Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

TLDR: This work focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides and the mechanics of hydrogenation-dehydrogenation reactions.

Abstract: Focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides. Characterization of the molecular structures of chromium; Mechanics of hydrogenation-dehydrogenation reactions; Mobility and reactivity on oxidic surfaces.

Figures

Figure 1. Behavior of Cr in the environment. Chromium ions can be released from natural chromium sources or by man in the environment, where it is susceptible to redox and homogeneous and/or heterogeneous reactions.

Figure 4. DRS (A) and RS (B) spectra of hydrated Cr on an alumina surface for increasing Cr loading (Reprinted from ref 93. Copyright 1995 Royal Chemical Society.)

Figure 5. Surface chemistry of hydrated Cr on amorphous surfaces, showing the relation between the polymerization degree of Cr on the one hand and the isoelectric point of the support and the Cr loading on the other hand (redrawn from ref 106).

Figure 6. Anchoring reaction of chromate on an alumina support. Reaction of Cr with the hydroxyl groups and dehydroxylation process of the surface oxide (redrawn from ref 106).

Figure 16. Application of the combined DRS-ESR method on the spectra of Cr/Al2O3 catalysts as a function of reduction temperature: (I) Series of DRS spectra with increasing reduction temperature [reduction at 200 C (A); 300 C (B); 400 C (C) and 600 C (D)]; (II) deconvoluted diffuse reflectance spectra (same abbrevations as under I); (III) experimental (A) and simulated (B) ESR spectrum of Cr5+ on alumina; (IV) DRS calibration lines of Cr6+ and Cr3+; (V) distribution of Cr6+, Cr5+, Cr3+, and Cr2+ ions on alumina as a function of pretreatment [(A) calcination at 550 °C; (B) reduction at 200 °C; (C) reduction at 300 °C, (D) reduction at 400 °C; (E) reduction at 600 °C; and (F) recalcination at 550 °C]. (Reprinted from refs 46 and 56. Copyright 1995 and 1993 American Chemical Society, respectively.)

Figure 2. Pictorial representation of a Phillips Polymerization Catalyst (Cr/SiO2). Interaction between ethylene molecules and supported Cr and oligomerization of ethylene. [Space-filling molecular model (as generated by Hyperchem): green, chromium; blue, oxygen; yellow, carbon; red, hydrogen; and gray, oxidic support].

Full text

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides
Bert M. Weckhuysen,*
,†
Israel E. Wachs,
‡
and Robert A. Schoonheydt
†
Centrum voor Oppervlaktechemie en Katalyse, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium and Zettlemoyer
Center for Surface Studies, Departments of Chemistry and Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015
Received July 31, 1995 (Revised Manuscript Received May 8, 1996)
Contents
I. Introduction 3327
A. General Introduction 3327
B. Scope of the Review 3328
II. Molecular Structure of Cr in Aqueous Media and
in the Solid State
3328
A. Molecular Structure of Cr in Aqueous Media 3329
B. Molecular Structure of Cr in the Solid State 3329
III. Characterization Methods 3330
IV. Molecular Structure of Cr on Oxidic Surfaces 3331
A. Molecular Structure of Cr on Amorphous
Oxides
3331
1. Hydrated Cr 3331
2. Anchored Cr 3332
3. Reduced Cr 3334
B. Molecular Structure of Cr on Molecular
Sieves
3335
1. Chromium Ion-Exchanged and
-Impregnated Molecular Sieves
3335
2. Hydrothermally Synthesized,
Chromium-Containing Molecular Sieves
3338
V. Quantitation of Cr Species on Oxidic Surfaces 3339
VI. Mobility and Surface Reactivity of Cr on Oxidic
Surfaces
3339
A. Mobility of Supported Cr 3339
B. Surface Reactivity of Supported Cr 3343
VII. Catalysis of Cr on Oxidic Surfaces 3344
A. Oxidation Reactions 3344
B. Hydrogenation
−
Dehydrogenation Reactions 3345
C. Polymerization Reactions 3346
VIII. Concluding Remarks 3346
IX. List of Abbreviations 3347
X. Acknowledgments 3347
XI. References 3347
I. Introduction
A. General Introduction
Among the transition metal ions of the 3d series,
Cr takes a particular position because of its vari-
ability in oxidation state, coordination numbers and
molecular structure.
1,2
The elucidation of these Cr
species on inorganic oxidic surfaces is a complex task,
which is of fundamental importance to understanding
the behavior of Cr in the environment, colloids, and
Cr-based heterogeneous catalysts.
3-6
The environmental behavior of Cr is illustrated in
Figure 1. Chromium is frequently encountered in
minerals and in geochemical deposits,
7
and due to
erosion and weathering, chromium becomes a surface
species or can be released into the environment. On
the other hand, Cr compounds are used in industries
such as leather tanning, electroplating, and pigment
production and, therefore, are found in solid wastes
and in waste waters. All these Cr oxides are highly
soluble and susceptible to various reactions at the
solid-water interface: redox processes and homoge-
neous and heterogeneous reactions. Chromium might
be oxidized or reduced by soil constituents and such
redox reactions have dramatic influences on the
behavior of Cr.
8,9
The water soluble and mobile Cr
6+
is toxic, while the hazard of the less mobile Cr
3+
is
relatively low. Manganese oxides are the known
naturally occurring oxidants of Cr
3+
, while Cr
6+
is
reduced to the less mobile Cr
3+
by organic soil
constituents (amino, humic, and fulvic acids) and
Fe
2+
.
Chromium-based catalysts are composed of Cr
oxides supported on inorganic oxides, such as silica,
alumina, and molecular sieves. Chromium on silica
(Cr/SiO
2
), as illustrated in Figure 2, is the famous
Phillips catalyst for the polymerization of ethylene
at relatively low pressures.
10-13
This catalyst is the
basis for the Phillips particle form process in the
production of high-density polyethylene (HDPE), one
of the most extensively used polymers. Other im-
portant catalytic activities are hydrogenation-dehy-
drogenation, oxidation, isomerization, aromatization,
and DeNO
x
reactions.
14-28
The basis for the activity
of Cr in such a wide spectrum of reactions lies in the
variability of oxidation states, of coordination envi-
* Auhor to whom correspondence should be addressed.
†
Katholieke Universiteit Leuven.
‡
Lehigh University.
Figure 1. Behavior of Cr in the environment. Chromium
ions can be released from natural chromium sources or by
man in the environment, where it is susceptible to redox
and homogeneous and/or heterogeneous reactions.
3327
Chem. Rev.
1996,
96,
3327
−
3349
S0009-2665(94)00044-0 CCC: $25 00 © 1996 American Chemical Society
+ +

ronments, and of degree of polymerization of Cr oxide
species. This variability is especially pronounced on
the surface. Thus, knowledge about the surface
chemistry of Cr in inorganic oxides is of key impor-
tance in environmental sciences and heterogeneous
catalysis.
B. Scope of the Review
A prerequisite for understanding the behavior of
Cr on surfaces of inorganic oxides is a thorough
knowledge of the chemistry and its dependence on
the type and composition of the inorganic oxide as
well as environmental conditions. In this review,
fundamental advances into the surface chemistry and
spectroscopy of Cr in inorganic oxides since 1985, the
publication year of the review of McDaniel,
6
are
emphasized. We will restrict ourself to amorphous
supports and molecular sieves and, thus, oxygen is
the main ligand of Cr.
With the basic principles of solution and solid-state
chromium chemistry as the starting point, it will be
shown that (1) the support type and composition play
a decisive role in the speciation of Cr; (2) a battery
of complementary techniques (DRS, RS, IR, XPS,
TPR, ESR, EXAFS-XANES, etc.) is necessary to
unravel the surface chemistry of Cr; (3) the oxidation
states of Cr can be spectroscopically quantified in
well-defined conditions; and (4) surface Cr ions are
mobile and possess catalytic activity.
II. Molecular Structure of Cr in Aqueous Media
and in the Solid State
Chromium occurs with different coordination num-
bers (2, 3, 4, 5, and 6), different oxidation states (Cr
n+
with n ) 2, 3, 4, 5, and 6), and molecular structures
(chromate, dichromate, trichromate, etc.).
1
Cr
6+
(d
0
)
ions are the mostly tetrahedrally coordinated and
tend to form polyoxoanions. Cr
3+
(d
3
) is the most
stable oxidation state and has been extensively
Figure 2. Pictorial representation of a Phillips Polymerization Catalyst (Cr/SiO
2
). Interaction between ethylene molecules
and supported Cr and oligomerization of ethylene. [Space-filling molecular model (as generated by Hyperchem): green,
chromium; blue, oxygen; yellow, carbon; red, hydrogen; and gray, oxidic support].
Bert M. Weckhuysen was born on July 27, 1968, in Aarschot, Belgium.
In 1991 he received his M.S. degree from the Faculty of Agronomy and
Applied Biological Sciences, Catholic University of Leuven, Belgium. From
1991 to 1995 he was employed as a doctoral student at the Center for
Surface Chemistry and Catalysis of the same faculty under the supervision
of Professor Robert A. Schoonheydt. There he gained experience in
spectroscopic techniques, zeolite synthesis, catalyst design, and charac-
terization. After obtaining his Ph.D. in Applied Biological Sciences from
the Department of Interphase Chemistry in 1995, he has worked as a
visiting research scientist with Professor Israel E. Wachs at the Zettlemoyer
Center for Surface Studies of Lehigh University. Presently, he is a
postdoctoral research fellow of the National Fund of Scientific Research
at the Catholic University of Leuven and a postdoctoral research associate
at the Chemistry Department of TexasA&MUniversity.
Robert A. Schoonheydt received his M.S. degree in 1966 and his Ph.D.
in 1970, both under the supervision of Professor Jan B. Uytterhoeven at
the Catholic University of Leuven, Belgium. After one year as a postdoc
with Professor Jack H. Lunsford in the Chemistry Department of Texas
A&M University, he returned to the Catholic University of Leuven as a
National Fund of Scientific Research researcher. He became full professor
at the same institute in 1989. His teaching responsabilities include physical
and analytical chemistry for engineering students at the Faculty of
Agronomy and Applied Biological Sciences. His research is concentrated
in three areas: (1) spectroscopy and chemistry of surface transition metal
ions; (2) molecular organization on clay surfaces; and (3) theoretical
modeling of molecule
−
surface interactions. Presently, he is secretary-
general of AIPEA and head of the Department of Interphase Chemistry.
3328 Chemical Reviews, 1996, Vol. 96, No. 8 Weckhuysen et al.
+ +

studied. The oxidation states of Cr
5+
(d
1
) and Cr
4+
(d
2
) are rather unstable and Cr
5+
easily dispropor-
tionates to Cr
3+
and Cr
6+
. The Cr
2+
(d
4
) ions are
strongly reducing and only stable in the absence of
oxygen.
A. Molecular Structure of Cr in Aqueous Media
The most important oxidation states in solution are
Cr
6+
,Cr
3+
, and Cr
2+
.
1
The specific chromium oxide
species that can exist depend on the solution pH, the
chromium oxide concentration, and the redox poten-
tial. Cr
6+
, for example, may be present in water as
chromate (CrO
4
2-
), dichromate (Cr
2
O
7
2-
), hydrogen
chromate (HCrO
4
-
), dihydrogen chromate (H
2
CrO
4
),
hydrogen dichromate (HCr
2
O
7
2-
), trichromate
(Cr
3
O
10
2-
), and tetrachromate (Cr
4
O
13
2-
). The last
three ions have been detected only in solutions of pH
< 0 or at chromium(VI) concentrations greater than
1 M. Polyanions containing more than four chro-
mium atoms are not known in solution.
All these features can be understood on the basis
of the Pourbaix diagram presented in Figure 3.
29
Above pH 8, only CrO
4
2-
is stable, and as the pH
decreases into the pH region 2-6, the equilibria
shifts to dichromate according to the overall equilib-
rium:
At still lower pH values and concentrated media,
tri- and tetrachromates are formed (respectively
Cr
3
O
10
2-
and Cr
4
O
13
2-
). In summary, decreasing of
the pH or increasing the chromium oxide concentra-
tion results in the formation of more polymerized
chromium oxide species. The Cr
6+
species is a strong
oxidant but the redox potential depends on the pH.
In acidic media, the following reaction is involved
while in basic solution
In acid solution, Cr
3+
is always an octahedral
hexaquo ion, Cr(H
2
O)
6
3+
. It tends to hydrolyze with
increasing pH, resulting in the formation of poly-
nuclear complexes containing OH
-
bridges.
30
This
is thought to occur by the loss of a proton from
coordinated water, followed by coordination of the
OH
-
to a second cation. The final product of this
hydrolysis is hydrated chromium(III) oxide or chro-
mic hydroxide (Cr(OH)
3
). The following equilibria
are thus observed with increasing pH:
The aqueous chemistry of the strongly reducing
Cr
2+
cation has not been as extensively studied
because of its instability. Cr
2+
ions are present in
water as octahedral high-spin hexaquo ions, Cr-
(H
2
O)
6
2+
, and are unstable with respect to oxidation
to Cr
3+
:
B. Molecular Structure of Cr in the Solid State
The principal chromium oxides are CrO
3
, CrO
2
, and
Cr
2
O
3
, but some intermediate states, like Cr
3
O
8
,
Israel E. Wachs received his B.E. degree in Chemical Engineering from
The City College of The City University of New York in 1973. He received
his M.S. degree in 1974 and his Ph.D. in Chemical Engineering in 1977
from Stanford University under the supervision of Professor R. J. Madix.
He joined the Corporate Research Laboratories of Exxon Research &
Engineering Co. in 1977 where he was involved in fundamental and
applied research in the areas of selective oxidation, hydrocarbon
conversion, hydrogenation of carbon monoxide (Fisher-Tropsch synthesis),
and hydrodesulfurization catalysis. In 1987, he became an Associate
Professor of Chemical Engineering at Lehigh University and was promoted
to full Professor in 1992. His teaching responsabilities include hetero-
geneous catalysis and surface characterization, fundamentals of air
pollution, advanced technologies in chemical engineering, chemical reaction
engineering, unit operations and fluid mechanics. His research program
has focused on the synthesis, characterization, and catalysis of supported
metal oxide catalysts with special emphasis on the molecular structure
−
reactivity relationships of oxidation reactions. He is the editor of the book
Characterization of Catalytic Materials
(Butterworths-Heinneman, 1992).
2CrO
4
2-
+ 2H
+
T Cr
2
O
7
2-
+ H
2
O (1)
Figure 3. The Pourbaix diagram of chromium, expressing
the Cr speciation as a function of pH and potential (T )
25 °C) (Redrawn from ref 29).
Cr
2
O
7
2-
+ 14H
+
+ 6e
-
f
2Cr
3+
+ 7H
2
O with E
o
) 1.33 eV (2)
CrO
4
2-
+ 4H
2
O + 3e
-
f
Cr(OH)
3
+ 5OH
-
with E
o
)-0.12 eV (3)
Cr(H
2
O)
6
3+
T [Cr(H
2
O)
5
OH]
2+
T
[Cr(H
2
O)
n
(OH)
m
]
3+-m
T Cr(OH)
3
(4)
Cr
3+
+ e
-
T Cr
2+
with E
o
)-0.41 eV (5)
Chromium in Inorganic Oxides Chemical Reviews, 1996, Vol. 96, No. 8 3329
+ +

Cr
2
O
5
, and Cr
5
O
12
, have also been observed.
31,32
The
red orthorhombic chromium trioxide (CrO
3
) crystals
are made up of chains of corner-shared CrO
4
tetra-
hedra. They lose oxygen upon heating to give a
succession of lower oxides until the green Cr
2
O
3
is
formed. The latter oxide is the most stable oxide and
has a spinel structure. The third major oxide of
chromium is the brown-black, CrO
2
, which is an
intermediate product in the decomposition of CrO
3
to Cr
2
O
3
and possesses a rutile structure.
Chromium is also frequently encountered in min-
erals and these Cr-bearing minerals contain either
hexa-, tri- or divalent Cr.
33
The most common
minerals contain (distorted) octahedral Cr
3+
(e.g.
chromite, ruby, muscovites and tourmaline), which
gives most of these minerals a green color. Although
Cr
2+
ions are rare and unstable in terrestrial miner-
als, their presence is suspected in the blue minerals
olivine and pyroxene. Cr
2+
ions are frequently octa-
hedrally coordinated, but tetrahedral Cr
2+
exists in
spinel-like minerals.
III. Characterization Methods
The characterization of the molecular structure of
supported chromium ions is rather involved, since
deposition of this metal ion on a support can result
in (1) isolated chromium ions, (2) a two-dimensional
chromium oxide overlayer, or (3) three-dimensional
chromium oxide crystallites. Moreover, each phase
can simultaneously possess several different molec-
ular structures. Thus, useful characterization tech-
niques, which can provide detailed information about
the molecular structure of the supported chromium
oxide, must be capable of discriminating between
these different states and of quantifying the indi-
vidual oxidation states. The spectra are complex and
usually encompasses several overlapping bands, so
that band decomposition routines and chemometrical
Table 1. Characterization Techniques for Cr Speciation and the Obtained Information on Coordination,
Oxidation States, and Dispersion
oxidation state
technique 65432coordination dispersion quantitative
minimum detectable
amount (wt % Cr)
DRS +--++ + - + <0.1
ESR -+-+- + ( + <0.1
IR +--++ + - - <0.2
RS +--++ + + - <0.2
XPS ++-++ - + + >0.4
EXAFS-XANES ((((( + - - >2.0
TPO-TPR -(((( - + + >0.8
SQUID -++++ - - + <0.1
SIMS ----- ( ( - >1
XRD ----- + ( - >5
ISS ----- - + - -
CO
2
-chemisorption ----- - + - -
Table 2. Spectroscopic Fingerprints of Supported Cr Species
spectroscopic
technique spectroscopic signature
type of
signature Cr species ref(s)
ESR axially symmetric/rhombic signal with
g around 2 and ppw < 60 G
isolated Cr
5+
(γ-signal) 34-47
nearly isotropic and broad signal (g ) 1.9-2.4)
with 40 < ppw < 1800 G
Cr
2
O
3
-like clusters (β-signal)/
Cr(H
2
O)
6
3+
complexes
broad signal around g ) 4 with high
D and E values
isolated Cr
3+
in highly distorted
octahedral coordination (δ-signal)
DRS 27000-30000; 36000-41000 cm
-1
charge
transfers
chromate 48-58
21000-23000; 27000-30000;
36000-41000 cm
-1
polychromate
15000-17000 cm
-1
d-d transitions (pseudo-) octahedral Cr
3+
,
including Cr
2
O
3
10000-13000 cm
-1
(pseudo-) octahedral Cr
2+
7000-10000 cm
-1
(pseudo-) tetrahedral Cr
2+
RS 865 cm
-1
Cr-O vibrations hydrated chromate 59-68
900; 942 cm
-1
hydrated dichromate
904; 956; 987 cm
-1
hydrated trichromate
980-990 cm
-1
dehydrated monochromate
1000-1010; 850-880 cm
-1
dehydrated polychromate
550 cm
-1
octahedral Cr
3+
IR 900-950 cm
-1
(1800-1900 cm
-1
in first overtone)
Cr-O vibrations Cr
6+
64,69-85
2178; 2184 cm
-1
CO vibrations Cr
2+
with CN ) 2 before
chemisorption (CrA species)
2191 cm
-1
Cr
2+
with CN ) 3 before
chemisorption (CrB species)
no chemisorption Cr
2+
with CN ) 4 before
chemisorption (CrC species)
XPS 580 eV Cr
6+
86-91
579 eV Cr
5+
577 eV Cr
3+
576 eV Cr
2+
3330 Chemical Reviews, 1996, Vol. 96, No. 8 Weckhuysen et al.
+ +

techniques need to be employed. This is especially
important if quantitative information is desired.
The different techniques used in the literature for
studying supported Cr are summarized in Table 1
(list of abbreviations provided at end of the paper),
together with the obtained information about spe-
ciation, dispersions and coordination. The given
information about speciation concerns only the de-
tectable oxidation states, while dispersion can be
defined as the ratio of the amount of Cr probed by a
particular characterization technique over the total
amount present on the surface. Furthermore, the
minimum detectable amounts are the minimum
values reported in the literature. It is also important
to state that the different characterization techniques
are only quantitative under well-defined conditions.
In the case of ESR, problems with quantification may
arise if spin-spin interactions or fast relaxation
processes occur. The binding energies (BE), mea-
sured by XPS, increases with increasing oxidation
state and for a fixed oxidation state with the elec-
tronegativity of the surrounding atoms. The BE
values are also influenced by the degree of dispersion
and bulk chromium oxides always exhibit lower BE
values compared to Cr ions dispersed in inorganic
oxides. As a result, the estimation of the valence
states only from BE values is very difficult. In
addition, highly dispersed Cr
6+
and Cr
5+
are readily
reduced under high vacuum in the ESCA chamber
and/or under influence of X-ray irradiation, and
consequently complicating the estimation of the
different oxidation states from XPS data.
86,87
The five most applied spectroscopic characteriza-
tion tools are ESR, DRS, RS, IR, and XPS, and their
spectroscopic fingerprints of the observed Cr species
are given in Table 2.
34-91
It is clear that no charac-
terization technique will be capable of providing all
the information needed for complete characterization.
Thus, successful characterization of chromium in
inorganic oxides requires a multitechnique approach.
IV. Molecular Structure of Cr on Oxidic Surfaces
A. Molecular Structure of Cr on Amorphous
Oxides
The molecular structure of Cr on amorphous oxides
is strongly dependent on the environmental condi-
tions (hydrated, dehydrated, oxidized, and reduced
environments) and on the type and composition of
the support (SiO
2
,Al
2
O
3
, SiO
2
‚Al
2
O
3
, MgO, ZrO
2
,
TiO
2
, AlPO
4
,Nb
2
O
5
, and SnO
2
).
1. Hydrated Cr
Under hydrated conditions, the surface of an
amorphous oxide is covered by a thin water film and
its hydroxyl population is subject to pH-dependent
equilibria reactions:
3,4,92
with X ) Si, Al, Ti, Mg, Nb, Sn or Zr; H
s
+
and H
+
represent the surface and solution proton, respec-
tively; K
1
) ([X-OH]*[H
s
+
])/[X-OH
2
+
]; K
2
) ([X-
O
-
]*[H
s
+
])/[X-OH] and the isoelectric point (IEP) )
(pK
1
+ pK
2
)/2 and represents the pH at which the
surface of the oxide has a net zero charge. The IEP’s
are dependent on oxide type and composition as
shown in Table 3. The lower the IEP of the amor-
phous oxide, the more the equilibria of the reactions
6-8 are driven to the right. The higher the H
+
concentration near the surface, the more the equi-
libria of reactions 1 and 4 are driven toward the
formation of dichromate and a Cr(H
2
O)
6
3+
complex,
respectively.
Spectroscopic measurements on supports with low
Cr
6+
loadings, confirm these findings, and the ob-
tained speciation is summarized in Table 3. DRS
experiments show that the monochromate:dichro-
mate ratio decreases with increasing Si:Al ratio of
silica aluminas, while by Raman spectroscopy, mono-
chromate is observed on MgO, Al
2
O
3
, ZrO
2
, and TiO
2
and mainly polychromates (dichromate, etc.) on SiO
2
.
As an example, Figure 4 shows the DRS and RS
spectra of hydrated Cr/Al
2
O
3
catalysts as a function
of the Cr loading. Instead, Cr
3+
species are difficult
to discriminate on hydrated surfaces by spectroscopy
and the different species of eq 4 cannot be clearly
distinguished. In any case, the broad isotropic ESR
signal around g ) 2 and the typical DRS absorptions
at around 17 000 and 23 000 cm
-1
are indicative for
the presence of hydrated octahedral Cr
3+
complexes.
When the Cr loading (as e.g. CrO
3
) increases two
effects come into play: (1) the pH near the surface
is lowered due to presence of chromium and decreases
with increasing Cr loading and (2) the dispersion
depends on the available surface area as well as
availability of reactive surface hydroxyl groups. Both
factors influence the chemistry of chromium in the
same direction, i.e. toward the formation of surface
polychromates. The detected Cr
6+
species are de-
scribed in Table 3, while Figure 5 illustrates the
speciation of hydrated Cr on surfaces of amorphous
supports. It is also important to stress that the
presence of anions and cations (Na
+
,F
-
, etc.) on the
Table 3. Observed Surface Chromium Oxide Species on Different Hydrated Amorphous Inorganic Oxides
oxide IEP Cr oxide at low Cr loading Cr oxide at high Cr loading ref(s)
MgO 11.0 chromate chromate 68
Al
2
O
3
8.9 chromate chromate and dichromate 56,57,63,66,68,94
TiO
2
6.2 chromate chromate and dichromate 68
ZrO
2
5.9 chromate dichromate and chromate 68
SiO
2
‚Al
2
O
3
4.5 chromate and some dichromate dichromate and chromate 68,93
Nb
2
O
5
4.2 chromate and some dichromate dichromate and chromate 95
SiO
2
3.9 chromate and some dichromate trichromate, dichromate and chromate 68,94
SiO
2
2.0 dichromate and some chromate tetrachromate, trichromate, dichromate 56,57,93
X-OH
2
+
T X-OH + H
s
+
(6)
X-OH T X-O
-
+ H
s
+
(7)
H
s
+
T H
+
(8)
Chromium in Inorganic Oxides Chemical Reviews, 1996, Vol. 96, No. 8 3331
+ +

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Surface chemistry and spectroscopy of chromium in inorganic oxides" ?

10-13 This catalyst is the basis for the Phillips particle form process in the production of high-density polyethylene ( HDPE ), one of the most extensively used polymers.