Influence of Ni substitution on redox properties of In2(1-x) Ni2xTiO5-δ oxides

Article abstract of DOI:10.1016/j.tca.2011.01.009

The thermal behavior of mixed metal oxides with nominal compositions of In_2(1-x)Ni_2xTiO_5-δ where 0.0 ≤ x ≤ 0.2, were investigated by recording their temperature programmed reduction (TPR). The samples were synthesized by ceramic route and analyzed for phase composition using powder X-ray diffraction. The TPR profile of pristine In ₂TiO₅ indicated the reduction of In³⁺, which takes place predominantly over the other species Ti⁴⁺ in In ₂TiO₅ sample. Ni substitution at In³⁺ site induced ease in reducibility as indicated by lowering of onset reduction temperature and T_max. Moreover, it also facilitated the reduction of otherwise non reducible cation, Ti⁴⁺. Ni metal initially formed by reduction, dissociates and activates hydrogen which then reacts and reduces the oxide (even Ti⁴⁺), a case of hydrogen spillover. The metallic Ni forms an alloy Ti₂Ni with reduced Ti metal and δIn ₃Ni₂ alloy with the reduced In metal as evident by XRD and XPS results of the reduced residue of 20% Ni doped (In_1.6Ni _0.4TiO_5-δ) sample. Due to formation of new alloy phases of δIn₃Ni₂ and Ti₂Ni, the rate of hydrogen adsorption diminishes which slows down further hydrogen activation and consequent further Ti⁴⁺ reduction.

Full text of DOI:10.1016/j.tca.2011.01.009

Thermochimica Acta 516 (2011) 40–45
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Influence of Ni substitution on redox properties of In2(1−x) Ni2xTiO5−ı oxides
A.M. Banerjeea, M.R. Paia, Jagannathb, S.R. Bharadwaja,∗
a Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
b Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The thermal behavior of mixed metal oxides with nominal compositions of In2(1−x)Ni2xTiO5−ı where
0.0 ≤ x ≤ 0.2, were investigated by recording their temperature programmed reduction (TPR). The samples
were synthesized by ceramic route and analyzed for phase composition using powder X-ray diffraction.
The TPR profile of pristine In2TiO5 indicated the reduction of In3+, which takes place predominantly
over the other species Ti4+ in In2TiO5 sample. Ni substitution at In3+ site induced ease in reducibility as
indicated by lowering of onset reduction temperature and Tmax. Moreover, it also facilitated the reduction
of otherwise non reducible cation, Ti4+. Ni metal initially formed by reduction, dissociates and activates
hydrogen which then reacts and reduces the oxide (even Ti4+), a case of hydrogen spillover. The metallic
Ni forms an alloy Ti2Ni with reduced Ti metal and ıIn3Ni2 alloy with the reduced In metal as evident by
XRD and XPS results of the reduced residue of 20% Ni doped (In1.6Ni0.4TiO5−ı) sample. Due to formation
of new alloy phases of ıIn3Ni2 and Ti2Ni, the rate of hydrogen adsorption diminishes which slows down
further hydrogen activation and consequent further Ti4+ reduction.
Received 13 August 2010
Received in revised form
15 December 2010
Accepted 7 January 2011
Available online 18 January 2011
Keywords:
Temperature programmed reduction
Reducibility
Alloy
Hydrogen spillover
XPS
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
chemical properties which in turn results in modified catalytic
behavior.
The physico-chemical properties of ceramic ternary oxides
(AxByOz) having perovskites, spinel or any other structure have
been investigated in great detail both because of fundamental sci-
entific knowledge and its widespread technological applications in
various fields. Specifically the ternary oxide that arise when one
of the cation is titanium are titantes which are high performance
materials having applications in various fields. Some of the recent
applications include catalysts [1–3], sensors and actuators [4,5],
solar cells [6] SOFC materials [7] and so on. So, it is imperative to
study their interesting physicochemical properties the knowledge
of which finds them utilized in suitable applications.
So, with the objective to study the thermal and reduction
behaviors of certain titania and indium oxide based single phased
compositions, which can serve as prospective catalyst for redox
reactions and as photocatalysts particularly for water splitting
reactions, studies have recently been taken up in our laboratories
on In2O3–TiO2 system resulting in formation of indium titanate,
In2TiO5. The thermo-physical characteristics and redox behavior
of In–Ti–O system was monitored. The catalytic properties of many
titanates have been shown to be dependent on their physicochem-
ical properties and the thermal properties of some titanates have
been investigated [14–16]. We have also investigated the redox
One of the interesting applications of these titanates is as
catalyst for various reactions. The important considerations in
designing an oxide catalyst are its thermal, chemical stability,
reducibility and oxidizability [8–13]. The thermal and chemi-
cal stability and redox behavior of these titanates would play
a vital role in determining their catalytic properties. Further, it
is well known that substitution by transition metal cations at A
or B sites of these oxide systems, further tailor their physico-
properties of Fe-doped lanthanum titanates (La2Ti2(1−x)Fe2xO7−ı,
0.0 ≤ x ≤ 1.0) and correlated its thermophysical properties with
catalytic activity for CO + N2O reaction [17,18]. The crystal struc-
ture and luminescent properties of indium titanate oxide, In2TiO5
have been reported [19]. The structure of In2TiO5, belongs unam-
biguously to In2VO5 type. The two independent In3+ ions are
octahedrally coordinated with six oxygen atoms, resulting in InO6
octahedra, which by edge sharing forms infinite ribbons. Three
2n−
corner connected (In2O4)n
infinite ribbons forms the tunnels
where O–Ti–O–Ti–O chains are inserted. In each tunnel formed by
four ribbons one finds two chains of TiO5 distorted square pyra-
mids oppositely oriented. In order to perturb this structure and
to modify catalytic properties, in the present study, an aliovalent
substitution by transition metal Ni2+ at A-site was attempted, intro-
ducing thereby some micro-structural changes in the lattice. It
∗
Corresponding author at: Fuel Cell Materials and Catalysis Section, 3-193
H, Chemistry Division, Modular Labs, Bhabha Atomic Research Centre, Mumbai
400085, India. Tel.: +91 22 25595100; fax: +91 22 25505151.
E-mail addresses: shyamala@barc.gov.in, shyamala@apsara.barc.ernet.in
(S.R. Bharadwaj).
0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.01.009

A.M. Banerjee et al. / Thermochimica Acta 516 (2011) 40–45
41
is apparent that the substitution of a smaller lower-valent Ni2+
3+
˚
˚
ion (ionic radius = 0.69 A) in place of In (ionic radius = 0.81 A) is
likely to form an anion deficient solid solution with partial loss
of oxygen in the lattice (ı).Recently we have studied the effect
3+
of B-site (Ti4+) substitution by Fe3+ (ionic radius = 0.64 A) and Cr
˚
˚
(ionic radius = 0.063 A) on thermal properties and reduction behav-
ior of In2TiO5. In2Ti1−xFexO5−ı and In2Ti1−xCrxO5−ı (0.0 ≤ x ≤ 0.2)
compositions were prepared by solid state route and the effect of
substitution on redox behavior was reported [20]. In the present
study, we report the A-site substitution-induced effects on indium
titanate. For this purpose, In2(1−x)Ni2xTiO5−ı (0.0 ≤ x ≤ 0.2) mixed
oxide catalysts were synthesized using solid-state reaction and
characterized by powder X-ray diffraction (XRD). The thermal and
reduction behaviors were studied by recording thermogravimet-
ric (TG), and temperature-programmed reduction (TPR) profiles.
The chemical oxidation states of individual metal ions of multi-
component oxides after reduction were identified using XRD and
X-ray photoelectron spectroscopy (XPS).
2. Experimental
Fig. 1. XRD patterns of In2(1−x)Ni2xTiO5−ı samples, # indicates peaks due to unre-
acted NiO phase and * indicates the peaks arising from NiTiO3 phase.
Mixed oxides with nominal composition, In2(1−x)Ni2xTiO5−ı for
0.0 ≤ x ≤ 0.2, were synthesized through ceramic route by mixing
reactant oxides in appropriate stoichiometry as depicted by fol-
lowing equation:
3. Results
3.1. XRD
Table 1 lists the abbreviations and phases identified from
XRD patterns of all the In2(1−x)Ni2xTiO5−ı (0.0 ≤ x ≤ 0.2) samples
prepared by solid state method. Fig. 1 shows the powder XRD
patterns of In2TiO5 and corresponding patterns observed due to
aliovalent substitution of Ni2+ in place of In3+ at A-site. From
Fig. 1, it is observed that the XRD pattern of x = 0, composition
matches well with that of orthorhombic In2TiO5 (JCPDS card No.
30-0640) oxide. The lines due to reactant oxides are missing in
these patterns thus confirming the completion of the solid-state
reaction. In2TiO5 is isostructural with In2VO5. It crystallizes in
the orthorhombhic space group, with a = 0.7241 nm, b = 0.3427 nm,
c = 1.4878 nm, cell volume = 0.3692 nm3 and Z = 4. The XRD patterns
of ITN05 (In1.95Ni0.05TiO5−ı) and ITN01 (In1.9Ni0.1TiO5−ı) composi-
tions match with the XRD patterns of unsubstituted indium titanate
sample as shown in Fig. 1. Thus, lower extent of Ni substitu-
tion resulted in single phase material comprised of In2TiO5 phase
due to formation of solid solution of Ni with the lattice of the
parent compound. But for samples having Ni content x ≥ 0.1 in
In2(1−x)Ni2xTiO5−ı a very low intensity line (marked with #) was
observed which corresponds to the 100% peak of NiO (JCPDS card
No. 47-1049). Thus for these compositions in addition to parent
phase there was segregation of unreacted NiO phase in very small
proportion. Moreover, for samples having higher Ni content i.e.
x ≥ 0.15, a third phase of NiTiO3 (JCPDS card No. 33-0960, peaks
marked with *) was observed in addition to the unreacted nickel
oxide phase (Fig. 1). This phenomenon of segregation of secondary
phases in the Ni doped samples can be attributed to the large dif-
(1 − x)In2O3 + TiO2 + 2xNIO → In2(1 − x)Ni2xTiO5 − ı
The pellets of homogeneous mixtures were calcined at 900 ◦C for
24 h, 1000 ◦C for 24 h and finally at 1250 ◦C for 12 h, with intermit-
tent grindings so as to ensure the uniformity and the completion of
the reaction.
The powder XRD patterns were recorded on a Philips X-
ray diffractometer (model PW 1710), equipped with a graphite
monochromator and Ni-filtered Cu K␣ radiation.
The thermal behavior of these samples (∼20 mg) was monitored
by recording their TG profiles both in air and 5% H2 + Ar atmo-
spheres, in the temperature range of 25–1000 ◦C at a heating rate
of 10 ◦C min−1 on Setaram TGA-92 instrument.
Redox behavior of catalyst was studied by recording temper-
ature programmed reduction/oxidation (TPR/TPO) profiles on a
TPDRO-1100 analyzer (ThermoQuest, Italy) in temperature range
of 25–1100 ◦C under the flow of H2 (5%) + Ar, gas mixtures at a flow
rate of 20 ml min−1, with a heating rate of 6 K min−1. The samples
were pretreated at 350 ◦C for about 2.5 h in helium, prior to record-
ing of the first TPR run. A thermal conductivity detector (TCD) is
employed to monitor the change in composition of reactive gas
mixture with time. The water formed during reduction process was
removed from the flowing gas by the help of a soda lime trap placed
just before the detector. Hence the signal obtained was primarily
due to change in thermal conductivity of the flowing gas by con-
sumption of hydrogen. The amount of hydrogen consumed by the
sample during a TPR run corresponded to the area under the TPR
profile. The calibration factor (␮moles of H2 per mV of TCD signal)
for the H2 was calculated using the standard sample. This factor has
been employed in all the substituted samples and the experimen-
tal amount of H2 consumed was obtained. The calculated value of
H2 consumed by the sample was compared with the experimental
value.
˚
ference in ionic radii between the substituent cation Ni (0.069 A)
˚
and the parent cation In (0.081 A).
3.2. Temperature programmed reduction behavior (TPR)
The typical first temperature programmed cycle (TPR) of substi-
tuted samples and unsubstituted sample are shown in Fig. 2. The
TPR profile of unsubstituted, In2TiO5 sample as seen in Fig. 2 com-
prises of a prominent band with onset at ∼625 ◦C and extending
beyond 1000 ◦C. This indicated the predominant reduction of one
of the species, identified to be In3+, in the temperature range of
XPS studies were carried out on electron spectrometer using
Mg K␣ X-rays (hꢀ = 1253.6 eV) as the primary source of radiation.
The appropriate corrections for charging effect were made with the
help of a C 1s signal appearing at 285.1 eV.

42
A.M. Banerjee et al. / Thermochimica Acta 516 (2011) 40–45
Table 1
Identification of phase in In2(1−x)Ni2xTiO5−ı (0.0 ≤ x ≤ 0.2) samples and their abbreviations.
S. no.
Nominal Composition
Ni content (2x)
Abbreviation
Phase identification by XRD
1
2
3
4
5
6
In2TiO5
0.00
0.05
0.1
0.2
0.3
ITO
Single phase, In2TiO5
Single phase, In2TiO5
Single phase, In2TiO5
In2TiO5 and NiO
In2TiO5, NiO and NiTiO3
In2TiO5, NiO and NiTiO3
In1.95Ni0.05TiO5−ı
In1.9Ni0.1TiO5−ı
In1.8Ni0.2TiO5−ı
In1.7Ni0.3TiO5−ı
In1.6Ni0.4TiO5−ı
ITN05
ITN1
ITN2
ITN3
ITN4
0.4
600–1100 ◦C over other reducible species in In2TiO5 sample. The
temperature maximum (Tmax) of the reduction bands corresponds
to ∼1070 ◦C. However, Ni2+ substitution at In3+ site has induced
considerable changes in the reduction profile of indium titanate.
In case of compositions having higher content of Ni, viz., ITN2 and
ITN4 TPR profiles (Fig. 2), exhibit two-three weak bands in low tem-
perature range of 450-800 ◦C in addition to main reduction band.
The presence of these bands is probably attributed to reduction of
Ni2+ → Ni0 along with reduction of In3+. At the same time, the Tmax
of the individual peaks was found to be lower in the case of substi-
tuted samples. For instance, weobservethelowering ofTmax around
by 80 ◦C for ITN4 in Fig. 2, as compared to the TPR profile of an un-
substituted phase. Also the onset reduction temperature of 625 ◦C
in unsubstituted phase has considerably decreased by Ni substi-
tution as shown in Fig. 2. The TPR profile as a whole has shifted
to lower temperature as a result of Ni substitution. These results
are comparable to our earlier work [20] where Fe and Cr substi-
tuted at B-site were found to increase the reducibility of indium
titanate. Thus, M (Ni2+, Cr3+ and Fe3+) substitution has undoubt-
edly facilitated the reduction of In2TiO5 phase. These changes in the
TPR profiles can be ascribed to the nonstoichiometry and imperfec-
tions generated in the single phased compositions as a result of A
and B-site substitution.
the corresponding TG exhibits substantial weight loss. A represen-
tative TG-DTA plot of ITN4 in H2 (5%) + Ar atmosphere is shown in
Fig. 3. The weight loss in TG plot starts above ∼500 ◦C and contin-
ues up to ∼1050 ◦C in the 20% Ni-substituted sample. As revealed
by temperature programmed reduction profiles (Fig. 2) of these
oxides, we can infer that this weight loss is due to reduction of
these oxides in H2 atmosphere. Mainly the weight loss is attributed
to reduction of species, In3+ and Ni2+, to In0 and Ni0 respectively.
The evidence of the above inference comes from temperature pro-
grammed reduction profiles of In2O3 and TiO2 oxides. TPR profiles
of In2O3 and TiO2 reactant oxides (exhibited as inset in Fig. 3) sug-
gests that In3+ species reduces above 600 ◦C with Tmax at 670 ◦C,
whereas TiO2 does not reduce up to 1000 ◦C recorded in the range
of 25–1000 ◦C under H2 flow (5% H2 in Ar).
3.4. Ex situ characterization of reduced residue samples
3.4.1. XRD
Fig. 4 shows the XRD patterns of reduced residue of In2TiO5,
ITN2 and ITN4 samples obtained after recording TPR. In the reduced
In2TiO5 and ITN2 samples the presence of prominent lines at
2ꢁ = 32·98◦, 39·2◦ conforms to reported pattern of In0 (JC-PDS
No.05-0642). Other strong lines at 27.44◦, 36.08◦ and 54.32◦ match
well with rutile TiO2 phase (JC-PDS No. 21-1276). Thus, both metal-
lic indium and rutile titania were present in these two reduced
samples. There was no other phases present in the reduced In2TiO5
sample. No prominent additional lines were also observed in the
reduced ITN2 sample, except for a weak hump at 41.2◦. This peak
can be attributed to the formation of an alloy of Ni and In namely
ıIn3Ni2 (JCPDS No. 070298). The small amount of nickel present
3.3. Thermo gravimetric studies
Both substituted and unsubstituted samples did not show any
weight loss in TG when recorded in O2 (5%) + Ar, thus indicating that
all substituted In2(1−x)Ni2xTiO5−ı oxide samples are stable in air up
to 1000 ◦C. Thus indicating that all In2(1−x)Ni2xTiO5−ı (x = 0.0–0.4)
samples once formed, do not pick up either moisture or CO2, and are
thermally stable compositions till 1000 ◦C. Although, in O2 (5%) + Ar
atmosphere these compositions are stable whereas in H2 (5%) + Ar,
Fig. 3. Thermogram plot of ITN4 composition recorded in H2 (5%) + Ar atmosphere.
Inset exhibiting TPR profiles of In2O3 and TiO2.
Fig. 2. Typical first TPR cycle of In2(1−x)Ni2xTiO5−ı samples as a function of x.

A.M. Banerjee et al. / Thermochimica Acta 516 (2011) 40–45
43
No. 04-0850). So, Ni metal formed by reduction was very active
and reacted with other reduced phases to form alloys. Further, the
presence of Ni facilitated the reduction of In3+ as the Tmax of the
individual peaks due to reduction of In3+ was found to be lowered
in the case of substituted samples. In addition to this, the otherwise
non-reducible cation Ti4+ also got reduced in the 20% substituted
sample which might probably be due to autocatalytic effect of Ni2+
.
This phenomenon can be referred to as an incident of hydrogen
spillover in mixed metal oxide. The Ni metal initially formed by
reduction of the cation was active enough to dissociate and acti-
vate the hydrogen, which in turn could even reduce Ti4+ cation in
addition to facilitating the reduction of In3+. It could also be sug-
gested that the extent of reduction of Ti4+ and subsequent alloy
formation would depend on the content of secondary phase. In
ITN2 sample In3+ was replaced partially by Ni2+ and the reduced
Ni metal facilitated the reduction of In3+ cation and then an alloy
formation took place. This fact is evident from the TPR curve where
the whole reduction pattern including Tmax for reduction of In3+
shifts to lower temperature. Further in the ITN4 sample the reduc-
tion of partially substituted Ni2+ cation generated Ni metal which
dissociated and activated the hydrogen which eased the reduction
of In3+ cation and also reduced Ti4+. This process of reduction of Ti4+
if continued would have resulted in the reduction of all Ti4+ but the
formation of the Ti2Ni alloy prevented further hydrogen activation
and that further reduction of the Ti4+ was hindered. So, only partial
reduction of Ti4+ was feasible. The presence of NiTiO3 phase might
have assisted the above process. Thus, Ni substitution at In-site of
In2TiO5 had a drastic effect on the reduction behavior of indium
titanates. TPR results in conjunction with XRD studies reveal the
reduction of In3+ → In0, while Ni2+ facilitated the reduction of oth-
erwise non-reducible cation Ti4+ and resulted in formation of Ni–Ti
alloy (Ti2Ni).
Fig. 4. XRD patterns of spent In2(1−x)Ni2xTiO5−ı samples after recording TPR in H2
(5%) + Ar atmosphere of ITO ITN2 and ITN4 compositions. The peaks due to ıIn3Ni2
alloy are marked by *.
in the sample was initially reduced at a lower temperature with
an onset at ∼450 ◦C and subsequently In3+ was also reduced (refer
Fig. 2). The reduced metallic nickel then reacted with the reduced In
metal to form the In-Ni alloy. An interesting feature was observed
in the XRD pattern of reduced ITN4 sample. The intensity of the
peaks due to reduced Indium metal was reduced drastically, along
with the appearance of prominent lines (marked by *) at 2ꢁ = 28.9,
41.0, 41.3 and 59.8. These peaks are attributed to the presence of
ıIn3Ni2 phase. Remaining line in the residue were at 2ꢁ = 41.5◦. An
alloy formation of Ni–Ti is indicated by this peak as 2ꢁ = 41.57◦ cor-
responds to 100% peak due to reflection from 511 plane of Ti2NI
alloy (JCPDS No. 180898). This suggests that Ti4+ an otherwise stable
cation towards reduction got reduced within 1100 ◦C. The forma-
tion of Ni–Ti alloy on reduction of ITN4 sample is quite plausible as
revealed by the XRD pattern, as secondary phase of NiTiO3, also got
reduced along with major indium titanate phase. This new phases
possibly would have facilitated the reduction of Ti4+ along with
reduction of Ni2+. Any of the lines in the spent samples do not
match with the reported XRD pattern of Ni metal alone (JC-PDS.
3.4.2. X-ray photoelectron spectroscopy (XPS)
In Fig. 5a and b, the X-ray photoelectron (XPS) spectrum of
fresh, and completely reduced ITN4, sample are shown, which were
recorded in order to identify the oxidation states of indium and tita-
nium metal ions present in the spent samples after the reduction.
Table 2 lists the elements identified and their oxidation states in the
fresh and reduced samples. The binding energies at 445.9 eV and
457.5 eV corresponding to In (3d5/2) and Ti (2p3/2) which matches
with the reported binding energies for In in +3 and Ti in +4 oxida-
tion state present in fresh ITN4 sample as shown in Fig. 5a and b.
But, the binding energy at 445.9 eV as observed in the fresh sam-
ple in Fig. 5a is shifted to lower binding energies of 444.1 eV in
completely reduced ITN4 sample. XPS spectra suggest the exis-
tence of indium in +3 oxidation state in the fresh sample and In(0)
in completely reduced sample. The changes in oxidation states of
Ti present in the fresh and reduced sample are shown in Fig. 5b.
Fig. 5. Binding energies of different metal ions in partially reduced sample of ITN4 obtained after recording TPR up to 1000 ◦C of (a) In (3d), and (b) Ti (2p).

44
A.M. Banerjee et al. / Thermochimica Acta 516 (2011) 40–45
Table 2
after reduction generates the active metal that can dissociate and
activate the hydrogen for facilitating an otherwise difficult reduc-
tion phenomenon. It is reported [22,23] that hydrogen spillover
can be broken into three primary steps: (i) dissociative chemisorp-
tion of gaseous H2 on a transition metal catalyst; (ii) migration of
H atoms from the catalyst to the substrate and (iii) diffusion of H
atoms on substrate surfaces and/or in the bulk materials. In this
case the metallic Ni formed after reduction is the transition metal
catalyst. The substrate is the remnant oxide from which Ni2+ cation
gets eliminated and then the reduction of bulk In3+ occurs by dif-
fusion of H-atoms into the remnant oxide material and thus the
reduction profile shifts to lower temperature. Diffusion of H atoms
and subsequent reduction of Ti4+ is definitely easier in NiTiO3 that
the indium titanate phase so we observe higher amount of Ti4+
reduction in the sample. This is further corroborated by the TPR
results where we observe increased hydrogen consumption dis-
crepancy with an increase in Ni content. Alloy formation of the
transition metal catalyst Ni with In and Ti prevented further reduc-
tion of Ti4+ but In3+ being reducible was completely reduced. Such
phenomenon of hydrogen spillover has been noticed over nickel
metal used as catalyst in hydrogen storage materials like activated
carbon, where hydrogen spillover is mainly responsible in increas-
ing the storage capacity [24]. Recently, pioneering studies by Yang
et al. revealed novel processes to store substantial quantities of
hydrogen via hydrogen spillover [25–27].
Representative XPS binding energy values of different elements in ITN4 samples
obtained before and after the first TPR cycle in temperature range of 25-1100 ◦C.
BE of XPS signal (eV)
Before TPR
After TPR
Elements
B.E
Oxidation state
In 3d5/2
445.9
+3
Ti 2P3/2
457.5
+4
In 3d5/2
444.1
0
Ti 2P3/2
457.5, 453.7
+4, 0
In3+
No peak due to
In0
Ti4+, Ti0
Ti0 metal
In the reduced sample, in addition to major peak at 457.5 eV a
shoulder is observed at 453.7 eV corresponding to Ti(0) state. The
appearance of this shoulder peak confirms the reduction of Ti4+
to Ti metal during the temperature programmed reduction (TPR)
experiments. In addition, in Fig. 5a, a shoulder appears at higher
binding energy in the XPS spectra of Indium ion for the fresh sam-
ple, but it completely disappears in the spectra recorded after the
complete reduction. The appearance of shoulder is attributed to the
interaction of In3+ with other ions present in the compound. Thus as
process of reduction proceeds, the interaction of Indium with other
ions decreases or it gets segregated out from the sample. However,
presence of In 1+ in the reduced sample is not observed as observed
earlier for partially reduced sample in case of iron substitution [20].
Intensity of In (3d) peak is considerably increased in Fig 5a, after
complete reduction as compared to fresh sample. Thus, ITN4, sam-
ple on reduction, deposits the metallic Indium on the surface and
hence the peak intensity due to Indium on surface increases. It is
well known that the catalytic properties of a sample are sensitive
to its surface composition; therefore this observation may play a
crucial role in deciding its performance for redox reactions. The
XPS results in accordance with TPR and thermogravimetry results
suggests the formation of metallic Ti(0) along with Ti4+ and In (0)
when ITN4 was reduced in H2 up to 1100 ◦C whereas unsubstituted
In2TiO5 did not reveal any reduction of Ti4+ species. Thus presence
of Ni influenced the reduction behavior of Ti4+ species present in
the sample by activating H2 molecules.
5. Conclusions
A-site substitution by a divalent Ni2+ cation at In3+ site of
In2TiO5 resulted in single phase compositions at low extent of
substitution (up to 5%), biphasic at a higher substitution (10%)
with very low impurity phase of unreacted NiO and triphasic
at even higher Ni content (15–20%), having very low concentra-
tions of NiO and NiTiO3 phase. The temperature programmed
reduction, XRD and XPS studies establishes complete reduction
of In3+ and Ni2+ to In0 and Ni0 states in temperature range of
450–1100 ◦C in all the samples. Ni substitution induced consider-
able ease in reducibility (Tmax) of substituted samples as compared
to In2TiO5 phase. The substitution-induced non-stoichiometry and
the microstrucural defects may cause the distortion in the lattice,
thus facilitating the reduction of oxides. The interesting obser-
4. Discussion
The reduction of In2(1−x)Ni2xTiO5−ı oxide samples is represented
by following equation:
vation was the reduction of otherwise non-reducible cation Ti4+
,
◦
in the 20% substituted sample, ITN4. Ni metal initially formed
by the reduction of Ni2+ dissociate and activate the hydrogen
which eased the In3+ reduction and even facilitated the reduc-
tion of Ti4+. Metallic Ni and In reacts to form ıIn3Ni2 alloy
while metallic Ni and Ti reacts to form an alloy Ti2Ni. Due to
the formation of these alloys the rate of hydrogen adsorption
diminishes, which slows down further hydrogen activation and
In2(1−x)3+Ni2x2+Ti4+O5−ı(s) + (3 − ı)H2(g)490−−1→100 2(1 − x)In0(s)
+ 2xNi0(s) + Ti4+O2(s) + (3 − ı)H2O(g)
C
H2 consumption data calculated as well as that observed from
the areas under TPR peak, for all samples for the first reduction
cycle (TPR) is listed in Table 3. The comparison shows that except
for ITN1 for all other samples experimental values are higher than
calculated values. This discrepancy increases with increase in Ni
content as observed from Table 3. It can be explained on the basis
of autocatalytic reduction of the samples in the presence of Ni2+
cations. We have reported earlier [20] that Ti4+ cation was stable in
H2 up to 1000 ◦C in Cr and Fe substituted In2TiO5 samples. As such,
to obtain the TPR profiles of some simple oxides, TiO2 and Cr2O3,
are difficult because the standard approximate free energy change
(ꢂG◦ in KJ/mol) for the process: MO + H2 → metal + H2O are respec-
tively ∼220 and ∼140 [21]. However for In2(1−x)Ni2xTiO5−ı samples,
in addition to the normal nucleation and growth process, during
reduction the presence of Ni2+ in even a minor amount modifies
the reduction. Ni metal initially formed by reduction dissociates
and activates hydrogen. This hydrogen then reacts and reduces
the oxide (even Ti4+). This phenomenon is termed as hydrogen
spillover. In oxide catalysts the presence of metal cations, which
Table 3
H2 consumption data of In2(1−x) NixTiO5−ı samples obtained from the first TPR cycle
in temperature range of 25–1100 ◦C.
Sample
Calculated Experimentalb
Residue as identified by XRD
In0 andTiO2 rutile
compositiona
In2TiO5
In1.95Ni0.05TiO5−ı 8396
In1.9Ni0.1TiO5−ı
In1.8Ni0.2TiO5−ı
In1.7Ni0.3TiO5−ı
In1.6Ni0.4TiO5−ı
8390
8404
9009
8401
8412
8424
8436
8358
9528
In, TiO2 and ıIn3Ni2
10,514
10,153c
In, TiO2, ıIn3Ni2 and Ti2Ni
a
Stoichiometry determines ı as x.
TCD signal for manual injection of standard hydrogen has been used for calibra-
b
tion to find out the H2 consumption.
c
The difference observed in experimental values (higher) of H2 consumed as com-
pared to calculated values is attributed to reduction of Ti4+ metal ions to Ti metal
driven by presence of Ni2+ ions.

A.M. Banerjee et al. / Thermochimica Acta 516 (2011) 40–45
45
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