The adsorption and oxidation of cyanogen on copper surfaces

Article abstract of DOI:10.1016/S0039-6028(03)00533-8

The adsorption of cyanogen on clean and oxygen pre-treated graphite supported copper films, and a polycrystalline copper surface, and the co-adsorption of cyanogen and oxygen on graphite supported copper films, and a polycrystalline copper surface has been studied using X-ray photoelectron spectroscopy. Cyanogen dissociates on the copper surfaces at 300 K, yielding an adsorbed cyano group, CN_(a). On the oxygen pre-treated copper surface cyanogen reacts quantitatively with the adsorbed oxygen at 300 K to form a surface cyanate species, NCO. On annealing to 600 K this species decomposes, leaving only N adatoms and residual adsorbed CN on the surface. The co-adsorption of cyanogen and oxygen from a cyanogen-oxygen mixture enhances the formation of NCO to the extent that all available surface oxygen is consumed to form NCO on annealing at 450 K. In the absence of available atomic surface oxygen NCO does not decompose at temperatures up to 600 K. NCO and NCO₂ are shown to be the intermediates in the oxidation of cyanogen on copper films and a polycrystalline copper foil.

Full text of DOI:10.1016/S0039-6028(03)00533-8

Surface Science 537 (2003) 64–74
www.elsevier.com/locate/susc
The adsorption and oxidation of cyanogen on copper surfaces
a,
A.F. Carley , M. Chinn , C.R. Parkinson
b
a,1
*
a
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, UK
b
Dstl, Porton Down, Salisbury, Wiltshire SP4 0JQ, UK
Received 23 January 2003; accepted for publication 30 March 2003
Abstract
The adsorption of cyanogen on clean and oxygen pre-treated graphite supported copper films, and a polycrystalline
copper surface, and the co-adsorption of cyanogen and oxygen on graphite supported copper films, and a polycrys-
talline copper surface has been studied using X-ray photoelectron spectroscopy. Cyanogen dissociates on the copper
surfaces at 300 K, yielding an adsorbed cyano group, CNðaÞ. On the oxygen pre-treated copper surface cyanogen reacts
quantitatively with the adsorbed oxygen at 300 K to form a surface cyanate species, NCO. On annealing to 600 K this
species decomposes, leaving only N adatoms and residual adsorbed CN on the surface. The co-adsorption of cyanogen
and oxygen from a cyanogen–oxygen mixture enhances the formation of NCO to the extent that all available surface
oxygen is consumed to form NCO on annealing at 450 K. In the absence of available atomic surface oxygen NCO does
not decompose at temperatures up to 600 K. NCO and NCO
cyanogen on copper films and a polycrystalline copper foil.
Ó 2003 Elsevier Science B.V. All rights reserved.
2
are shown to be the intermediates in the oxidation of
Keywords: X-ray photoelectron spectroscopy; Chemisorption; Surface chemical reaction; Copper; Carbon; Cyanogen; Polycrystalline
thin films; Metallic films
þ
1
. Introduction
counterparts, CO and NO , and the wealth of
comparative literature that is available relating to
CO–metal interactions. Research into cyanogen
and related molecules is also in part driven by their
acute toxicity and hence their use in chemical
warfare agents. Protection against these chemical
warfare agents exists in the form of ÔactivatedÕ
carbon filters housed within gas masks. Carbon
activated with copper and chromium has been
shown [17] to deactivate hydrogen cyanide and
cyanogen; however the mechanism by which this
occurs is not fully understood. Furthermore, iso-
cyanides (metal–NC) and isocyanates (metal–
NCO) have been shown to be intermediates in
Cyanogen has been the subject of many inves-
tigations [1–16], partly because the metal–cyanide
interface provides the simplest model system for
the adsorption of organic nitriles, but also because
of expected similarities in the chemistry exhibited
À
by CN to the chemistry shown by its isoelectronic
*
Corresponding author. Tel.: +44-29-2087-4139; fax: +44-
9-2087-4030.
E-mail address: carley@cardiff.ac.uk (A.F. Carley).
2
1
Present address: Department of Physics, University of
Warwick, Coventry CV4 7AL, UK.
x
the catalytic reduction of NO by hydrocarbons
0
039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00533-8

A.F. Carley et al. / Surface Science 537 (2003) 64–74
65
[12–14], and have also been observed in the inter-
action of NO, O and CO with Cu [15]. Saussey
from a co-adsorption study to suggest that the
decomposition of NCO only occurs via an NCO
2
2
and co-workers [12] have shown that in the pres-
ence of Cu, acrylonitrile (among the reaction
products formed between the reaction of NO and
propane over H-ZSM-5 zeolites) is decomposed
into ethylene and copper cyanide. Under the re-
action conditions employed the copper cyanide is
understood to isomerize to copper isocyanide and
further convert into copper isocyanate (Cu–NCO).
In the presence of water and NO the isocyanate is
transient intermediate. This work was undertaken
as part of an ongoing project to study the inter-
2 2
action of a range of molecules (CO, NO, O , H O)
with copper clusters and films deposited on a
graphite substrate.
2. Experimental
converted to N
14].
The reaction of cyanogen on the Cu(1 1 1) sur-
face was first investigated by Solymosi and co-
workers [3–5]. Adsorbed CN was shown to be a
very stable species, which desorbed completely as
cyanogen above 700 K. However, in the presence
of adsorbed oxygen, cyanogen was observed to
undergo oxidation at a temperature slightly above
2
via an Cu–(NH
3
)
n
intermediate
The experimental setup used in this study has
been described in detail elsewhere [18]. Briefly,
experiments were performed on a Vacuum Gen-
erators ultrahigh-vacuum (UHV) photoelectron
spectrometer equipped with a dual anode X-ray
source, concentric hemispherical analyzer, Ar ion
sputtering gun, Knudsen molecular effusion cell
(K-cell; WA Technology), and a quadrupole mass
analyzer (Ledamass). The XP spectra were ob-
tained using an Al-anode and the binding energies
were calibrated relative to the Cu(2p3=2) peak for
bulk copper at 932.7 eV [19].
[
4
00 K yielding carbon dioxide, nitrogen, and cy-
anogen. A cyanate (Cu–OCN) or an isocyanate
Cu–NCO) species was proposed as an interme-
(
diate in the oxidative decomposition pathway. A
recent reflection absorption infrared spectroscopy
The graphite sample (Goodfellows 99.999%)
was mounted with metal fastenings on a UHV
sample manipulator with facilities for liquid ni-
trogen cooling to 80 K and indirect resistive
heating to 700 K. The sample temperature was
measured using a thermocouple mounted close to
the sample holder. The surface was cleaned in
vacuum with cycles of Ar ion sputtering followed
by annealing in vacuum at 700 K. The resulting
surface was non-crystalline, as evidenced by the
loss of satellite structure at approximately 8 eV
toward higher binding energy from the main C(1s)
peak [20].
(
RAIRS) study [10] has shown that both the de-
composition of isocyanic acid and the reaction of
cyanogen with oxygen yield an isocyanate complex
bonded to the Cu through the nitrogen end of the
molecule. Furthermore, ab initio electronic struc-
ture calculations for OCN on Ni(1 0 0) by Yang
and Whitten [16] found that a linear NCO species
bound end-on through the N atom was more sta-
ble than either an end-on O-bonded OCN species
with the axis perpendicular or a side-on OCN
species bonded parallel to the surface. Madix et al.
[
11], in a TPD, XPS, and UPS investigation of
Controlled quantities of copper were deposited
onto the cleaned graphite surface at room tem-
perature via the evaporator, operating at a tem-
perature of 1373 K, the copper film thickness being
estimated from the attenuation of the C(1s) sub-
strate peak. Surface adatom concentrations were
determined using quantification procedures which
have been discussed elsewhere [21, and references
therein] and are summarized here:
cyanogen adsorption on Cu(1 1 0), also found ev-
idence for the formation of an NCO species on
oxygen pre-treated copper surfaces, but in addi-
tion proposed a second transient intermediate,
x
NCO , to explain the oxidative decomposition
pathway of NCO.
In this paper we present quantitative XPS evi-
dence supporting the formation of the cyanate
species, NCO, from the reaction of cyanogen and
oxygen on supported thin copper films and a
polycrystalline copper surface, and report evidence
(i) Peak intensities are calculated after subtrac-
tion of a Shirley background using a simple

66
A.F. Carley et al. / Surface Science 537 (2003) 64–74
trapezoidal procedure and normalized to
3. Results
standard experimental conditions (dwell
time and number of scans). Spectra consist-
ing of overlapping components were curve-
fitted using gaussian functions, in order to
determine the intensities of individual compo-
nents.
3.1. Cyanogen adsorption on a clean copper film
The N(1s) XPS spectra taken following the ex-
ꢀ
posure of a clean 20 A thick graphite supported
copper film to 100 L of cyanogen at 300 K and
annealed to various temperatures are shown in
Fig. 1. At 300 K a single peak is observed in the
N(1s) spectrum with a binding energy of 398.0
eV. Following annealing to 600 K no change
in intensity or shape is observed in the N(1s)
peak. At 300 K the C(1s) spectrum (Fig. 1, inset)
shows a single peak at a binding energy of
(
ii) Intensities are corrected for the variation of
analyzer transmission with photoelectron ki-
netic energy E
transmission varies as E .
K
––for our instrument the
À1
K
(
iii) Adatom concentrations r
from [21]
X
are calculated
I
X
=I ¼ lXr
S
X
=l N
S
S
k cos /
S
284.0 eV. The N and C adatom concentrations
where X and S refer to the adspecies and
substrate respectively, I is the corrected in-
tensity of a photoemission peak and l is the
Scofield photoionization cross-section for the
particular core-level, corrected for angular
effects. N is the number density of substrate
S
atoms, k the mean free path of the substrate
S
core-level photoelectron in the substrate, and
is the electron detection measured with re-
/
spect to the substrate normal. We have ap-
plied this procedure to systems where absolute
surface coverages are known, for example the
p(2 Â 1) oxygen structure for Cu(1 1 0)–O and
the c(2 Â 2) structure for Cu(1 1 0)–Cl, and to
systems where relative adatom concentrations
are known, such as physisorbed molecules:
acetonitrile, sulfur dioxide, methylamine,
carbonyl sulfide, and methyl mercaptan.
From these measurements we estimate ad-
atom concentrations to be accurate to better
than 10%.
Cyanogen was prepared by the thermal de-
composition of silver cyanide (Aldrich 99.99%)
22] in a stainless steel cylinder mounted on a
[
UHV gas line. The resulting gas was purified
by successive freeze pump thaw cycles, and no
impurities were detected with the mass spectro-
meter.
All experiments were conducted at sample tem-
peratures above 150 K to prevent X-ray-induced
polymerization of condensed cyanogen multilayers
ꢀ
Fig. 1. N(1s) spectra obtained from a clean 20 A thick copper
film surface: (a) after exposure to 100 L cyanogen at 300 K, (b)
on heating to 450 K, and (c) after heating to 600 K. Inset is
shown the C(1s) spectrum corresponding to (a).
[
2].

A.F. Carley et al. / Surface Science 537 (2003) 64–74
67
derived from an analysis of the N(1s) and C(1s)
peaks observed at 300 K are in the ratio 1:1. Fur-
ther analysis of the C(1s) spectra on annealing the
substrate to 600 K reveals strong interference
due to the underlying graphite substrate C(1s)
peak. Intercalation of Cu into the graphite matrix
has not been observed after Cu deposition
and subsequent annealing [23]. The presence of a
C(1s) peak from the underlying graphite sup-
port precluded us from identifying any cyanogen-
induced C(1s) features with any certainty, and
consequently, the interaction of cyanogen and
oxygen on a polycrystalline sample was also
studied.
3.2. Cyanogen adsorption on a pre-oxidized copper
film
3.2.1. Thin copper film
Fig. 2 shows O(1s) and N(1s) spectra taken
following the exposure of an oxygen pre-treated
ꢀ
(200 L oxygen at room temperature) 20 A thick
copper film to cyanogen (100 L). In order to follow
the formation of any intermediates, spectra were
taken as a function of temperature. The adsorp-
tion of oxygen onto the copper surface at room
temperature results in a single peak in the O(1s)
spectrum at 529.5 eV, characteristic of oxygen
adatoms [24]. After exposing the surface to
ꢀ
Fig. 2. O(1s) and N(1s) spectra obtained from a 20 A thick copper film: (a) after exposure to 200 L oxygen at 300 K, (b) after exposure
to 100 L cyanogen at 300 K, (c) after heating to 450 K, and (d) after heating to 600 K.

68
A.F. Carley et al. / Surface Science 537 (2003) 64–74
cyanogen at room temperature, a second peak
with a binding energy of 532.5 eV is observed in
the O(1s) spectrum, and a single peak in the N(1s)
region at a binding energy of 398.0 eV is observed.
Following annealing at 450 K the O(1s) peaks at
quent annealing at 600 K, are shown in Fig. 3. The
adsorption behavior of cyanogen on the pre-oxi-
dized polycrystalline copper surface is very similar
to that observed on the pre-oxidized Cu film.
However, for this polycrystalline Cu surface the
C(1s) region could be studied without interference
from the underlying graphite. Exposing the Cu
surface to 200 L oxygen gives rise to a single peak
in the O(1s) region at 529.5 eV, indicative of oxygen
adatoms O(a). After dosing the surface with cya-
nogen at room temperature, a second peak emerges
in the O(1s) region at 532.5 eV. At the same time
two peaks are observed in the C(1s) region at 284.0
and 287.0 eV. In the N(1s) region only a single peak
at a binding energy of 398.0 eV is observed. On
annealing at 450 K, the O(1s) peak components at
binding energies of 529.5 and 532.5 eV and the
C(1s) peak component at a binding energy of 287.0
eV are observed to decrease in intensity, reflecting a
decrease in the surface concentrations of the cor-
responding species in a 1:1:1 ratio. At this tem-
perature a second peak at a binding energy of 396.5
eV in the N(1s) region emerges on the shoulder of
the main peak at 398.0 eV. On further annealing at
600 K almost complete loss of oxygen species is
observed in the O(1s) region and the high binding
energy peak in the C(1s) region is observed to dis-
appear. In the N(1s) region the second peak at a
binding energy of 396.5 eV is observed to grow
slightly at the expense of the fist peak at a binding
energy of 398.0 eV. Table 2 tabulates the surface
concentrations for the O(1s), N(1s) and C(1s) spe-
cies observed at temperatures from 300 to 600 K
for the adsorption of cyanogen on a oxygen pre-
treated polycrystalline Cu.
5
29.5 and 532.5 eV are observed to decrease in
intensity in a 1:1 ratio. A shoulder is observed to
develop in the N(1s) region at 396.5 eV at this
temperature, characteristic of N adatoms [25] and
a loss of intensity of the N(1s) component at 398.0
eV is observed. The decrease in the surface con-
centration of the species with an N(1s) binding
energy of 398.0 eV equals the increase in concen-
tration of the species corresponding to the N(1s)
component at 396.5 eV, and also equals the de-
crease in concentration of each of the oxygen
species. It is this quantitative information which
will allow us to comment later on the mechanism
of cyanogen oxidation.
Further annealing at 600 K results in the com-
plete loss of the peaks in the O(1s) region at 529.5
and 532.5 eV. The 396.5 eV peak intensity in the
N(1s) region is observed to increase, and a con-
comitant loss of intensity of the N(1s) component
at 398.0 eV is observed at this temperature. Table
1
tabulates the surface concentrations for the O(1s)
and N(1s) species observed at temperatures from
00 to 600 K for the adsorption of cyanogen on an
oxygen pre-treated Cu film.
3
3
.2.2. Polycrystalline copper foil
N(1s), C(1s), and O(1s) XPS spectra, obtained
following the exposure of an oxygen pre-treated
200 L oxygen at room temperature) polycrystal-
line copper surface to 100 L cyanogen and subse-
(
Table 1
À2
ꢀ
XPS-derived surface concentrations (cm ) for a pre-oxidized 20 A thick copper film exposed to cyanogen and then annealed at the
temperatures indicated
Binding energy (eV)
O(1s)
29.5
N(1s)
396.5
5
532.5
398.0
14
14
14
2
1
4
6
00 L O
2
at 300 K
5.2 Â 10
3.2 Â 10
1.6 Â 10
–
–
–
–
14
14
14
14
14
00 L (CN)
50 K
00 K
2
at 300 K
2.5 Â 10
1.1 Â 10
–
–
4.6 Â 10
3.5 Â 10
1.8 Â 10
14
1.2 Â 10
2.3 Â 10
14

A.F. Carley et al. / Surface Science 537 (2003) 64–74
69
Fig. 3. O(1s), C(1s), and N(1s) spectra obtained from a polycrystalline copper surface: (a) after exposure to 200 L oxygen at 300 K, (b)
after exposure to 100 L cyanogen at 300 K, (c) after heating to 450 K, and (d) after heating to 600 K.
Table 2
XPS-derived surface concentrations (cm ) for a pre-oxidized copper foil exposed to cyanogen and then annealed at the temperatures
indicated
À2
Binding energy (eV)
O(1s)
29.5
C(1s)
284.2
N(1s)
396.5
5
532.5
286.9
398.0
14
14
14
2
1
4
6
00 L O
2
at 300 K
5.3 Â 10
3.3 Â 10
1.2 Â 10
–
–
–
–
–
–
14
14
14
14
14
14
14
14
14
00 L (CN)
50 K
00 K
2
at 300 K
2.2 Â 10
1.4 Â 10
–
2.1 Â 10
2.0 Â 10
2.0 Â 10
1.9 Â 10
1.0 Â 10
–
–
3.9 Â 10
3.0 Â 10
2.0 Â 10
14
14
0.7 Â 10
1.8 Â 10
14
3
.2.3. Co-adsorption of cyanogen and oxygen on a
thin copper film
passivate the copper surface [9], a mixture with
excess oxygen was employed. The oxygen to cya-
nogen mixture ratio was determined by mass
spectroscopy to be 20 to 1. No other impurities
were detected in the mixture.
Exposing the Cu surface to 100 L of the oxygen/
cyanogen mixture gives rise to two peaks in the
O(1s) region at 529.5 and 532.5 eV with intensities
O(1s) XPS spectra taken following exposure of a
ꢀ
0 A thick copper film to an oxygen/cyanogen
2
mixture at 300 K, and subsequent annealing at
various temperatures are shown in Fig. 4. Due to
the higher sticking probability of cyanogen com-
pared with oxygen, and the ability of cyanogen to

70
A.F. Carley et al. / Surface Science 537 (2003) 64–74
in the ratio of approximately 1:2. At the same time
an intense peak is observed in the N(1s) region at
98.0 eV. On annealing at 450 K, complete loss of
3
the O(1s) component at 529.5 eV is observed, and
a slight increase in the intensity of the O(1s) peak
at 532.5 eV is observed. The N(1s) spectrum re-
mains unchanged at this temperature. On further
annealing at 600 K no further changes are ob-
served in either the O(1s) or the N(1s) spectra.
Table 3 tabulates the surface concentrations for
the O(1s), and N(1s) species observed at temper-
atures from 300 to 600 K for the co-adsorption of
ꢀ
cyanogen and oxygen on a 20 A thick Cu film.
4. Discussion
4.1. Clean copper surface
Previous work has shown that cyanogen disso-
ciates on the Cu(1 1 0) [11] and copper film surfaces
2,9] at temperatures above 150 K to yield ad-
sorbed CN.
[
ðCNÞ ! 2CNðaÞ
2
The peaks at 398.0 and 284.0 eV in the N(1s) and
C(1s) spectra respectively are thus assigned to the
adsorbed CN group, consistent with the previous
studies [2,9,11].
For most diatomic molecules such as NO and
CO there is a consensus that the typical bonding
geometry to the surface is of an end-on configu-
ration, where the molecular axis is perpendicular
(
or almost perpendicular) to the surface. Evidence
ꢀ
Fig. 4. O(1s) spectra obtained from a 20 A thick copper film:
a) after exposure to 100 L of a oxygen:cyanogen mixture (20:1)
at 300 K, (b) after heating to 450 K, and (c) after heating to
00 K.
from transition metal co-ordination chemistry
leads one to predict an end-on bonding geometry
for CNðaÞ, most likely through the C atom. For
(
6
Table 3
À2
ꢀ
XPS-derived surface concentrations (cm ) for a pre-oxidized 20 A thick copper film exposed to an oxygen–cyanogen (20:1) mixture
and then annealed at the temperatures indicated
Binding energy (eV)
O(1s)
29.5
N(1s)
396.5
5
532.5
398.0
13
14
14
14
14
Mixture at 300 K
5.4 Â 10
1.0 Â 10
1.1 Â 10
1.1 Â 10
–
–
–
5.6 Â 10
5.7 Â 10
5.4 Â 10
1
1
4
4
4
6
50 K
00 K
–
–

A.F. Carley et al. / Surface Science 537 (2003) 64–74
71
example, nearly all the d-block transition elements
are known to form cyano-complexes [27,28], with
the cyanide ion acting as a monodentate ligand via
the carbon atom. There are of course exceptions to
this, for example the cyanide ion can act as a
donor at both ends of the ion, the equivalent of a
bridging agent in surface chemistry. Recent surface
studies on the adsorption of cyanogen on Cu and
Pd surfaces using HREELS [6–8] suggest that the
CN species is adsorbed on the clean surface in a
parallel or close to parallel bonding configuration.
Such a bonding configuration is consistent with
the maximum surface concentration reported for
adsorption of CN on a clean Cu(1 1 1) surface of
of cyanogen and pre-adsorbed oxygen can be
postulated. If NCO is considered to be the major
product from the reaction between cyanogen and
adsorbed oxygen then the appearance of the high
binding energy O(1s) feature can be explained by:
(i) The formation of the cyanate species NCO at
300 K:
300 K
ðCNÞðgÞ þ 2OðaÞ ƒƒ! NCOðaÞ þ CNðaÞ þ OðaÞ
The stoichiometric loss of both O(1s) peaks on
heating to 450 K and then to 600 K, and also the
concomitant stoichiometric appearance of the
N(1s) peak at 396.5 eV (N adatoms) can be ex-
plained by:
14
À2
3
.6 Â 10 cm [3]. There is also some evidence
that adsorbed oxygen can cause the adsorbed CN
group to change orientation [8]. In the following
section we aim to demonstrate, via XPS, that cy-
anogen reacts with oxygen to form a cyanate,
NCO, and that the C, and not the N of CNðaÞ is
directly bonded to the reacting oxygen.
(ii) The formation of short-lived NCO species,
2
NCOðaÞ þ OðaÞ ! ½NCO
2
ꢀðtÞ
which decomposes as follows:
½
NCO
2
ꢀðtÞ ! NðaÞ þ CO2ðgÞ
4
4
.2. Preoxidized copper
The surface concentration of N , formed from
ðaÞ
the decomposition of [NCO ] at 600 K, is consis-
2
.2.1. Copper film
In this paper we suggest the species resulting
tent with the proposed scheme––concommitant
with the 1:1 stoichiometric decrease in both of the
O(1s) peaks at binding energies of 532.5 and 529.5
eV, an N(1s) peak is observed in the N(1s) spec-
trum at a binding energy of 396.5 eV, due to the
formation N adatoms. The corresponding surface
concentration of this species is equal to the de-
crease in the surface concentration of either of the
O(1s) peaks. As the sample is heated to 600 K all
the NCO is thought to convert to adsorbed N
from the interaction of cyanogen and chemisorbed
oxygen is NCOðaÞ. Analysis of the O(1s) XPS
spectra clearly suggests the presence of a stable
O
x
(CN) species from the reaction of CNðaÞ with
oxygen at 300 K. However, while the O(1s) spec-
trum shows two resolvable peaks at binding
energies of 529.5 and 532.5 eV, assigned to
chemisorbed oxygen and an oxygen bound to a
cyano group [11] respectively, the stoichiometry of
adatoms and CO , leaving only adsorbed CN re-
2
sponsible for the N(1s) peak at the binding energy
the O
x
(CN) species cannot be fully determined
from direct surface concentration measurements in
this experiment because: (i) the N(1s) spectrum
shows only one peak, at a binding energy of 398.0
eV, which is consistent with the assignment for
CNðaÞ [2,9], and (ii) the C(1s) region is not avail-
able for reasons outlined earlier. Clearly, therefore
if NCO is present on the surface it not resolvable
from the binding energy of CNðaÞ. However, by
analyzing the variation of surface concentration of
the individual peaks as a function of temperature,
shown in Table 2, a surface species and a surface
reaction mechanism resulting from the interaction
of 398.0 eV. Confirmation of the N:O stoichio-
metry of the surface cyanate species can therefore
achieved by subtraction of the 398.0 eV peak area
after surface annealing from the area of the same
peak observed before surface annealing. This
subtraction reveals a 1:1 stoichiometry for the
O(1s) and N(1s) peaks associated with the cyanate
species.
It could be argued that the reduction of intensity
of the O(1s) peak at a binding energy of 529.5 eV
on annealing the surface at 600 K, is due to oxygen
adatoms recombining and desorbing from the

72
A.F. Carley et al. / Surface Science 537 (2003) 64–74
surface rather than from a cyanate–oxygen reac-
tion. In order to demonstrate that the reduction in
intensity of this peak is only due to a cyanate–
N(1s) regions at binding energies of 532.5 and
398.0 eV are assigned to NCOðaÞ, and (NCOðaÞ
CNðaÞ) species respectively. Further confirmation
of these assignments is evident from the reaction of
the polycrystalline Cu surface, where detailed
C(1s) spectra are available.
þ
ꢀ
oxygen reaction, a 20 A thick copper film was
exposed to 200 L oxygen and heated in stages to
6
00 K. The O(1s) spectra taken following the ex-
ꢀ
posure of the clean 20 A thick copper surface to
00 L oxygen are shown in Fig. 5. Following ex-
2
4.2.2. Polycrystalline copper
posure one oxygen peak is observed with a binding
energy of 529.5 eV. On heating the surface to 450
K no loss of intensity of this peak was observed.
Only after heating to 600 K was a slight loss in
intensity observed. Therefore the observed loss in
O(1s) peak intensity at 529.5 eV on annealing the
oxygen/cyanogen surface at 600 K must result
from a cyanate–oxygen reaction.
The result of adsorbing oxygen followed by cy-
anogen on the polycrystalline copper surface is
identical to that observed for the copper film.
However, for the polycrystalline Cu surface the
C(1s) region could be studied allowing us to
identify the formation and stoichiometry of the
surface cyanate species, suggested to be NCO. The
adsorption of cyanogen onto the pre-oxidized
polycrystalline copper surface gave rise to two
peaks in the C(1s) spectrum with binding energies
of 284.0 and 287.0 eV. The peak at 284.0 eV is
characteristic of adsorbed CN, and consistent with
previous studies [2,9,11]. The peak at 287.0 eV is
consistent with a binding energy for a C species
attached to an electronegative oxygen atom. This
peak is therefore is assigned to the C atom in ad-
sorbed NCO. This assignment is further supported
by the surface concentrations of the respective
peaks as: (i) the surface concentrations for the
species giving rise to the C(1s) peak component,
and the O(1s) component at binding energies of
In summary, for the reaction of cyanogen on a
pre-oxidized Cu film the peaks in the O(1s) and
287.0 and 532.5 eV, respectively, are equal, and (ii)
these concentrations decrease with 1:1 stoichio-
metry on annealing at 450 K. These observations
are consistent with the formation and decompo-
2
sition of NCO via an NCO intermediate, as out-
lined earlier. It might be suggested that the large
binding energy shift, ꢁ3 eV, observed in the C(1s)
spectrum between the peak components at 284.0
and 287.0 eV is consistent with the formation of a
O (CN) species rather than an OCN species, es-
2
pecially considering the binding energy shift be-
tween CðaÞ and OCH3ðaÞ has been shown to be only
1.3 eV [26]. However, since the ratio of the C to O
atom concentrations, derived from the respective
photoemission peaks are consistent with an NCO
species, we ascribe the large binding energy shift to
final state relaxation effects.
ꢀ
Fig. 5. O(1s) spectra obtained from a 20 A thick copper film:
In contrast to the C(1s) region the absence of a
distinct, shifted N(1s) component for adsorbed
(
a) after exposure to 200 L oxygen at 300 K, (b) after heating to
4
50 K, and (c) after heating to 600 K.

A.F. Carley et al. / Surface Science 537 (2003) 64–74
73
NCO suggests that there is little interaction oc-
curring between the nitrogen of the cyano group
and the chemisorbed oxygen, and therefore any
bonding of chemisorbed oxygen to the cyano
group is most likely to occur via a C–O interac-
tion. Co-ordination of NCO to Cu can be in the
form of cyanate Cu–OCN, fulminate Cu–CNO,
isofulminate Cu–ONC, or isocyanate Cu–NCO.
The XPS data presented here suggests that either
or both the surface cyanate or isocyanate are
generated on the Cu surface under the conditions
applied here. A RAIRS study of the decomposi-
tion of isocyanic acid and the reaction of cyanogen
with pre-adsorbed oxygen on the Cu(1 0 0) surface
has shown that isocyanate complexes are the most
common and are bonded linearly to the surface
adsorption conditions, as too is the total concen-
tration of the adsorbed cyano group. Approxi-
mately twice the concentration of adsorbed cyano
species is observed under co-adsorption condi-
tions. This can easily be understood if one refers to
the nature of the oxygen species undergoing reac-
tion. During co-adsorption the dissociating cya-
nogen can react with oxygen transients (formed
during the oxygen dissociation).
However, the most noticeable difference ob-
served between the two reaction conditions is in
the stability of the NCO species generated at 300 K
under co-adsorption conditions. In the case of pre-
adsorbed oxygen, the NCO species are observed to
decompose upon annealing at 600 K to the extent
that oxygen is no longer observed on the surface.
In contrast, under co-adsorption conditions the
NCO species do not decompose as evidenced by
the O(1s) spectra. If the mechanism outlined ear-
lier, involving the decomposition intermediate
[
10].
4
.2.3. Co-adsorbed cyanogen and oxygen
To gain further insight into the reaction mech-
anism of cyanogen and oxygen on copper surfaces
a co-adsorption study was conducted. Experi-
mental evidence has shown that a distinction has
to be made between the chemistry associated with
the sequential adsorption of oxygen followed by a
second reactant and that of co-adsorption of an
oxygen–reactant mixture [29–31]. In the case of co-
adsorption oxygen transients can control the ki-
netics and mechanisms of surface reactions. In
contrast, on copper surfaces partially covered with
pre-adsorbed oxygen, two different oxygen species
exist––an ÔactiveÕ oxygen species (which can acti-
vate the adsorption of other molecules) associated
with adsorption sites at the edges of oxygen is-
lands, and an ÔinactiveÕ oxygen species corre-
sponding to oxygen adatoms in the interior of
these islands. Pre-adsorbed oxygen has been
shown not to promote the adsorption of cyanogen;
with an increase in oxygen coverage the extent of
cyanogen adsorption has been shown to gradually
decrease [3,9].
2
NCO , is correct then NCO requires available
surface atomic oxygen to decompose. The O(1s)
spectra observed under co-adsorption conditions
suggest that most of the surface atomic oxygen
formed from the dissociative adsorption of dioxy-
gen has been consumed at 300 K, resulting in only
a weak O(1s) peak at a binding energy of 529.5 eV
2À
corresponding to O (a). This feature disappears
on heating to 450 K leaving behind an adlayer
comprising only NCO(a). It then follows that
without any further available oxygen adatoms, the
NCO species cannot form the NCO intermediate
2
2
necessary for decomposition into CO and ad-
sorbed N adatoms, and therefore remain bonded
to the surface.
5. Conclusion
Cyanogen adsorbs and dissociates on copper
film surfaces at 300 K yielding a surface cyano
group, CNðaÞ. The cyano groups react with oxygen
under pre-adsorption and co-adsorption condi-
tions to yield a surface cyanate, NCO. On heating
the surface above 450 K, and in the presence of
available surface oxygen, NCO decomposes via a
The XPS data in Figs. 2 and 4 illustrate the clear
distinction that must be made between the reac-
tivity of oxygen/cyanogen mixtures on clean cop-
per surfaces and the reactivity of cyanogen on
copper surfaces with a pre-adsorbed oxygen cov-
erage. The formation of the NCO species at 300 K
on the copper surface is enhanced under co-
NCO intermediate. In the absence of available
2
surface oxygen NCO is stable on the copper

74
A.F. Carley et al. / Surface Science 537 (2003) 64–74
surface up to 600 K. The cyanogen/oxygen co-
adsorption experiments confirm that the surface
NCO species does not decompose on the surface
below 600 K by any other route, e.g., via dispro-
portionation.
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