• International Journal of Technology (IJTech)
  • Vol 14, No 2 (2023)

Fabrication of Silver-Silica Composite using the Carbo-thermal Degradation of Oil Palm Leaves for the Reduction of p-nitrophenol

Fabrication of Silver-Silica Composite using the Carbo-thermal Degradation of Oil Palm Leaves for the Reduction of p-nitrophenol

Title: Fabrication of Silver-Silica Composite using the Carbo-thermal Degradation of Oil Palm Leaves for the Reduction of p-nitrophenol
Salprima Yudha S, Charles Banon, Aswin Falahudin, Muhamad Alvin Reagen, Noor Haida Mohd Kaus, Subhan Salaeh

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Cite this article as:
Yudha S.S., Banon, C., Falahudin, A., Reagen, M.A., Kaus, N.H.M., Salaeh, S., 2023. Fabrication of Silver-Silica Composite using the Carbo-thermal Degradation of Oil Palm Leaves for the Reduction of p-nitrophenol. International Journal of Technology. Volume 14(2), pp. 290-299

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Salprima Yudha S 1. Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Bengkulu, Jalan W.R. Supratman, Kandang Limun, Kota Bengkulu, 38122, Indonesia, 2. Research Center of Sumatera
Charles Banon Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Bengkulu, Jalan W.R. Supratman, Kandang Limun, Kota Bengkulu, 38122, Indonesia
Aswin Falahudin Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Bengkulu, Jalan W.R. Supratman, Kandang Limun, Kota Bengkulu, 38122, Indonesia
Muhamad Alvin Reagen Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Bengkulu, Jalan W.R. Supratman, Kandang Limun, Kota Bengkulu, 38122, Indonesia
Noor Haida Mohd Kaus School of Chemical Sciences, Universiti Sains Malaysia, 11800, Penang, Malaysia
Subhan Salaeh Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani 94000, Thailand
Email to Corresponding Author

Abstract
Fabrication of Silver-Silica Composite using the Carbo-thermal Degradation of Oil Palm Leaves for the Reduction of p-nitrophenol

Oil palm leaves are natural sources of bio-silica. In situ carbothermal degradation was conducted at 600°C using silver nitrate as a metallic silver (Ag) precursor and oil palm leaves as a silica (SiO2) source. X-ray diffraction of the solid product (Ag@OPLA600) revealed the presence of Ag and SiO2 from the oil palm leaves. Fourier transform infrared spectroscopy showed an absorbance consistent with the presence of SiO2. Scanning electron microscopy revealed a solid surface with cavities. Ag@OPLA600 was used as a heterogeneous mediator to reduce p-nitrophenol to p-aminophenol within 15 minutes. This work provides a new approach for the utilization of sustainable natural resources as a metal Ag supporting material to produce a mediator for the conversion of p-nitrophenol to p-aminophenol in a simple manner.


Carbo-thermal degradation; Heterogeneous mediator; Oil palm leaves; Silver-based composite

Introduction

Silica (SiO2) is the most abundant compound in nature and is found in various living organisms. The most common sources of natural silica in living organisms are sugarcane bagasse and rice husks (Permatasari, Sucahya, and Nandiyanto, 2016; Prasad and Pandey, 2012). The high demand for silica in chemistry and chemical technology has prompted intensive research efforts toward the utilization of biogenic silica as main or supporting material (Prabha et al., 2020; Bahadur et al., 2011). For example, silica is commonly used as a supporting material to prepare heterogeneous catalysts (Corro et al., 2017; Adam, Ahmed, and Min, 2008). Recently, a heterogeneous silver-based catalyst was prepared by simply impregnating waste rice husk with silver ions, which were converted to supported nanoparticles through carbothermal reduction under a nitrogen atmosphere (Unglaube, Kreyenschulte, and Mejía, 2021). Silver–silica derivatives (Ag2O–SiO2) were demonstrated to exhibit good activity toward bacteria (Hou et al., 2018; Peszke et al., 2017; Quang et al., 2011), as virucidal materials (Assis et al., 2021), as rice antibacterial agents (Cui et al., 2016), and other developments to obtain Ag/SiO2 with specific characteristics (Flores et al., 2008; Jiang, Chen, and Mao, 2008).

The utilization of biomass from agricultural wastes to produce various materials with diverse applications has spawned considerable research (Kartohardjono, Pamitran, and Putra, 2019). In Indonesia, palm oil stands out as a popular agricultural commodity, whose production generates various types of waste. Consequently, the valorization of such wastes in, for example, the delignification of oil palm empty fruit bunches (Hermansyah et al., 2019) and the extraction of palmitic from oil palm (Mulia et al., 2018) is attracting increasing research attention.

In this context, oil palm leaves (OPL) are particularly interesting owing to their high silica content, which can be produced from burning OPL. Recent studies have demonstrated that OPL can be used to obtain significant amounts of silica to prepare diverse materials (Onoja et al., 2018; Onoja et al., 2017). For example, the authors’ research group reported the use of OPL to produce silica-containing materials, which were then used as precursors in the manufacture of calcium silicate derivatives (Yudha et al., 2020a), ZnO–SiO2, and Zn2SiO4 (Yudha et al., 2020b). Moreover, the decomposition product of OPL was used as a support material for gold particles as catalysts for the reduction of nitroarene compounds (Yudha et al., 2021).

In this study, a silver-based composite (Ag@OPLA600) was fabricated via a facile and straightforward method involving a simple impregnation of silver ions into OPL ash powders followed by carbothermal degradation at 600°C to destroy the organic materials and reduce silver ions to metallic silver. The obtained composite was then used as a mediator for the heterogeneous reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP), which is one of the intermediates in the synthesis of N-(4-hydroxyphenyl) acetamide (commonly known as paracetamol) via a simple acetylation process (Abdullaev et al., 2014). Heterogeneous systems are often used as reaction mediators or catalysts due to their easy separation, recyclability, and reusability. In this case, silver particles as the active material are either attached to the surface or trapped in silica for the reduction of p-NP to p-AP.

Experimental Methods

2.1.  Materials

All glassware and crucibles used in the experiments should be cleaned with a freshly prepared aqua-regia solution. The chemical reagents such as silver nitrate, acetone, and p-nitrophenol were purchased from Merck (Germany), and NaBH4 was purchased from Sigma–Aldrich (Singapore); demineralized water was purchased from Brataco Chemika (Indonesia). The chemicals used in this work were of analytical grade and were used as received without any further purification. The OPL powder preparation follows the previously reported procedure (Yudha et al., 2020a).

 2.2. Methods

The Ag@OPLA600 composite was synthesized using a solid-state reaction between OPL and silver nitrate (AgNO3) according to a modified literature procedure (Unglaube, Kreyenschulte, and Mejía, 2021). Specifically, nitrogen gas was not used in this study for the solid-state reaction. Briefly, 1 g of AgNO3 was placed in a solvent mixture of acetone/demineralized water (50 mL/5 mL) in a crucible and stirred until completely dissolved. Subsequently, 4 g of OPL powder was added to the AgNO3 solution with stirring until all the OPL powder was suspended. The reaction mixture was heated to 70°C to form a gel on the bottom of the crucible. The gel was left to stand for 12 hours and then placed in a furnace, where the temperature was increased to 600°C for 2 hours and maintained at 600°C for another 3 hours before cooling down for 12 hours. The as-obtained Ag@OPLA600 composite was characterized via X-ray diffraction (XRD MiniFlex, Rigaku) and Fourier transform infrared (FTIR, Compact FT-IR Alpha 2, Bruker) spectroscopy to obtain the phase and vibration patterns, respectively, and scanning electron microscopy (SEM, JEOL, JED-2300) and energy-dispersive X-ray spectroscopy (EDX) for the surface and elemental analysis, respectively.

The catalytic performance of Ag@OPLA600 was evaluated in the reduction of nitroarene using a slightly modified reported procedure (Kadam and Tilve, 2015). Ag@OPLA600 (50 mg) was added to a reaction mixture of p-NP (1 mL, 2 mM), NaBH4 (5 mL, 30 mM), and demineralized water (10 mL) with gentle stirring. The UV–Visible spectra of the solution were recorded over the range of 200–600 nm using an Agilent-60 spectrophotometer. The reaction progress was monitored every 5 minutes until the nitrophenolate ion intermediate completely disappeared. The solution was picked up using dropper pipettes directly without centrifugation for UV-Visible measurement and returned after analysis. The synthesis of the as-prepared materials and the reduction of p-NP to p-AP are illustrated in Figure 1.

Figure 1 Illustration/scheme for the experimental procedure in preparing composite and its function in reduction of p-nitrophenol (p -NP) to p-aminophenol (p-AP)

Results and Discussion

3.1. Composite Preparation and Characterization

        The carbothermal degradation process afforded a grey-white powder. Its XRD pattern showed a broad peak at   = 22° and several sharp peaks at 38.2°, 44.3°, 64.4°, and 77.4° (Figure 2a). This is in line with the JCPDS card number 85-0335 for amorphous silica (Musi?, Filipovi?-Vincekovi?, and Sekovani?, 2011) and JCPDS card number JCPDS 04-0783 for metallic silver (Ag (0)) (Syed et al., 2019; Yang, Zhang, and Huang, 2013; Park et al., 2011). Heating AgNO3 at low temperatures resulted in combustion with oxygen to form silver (I, III) oxide (AgO) and silver(I) oxide (Ag2O). In contrast, heating above 400°C led to cleavage of the Ag–O bond, eventually producing Ag metal as the decomposition product (Waterhouse, Bowmaker, and Metson, 2011). The reaction did not require the presence of inert gases such as nitrogen and argon. The XRD pattern showed peaks corresponding to OPL powder, which exhibits amorphous peaks, except two broad peaks at  = 21.6° and 29.9° (Yudha et al., 2020a). The XRD analysis revealed the disappearance of the peaks of the starting materials and the appearance of new peaks of the composite material.
        Figure 2b shows peaks at 3418 cm-1 (OH stretching), 2922 cm-1 (C–H stretching), 1632 cm-1 (–O–tensile vibration), and 1040 cm-1 (C–OH bending). Figure 2c shows the FTIR spectra of the Ag@OPLA600 solid. The peaks at 3430 and 1634 cm-1 can be assigned to the stretching and bending vibration of the OH group from water adsorbed on the surface of the solid silica material. The sharp peak at 1094 cm-1 and the small peak at 799 cm-1 can be attributed to the Si–O–Si stretching vibration (Deshmukh, Peshwe, and Pathak, 2012). Although the characteristic peaks of the Ag–O–Si bond were not observed in the FTIR spectrum, the XRD result (Figure 2a) confirmed that Ag@OPLA600 contained Ag metal. Taken together, the XRD and FTIR spectroscopy results confirmed the successful synthesis of the composite.

Figure 2 (a) XRD pattern of Ag@OPLA600, (b) FTIR spectrum of OPL powder (c) FTIR spectrum of Ag@OPLA600

      The SEM image (Figure 3) showed that the surface of the Ag@OPLA600 composite was irregular and contained large pores. Small grains due to Ag metal were also observed. Table 1 summarizes the results of the EDX analysis, which demonstrated that the composite contained Si, Ag, and some impurities in very small quantities, such as K, Ca, Mg, and Cu. The amount of Ag in the as-prepared materials was 19.73%, which was smaller than in the original precursor (63.5%) owing to the presence of other elements in Ag@OPLA600. The presence of other materials that could support metallic Ag, especially silica, was thereby demonstrated.

Figure 3 SEM-images of Ag@OPLA600 (magnification 5000×)

Table 1 EDX analysis result of the as prepared Ag@OPLA600

Element

K(eV)

% Mass

C

0.277

5.12

O

0.525

49.1

Mg

1.253

0.42

Si

1.739

19.45

P

2.013

0.40

Cl

2.621

0.39

Ag

2.983

19.73

K

3.312

2.08

Ca

3.690

2.05

Cu

8.040

0.87

Zn

8.630

0.48

3.2. Composite Application

The Ag@OPLA600 composite was applied as a heterogeneous mediator for the reduction of p-NP with the assistance of hydrogen produced in situ from NaBH4 dissolved in water. The reaction could be followed with the naked eye as follows: When NaBH4 was added to the solution of p-NP, the color changed from transparent to yellow due to the formation of a p-nitrophenolate intermediate. Interestingly, the yellow solution slowly turned transparent again when Ag@OPLA600 was added due to the conversion of p-nitrophenolate to p-AP, as shown in Figure 4.

A UV-Visible spectrophotometry analysis was performed to gain more insight into the transformation of p-NP to p-AP. Figure 4a displays a spectrum of p-NP in water, which shows a clear absorbance at 316 nm. The solution of this compound was clear. Then, the addition of NaBH4 to the p-NP solution resulted in a deprotonation reaction and the formation of nitrophenolate ions (yellow solution), as indicated by a new peak at 401 nm (Figure 4b) (Nurwahid et al., 2022; Hou et al., 2016). The Ag@OPLA600 composite helped deliver hydrogen ions from NaBH4 to the nitro group in the p-nitrophenolate ion structure. The reduction of the nitro group to the amine group in the presence of Ag@OPLA600 occurred within 15 minutes, evidenced by the rapid disappearance of the p-NP peak. The disappearance of the peak of the 4-nitrophenolate ions was accompanied by a new peak at 301 nm due to p-AP (Figure 4c).

The reaction of nitrophenolate ions is known to be kinetically hindered and affected by the repulsion between BH4- and p-nitrophenolate ions; therefore, it does not occur in the absence of a suitable catalyst (Kalantari et al., 2019; Zhang et al., 2018; Baruah et al, 2013; Li and Chen, 2013; Huang et al., 2010). To gain more insight into this reaction, a qualitative analysis was performed using pristine p-NP and p-NP treated with Ag@OPLA600 in the absence of NaBH4, as shown in Figure 5. p-nitrophenol (p-NP) (Figure 5a) was converted to p-nitrophenolate ions (Figure 5b) in the presence of Ag@OPLA600, which is sufficiently basic to promote the ionization of the phenol group in a water environment. However, due to the lack of a hydrogen source in the reaction mixture, the conversion to p-AP did not proceed even when the reaction mixture was left to stand at room temperature for 24 h.  


Figure 4 UV-Visible Spectrophotometry analysis of p-nitrophenol reduction in the presence of NaBH4 and Ag@OPLA600; (a) p-nitrophenol spectrum, (b) p-nitrophenolate ion spectrum, (c) p-aminophenol spectrum

Figure 5 UV-Visible Spectrophotometry analysis of p-nitrophenol reactivity in the presence of Ag@OPLA600; (a) p-nitrophenol spectrum, (b) p-nitrophenolate ion spectrum

        According to the results depicted in Figures 4 and 5, the mechanism shown in Figure 6 was proposed. The reduction of p-NP to p-AP is catalyzed by Ag nanoparticles or supported Ag particles, as previously reported (Al-Namil, Khoury, and Patra, 2019; Rajamanikandan, Shanmugaraj, and Ilanchelian, 2017). In particular, an isotope-labeling study revealed that water (not NaBH4) is the source of hydrogen and that the reaction systems are stabilized by BH4- (Kong et al., 2017). Ag@OPLA600 appears at the bottom of the vial after the reaction is complete, which indicates that the reaction is heterogeneous. The solid material can be easily separated by slowly decanting the solution after the reaction.

Figure 6 The possible general reaction mechanism of reduction of the p-nitrophenol to p-aminophenol in the presence of Ag@OPLA600

This carbothermal degradation of OPL in the presence of AgNO3 is advantageous regarding eco-friendliness and cost-effectiveness. First, a cost-effective fabrication is achieved using a one-pot process to form Ag@OPL600. Second, nontoxic and sustainable raw materials are used, and the generation of chemical waste is minimized. 

Conclusion

A green solid–solid reaction between OPL and AgNO3 at 600 °C produced a solid Ag@OPLA600 composite, which was used as an inexpensive mediator for the reduction of p-NP to p-AP. The results demonstrate the potential of this approach for waste and biomass valorization, especially in the case of palm oil biomass. These findings could pave the way for the development of sustainable and cost-effective technologies for reducing waste and utilizing renewable resources. While further research is needed to fully explore the potential of this approach, the results of this study represent an important step forward in this field.

Acknowledgement

The highest praise goes to Universitas Bengkulu (UNIB) for providing research funds to enable increased collaboration between UNIB researchers and researchers from other countries to run smoothly. This International Research Collaboration research grant is being carried out under a research contract with Lembaga Penelitian dan Pengabdian kepada Masyarakat (LPPM) UNIB, Contract No: 1747/UN30.15/PG/2021 (June 22, 2021). 

References

Abdullaev, M.G., Abdullaeva, Z.S., Klyuev, M.V., Kafarova, S.S., Gebekova, Z.G., 2014.  Kinetics of the production of p-acetaminophenol and p-hydroxyphenylsalicylamide by reductive acylation of p-nitrophenol on palladium-containing anionites. Pharmaceutical Chemistry Journal, Volume 47, pp. 610–611

Adam, F., Ahmed, A.E., Min, S.L., 2008. Silver modified porous silica from rice husk and its catalytic potential. Journal of Porous Material, Volume 15, pp. 433–444

Al-Namil, D.S. Khoury, E.E., Patra, D., 2019.  Solid-state green synthesis of AgNPs: higher temperature harvests larger AgNPs but smaller size has better catalytic reduction reaction. Scientific Reports, Volume 9, p. 5212

Assis, M., Simoes, L.G.P., Tremiliosi, G.C., Coelho, D., Minozzi, D.T., Santos, R.I., Vilela, D.C.B., do Santos, J.R., Ribeiro, L.K., Rosa, I.L.V., Mascaro, L.H., Andrés, J., Longo, E., 2021. SiO2-Ag composite as a highly virucidal material: a roadmap that rapidly eliminates SARS-CoV-2. Nanomaterials, Volume 11(3), p. 638

Bahadur, N.M., Furusawa, T., Sato, M., Kurayama, F., Siddiquey, I.A., Suzuki, N., 2011. Fast and facile synthesis of silica coated silver nanoparticles by microwave irradiation. Journal of Colloid and Interface Science, Volume 355, pp. 312–320

Baruah, B., Gabriel, G.J., Akbashev, M.J., Booher, M.E., 2013. Facile synthesis of silver nanoparticles stabilized by cationic polynorbornenes and their catalytic activity in 4-nitrophenol reduction. Langmuir, Volume 29, pp. 4225?4234

Corro, G., Vidal, E., Cebada, S., Pal, U., Banuelos, F., Vargas, D., Guilleminot, E., 2017. Electronic state of silver in Ag/SiO2 and Ag/ZnO catalysts and its effect on diesel particulate matter oxidation: an XPS study.  Applied Catalysis B: Environmental, Volume 216, pp. 1–10

Cui, J., Liang, Y., Yang, D., Liu, Y., 2016.  Facile fabrication of rice husk based silicon dioxide nanospheres loaded with silver nanoparticles as a rice antibacterial agent. Scientific Reports, Volume 6, p. 21423

Deshmukh, P., Peshwe, D., Pathak, S., 2012. FTIR and TGA analysis in relation with the % crystallinity of the SiO2 obtained by burning rice husk at various temperatures. Advanced Materials Research. Volume 585, pp. 77–81

Flores, J.C., Torres, V., Popa, M., Crespo, D., Calderón-Moreno, J.M., 2008. Preparation of core–shell nanospheres of silica–silver: SiO2@Ag. Journal of Non-Crystalline Solids, Volume 354, pp. 5435–5439

Hermansyah, H., Putri, D.N., Prasetyanto, A., Chairuddin, Z.B., Perdani, M.S., Sahlan, M., Yohda, M., 2019. Delignification of oil palm empty fruit bunch using peracetic acid and alkaline peroxide combined with the ultrasound. International Journal of Technology, Volume 10(8), pp. 15231532

Hou, J., Yu, B., Liu, E., Dong, W., Lu, P., Wang, Z., Yang, V.C., Gong, J., 2016. High efficiency, stable and controllable multi-cores rattle-type Ag@SiO2 catalyst for reduction of 4-nitrophenol. RSC Advances, Volume 6, pp. 95263–95272

Hou, Y.-X., Abdullah, H., Kuo, D.-H., Leu, S.-J., Gultom, N.S., Su, C.H., 2018. A comparison study of SiO2/nano metal oxide composite sphere for antibacterial application. Composites Part B, Volume 133, pp. 166–176

Huang, J., Vongehr, S., Tang, S., Lu, H., Meng, X., 2010. Highly catalytic Pd-Ag bimetallic dendrites. The Journal of Physical Chemistry C, Volume 114, pp. 15005–15010

Jiang, X., Chen, S., Mao, C., 2008. Synthesis of Ag/SiO2 nanocomposite material by adsorption phases nanoreactor technique. Colloids and Surfaces A: Physicochemical and Engneering Aspects, Volume 320, pp. 104–110

Kadam, H.K., Tilve, S.G., 2015. Advancement in methodologies for reduction of nitroarenes. RSC Advances, Volume 5, pp. 83391–83407

Kalantari, K., Afifififi, A.B.M., Bayat, S., Shameli, K., Yousefifi, S., Mokhtar, N., Kalantari, A., 2019. Heterogeneous catalysis in 4-nitrophenol degradation and antioxidant activities of silver nanoparticles embedded in Tapioca starch. Arabian Journal of Chemistry, Volume 12, pp. 52465252

Kartohardjono, S., Pamitran, A.S., Putra, N., 2019. Biomass: from waste to valuable materials.  International Journal of Technology, Volume 10(8), pp. 14651468

Kong, X., Zhu, H., Chen, C.-L., Huang, G., Chen, Q., 2017. insights into the reduction of 4-nitrophenol to 4-aminophenol on catalysts. Chemical Physics Letters,  Volume 684, pp. 148–152

Li, M., Chen, G., 2013. Revisiting catalytic model reaction p-nitrophenol/NaBH4 using metallic nanoparticles coated on polymeric spheres. Nanoscale, Volume 5, pp. 11919–11927

Mulia, K., Adam, D., Zahrina, I., Krisanti. E.A., 2018. Green extraction of palmitic acid from palm oil using betaine-based natural deep eutectic solvents. International Journal of Technology, Volume 9(2), pp. 335344

Musi?, S., Filipovi?-Vincekovi?, N., Sekovani?, L., 2011. Precipitation of amorphous SiO2 particles and their properties. Brazilian Journal of Chemical Engineering, Volume 28 (1), pp. 89–94

Nurwahid, I.H., Dimonti, L.C.C., Dwiatmoko, A.A., Ha, J.-M., Yunarti, R.T., 2022. Investigation on SiO2 derived from sugarcane bagasse ash and pumice stone as a catalyst support for silver metal in the 4-nitrophenol reduction reaction. Inorganic Chemistry Communications, Volume 35, p. 109098

Onoja E., Attan, N., Chandren, S., Razak, F.I.A., Keyon, A.S.A., Mahat, N.A., Wahab, R.A., 2017. Insights into the physicochemical properties of the malaysian oil palm leaves as an alternative source of industrial materials and bioenergy. Malaysian Journal of Fundamental and Applied Sciences, Volume 13(4), pp. 623–631

Onoja, E., Chandren, S., Ilyana, F., Razak, A., Wahab, R.A., 2018. Extraction of nanosilica from oil palm leaves and its application as support for lipase immobilization. Journal of Biotechnology, Volume 283(10), pp. 81–96

Park, H.-H., Zhang, X., Choi, Y.-J., Park, H.-H., Hill, R.-H., 2011. Synthesis of Ag nanostructures by photochemical reduction using citrate-capped Pt Seeds. Journal of Nanomaterials, Volume 2011, p. 265287

Permatasari, N., Sucahya, T.N., Nandiyanto, A.B.D., 2016. Review: agricultural wastes as a source of silica material. Indonesian Journal of Science & Technology, Volume 1(1), pp. 82–106

Peszke, J., Dulski, M., Nowak, A., Balin, K., Zubko, M., Su?owicz, S., Nowak, B., Piotrowska-Seget, Z., Talik, E., Wojtyniak, M., Mrozek-Wilczkiewicz, A., Malarzbf, K., Szade, J., 2017. Unique properties of silver and copper silica-based nanocomposites as antimicrobial agents. RSC Advances. Volume 7, pp. 28092–28104

Prabha, S., Durgalakshmi, D., Rajendran, S., Lichtfouse, E. 2020. Plant?derived silica nanoparticles and composites for biosensors, bioimaging, drug delivery and supercapacitors: a review. Environmental Chemistry Letters, Volume 19, pp. 1667–1691

Prasad, R., Pandey, M., 2012. Rice husk ash as a renewable source for the production of value-added silica gel and its application: an overview. Bulletin of Chemical Reaction Engineering & Catalysis, Volume 7(1), pp. 1–25

Quang, D.V., Sarawade, P.B., Hilong, A., Kim, J.-K., Chai, Y.G. Kim, S.H., Ryu, J.-Y., Kim, H.T., 2011. Preparation of silver nanoparticle containing silica micro beads and investigation of their antibacterial activity. Applied Surface Science, Volume 257, pp. 6963–6970

Rajamanikandan, R., Shanmugaraj, K., Ilanchelian, M., 2017. Concentration dependent catalytic activity of glutathione coated silver nanoparticles for the reduction of 4-nitrophenol and organic dyes.  Journal Cluster Sciences, Volume 28, pp. 1009–1023

Syed, S., Prasad, N.M.N. Dhananjaya, B.L. Kumar, M.K., Yallappa S., Satish S., 2019. Synthesis of silver nanoparticles by endosymbiont pseudomonas ?uorescens CA 417 and their bactericidal activity. Enzyme and Microbial Technology. Volume 95, pp. 128–136

Unglaube, F., Kreyenschulte, C.R., Mejía, E., 2021. Development and application of efficient Ag-based hydrogenation catalysts prepared from rice husk waste. The European Society Journal for Catalys. Volume 13, pp. 1–10

Waterhouse, G.I.N., Bowmaker, G.A., Metson, J.B., 2011. The thermal decomposition of silver (I, III) oxide: a combined XRD, FT-IR and raman spectroscopic study. Physical Chemistry Chemical Physic, Volume 3, pp. 3838–384

Yang, P., Zhang, Y., Huang, B., 2013. Size-adjusted hollow Ag spheres fabricated through reducing Ag2O in-situ. Materials Research Bulletin, Volume 48, pp. 3756–3760

Yudha S, S., Falahudin, A., Asdim, Han, J.I., 2021. In situ preparation of gold–silica particles from a mixture of oil palm leaves and chloroauric acid for reduction of nitroaromatic compounds in water. Waste and Biomass valorization, Volume 12, pp. 3773–3780

Yudha S, S., Falahudin, A., Kaus, N.H.M., Thongmee, S., Ikram, S., Asdim, 2020a. Preliminary synthesis of calcium silicates using oil palm leaves and eggshells. Bulletin of Chemical Reaction Engineering & Catalysis, Volume 15(2), pp. 561–567

Yudha S, S., Robkhob, P., Imbon, T., Falahudin, A., Asdim, 2020b. ZnO-SiO2 and Zn2SiO4 synthesis utilizing oil palm leaves for degradation of methylene blue dye in aqueous solution. The Journal of the Indonesian Chemical Society, Volume 3(2) pp. 94–100

Zhang, J., Yan, Z., Fu, L., Zhang, Y., Yang, H., Ouyang, J., Chen, D., 2018. Silver nanoparticles assembled on modi?ed sepiolite nano?bers for enhanced catalytic reduction of 4-nitrophenol. Applied Clay Sciences, Volume 166, pp. 166?173