Borohydride-free catalytic reduction of organic pollutants by platinum nanoparticles supported on cellulose fibers

Article abstract of DOI:10.1016/j.molliq.2019.111988

This study presents hydrogen gas-assisted reduction of organic pollutants viz. 4-nitrophenol (4-NP) and methyl orange (MO) by platinum nanoparticles (PtNPs) immobilized on cellulose fibers (CF). Because sodium borohydride (NaBH₄) causes secondary pollution in water by its decomposition products, hydrogen (H₂) gas was used as a clean, highly active, and alternative reducing agent. As the catalyst, PtNPs of about 2 nm in diameter were synthesized and supported on CF by employing a facile method. The nanocomposites (PtNPs@CF) demonstrated excellent catalytic activity and cyclic stability in the reduction of 4-NP and MO using H₂ gas as the clean, environmentally friendly, highly reactive, and an alternative reducing agent to NaBH₄. The catalytic reduction followed the pseudo-first-order reaction kinetic with apparent rate constants of 3.6 × 10^?1 min^?1 and 5.3?10^?1 min^?1 for 4-NP and MO, respectively. The results of this study further indicate that the H₂ gas (reducing agent) and PtNPs@CF nanocomposite (catalyst) can potentially be used for the reduction of a wide variety of other organic and inorganic compounds in water.

Full text of DOI:10.1016/j.molliq.2019.111988

Journal Pre-proof
Borohydride-free catalytic reduction of organic pollutants by platinum nanoparticles
supported on cellulose fibers
Md Tariqul Islam, Kazi Afroza Sultana, Juan C. Noveron
PII:
S0167-7322(19)33863-2
DOI:
https://doi.org/10.1016/j.molliq.2019.111988
MOLLIQ 111988
Reference:
To appear in: Journal of Molecular Liquids
Received Date: 10 July 2019
Revised Date: 7 October 2019
Accepted Date: 20 October 2019
Please cite this article as: M.T. Islam, K.A. Sultana, J.C. Noveron, Borohydride-free catalytic reduction of
organic pollutants by platinum nanoparticles supported on cellulose fibers, Journal of Molecular Liquids
(2019), doi: https://doi.org/10.1016/j.molliq.2019.111988.
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Borohydride-free catalytic reduction of organic pollutants by platinum
nanoparticles supported on cellulose fibers
Md. Tariqul Islam,abc* Kazi Afroza Sultana,d Juan C. Noveron ab*
a Department of Chemistry and Biochemistry, University of Texas at El Paso, 500 West
University Avenue, El Paso, TX 79968, USA
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b NSF Nanosystems Engineering Research Center for Nanotechnology Enabled Water
Treatment (NEWT)
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c Department of Chemistry, University of Texas of the Permian Basin, 4901 E University
Blvd, Odessa, TX 79762
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d Department of Environmental Science and Engineering, University of Texas at El Paso,
500 West University Avenue, El Paso, TX 79968, USA
Keywords: Hydrogen gas, reducing agent, catalytic reduction, platinum nanoparticles, sodium
rhodizonate, cellulose fibers.
Abstract
This study presents hydrogen gas-assisted reduction of organic pollutants viz. 4-
nitrophenol (4-NP) and methyl orange (MO) by platinum nanoparticles (PtNPs) immobilized on
cellulose fibers (CF). Because sodium borohydride (NaBH4) causes secondary pollution in water
by its decomposition products, hydrogen (H2) gas was used as a clean, highly active, and
alternative reducing agent. As the catalyst, PtNPs of about 2 nm in diameter were synthesized
and supported on CF by employing a facile method. The nanocomposites (PtNPs@CF)
demonstrated excellent catalytic activity and cyclic stability in the reduction of 4-NP and MO
using H2 gas as the clean, environmentally friendly, highly reactive, and an alternative reducing
agent to NaBH4. The catalytic reduction followed the pseudo-first-order reaction kinetic with
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apparent rate constants of 3.6×10−1 min-1 and 5.3✕10-1 min-1 for 4-NP and MO, respectively. The
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results of this study further indicate that the H2 gas (reducing agent) and PtNPs@CF
nanocomposite (catalyst) can potentially be used for the reduction of a wide variety of other
organic and inorganic compounds in water.
1. Introduction
Platinum nanoparticles (PtNPs) possess extraordinary catalytic properties.1,2 Although
bulk or macroscopic platinum is one of the most inert and catalytically inactive materials on
earth, platinum in the in the nanoscopic size range (1-100 nm) is extremely catalytically active.3,4
With the decrease in particle size, the proportion of platinum atoms having the low-coordinated
surface sites (edges and corners of the nanocrystal) increases that provide excellent and exotic
catalytic activity compared to the atoms in the bulk of the particles, Figure 1 a (inset). These low-
coordinated platinum atoms are highly active in carrying out a number of catalytic reactions.3,4
The size, shape, and composition of the PtNPs further determine the activity and selectivity of
the catalytic processes.5 These parameters also determine electronic, optical, and physical
properties of the nanoparticles. In terms of catalysis, some of the widely studied and employed
chemical transformations that PtNPs catalyze are reductions,6,7 oxidations,8,9 and coupling.10
Catalytic reduction has become one of the prominent methods for the degradation or
deactivation of organic and inorganic pollutants in water.11,12,13 For example, the catalytic
reduction of has been reported to be an efficient method for the degradation or deactivation of
pollutants such as hexavalent chromium, organic dyes, and antibiotics. Especially, catalytic
reduction of 4-NP is one of the most widely studied reaction that is performed in academia to
evaluate the activity of a newly-developed catalysts such as the noble/transition metal
nanoparticles and their nanocomposites.14,15 In addition of being a model reaction, the reaction is
employed in industry as the reduced product viz. 4-aminophenol (4-AP) has industrial
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applications. For instance, 4-AP is used in dyeing textiles, hair, furs, feathers; used as a
photographic developer, antioxidants, oil additives; used in pharmaceuticals;16 an intermediate in
the manufacture of sulfur and azo dyes.17,18 Because the reducing property, it is used as a
corrosion inhibitor in paints and as an anticorrosion-lubricating agent in 2-cycle engine fuels.16
Additionally, 4-AP is used as a wood stain and in imparting a roselike color to timber.19 For the
catalytic reduction of 4-NP and other organic pollutants, excess NaBH4 is used most often as the
reducing agent.20,21,22 However, the major drawback of NaBH4 is that it causes secondary
-
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pollution by its excess use and its decomposition by-products such as B(OH)3, B(OH)4 , BO2 ,
3-
BO3 . Moreover, the removal of the excess NaBH4 and its decomposition products becomes
even more challenging and cumbersome job. Therefore, there is a need for the development of a
cleaner and alternative reducing agent for the reduction organic and inorganic pollutants without
causing secondary pollution.
In this regard, neat H2 gas could be considered as a cleaner an alternative reducing agent
for the remediation of environmental pollutants. The utilization of H2 gas is very common in
organic synthesis for the hydrogenation/reduction of organic compounds; however, there are few
reports for its utilization towards the reductive degradation of organic pollutants such as 4-NP
and MO in water. For example, Lowry Gregory and Martin Reinhard reported the
hydrodehalogenation of halogenated organic compounds in water using a palladium catalyst and
H2 gas.23 However, for the reduction of 4-NP or nitro aromatic compounds, the use of H2 gas as
the reducing agent has not been reported, thus far, to the best of our knowledge.
In this study, H2 gas-assisted catalytic reduction of 4-NP and MO on PtNPs immobilized
on cellulose fibers is reported. PtNPs of about 2 nm in size were synthesized in water by the use
of sodium rhodizonate as the reducing-plus-stabilizing agents. Cellulose fibers were used as a
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solid catalyst support to 1) avoid the energy-intensive separation processes of the PtNPs, 2)
prevent the loss of the PtNPs, and 3) prevent the aggregation and/or agglomeration of the PtNPs.
The CF, used in this study, was obtained from Kimwipe paper because it demonstrated high
binding affinity for PtNPs and good chemical and mechanical stability. Transmission electron
microscopy revealed that PtNPs of about 2 nm in size were successfully immobilized to the
cellulose fibers. The PtNPs bound CF nanocomposite (PtNPs@CF) was applied as highly active
and reusable catalyst for the reduction of 4-NP and MO in water using neat H2 gas as the clean
and environmentally friendly reducing agent alternative to NaBH4.
2. Materials and Methods
2.1. Materials
All chemicals were used as received, unless otherwise mentioned. Sodium rhodizonate
dibasic (97%), hydrogen hexachloroplatinate (IV) hydrate (H2PtCl6 3H2O ≥ 99.9%), 4-NP, and
MO, were purchased from Sigma Aldrich. Kimtech kimwipe paper was purchased from Fisher
Scientific, USA. Ultrapure H2 gas (99.99%) was obtained from Matheson Tri-Gas and Milli-Q
water (> 18.20 MI cm resistivity) was obtained from Milli-Q (Advantage A-10) water filter
system.
2.2. Characterization Techniques
Hitachi H-7650 transmission electron microscope (TEM) with an accelerating voltage of
80 kV was used to obtain TEM images. Carbon coated copper grid with 200 mesh (Electron
Microscopy Science) were used for TEM imaging. The grid was prepared by drop casting
method. ImageJ software was used to analyze the average diameter and size distribution of
PtNPs from the TEM image. Scanning electron microscopy (SEM) image was obtained by using
Hitachi S-3400N Type II scanning electron microscope (SEM) having an accelerating voltage of
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15 kV. The X-ray diffraction pattern was obtained Bruker D8 Discover XRD having Cu Kα
radiation. The XPS analysis was performed by using Thermo Scientific Escalab 250Xi
spectrometer with a six-channel detector. Photoelectrons were generated with a monochromatic
Al Ka (1486.68ꢀeV) X-ray source. Agilent Cary 50 Conc UV-Visible spectrophotometer was
employed to obtain the UV-visible spectrum. Standard quartz cuvette having path length of 10
mm was used for the UV-visible studies.
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2.3. Synthesis of PtNPs and PtNPs@CF
The synthesis of PtNPs was adopted from our previously reported studies.4 In details, 20
mL of 0.5 mM aqueous solution of chloroplatinic acid was brought to a boil in a 100 mL round-
bottomed flask and then 5 mL of 9.7 mM aqueous solution of sodium rhodizonate was added
with vigorous stirring (1200 rpm). The reaction mixture was further boiled for 5 min, which was
followed by the cooling under the ambient conditions. It could be noted that the synthesis of
PtNPs could also be carried out at room temperature by stirring the solution of chloroplatinic
acid and sodium rhodizonate for about 20 min of time. For this study, the PtNPs that were
synthesized at boiling conditions.
After cooling the reaction, a single sheet of Kimwipe paper (~456 mg) was immersed
into the reaction flask whilst shaking by hand for about 5 min. This caused the binding of PtNPs
on the Kimwipe paper and the color of the paper changed from white to grey. After rinsing with
deionized water, the PtNPs bound Kimwipe paper (PtNPs@CF) was used for catalytic
applications. ICP-OES was performed on the supernatant and the rising solution, which
confirmed more than 99 % binding ability of the PtNPs on the Kimwipe paper. Thus, the weight
percent of the PtNPs on the Kimwipe paper was calculated to be 0.87 %.
2.4. Catalytic reduction of 4-NP and MO on PtNPs@CF
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The catalytic reduction of 4-NP and MO was carried out under ambient conditions and at
normal pH. Same experimental conditions were adopted for the reduction of 4-NP and MO. The
only difference was in their initial concentrations, which were 0.1 mM and .0611 mM for 4-NP
and MO, respectively. The experimental procedure for the reduction of 4-NP is mentioned in
detail. In 125 mL Erlenmeyer flask, 100 mL of 0.1mM 4-NP solution was taken. A full Kimwipe
paper bound with PtNPs was immersed into the 4-NP solution. The flask was sealed with a
rubber septum. Afterward, H2 gas was bubbled into the flask with gentle stirring of the reaction
mixture. Hydrogen gas was bubbled by using a balloon connected with a long-stemmed stainless-
steel needle. In order to allow continuous bubbling of the H2 gas a second needle was used to
poke the rubber septum, which facilitated the exit of the escaping gas. The reduction of 4-NP
was monitored in every 2 minutes period of time by using UV-visible spectrophotometer. About
1 mL aliquots were taken out at a regular interval and the sample was reintroduced into the
reaction flask after the UV-visible spectroscopic analysis. The UV-visible spectrophotometer
monitored the lowering of the characteristic absorption band of 4-NP at 320 nm. With the
gradual lowering of the absorption band at 320 nm a new band originated at 300 nm, which
indicated the formation of 4-AP from 4-NP. For MO reduction, the characteristic absorption
band that was monitored for its reduction followed by the calculation of percent reduction was
464 nm.
Cyclic stability of the PtNPs@CF was studied by repeating the reduction of MO for 6
cycles. The PtNPs@CF was rinsed with deionized water before the subsequent cycles of use. In
every cycle the percent reduction of MO was calculated after 10 min of catalytic reaction.
3. Characterization of PtNPs and PtNPs@CF
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Various techniques such as Transmission electron microscopy (TEM), Scanning electron
microscopy (SEM), X-ray diffraction spectroscopy (XRD), and X-ray photoelectron
spectroscopy (XPS) were utilized to thoroughly characterize the PtNPs and the PtNPs@CF. For
example, TEM and SEM revealed the morphology of the PtNPs and PtNPs@CF. The binding of
the PtNPs to the CF was visualized by the TEM images. High-resolution TEM microscopy
image further revealed the atomic resolution image of the PtNPs showing the lattice fringes. The
X-ray diffraction pattern demonstrated the crystalline structure of the PtNPs and cellulose fibers
in the nanocomposite.24 The XPS study further provided the elemental composition and the
functional groups of the PtNPs@CF.25
The morphological information such as the size and shape of the PtNPs were obtained
from the TEM images. As shown in Figure 1a, the PtNPs (before binding with CF) could be
observed as spherical in shape. The size of the PtNPs were distributed between 0.75 to 4.5 nm
with an average of ~2 nm in diameter, Figure 1b. The PtNPs were seen mostly individual
although some aggregations were also observed. High-resolution TEM image showed the arrays
of Pt atoms in the nanoparticles, Figure 1a (inset). Further, the lattice spacing of 0.23 nm
indicated the gap between Pt (111) lattice fringes.26
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Figure 1. a) Typical and high-resolution TEM images of the as-prepared PtNPs in water, b) size-
distribution of the PtNPs, c) SEM image of the PtNPs@CF, d) TEM image of the PtNPs@CF, e)
XRD pattern of the PtNPs@CF, f) digital photographic images of the pristine kimwipe paper
and PtNPs bound kimwipe paper, and possible binding interaction of the PtNPs with cellulose.
Inset (1a): nanocrystal showing relatively more exposed atoms at the corners and edges.
The morphology of the PtNPs@CF was analyzed by the SEM image, Figure 1c. It
revealed the fibrous/filamentous morphology of the cellulose fibers having high aspect ratio with
a diameter of about 20 µm. Because the PtNPs were too small, they could hardly be seen in the
SEM image. Therefore, TEM image was obtained on the PtNPs@CF to visualize the PtNPs,
Figure 1d. The TEM image clearly demonstrated that the PtNPs were decorated on the surface of
the CF. The PtNPs supported on the CF was found to be somewhat more aggregated compared to
the PtNPs in solution.
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The XRD pattern of the PtNPs@CF revealed the crystalline property of the cellulose
fibers and the PtNPs, Figure 1e. It was found that the CF was highly crystalline. The diffraction
peaks located at 2θ = 15.4°, 16.2° and 22.7° corresponds to the Iα, or Iβ allomorphs
cellulose.27,28 Additionally, relatively weak diffraction peaks at 39.8°, 46.4°, and 67.7°
corresponds to the metallic crystalline nature of the PtNPs, Figure 1e. Diffraction peaks at 39.8°,
46.4°, and 67.7° are characteristic to the (111), (200), and (220) lattice planes of the completely
reduced or metallic (Pt0) nanoparticles, which further confirms the face-centered cube (fcc)
crystalline structure of the PtNPs.29
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Figure 2. a) XPS survey spectrum of PtNPs@CF, b) high-resolution XPS spectra of C 1s, and c)
Pt 4f.
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The X-ray photoelectron survey spectrum indicated the presence of elements such as
carbon, oxygen, and platinum in the PtNPs@CF, Figure 2a. The oxidation states and the bonding
of the elements were further analyzed by the high-resolution X-ray photoelectron spectroscopy
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(HRXPS) study. For example, relatively highly intense peaks of Pt 4f at 71.22 eV (Pt 4f7/2) and
74.5 eV(Pt 4f5/2) suggested the presence of metallic platinum (Pt0) in the PtNPs, Figure 2c.30,31
Two less intense peaks of Pt 4f at 72.4 eV (Pt 4f7/2) and 76.2 eV (Pt 4f5/2) indicated the presence
of trace ionic platinum (Pt2+) in the PtNPs.24,25 Therefore, the HRXPS studies revealed that the
PtNPs consisted of both metallic and ionic species of platinum. The Pt0 XPS peaks could have
originated from the metallic core of the PtNPts whereas, the Pt2+ peak originated from surface of
the PtNPts. The ionic Pt2+ on the surface of the PtNPs can coordinate to the excess rhodizonate
ions to become stabilized in water. This further indicates the binding ability of PtNPs with the
cellulose fibers through the oxygen carrying functional groups of the cellulose.
Further, the PtNPs@CF was characterized by the Brunauer-Emmett-Teller (BET)
specific surface area analysis, which was achieved by the nitrogen gas adsorption isotherms. The
BET specific surface area of ~0.0816 m2/g with pore volume of ~1.344 mm3/g and a pore size of
~145.449 Å were measured for the PtNPs@CF.
4. Results and Discussion
Sodium borohydride is the most ubiquitously used reducing agent for the reduction of 4-
nitrophenol and organic pollutants in water.32,33,34,35 However, the excess sodium borohydride
and its decomposition products remain in the reaction mixture. Therefore, the separation of the
desired reaction products from the unexpected excess borohydride and its decomposition
products becomes a difficult job. When used for the purpose of water treatment, the removal of
the excess NaBH4 and its decomposition products, also called as secondary pollutants, becomes a
cumbersome job as well. Therefore, there is a need for the use of a clean, effective and
environmentally benign reducing agent. In this aspect, although the use of hydrogen gas as a
reducing agent is very common in organic synthesis, its use in the reduction of organic pollutants
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(viz. 4-NP and MO) in water is very scarce or not reported to the best of our knowledge. This
study reports the use of hydrogen gas for the effective reduction of 4-NP in water, which could
possibly be extended to the reduction of a wide variety of other organic and inorganic species
such as organic dyes, polychlorinated organic compounds, hexavalent chromium, etc.
The synthesis of PtNPs was carried out by rapidly injecting the sodium rhodizonate
solution into a boiling solution of chloroplatinic acid in water. The molar ratio (R) of the sodium
rhodizonate to chloroplatinic acid was 3.88. Within 5 min period of heating, the reaction mixture
transformed to an orange to brownish yellow color, which indicated the formation of the PtNPs.
Since sodium rhodizonate was the only species used for the synthesis of the PtNPs, it could be
considered that sodium rhodizonate performed as the reducing as well as stabilizing agent at the
same time. The as-prepared PtNPs solution could be stored at room temperature in a laboratory
drawer for more than 6 months period of time without observing the aggregation of the
nanoparticles. As-synthesized PtNPs were supported on cellulose fibers obtained from Kimwipe
paper. The Kimwipe paper was chosen as the source of cellulose fibers due to the macroporosity
and chemical functionality, which could rapidly host or impregnate the PtNPs. Due to the
polarized functional groups [(e.g. electron-rich oxygen atoms of polar hydroxyl (O-H) and ether
groups and (O-C-O)] of the cellulose fibers, we assume that the PtNPs were strongly bound to
cellulose fibers via the electrostatic and nonbonding interactions, Figure 1g. The Pt2+ ions at the
surface of the PtNPs (revealed by the XPS study) can bind to the partially polarized oxygen
atoms of the cellulose fibers through the ion-dipole interactions.36
The kinetics of the hydrogen gas mediated catalytic reduction of 4-NP was evaluated by
the pseudo-first-order model with respect to 4-NP. The linear form of the pseudo-first-order
reaction kinetic model could be expressed by the following equation.
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ꢂꢃ
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−ꢀꢁ = ꢅꢆ
ꢂꢄ
(1)
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where Co and Ct represent the initial and time-dependent concentration of 4-NP, k (1/min)
represents the pseudo-first-order rate constant.
The percent reduction of 4-NP was calculated using equation 2.
ꢑ ꢏꢑ
ꢇꢈꢉꢊꢋꢆꢌꢍꢁ (%) = ꢂ ꢏꢂ
=
ꢐ × 100%
ꢑ
ꢎ
(2)
ꢎ
ꢐ
ꢎ
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ꢂ
ꢎ
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where A0 and At represent the absorbance of 4-NP at concentrations C0 and Ct, respectively.
4.1. Catalytic reduction of 4-NP using H2 gas
Hydrogen gas assisted catalytic reduction 4-NP and the corresponding kinetics are shown
in Figure 3. Time-dependent UV-Visible spectrum of 4-NP during its catalytic reduction on
PtNPs@CF is shown in Figure 3a. A gradual decrease in the characteristic absorbance of 4-NP at
320 nm was observed. A new band originated at 300 nm, characteristic to 4-AP confirmed the
reduction of 4-NP to 4-AP. On the other hand, the uncatalyzed reaction, where only cellulose
fibers and hydrogen gas was used, did not show any lowering in the absorbance of 4-AP
characteristic peaks after 16 min of reaction time, Figure 3b. About 1-2 percent of the 4-AP was
found to be adsorbed on the Kimwipe paper. This indicates the ability of the PtNPs in the
reduction of 4-AP in presence of hydrogen gas.
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1.6
1.4
1.2
1
1.6
1.4
1.2
1
Uncatalyzed
Catalyzed
Stock solution
0 min
0 min
2 min
4 min
6 min
8 min
10 min
12 min
14 min
16 min
b)
a)
2 min
4 min
6 min
8 min
10 min
12 min
14 min
16 min
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
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300
350
400
450
500
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
3
2.5
2
100
80
60
40
20
0
y = 0.357x - 0.1158
d)
c)
1.5
1
0.5
0
Uncatalyzed
Uncatalyzed
y = 0.0029x + 0.0048
-0.5
0
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4
6
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12
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16
0
2
4
6
8
Time (min)
Time (min)
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Figure 3. Time-dependent UV-visible spectrum of 4-NP during a) PtNPs@CF catalyzed and b)
uncatalyzed reactions. Corresponding c) percent reduction and d) pseudo-first order kinetics of
the 4-NP reduction.
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The time-dependent percent reduction and the pseudo-first-order reaction kinetics for the
catalytic reduction of 4-NP are shown in Figure 3c and 3d, respectively. It was observed that the
PtNPs@CF reduced more the 93 percent of the 4NP in about 8 min of reaction time (calculated
using Equation 2), Figure 3c. On the other hand, the uncatalyzed reaction did not reduce 4-NP at
all even after 16 min of reaction time. About 1-2 percent of 4-NP removal was obtained for the
uncatalyzed reaction, which was mainly because of adsorption of 4-NP on the CF, Figure 3c. The
apparent rate constant (kapp) for the PtNPs@CF catalyzed reaction was calculated from the slope
of the -ln(Ct/C0) versus time (t) curve, Equation 1. The kapp for the PtNPs@CF catalyzed and
uncatalyzed reactions were 3.6 × 10−1 min-1 and 2.9 × 10−3 min-1, respectively. From the kapp
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values it was found that the PtNPs@CF catalyzed reaction was about 125-fold faster than the
uncatalyzed reaction.
4.2. Catalytic reduction of MO using H2 gas
The UV-Visible spectrum of MO during the catalytic reduction is shown in Figure 4a. It
was observed that the characteristic absorbance of MO at 464 and 275 nm diminished gradually
while a new peak originated at 245 nm. Due to the conjugation throughout the structure and the
azo (-N=N-) bond MO gives characteristic peaks at 464 and 275 nm, respectively. The
disappearance of these two characteristic peaks indicate the loss of conjugation due to the break
of the azo (-N=N-) bond. The rise of a new peak at 245 nm is characteristic to the reduction
products such as hydrazine derivatives of MO, 4-aminobenzenesulfonate, and 4-N,N-
dimenthylaminobenzene.37
100
b)
1.6
a)
0 min
2 min
4 min
6 min
8 min
10 min
80
60
40
20
0
1.2
0.8
0.4
0
Uncatalyzed
0
2
4
6
8
10
220
320
420
520
620
Time (min)
Wavelength (nm)
6
5
4
3
2
1
0
100
80
60
40
20
0
c)
y = 0.5292x - 0.2173
y = 0.0017x - 4E-05
8 10
Uncatalyzed
1st
2nd
3rd
4th
5th
6th
0
2
4
6
Cycles number
Time (min)
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Figure 4. a) Time-dependent UV-visible spectrum of MO solution during the PtNPs@CF
catalyzed reaction, b) time-dependent percent reduction of MO, c) corresponding pseudo-first-
order kinetics of MO reduction, and cyclic stability of PtNPs@CF for the reduction of MO.
The kinetics of the reaction showed that about 99 % MO was reduced after 10 min of
reaction, Figure 4b. The reaction where no catalyst was used showed about 1.6 % decolorization
of MO due to adsorption of the cellulose fibers, Figure 4b. The reaction followed the pseudo-
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first-order kinetics and kapp was calculated to be 5.3✕10-1min-1, whereas; the kapp of the
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uncatalyzed reaction was calculated to be 1.710-1 min-1.
The cyclic stability of the PtNPs@CF was studied for six cycles and it was found that the
catalyst was fairly stable throughout the cycles of use, Figure 4d. Negligible decrease in the
catalytic activity was observed, which indicated robustness of the PtNPs@CF for multiple cylces
of use.
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Scheme 1. H2 gas-assisted catalytic reduction of 4-NP on PtNPs@CF.
285
286
287
288
A well-accepted mechanism for the hydrogen gas-assisted reduction of 4-NP on
PtNPs@CF can be briefly be described as of Scheme 1. The H2 gas adsorbs to the PtNPs surface
with partial or complete cleavage of the H-H bond, also known as the chemisorption of the H2
gas. The pi bond of the nitro group of the 4-NP complexes with the PtNPs. Afterward, the
15

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290
291
292
293
294
295
296
297
chemisorbed H atoms are transferred sequentially to the pi bond of the nitro group forming 4-AP
causing the reduction of the 4-NP. The reaction product viz 4-AP is then released from the
PtNPs.
4.3. Comparison with Literature
The catalytic performance of PtNPs@CF for the reduction of 4-NP and MO was further
compared with various other noble and transition metal nanoparticles catalysts, Table 1.
However, most of the studies available in literature reported the use of NaBH4 as reducing agent.
Therefore, following table compares the catalytic activity of different catalysts in reducing 4-NP
and MO by H2 gas and NaBH4.
298
299
Table 1. Catalytic performance of PtNPs@CF compared with other noble and transition metal
catalysts.
Catalysts
Substrate Reducing Rate constant Reference
agent
(min-1)
H2
5.3✕10-1
PtNPs@CF
MO
This work
3.6✕10−1
1.8✕10-2
6.3✕10-1
6.6✕10-3
1.1✕10-1
5.7 ✕10-1
1.77✕10-1
3.69✕10-1
2.52✕10-2
4.58✕10-2
2.44✕10-2
6.60✕10-2
2.75✕10-1
2.28✕10-1
2.16✕10-1
3.35✕10-2
PtNPs@CF
4-NP
4-NP
4-NP
4-NP
4NP
H2
This work
38
Au/SNTs
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
39
40
41
42
22
32
35
AuNPs@TiO2
AuNPs@MWCNT
Dendrimer-PdNC
PtNPs@polymer brush
PET2-APTES-Ag/Cu
PC-CuO-DEA
4-NP
4-NP
4-NP
4-NP
PET-Fe
PET-APTES-Fe
PET-PAMAM-Fe
PET-SH-Fe
43
44
45
46
AuNPs@CF
Ni-sponge
MO
MO
MO
MO
NaBH4
NaBH4
NaBH4
NaBH4
Cu-NRds
Ag nanostructures
16

47
1.02✕10-1
AuNPs
MO
NaBH4
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
The comparison table demonstrates that the H2 gas assisted catalytic reduction of 4-NP
and MO on PtNPs@CF is faster than the catalytic performance of some noble metal
nanoparticles-based catalysts. The PtNPs@CF also showed slower activity compared to some
other catalytic systems, which could be due to the use of a large excess of NaBH4 in those
systems. However, due to environmental benignity, cleanliness, and facility of the process, H2
gas assisted catalytic reduction can be considered as an alternative reducing agent for the
reduction of environmental pollutants in water. Additionally, as the reducing agent H2 gas can be
utilized for the catalytic reduction of a wide spectrum of organic and inorganic compounds.
5. Conclusions and summary
We have presented the catalytic reduction of hazardous organic pollutants (4-nitrophenol
and methyl orange) on platinum nanoparticles immobilized on cellulose fibers. Platinum
nanoparticles of about 2 nm in size were synthesized, which demonstrated high catalytic activty
in the reduction of organic pollutants in presence of hydrogen gas as the reducing agent.
Hydrogen gas was used as the clean and environmentally benign reducing agent alternative to
NaBH4. Excellent catalytic activity was obtained under the ambient experimental conditions and
the reaction kinetics followed the pseudo-first-order model. Since hydrogen gas does not
generate any secondary pollutants, it could potentially be utilized as a clean, efficient, and
environmentally friendly reducing agent for the reduction of organic pollutants such as
halogenated organic compounds, pharmaceuticals, antibiotics, and so on.
Acknowledgements
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330
Financial support from the National Science Foundation (NSF) grants ERC
Nanotechnology-Enabled Water Treatment Center 1449500, CHE-0748913, DMR PREM-
1205302, and USDA 2014-38422-22078 are gratefully acknowledged.
Author Information
Corresponding Author:
Md Tariqul Islam, Email: islam_m@utpb.edu / tariqul.lab@gmail.com
Juan C. Noveron, Email: jcnoveron@utep.edu
Notes
The authors declare no conflicting financial interest.
Graphical abstract
331
332
Hydrogen gas as the clean and alternative reducing agent for the catalytic reduction of 4-
nitrophenol and methyl orange on platinum nanoparticles supported on cellulose fibers.
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25

Highlights:
•
•
•
•
Synthesis of cellulose fibers supported platinum nanoparitcles.
Hydrogen gas-assisted fast reduction of 4-nitrophenol & methyl orange.
Excellent catalytic activity and stability of the nanocomposite.
Hydrogen gas is a clean reducing agent alternative to NaBH4.
1

Conflict of interest form
for
Borohydride-free catalytic reduction of organic pollutants by platinum
nanoparticles supported on cellulose fibers
Md. Tariqul Islam,abc* Kazi Afroza Sultana,d Juan C. Noveron ab*
a Department of Chemistry and Biochemistry, University of Texas at El Paso, 500 West
University Avenue, El Paso, TX 79968, USA
b NSF Nanosystems Engineering Research Center for Nanotechnology Enabled Water
Treatment (NEWT)
c Department of Chemistry, University of Texas of the Permian Basin, 4901 E University
Blvd, Odessa, TX 79762
d Department of Environmental Science and Engineering, University of Texas at El Paso,
500 West University Avenue, El Paso, TX 79968, USA
Authors declare no conflicting interest of any type.
Sincerely,
Md Tariqul Islam, Ph.D.
Senior Lecturer of Chemistry
University of Texas Permian Basin
4901 E University Blvd, Odessa, TX 79762
1

Fax: 432-552-3230 Tel: 765-749-2505
E-mail: islam_m@utpb.edu
2