron-

based core-level photoemission and near-edge X-ray

absorption fine-structure (NEXAFS).

Previous studies of scanning tunneling microscopy

(STM) have revealed that Ni grows on CeO2(111) at 300 K

S. D. Senanayake � S. Agnoli � L. Barrio � J. Hrbek �J. A. Rodriguez (&)

The Department of Chemistry, Brookhaven National Laboratory,

Upton, New York 11973, USA

e-mail: rodrigez@bnl.gov

J. Evans

Facultad de Ciencias, Universidad Central de Venezuela,

Caracas 1020 A, Venezuela

T.-L. Chen

Chemical Sciences Division, Oak Ridge National Laboratory,

Oak Ridge, TN 37831, USA

123

Top Catal (2011) 54:34–41

DOI 10.1007/s11244-011-9645-6

forming three dimensional particles [5]. Annealing to

higher temperatures (500–800 K) induces the sintering of

the Ni particles [5] and also a possible migration of the

admetal into the ceria substrate [5, 6]. Photoemission

experiments for the deposition of nickel on reduced ceria

films point to intermixing with the ceria covering Ni or

NiOx particles [6]. CeO2 exhibits a fluorite structure, in

which each Ce4? cation is surrounded by eight oxygen

atoms [7]. On the other hand, NiO adopts a rock-salt

structure with each Ni2? cation surrounded by ‘‘only’’ six

oxygen atoms [7]. Experiments of X-ray diffraction (XRD)

and X-ray absorption fine-structure (XAFS) performed in

our laboratory for powders of Ce1-x NixO2-y mixed-oxides

show a low stability for Ni inside the typical fluorite

structure of ceria [8]. A limit of 10–12% was found

for a stable Ce $ Ni exchange within a fluorite lattice.

For higher concentrations of Ni, formation of NiO/

Ce1-xNixO2-y mixtures was observed [8].

The reaction of low concentrations of CO in a mixture

with H2 to form CH4 was developed as a gas purification

process in the 1950s [1, 9]. At the present time, the metha-

nation reaction has a critical role in the production of syn-

thetic natural gas from hydrogen-deficient carbonaceous

materials. In addition, the reaction is an obvious starting

point in studies of fuel and chemical synthesis from carbon

sources [1, 9]. In the 1980s and 1990s, the CO methanation

reaction over single-crystal surfaces was the subject of many

investigations [3]. Detailed studies have been carried out for the

CO ? 3H2 ? CH4 ? H2O reaction on Ni(100) and Ni(111)

[3, 10, 11]. Strong similarities were found in the kinetic data for

the close-packed (111) and for the more open (100) crystal

plane of Ni, indicating that the methanation reaction is structure

insensitive [10, 11]. Subsequent studies for Ni/TiO2(100) cat-

alysts indicate that the formation of bonds between the admetal

and oxide support can enhance the methanation activity of Ni

[12, 13]. It is worthwhile to investigate if the same phenomenon

occurs in Ni/CeO2(111) catalysts.

The water–gas shift (WGS, CO ? H2O ? CO2 ? H2) is

an important reaction used in the chemical industry for the

production of clean H2 [1, 14]. At the present time, mixtures

of Fe–Cr oxides and Cu/ZnO are the commercially used

catalysts for the WGS reaction at temperatures between

350–500 �C and 180–250 �C, respectively. However, these

oxide catalysts are pyrophoric and normally require lengthy

and complex activation steps before usage [1, 14]. Conse-

quently better catalysts are being sought [4, 15]. In this

respect, ceria-supported metal catalysts are receiving a lot of

attention [15–19]. Our studies indicate that Ni/CeO2 cata-

lysts display a higher WGS activity than Cu/ZnO catalysts.

The deposition of small coverages of Ni on CeO2(111)

produces systems that are very efficient for the dissociation

of water and catalyzing the OH ? CO ? CO2 ? 0.5 H2

reaction.

2 Experimental Methods

The experiments presented in this work were undertaken in

three different chambers [17–21]. Beamline U12A at the

National Synchrotron Light Source (NSLS) was used to

collect the core-level photoemission and NEXAFS data.

The ultra-high vacuum (UHV) end-station (background

pressure * 1 9 10-10 Torr) with all necessary surface

science tools has been described previously in great detail

[20, 21]. The C 1s core-level spectra were recorded with a

photon energy of 400 eV and an energy resolution of

0.3 eV. NEXAFS spectroscopy was undertaken in the

partial yield mode with a grid bias of -225 eV at an energy

resolution of 0.5 eV. Higher order contributions of the Ce

L edge were subtracted from the presented spectra. The

energy calibration in the photoemission spectra was

undertaken using the satellite feature in the Ce 4d region at

122.3 eV, while NEXAFS spectra were aligned to the C

feature from the mesh at 284.7 eV. The XPS (Mg Ka) and

UPS (He I and II) data reported in ‘‘Sect. 3’’ for the Ni/

CeO2(111) systems were collected in a second chamber

described elsewhere [22]. UPS measurements were

obtained at normal incidence with respect to the sample

normal while XPS was at 45� with respect to the sample

normal.

The studies of CO methanation and the WGS were

conducted in a third UHV chamber which has a batch

reactor attached [17–19]. After preparing and characteriz-

ing the Ni/CeO2(111) surfaces in the UHV chamber, the

sample was transferred to the reactor at *300 K, then the

reactant gases were introduced (20 Torr of CO and 10 Torr

of H2O for the WGS [17–19]; 26 Torr of CO and 94 Torr

of H2 for CO methanation [10, 11]) and the sample was

rapidly heated to reaction temperatures in the range of

550–650 K. The reaction conditions used to investigate CO

methanation and the WGS were taken from previous

studies [10, 11, 17–19]. The reported rates in Sects. 3.3 and

3.4 are for steady-state conditions. The WGS was followed

during reaction times of 1–20 min, while reaction times of

1–75 min were used to monitor CO methanation. The

amount of molecules produced was normalized by the

active area exposed by the front of the sample, which was

the one that contained nickel.

The kinetic tests were done using a CeO2(111) single

crystal which was cleaned following standard procedures

used in our previous studies for Cu/CeO2(111) and Au/

CeO2(111) surfaces [17–19]. To avoid problems with

charging in the photoemission and NEXAFS experiments,

films of CeO2(111) were grown in situ onto a hot Ru(0001)

substrate (700 K) using a commercial built four pocket

Oxford evaporator in an O2 atmosphere (1 9 10-7 Torr).

Previous studies have been devoted to understanding this

growth and the formation of well defined 111 surfaces

Top Catal (2011) 54:34–41 35

123

[5, 21, 23]. Ni was evaporated on to CeO2(111) surfaces at

300 K. The Ni/CeO2(111) systems were exposed to CO

(99.99% Sigma-Aldrich) in the UHV chambers at 100 or

300 K.

3 Results and Discussion

3.1 The Interaction of Ni with CeO2(111)

Figure 1 depicts XPS data, Ni 2p and Ce 3d regions in top

panel and the Ni 2p3/2 peak in closer detail in the bottom

panel, upon deposition of Ni (*0.4 ML) at 300 K followed

by incremental annealing to 500, 700 and 800 K. The

deposition of the metal at room temperature produces a Ni

2p3/2 peak close to the binding energy of 853 eV reported

for bulk metallic nickel [2]. A weak shoulder is observed

near 854 eV, the position reported for the Ni 2p3/2 peak of

NiO [2]. This feature increased in intensity when the

sample was annealed to 500 K. The Ce 3d region is very

sensitive to changes in the oxidation state of the Ce cations,

showing distinctive line shapes for Ce4? and Ce3? species

[22, 24]. An analysis of the Ce 3d spectra indicate that the

clean CeO2(111) surface contains only Ce4? cations before

the deposition of nickel. After the adsorption of Ni and

annealing to 800 K, there are some minor changes in the

Ce 3d region that point to reduction of a small amount of

Ce4? into Ce3? cations. From studies of STM [5], it is

known that Ni grows on CeO2(111) at 300 K forming three

dimensional particles that sinter upon annealing to higher

temperatures (500–800 K) [5]. Since Ni and Ce can form

mixed-metal oxides [6, 8], migration of the Ni admetal into

the CeO2(111) substrate upon annealing is possible. In

principle, the appearance of Ni2? features in the XPS data

of Fig. 1 could be a consequence of the formation of NiO

or a Ce1-xNixO2-y mixed metal oxide. The line-shape of

the Ni 2p XPS spectrum of NiO has characteristic features

that disappear if Ni2? is dissolved in a lattice of another

oxide [25, 26]. These typical features of NiO are not seen

in Fig. 1, reinforcing the idea that the nickel is migrating

into the oxide substrate. After depositing small coverages

of Ni (\0.5 ML) on CeO2(111) and annealing to

500–800 K, we always saw a signal for Ni2? in XPS. When

the Ni coverages were increased above 0.5 ML, the ratio of

the Ni2?/Ni0 signals in XPS decreased in a continuous way

(i.e., no substantial amounts of additional Ni2? were pro-

duced). This is consistent with the low stability for Ni

inside the typical fluorite structure of ceria [8]. A limit of

10–12% has been found for a stable Ce $ Ni exchange

within a fluorite lattice [8].

Figure 2 shows He II (hm = 40.8 eV) valence band

spectra for CeO2(111) and Ni/CeO2(111) surfaces as a

function of temperature. The valence spectrum at the

bottom matches that reported in the literature for

CeO2(111) [21] with negligible signal (a band gap) from 0

to 2.5 eV. The deposition of Ni (*0.4 ML) at 300 K

produces new features in the region between 0 and 3 eV

where the occupied Ni 3d states are expected [27, 28].

There is also a new peak at *10.5 eV that probably cor-

responds to CO chemisorbed from the background gases

present inside the UHV chamber [27, 28]. Annealing from

300 to 500 or 800 K produces drastic changes in the

Fig. 1 a Evolution of the Ce 3d and Ni 2p photoemission lines as a

function of the annealing temperature measured using a Mg Ka source

(1253.6 eV). The spectrum of the clean CeO2 prior to the Ni

deposition (*0.4 ML) is reported at the bottom. Spectra have been

normalized to the same height of the f2 (v) peak in the Ce 3d features

[24]. b Evolution of the Ni 2p3/2 photoemission peak as a function of

the annealing temperature (hv = 1253.6 eV)

36 Top Catal (2011) 54:34–41

123

line-shape of the valence band features. The valence signal

for Ce3? cations is expected from 1.5 to 2 eV [21, 22] and

it is probably overlapping with the signal for occupied Ni

3d states. The Ni 2p XPS spectra in Fig. 1b indicate that

metallic Ni is still present in the Ni/CeO2(111) system

upon annealing to 700 and 800 K, but the valence UPS data

of Fig. 2 show that interactions of metallic Ni with the

oxide substrate significantly reduce the density of occupied

Ni 3d states near the Fermi level. Thus, one has the

coexistence of Ni2? and Ni0 species, and the metallic Ni is

electronically perturbed with respect to bulk nickel

[27, 28].

3.2 Adsorption of CO on Ni–CeO2(111) Surfaces

As we will see below, the WGS and CO methanation

reactions take place over Ni/CeO2(111) surfaces at tem-

peratures between 500 and 700 K. In this section, we

examine the adsorption of CO on clean CeO2(111) and on

Ni/CeO2(111) surfaces pre-annealed to 800 K. Figure 3

displays data recorded after dosing CO to clean CeO2 (top)

and Ni supported CeO2 (bottom) at 90 K followed by

heating to 200 and 500 K. The C K-edge NEXAFS spectra

of the adsorbed CO were collected in both the grazing (65�)

and normal (0�) incidence angles (not shown). The sharpest

peak visible is at 287 eV with a strong favor to the grazing

incidence by a ratio of 1.8:1. A smaller less angular

dependent peak is also present at 290 eV. When compared

to previous studies [29, 30] we can assign the 287 eV peak

to the p*(CO) resonance while the 290 eV feature corre-

sponds to p*(CO3). Based on these data it appears that the

physisorbed CO prefers a mode of adsorption with its plane

of orientation horizontal with respect to the surface while

the CO3 is much more disordered in orientation preferring

both the grazing and normal incidences. Very weak r*

features are also prevalent around 301 eV (not shown). The

physisorbed CO (287 eV) is present on the surface from 90

to 200 K and the carbonates (290 eV) survive up to 400 K.

For the system containing Ni (*0.4 ML), see bottom panel

in Fig. 3, the 200 K spectrum reveals a broad contribution

that can be attributed to CO chemisorbed (286.9 eV) on

different types of nickel species or on the interface between

Fig. 2 He II (40.8 eV) excited valence band photoemission spectra

for the Ni/CeO2 system as a function of the annealing temperature.

The spectrum of the CeO2 epitaxial film prior to the Ni deposition is

reported at the bottom. At 300 K, *0.4 ML of Ni were vapor

deposited on the CeO2(111) surface

0.6

π∗ C-O90 K C K-edge

0.4

0.2

200 K

105 K

π∗ CO3

Inte

nsity

(ar

bitr

ary

units

)

0.0

Photon energy (eV)

A

4 90 K π∗ C-O

C K-edge

2

200 K

π∗ CO

Inte

nnsi

ty (

arbi

trar

y un

its)

500 K

π CO3

B

Photon energy (eV)

285284 286 287 288 289 290 291 292 293

285284 286 287 288 289 290 291 292 293

Fig. 3 C K-edge NEXAFS for CO (2L exposure) adsorbed on to

clean CeO2 (top) and Ni/CeO2 (bottom) at 90 K followed by

annealing to the indicated temperatures. All spectra are in the grazing

incidence (65�). Before adsorbing CO, 0.4 ML of Ni were deposited

on the CeO2(111) surface and the sample was annealed to 800 K

Top Catal (2011) 54:34–41 37

123

Ni–CeO2. CO desorbs from NiO at 200–260 K. A shoulder

at 289.5 eV is also seen in Fig. 3 and it is likely a contri-

bution from the p*(CO3) bound to CeO2, Ni or the Ni–CeO2

interface. Annealing to 500 K shows desorption of the

chemisorbed CO and only the CO3 is left on the surface.

Figure 4 shows the C 1s region upon adsorption of CO

(2L) onto Ni/CeO2(111) and Ni/CeO2-x (111) systems

containing *0.4 ML of nickel. CO was dosed to

Ni/CeO2(111) at 90 K followed by annealing to the indi-

cated temperatures. On the Ni/CeO2 surface two broad

peaks centered nominally at 286.2 and 292 eV appear. The

same exposure of CO to clean CeO2 (not shown) exhibited

only features coming from CO–Ce at *291–292 eV, and

hence the 286.2 eV peak is likely a photoemission feature

for CO–Ni. A peak at 286.5 eV was found when metals

(M = Rh, Pd) were deposited on CeO2(111) and CO was

bound directly to them (M–CO) [31, 32]. The broad

291–292 eV peak has been reported previously in several

studies dealing with CO adsorption on ceria surfaces and

was attributed to CO3 species based on the expected BE

fingerprint for a carbonate species [29, 30] but, on the basis

of the NEXAFS data in Fig. 3, it can also have contribu-

tions of physisorbed CO up to 200 K. Annealing beyond

200 K shows the gradual desorption of CO species and by

500 K only traces of CO3 are left on the surface. There was

no clear signal for the decomposition of CO on the Ni/

CeO2(111) surface. On the other hand, when CO was

adsorbed onto a Ni/CeO2-x(111) surface at 300 K, bottom

of the figure, we observed the CO–Ni peak and a weak

contribution of CO3 species together with an additional

C–M species at 281.8 eV, which appears in between the C

1s binding energies of Ni3C (283.9 eV) and NiCx

(281.2 eV) [2]. The fact that CO dissociates on Ni/

CeO2-x(111) has important implications for the WGS and

CO methanation reactions. The NiO/Ce1-xNixO2-y species

formed upon annealing of Ni/CeO2(111) to 700 or 800 K

(Figs. 1, 2) are not stable under the conditions of the WGS

and CO methanation reactions being reduced by CO and

hydrogen to Ni/CeO2-x(111).

3.3 WGS Reaction on Ni/CeO2(111) Surfaces

Previous studies have examined the WGS reaction on

model catalysts such as Cu(111) [33], Cu(100) [17–19] or

Cu(110) [33], and on copper nanoparticles dispersed on

well-defined oxide surfaces [17–19, 34]. In this work, we

investigated the WGS activity of Ni/CeO2(111) surfaces

prepared following the steps shown in Figs. 1 and 2.

Figure 5 displays the WGS activity of model Ni/CeO2(111)

catalysts as a function of admetal coverage. The clean

CeO2(111) surface displayed no activity for the WGS under

the reaction conditions investigated here [17–19]. Upon

adding Ni to CeO2(111), there is a continuous increase in the

catalytic activity until a maximum is reached at coverages of

0.2–0.25 ML. For these systems, the production of methane

is negligible. On the other hand, when the coverage of Ni is

around 0.4 ML, one sees the appearance of CH4 as a reaction

product. For Ni coverages in the range of 0.4–2.5 ML, there

is a continuous decrease in the WGS activity while the rate

of formation of methane increases. This trend can be

attributed to a decrease in the WGS catalytic activity when

the Ni particle size becomes very large [5]. A similar phe-

nomenon has been seen for copper dispersed on well-defined

oxide surfaces [17–19, 34].

The kinetic data in Fig. 5 were collected using a reaction

cell attached to an ultra-high vacuum chamber for surface

characterization [17–19]. The gases were pumped out from

the reaction cell and the Ni/CeO2(111) surfaces were post-

characterized using standard XPS. In the C 1s region we

found typical peaks for adsorbed formate- or carbonate-like

species near 290 eV [20]. These species have been pro-

posed by some authors as intermediates in the WGS

reaction [15, 20] but they also could be the product of

readsorption of CO2. In the C 1s region we also detected a

small peak near 281.5 eV that could arise from carbon

bound to nickel [2]. In the post-reaction XPS, the Ni 2p

core-level showed that all the Ni2? present in the fresh

samples as NiO or Ce1-xNixO2-y was reduced during

reaction to metallic Ni or NiCx which both have a Ni 2p3/2

binding energy close to 853.2 eV [2]. An analysis of the

line-shape for the Ce 3d core levels [22, 35] indicated that

the ceria support underwent a partial reduction (CeO1.92–

CeO1.96 as final stoichiometries). Thus, systems consisting

of Ni or NiCx particles supported on CeO2-x(111) were

probably the active phase during the WGS reaction.

2.0

1.6

1.8

CO

CO3

CO

90 K

200 K

400K

1.0

1.2

1.4

Ni / CeO2Clean

C

Ce4s

500 K

0.4

0.6

0.8

Ni / CeO2-x

300 K

C

C1s

Inte

nsity

(ar

bitr

ary

units

)

C1s

Binding energy (eV)292294 290 288 286 284 282 280 278

Fig. 4 C 1s spectra for the adsorption of CO (2L exposure) on Ni

deposited CeO2(111), top, and CeO2-x(111), bottom. The electrons

were excited with a photon energy of 400 eV. Before adsorbing CO,

0.4 ML of Ni were deposited on the CeO2(111) and CeO2-x(111)

surfaces and the sample was annealed to 800 K

38 Top Catal (2011) 54:34–41

123

Figure 6 compares the WGS activity of Ni/CeO2(111)

and Cu/CeO2(111) [17–19]. Cu–CeO2 is a well known

catalyst for the WGS [15, 36]. For small coverages of the

admetal (\0.25 ML), Ni/CeO2(111) is a better WGS cat-

alyst than Cu/CeO2(111). However, the deposition of

0.4–0.6 ML of copper on CeO2(111) produces surfaces

with the highest catalytic activity in Fig. 6. At large cov-

erages of the admetal ([0.4 ML), the Cu/CeO2(111) sys-

tems are better WGS catalysts than the Ni/CeO2(111)

systems. This trend seems to reflect the fact that metallic

Cu is a better WGS catalyst than metallic Ni [37].

Figure 7 shows Arrhenius plots for the best Ni/

CeO2(111) and Cu/CeO2(111) catalysts in Fig. 6. For

comparison we also include the corresponding data for Cu/

ZnO(000ı) and Cu(100) surfaces [17–19]. Cu/ZnO is a

common industrial catalyst for the WGS [1, 15], and

Cu(100) has become a benchmark to study the WGS on

extended surfaces of metals [17–19, 37]. The data in Fig. 7

indicate that the metal/oxide catalysts are more active than

Cu(100) at temperatures between 575 and 650 K. Ni/CeO2

always has a WGS activity smaller than that of Cu/CeO2,

but it has a higher activity than Cu/ZnO. If one normalizes

the production of H2 by the admetal coverage (0.25 ML of

Ni; 0.5 ML of Cu), then Ni/CeO2 becomes an even better

catalyst than Cu/ZnO. The slopes of the Arrhenius plots

gave apparent activation energies of 15.2 kcal/mol for

Cu(100) [17–19], 12.4 kcal/mol for Cu/ZnO(000ı) [17–19],

10.4 kcal/mol for Ni/CeO2(111), and 8.6 kcal/mol for Cu/

CeO2(111) [17–19]. The rate-determining step for the

WGS on Cu(100) is the dissociation of water [37]. In the

metal/oxide systems, the oxide may help with the

dissociation of water [17–19, 35, 38] lowering in this way

the apparent activation energy for the reaction. Indeed,

tests experiments showed that Ni/CeO2(111) is more active

for the dissociation of water than clean CeO2(111),

Cu(100) [17–19, 39] and Ni(100) [39].

mo

lecu

le c

m-2

6

7

820 Torr CO, 10 Torr H2O 625 K

17

2

3

4

5

CO2H2

Nikcel (111) / ML0.0 0.4 0.8 1.2 1.6 2.0 2.4

mo

lecu

les

pro

du

ced

/ 10

0

1

2

CH4

coverage on COe2 /

Fig. 5 WGS activity of model Ni/CeO2(111) catalysts as a function

of admetal coverage. The Ni/CeO2(111) catalysts were prepared by

depositing Ni on CeO2(111) at 300 K followed by annealing to

800 K. Each Ni/CeO2(111) surface was exposed to a mixture of

20 Torr of CO and 10 Torr of H2O at 625 K for 5 min. Steady-state

was reached 2–3 min after introducing the gases in the batch reactor2

11

17 m

ole

cule

cm

-2

7

8

9

10 20 Torr CO, 10 Torr H2O 625 K

C

s p

rod

uce

d /

101

3

4

5

6Cu

H2

mo

lecu

les

0

1

2

3 Ni

Admetal coverage on CeO2(111) / ML0.0 0.4 0.8 1.2 1.6 2.0 2.4

Fig. 6 Comparison of the WGS activity of Ni/CeO2(111) and Cu/

CeO2(111) [17–19] catalysts. Each Ni/CeO2(111) or Cu/CeO2(111)

surface was exposed to a mixture of 20 Torr of CO and 10 Torr of

H2O at 625 K for 5 min)}

1.5

s cm

-2 s

-1

0.5

1.0

Cu/CeO2

mo

lecu

le

0.0 Cu/ZnO

Ni/CeO2

n{r

ate/

(1015

-1.0

-0.5

Cu(100)

1000 K/T

Ln

-1.5

( )

WGS reaction

1.55 1.60 1.65 1.70 1.75

Fig. 7 Arrhenius plot for the WGS reaction rate on a CeO2(111)

surface containing 0.25 ML of nickel. For comparison we also include

the corresponding results for Cu(100), 0.5 ML of Cu on ZnO(000ı),

and 0.5 ML of Cu on CeO2(111) [17–19]. All the data were acquired

with a pressure of 20 Torr of CO and 10 Torr of H2O

Top Catal (2011) 54:34–41 39

123

3.4 CO Methanation Reaction on Ni/CeO2(111)

Surfaces

Since CO methanation is a secondary reaction competing

with the WGS on Ni/CeO2(111) catalysts, we decided to

study it in more detail using mixtures of CO/H2 as reactants

[10, 11]. The CO methanation activity of Ni-based catalysts

is well established [2, 10–13]. The top panel in Fig. 8 shows

data for the production of CH4 from CO and H2 over a series

of Ni/CeO2(111) surfaces. For Ni coverages below 0.2 ML

the production of methane is very small. For these Ni/

CeO2-x(111) systems, there was CO dissociation and

deposition of C on the surface (In post-reaction character-

ization, a peak was seen in the C 1s region near 281.5 eV that

could arise from carbon bound to nickel [2]). The interac-

tions between small nickel particles and the carbon were

probably too strong to allow the efficient dissociation of H2

and the subsequent formation of methane (Ca ? 4Ha ?CH4,gas) [10]. At Ni coverages above 0.2 ML, there was a

substantial enhancement in the rate for methane formation.

As the Ni particle size increased [5], the strength of the

Ni $ C interactions probably decreased, and there were

more Ni sites available for the dissociation of H2 and the full

hydrogenation of C. In the bottom panel of Fig. 8, we present

the estimated turnover number (TON) for the methanation of

CO on Ni/CeO2(111) surfaces with an admetal coverage

below 1 ML. To estimate the TON, we divided the rate of

production of methane under steady-state conditions by the

total number of Ni atoms present in the catalysts. For Ni(100)

and Ni(111), under identical reaction conditions, a TON of

*0.7 CH4 molecules site-1 s-1 has been measured [10].

This value is comparable to the range of TONs estimated for

Ni coverages of 0.5–1 ML on CeO2(111): 0.72–0.84 CH4

molecules site-1 s-1. As in the case of Ni/TiO2(100) [12],

one finds that a Ni/CeO2(111) system can exhibit a CO

methanation activity higher than that of Ni(100) or Ni(111).

Experiments similar to those in Fig. 8 were also done at

temperatures of 550, 575, 600, and 650 K. We constructed

the Arrhenius plot shown in Fig. 9 for a Ni/CeO2(111) cat-

alyst with 0.6 ML of Ni. The plot gives an apparent activation

energy of 22.6 kcal/mol, which is close to the value of

24.7 kcal/mol reported for Ni(100) or Ni(111) [10], and to

the value of 25.2 kcal/mol obtained for Ni/TiO2(100) [12].

4 Summary and Conclusions

X-ray and ultraviolet photoelectron spectroscopies were

used to study the interaction of Ni atoms with CeO2(111)

surfaces. Upon adsorption on CeO2(111) at 300 K, nickel

remains in a metallic state. Heating to elevated tempera-

tures (500–800 K) leads to partial reduction of the ceria

substrate with the formation of Ni2? species that exists as

NiO and/or Ce1-xNixO2-y. The NiO/Ce1-xNixO2-y spe-

cies formed during the annealing of Ni/CeO2(111) are not

stable under the conditions of the WGS and CO metha-

nation reactions being reduced by CO and hydrogen to Ni/

CeO2-x(111).

The results of core-level photoemission and NEXAFS

point to weakly bound CO species on CeO2(111) which are

clearly distinguishable from the formation of chemisorbed

carbonates. In the presence of Ni, a stronger interaction is

observed with chemisorption of CO on the admetal. When

ule

cm

-2

4 24 Torr CO, 96 Torr H2625 K

uce

d /

1017

mo

lecu

2

3

625

4 m

ole

cule

s p

rodu

1

e c-1

Nickel coverage on CeO2 (111) / ML0.0 0.4 0.8 1.2 1.6 2.0 2.4C

H

0

mo

lecu

les

site

-1 s

0.8

1.0 24 Torr CO, 96 Torr H2 625 K

e r N

um

ber

/ C

H4

m

0.4

0.6

Est

imat

ed T

urn

ove

0.0

0.2

Nickel coverage on CeO2(111) / ML0.0 0.2 0.4 0.6 0.8 1.0E

Fig. 8 Top panel CO methanation activity of model Ni/CeO2(111)

catalysts as a function of admetal coverage. The Ni/CeO2(111)

catalysts were prepared by depositing Ni on CeO2(111) at 300 K

followed by annealing to 800 K. Each Ni/CeO2(111) surface was

exposed to a mixture of 24 Torr of CO and 96 Torr of H2 at 625 K.

The reported values correspond to the number of CH4 molecules

produced after a reaction time of 5 min under steady state conditions.

Bottom panel Estimated turnover number for CO methanation over

Ni/CeO2(111) catalysts with a nickel coverage of 0.05–1.0 ML

40 Top Catal (2011) 54:34–41

123

the Ni is in contact with a partially reduced CeO2-x(111)

surface, adsorbed CO dissociates at 300 K forming NiCx

compounds that may be involved in the formation of CH4

at higher temperatures.

At medium and large Ni coverages ([0.3 ML), the Ni/

CeO2(111) surfaces are able to catalyze the production of

methane from CO and H2, with an activity slightly higher

than that of Ni(100) or Ni(111). On the other hand, at small

coverages of Ni (\0.3 ML), the Ni/CeO2(111) surfaces

exhibit a very low activity for CO methanation but are very

good catalysts for the WGS reaction.

Acknowledgments The authors are grateful to J. Hanson, G. Zhou

and M. Perez for thought-provoking discussions about the catalytic

properties of Ni/CeO2(111). The research carried out at Brookhaven

National Laboratory was supported by the Division of Chemical

Sciences, Geosciences, and Biosciences, Office of Basic Energy

Sciences of the US Department of Energy (DE-AC02-98CH10886

contract). J. Evans thanks INTEVEP and IDB for research grants that

made possible part of this work at the Universidad Central de

Venezuela. T.-L. Chen was supported by the Division of Chemical

Sciences, Geosciences, and Biosciences, Office of Basic Energy

Sciences, US Department of Energy, under contract DE-AC05-

00OR22725 with Oak Ridge National Laboratory, managed and

operated by UT-Battelle, LLC. Use of the National Synchrotron Light

Source, Brookhaven National Laboratory, was supported by the US

Department of Energy, Office of Science, Office of Basic Energy

Sciences, under Contract No. DE-AC02-98CH10886.

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s-1)} 1 0.6 ML Ni on CeO2(111)

cule

s cm

-2

0

24 Torr CO, 96 Torr H2

(1015

mo

le

-1

Ln

{rat

e/

-2CO methanation

1000 K/T1.55 1.60 1.65 1.70 1.75 1.80 1.85

Fig. 9 Arrhenius plot for the CO methanation reaction rate on a

CeO2(111) surface containing 0.6 ML of nickel. All the data were

acquired with a pressure of 24 Torr of CO and 96 Torr of H2

Top Catal (2011) 54:34–41 41

123