Photo enhanced detoxification of chromium (VI) by formic acid using 3D palladium nanocatalyst

Article abstract of DOI:10.1016/j.jphotochem.2017.01.034

The detoxification of Cr(VI) to Cr(III) at room temperature by formic acid is investigated in the presence of 3D urchin shaped monometallic palladium (Pd_urc) nanocatalyst. The rate of detoxification reaction is found to enhance with the intensity of light. Kinetic analysis reveals that the detoxification of Cr(VI) in presence of Pd_urc nanocatalyst is most efficient in sunlight with a pseudo-first-order rate constant of 0.274 ± 0.067 min^?1 as compared to commercially available Pd/C nanoparticles and radiolytically synthesized spherical Pd nanoparticles. The synergistic effect of high surface area of porous Pd_urc nanocatalyst and higher rate of photodecomposition of formic acid in presence of Pd_urc is proposed for the photo enhanced catalytic conversion of Cr(VI) to Cr(III). The reaction mechanism is confirmed by increase in the evolution of carbon dioxide (CO₂) in the presence of light, monitored using gas chromatography. The sunlight enhanced conversion of Cr(VI) to Cr(III) in aqueous media, under ambient reaction condition, room temperature, low catalyst loading and recyclability of the Pd_urc nanocatalyst will be helpful in developing new strategies for the detoxification of waste water.

Full text of DOI:10.1016/j.jphotochem.2017.01.034

Accepted Manuscript
Title: Photo Enhanced Detoxification of Chromium (VI) by
Formic Acid using 3D Palladium Nanocatalyst
Authors: Uddhav S. Markad, Ajit M. Kalekar, Devidas B.
Naik, Kiran Kumar K. Sharma, Krushna J. Kshirasagar, Geeta
K. Sharma
PII:
S1010-6030(16)31092-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.01.034
JPC 10524
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
19-11-2016
2-1-2017
31-1-2017
Please cite this article as: Uddhav S.Markad, Ajit M.Kalekar, Devidas B.Naik,
Kiran Kumar K.Sharma, Krushna J.Kshirasagar, Geeta K.Sharma, Photo
Enhanced Detoxification of Chromium (VI) by Formic Acid using 3D Palladium
Nanocatalyst, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2017.01.034
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Photo Enhanced Detoxification of Chromium (VI) by Formic Acid using 3D
Palladium Nanocatalyst
Uddhav S. Markada,b, Ajit M. Kalekara, Devidas B. Naikb, Kiran Kumar K. Sharmac,
Krushna J. Kshirasagara, Geeta K. Sharmaa*
aNational Centre for Free Radical Research, Department of Chemistry, Savitribai Phule Pune
University, Pune - 411007, Maharashtra, India.
bRadiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai
– 400085, Maharashtra, India
cSchool of Nanoscience and Technology, Shivaji University, Kolhapur-416004, Maharashtra,
India
1

Graphical Abstract
2

Highlights
The present study investigates the photoenhanced detoxification of Cr(VI) by
formic acid using 3D porous urchin shaped Pd nanostructures as catalyst. We
have for the first time investigated the effect of light on reduction reaction and it
is found to be dependent on the light source and light intensity. The detoxification
reaction is most efficient in sunlight with the highest pseudo-first order rate
constant of 0.274 ±0.067 min-1 reported so far. The methodology for the
synthesis of nanostructures and the detoxification reaction are environmentally
benign and excludes the use of toxic reagents.
3

Abstract
The detoxification of Cr(VI) to Cr(III) at room temperature by formic acid is investigated
in the presence of 3D urchin shaped monometallic palladium (Pdurc) nanocatalyst at room
temperature. The rate of detoxification reaction is found to enhance with the intensity of light.
Kinetic analysis reveals that the detoxification of Cr(VI) in presence of Pdurc nanocatalyst is
most efficient in sunlight with a pseudo-first-order rate constant of 0.274 ±0.067 min-1 as
compared to commercially available Pd/C nanoparticles and radiolytically synthesized
spherical Pd nanoparticles. The synergistic effect of high surface area of porous Pdurc
nanocatalyst and higher rate of photodecomposition of formic acid in presence of Pdurc is
proposed for the photo enhanced catalytic conversion of Cr(VI) to Cr(III). The reaction
mechanism is confirmed by increase in the evolution of carbon dioxide (CO2) in the presence
of light, monitored using gas chromatography. The sunlight enhanced conversion of Cr(VI) to
Cr(III) in aqueous media, under ambient reaction condition, room temperature, low catalyst
loading and recyclability of the Pdurc nanocatalyst will be helpful in developing new
strategies for the detoxification of waste water
.
4

1. Introduction
Chromium (Cr) is reported to be a major inorganic contaminant after lead [1] and
common pollutant among the hazardous waste. Chromium is mainly discharged from
industrial activities viz. textile dyeing, leather tanning, electroplating and steel industry
[2], [3]. Chromium in its trivalent state Cr(III) is non-carcinogenic and is an essential
trace nutrient for human beings[4], while its hexavalent state Cr(VI) is a well-
documented carcinogen [5] and is also a strong oxidant. High solubility of Cr(VI) in
aqueous medium facilitates percolation into the ground water through waste discharge.
The toxicity associated with Cr(VI) necessitates its removal from the waste water
before discharging into the environment.
The use of thiol-sulfonic acid bifunctionalized mesoporous silica [6], and ferrous
sulphate in combination with sodium dithionite [7] is well documented in the literature
for reduction of Cr(VI) to Cr(III). However, the reducing agents in these methods
involves other metal ions which functions as homogenous catalyst for the reduction of
Cr(VI) to Cr(III) [8], [9]. These metal ion catalyst are not recoverable and further add
up to environmental problems [3]. It has also been reported that microorganisms can
be employed for the anaerobic bio-removal of Cr(VI) [10], but has limitations to
laboratory level. Therefore, use of organic nontoxic reducing agents such as oxalic
acid and formic acid are currently in focus for the reduction of Cr(VI) to Cr(III).
Formic acid is advantageous as it completely degrades to CO2 and H2 in the presence
of a metallic heterogeneous catalyst, which are reusable.
Applied nanoscience has evolved as a field with immense potential for a sustainable
and quality environment through pollution reduction and remediation. Metallic
nanostructures provide immense scope in the field of heterogeneous catalysis [11], and
5

are widely used as nanocatalyst with or without support. Recent reports shows
application of metallic nanocomposite such as, CdS–graphene, [12], two-dimensional
MoS2 nanosheet-coated Bi2S3 [13], TiO2 based nanocomposites[14],[15],[16]
photocatalyst as greener alternative for the reduction of Cr(VI) by formic acid using
UV and visible light. However, the several steps involved during the synthesis of such
nanocomposites remains a hurdle in reproducibility. Graphene oxide supported nickel
nanoparticles [17], palladium (Pd) nanoparticles supported on amine-functionalized
SiO2 [18], TiO2 nanotube supported Pd nanoparticles [19], Pd nanoparticles supported
on mesoporous γ-Al2O3 [20] and polyvinyl pyrrolidone stabilized Pd colloidal
nanoparticles [21] are reported to catalyze the reduction of Cr(VI) to Cr(III) by
molecular hydrogen gas and formic acid. Similarly, few reports on the use of mono
metallic colloidal Pd nanostructures viz. Pd nanoworms [22] and spherical Pd
nanoparticles [23] as heterogeneous nanocatalyst in the reduction of Cr(VI) to Cr(III)
by formic acid at elevated temperature are available in the literature. The later study
demonstrates the size and morphology effect of Pd nanostructure in the reduction of
Cr(VI) to Cr(III) by formic acid. For an effective monometallic heterogeneous
nanocatalyst, low loading and ambient reaction temperature are preferable. Our group
has earlier demonstrated the robust reproducibility of radiolysis technique to obtain
monometallic 3D Pd nanostructures with well-defined microscopic orientation which
shows efficient heterogenous catalytic activity [24] at ambient temperature and good
hydrogen storage property [25] owing to their porosity and high surface area. In this
study, we investigate the use of unsupported/ bare Pdurc nanostructures as nanocatalyst
for room temperature reduction of Cr(VI) by formic acid. Further, the role of light
6

which has not been reported so far for the monometallic Pd nanocatalysed
detoxification of Chromium (VI) by formic acid using Pdurc is investigated.
2. Experimental
2.1 Materials
Tetraamminepalladium (II) chloride monohydrate (Pd(NH3)4Cl2.H2O) (98%), and
commercial catalyst Pd/C (10%Pd ) was purchased from Sigma Aldrich and used as
received. Formic acid (98 %), Potassium dichromate (K2Cr2O7, 99.5%) was obtained
from SRL Pvt. Ltd., Cetylpyridiniumchloride monohydrate (CPCl, 99%) was
purchased from SD-Fine Chemicals, and Propan-2-ol (99.7%) was obtained from
Spectrochem, Nitrogen gas with purity 99.995% was purchased from Inox air
products. All the materials were used as received. Deionized water from Milli-Q
system with resistance 18.2 MΩ M-1cm-1 was used for preparation of all the solutions.
60
A Co gamma source with a dose rate 60 Gy min-1 (evaluated by Fricke dosimetry)
available at the Department of Chemistry, Savitribai Phule Pune University was used
for the synthesis of Pdurc. The gamma irradiation facility is a commercial contained
facility supplied by Department of Atomic Energy, India and is regulated by Atomic
Energy Regulatory Board, India.
2.2 Synthesis of palladium nanostructures
Palladium nanostructure is synthesized as reported by us earlier [24], [25]. Briefly, it is
prepared by dissolving 0.1 M Pd(NH3)4Cl2 in 1.03 g CPCl completely. To this micellar
solution, 0.015 µL propan-2-ol (final concentration in solution 0.1 M) is added to

scavenge the OH radicals [25]. The oxygen interference is removed by bubbling
nitrogen gas for 15 minutes before irradiation. The micellar solution containing the Pd
salt is irradiated in the same glass tubes sealed with silicon septum. The stable,
7

60
perfectly transparent micellar solution is exposed to γ radiation from Co γ source
facility. After 80 kGy of absorbed dose, the Pd nanostructures formed were harvested
using propan-2-ol. The suspended Pd nanostructures are centrifuged and the
supernatant was removed by decantation. The residues were washed several times with
propan-2-ol and water to remove any CPCl traces. Spherical Pd nanoparticles for
comparative study are synthesized by irradiating the micellar solution of 0.1 M
Pd(NH3)4Cl2 in 1.03 g CPCl to a dose of 28 kGy. The isolated Pd nanostructure is
then dried in the oven at 60˚C for 12 hours prior to characterization and determination
of catalytic activity.
2.3 Characterization
The UV−Vis absorption spectra of Pd precursor and Pdurc were recorded using a
Shimadzu- UV1800. Field emission gun-scanning electron microscope (FEG-SEM)
imaging of the synthesized Pdurc were carried out using FEI model NOVANANOSEM
450. The samples for SEM analysis were prepared by depositing a suspension of Pdurc
in isopropyl alcohol (IPA) on silicon wafers and drying it at room temperature.
Transmission electron microscopy (TEM) analysis was performed using TECNAI-G2
20 ULTRA-TWIN FEI with an accelerating voltage up to 200 kV. A FEI TECNAI
3010 transmission electron microscope operating at 300 kV (resolution 1.7 Å) was
used for high resolution transmission electron microscopy (HR-TEM) measurements.
The Pd nanostructures were sonicated using Oscar ultrasonicator prior to deposition on
a carbon coated copper grid having 200 mesh size. X-Ray diffraction (XRD) of the
dried Pd nanostructures were recorded using PANalytical X’pert Pro diffractometer
with a CuKα (1.5418 Å) line. The voltage and current applied to X-ray anode was 45
kV and 40 mA respectively. The nitrogen adsorption isotherms at 77 K were measured
8

on a Quantachrome Autosorb iQ gas analyzer. The BET surface area was calculated
using [0.1 – 0.30] relative pressure range, the correlation coefficient being 0.999. X-
ray photoelectron spectroscopic (XPS) analysis was carried out using a FEI Inc model
PHI 5000 Versa Prob II. Light intensity was measured on digital light meter ‘LX-
101A’. The estimation of CO2 was done on Shimadzu TCD-2014 GC analyser with
Porapak-Q (1 m × 6 mm) stainless steel column with N2 as the carrier gas.
2.4 Detoxification of Cr(VI)
The synthesized Pdurc nanostructures are used as catalyst in the reduction of Cr(VI) to
Cr(III) by formic acid. In a typical experiment, 0.1 M K2Cr2O7 and 50 mg/L Pdurc
dispersion were prepared as stock. K2Cr2O7 and formic acid were taken to make a final
concentration of dichromate as 0.4 mM and 0.2 M formic acid in a 25 ml beaker.
Appropriate Pdurc were then added to this solution to maintain a catalyst concentration
of 5 mg/L. The spectral evolution with time is followed using a UV-Visible
spectrophotometer. The reaction were carried out at room temperature in dark, sunlight
and fluorescent light and monitored with time at 350 nm corresponding to Cr(VI)
absorption peak. The dark reaction condition was maintained by wrapping the beaker
with aluminium foil in a dark room. In order to understand the influence of light,
similar experiments were performed using a 100 W tungsten lamp. The light intensity
were varied from 1600 to 9800 Lux which was measured using a digital light meter.
The recycling study of Pdurc nanocatalyst is carried out by mixing 0.012 mL
K2Cr2O7 (0.1 M) and 2.665 mL Milli-Q water followed by the addition of 0.054 mL
formic acid (22.4 M). The mixture was stirred for 1 minute. To this mixture, 0.3 mL
Pdurc (50 mg/L) was added and the reaction was monitored by observing the change in
the absorbance of K2Cr2O7 at 350 nm as a function of time. After the completion of the
9

first cycle of reduction, dichromate and formic acid was replenished in the reaction
cell and monitored upto six successive cycles.
2.5 Kinetic analysis
The kinetics of the catalytic reduction of Cr(VI) by formic acid in the presence of
Pdurc nanocatalyst is analysed using Langmuir-Hinshelwood mechanism assuming that
both the reactants adsorb on the surface of Pdurc nanostructures. The reaction steps
involved in the interaction of Pdurc and formate ions which finally leads to the
liberation of hydrogen [26] and subsequent reduction of Cr(VI) to Cr(III) on the
surface of catalyst are shown in Scheme 1.
Scheme 1: Reduction of Cr (VI) at the surface of Pdurc. The asterisk (*) indicates the
active site of Pdurc.
The reaction for the detoxification of Cr(VI) to Cr(III) by formic acid in the presence
of Pdurc is treated as pseudo first-order kinetics since the concentration of formic acid
is maintained 1000 folds higher than that of Cr(VI) concentration in all the
experiments. The desorption of Cr(III) from the surface of the Pdurc nanocatalyst is the
10

rate determining step in the reaction proposed for the catalytic mechanism. The rate
law for the Cr(VI) reduction by formic acid is represented by equation (6)
[
( )]
[
( )][ ]
( )
Since, [HCOO-] >> [Cr(VI)], k

[HCOO-] = kapp, known as apparent pseudo first-
order rate constant and were determined using a classical Langmuir-Hinshelwood
equation which is deduced as reported earlier for platinum and palladium
nanostructures [24], [27], and expressed by equation (7).
( )
where C0 and C are the initial and final concentrations having equivalents in terms of
absorbance (A0 and At respectively) monitored at a fixed wavelength ( ) at time t. kapp

is the apparent pseudo first- order rate constant assumed to be proportional to the
accessible surface area (S) for the Pdurc. Therefore, for constant catalyst concentration
and uniform accessible surface area of Pdurc nanostructures, a plot of ln (A0/At) with
respect to time gives a straight line whose slope is the kapp.
3. Result and discussion
3.1 Characterization of Pd nanostructure
The FEG-SEM and TEM images of the synthesized Pd nanostructure in micellar
CPCl are depicted in Figure1, panel a-b respectively. The TEM image shows arms
made up of 3-4 nm Pd nanoparticles, growing away 3-dimensionaly from the center
resembling an urchin like structure. The diameter of these Pd nanostructures ranges
from 35-50 nm. However, the individual nanowires have diameter of about 3 nm
representing the coaxial formation of small pores. The reduction of the palladium salt
is also determined by UV-Vis absorption spectra shown in supporting information (SI-
11

Figure S1) which shows reduction of the Pd salt after 80 kGy irradiation. The
observed morphology and the UV-Vis absorption spectra are consistent with that
reported by us [24],[25] showing the robust reproducibility of the synthesis method.
The HR-TEM image, Figure 1, panel c-d shows lattice fringes of 0.236 nm spacing
corresponding to fcc palladium. The TEM images of the sample irradiated at lower
dose (28 kGy) shows spherical Pd nanoparticles of an average size of 3 nm (SI-Figure
S2).
Figure. 1. FEG-SEM and TEM images of Pd nanostructure obtained after radiolysis of N2
saturated solution containing CPCl, 0.1 M palladium salt (panel a and b); HR-TEM image
showing continuous lattice and (c); enlarged lattice fringes with d spacing of 0.236 nm (d).
(Dose 80 kGy)
The synthesized nanomaterial is subjected to XRD analysis to investigate the
crystalline nature of Pdurc nanostructures. The XRD pattern of the prepared
nanostructures shows peaks at 2

values 39.93, 46.81, 68.01
and 81.19o (Figure 2)
12

which could be indexed to the characteristic peaks (111), (200), (220) and (311)
planes of crystalline palladium respectively (JCPDS No. 05-0681) [28], suggesting the
crystallinity of synthesized Pdurc nanostructure.
Figure. 2. XRD pattern of the synthesized Pdurc in CPCl by radiolysis with JCPDS data (red
bar) for Pd metal (JCPDS No. 05-0681).
The synthesized Pd nanostructures were also analysed by XPS. Figure 3a shows the full scan
spectrum of Pdurc nanostructures which confirms the presence of C, Cl, O and Pd in the
sample. The deconvoluted Pd 3d core level spectrum shows the characteristic peaks at
binding energy of 334.74 and 339.9 eV, which can be assigned to Pd0 3d5/2 and Pd0 3d3/2
respectively for metallic palladium[29].
BET measurements at 77 K were performed to estimate the total surface area of the
synthesized Pdurc. As mentioned in the experimental section, the nanostructures were washed
several times with propan-2-ol and water before analysis to ensure complete removal of
surfactant. The adsorption isotherms of Pdurc at 77 K is depicted in SI-Figure S3. The total
surface area was calculated to be 37.9 m2/g which is little higher than the values reported by
us [24]
13

Figure. 3. X-ray photoelectron spectra (XPS) of Pdurc nanostructure obtained after radiolysis
containing CPCl, 0.1 M palladium salt (a) High resolution XPS spectra of Pd 3d (b)
3.2 Detoxification of Cr(VI) by formic acid using Pdurc catalyst
The evolution of UV-Vis spectra with time for the reduction of Cr(VI) to Cr(III) by formic
acid using Pdurc nanocatalyst in sunlight is depicted in Figure 4a, which shows the 350 nm
peak reduced to residual absorption within 25 minutes. Figure 4b represents the reaction of
Cr(VI) with formic acid without Pdurc nanocatalyst in sunlight. As shown in Figure 4b inset,
no appreciable change in the absorbance is observed even up to 39 minutes. These results
suggest that the reduction of Cr(VI) to Cr(III) by formic acid occurs only in the presence of
Pdurc nanocatalyst. The formation of Cr(III) is confirmed by addition of an excess amount of
sodium hydroxide which leads to a green solution of hexahydroxochromate (III) ions [30].
Similar reactions were carried out with different light source under the same reaction
conditions as used for sunlight. The decrease in the absorbance at 350 nm with time in
sunlight, fluorescent light and in dark is shown in Figure 5a. It is apparent from the figure
that the catalytic degradation of Cr(VI) to Cr(III) by formic acid with Pdurc nanocatalyst is
most efficient in sunlight (86 x 103 Lux) completing within 15 minutes while the reaction
14

under fluorescent light (82 Lux) requires around 55 minutes. In the absence of any source of
light (0 Lux), the reaction remains incomplete even upto 90 minutes.
Figure. 4. UV-Visible spectral evolution with time during the reduction of 0.4 mM
potassium dichromate by 0.2 M formic acid at room temperature, when exposed to
sunlight (86x103 Lux) with (a) 5 ppm Pdurc (b) No Pdurc
Systematic experiments were carried out to investigate the effect of light intensity and
role of light source in the catalytic degradation of Cr(VI) to Cr(III). 100 W tungsten
lamp is used as a fixed source of light. The light intensity is varied by adjusting the
distance between source of light and surface of reaction mixture, keeping the source
constant. Experiments were carried out with light intensity varying from 1600 to 9800
Lux. The time dependent absorbance at 350 nm is shown in Figure 5b. As can be seen
from the figure, the conversion of Cr(VI) to Cr(III) by formic acid in the presence of
Pdurc nanocatalyst is dependent on light intensity. Fastest conversion is achieved at
9800 Lux within 30 min and the reaction time increases to 55 minutes with decreasing
light intensity.
15

It is evident from Figure 5a and 5b, that the decrease in the absorbance of Cr(VI) is
not instantaneous indicating that the catalytic reduction reaction is not immediate, but
it starts after a time lapse which is generally termed as ‘induction time’, as observed in
nanocatalytic reactions reported by us [24],[27] and other groups [31], [32]. We have
reported earlier that t0 is dependent on the sequence of addition of substrate and the
catalyst in the reaction[27].
Figure. 5. Time dependent decrease in the absorbance of aqueous solution containing 0.2 M
formic acid and 0.4 mM potassium dichromate in the presence of 5 ppm Pdurc nanocatalyst
exposed to(a) (i) sunlight, 86 x 103 Lux(-■- ), (ii) fluorescent light, 82 Lux(-●-) and (iii) In
dark, 0 Lux (-▲- ) monitored at 350 nm.(b)100 W tungsten lamp -■- (1600 Lux), -●-
(2600 Lux), -▲- (4600 Lux) and -▼-(9800 Lux) monitored at 350 nm.
16

The effect of the sequence of addition of the substrates on the catalytic reaction was
tested under two conditions, i) Pdurc was initially added to formic acid and finally
Cr(VI) was added to this mixture and ii) Cr(VI) was first mixed with formic acid and
finally Pdurc catalyst was added. In both the cases reaction was exposed to fluorescent
light. The time dependent absorbance monitored at 350 nm under these two conditions
are shown in Figure 6 which shows that the t0 is absent in the former condition
indicating that binding constant of Cr (KCr (VI)) on the Pdurc surface, as shown in
reaction (1), Scheme 1 is an important factor.
Figure 6. Time dependent decrease in the absorbance following sequential addition of
(-■-) aqueous solution of 5 ppm Pdurc nanocatalyst, 0.2 M formic acid and 0.4mM
potassium dichromate and (-●-) aqueous solution of 0.2 M formic acid and 0.4 mM
potassium dichromate and 5 ppm Pdurc nanocatalyst, exposed to a florescent lamp.
3.3 Kinetics of removal of Cr(VI) by formic acid using Pdurc nanocatalyst
The spectral data obtained are analyzed kinetically to gain insight into the reduction
of Cr(VI) by formic acid in the presence of Pdurc nanocatalyst using equation 7. The
pseudo first-order apparent rate constant kapp (min−1) are obtained from the slope of the
linear regression of ln (A0 /A) against reaction time (Figure 7a) for sunlight,
17

fluorescent light and dark and are tabulated in Table 1. As evident from the table, the
highest kapp is observed for the reaction carried out in sunlight (0.274 ± 0.067 min-1)
followed by reactions in fluorescent light (0.107±0.013 min-1) and the lowest in dark
(0.067±0.006 min-1).
Similarly the plot of ln (A0 /A) against reaction time at different intensities of
100 W tungsten lamp is shown in Figure 7b and the respective kapp values are
tabulated in Table 1. The pseudofirst- order rate constant increases from 0.079±0.006
min-1 to 0.184±0.02 min-1 as the intensity of light increases from 1600 Lux to 9800
Lux suggesting that the detoxification of Cr(VI) by formic acid in the presence of
Pdurc nanocatalyst is dependent on light intensity. This enhancement is proposed to be
due to the photo-assisted cleavage of formic acid over the surface of the catalyst
(reaction (3), Scheme 1) and desorption of Cr(III) adsorbed on the Pdurc surface. This
is further supported by monitoring the amount of CO2 released during the oxidation of
formic acid over the Pdurc nanocatalyst using gas chromatography (GC) technique.
The chromatogram for the oxidation under dark, fluorescent and 100 W tungsten lamp
irradiated conditions are depicted in the SI-Figure S4. The chromatograms showed a
marked enhancement in CO2 released when formic acid is irradiated using a 100 W
tungsten lamp which is in agreement with our results.
For comparing the efficiency and investigate the role of nanocatalyst
morphology, catalytic reduction were also studied using commercially available Pd/C
catalyst and Pd nanoparticle synthesized by radiolytic reduction. For Pd/C and
spherical Pd nanoparticles the kapp values calculated are 0.0019 min-1 and 0.0025 min-1
respectively (SI-Figure S5 and S6). These results suggest that the porous nature Pdurc
nanostructure leads to higher catalytic activity.
18

Figure 7. Plot of ln(A0/A) against time for aqueous solution containing 0.2 M formic
acid and 0.4 mM potassium dichromate in the presence of 5 ppm Pdurc nanocatalyst
exposed to (
a) (-■-) sunlight; 86 x 103 Lux, (-●-) fluorescent light; 82 Lux and (-▲-)
dark; 0 Lux monitored at 350 nm. (b) Exposed to 100 W tungsten lamp -■- (1600
Lux), -●- (2600 Lux) and -▲- (4600 Lux) and -▼- (9800 Lux) monitored at 350 nm.
Table 1: Pseudo-first order rate constants (kapp) for the Pdurc catalysed reduction of
Cr(VI) to Cr(III) by formic acid using various light source and by varying the light
intensity.
Light source and intensity (Lux)
Sun Light (86000)
kapp (min-1)
0.274 ±0.067
0.107±0.013
0.067±0.006
Fluorescent light (82)
Dark (0)
Tungsten lamp 100W
1600
2600
4600
9800
0.079±0.006
0.106±0.008
0.141±0.018
0.184±0.02
3.4 Recyclability of Pdurc Nanocatalyst in the Detoxification of Cr(VI)
Recyclability and conversion of the substrate are important parameters for the
industrial application of the developed nanocatalyst. Recyclability of the Pdurc
19

nanocatalyst for detoxification of Cr(VI) by formic acid is investigated by following
the % conversion of Cr(VI). Since the concentration of Pdurc nanocatalyst used in the
reaction is very small, the reaction cycles are performed by adding the substrate
consecutively in the reaction mixture after the completion of one cycle. The
conversion of Cr(VI) is tested for 6 reaction cycles and the % conversion is almost 90-
97% as shown in Figure 8 which shows that the Pdurc nanocatalyst maintains its
integrity upto six reaction cycles.
Figure 8. The % conversion versus number of cycles indicating the stability of the
nanocatalyst. Reaction conditions: 0.012 mL K2Cr2O7 (0.4 mM) + 2.665 mL water +
0.025 mL F.A (0.2M) + 0.3 mL Pdurc (5 mg/L).
4
Discussion
The identical morphology of the synthesized Pdurc nanostructures characterized using
TEM, HR-TEM, and XRD to those reported earlier by our group [24],[25] shows the
robust reproducibility for the production of tuneable metal nanostructures by
radiolysis. The XPS analysis of the nanostructures confirms the presence of metallic
palladium. The kinetic analysis of the conversion of Cr(VI) to Cr(III) by formic acid in
the presence of the synthesized Pdurc nanocatalyst shows the highest catalytic activity
when exposed to sunlight with a pseudo first-order rate constant of 0.274 ± 0.067 min-
20

1. It is also worth mentioning that the rate constant is higher than the values reported
for the conversion of Cr(VI) by formic acid by Dandapat et al. (0.085min−1) at 50C
[20] using Pd nanoparticles supported on mesoporous -Al2O3 and recently by Yang
et al (0.1515min−1) [33], using viral-templated palladium nanocatalyst. Fu et. al. have
studied the catalytic reduction of Cr(VI) using PVP stabilised Pd nano-tetrapods and
reported a higher kapp value of 0.571 min−1. These studies however are limited to
higher temperature of 50C [34] and do not take into consideration, the role of light in
the catalytic reduction. Further, the enhancement in the rate constant when exposed to
different source of light and intensity reveals that the catalytic activity of Pdurc
nanostructure for the detoxification of Cr(VI) is dependent on both source of light and
the intensity.
From these results, a synergistic effect of the porous nature of the synthesized Pdurc
nanostructures facilitating the diffusion of the reactants on to the surface of
nanocatalyst leading to an establishment of fast equilibrium between the reactants
(reaction (1) and (2), Scheme 1) in addition to photo assisted cleavage of the formic
acid (reaction (3), Scheme 1) and desorption of Cr(III) (reaction (5), Scheme 1)
on/from the Pdurc surface is responsible for the enhanced catalytic activity of the
synthesized Pdurc nanostructure. The retention of % conversion ranging from 90-97%
upto 6 reaction cycles is due to the retention of porosity and fast desorption of the
products Cr(III) and CO2 from the nanocatalyst surface preventing the poisoning of the
catalytic surface.
5
Conclusion
The catalytic activity of monometallic Pdurc nanocatalyst synthesized by the radiation
chemical route is investigated for the detoxification of Cr(VI) by formic acid at room
21

temperature rather than the generally known detoxification at high temperature. The
catalytic reaction is found to be dependent on the source of light and their intensity. It
is observed that the detoxification reaction, is very efficient in sunlight with the
pseudo first-order rate constant of 0.274 ± 0.067 min-1. The enhanced catalytic
activity, with a low catalyst loading (5 ppm), of Pdurc is due to the porous nature of
Pdurc which provides more surface area for the adsorption of the substrates. It is
proposed that the synergistic effect of the surface area and photo-assisted
decomposition of formic acid leads to the highest catalytic activity of Pdurc in the
detoxification of Cr(VI). Further, Pdurc exhibited robust recycling stability as %
conversion of Cr(VI) is retained even after 6 cycles of reaction. These results will help
towards understanding of the effect of light on nanocatalyzed reactions which will
facilitate the designing of viable, cost effective pathways for water remediation.
Acknowledgement
U.S.M. acknowledges BARC-Pune University Collaborative Ph.D. program for the
financial support. K.J.K thanks UGC-DAE-CSR–Kolkata for financial assistance. The
authors acknowledge the support received through the UGC-UPE II program in the
thrust area of novel material.
22

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