PVP stabilized Pt nano particles catalyzed de-oxygenation of phenoxazine group by hydrazine in physiological buffer media: Surfactant competes with reactants for the same surface sites

Article abstract of DOI:10.1039/c2dt32007j

PVP capped platinum nano particles (PNP) of 5 nm diameter were prepared and characterized as homogeneous and of spherical nature. At physiological pH range (6.0-8.0), these PNP catalyze the deoxygenation of phenoxazine group containing resazurin (1) by hydrazine. The observed rate constants (k_o), increase linearly with [PNP] at constant [1] and [Hydrazine]; but first increase and then after reaching a maximum it decrease with increase in [1] as well as in [Hydrazine]. The k_o values increase linearly with 1/[H ⁺] indicating N₂H₄ as the reducing species that generates from the PNP assisted deprotonation of N₂H ₅⁺. The kinetic observations suggest Langmuir-Hinshelwood type surface reaction mechanism where both 1 and hydrazine are adsorbed on nano particles surface and compete for the same sites. Interestingly, the surfactant molecules, polyvinylpyrrolidone (PVP), though do not take part into reduction reaction but having same type of functional groups as reactants, competes with them for the same surface sites. Adsorption on PNP with same type of functional group is further supported by the FTIR spectra of Pt-PVP and Pt-1. Thus on increasing [PVP], k_o decreases linearly and only when [PVP] is held constant, the plot of k_ovs. [PNP] passes through the origin indicating the insignificance of uncatalyzed reaction. The plot of ln k _ovs. [1] or [Hydrazine] shows two different linear zones with different exponent values with respect to [1] and [Hydrazine]. This indicates that along with the complex heterogeneous surface adsorption processes, the mutual interactions between the reactants are also changing with the relative concentrations of reactants or, in general, with the molar ratio ([Hydrazine]/[1]).

Full text of DOI:10.1039/c2dt32007j

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PVP stabilized Pt nano particles catalyzed
de-oxygenation of phenoxazine group by hydrazine in
physiological buffer media: surfactant competes with
reactants for the same surface sites†
Cite this: Dalton Trans., 2013, 42, 4068
Ranendu Sekhar Das,*a Bula Singh,b Rupendranath Banerjeea and
Subrata Mukhopadhyaya
PVP capped platinum nano particles (PNP) of 5 nm diameter were prepared and characterized as homo-
geneous and of spherical nature. At physiological pH range (6.0–8.0), these PNP catalyze the deoxygena-
tion of phenoxazine group containing resazurin (1) by hydrazine. The observed rate constants (ko),
increase linearly with [PNP] at constant [1] and [Hydrazine]; but first increase and then after reaching a
maximum it decrease with increase in [1] as well as in [Hydrazine]. The ko values increase linearly with
1/[H+] indicating N2H4 as the reducing species that generates from the PNP assisted deprotonation of
N2H5+. The kinetic observations suggest Langmuir–Hinshelwood type surface reaction mechanism where
both 1 and hydrazine are adsorbed on nano particles surface and compete for the same sites. Interest-
ingly, the surfactant molecules, polyvinylpyrrolidone (PVP), though do not take part into reduction reac-
tion but having same type of functional groups as reactants, competes with them for the same surface
sites. Adsorption on PNP with same type of functional group is further supported by the FTIR spectra of
Pt-PVP and Pt-1. Thus on increasing [PVP], ko decreases linearly and only when [PVP] is held constant, the
plot of ko vs. [PNP] passes through the origin indicating the insignificance of uncatalyzed reaction. The
plot of ln ko vs. [1] or [Hydrazine] shows two different linear zones with different exponent values with
respect to [1] and [Hydrazine]. This indicates that along with the complex heterogeneous surface adsorp-
tion processes, the mutual interactions between the reactants are also changing with the relative concen-
trations of reactants or, in general, with the molar ratio ([Hydrazine]/[1]).
Received 31st August 2012,
Accepted 7th December 2012
DOI: 10.1039/c2dt32007j
www.rsc.org/dalton
role in the hydrogen oxidation reaction (HOR) and oxygen
reduction reaction (ORR) in fuel cells and chemical
Introduction
During the last few decades, research interests in the field of reactions.16–20 Since platinum itself remains an inert metal in
nano particles have swung enormously resulting in an upsurge biological systems, platinum based nano particles are promis-
in our knowledge bank about the fundamentals at sub-molecu- ing tools in this field. Recent studies have shown such potenti-
lar level physics and chemistry. Numerous efforts have been ality in treatment of cancerous cell21,22 and in destruction of
put to prepare different nano particles of various mor- harmful radicals.23
phologies with newer techniques1–8 to discover their physical
Motivated by this, we have synthesized homogenous PVP
and potential chemical properties.9–15 A considerable attention stabilized platinum nano particles (PNP) by a fast and easier
have also been cast towards nano particles prepared from the but efficient single pot method and studied the catalyzed de-
novel metals, like platinum, due to its high stability and cata- oxygenation reaction of phenoxazine group.
lytic efficiencies. Platinum based nano particles play a crucial
Phenoxazine group, present in resazurin (7-hydroxy-3H-
phenoxazin-3-one-10-oxide) (1) contains a heterocyclic N-oxide
group that on reduction loses its oxygen and produces fluo-
rescent resorufin (7-hydroxy-3H-phenoxazin-3-one) (2). Resa-
zurin, also known as Alamar Blue,24,25 had been used over last
half of the century to determine bacterial or yeast contami-
nation in biological fluids and in milk.26,27 Moreover, since
resazurin is nontoxic to cells it has been in profuse use for the
aDepartment of Chemistry, Jadavpur University, Kolkata 700 032, India.
E-mail: ranendu147@gmail.com
bDepartment of Chemistry, Visva-Bharati, Santiniketan 731 235, India
†Electronic supplementary information (ESI) available: Fig. S1–S7, Tables S1–S7
and explanation of effects of surface restructuring on k0 are provided. See DOI:
10.1039/c2dt32007j
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assessment of cell proliferation and cytotoxicity.28–30 In meta- electrical double layer at the particle interfaces.47 After the
bolically active cells, resazurin is reduced to resorufin and complete reduction and thus formation of nano particles,
dihydroresorufin by mitochondrial enzymes activity by accept- aqueous solution of PVP was added to Pt nano particles solu-
ing electrons from NADPH, FADH, FMNH, NADH as well as tion to cap them for enhanced stability. PVP, a colorless, non-
from numerous cytochromes.31–33 A great deal of photochemi- ionic, non-toxic and water soluble, polymeric compound of
cal studies have also been carried out on resazurin because the high molecular weight has been used abundantly to stabilize
deoxygenation of N-oxide has a close resemblance to the different kinds of metal nano particles.48–51
enzyme mediated oxygen atom transfer involving monooxygen-
The general procedure for the preparation of platinum
ases.34,35 Due to these importance, in recent times nano par- nano particles (PNP) was as following: 50 mL of an aqueous
ticles catalyzed reduction of resazurin have also been studied K2PtCl4 solution (2.5 × 10−4 M) was heated to boiling with vig-
by laser and single molecule fluorescence microscopy.36–38
orous stirring. Then 2 mL of 1% (w/v) aqueous tri sodium
Following the importance of nano particles in biological citrate solution was quickly injected in this solution to reduce
field,21–23 the detailed kinetics of PNP catalyzed reduction of Pt(II) to Pt nano particles. The solution was allowed to boil for
resazurin by hydrazine was studied in physiological pH con- one hour with vigorous stirring when the transparent light
dition. The study shows that the reaction takes place on the brownish-yellow colored solution turned to brown and finally
very nano particle surface and molecules with similar func- to opaque black indicating the formation of Pt nano particles.
tional groups compete for the same reaction sites.
At this stage, 5 mL of 1% (w/v) aqueous PVP solution was
added to this Pt nano particles solution and the boiling was
continued for another 30 minutes with stirring. Finally, the
opaque black PNP solution was cooled to room temperature
and was stored in a cold, dry and dark place. This PNP solu-
tion, which was treated as the stock is stable for months and
was always used after sonication.
Experimental section
Materials
Potassium tetrachloroplatinate(II) (Aldrich), sodium citrate
dihydrate (Rankem, India), polyvinylpyrrolidone (k-30, avg.
MW = 40 000, Fluka), resazurin (Loba Chemie, India), hydra-
zine hydrate (Loba Chemicals), sodium nitrate (Merck), potass-
ium dihydrogen phosphate (SRL, India) and sodium hydroxide
(SRL) were used as received. Milli-Q water and double distilled
water was used for the preparation of PNP and kinetic experi-
ments respectively. All other chemicals used were of analytical
grade.
Instrumentation
Absorbance and UV-VIS spectra were recorded using a Shi-
madzu spectrophotometer (UV-1700) with 1.00 cm quartz cuv-
ettes. Transmission electron microscopic (TEM) analyses were
performed in a Hitachi (H-9000 NAR) instrument on samples
prepared by placing a drop of PNP solution on Cu grid, pre-
coated with carbon films, followed by solvent evaporation
under vacuum. PNP solution was centrifuged in Heraeus,
Biofuge primo R machine and Fourier Transform Infrared
Preparation PVP stabilized platinum nano particles (PNP)
Preparation of stable polymer capped platinum nano particles Spectroscopy (FTIR) data were collected using a Shimadzu
was proposed long back in 1940s by Rampino and Nord.39 FT-IR 8400S. PNP solution was regularly sonicated in Rivotek
Since then platinum nano particles have been prepared by ultrasonic bath (50 Hz) before any experimentation.
wide and different varieties of techniques both in aqueous and
The buffer solutions were prepared following literature
in organic medium which is very well documented.40 Different method52 and were measured with a Toshniwal pH-meter
types of surfactants were also used to stabilized the nano (CL-54, India), calibrated as usual using standard buffers.
particle.41–45 Nevertheless, a necessity of simple and quick
Characterization of PNP
preparation method of stable platinum nano particles from
corresponding Pt(II) salt in pure aqueous medium is undeni- The formation of PNP was characterized by TEM imaging and
able from the viewpoint of effectiveness, intricate setups and UV-VIS spectroscopy. The TEM images of the PNP are shown at
costly materials.
two different magnifications (Fig. 1). The size and the shape of
Herein, we developed a rapid, one-pot, two step, efficient the nano particles ware determined from enlarged images and
synthetic procedure for polyvinylpyrrolidone (PVP) capped it can be seen that the sample contains solid nano particles of
platinum nano particles (PNP). Platinum nano particles were spherical nature. The % size distribution plot (Fig. 2) shows
prepared by the reduction of aqueous Pt(II) ions with tri that the extent of heterogeneity in nano particles sizes lie in a
sodium citrate. Citrate ions act as both the reducing agent and very narrow range. Whereas the nano particles of size of 5 nm
the primary stabilizing agent. Though it is generally believed diameter alone consists about 50% of the total population, the
that a metal salt like Pt(II) is reduced first to zero-valent metal other nano particles with diameters of 3, 4 and 6 nm contrib-
atom as Pt(0) at its embryonic stage and then forms metal utes for the rest of the part.
clusters after reaching a critical concentration of Pt(0), an
Fig. 3 presents the UV-VIS spectrum of aqueous solution of
alternative mechanism with initial formation of Pt(I)–Pt(II) Pt(II) complex and stock PNP solution. Spectrum of the
bond has also been proposed.46 However, citrate ions primarily aqueous solution of the precursor Pt(II) complex shows three
stabilize the nano particles electrostatically by forming distinct absorbance peaks at 322, 384 and 472 nm which
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Fig. 2 The % distribution of different sizes of PNP shows a predominance of
5 nm nano particles.
Fig. 3 Characteristic UV-VIS spectrum of K2PtCl4 complex (brown) and PNP,
after formation (grey).
Fig. 1 (a, b) HR-TEM image of PVP stabilized platinum nano particles (PNP).
platinum surface through the nitrogen of heterocyclic ring
apart from the carbonyl group is also feasible. But, whatever
be the mode of coordination, it is evident that PVP is present
and is capping the nano particles surface through charge
transfer interaction.
Here it is worthwhile to mention that though the role of
PVP is to protect the nano particles surface, the interaction of
PVP with platinum surface is not very strong61,62 and it does
not passivate the surface like other capping agents (for
example, –SH).63
become absent in the UV-VIS absorption spectrum of stock
PNP solution. The absence of peaks confirm the formation of
the platinum nano particles.53–56
The presence of PVP molecules on platinum surface was
confirmed by FTIR spectral analysis. The FTIR spectrum of
PNP (Fig. 4) shows an intense absorption band at 1643 cm−1
which is attributed to the presence of CvO bond in pyrroli-
done ring of PVP. Other peaks at ∼3400 (s, w), 1400 (m) and
1288 (m) and 1099 (m) are assigned to C–H and O–H stretch-
ing (from residual water molecules), C–H bending, N–OH and
C–N stretching respectively.57–59 PVP molecules have both a
Deoxygenation reaction of 1
carbonyl group and a heterocyclic nitrogen atom and it can The UV-visible absorption spectrum of resazurin in aqueous
coordinate to nano particles in either or combined way.58,60 phosphate buffer solutions shows a characteristic strong
Since the platinum nano particles are of very small sizes absorption band at 602 nm and a weak one at 380 nm which
(∼5 nm) and the electronegativity of nitrogen atom is less than arises due to the π–π* transition of the phenoxazin-3-one and
the oxygen atom, a weak coordination of PVP molecules to the weak n–π* transitions of the N-oxide, respectively. When
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Fig. 4 FTIR spectrum of PVP capped Pt nano particles (PNP).
PNP (other conditions being the same as in general kinetic
experiments) is very slow (Fig. S3†). Also, when measured
amount of PNP or aqueous PVP solution ([PVP] ≤ 0.0018%)
alone was added to the buffered solution of 1, the absorbance
at 602 nm does not change at least for two hours. It ensures
that platinum nano particles alone (∼10−10 M, vide infra) or
the stabilizing agent PVP molecules present in the final reac-
tion mixture cannot reduce 1 under similar kinetic condition.
Moreover, it has also been ensured that the addition of mere
PVP solution ([PVP] ≤ 0.0018%) cannot catalyze the reduction
of 1 by hydrazine. During the preparation of final reaction
mixture, measured amount of PNP solution from the stock
solution and sodium nitrate solution (to maintain ionic
strength, μ) were added to the buffered solution of 1 followed
by the final quick addition of hydrazine solution. The final
volume of the reaction mixture was 3.0 mL. A typical UV-VIS
spectral change of 1 with time for the catalyzed reaction is
shown in Fig. 5. Kinetics was measured in phosphate buffer
(pH 6.0–8.0) in situ in an electrically controlled thermostat.
Kinetic experiments for reduction of 1 were performed in
the presence of a large excess of hydrazine over [1] to ensure
pseudo first-order conditions. The rate constants (ko) were
determined from the plots of decrease in absorbance with
time at 602 nm, the visible absorption peak of 1, where no
other species in the reaction mixture absorbs. Non-linear, stan-
dard least-squares fit to a first-order equation of such decay
with time, yielded the observed first-order rate constants, ko
(Fig. 6). However, it can be noted that there is an induction
Scheme 1
resazurin is catalytically reduced by hydrazine, resorufin is pro-
duced which shows its characteristic visible absorption at
572 nm with a shoulder at 535 nm (Fig. S1, ESI†).64 Here it is
worth mentioning that resorufin, which was produced on
deoxygenation of 1 is also catalytically reduced to colorless pro-
tonated resorufin anion (3) at pH 7.0.65 Thus there are two
catalyzed reaction (R1 and R2) and visibly the whole reduction
process is accompanied with a distinct color change from blue
to pink to colorless (Scheme 1). The reduction of resorufin can
be followed spectrophotometrically at 572 nm (Fig. S2†). But
here we have only contended our study to the detailed kinetics
of catalyzed reduction of 1.
Kinetics
The reduction of resazurin (1) by hydrazine is strongly cata- period (t0) in catalyzed reaction and which was not considered
lyzed by PVP stabilized platinum nano particles (PNP). The in the determination of ko values. Presence of arial oxygen or
uncatalyzed reaction between 1 and [Hydrazine] ([Hydrazine] is the impurities in the reaction medium cannot explain the
the analytical concentration of added hydrazine) in absence of induction period because on passing argon or on addition of
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Fig. 7 Variation of ko with the increase of [1] at fixed [Hydrazine] and [PNP].
Condition: [Hydrazine] = 5.0 mM (A), 10.0 mM (B); [PNP] = 5.46 × 10−11 M;
μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
Fig. 5 Spectral change for PVP capped platinum nano particles (PNP) catalyzed
reduction of 1 by hydrazine at 60 s time interval. Condition: [1] = 0.025 mM;
[Hydrazine] = 5.0 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0
0.1 °C.
from the reactions catalyzed by nano particles of different
sizes (reactions performed under similar conditions) are sup-
posed to be also too small to be differentiated. Hence, we can
assume that the rate constant of the catalyzed reaction is vir-
tually unaffected by the insignificant amount of heterogeneity
in nano particles sizes.
However, to minimize the impacts of the unavoidable
errors, we have tried to put more emphasis on the trends of
different variations to draw conclusions rather than the
numerical value of any particular data.
Results and discussion
Concentration of PNP
The approximate concentration of PNP was calculated using
eqn (1)67
½PNPꢀ ¼ ½Pt2þ ionsꢀ=ðVPNP=VPt atom
Þ
ð1Þ
Fig. 6 PNP catalyzed reduction of 1 by hydrazine at 602 nm. Condition: [1] =
0.025 mM; [Hydrazine] = 5.0 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0;
T = 25.0 0.1 °C.
where, V refers to the corresponding volumes. Calculations
with effective ionic radii for Pt(II) as 0.080 pm68 and sizes of
PNP assigned from TEM imaging as ∼5 nm gives the concen-
tration of stock PNP solution to be approximately 8.2 × 10−9
M. Similar formulation, based on ion and particle diameters
also gives about the same order of value.69 Different amounts
of PNP solution from stock were used to catalyze the reaction
and in reaction media the concentration of PNP was in the
order of 10−11 M.
chelating agent, like dipicolinic acid, the induction period
remains as usual.66 This induction period can be attributed to
the nano particles surface restructuring and will be discussed
again later.
Here, it is worth mentioning that in our previous work66 we
have shown that all the nano particles with different sizes fol-
lowed the same reaction mechanism and gives a first-order
reaction. The effect of size of nano particles is manifested in
Variation of ko with [1] and [Hydrazine]
the rate of the reactions which usually decrease with the When concentration of 1 is increased at fixed [Hydrazine] and
increase in the nano particles size. With larger size distri- [PNP] (Table S1†), the observed first-order rate constant, ko,
bution, the difference in rate constants will also be large. In first increases with the increase in [1]; then reaching a maxima
this study, as we have mentioned earlier that the differences it decreases with further increase in [1] (Fig. 7).
between the size of nano particles are limited within a very
Similar type of variation of ko is observed when [Hydrazine]
narrow range, the difference in rate constant values derived at fixed [1] and [PNP] is increased. The observed rate
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and flexible species require much less space and thus only at
high [Hydrazine] ko values commence to decrease.
Here it will be very interesting to consider that how the
reactant molecules occupy the sites on platinum surface. In
case of hydrazine it is well established that being a symmetri-
cal molecule it only uses lone pair of electron of nitrogen
atoms to be adsorbed on platinum surface.74,75 However, to
the best of our effort, we could not find any report regarding
any possible mode of attachment between Pt and resazurin in
the literature. We studied the FTIR of pure resazurin and PNP–
resazurin mixture to understand the mode of coordination
through which it is adsorbed on nano particles surface (Fig. S4
and S5† respectively). It can be seen that peaks both at
1354 cm−1, which can be assigned to N+–O− bond76 and peaks
at 1606 cm−1, attributed to carbonyl group, have red shifted.
Since peaks ∼1350 cm−1 are more diminished and more red
shifted, it can be assumed that the probability that resazurin
uses N+–O− end to sit on platinum surface is more than the
CvO end.
Fig. 8 Dependency of ko on [Hydrazine] at fixed [1] and [PNP]. Condition: [1] =
0.025 mM (A), 0.04 mM (B); [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T =
25.0 0.1 °C.
The different nature of interaction of 1 and hydrazine with
the platinum surface also results in different adsorption
strengths. Between 1 and hydrazine, literature report shows
that the adsorption coefficient of 1 is much more stronger
than hydrazine.38 This preferential adsorption of 1 over hydra-
zine is probably another reason that why even at small increase
in [1] compared to that in [Hydrazine], the surface tends to
saturate and the reaction rate starts to fall.
constants, ko first increase and then it decrease with the
increase in [Hydrazine] (Table S2,† Fig. 8).
These kinetic observations are in firm support of surface
catalytic reaction. In general, heterogeneous surface reaction
can be explained either by Eley–Rideal or by Langmuir–Hin-
shelwood mechanism. In Eley–Rideal mechanism, one of the
reactants, present in gas or solution phase interacts with the
surface adsorbed reactant whereas in Langmuir–Hinshelwood
mechanism both reactants are adsorbed on the catalyst
surface. If the reduction of 1 proceeds through Eley–Rideal
mechanism, then with increase of [1] or [Hydrazine] values of
ko should have reached saturation. But since the rate constant
values first increases and then decreases, it is more plausible
that the catalytic reduction follows Langmuir–Hinshelwood
mechanism where both resazurin and the reductant hydrazine
molecules compete to sit on the surface sites.38,70–73 Thus, at
fixed [PNP], when [1] or [Hydrazine] is increased, the reaction
sites on PNP are more occupied by the corresponding
molecules. This results in increased reaction rate up to an
optimum value and then due to the paucity of other reactants
([Hydrazine] or [1], respectively) the observed rate constants
decrease. Both the observations are in accord with the recent
observations made by Chen et al.38
Effect of [PNP] and [PVP] on ko
The observed rate constants, ko, for catalyzed reaction not only
depend upon the concentration of PNP but it is also affected
by the concentration of capping agent, PVP in the media. At
fixed [1] and [Hydrazine], with increasing [PNP] ([PVP] vary
approximately as 0.0006–0.0018% only, Table S3†) the
observed rate constants increase linearly. This trend is consist-
ent with our previous findings and reveals only the typical be-
havior of the catalyst.66 But, whereas, at zero concentration of
catalyst, it was also expected that the rate should have been
zero as the uncatalyzed reaction between 1 and hydrazine is
very slow, the plot yields an intercept value about 1 × 10−3 s−1
(Fig. 9). This large intercept value alone cannot be explained
by the much lower uncatalyzed reaction rate constant value
(2.4 × 10−4 s−1).
Here the effect of variation of [1] and hydrazine is note-
worthy. While for [1], after the optimum concentration
∼0.015 mM the decrease of ko starts, for hydrazine the
decrease takes place at high concentration as ∼20 mM. Appar-
ently, it seems that at comparatively lower concentration of 1
the surface of platinum nano particles appears to be covered
to such extent that the reductant molecule hydrazine lacks the
sitting place. This effect probably arises from the bulky nature
of the molecule 1. The aromatic ring moiety of 1 has a planner
structure and it has been proposed to be oriented in parallel
way to the surface during adsorption.71 Due to this steric hin-
drance, even at low [1], it covers up the other adsorption sites.
Hydrazine molecule, on the contrary being a much smaller
Interestingly, at fixed [PNP] and other kinetic conditions,
when total [PVP] is increased by additional PVP solution the
observed rate regularly falls (Fig. 10, Table S4†). Thus increas-
ing [PNP] and [PVP] have the mutually opposite effect on the
observed rate and this complexity results in the aforesaid large
intercept value. Thus, true dependence of the catalyzed rate
constants on [PNP] can only be observed when the [PVP] is
kept constant. So, when [PVP] is kept fixed at 0.01% through
calculated addition of extra PVP solution, the observed rate
constants, ko not only linearly increases with the increase with
[PNP] but also passes through origin (Fig. 11, Table S3†).
Thus the observed rate constants depend both on the rela-
tive concentrations of PNP and PVP. PVP molecules themselves
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Fig. 11 Plot of ko vs. [PNP] at fixed [PVP] is straight line passing through origin.
Condition: [1] = 0.025 mM; [Hydrazine] = 5.0 mM; μ = 0.25 M; [PVP] = 0.01%;
pH = 7.0; T = 25.0 0.1 °C.
Fig. 9 Variation of ko with [PNP] is a straight line but with intercept. Condition:
[1] = 0.025 mM; [Hydrazine] = 5.0 mM; μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
Fig. 12 Variation of ko with [H+]. Condition: [1] = 0.025 mM; [Hydrazine] =
5.0 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; T = 25.0 0.1 °C.
Fig. 10 Variation of ko with [PVP]. Condition: [1] = 0.025 mM; [Hydrazine] =
5.0 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
Effect of [H+] on ko
do not take part in reduction reaction or their presences in
large amount do not disturb the diffusion behavior of reac-
tants in solution. The later can be confirmed from the fact that
uncatalyzed reaction rates are not affected by the absence or
presence of PVP in large concentration (0.02%). These obser-
vations lead us to the possibility that PVP molecules not only
cap the platinum surface but also occupy the same sitting sites
as resazurin and hydrazine does. It is more plausible because
PVP molecules possess similar coordinating groups like both
hydrazine and resazurin. Again it is also supported by the
earlier mentioned fact, that there is evidence58,60 for PVP for
using both types of coordination mode. Since PVP has no role
in the main reduction reaction of 1, the concentration of PVP
does not appear in the reaction mechanism and the extra occu-
pation of reaction sites is only manifested in the decrease in
reaction rates.
PNP catalyzed reaction rates increases fairly linearly with
1/[H+] in the pH range 6.0–8.0 (Table S5,† Fig. 12) which indi-
cates that the reactive reducing species is hydrazine, not the
hydrazinium (N2H5 ) ion. Since the pKa value for N2H5+ is 7.95,
+
at lower pH most of the hydrazine will remain protonated and
free hydrazine (N2H4) will only be available at higher pH.
Thus, the production of free N2H4 molecules at lower pH will
require an assisted deprotonation of N2H5+ molecules. Assisted
deprotonation may result from the adsorption of hydrazinium
ion on PNP surface.66
Effect of ionic strength (μ) on ko
The observed rate constants of catalyzed reactions increases
linearly with the ionic strength of the media (Table 1). Ionic
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Since the kinetic observations support the Langmuir–Hin-
shelwood surface mechanism, the rate of the reaction can
simply be expressed as follows70
Table 1 Representative ko values at different ionic strength (μ)
μ (M)
0.16
1.28
0.19
1.35
0.22
1.44
0.25
1.50
0.28
1.62
0.31
1.70
103ko (s−1
)
Rate ¼ ꢁd½1ꢀ=dt ¼ ko½1ꢀ
α
β
α
¼ ðkKR½1ꢀ KH½Hydrazineꢀ Þ=ð1 þ KR½1ꢀ
þ KH½Hydrazineꢀ Þ
β
2
ð2Þ
where, k is the true rate constant and KR and KH are the
adsorption coefficients of 1 and hydrazine respectively. The
exponents α and β are the corresponding reaction order with
respect to [1] and [Hydrazine]86 which can also be correlated
with the heterogeneity of the nano particles surface, that is the
adsorption energy of the adsorption sites is not
homogeneous.87
The above equation can be rearranged as
Scheme 2
α
1=2
ð½1ꢀ =ko½1ꢀÞ ¼ ð1
þ KH½Hydrazineꢀ Þ=ðkKRKH½Hydrazineꢀ Þ
strength of the reaction medium was maintained by the calcu-
lated addition of NaNO3 solution. The positive slope indicates
that the reacting species are of same charge.
β
β 1=2
α
β 1=2
þ ½1ꢀ ðKR=kKH½Hydrazineꢀ Þ
ð3Þ
Proposed mechanism for R1
β
1=2
α
α 1=2
Or; ð½Hydrazineꢀ =ko½1ꢀÞ ¼ ð1 þ KR½1ꢀ Þ=ðkKHKR ½1ꢀ Þ
On the basis of the kinetic observations the catalyzed reaction
steps can most simply be presented by Scheme 2. The possible
products that are formed from 1 and hydrazine are well
reported in literature. On deoxygenation of 1, along with 2,
mainly water is produced32,77 and on oxidation of hydrazine,
mainly N2 is formed along with little H2.74,78–81 Though recent
studies have shown that metal-catalyzed hydrogen production
from the decomposition of hydrazine takes place only under
certain reaction conditions,82 the possibility of formation of
small amount of hydrogen, however, cannot be neglected.
Thus separate experiments were conducted where pure hydro-
gen gas was passed through the reaction mixture under
similar reaction condition. It has been found that no reaction
takes place at least for 2 hours. Thus we can firmly eliminate
the possibility of reduction of 1 by hydrogen, if any generated.
In Scheme 2, the aromatic side rings of 1 and 2 have been
β
α 1=2
þ ½Hydrazineꢀ ðKH=kKR½1ꢀ Þ
ð4Þ
If, both α = β = 1, then both the equation reduces to classi-
cal Langmuir–Hinshelwood equation,
1=2
ð½1ꢀ=ko½1ꢀÞ ¼ ð1
1=2
þ KH½HydrazineꢀÞ=ðkKRKH½HydrazineꢀÞ
þ ½1ꢀðKR=kKH½HydrazineꢀÞ
1=2
ð5Þ
ð6Þ
1=2
1=2
Or; ð½Hydrazineꢀ=ko½1ꢀÞ ¼ ð1 þ KR½1ꢀÞ=ðkKHKR ½1ꢀÞ
1=2
þ ½HydrazineꢀðKH=kKR½1ꢀÞ
So, the plot of ([1]/ko[1])1/2 vs. [1] or ([Hydrazine]/ko[1])1/2 vs.
omitted for simplicity. The scheme is only representative and [Hydrazine] should have been be a continuous straight line.88
protons may not come from a particular hydrazine but from But we have found that none of the plot is a single straight
other neighboring hydrazine molecules also.
line (Fig. 13 and 14).
During the adsorption on the platinum surface (step 1),
It can be seen that both in Fig. 13 and 14 that though some
resazurin (1) is adsorbed through the N+–O− bond (from FT-IR experimental points consists a linear zone but it deviates from
data, as previously mentioned) and hydrazine molecules are linearity over the wide concentration range and more impor-
adsorbed through the nitrogen atoms. Since during the self tantly there are unexpected breaks in the graph. The appear-
decomposition of hydrazine on metal films, it has been found ance of the breaks cannot be related to any inhomogeneity in
that at initial stage the hydrogen atoms are ruptured, we can nano particles size because for any first-order reaction, the
assume that this hydrogen can be transferred to 1 in step plots should have been a straight line. These breaks in the
2.83–85 With this hydrogen, 1 is reduced and 2 is produced plots tell that the interaction between the adsorbate molecules
(step 3). The adsorbed OH species then receives another and the adsorption sites are not very homogenous. Thus, it is
proton from the hydrazine residue (step 3) and departs as reasonable that the exponent values are not constant over the
water molecule. The remaining hydrazine residue reacts total reactant concentrations or in general, at different molar
further with another molecule of 1 or itself decomposes to ratio ([Hydrazine]/[1]). If we assume that the adsorption of [1]
produce N2, NH3 and little H2 (step 4).
and [Hydrazine] is not so strong, then the eqn (2) can be
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Table 2 Change in α and β values with molar ratio
Exponent value
Molar ratioa
Exponent value
Molar ratioa
αb
αc
βd
βe
1.76 0.0
1.37 0.0
1.54 0.4
0.53 0.25
500–333
2000–1000
200–600
2.55 0.14
2.43 0.21
0.62 0.09
0.33 0.09
333–125
666–250
800–1400
500–1125
125–375
a Molar ratio = [Hydrazine]/[1]. b [Hydrazine] = 5.0 mM; [PNP] = 5.46 ×
10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0
0.1 °C. c [Hydrazine] =
10.0 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0
0.1 °C. d [1] = 0.025 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0;
T = 25.0 0.1 °C. e [1] = 0.04 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M;
pH = 7.0; T = 25.0 0.1 °C.
Fig. 13 Plot of ([1]/ko[1])1/2 vs. [1]. Condition: [Hydrazine] = 10.0 mM; [PNP] =
5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
Fig. 15 Plot of ln ko vs. ln[Hydrazine] at different temperature. Condition: [1] =
0.025 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0.
It is conceivable since with the change in molar ratio the
extent of surface occupation by the reactants also changes.
Thus the interaction between a reactant molecule with its
adsorption site or even with the neighboring reactants also
changes and that should profoundly affect the observed cata-
lytic rate value. This gives the validation of the overall non-
linearity of Fig. 13 and 14. The calculated changes in exponent
values are over a wide molar ratio range and thus they are only
representative data to support the findings. In reality the
change may be more continuous with each smaller change in
molar ratio values. It has also been found that the exponent
values are more or less temperature independent (Table S6†)
which can be seen from the plot of ln ko vs. ln[Hydrazine] at
different temperatures (Fig. 15). Since it will be very much erro-
neous to assume certain α or β value, further attempt to evalu-
ate KR or KH was not put forward.
Fig. 14 Plot of ([Hydrazine]/ko[1])1/2 vs. [Hydrazine]. Condition: [1] = 0.04 mM;
[PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
simplified to
α
β
Rate ¼ ko½1ꢀ ¼ kKR½1ꢀ KH½Hydrazineꢀ or;
ð7Þ
ko ¼ kKR½1ꢀαꢁ1KH½Hydrazineꢀ
β
If we plot ln ko vs. ln[1] or ln ko vs. ln[Hydrazine] (Fig. S6
and S7† respectively) keeping [Hydrazine] or [1] constant
respectively, clearly in each case, we get two distinct array of
points which can be fitted to straight lines. From the slope
values of different linear zones the corresponding exponent
values with respect to reactants can be obtained (Table 2).85
This type of adsorption profile with multiple linear section
may result from irregular energy distribution associated with
adsorption processes.89
Role of induction period, t0
It can be seen from Table 2 that both the exponents with The induction period, t0, had been suggested as a measure of
respect to 1 (α) and hydrazine (β) change over different molar surface restructuring of the PNP and 1/t0 can be treated as the
ratios. Also, the large exponent values are noteworthy, which rate of restructuring process.70,73 Surface restructuring is an
actually indicates the favorable surface adsorption processes.90 inevitable phenomena occurring at the surfaces due to the
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Fig. 16 Plot of 1/t0 vs. [Hydrazine]. Condition: [1] = 0.025 mM; [PNP] = 5.46 ×
10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
Fig. 18 Variation of 1/t0 vs. [PNP] (A) in absence and (B) in presence of excess
PVP (for Condition, see Fig. 9 and Fig. 11 respectively).
Fig. 19 Plot of ln(1/t0) vs. 103/T gives. Condition: [1] = 0.025 mM; [Hydrazine]
= 5.0 mM; [PNP] = 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0.
Fig. 17 Plot of (1/t0 − 1/t0,sp) vs. [Hydrazine]. Condition: [1] = 0.025 mM; [PNP]
= 5.46 × 10−11 M; μ = 0.25 M; pH = 7.0; T = 25.0 0.1 °C.
relative instability of surface atoms in comparison to the bulk vs. [PNP] (Fig. 18). As expected in absence of PNP, 1/t0 is also
of the materials.91 The restructuring process consists of both zero.
spontaneous as well as adsorbate induced which are well docu-
From the above discussions it is also noteworthy that the
mented in literatures.91–93 For nano particles formed by 5d rate of spontaneous surface restructuring is really small and
elements, like platinum, where the relativistic effects favors this probably results from the relative stability of platinum
the incident,92 restructuring is more important and along with surface than the others, like, gold surface. The findings are in
the spontaneous restructuring process, adsorbate induced accordance with literature reports.38 Moreover, since both of ko
restructuring also demands an attention. The initial induction and 1/t0 varies linearly with [Hydrazine], even if we subtract
period, t0, during the catalyzed de-oxygenation reaction of 1 the contribution of restructuring (however small it may be)
includes both of the spontaneous and reactants induced from the rate constants, the nature of different plots will
restructuring effect.70,73 Fig. 16 shows the variation of 1/t0 remain same (see ESI† for a detail explanation). Thus the con-
against [Hydrazine] and the intercept of the plot gives a clusions derived from the variations from different trends still
measure of spontaneous restructuring (t0,sp).70,73 Thus we can holds in spite of the presence of surface restructuring effects.
have the only effect of adsorbate induced restructuring by cal-
The energy of surface restructuring can be calculated from
culating the (1/t0 − 1/t0,sp), which not only increase linearly 1/t0 following the simple Arrhenius law. Thus from the slope
with [Hydrazine] but also passes through the origin (Fig. 17). of the plot of ln(1/t0) vs. 103/T (Fig. 19), the surface restructur-
The confirmation whether the induction period is really a ing energy can be obtained as approximately 58 4 kJ M−1
.
result of surface restructuring also comes from the plot of 1/t0 This is in close similarity with literature reports.71,72
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The activation energy for the PNP catalyzed reduction of 1 by
hydrazine can be calculated from simple Arrhenius equation, ln
ko = ln A − Ea/RT where ko and A are the observed rate constant
and pre-exponential constant respectively. The plot of ln ko vs.
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mate activation energy, Ea is 77 9 kJ M−1. The surface restructur-
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Conclusions
We have prepared homogenous spherical 5 nm sized PVP
capped platinum nano particles (PNP) and studied the kinetics
of catalyzed reduction and hence deoxygenation of 1 by hydra-
zine. The reaction takes place on nano particle surface and
follows Langmuir–Hinshelwood mechanism where both reac-
tants 1 and hydrazine competes for the same reaction sites.
Since, PVP molecules, present as capping agent, have the same
type of functional groups like 1 and hydrazine, compete for
the same surface sites as the reactants though do not take part
in reduction process. The reaction rate depends on the mutual
interaction between the reactants as well as between reactants
and adsorption sites but this interaction continuously changes
as the ratio of concentration of hydrazine and 1 is varied.
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Acknowledgements
The award of a Senior Research Fellowship (CSIR, New Delhi)
to R. S. Das is gratefully acknowledged.
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