The enhanced catalytic dissociation of adsorbed hydrogen containing molecules
Some general conceptions and mechanisms of dissociative adsorption and catalysis are analyzed. The role of such factors as electronic shell configurations of isolated atoms, crystalline structure of the surface, local structural irregularities and defects, orientation of the surface bonds, dimensio...
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Інститут фізики конденсованих систем НАН України
1999
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| Цитувати: | The enhanced catalytic dissociation of adsorbed hydrogen containing molecules / V.G. Litovchenko, A.A. Efremov // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 561-576. — Бібліогр.: 11 назв. — англ. |
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Litovchenko, V.G. Efremov, A.A. 2017-06-12T12:02:27Z 2017-06-12T12:02:27Z 1999 The enhanced catalytic dissociation of adsorbed hydrogen containing molecules / V.G. Litovchenko, A.A. Efremov // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 561-576. — Бібліогр.: 11 назв. — англ. 1607-324X DOI:10.5488/CMP.2.3.561 PACS: 82.65.Jv, 82.20.W, 82.30.Nr, 33.15.F https://nasplib.isofts.kiev.ua/handle/123456789/120594 Some general conceptions and mechanisms of dissociative adsorption and catalysis are analyzed. The role of such factors as electronic shell configurations of isolated atoms, crystalline structure of the surface, local structural irregularities and defects, orientation of the surface bonds, dimensional effects, the presence of foreign atoms in the local atomic environment as well as the local symmetry of adsorption center are discussed. These considerations were used in developing the program for computer simulation of the process. The catalytic properties of PdCux surface alloys are analyzed using computer modelling and thus the enhancement of dissociative adsorption of H₂ for such a system is predicted. The experimental data, demonstrating the enhancement of sensitivity of MIS sensor with CuPd electrode in comparison with the pure Pd electrode, are presented. Аналізуються деякі загальні концепції і механізми дисоціативної адсорбції і каталізу. Обговорюється роль таких факторів, як конфігурації електронних оболонок ізольованих атомів, кристалічна структура поверхні, локальні структурні нерегулярності, орієнтація поверхневих зв’язків, ефекти розмірності, присутність чужорідних атомів у локальному атомному оточенні, а також роль локальної симетрії центра адсорбції. Ці обговорення використовуються при розвитку програми для комп’ютерного моделювання процесу. Каталітичні властивості поверхневих сплавів PdCux аналізуються, використовуючи комп’ютерне моделювання, і отже, передбачено підсилення дисоціативної адсорбції H₂ для такої системи. Представлені експериментальні дані, які демонструють підсилення чутливості сенсора метал-ізолятор-напівпровідник з електродом CuPd у порівнянні з чистим електродом Pd. This work was performed within the framework of research and development project 01RS9604/8 between TU Ilmenau, Germany and ISP NAS of Ukraine. en Інститут фізики конденсованих систем НАН України Condensed Matter Physics The enhanced catalytic dissociation of adsorbed hydrogen containing molecules Підсилена каталітична дисоціація адсорбованого водню, що містить молекули Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules |
| spellingShingle |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules Litovchenko, V.G. Efremov, A.A. |
| title_short |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules |
| title_full |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules |
| title_fullStr |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules |
| title_full_unstemmed |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules |
| title_sort |
enhanced catalytic dissociation of adsorbed hydrogen containing molecules |
| author |
Litovchenko, V.G. Efremov, A.A. |
| author_facet |
Litovchenko, V.G. Efremov, A.A. |
| publishDate |
1999 |
| language |
English |
| container_title |
Condensed Matter Physics |
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Інститут фізики конденсованих систем НАН України |
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Article |
| title_alt |
Підсилена каталітична дисоціація адсорбованого водню, що містить молекули |
| description |
Some general conceptions and mechanisms of dissociative adsorption and
catalysis are analyzed. The role of such factors as electronic shell configurations of isolated atoms, crystalline structure of the surface, local structural
irregularities and defects, orientation of the surface bonds, dimensional effects, the presence of foreign atoms in the local atomic environment as
well as the local symmetry of adsorption center are discussed. These considerations were used in developing the program for computer simulation
of the process. The catalytic properties of PdCux surface alloys are analyzed using computer modelling and thus the enhancement of dissociative
adsorption of H₂ for such a system is predicted. The experimental data,
demonstrating the enhancement of sensitivity of MIS sensor with CuPd
electrode in comparison with the pure Pd electrode, are presented.
Аналізуються деякі загальні концепції і механізми дисоціативної адсорбції і каталізу. Обговорюється роль таких факторів, як конфігурації електронних оболонок ізольованих атомів, кристалічна структура поверхні, локальні структурні нерегулярності, орієнтація поверхневих зв’язків, ефекти розмірності, присутність чужорідних атомів у
локальному атомному оточенні, а також роль локальної симетрії центра адсорбції. Ці обговорення використовуються при розвитку програми для комп’ютерного моделювання процесу. Каталітичні властивості поверхневих сплавів PdCux аналізуються, використовуючи комп’ютерне моделювання, і отже, передбачено підсилення дисоціативної адсорбції H₂ для такої системи. Представлені експериментальні дані, які демонструють підсилення чутливості сенсора
метал-ізолятор-напівпровідник з електродом CuPd у порівнянні з чистим електродом Pd.
|
| issn |
1607-324X |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/120594 |
| citation_txt |
The enhanced catalytic dissociation of adsorbed hydrogen containing molecules / V.G. Litovchenko, A.A. Efremov // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 561-576. — Бібліогр.: 11 назв. — англ. |
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2025-11-24T16:06:46Z |
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| fulltext |
Condensed Matter Physics, 1999, Vol. 2, No. 3(19), pp. 561–576
The enhanced catalytic dissociation of
adsorbed hydrogen containing
molecules
V.G.Litovchenko, A.A.Efremov
Institute of Semiconductor Physics
of the National Academy of Sciences of Ukraine,
45 Nauki Str., 252028 Kyiv, Ukraine
Received April 21, 1998
Some general conceptions and mechanisms of dissociative adsorption and
catalysis are analyzed. The role of such factors as electronic shell configu-
rations of isolated atoms, crystalline structure of the surface, local structural
irregularities and defects, orientation of the surface bonds, dimensional ef-
fects, the presence of foreign atoms in the local atomic environment as
well as the local symmetry of adsorption center are discussed. These con-
siderations were used in developing the program for computer simulation
of the process. The catalytic properties of PdCux surface alloys are ana-
lyzed using computer modelling and thus the enhancement of dissociative
adsorption of H2 for such a system is predicted. The experimental data,
demonstrating the enhancement of sensitivity of MIS sensor with CuPd
electrode in comparison with the pure Pd electrode, are presented.
Key words: dissociative adsorption, computer simulation, catalysis, gas
detection, surface alloys
PACS: 82.65.Jv, 82.20.W, 82.30.Nr, 33.15.F
1. Introduction
The problems of enhancing the catalytic activity of the surface solids were dis-
cussed for a long time both from theoretical and applied points of view [1–5]. The
classical catalytic systems ordinarily use the transition metals in different modifi-
cations, mainly as a microporous composite, polycrystalline substrates, sometimes
as monocrystalline samples. The most novel approach in this direction is using the
composite systems consisting of either the film of transition metal covered by ultra-
thin layer of another noble metal [6] or submonolayer coatings of catalytically active
atoms on inactive substrates [7,8].
The important and specific case of a catalytic process is a dissociative adsorption
of molecules leading, in the end, to intensive surface reactions between atomic parti-
c© V.G.Litovchenko, A.A.Efremov 561
V.G.Litovchenko, A.A.Efremov
Figure 1. 1D presentation of potential barrier when H2 molecule interact with
the metal surface. 1,1’ is a repulsive curve for molecule, 1 is a pre-chemisorption
state. 2, 3, 4 are attractive curves for atomic hydrogen for Cu, Pd (declined
bonds) and Pd (normal bonds) correspondingly 2’, 3’, 4’ are respective barriers;
5 is a repulsive curve for bounded Ha
cles. This process also plays one of the key roles in gas detection by metal-insulator-
semiconductor (MIS) sensors with a catalytically active (Pt, Pd etc.) metallic layer.
Numerous experimental data show that catalytic yield of a reaction depends strongly
on both the structure and on the composition of the catalyst surface [1–5]. In spite
of such noble metals as Cu, Au of Ag are usually bad catalysts, the use of surface
alloys consisting of transition and noble metals instead of a pure transition metal
leads in many cases to more pronounced catalytic properties. Even isolated atoms
deposited on the inert surface may demonstrate in some cases a very high catalytic
activity [8]. Therefore, such a fundamental factor as an electron shell configuration of
atoms, together with local short range order and crystalline structure at the surface
are simultaneously important for catalytic behaviour of solids. At least, two fac-
tors, which were not enough analyzed, have to be considered in this connection: (i)
peculiarities of chemical bonds (their rehybridization, orientation, directness, weak-
ening, reattachment etc.) on the surface, where actually the catalytic processes take
place. (ii) the effect of the active doping atoms (ligands), which are present in the
local vicinity of transition metal atom (up to the forming of alloys) on the catalysis
reaction event.
In the present paper we shall discuss some general conceptions and mechanisms of
dissociative adsorption and catalysis starting from electronic configurations of atoms
being part of an active adsorption center. Then, we shall consider the role both of the
562
The enhanced catalytic dissociation. . .
structural and impurity peculiarities of the surface in these processes. More detailed
analysis of the catalytic properties of PdCux surface alloy will be presented. We shall
study dissociative adsorption of hydrogen at the active centers of such a surface in the
framework of both the macroscopic and the microscopic models using two variants
of computer modelling. In the former case the model describing the processes in
MIS gas sensors with Pd (or PdCu) electrode will be considered. In the latter case
the computer simulation will be applied to dissociative adsorption itself. We shall
show that CuPd alloy is, in some extent, the optimum catalyst for decomposition of
hydrogen containing molecules. In conclusion, the experimental data demonstrating
the enhancement of sensitivity of MIS gas sensor with Pd electrode, covered with
an ultra-thin CuPd alloy film, in comparison with pure Pd film will be presented.
2. Qualitative consideration of the adsorbo-catalytic cen ters
on the transition metals
2.1. Configurations of the electronic shells in isolated ato ms and their ef-
fect on adsorption
To understand the role of different factors listed above effecting the interaction
of the molecule with the surface it is useful to analyze a one-dimensional potential
diagram of this process (in the case of H2) shown in figure 1. The resulting shape
of the potential energy curve is determined by the superposition of three different
branches:
(i) the curve, describing the repulsion of a nondissociated H2 molecule at its ap-
proaching the center (1–1’, figure 1);
(ii) the curve, corresponding to the repulsion of isolated atom(s) within a potential
well after dissociation of the molecule (5, figure 1);
(iii) the curve, corresponding to the attraction between the active center and the
atomic particle outside the well (2,3,4, figure 1).
The intersection of the first and the second potential curves (“cross-over”) de-
termines both position and height of a potential barrier for dissociative adsorption,
and superposition of the 2nd and the 3rd potential curves gives us the depth and the
position of the potential well for the captured atom. In this qualitative model the
dissociation takes place near the cross-over point. One may derive that the closer
the molecule can approach the surface without dissociation, the higher proved to
be the potential barrier for adsorption. This fact is demonstrated by a comparison
of the attractive curves 2–2’, 3–3’, 4–4’ in figure 1. Therefore, the reason for good
catalytic properties of a definite group of transition metals is participation in the cre-
ation of the chemisorption bonds d-electron orbitals. These orbitals, in comparison
with apparently even more strong bonding s, p or sp-hybrid orbitals, are long-range
acted orbitals (curve 4, figure 1). As a result they form a relatively small barrier
for adsorption, the activation energy for catalytic processes Ea decrease drastically,
563
V.G.Litovchenko, A.A.Efremov
and the cross-section of the reaction of catalytic decomposition is highly increased.
Such surface atoms as Ni, Pd, Pt (as well as W, Mo), whose long-ranged d orbitals
interact with H2-molecule at relatively long distances (curve 3,4) can form the cen-
ters for easy or spontaneous adsorption. On the contrary, for Cu, Ag, or Au with
s-orbitals the adsorption of H2 is practically missing (curve 2).
2.2. The electronic transitions in the system “molecule-ac tive center”: the
role of the local structure irregularities and defects
Dissociative adsorption on the active centers at the surface may be treated
from somewhat another point of view, namely as a result of electron transitions
from atoms of a substrate to the antibonding orbital of a molecule (dissociation)
and subsequent transition of electrons from molecule constituents to the empty or-
bitals of surface atoms (chemisorption). When H2 molecule moves to the surface, the
nonequivalent shifting of the splitted energy levels takes place: antibonding (empty)
level of H2 (1s
2) shifts down, but bonding one of Cu and Ni (Pt) 4s2(6s1) shifts up. If
outer electronic shells of surface atoms are formed by isotropic s-orbitals, the main
role in the process is played by the collective properties of electronic gas, namely
the intersection of antibonding level of the molecule with the Fermi level of the
metal. The latter results in the crossover at a definite small distance only. On the
contrary, in the case of more localized, directed and long-range acted d-orbitals (Ni-
3d, Pt-6d), the intersection takes place at the much longer distances, because the
mentioned electron transfers may occur from the quasi-local state above Fermi-level.
According to figure 1 the dissociative chemisorption is accompanied by the overtak-
ing of a small activation barrier. These considerations enable us to explain the role
of various local structural irregularities and defects on the surface of a transition
metal in a remarkable increase of the chemisorption of H2, O2 and O-containing
molecules. Dislocations, steps, atomic layer terraces, distorted and weakened bonds,
surface substructures (2×1, 7×7 etc.), reoriented surface domains with unsaturated
boundary bonds in polycrystalline Pt, Pd, Ni films and so on, can act as chemi-
cal active unsaturated surface radicals determining the increase of a catalytic yield.
They not only give addition places for adsorption, but also form a lot of quasilocal
levels, which became responsible for the electron transitions mentioned.
2.3. Active centers: the role of crystalline structure in th eir atomic environ-
ment on the surface
The effect of the crystal plane structure and crystal orientation was demon-
strated in many works. As far as the more compact surfaces have got a less free
energy (surface (100)), Pt(111) plane was preferably used in the catalytic reaction.
In particular, at HN3 oxidation the (111) plane continually reconstructed to (100)
plane due to the heating by a surface chemical reaction. In general, the plane with
higher indexes has a tendency to enhancing the catalytic reaction.
It is possible to analyze the catalytic processes for H2 adsorption within the
framework of “surface model” [1,5]. At the H2 molecule moving to a surface, at
564
The enhanced catalytic dissociation. . .
small distances, the chemical potential of electrons has to coincide with the metal
Fermi surface. To satisfy this condition the electron transition of H-electron to metal
has to be realized. Theoretical analysis (for W [1] Pt [5,9,10]) shows that only the
nearest d-orbital, directed to s-orbitals of H adatom, takes part in forming the
adsorption bonds. Besides, directed along the plane, and a weaker interaction of
H-adatom with the other surface atoms of transition metal takes place as well. Just
this, crystalline order dependent interaction, determines the important peculiarities
of surface molecules comparative to the bulk. Minimum of energy occurs under
bonding of the adatom H with four (for (100) plane) d-orbitals of the surface atoms
of the substrate. The configurations with 2 or 1 d-orbitals are less stable, but still
probable for the surface as well, because of a large dispersion of energy levels for the
near-to-surface H-atom. Hence, resonance tunnelling of the electron at the forming
of s-d bounds becomes possible in this configuration.
2.4. Active centers: the role of the orientation of surface b onds
The orientation of unsaturated surface bonds is very important for surface re-
actions. This parameter is strongly dependent on the type of crystalline structure.
In a set of theoretical works (see [1]) the role of orientation of d-orbitals has been
analyzed. Catalytic properties can be classified according to the degree of orienta-
tion of the free d-orbitals. Of course, in addition to this factor, it is also necessary
to take into account the number of free d-orbitals and the degree of their directness
(i.e., number of d-shells) [1,3]:
1) normal to-surface directed nonoccupied d-orbitals (Mo,W);
2) large-angle directed (45o, 36o) (Pd, In, Ru, Os, Re, Te);
3) low-angle, partly filled d-orbitals (Fe, Co, Ni, Pd, Pt);
4) fully-occupied d-shells with asymmetric properties (Zn, Ge, Cd, In, Sn, Pb).
It was found [1] that the normally oriented d-orbitals are usually more suitable
for adsorption, because interaction of the molecule with declined bonds requires its
more sufficient approach to the surface. Another factor is a geometric correspondence
between positions of the atoms in the molecule and positions of the end points of
surface dangling bonds. Hence, in some special cases declined bonds proved to be
more favourable for a catalytic reaction.
2.5. Active centers on the surface of small clusters
The structure of small clusters is typical of microcrystalline composites and
nonepytaxial films. In this case we have got a substrate, which is intermediate be-
tween insulated molecules and a bulk condensed matter. At growth, the size of the
cluster increases and the energy of ionization decreases up to the value of workfunc-
tion (for example, for Ag from 7.5 eV to 4.5 eV). On the contrary, the electron affinity
565
V.G.Litovchenko, A.A.Efremov
increases, as well as the average energy of the chemical bond does (2.96 eV for metal-
lic Ag). Surface energy is also remarkably larger for the smallest clusters (1–5 nm).
A very important point is that for a small cluster, electron orbitals have got specific
features. In particular, high catalytic properties of Pt could not be explained within
the framework of electron configuration of insulating atoms due a complete filling
of d-shell (10d electrons), hence, the lower empty orbitals of Pt could be s-orbitals.
But even in the simplest clusters 2Pt, the s-orbitals, being diffuse overlapped with
each other, form a symmetric molecular orbital with the small bonding energy. The
d-orbital electrons, now having a higher energy can transfer to this empty orbital,
and in such a way, lead to the vacating of some d-orbitals. The number of such free
d-orbitals increases at the increase of the size of clusters. The same process also
takes place for the other transition elements even for the case, when d-shell is not
completely occupied (as Ni, Pt). Therefore, for large clusters or bulk solid state the
combination of the low-energy free d-orbitals (which is necessary to form long-range
chemical bonds, for effective catalytic properties) with a high energy occupied the
antisymmetric combination of d-orbitals (t2g: dxy, dxz, dyz; eg: dx2−y2, dz2), which
is strongly directed, and promote the overlapping with the orbitals of the adsorbed
molecules. This mechanism relates to the so called “catalytic stereochemistry” and
is usually revealed for Pt, Pd, Ni etc.
2.6. Active centers: the role of the foreign atoms in the loca l atomic envi-
ronment
There is, however, quite another class of catalytically promoted reactions which
are practically insensitive to the structure of catalysts but only to its composition.
Even for structural-sensitive reactions, the emerging of some types of foreign
atoms (ligands) in the local environment of the adsorption center proved to be more
effective for the control of catalytic activity, than structural transformations [1,3,4,5].
There are at least four different mechanisms of the impurity effecting the catalytic
activity of transition metals.
(i) First of all, the microcluster, which is formed on (or under) the surface from an
active atom and surrounding ligands may provide a more exothermic reaction
than in ordinary case.
(ii) Some peculiarities of electron transfer between the central atom of the active
complex and ligands may effect the degree of d-orbital filling. The main idea
for using the surface alloys (or highly-doped transition metals) consists in the
transformation, (like in the case of Pd) of the completely filled d-shells for
elements of 1B group (Cu, Ag, Au) and 2B (Zn, Cd, Hg) into partly unfilled
d-shells. The latter takes place due to overlapping with unfilled (Pd, Ni, as well
as Ro, W, Mo etc.) orbitals, and provides new systems with a high efficiency
and stability against oxidation and other undesirable aging processes. The
most interesting are the systems consisting of a transition metal doped with
copper.
566
The enhanced catalytic dissociation. . .
(iii) Replacement of the intrinsic atoms in close vicinity of the center atom by
foreign atoms changes the local symmetry of the center, the character of its
bounding with the nearest neighbours as well as the orientation of surface
dangling bonds. For example, the presence of oxygen results in such bonds
reattachment when d-electrons prefer to be transferred to p-orbitals of the
neighbours, rather than on the antibonding orbital of the adsorbing molecule
[2, 11]. On the contrary, the presence in the local atomic environment of a
relatively electropositive elements (e.g. Cu in Pd) gives some accumulation
of electron density on d-orbital of Pd and the increase of its directness. The
model for describing the catalytic activity of CuxPd complexes based on this
effect will be theoretically considered below.
(iv) The next mechanism consists in long range Coulomb interaction. The differ-
ence in electronegativities (1.8 eV for Cu and 2.1-2.3 eV for Pd, Pt) leads to
some Pd-Cu bond polarization within the CunPdm cluster with the accumu-
lation of a positive charge on Cu. The resulting electrostatic field transforms
the potential diagram of the center just outside the main potential well (figure
1) in such a way that the residence time of a molecule in pre-chemisorption
state increases.
2.7. The local symmetry of adsorption centers: topo- and ste reochemistry
aspects
Catalytic processes for surface complexes (including H-containing molecules)
have also got some important features, because of their discrete electron energy
spectrum. For instance, in the case of surface reaction with organic molecules the
orbitals have a tendency to conservation of the symmetry factor during chemical
transformations. The reaction is symmetry-permitted, when the symmetry of all
bonding orbitals of the reagents coincides with the symmetry of the final product.
A catalyst can promote the reaction, transforming the symmetry-forbidden reaction
for gas, into a symmetry-permitted reaction on the surface by way of forming the
subsequent adsorbed particles with a suitable symmetry. In particular, some un-
bonding d-electrons can transfer to the surface molecule, and then transfer on the
d-orbitals of the metal surface with the other symmetry. Another way is forming
a surface radical, having another origin than the molecules in a gaseous phase. An
example is the case of H2 + O2 reaction on the surface, which for a gaseous state is
symmetry-forbidden due to noncoexistance of the pair-degree of orbitals, but disso-
ciation of H2 → H+H on the surface permits the reaction O2 with hydrogen [3]. A
very important particular case of catalytic enhancement is based on the coinciding
of molecular symmetry of the final product of the reaction with the symmetry of the
local atomic arrangement on the surface. A typical example is a hydrocyclization
reaction on n-heptan to toluol on Pt(111) surface with hexagonal symmetry.
567
V.G.Litovchenko, A.A.Efremov
0 100 200 300 400 500
0
100
200
300
400
500
Slow decrease until
air inlet begins
The end of H
2
pulse
air pulse beginning
5
4
3
2
1
S
ur
fa
ce
c
on
ce
nt
ra
tio
ns
Time, s
Figure 2. Theoretical calculation of concentrations of atomic hydrogen He, oxy-
gen Oe and hydroxyls OHe at the external metallic surface, and of hydrogen
captured at the metal-insulator interface (Hi) of the MIS gas sensors under the
action of the sequence of H2 and air pulses: 1 – He; 3 – Hi; 4 – Oe; 5 – OHe; curve
3 is the concentration of hydrogen dissolved in the bulk of the layer.
3. Modelling and simulation of the adsorption-desorption p ro-
cesses
We have applied the above qualitative consideration to the analysis of the cat-
alytic properties of PdCux surface alloys by means of computer modelling. We have
used two different approaches to the modelling of the H2 adsorption processes. The
first is the computation of a MIS sensor response under hydrogen action [2,11]. The
second approach in the modelling is the direct simulation of H 2 molecule behaviour
during its adsorption on the pair of surface centers.
3.1. The modelling of adsorption/desorption processes at t he surface of the
active metal layer in MIS gas sensors.
The main physical processes, which occur in MIS gas sensor with catalytic metal
layer may be outlined as follows [2, 11]. At the beginning, on the external surface of
the active metallic layer, the dissociative adsorption and desorption of H 2 molecules
takes place. Then, atomic hydrogen is transferred through the film to the metal-
insulator interface, where the internal adsorption and polarization of the atoms takes
place. The result of such polarization is the shift of the flat-band voltage (∆VFB)
of the MIS structure, which may be also calculated and compared with the results
of C-V measurements. These processes have been considered within the framework
of a chemical kinetic approach. The system behaviour was described by a system
of differential equations for surface (nke) and interface (nke) concentrations of k-th
568
The enhanced catalytic dissociation. . .
atom:
dnke/dt = Rk(n1e, n2e, . . . , nke, . . .) + ckPmN
µ
ef − dkn
µ
ke − dknki/dt
dnki/dt = KknkeNif − SknkiNef . (1)
Here adsorption and desorption are determined by a set of adsorption/desorption
coefficients ckm and dkm for molecules and by the order adsorption reaction µ (multi-
centered dissociative, or monocentered). These parameters are functions of the ambi-
ent temperature. The further process consists in the reactions at the external surface
between the products of dissociation of the molecules. The rates of these surface re-
actions are described by a nonlinear function Rk(n1e, n2e, ..., nke, ...) according to
the theory of absolute chemical reaction rates.
Here, power µ is equal to 2 for dissociative adsorption (associative desorption) of
a molecule containing the atom of k-th type. Nif and Nef are the numbers of centers
available for adsorption, i.e. free from adsorbed particles:
Nef = Ne −
∑
k
νknke , Nif = Ni −
∑
k
νknki ; (2)
in its turn νk is a number of centers occupied (or blocked) by one captured particle.
The parameters Kk and Sk are forward and reverse transport coefficients for each
component which can move through the layer . Calculated response of a MIS sensor
on a pulse of hydrogen, followed by a pulse of oxygen is shown in figure 2. Here
chemical reactions between atomic H and O (resulting in OH formation and H 2O
desorption) on the external surface are taken into account.
The comparison of the theoretical and experimental responses of the sensor on
the gas pulses makes it possible to derive the values of phenomenological coefficients
related to adsorption/desorption process itself. Hence, the reasonable estimations of
such microscopic parameters as the depth and width of potential well and the height
of barrier for adsorption have been done [2].
Herein below we shall consider in more details the second approach in the mod-
elling.
3.2. The computer simulation of dissociative adsorption at the Cu x Pd
surface
The method, which we have used here may be treated as a version of molecular
dynamics or molecular mechanics approaches. In the simulations we compare dis-
sociative adsorption on the surface of pure palladium and on PdCux surface alloy,
which may be formed, in particular, by the growth of ultra-thin copper films on
the palladium. The deposition of metals on metals present an alternative approach
to the studies of alloy surfaces and a way of producing controllable compositions
more readily. Such surfaces in some cases exhibit configurations and properties at
bulk alloy surfaces. This type of surface alloys and composites has become an area
of a considerable part of the current research, because of their fundamental inter-
ests in catalysis as well as due to their importance in a novel device fabrication for
569
V.G.Litovchenko, A.A.Efremov
coating and sensor development. The aim of simulations is to explain the enhanced
sensitivity of (MIS) gas sensors with the active CuPd layer.
The general schema of the process is shown in figure 3. Here a two atomic
molecule (e.g. H2) moves toward the group of atomic force centers, which represent
atoms of Pd or/and Cu at some fragment of the surface. The classical Hamiltonian
for such a system may be presented as follows:
H(p1,p2, r1, r2) =
∑
i
p2i /2m+
∑
i
N
∑
k
Uik(ri,Rk,Θk) + Vm(|r1, r2|), (3)
here p1, p2, r1, r2 are momentums and radius vectors of the atoms, Vm(|r1, r2|) is
interatomic interaction in the molecule. For this purpose we use the Morse potential:
Vm(r) = Do exp
(
−(r − ro)/LD
)
(
exp
(
−(r − ro)/LD
)
− 2
)
, (4)
where Do is a dissociation energy of the molecule, ro is an equilibrium interatomic
distance, LD is an effective width of the respective potential well, determining both
the oscillation frequency ωm=(1/LD)(2Do/m)1/2 and the maximum possible oscilla-
tion amplitude. Uik(ri, Rk,Θk) describes the interaction between i-th (i=1,2) atom
in the molecule and k-th atomic force center (k=1,...,N) located at point Rk. The
angle Θk according to the previous considerations corresponds to the orientation of
the d-orbital as to the surface.
It should be emphasized that the surface of potential energy Uik(ri, Rk,Θk) has
quite different view for a whole molecule (curve 1, figure 1) and for a pair of iso-
lated hydrogen atoms (curve 2, figure 1). As it is obvious from figure 1, H2 molecule
never be captured by the chemisorption potential well and should be reflected by the
surface. On the other hand, the atoms after the molecule dissociation cannot leave
the surface of the solid being isolated. These two potential surfaces intersect each
other forming the potential barrier for dissociative adsorption. For U ik(ri, Rk,Θk)
in the case of dissociative adsorption we have used analytic approximation, giving
the anisotropic potential well with the depth Wo and barrier Ea shown in figure 4.
The cross-section of this well along the bond direction corresponds to the curve 1 in
figure 1. This shape of the potential surface was used only when dissociation took
place in the vicinity of the cross-section point. Otherwise, a pure repulsive ∼ r−n po-
tential for molecule was chosen. The respective equations of motion for the molecule
in this two-sheet potential field have been solved numerically using a combined 4th
order Runge-Kutt-Adams method. The source code has been written in C++ pro-
gramming language. Windows Application executable file has been obtained with
Microsoft Visual C/C++. The program permits us to change the parameters of the
wells, barriers and orientation angles of the bonds.
The interaction of the molecule with five atomic force centers was considered
at different initial translational kinetic energies of the molecule and its vibrational
excitations. Though the rotational movements may also be included in the model,
their role was not studied in detail. Pd atoms were modeled by anisotropic and
extended ellipsoid-like potential wells with the barrier depending on the well ori-
entation. On the contrary, copper atoms were represented by practically isotropic
570
The enhanced catalytic dissociation. . .
Figure 3. The interaction of H2 molecule with different types of atomic force
centers, which simulate the different types of surfaces (a,b and c). a) Pd surface
with normal orientation of d-orbitals; b) the same with declined orientation of
d-orbitals; c) The case of the surface of Cu3Pd alloy. A) Atoms at the surface
(top view, XZ plane); B) The 2D map of the potential energy for atomic force
centers and the movement of the atomic particles during the adsorption process
according to modelling (XY plane cross-section): 1 is a hydrogen molecule, 2 is
an atomic hydrogen after dissociation (indicated positions correspond to different
stages of the process), 3 is a barrier for adsorption, 4 is a potential well for the
captured hydrogen (the depth of the well relates to Pd-H bond energy), 5 is a
repulsive part of the atomic potential. C) Respective one-dimensional potential
curve along the normal to the surface.
571
V.G.Litovchenko, A.A.Efremov
Figure 4. 3D plot of potential energy for the interaction of H2 and (H+H) with
the active center on the metallic (Pd or Cu) surface.
wells with a more pronounced barrier. The dissipation of energy after adsorption
was taken into account by introducing the damping factor within the well near the
surface. The reverse process of desorption has been simulated by supplying some
correlated subsequent impulses of force to the captured atoms in the wells. As a re-
sult of calculations the trajectories of the atoms have been obtained. Moreover, the
program provided a comprehensive visualization of the simulated processes. There
are some interesting points proved to be revealed.
1. In the case when only Pd atoms are present at the surface and their d-orbitals
are normally oriented, the geometric factor (i.e. the correspondence of the
bond length in the molecule and the distance between d-orbitals ) is the most
important.
If d-orbitals of the Pd atoms are normally oriented, a very low potential barrier
does not prevent the Ha to get into the well in the case of dissociation. In the
classic model used here, dissociation takes place due to separation of atoms by
potential wells. The success of this process depends on the degree of vibrational
excitation of the molecule. The excitation also permits us to approximately
simulate the role of electronic excitation in decomposition (due to the electron
transition to the antibonding orbital of the molecule).
2. In the case of declined d-orbitals, the geometric factor became more favourable.
Besides, some “easy way” through the barrier appears (figure 3b.B). Along
this path in the saddle point the molecule is subjected to a strong deformation
which is useful for the subsequent dissociation.
3. Due to the specific local atomic arrangement on Cu3Pd surface, when each
572
The enhanced catalytic dissociation. . .
Pd atom is surrounded by Cu atoms, the symmetry considerations make it
possible to conclude that d-orbitals of Pd are more or less normally oriented.
Geometric factor here is very unsuitable: Pd-Pd distance is about two times
higher than for pure palladium. In this case, however, dissociation is stimulated
by the action of the potential barrier of copper.
4. For CuPd surface, declined Pd bonds are more probable and geometric factor
is more favourable than in the previous case. Simulations reveal in this case
the enhanced dissociation due to cumulative action of both copper barrier
and Pd wells. As a result the hydrogen atoms prove to be bonded not with
the neighbour surface Pd atoms but with the more distant ones. Therefore,
the reverse process of quick correlated desorption became forbidden and such
an important microscopic parameter as ck/dk in (1) should be effectively in-
creased, leading both to an increase of catalytic yield in the case of catalysis
and the sensitivity in the case of gas detection. The respective experimental
data are presented below.
4. Some experimental data
According to the principles of MIS gas sensors operation outlined in the previous
section, an exposition of the external surface of the active layer with H2 molecules
leads to internal adsorption of atomic hydrogen at the metal-dielectric interface and
to the shift of flat-band voltage (∆VFB) of the MIS structure. The response of the
sensor i.e. ∆VFB(t) value as a function of time of the exposition obtained from C(V)
measurements, may be compared with the theoretical calculations based on the
solution of the system (1) and (2). In our experiments the Pd film (field electrode)
was deposited by magnetron deposition up to thickness 50 nm. In some cases Pd
was covered by ultra-thin (5 nm) and probably, initially discontinuous, copper layer.
During preparation of the samples, Cu atoms penetrated into the subsurface of
Pd layer to the depth of 5 nm. As a result, a homogeneous (SEM data) surface
alloy CuxPd with the composition of about x=1 (AES data) is formed. The typical
responses of the sensors on H2 pulse followed by an oxygen pulse are shown in
figure 5. The parameters of the responses of the field-effect gas sensors Pd-Si3-N4-
SiO2-Si and Cu/Pd-Si3-N4-SiO2-Si are demonstrated in table 1.
The maximum response value (besides partial pressure of H2) depends directly on
two microscopic parameters: (i) the number of catalytically active centers and (ii) on
ck/dk ratio for the detected molecules (see above). Other parameters of the response
(figure 5), namely, the time of the response τ1 (after the beginning of exposition)
and the times of the recovery τ2 (after the end of gas pulse) depend on the bulk
transport properties of the metallic layer as well.
In this table the averaged data for 5 samples and respective measurement regimes
are presented. The characteristics were measured both for “as prepared” sensors, and
after their thermal treatment in the dry air gas environment. The comparison can
be summarized as follows:
573
V.G.Litovchenko, A.A.Efremov
0 50 100 150 200 250 300
0
100
200
300
400
500
600
700
dry air inlet
dry air inlet
2
1τ
τ
2
1
V
, m
V
Time, s
Figure 5. Transient responses of freshly prepared MIS structures with Pd/Cu
(1) and Pd(2) electrodes on hydrogen pulse (500 ppm) followed the air pulse
(1 atm). τ1 and τ2 are the characteristic times of the response and recovery,
correspondingly.
1. The value of ∆VFB for systems with surface Cu/Pd composite, indicates their
higher catalytic activity (∆VFB larger on 10–20%), than for the systems with
pure Pd surface.
2. After “aging” heat treatments these differences become even more pronounced
(30–40%). i.e., more noticeable catalytic enhancement takes place after CuPd
surface alloy formation.
3. Time of the response is systematically smaller (but not so noticeable) for
MIS structures with a surface alloy. On the other hand, times of recovery
are remarkably smaller for Pd/Cu surface. Therefore, more enhanced reaction
between atomic hydrogen and oxygen (accompanied by H2O desorption) takes
place in this case.
4. The fact of a more intensive reaction with hydrogen shows that interaction of
oxygen with substrate atoms proper on CuPd surface is more weak. Indeed,
these structures proved to be more stable and resistant against “aging” heat
treatments as well (see table 1).
5. Conclusion
We would like to emphasize in conclusion that the qualitative conception of de-
scribing the chemical bonds, based on the atomic d-orbitals characterization, can be
used to predict some general regularities of dissociative adsorption and catalysis on
574
The enhanced catalytic dissociation. . .
Table 1. Parameters of the Pd-Si3-N4-SiO2-Si and (Pd/Cu)-Si3-N4-SiO2-Si sen-
sors annealed at 150oC
NN
Parameter
(average)
Pd sensor
(initial)
Pd sensor
(annealed)
Pd/Cu sensor
(initial)
Pd/Cu sensor
(annealed)
1 Sensitivity
VFB (mV)
500 400 550 570
2 Time of re-
sponse (s)
5–35 20 25 15
3 Time of re-
covery (s)
45 10 5 2
the surface of alloys. The most important factors, which effect the peculiarities of
these bonds are (i) type of crystalline structure of the surface in close vicinity to the
active centers, in particular, the local symmetry of atomic arrangement, (ii) intrinsic
defects, and (iii) type, number (symmetry) and electronegativity of foreign atoms
present in the neighbourhood of the atoms of d-element. The factors (i)-(iii) act on
the three most critical parameters of surface dangling bonds such as orientation,
directness, and length. These parameters in their turn determine the characteristics
of a potential barrier and a well of an adsorption center. Computer simulation of
adsorption-desorption processes based on this approach has enabled us to study the
catalytic properties of CuxPd alloy surface relating to the dissociative adsorption of
hydrogen-containing molecules as well as to explain the enhancing action of copper
on this process. This direct simulation together with a computer modelling of MIS
gas sensor operation (based on more phenomenological approach) gives us also a
possibility to explain the experimental data, demonstrating the increase of the cat-
alytic activity of CuPd surface as to H2 decomposition (in comparison with a pure
Pd) in MIS gas sensors.
6. Acknowledgements
This work was performed within the framework of research and development
project 01RS9604/8 between TU Ilmenau, Germany and ISP NAS of Ukraine.
References
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3. Tompkins F.C. Heterogeneous catalysis. – In: Surface Science: Recent Progress and
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284.
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Russian).
Підсилена каталітична дисоціація адсорбованого
водню, що містить молекули
В.Г.Литовченко, А.А.Єфремов
Інститут фізики напівпровідників НАН України,
252028 Київ, проспект Науки, 45
Отримано 21 квітня 1998 р.
Аналізуються деякі загальні концепції і механізми дисоціативної ад-
сорбції і каталізу. Обговорюється роль таких факторів, як конфігу-
рації електронних оболонок ізольованих атомів, кристалічна структу-
ра поверхні, локальні структурні нерегулярності, орієнтація поверх-
невих зв’язків, ефекти розмірності, присутність чужорідних атомів у
локальному атомному оточенні, а також роль локальної симетрії цен-
тра адсорбції. Ці обговорення використовуються при розвитку про-
грами для комп’ютерного моделювання процесу. Каталітичні вла-
стивості поверхневих сплавів PdCux аналізуються, використовую-
чи комп’ютерне моделювання, і отже, передбачено підсилення ди-
соціативної адсорбції H2 для такої системи. Представлені експе-
риментальні дані, які демонструють підсилення чутливості сенсора
метал-ізолятор-напівпровідник з електродом CuPd у порівнянні з чи-
стим електродом Pd.
Ключові слова: дисоціативна адсорбція, комп’ютерне
моделювання, каталіз, виявлення газу, поверхневі сплави
PACS: 82.65.Jv, 82.20.W, 82.30.Nr, 33.15.F
576
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