Models of the adsorbtion-catalytic centers on transition metals
The models of adsorbo-catalitic centers for transition metals, which use the peculiarities of the electron-orbital structures as insulated and bounded atoms, have been analized. The important role of the unfilled d-orbitals, their orientation, changing of the occupation of d-shalls, and influence...
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| Cite this: | Models of the adsorbtion-catalytic centers on transition metals / V.G. Litovchenko // Condensed Matter Physics. — 1998. — Т. 1, № 2(14). — С. 383-388. — Бібліогр.: 8 назв. — англ. |
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| citation_txt | Models of the adsorbtion-catalytic centers on transition metals / V.G. Litovchenko // Condensed Matter Physics. — 1998. — Т. 1, № 2(14). — С. 383-388. — Бібліогр.: 8 назв. — англ. |
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| description | The models of adsorbo-catalitic centers for transition metals, which use the
peculiarities of the electron-orbital structures as insulated and bounded
atoms, have been analized. The important role of the unfilled d-orbitals,
their orientation, changing of the occupation of d-shalls, and influence of
the ligands with completely occupied d-orbitals were considered in connection
with the catalytic activity of the transient metals. The new approach for
enhanced catalytic properties, based on the film alloys of catalytic metals,
was proposed.
Проаналізовано моделі адсорбо-каталітичних центрів перехідних металів, враховуючи електронні орбіталі як ізольованих, так і сконденсованих у тверде тіло атомів. Відзначається домінуючий вплив на каталітичну активність незаповнених d -оболонок. Запропоновано новий підхід для підвищення каталітичної активності, що базується на використанні плівкових сплавів перехідних металів.
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| first_indexed | 2025-11-28T15:47:31Z |
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Condensed Matter Physics, 1998, Vol. 1, No 2(14), p. 383–388
Models of the adsorbtion-catalytic
centers on transition metals
V.G.Litovchenko
Institute of Semiconductor Physics of the National Academy of Sciences
of Ukraine, 45 Nauki Av., UA–252028 Kyiv, Ukraine
Received May 21, 1998
The models of adsorbo-catalitic centers for transition metals, which use the
peculiarities of the electron-orbital structures as insulated and bounded
atoms, have been analized. The important role of the unfilled d-orbitals,
their orientation, changing of the occupation of d-shalls, and influence of
the ligands with completely occupied d-orbitals were considered in connec-
tion with the catalytic activity of the transient metals. The new approach for
enhanced catalytic properties, based on the film alloys of catalytic metals,
was proposed.
Key words: adsorbtion, catalys, transition metals and alloys
PACS: 05.70.Ce, 65.50.+m
1. Introduction
Characteristics of hydrogen on the surface of transition metals (adsorption,
catalytic decomposition, transport etc.) are the subject of a precise scientific in-
vestigation and play an important role in determining the parameters of solid-state
gas sensors of H-containing molecules. Decomposition and subsequent transport of
a proton (or atomic hydrogen) occur strongly depending on the interconnection of
the neighbouring adsorption centers for protons, the shape of the barrier, dynamic
parameters (determined by a barrier electron–phonon coupling) etc.
A thorough theoretical analysis of the proton properties within the H-bond
model, (transfer, orientational hopping between the neighbouring bonds etc.) has
been made in a number of works by I.V. Stasyuk and co-authors [1,2]. In particular,
an analysis within the framework of a 1-dimensional chemical model with taking
into consideration a tunnelling effect, demonstrates the orientation motion effect
which strongly depends on the energy spectrum of the H-bonded proton system.
This leads to the redistribution of protons in a chain-cluster and, hence, the ground
state (i.e., barriers, wells) is changed. The external electric field can also influence
the general thermodynamic functions of the system containing hydrogen. For the
c© V.G.Litovchenko 383
V.G.Litovchenko
effect of the surface charging induced by adsorption, which is the basis of semi-
conductor gas sensors, the proton (Ha) polarization is the principal phenomenon.
Earlier calculations performed for small molecular complexes by Zundel and Eck-
ert [2] exhibit an anomalously large proton polarizability which is in contradiction
with the experimental data. To consider a more realistic situation, I.V. Stasyuk
with co-authors have proposed a new approach (a pseudo-spin based model) for
the description of H-bonded molecular systems which give reasonable values of en-
ergy levels and the degree of polarization. We believe that this model, at least in a
qualitative way, can be used for analysing the catalytic processes of H-containing
molecules on transition metals (decomposition, transport etc.) and can also be used
for the study of the layered structures “metal-insulator-semiconductor” (MIS).
2. General features of the transition metal catalysts
The enhanced catalytic activity of the definite transition metals is based on the
fundamental characteristics of the electron shell configuration. Indeed, in the case,
when a d-orbital plays an important role in the formation of chemical bonds (es-
pecially surface bonds), all those atoms are candidates for the creation of highly
catalytic systems. The reason for this is that d-orbitals (in comparison with s–
and even p–, as well as sp–hybrid–orbitals) are long–range active, so they can
create a small barrier for adsorption (Vd–barrier ≪ Vs–barrier), and due to this
the activation energy for a catalytic process εcat decreases drastically, whereas the
reaction cross–section Scat.∼r, and therefore, the catalytic reaction yield have to
be increased considerably. Figure 1 demonstrates the Lennard–Jones configuration
diagram for s– and d– bonds and illustrates the above formulated statement. Be-
cause the energy of the bonds E = Emin−Emax also becomes small, the products of
the catalytic reaction more easily leave the surface. On the other hand, the bond
energy Eb is often nearly the same as, for example, for H on Cu, Zn, Ag, Ni and
Pd [3–5]). So, chemosorption of H2 on Ni and Pd proceeds rather quickly, but on
Cu, Zn and Ag chemosorption of H2 is nearly absent.
Let us compare electron configurations for a set of transition and semitransition
metals.
In all of these cases the role of d-orbitals is important. It is known that the
most expanded chemical bonds create either pure d-orbital (as in Pd) or atoms
where d-orbitals are near to saturation and only one s-orbital (Pt) or two (Fe, Co)
give a nondominant share and lead to some symmetrisation of the bonds.
3. Catalysis and the influence of external impurities or alloys
The role of promoters or inhibitors of catalysis has been long known. In par-
ticular, alkali metals were known as promoters for the synthesis of ammonia on
Ru(4d75S1) and on Ro (4d85S1), but the inhibition of the CH4 on Ni catalysis by
S and P and other reactions of such type were observed.
384
Models of the adsorption-catalytic centers
Table 1. Electron configurations for different d-elements
Elements Electron configuration Group
Fe (2d64s2) 4a-th
Ni (2d84s2) 4a
Cu (3d104s1) 4b
Zn (3d104s2) 4b
Co (3d64s2) 4b
Pd (4d10) 5a
Ag (4d105s1) 5b
Pt (5d96s1) 5a
Table 2. Electronegativity of elements
K Ti Fe Ni Cu Sn Pd Pt
0.8 1.6 1.8 1.8 2.0 1.7-1.8 2.1-2.3 2.1-2.4
3.1. Electrostatic model
The comparison with electronegativity seems to indicate a local electrostatic
mechanism of the adsorption-catalytic promotion. In accordance with this con-
ception, highly electropositive alkali metals considerably decrease the electrostatic
potential of a catalytic substrate. In this case the potential well of the adsorp-
tion state increases and, consequently, decreases the barrier for desorption. On the
other hand, the electronegative elements (like O, S, P, Cr, F, Cl etc.) lead to the
weakening of chemical bonds. As a result, the depth of the pre-chemosorption well
decreases. It causes a decrease of the barrier for chemosorption and an increase of
one for desorption. The reason of this is the redistribution of the electron density of
chemical bonds – accepting in the case of electronegative elements and adding – in
the electropositive elements case. So for H2 (and H-related molecules) dissociative
reaction, it is possible to propose the following set of elements for enhancing the
catalytic activity:
In accordance with the known relation for sticking coefficient S
S ∼ (1−A exp(Ea − Ed)/kT ).
From this relation it is possible to see that at Ed>Ea we have an activated ad-
sorption (like for H2 on Cu) and at Ed<Ea – a nonactivated adsorption (H2 on Pt,
Pd). The changing of the electron density on the Fermi surface of Me is the most
important factor of the electrostatic mechanism.
385
V.G.Litovchenko
2 H
d-orbital
sd-hybrids
H -
gas
2
orbital S
εad
εds
εb
(a)
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Cu (3d 4s )
10 1 8
Ni (3d 4s )
22
Pd (4d )
10 1
Pt (6s 5d )
1 9
3d3d (occupied)
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4s (half-oc.)
4s
4d 5d
6s
10
11
10
88
22
99
11
9
ρ ρ
ρ ρ
ε ε
ε ε(b)
Figure 1. (a) The curves of potential energy vs. distance M–H of the dissociative
adsorbtion of H2 molecule in a l-dimentional model on non-transition metals (up-
per curve–energy activation εas) and transition metals (bottom curve, εd ≪ εa).
(b) Electron density ρ distribution for d and s orbitals for Cu, Ni, Pd, Pt.
3.2. Molecular bond model
For isolated transition Me atoms there are 5 linearly independent d-orbitals
(taking into account a spin, a fully occupied d-shell contains 10d electrons): dxy,
dyz, dxz, dx2−y2 , dx2 , which have the same energy (with the quantum number, cor-
respondingly, 1=2; m=0, ±1; ±2). Orbitals dx2−y2 , dz2 have the maximum electron
density along the coordinate axis x, y, z, and orbitals dxy, dyz, dxz are directed along
bisectors. The latter are denoted as t2g (or de) orbitals, the former are known as
eg orbitals. The t2g orbitals overlap and create metallic conductivity bands, eg
orbitals do not overlap, and are low localized. For a face centered cubic lattice,
plane (100), orbital eg (field) is directed along the normal to the surface, and 4
bonds t2g are oriented at 45◦. For an octahedron (III) face, both types of orbitals
are not normally directed, but, correspondingly, at 35◦ and 55◦ to the surface.
For (110)–t2g (filled) are normal, but eg are oblique at 45◦ to the surface. As more
normally directed and filled orbitals are catalytically active, the (III) plane is more
preferable.
In the crystal lattice field the degeneration is removed and we have a splitting
of the energy levels. For example, in the octahedral field the energy of orbital eg
becomes larger, and of t2g – lower. Because the value of splitting is different for
different crystal planes, we have a variation for the values of splitting and, hence,
for polycrystalline samples we have a catalytically heterogeneous surface.
As a model of adsorption let us consider the adsorption of H2 on the non-
catalytic (Cu) and catalytic (Ni) substrate, [5], figure 2. The energy levels at the
moving of H2 to the surface become shifted in a different manner: the anti-bonding
(non-occupied) IS2 level (H2) shifts down and 4S2 (Cu and Ni) – shifts up; this
determines the crossover position at a definite small distance. These values are
386
Models of the adsorption-catalytic centers
crossower
Cu
Cu
H
H
H
H
Ni
Ni
Ni
Cu-Cu Cu-Cu Cu-Cu
HH HH H H H H
HH HH HH
Ni-Ni Ni-Ni Ni-Ni Ni-Ni
(4s ) 2 **
(1s ) 2 **
2 1s
4s 2
(4s ) 2 **
(1s ) 2 **
4s 2
2 1s
3d
Vac Vac0 0r
M-H
r
M-H
EF EF
(a) (b)
Figure 2. Energy levels shift under dissociative chemoadsorbtion molecule H2 on
Cu (a) and Ni (b) in the model of surface clusters H2Cu2 and H2Ni2, respec-
tively [5].
ordinarily deposited slightly below the Fermi energy. So, here we have a change
of the anti-bonding orbital to slight bonding – at moving to the substrate. The
decrease of the energy of H-M complex and the increase of the filling energy of
Me (4S2–bonding) mean that after the crossover point we have a decompositional
adsorption of H2-molecule. For Ni the repulsion barrier near the surface decreases
due to the influence of anti-bonding 3d orbitals, for which the crossover takes place
at a much larger distance. Here Ea becomes rather small and Ea∼Ed.
3.3. Ligands and alloys
For ligands (mixtures) negatively charged (electronegative elements), we have
an increasing energy of d–level (due to the repulsion force). Depending on the
position of the ligand atoms (1 atom – spherical symmetry field, 4 – tetrahedral,
5 – pyramid, 6 – octahedral etc.), splitting becomes more and more complicated
and grows, the higher positions have, starting from octahedral, eg orbitals. The
latter are more directed than t2g, so they are more preferable for the creation of
long-acting chemical bonds with relatively low desorption barriers.
Mixed orbitals can be estimated from the theory of the ligand field (based on
the molecular orbital method).
If bonds are created due to a non-paired ligand, electrons and vacant orbitals
of Me, – that gives donor-acceptor bonds. If an electron shifts from the ligand
to Me – then Me is an acceptor, and the ligand is a donor; in the opposite case
Me is a donor (dative bonds). For such bonds more acceptable are the elements
with a fully (or nearly) occupied d-orbital configuration (d10−n, n=1, 2, 3 ...)
namely, Cu, Ag, Cd, Hg. In particular, Cu repulses the d-orbital of Pd, and in a
circular configuration leads to the creation of a more normal direction of d-bonds,
stimulating in such a manner their catalytic activity. Indeed, the change of the
adsorption and catalytic properties of Pd by Cu (and other semitransition metals
387
V.G.Litovchenko
[5-8]) was observed. Desactivated Pd (Pt) atoms in relation to CO chemosorption,
and, on the contrary, activation of H2 adsorption take place. The similar type of
phenomena is observed for extended values of Cu. Castro et al. [7-8] have shown
that for CuPt–alloys (Cu3Pt (III) etc.) the adsorption energy of CO decreases
in comparison with pure transition Me. For alloys a cut sp-band configuration is
important for obtaining a high adsorption position. For CO-adsorption on NiAl
(110), the effect of decreasing Ea is treated as an additional filling of a d-band
of Ni and changing in a sp-d hybridization, which leads to increasing the role of
d-orbitals. On the Cu3Pt alloy the adsorption H2 does take place almost as for
pure Pt, and at the same time oxygen can be blocked by Cu atoms. Catalytic
processes take place, whereas CO and larger molecules prevent the adsorption of
H2 molecules on Pt (Pd) atoms [7].
References
1. I.V.Stasyuk, O.L.Ivankiv // Modern Phys. Let., 1992, vol. B6, p. 85.
2. I.V.Stasyuk, O.L.Ivankiv, N.I.Pavlenko // J. of Phys. Studies, 1997, vol. 1, No 3, p. 418.
3. O.Krylov, V.Kiselev Adsorption and catalysis on transition metals and their oxides.
Springer, Surf. Sci., vol. 9, 1989.
4. V.G.Litovchenko, A.A.Efremov, T.I.Gorbanyuk, I.P.Lisovskii, D.Schipanski, Z.Gergin-
chen, P.Kornezky // J. Phys. Low. Dimensional structures, 1995, vol. 12, p. 193.
5. A.Zangwill Physics of surfaces. Cambridge Univ. Press, 1988.
6. J.A.Rodrigues, R.Campbell, D.Goodman // Surf. Science, 1994, vol. 307-309, p. 377.
7. M.O.Castro, G.Dooyen // Surf. Science, 1994, vol. 307-309, p. 384; p. 412.
8. R.Linke, V.Shneider, H.Bussee et al // Surf. Science, 1994, vol. 307-309, p. 407.
Моделі адсорбо-каталітичних центрів перехідних
металів
В.Г.Литовченко
Інститут фізики напівпровідників НАН України,
252028 м. Київ-28, просп. Науки, 45
Отримано 21 травня 1998 р.
Проаналізовано моделі адсорбо-каталітичних центрів перехідних
металів, враховуючи електронні орбіталі як ізольованих, так і скон-
денсованих у тверде тіло атомів. Відзначається домінуючий вплив на
каталітичну активність незаповнених d -оболонок. Запропоновано
новий підхід для підвищення каталітичної активності, що базується на
використанні плівкових сплавів перехідних металів.
Ключові слова: адсорбція, каталіз, перехідні метали та сплави
PACS: 05.70.Ce, 65.50.+m
388
|
| id | nasplib_isofts_kiev_ua-123456789-118932 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1607-324X |
| language | English |
| last_indexed | 2025-11-28T15:47:31Z |
| publishDate | 1998 |
| publisher | Інститут фізики конденсованих систем НАН України |
| record_format | dspace |
| spelling | Litovchenko, V.G. 2017-06-01T14:24:41Z 2017-06-01T14:24:41Z 1998 Models of the adsorbtion-catalytic centers on transition metals / V.G. Litovchenko // Condensed Matter Physics. — 1998. — Т. 1, № 2(14). — С. 383-388. — Бібліогр.: 8 назв. — англ. 1607-324X DOI:10.5488/CMP.1.2.383 PACS: 05.70.Ce, 65.50.+m https://nasplib.isofts.kiev.ua/handle/123456789/118932 The models of adsorbo-catalitic centers for transition metals, which use the peculiarities of the electron-orbital structures as insulated and bounded atoms, have been analized. The important role of the unfilled d-orbitals, their orientation, changing of the occupation of d-shalls, and influence of the ligands with completely occupied d-orbitals were considered in connection with the catalytic activity of the transient metals. The new approach for enhanced catalytic properties, based on the film alloys of catalytic metals, was proposed. Проаналізовано моделі адсорбо-каталітичних центрів перехідних металів, враховуючи електронні орбіталі як ізольованих, так і сконденсованих у тверде тіло атомів. Відзначається домінуючий вплив на каталітичну активність незаповнених d -оболонок. Запропоновано новий підхід для підвищення каталітичної активності, що базується на використанні плівкових сплавів перехідних металів. en Інститут фізики конденсованих систем НАН України Condensed Matter Physics Models of the adsorbtion-catalytic centers on transition metals Моделі адсорбо-каталітичних центрів перехідних металів Article published earlier |
| spellingShingle | Models of the adsorbtion-catalytic centers on transition metals Litovchenko, V.G. |
| title | Models of the adsorbtion-catalytic centers on transition metals |
| title_alt | Моделі адсорбо-каталітичних центрів перехідних металів |
| title_full | Models of the adsorbtion-catalytic centers on transition metals |
| title_fullStr | Models of the adsorbtion-catalytic centers on transition metals |
| title_full_unstemmed | Models of the adsorbtion-catalytic centers on transition metals |
| title_short | Models of the adsorbtion-catalytic centers on transition metals |
| title_sort | models of the adsorbtion-catalytic centers on transition metals |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/118932 |
| work_keys_str_mv | AT litovchenkovg modelsoftheadsorbtioncatalyticcentersontransitionmetals AT litovchenkovg modelíadsorbokatalítičnihcentrívperehídnihmetalív |