Upgraded Valorization of Biowaste: Laser-Assisted Synthesis of Pd/Calcium Lignosulfonate Nanocomposite for Hydrogen Storage and Environmental Remediation.

Laser ablation in liquid (LAL), one of the promising pathways to produce nanoparticles, is used herein for the modification of the abundant biowaste, calcium lignosulfonate (CLS), adorning it with palladium nanoparticles (Pd NPs). The ensuing Pd/CLS nanocomposite, fabricated via a simple stirring method, is deployed for hydrogen storage and environmental cleanup studies; a hydrogen storage capacity of about 5.8 C g-1 confirmed that Pd NPs serve as active sites for the adsorption of hydrogen. Additionally, the novel, sustainable, and reusable nanocomposite also exhibits superior catalytic activity toward the reduction of hexavalent chromium [Cr(VI)], 4-nitrophenol (4-NP), and methylene blue (MB) in an aqueous solution in a short time; the synthesized nanocatalyst could be reused for at least eight successive runs.

Introduction

It is universally known today that growing global pollution problems at an alarming rate are responsible for major irreparable damages to the environment. The rapid industrialization is liable for several environmental impacts on humans’ life, such as climate change, water pollution, water scarcity, and unhealthy air, among others. Besides, depleting natural resources need to be envisaged as a sensitive matter. Meeting the society’s demands requires alternative biofriendly and sustainable energy resources[1] that do not contribute to much polluting emissions such as fuel cells. Hydrogen produced from water is one of the renewable energy forms that embraces this definition.[2,3]

Aiming for this sustainable target, the design and synthesis of novel materials in a greener fashion from renewable and abundant resources has been the main objective.[4−6] Among the bio-based materials, after cellulose, lignin is the most widely known naturally occurring biopolymer[7] comprising of cross-linked amorphous copolymer bonded together by means of different C–O–C and C–C linkages. The ineptest characteristic of lignin would be impermeability, which possibly can be addressed by introducing various functional groups in lignin,[8] sulfonation being more adaptable for application in industrial products.[7,8] The enhancement of its water-soluble properties is the consequence of hydrophilic groups, such as sulfonate, phenolic hydroxyl, as well as alcoholic hydroxyl.[9] In the presence of sulfonic groups, carbon chain, hydrophobic groups, along with hydrophilic groups, lignosufonate (LS) shows good surface activity and acts as a catalyst.[10] According to the production process deployed, diverse ion-based LS can be obtained as calcium, sodium, or magnesium lignosulfonates.[8] LS, a three-dimensional network structured polyphenolic compound, can be simply oxidized electrochemically[7] and makes an ideal substrate to decorate with nanoparticles thus modifying and fine-tuning its properties for high-end applications,[6] namely, for energy storage and in catalysis.[7,11]

Synthesis of nanocomposite-based materials for the hydrogen storage study has been one of the active research fields; nanocomposites encompassing nickel, palladium, platinum, and some metal oxide have the ability for hydrogen adsorption and desorption.[12] Among these metals, Pd shows a unique ability for adsorption of hydrogen atom because of its structure,[13,14] and adorning Pd NPs on such materials can turn them into effective catalysts for adsorption/desorption of hydrogen atoms[15] and also reduction/degradation of environmental pollutants;[16−21] such activity for hydrogen storage has been studied in nanocomposites, including Si wafer[12] and single- and multiwalled carbon nanotubes.[22,23]

Environmental contamination comprises water-, soil-, and air pollution that has been commonly caused by using poisonous/hazardous chemicals, namely, nitrophenols, heavy metals, and organic dyes, among others.[15,17−19,24] Two main types of pollutants such as nitro compounds and toxic dyes are mostly used in diverse industries, e.g., drugs, papers, ceramics, cosmetic printing, food processing, and especially textiles. The toxic nitrophenols (especially 4-NP) are largely responsible as the main environmental pollutants[25,26] and must be converted into harmless aminophenols (especially 4-AP) that are prospective intermediates and precursors for pharmaceuticals, synthetic resins, pesticides, and dyes ideally via a reductive protocol.[17,18,26−28] Another hazardous heavy metal in wastewater is [Cr(VI)], produced during some large-scale industrial processes. Trivalent chromium [Cr(III)], in contrast, has 100-fold less toxicity and can be efficiently eliminated via the formation of insoluble hydroxides in water under controlled conditions.[17,29−31] Consequently, it is obligatory to develop efficient, ecofriendly, and sustainable methods for the treatment of nitro compounds, organic dyes, and heavy metals, thus addressing both the energy demand and environmental issues; the assembly of nanocomposites and modified nanoparticles does inspire newer strategies for the degradation of pollutants and environmental remediation.[32−38]

Nanoparticle fabrication by laser ablation in liquid (LAL) is an enticing approach[39−41] that provides a simple method that is adaptable for various media and conditions; Pd NPs have been produced by LAL and examined for their catalytic[42] and energy storage applications,[43] electrochemical means being the promising option to store hydrogen.[12] The main advantage is that it does not require extreme conditions, e.g., compression and condensation.[44] Cyclic voltammetry (CV) is an indispensable technique for a wide range of materials to study adsorption/desorption of hydrogen. In an alkaline environment, the CV technique shows water decomposition and adsorption and desorption of hydrogen atoms by means of working electrode[12,45] whereas UV–vis spectroscopy complements as a convenient method to monitor the conversion processes.

Introducing a new nanocomposite based on an abundant, renewable, and biofriendly material and utilizing a safe method to generate Pd NPs are the first aim of this paper. In search of improving hydrogen storage properties, LS was decorated with Pd NPs wherein the LAL process was deployed to fabricate Pd NPs. Finally, Pdx/CLS nanocomposite was constructed via a straightforward method (Figure a). The hydrogen adsorption/desorption of metal NPs was studied by electrochemical means in 1 M KOH as medium. CV and cyclic life performance were carried out to assess the electrochemical behavior of the electrodes. Besides, catalytic performance of the prepared Pdx/CLS nanocomposite was investigated to reduce and degrade environmental pollutants, namely, 4-NP and methylene blue (MB) with NaBH4 and Cr(VI) with biomass-derived formic acid (HCOOH) in water.

Figure 1

Schematic illustration for the preparation of (a) Pdx/CLS nanocomposite and (b) Pdx/CLS electrode.

Results and Discussion

Hydrogen Storage Mechanism

CV is an acceptable method to study the electrochemical behavior of materials. In the present work, to investigate hydrogen adsorption/desorption, we applied CV tests to the CLS and Pdx/CLS electrodes and also to the pure stainless-steel mesh, which is the substrate of the nanocomposite electrode. By applying potential in an alkaline solution, at first, water decomposition (H2O + e– → H + OH–) was observed. This generated hydrogen tends to adsorb onto nanocomposites because of the electrode electric polarization under the applied potential. When the potential begins to decline, the liberated hydrogen atoms lead to recombination with OH– to form H2O. In the anodic and cathodic direction, the equation involved iswhere [W] represents the working electrode host cites.

Nanocomposite Characteristic

XRD pattern was studied crystallographically for the synthesized product. In addition, XRD was carried out to evaluate the influence of Pd NPs on the structure of Pdx/CLS. Subsequently, the XRD pattern of Pdx/CLS and Bragg’s law were applied for estimation of the lattice parameter of Pd; Figures a−d shows the XRD pattern of CLS and Pdx/CLS, respectively; sharp peaks indicate that all synthesized samples are well crystallized. The strong and sharp peaks of CLS suggest that it exists as ultrathin nanosheets, which make an ideal space for loading of Pd NPs.[46] The XRD pattern of Pd100/CLS shows two strong reflection peaks at 40.119 and 68.121° associated with arrangement of (111) and (220) planes of Pd (JCPDS 46-1043). These diffraction peaks were assigned to Pd structure, thus confirming the successful formation of Pd100/CLS. When curves were meticulously compared, a significant decrease in intensity was observed around 26.62, 29.3, and 36.4° owing to the immobilization of Pd NPs. It is interesting to note that Pd has a face-centered cubic structure; the lattice parameter of Pd can be achieved using XRD pattern and following the equation (Bragg’s law)[47]where a is the lattice parameter of the cubic crystal; h, k, and l are the Miller indices of the Bragg plane; λ is the wavelength of the X-ray radiation used; and d is the distance between layers in a crystal. The strong and sharp diffraction peak of the (111) Bragg plane was employed to estimate lattice parameters of all samples. Moreover, the mean crystalline sizes were gauged by Scherrer’s equation[48] and XRD data. The equation used waswhere λ = 0.154 nm, β is the full width of half-maximum intensity (FWHM) in radians, and θ is the Bragg diffraction peak. The calculated results by using eqs , 3, and 4 are presented in Table . Our procedural increase in time did not affect the mean crystalline sizes, which is considered an obvious benefit.

Figure 2

XRD pattern for (a) CLS, (b) Pd15/CLS, (c) Pd30/CLS, and (d) Pd100/CLS.

Table 1

Structural Properties of Pd15/CLS and Pd100/CLS

sample name	2θ	hkl	D (nm)	a (Å)
Pd15/CLS	40.02	111	19.89	3.89
Pd30/CLS	40.01	111	19.89	3.88
Pd100/CLS	40.04	111	19.88	3.89

FT-IR provides analysis to justify the presence of functional groups in the materials. Figure shows representative FT-IR spectra, recorded in the wavenumber range 400–3800 cm–1, for CLS and Pd100/CLS. The comparison of the spectrum of lignin[8] and CLS (Figure a) shows the same main structural features of the samples. This is undoubtedly beneficial because the sulfonation did not alter the main structure and change just appeared in the lignin chains.[8] The most characteristic infrared bands of CLS appeared at 3000–3500 cm–1 (Figure ) associated with the OH stretching.[8,46,49] As shown in Figure b, the spectrum of Pd100/CLS shows the same peaks as CLS. The Pd NPs’ immobilization on CLS did not generate any significant peaks but simply influenced the intensity of functional groups; the intensity of the main peaks of CLS and Pd100/CLS is presented in Figure .

Figure 3

FT-IR spectrum for (a) CLS and (b) Pd100/CLS.

The surface structure is studied in terms of the adsorption/desorption N2 at 77 K on the CLS and Pd100/CLS surface, and the results are shown in Figure and summarized in Table . Figure a shows the N2 adsorption/desorption isotherm of the samples, which suggests that the adsorption hysteresis loop in the P/P0 range is from 0.7 to 0.99. This is likely due to the filling and draining of the mesopores by capillary condensation and usually attributed to bottleneck pores.[29] While comparing N2 adsorption for CLS and Pd100/CLS, in the range of P/P0 = 0.8–0.99, a sharp increase of accumulation of N2 in the pores and the vacant space of Pd100/CLS becomes apparent. This leads to the conclusion that Pd100/CLS has a large surface area because of the aggregated Pd NPs.[50,51] Pore structure (BJH) of the samples is depicted in Figure b, and the major peaks of CLS and Pd100/CLS were about 2.7 and 1.2 nm, respectively. BET results determined that surface properties of Pd100/CLS significantly increased in comparison with initial CLS. Interaction between Pd NPs and the surface of CLS leads to increase of mesoporous volume; consequently, the size and pore volume of CLS increased after Pd NPs’ loading.[52] The increasing surface area is probably due to the doping effect and the formation of new pores because of the loading Pd NPs on the CLS surface,[50] and this enhancement of surface area and pore volume offers a good opportunity to adsorb and store hydrogen.

Figure 4

N2 adsorption/desorption isotherms (a) and pore size distribution (b) of CLS and Pd100/CLS.

Table 2

Surface Characterizations for CLS and Pd100/CLS Determined by Nitrogen Physisorption at 77 K

	BET surface area (m2 g–1)	total pore volume (cm3 g–1)	mean pore diameter (nm)
CLS	6.96	0.05	28.67
Pd100/CLS	18.89	0.18	38.77

The different magnification of SEM analyses for CLS nanocomposites are shown in Figure a,b, depicting that CLS has a ravinelike morphology with disparate porosity. It is envisaged that such a structure provides ideal space for attaching nanoparticles. The SEM images for Pd100/CLS (Figure c,d) confirmed the presence of Pd NPs on the CLS surface; Pd NPs appeared aggregated and did not show the regular morphology.

Figure 5

SEM images at different magnification levels for CLS (a, b) and Pd100/CLS (c, d).

To scrutinize the present Pd NPs on the CLS surface, TEM analysis was performed for Pd100/CLS (Figure ). Light gray thin films are CLS nanosheets interconnected in a disorderly arrangement, as corroborated by another report.[53] Spherical-type structures were formed in some regions, indicating that Pd NPs aggregated with each other, the space between Pd NPs and CLS surface being the generated porosity. The average diameter of Pd NPs is estimated, with Image J software, to be about 8 nm. These results are in acceptable agreement with the BET and SEM analysis.

Figure 6

TEM image for the Pd100/CLS nanocomposite.

Figure depicts SEM image of the ensuing electrodes with rough structures for easy access of active cites at the surface between electrodes and electrolyte medium.

Figure 7

SEM images of ensuing electrodes. CLS electrode (a, b) and Pd100/CLS electrode (c, d).

Figure shows the EDS analysis of nanocomposites and resulting electrodes. Figure b,d confirms the presence of Pd NPs in the nanocomposite.

Figure 8

EDS analyses of (a) CLS, (b) Pd100/CLS nanocomposite, (c) CLS electrode, and (d) Pd100/CLS electrode.

Cyclic Voltammetry Studies

To comprehend the electrochemical conduction of samples, CV study was performed in a three-electrode cell filled with 1 M KOH at a sweep rate of 50 mV s–1 using a stainless-steel mesh substrate deposited with CLS and Pd100/CLS. Figure a shows CV curves of the mesh substrate (curve I) and CLS electrode (curve II). According to the CV curves, the results suggest that the mesh substrate has no evident peaks in this potential window; i.e., it does not adsorb and desorb hydrogen. Throughout Figure , the CV curve of CLS shows minor faradic peaks, which means that the CLS can adsorb and desorb a little hydrogen. To take into account Pd NPs’ performance in adsorbing and desorbing hydrogen, the CV curve of Pd15-100/CLS is presented in Figure . As apparent, all curves show oxidation and reduction peaks, demonstrating that hydrogen was adsorbed and desorbed on the surface of the working electrodes. In the cathodic direction, in the scan to negative potential, the CV curve of Pd15-100/CLS shows a reduction peak (C) due to water decomposition, H formation, and adsorption at the electrode surface.[44] In contrast, in the anodic direction, an oxidation peak (A) shows hydrogen desorbing on the surface of Pd15-100/CLS electrode, due to the faradic reaction. The reduction peak is associated with hydrogen adsorption on the Pd surface.[12] The produced hydrogen was adsorbed and stored on the surface of the working electrode due to the polarization created by the applied potential. These potential peaks, in the cathodic and anodic process, show a quasireversible electrochemical reaction appearing on the Pdx/CLS electrodes. To assess the influence of Pd NPs in hydrogen adsorption/desorption, the oxidation and reduction peak currents, i.e., IA and IC, are compared and listed in Table . The immobilization Pd NPs on CLS has a meaningful effect on the oxidation and reduction peaks, thus emphasizing that Pd is an effective catalyst for hydrogen adsorption and desorption.

Figure 9

CV curves of (a) mesh substrate (curve I) and CLS electrode (curve II) (b–d) Pd100/CLS electrodes in 1 M KOH at the 50 mV s–1 sweep rate; A and C represent the oxidation and reduction peaks, respectively.

Table 3

CV Data for CLS and Pdx/CLS

	Irp (A g–1)	Erp (V)	Iop (A g–1)	Eop (V)
CLS	–0.1	–1.04	0.03	–0.7
Pd15/CLS	–0.15	–1.01	0.18	–0.7
Pd30/CLS	–0.5	–1.1	0.31	–0.7
Pd100/CLS	–1.85	–1.02	1.3	–0.7

Cyclic Life Performance

The amount of hydrogen desorbed can be measured by the integration of the anodic peak. The total charge related to hydrogen desorption can be associated with the integral of the curve where atoms are being desorbed.[54] Given that the electrode/electrolyte interface act as a capacitor, the measurement of QH is done by assuming a baseline to separate the double layer and faradic region. This part of the CV curve, pertaining to Pd100/CLS, is presented in Figure a. The red area corresponds to the double-layer capacitance, and the white area shows the amount of hydrogen desorption. The integration is defined by following equationwhere ϑ is the sweep rate, dE is the potential window, and Ia and Idl are the current due to the anodic peak and double-layer charging, respectively.

Figure 10

(a) First cycle of Pd100/CLS electrode in a 1 M KOH solution at the sweep rate of 50 mV s–1 and (b) cyclic life performance of Pd100/CLS.

To investigate the cyclic life performance of Pd100/CLS, a CV curve for 100 cycles was obtained. By using eq and CV data, QH was estimated and drawn as a function of the cycle’s number in Figure b. As the curve shows, the amount of hydrogen charge decays with the increase in the cycle number. In the first 20 cycles, a 43% decrease in QH is probably due to the oxidation of the electrode; QH decrease during scans maybe due to the separation of Pd100/CLS from the electrode surface.

Catalytic Studies of Pdx/CLS Nanocomposites for Reduction of Environmental Pollutants

In this work, Cr(VI), MB, and 4-NP were subjected to the reduction/degradation process using Pdx/CLS nanocomposite as a sustainable and recoverable catalytic nanosystem and HCOOH or NaBH4 as reducing agents in aqueous media. The reduction is important and the main chemical reaction in industrial organic chemistry, which is performed via an electron-transfer change.[17,18] The progress of the conversion processes was checked via UV–vis analysis; Cr(VI), 4-NP, and MB have a typical absorption peak at 350, 317, and 664 nm, respectively.

Cr(VI) reduction to Cr(III) was examined by deploying HCOOH and K2Cr2O7 as a hydrogen donor and chromium source, respectively, in the presence of Pd15/CLS and Pd100/CLS nanocomposites at 50 °C. The mechanism of Cr(VI) reduction by Pdx/CLS nanocomposite was studied using hydrogen transfer of HCOOH and electron transfer between oxygen as a ligand to Cr(VI) metal in solution (Scheme ). HCOOH was first decomposed to hydrogen and carbon dioxide after adsorption onto nanocomposite surface; Cr2O72– was then converted to Cr(III) by the formation of bubbling H2 gas (eqs 1 and 2, Scheme ).

Scheme 1

Mechanism for the Cr(VI) Reduction to Cr(III) by Pdx/CLS Nanocomposite

After the addition of HCOOH and beginning of the Cr(VI) reduction, the peak at 350 nm completely vanished within 2 min (Figure ), with the disappearance of the yellow color. Without the addition of Pdx/CLS nanocomposites and formic acid, the reduction of Cr(VI) did not occur and no color change was discernable even after 3 h (Table , entries 1 and 6). As shown in Table , in the presence of the CLS, the reduction of Cr(VI) was not completed even after 1 h (Table , entry 2). The CLS serves as a support and stabilizing agent and decreases the agglomeration of Pd NPs, providing a synergistic effect in the reduction process. The best results were attained using 5.0 mg of Pd100/CLS and 1.0 mL of aqueous HCOOH solution at 50 °C within 120 s (158 s) (Table , entries 4 and 7).

Figure 11

UV–visible spectra of the HCOOH-induced Cr(VI) reduction using 5.0 mg of Pd100/CLS.

Table 4

Optimization of Conditions for the Cr(VI) Reduction by Pdx/CLS Nanocomposites and HCOOH at 50 °C

entry	catalyst (mg)	HCOOH (mL)	time
1		1.0	3 ha
2	CLS (5.0)	1.0	1 hb
3	Pd15/CLS (3.0)	1.0	3:40 min
4	Pd15/CLS (5.0)	1.0	2:38 min
5	Pd100/CLS (7.0)		3 ha
6	Pd100/CLS (3.0)	1.0	2:15 min
7	Pd100/CLS (5.0)	1.0	2 min

No reaction.

Not complete.

Additionally, the catalytic prowess of Pd15/CLS and Pd100/CLS nanocomposites was studied toward 4-NP reduction to 4-AP; the mechanism for this process by Pdx/CLS nanocomposites, via an electron-transfer reaction using NaBH4 at ambient temperature, is depicted in Scheme . The reduction process can be performed by the adsorption of the BH4– and 4-NP ions on the Pdx/CLS surface and the transfer of active hydrogen species via the π–π stacking interactions; the reduction process can be catalyzed using Pd NPs along with the formation of a metal hydride complex. As shown in Scheme , the formed 4-AP was finally desorbed from the Pdx/CLS surface and another catalytic cycle can start anew.

Scheme 2

4-NP Reduction Mechanism Using Pdx/CLS Nanocomposite

During the reduction reaction, the electron transfers between BH4– ions (reductant) and Pd NPs (oxidant) can result in the disappearance of a chiastic SPR at λmax ∼317 nm and appearance of a new SPR at 297 nm (Figure ). Upon the addition of Pdx/CLS nanocomposites to the reaction solution and start of the reduction reaction, the peak intensity at 317 nm quickly shifted to 400 nm and decreased with the concomitant disappearance of the yellow color within 60 s at ambient temperature. The peak observed at λmax ∼400 nm was associated with the produced 4-nitrophenolate ions following the addition of NaBH4 to 4-NP solution, disappearing after the 4-NP reduction. The reduction process was examined with time variation using various amounts and conditions, and results are summarized in Table . In the absence of the Pdx/CLS nanocomposites and NaBH4 reductant, no reaction and obvious color change was observed (Table , entries 1 and 6). The catalytic performance of the Pd15/CLS and Pd100/CLS nanocomposites became apparent in the reduction of 4-NP to 4-AP (Table , entries 3–12); the Pd15/CLS and Pd100/CLS nanocomposites are much more reactive than CLS (Table , entry 2). Additionally, CLS, as a support, prevents the aggregation of Pd NPs. In addition, the catalytic activity of the Pd100/CLS nanocomposite is better than Pd15/CLS in a comparatively shorter time period. As shown, the reduction of 4-NP to 4-AP was quickly completed within 60 s using 7.0 mg of Pd100/CLS nanocomposite as a highly active nanocatalyst and 100 equiv of NaBH4 (Table , entry 12). Additionally, the reaction time for the reduction reaction increased to 83 and 75 s in the presence of 50 or/and 79 equiv of the NaBH4 solution, respectively (Table , entries 10 and 11).

Figure 12

UV–visible spectra of the NaBH4-induced reduction of 4-NP using 7.0 mg of Pd100/CLS nanocomposite.

Table 5

Optimization of 4-NP Reduction (2.5 × 10–3 M) Using Pdx/CLS Nanocomposites and NaBH4 at Ambient Temperature

entry	catalyst (mg)	NaBH4 (equiv)	time (min)
1		100	170a
2	CLS (7.0)	100	14:51
3	Pd15/CLS (5.0)	100	2:55
4	Pd15/CLS (7.0)	79	4:30
5	Pd15/CLS (7.0)	100	2:30
6	Pd100/CLS (5.0)	0.0	50b
7	Pd100/CLS (5.0)	50	3:5
8	Pd100/CLS (5.0)	79	2:20
9	Pd100/CLS (5.0)	100	2:5
10	Pd100/CLS (7.0)	50	1:23
11	Pd100/CLS (7.0)	79	1:15
12	Pd100/CLS (7.0)	100	1

No reaction.

Not complete.

For the other pollutant, MB, a similar method was repeated to assess the catalytic ability of the Pdx/CLS nanocomposites and the results are reported in Table ; generally, Pdx/CLS nanocomposites could reduce MB solution in a short span of time. The investigation of the MB reduction was performed by recording the process of the UV absorption spectra at λmax ∼664 nm (Figure ). According to the obtained results, no color change was observed without assistance of Pdx/CLS nanocomposites and NaBH4 reductant even after 60 min (Table , entries 1 and 4). The obtained results (Table ) depict that the Pd15/CLS has weaker performance for the MB reduction, and a relatively slower reaction rate was observed in comparison with Pd100/CLS. The best activity for the MB reduction was obtained using 3.0 mg of Pd100/CLS nanocomposite; further increments of its amount did not improve the results (Table , entries 5 and 6). Since both, the adsorption of reactants and electron-transfer process from the BH4– to MB take place on the Pd surface, the Pd100/CLS nanocomposite is much more reactive than CLS. CLS, as a support, prevents the aggregation of Pd NPs, and its synergic effect is important in the reduction process.

Table 6

Optimization for the Reduction of MB (3.1 × 10–5 M) Using Pdx/CLS Nanocomposites and NaBH4

entry	catalyst (mg)	NaBH4 (M)	time
1		5.3 × 10–3	60 mina
2	CLS (3.0)	5.3 × 10–3	150 minb
3	Pd15/CLS (3.0)	5.3 × 10–3	5 s
4	Pd100/CLS (3.0)	0.0	60 mina
5	Pd100/CLS (3.0)	5.3 × 10–3	1 s
6	Pd100/CLS (5.0)	5.3 × 10–3	1 s

No reaction.

Not complete.

Figure 13

UV–vis spectra of the NaBH4-mediated reduction of MB using 3.0 mg of Pd100/CLS.

Catalyst Recyclability

The Pd100/CLS nanocomposite revealed excellent reusability and recyclability for catalyzing the 4-NP reduction using NaBH4 under equivalent empirical conditions. The easy separation of the synthesized Pdx/CLS allows it to be collected and removed from the reaction mixture via simple filtration. After carrying out the reduction process, the used nanocatalyst can be collected, washed with distilled water and ethanol, and then dried for the consecutive cycle. Figure depicts that Pd100/CLS catalyzed the reduction of 4-NP with declined product yield of only 3% after eight consecutive runs.

Figure 14

Recycling experiments performed with Pd100/CLS nanocomposite for 4-NP reduction.

Conclusions

Commercial lignosulfonate (CLS) was modified by means of Pd NPs to accomplish hydrogen storage in alkaline medium, and the same catalyst was equally efficient for the environmental cleanup of common pollutants in aqueous medium. Characterization techniques, viz., XRD, FT-IR, BET/BJH, SEM, EDS, and TEM, were deployed to study the surface of the representative samples. The results show that the LAL process managed to fabricate Pd NPs and their subsequent deposition procedure on CLS was successful. The electrochemical study has shown an encouraging hydrogen storage capacity due to the loading of Pd NPs, which was about 5.8 C g–1; Pd NPs serve as active sites for the hydrogen adsorption. The present study also illustrates that Pdx/CLS nanocomposites have exceptional catalytic activity and stability for the reduction of common environmental pollutants in water, namely, 4-NP and MB with NaBH4 and Cr(VI) with HCOOH.

Experimental Section

Instruments and Reagents

Commercial calcium lignosulfonate (CLS) with an ideal formula was used. Powder X-ray diffraction patterns were applied to characterize the structure of samples (XRD, Unisantis xmd300 model), the wavelength of 1.5405 Å, and the 2θ range 10–80°. A UV–vis spectrophotometer (Hitachi, U-2900, λ = 200–700 nm) was employed to record the reduction of MB, 4-NP, and Cr(VI). A scanning electron microscope (Cam scan MV2300), equipped by an energy dispersive X-ray spectrometer (EDS), was used to determine the morphologies and the elemental analysis of nanocomposites and electrodes, respectively. To measure the chemical environment of samples, Fourier transform infrared (FT-IR, Thermo Nicolet) spectra were acquired. Brunauer–Emmett–Teller (BET, BELSORP-mini II) and Barret–Joyner–Halenda (BJH) analysis were deployed to study pore size and surface morphology of samples. The presence of Pd NPs on CLS was visualized using a transmission electron microscope (Zeiss-EM10C-100 kV). In addition, cyclic voltammetry was deployed to determine the electrochemical characteristics of the samples.

Synthesis of Pd-Loaded CLS

To produce colloidal Pd NPs, a bulk piece of palladium was ultrasonically cleaned in distilled water and acetone. The Pd target (99.9%, Aldrich) is positioned at the cell bottom filled with deionized water. The ablation process was carried out according to our earlier described procedure.[42] As the LAL process was performed, the color of deionized water turned black, affirming the presence of Pd NPs in deionized water. To prevent the agglomeration and reduce the rate of production for Pd NPs due to absorption laser beam by NPs, the LAL process was carried out in 3, 6, and 20 stages, each stage being of 5 min for Pd15/CLS, Pd30/CLS, and Pd100/CLS, respectively. In each stage, 5 mL of deionized water was used in a way that about 2 mm water covers the whole target surface. After 5 min, the colloidal Pd NPs were extracted, and the LAL process, repeated with fresh deionized water. Finally, LAL was performed for different time periods: 15, 30, and 100 min. After the LAL process, CLS was added to the Pd NP colloidal solution and stirred with a magnetic stirrer for 24 h at room temperature. Finally, the ensuing nanocomposite was filtered and dried at ambient temperature in a dust-free atmosphere. According to LAL time, the nanocomposites were labeled Pdx/CLS, where x represents the ablation time (x = 15, 30, and 100 min). The schematic of the whole procedure is depicted in Figure a.

Electrode Preparation

CLS and Pdx/CLS were used to create the electrodes: about 2 mg of the samples was added into deionized water (3 mL) and sonicated for 15 min. A stainless-steel mesh (200 mesh, 0.36 cm2) was used as the substrate, which was washed by acetone and deionized water, respectively. Then, the electrodes were prepared by the drop casting method on the substrate and dried at room temperature (Figure b).

Electrochemical Instrumentation

All electrochemical measurements were made at ambient temperature by an autolab (PSTAT204) using a three-electrode cell composed of a working electrode, a reference electrode (Ag/AgCl), and a counter electrode (Pt plate). The KOH electrolyte was prepared at 1 M concentration. For minimizing the ohmic drop of electrolyte, reference electrode was placed close to the surface of the working electrode. CV measurements of the working electrodes were carried out in a potential range of −1.3 to 0 V with a scanning rate of 50 mV s–1.

General Procedure for the Reduction of Cr(VI)

To assess the reduction of Cr(VI) to Cr(III), an aqueous K2Cr2O7 solution (3.4 × 10–3 M, 25 mL) was first mixed with Pdx/CLS nanocomposite (5.0 mg) and then a HCO2H solution (1.0 mL, 88%) was added at 50 °C under constant stirring. UV–vis analysis was employed to monitor the process; the Cr(VI) aqueous solution has an absorption peak at 350 nm. After fading of the solution color (light yellow), the Pdx/CLS nanocomposite was removed, washed with EtOH and deionized H2O, and utilized again in sequential cycles.

4-Nitrophenol (4-NP) Reduction

The efficacy of the Pdx/CLS nanocomposite was also evaluated in the 4-NP reduction to 4-AP using NaBH4. In this experiment, 7.0 mg of Pdx/CLS were dispersed into aqueous 4-NP solution (2.5 mM, 25 mL) and then freshly prepared NaBH4 solution (25 mL, 250 mM) was added under stirring at room temperature. The yellow color of the solution disappeared and it became colorless, indicating the completion of reduction. After affirming the end of the reduction process by UV–vis spectroscopy, Pdx/CLS was filtered off, washed with deionized H2O, dried, and then reused for subsequent runs.

Methylene Blue (MB) Reduction

In this investigation, Pdx/CLS nanocomposite (3.0 mg) and aqueous MB solution (25 mL, 3.1 × 10–5 M) were shaken under stirring at room temperature for 60 s. A freshly prepared aqueous NaBH4 solution (25 mL, 5.3 × 10–3 M) was quickly added to the mixture and was allowed to stir at room temperature while the above solution rapidly became colorless. At the end, the nanocomposite was similarly separated, washed, dried, and utilized in the following cycles with no significant changes in the catalytic activity.