1 Title Page Value-Added Humic Acid Derived from Lignite Using

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10099-10110

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Value-Added Humic Acid Derived from Lignite Using Novel Solid-Phase Activation Process with Pd/CeO2 Nanocatalyst: A Physiochemical Study Yafu Tang,† Yuechao Yang,*,†,§ Dongdong Cheng,*,† Bin Gao,‡ Yongshan Wan,§ and Yuncong C. Li§

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National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources; National Engineering & Technology Research Center for Slow and Controlled Release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Daizong Street No. 61, Taishan District, Taian, Shandong 271018, China ‡ Agricultural and Biological Engineering, Institute of Food and Agricultural Sciences (IFAS), University of Florida, Gainesville, Florida 32611, United States § Department of Soil and Water Science, Tropical Research and Education Center, IFAS, University of Florida, Homestead, Florida 33031, United States ABSTRACT: Soil, air, and water pollution caused by lignite is considered a serious environmental problem. Activation methods thus have been developed to extract humic acid from lignite to support the agricultural production as the soil amendment or fertilizer synergist. The traditional activation methods of humic acid from lignite, however, are not environmentally friendly. As the first study, this work developed a novel solid-phase activation method with a Pd/CeO2 nanocatalyst for lignite-derived humic acid. This study analyzed the morphology and structures of the as-synthesized Pd/CeO2 nanocatalyst with various characterization tools. The mechanisms of the Pd/CeO2 nanocatalyst for lignite activation were determined. The Pd/CeO2 catalyst effectively promoted the production of water-soluble humic acids from lignite via KOH solid-phase activation at room temperature. It increased the amount of small molecular active groups and the corresponding small molecules of humic acid. The existence of a strong synergistic effect at the interface sites between Pd/CeO2 nanoparticles and lignite was one of the key factors for the outstanding catalytic performance. In conclusion, this study has great application perspectives for reducing lignite pollution and increasing humic acid utilization by crops, which can improve the sustainability of the environment and agricultural systems. KEYWORDS: Lignite, Catalyst, Activation, Humic acid

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of traditional inorganic fertilizers.7 In particular, humic substances can not only enhance nutrient uptake and utilization in plants through chelating minerals, but also can be the main source of organic carbon to plants through their own degradation.8 However, humic substances in lignite cannot be directly utilized by crops. Activation processes thus are often needed to convert these humic substances into water-soluble forms that further stimulate the growth of plants.9 Physical and chemical activation methods have been developed recently to better utilize humic substances in lignite.6,10 The traditional activation of humic acid from lignite often uses oxidants such as HNO3, H2O2, and KMnO4 to pretreat the samples to increase yield. However, most of these methods use liquid phase reactions and require external heating with a high demand of energy, relatively long reaction time, and high standard of equipment.6 Furthermore, the activation process may not be environmentally friendly and release undesired byproducts into the natural

INTRODUCTION As the lowest rank of coal, lignite is an abundant natural resource and is often piled up as waste in coal mining areas because of its low calorific value and high ash content.1,2 It not only occupies and destroys arable lands, but also causes environmental pollution such as air pollution and surface and underground water contamination.2 The mineralization process of lignite often releases acidic leachates containing heavy metals and trace elements which result in serious contamination of soil and water aquifers.3,4 Therefore, the environmental problems caused by lignite are of great concern and deserve attention. However, recent studies have suggested to use lignite for the development of value-added products including humic and fulvic acids.5 Humic acid contains various types of acidic functional groups that play an important role in regulating many crucial ecological and environmental processes. Previous studies have demonstrated that humic acid can be used as a remediation agent in many environmental applications as well as a soil amendment to improve soil properties.6 Several studies have indicated that humic substances may have stronger effects on plant growth and crop yield than some © 2017 American Chemical Society

Received: June 26, 2017 Revised: August 30, 2017 Published: October 2, 2017 10099

DOI: 10.1021/acssuschemeng.7b02094 ACS Sustainable Chem. Eng. 2017, 5, 10099−10110

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Diagram of the Synthetic Route of Pd-Doped CeO2

environment. These drawbacks have limited the development of activation technologies for utilizing humic substances of lignite. It is thus necessary to develop novel activation methods to utilize lignite that are low in cost, highly effective, and environmentally friendly. With the development of nanotechnology, nanosized catalysts have attracted much attention recently.11 Metal and metal oxide-based composites are recognized as promising catalysts.12 In particular, Pd/CeO2 composites have been used as low-cost and highly efficient catalysts in various applications.13−15 Pd/CeO2 composites as a three-way catalytic convertor are currently used in vehicles to oxidize methane.16−18 In this case, ceria is an ideal support for methane oxidation catalysis because of its high oxygen storage capacity and high oxygen mobility.19 Vayssilov studied model Pt/CeO2 catalysts on electron and oxygen transfer and showed favorable interactions on nanostructured ceria that enhances activity.20 It has been demonstrated that, among the combination of noble metals (e.g., Pt, Ru, Pd and Au) and rare earth oxides (e.g., ZrO2, TiO2, and CeO2) for a single nanostructure for catalytic reactions, the Pd/CeO2 catalyst is considered the most effective one for the water−gas-shift (WGS) reaction.21−23 Tan et al. used CeO2 of different morphologies to support Pd nanocatalysts and found that Pd/CeO2 can fully convert indoor formaldehyde into CO2 at ambient temperature.24 This is partially due to the fact that Ce is a multivalent ion (III and IV), and thus, CeO2 has excellent redox potential with superior oxygen storage and release capacities.12 It has also been demonstrated that the CeO2 (100) surface is highly defective and contains more oxygen vacancies than the (111) and (110) surfaces.25 Additionally, it has been reported that the energy required for the formation of reactive oxygen vacancies on a (100) surface is lower compared with a (111) surface.26 Therefore, the CeO2 (100) surface is favorable for catalytic applications. Findings from previous studies have all pointed out that the Pd/CeO2 nanocomposite is an excellent catalyst that can be used to oxidize and convert methane, formaldehyde, water gas, and other hydrocarbons with complicated carbon structures.17,22,24 Because lignite has high volatile hydrocarbons and is rich in carboxyl, hydroxyl, and phenol functional groups,27 it is anticipated that Pd/CeO2 nanocomposites can also oxidize and activate the humic substances from lignite to promote its applications. However, little research has been conducted on the use of a Pd/CeO2 catalyst to active lignite via a solid-phase reaction at room temperature. The objective of this work was to take advantage of the excellent oxidation catalytic ability of Pd/CeO2 to develop and optimize a solid-phase activation process to produce valueadded water-soluble humic substances from lignite. The activation of lignite often involves the convention of the insoluble

Figure 1. (a) FESEM image of CeO2 catalyst, (b) FESEM image of Pd/CeO2 catalyst, (c) TEM image of CeO2 catalyst, (d) TEM image of Pd/CeO2 catalyst, (e) HRTEM image of CeO2 catalyst, and (f) HRTEM image of Pd/CeO2 catalyst.

calcium and magnesium humic materials into soluble potassium (sodium) salts through KOH treatment. In this work, a novel ball milling process was applied in the solid-phase activation process of lignite to promote the KOH conversion in the presence of Pd/CeO2 catalysts of different morphologies. Various laboratory experiments were conducted to determine the solidphase activation mechanisms. In particular, the catalytic mechanisms of the Pd/CeO2 nanocomposites in surface oxidation of humic substances in lignite were explored. Findings from this work can be used to for the development of low cost, highly effective, and environmentally friendly technologies to reduce the environmental pollution of lignite and produce value-added humic acid from lignite.

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EXPRIMENTAL SECTION

Materials. Lignite (Shanxi, China), Ce(NO)3·6H2O (analytical grade, Aladdin Chemistry Co, Ltd., Shanghai, China), NaBH4 (analytical grade, Aladdin Chemistry Co, Ltd., Tianjin, China), PdCl2 (Aladdin Chemistry Co, Ltd., Shanghai, China), and KOH (analytical grade; Tianjin Kaitong Chemical Industry Co, Ltd., Tianjin, China) were used in this study. The solutions were made with deionized water. Synthesis of CeO2 Catalysts. The CeO2 catalyst was prepared according to the previous methods.12,24 The ceria materials were 10100


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ACS Sustainable Chemistry & Engineering

Figure 2. SEM/EDX analysis of CeO2 and Pd/CeO2 catalyst: (a) SEM image of CeO2 catalyst, (b) EDX spectrum of square region inside image, (c) SEM image of Pd/CeO2 catalyst, and (d) EDX spectrum of square region inside image.

Figure 3. XRD patterns of (a) Pd/CeO2 catalyst and (b) CeO2 catalyst.

Figure 4. Raman spectra of (a) Pd/CeO2 catalyst and (b) CeO2 catalyst.

prepared via the template-free alkaline hydrothermal method.24 In brief, 1 g of Ce(NO)3·6H2O was dissolved in 30 mL of deionized water under stirring conditions. Then, 10 mL NaOH (800 g/L) of solution was added. The mixture was stirred for 30 min at room temperature and then rapidly sealed into a 50 mL autoclave. The hydrothermal treatment was conducted at 473 K for 24 h. The final product was collected by filtration, centrifuged, and rinsed several times with deionized water to remove any possible ionic remnants and then dried and calcined at 337 K for 4 h. Preparation of Pd/CeO2 Nanoparticles. The Pd/CeO2 nanoparticles were synthesized using a wet impregnation method.24 Here, 0.3 g of CeO2 samples were mixed with 100 mL of deionized water and sonicated for 10 min at room temperature. Then, 0.005 g of PdCl2 was added to the above solution, and the reaction mixture was stirred for an hour. Afterward, the pH of the mixture was buffered to neutral using a 5% NaOH solution. Then, 2 mL of 0.005 g/mL NaBH4 solution was added into the suspension under stirring conditions. The final precipitates were thoroughly washed four times with deionized water and ethanol. The resulting Pd/CeO2 nanoparticles were dried at 337 K overnight.

Table 1. N2 Adsorption−Desorption Analysis of Catalysts Catalyst

BET surface area (m2 g−1)

Pore volume (cm3 g−1)

Average pore size (nm)

CeO2 Pd/CeO2

15.34 18.06

0.006 0.007

0.37 0.38

Catalyst Characterization. The size and morphology of the synthesized sample were obtained by the following methods. Fieldemission scanning electron microscopy (FESEM, S-4800, Japan) was conducted at an accelerating voltage of 15 K. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a field emission transmission electron microscope (Tecnai G2 F20, USA) at an a working voltage of 200 kV. A JSM6360LV scanning electron microscope (SEM) (JEOL) equipped with an X-act energy-dispersive X-ray spectrometer (EDX) (Oxford) was used to analyze the morphology and surface elemental composition of the synthesized sample. The powder X-ray diffraction (XRD) analysis of the synthesized sample was performed using an X-ray diffractometer (D8 ADVANCE, Germany) with Ni-filtered Cu Kα radiation 10101


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Figure 5. XPS analyses of (a) Ce 3d of the CeO2 and (b) O 1s of the CeO2. (c) Wide survey scan of Pd/CeO2, (d) Ce 3d of the Pd/CeO2, (e) O 1s of the Pd/CeO2,and (f) Pd 3d of the Pd/CeO2.

Table 2. Fitting Results of O 1s XPS Spectra of CeO2 and Pd/CeO2 Catalysts Sample

Valence state of Ce

Relative percentage (%)

CeO2 CeO2 Pd/CeO2 Pd/CeO2

Ce3+ Ce4+ Ce3+ Ce4+

21.37 78.63 23.57 76.43

Table 3. Fitting Results of O 1s XPS Spectra of CeO2 and Pd/CeO2 Catalysts Sample

Oxygen species

Binding energy (eV)

Relative percentage (%)

CeO2 CeO2 CeO2 Pd/CeO2 Pd/CeO2 Pd/CeO2

Olat (lattice oxygen) Ovan (oxygen vacancy) Oche (chemisorbed) Olat (lattice oxygen) Ovan (oxygen vacancy) Oche (chemisorbed)

529.1 530.4 532.7 529.1 530.5 532.7

55.25 39.6 5.15 51.8 41.1 7.1

Figure 6. H2-TPR profiles of the samples: (a) Pd/CeO2 catalyst and (b) CeO2 catalyst. reported from the spectra are accurate to within 2 cm−1.X-ray photoelectron spectroscopy (XPS) was recorded using a Thermo escalab 250Xi photoelectron spectrometer (USA) with a monochromatic Al Kα (hν = 1486.6 eV) radiation source. The charging shift was calibrated with C 1 s value of adventitious carbon at a binding energy of 284.8 eV. Smart background correction was used for peak fits with the Avantage program. H2 temperature-programmed reduction (H2-TPR) was performed using a ChemiSorb 2720 (USA) apparatus equipped

(λ = 0.1541 nm). The 2θ angular region between 5° and 90° was operated at a scan rate of 4 min−1. Raman spectra of the synthesized sample was conducted by spectrometer equipped with a liquid N2-cooled charge-coupled device detector and a confocal microscope (Renishaw inVia, Britain). A 350 mW near-infrared 785 nm laser was used for analysis under ambient conditions. The wavenumber values 10102


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ACS Sustainable Chemistry & Engineering Table 4. Elemental Composition (%) and Atomic Ratio of RL, AL, ACL, and APL Ultimate analysis (wt %, daf) Different lignite

C

H

O

N

S

H/C

Ash

RL AL ACL APL

39.10 38.95 39.15 39.20

2.59 2.60 2.15 1.63

24.63 25.73 26.53 26.73

0.95 0.94 0.94 0.95

0.99 0.98 0.99 0.98

0.07 0.07 0.05 0.04

30.29 30.15 29.35 29.31

with a TCD detector. TPR was carried out at a heating rate of 5 °C min−1 using 10 vol % H2−Ar mixture and at a flow rate of 15 mL·min−1 to examine the redox behaviors of the samples. The Brunauer−Emmett−Teller (BET) surface area and pore volume of the sample were determined with N2 adsorption−desorption isotherms at −196 °C using Micrometrics ASAP 2020 (USA). Activation of Lignite. Lignite was milled and dried and then sieved to pass an 80 mesh. Solid KOH was used as the activation agent. The experiment included four treatments: (1) raw lignite (RL) as the control, (2) 10% activation agent with lignite (AL), (3) 10% activation agent and 1% CeO2 nanocatalyst with lignite (ACL), and (4) 10% activation agent and 1% Pd/CeO2 nanocatalyst with lignite (APL). The same amount of lignite was used in the above treatments. Each treatment was placed in a ball mill (QM-10-15, China) and was ground for 60 min at a speed of 80 r/min. The experiments were repeated three times. Catalytic Activity Evaluations. After each of the treatments (RL, AL, ACL, APL), elemental compositions of the samples were determined using an elemental analysis instrument (Model 1106, Germany).Total humic acid and water-soluble humic acid in the four treatments were determined with the previously reported methods.28,29 To observe the static grading phenomena of the four treatments, 1 g of each sample was added in 1000 mL of water and placed for 1 year. To further explore the catalytic effect, the humic acid was fractionated into three size ranges: below10,000 Da, 10,000−50 000 Da, and over 50,000 Da with the continuous flow analytical system (DMJ60, China). Each molecular size fraction was determined following the methods of previous studies.30,31 The light absorbance at 465 and 665 nm of the four treated humic acids was obtained using a Spectronic 20 Genesys spectrophotometer on a solution of 3.0 mg of each HA in 10 mL of 0.05 M NaHCO3, and then the E4/E6 ratios were calculated.32,33 To further understand the activation and catalytic mechanisms, water-soluble calcium and magnesium in lignite was determined with an atomic absorption spectrometer (AA-700, Japan). A JSM-6360LV scanning electron microscope (SEM, S-4800, Japan) equipped with an X-act energy-dispersive X-ray spectrometer (EDX) (Oxford) was also used for the morphological survey and elemental identification of the surface of the four treatments. Solid-state NMR spectroscopy (13C-CPMAS-NMR) was performed on a Bruker AV-300 (Germany) equipped with a 4 mm wide-bore MAS probe, and NMR spectra were obtained by applying the following parameters: 13,000 Hz of rotor spin rate, 1 s of recycle time, 1 ms of contact time, 20 ms of acquisition time, 5000 scans. Samples were packed in 4 mm zirconia rotors with Kel-F caps. The four treatment samples were redissolved in 1 mL of hexane and transferred in a glass vial for gas chromatography−mass spectrometry (GC-MS, MSQ8100 GC/MS, China) analysis.

Figure 7. Static classification of RL, AL, ACL, and APL treatments with 1 mg lignite dissolved in 1000 mL of water: (a) on the first day and (b) after one year. Statistical Analysis. Tukey’s multiple range testing was performed to compare the average values among the parameters. The statistical significance was at a probability level of p < 0.05. The data were analyzed using the Statistical Analysis System (SAS) package version 9.2 (2010, SAS Institute, Cary, NC).

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RESULTS AND DISCUSSION Characterization of Catalysts. The synthetic route of Pdloaded CeO2 is shown in Scheme 1. The Pd/CeO2 nanocatalyst was purposefully synthesized by a two-step process, which included alkaline hydrothermal and wet impregnation.12,24 The morphologies of the CeO2 and Pd/CeO2 catalysts are shown in Figure 1. The low-magnification FESEM image (Figure 1a) shows the rough surface with no impregnated particles of the synthesized CeO2 catalyst. However, after Pd nanoparticles were decorated on the CeO2 catalysts, a typical FESEM of the synthesized Pd/CeO2 catalyst in Figure 1b shows numerous small particles attaching to the strip structure. In addition, the TEM and HRTEM images (Figure 1c−1f) show the morphology and particle size of the as-synthesized CeO2 and Pd/CeO2. The TEM image in Figure 1c shows the panoramic of the as-synthesized CeO2. Obviously, the as-synthesized CeO2 sample is composed of numerous cubic morphologies about 30 nm in diameter. Figure 1e is the HRTEM image of the as-synthesized CeO2, exhibiting a clear lattice fringe with an interplanar spacing. The estimated d spacing for the CeO2 phase was found to be 0.27 nm, indicating that the CeO2 nanoparticles preferentially expose the (100) facets.12

Table 5. Total Humic Acid (%), Water Humic Acid, and E4/E6 Ratio and Different Average Molecular Mass of the Samplesa

a

Samples

THA (%)

WHA (%)

E4/E6

TC

50,000

RL AL ACL APL

30.41 34.73 37.68 39.49

26.90 28.26 32.68 34.70

1.14 1.30 1.41 1.46

42.31 42.32 42.29 42.30

0.88 0.99 1.14 1.54

0.33 0.56 0.55 0.95

41.71 41.13 40.32 39.82

THA: total humic acid; WHA: water-soluble humic acid; E4/E6: ratio of light absorbance at 465 and 665 nm; TC: total carbon content. 10103


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Figure 8. SEM images and corresponding particle size distributions of different treatments: (a) RL, (b) HL, (C) HCL, and (d) APL.

According to the findings of previous studies, the as-synthesized CeO2 surface (100) is highly defective and has a high concentration of oxygen vacancies, which is advantageous from a catalytic point of view.34,35 The typical TEM of the Pd/CeO2 catalyst (Figure 1d) indicate numerous small particles attached to cubic morphology, and the average diameter of the Pd/CeO2 is about 18.75 nm after Pd nanoparticles were loaded on CeO2 nanoparticles, which is consistent with the FESEM results. The HRTEM of the Pd/CeO2 catalyst (Figure 1f) also indicates that the Pd nanoparticles were along the edges of the CeO2 with an average particle size of about 1.6 nm. To further demonstrate the composition of the synthesized sample, SEM/EDX analyses of CeO2 and Pd/CeO2 (Figure 2) were used. As shown in Figure 2b, EDX spectra from synthesized

CeO2 show the elements Ce and O, indicating that the as-synthesized sample has high purity. After Pd was added, Pd, O, and Ce are all observed by EDX of Pd/CeO2 (Figure 2d). EDX spectra show that the Pd nanoparticles were highly dispersed on the surface of the CeO2 support, which correspond to HRTEM image results. The XRD patterns of the synthesized CeO2 catalyst are shown in Figure 3b. Several diffraction peaks of synthesized CeO2 were observed at 2θ values of 28.7°, 33.1°, 47.3°, 56.3°, 59.1°, 69.4°, 76.6°, 79.1°, and 88.6°, which correspond to (111), (200), (220), (311), (222), (400), and (331) planes. These results suggest that the synthesized samples had a typical fluorite structure of CeO2,12,24,36 which indicates the as-synthesized sample has high purity. There were no other diffraction peaks 10104


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that the Pd/CeO2 catalyst had a higher BET surface area than CeO2. The pore volumes of CeO2 and Pd/CeO2 samples were 0.006 and 0.007 cm3g−1, respectively (Table 1). Average pore size of CeO2 was 0.37 nm, while Pd/CeO2 had a similar pore size (0.38 nm). To understand the chemical states of the samples, XPS analyses of Ce 3d, O 1s, and Pd 3d were performed (Figure 5). Eight peaks (Figure 5a) were observed in the complex Ce 3d spectra of the CeO2 samples.12 Two tags with V′and U′ indicated the 3d104f1 electronic state of Ce3+,19 and the tags with V′′′, V′′, V, U′′′, U′′, and V indicated the final state of Ce4+.12,19 Similarly, the Ce 3d spectra of Pd/CeO2 samples showed eight peaks (Figure 5d), suggesting that both Ce3+and Ce4+ species existed in the samples. In order to estimate the relative strength of Ce3+and Ce4+ of the CeO2 and Pd/CeO2 samples, the relative percentages of Ce3+ were calculated as follows: Ce3+ concentration= [A(Ce3+)]/[A(Ce3+) + A(Ce4+)]. The contents of Ce3+and Ce4+ of the samples are listed in Table 2. As shown in Table 2, Ce3+ in the Pd/CeO2 nanoparticles (23.57%) was higher than in CeO2 nanoparticles (21.37%), indicating the highly reducible nature of Pd/CeO2. According to previous reports, the adsorbed oxygen species depended on the oxygen vacancies.24 To investigate the status of oxygen species of the samples, the surface oxygen species were further characterized by the O 1s XPS analysis. The XPS spectra of O 1s are shown in Figure 5b and e for CeO2 and Pd/CeO2, respectively. The O 1s XPS data exhibit a main peak and two smaller shoulder peaks. The main peak at the band energy of 529.1 eV can be ascribed to the lattice oxygen (Olat). The peak at the band energy of 530.4−530.5 eV can be attributed to an oxygen vacancy (Ovan). The peak at the band energy of 532.7 eV can be attributed to surface chemisorbed oxygen (Oche).24,42 The relative percentages of Ovan and Oche in Pd/ CeO2 were higher than those in CeO2 (Table 3). The improvement of Ovan can be attributed to the increased content of Ce3+ in Pd/CeO2. Meanwhile, to further determine the chemical compositions, the wide scan survey of the Pd/CeO2 catalyst over a range of 0−1200 eV was attained (Figure 5c). The main spectra peaks centered at ca. 279.49, 334.44, 527.03, and 915.09 eV, which correspond to C 1s, Pd 3d, O 1s, and Ce 3d, respectively, were observed, confirming the successful synthesis of the Pd/CeO2 catalyst. To observe the chemical states of the Pd element on the surfaces of Pd/CeO2, XPS analysis of Pd 3d was performed (Figure 5f). Pd/CeO2 displayed two peaks at 335.6 and 337.6 eV, indicating the typical characteristics of metallic Pd and Pd oxide, respectively.24 Those may affect the oxidation and reduction properties of the catalyst. To investigate the reducibility of CeO2 and Pd/CeO2 catalysts, H2-TPR was employed (Figure 6). Obviously, the reduction profile of the two catalysts contains two main peaks. In general, the peaks at high temperature associated with a reduction in lattice oxygen of CeO2 and the low temperature peaks revealed the reduction of Pd species in the oxidized state.24 It is clear from Figure 6 that the CeO2 catalyst showed two reduction peaks centered at 450 and 558 °C and the Pd/ CeO2 catalyst showed two reduction peaks centered at 309 and 463 °C, which indicated that the reduction peaks shift to low temperature after Pd is loaded. The result further indicated that Pd loading can improve the reducibility of a CeO2 catalyst, which may benefit the catalytic activity for the humic acid of lignite. Catalytic Activity of Catalysts for Humic Acid of Lignite. To test the catalytic ability of synthesized CeO2 and

Figure 9. 13C-CPMAS-NMR spectra of solids at different treatments: (a) APL, (b) ACL, (c) AL, and (d) RL.

Table 6. Distribution of Carbon in RL, AL, ACL, and APL Determined by Solid-State CP/MAS13C NMR Spectroscopy Area (%) Functional groups

Chemical shift (ppm)

RL

AL

ACL

APL

Aliphatic C O-Alkyl −CH2OH Aromatic C Carboxyl C Carbonyl C

10−50 50−100 100−110 110−160 160−200 200−220

0.15 8.30 0.00 83.10 8.33 0.12

0.15 11.75 0.00 77.1 10.92 0.08

0.225 18.18 0.00 66.9 14.62 0.08

0.15 7.75 0.05 78.9 13.14 0.06

in the XRD patterns of the synthesized Pd/CeO2 (Figure 3a) compared with those of synthesized CeO2, even though many Pd nanoparticles were loaded. This is probably because the Pd nanoparticles were highly dispersed on the surface of CeO2 support.37 Figure 4 shows Raman spectra of two synthesized catalysts. A sharp Raman peak of synthesized CeO2 catalyst was observed at near 464 cm−1 indicating the presence of the Raman active F2g mode of fluorite-structured CeO2 with the space group Fm3m (Figure 4b).38−40 This result is consistent with findings of the XRD analysis. However, the Raman peak of the Pd/CeO2 catalyst was observed at 459 cm−1, which was shifted toward lower wavenumbers after the addition of Pd nanoparticles (Figure 4a). In addition to the strong phonon mode at 446 cm−1 from the F2g symmetric stretching of the Ce−O bond, a signal related to oxygen vacancy was observed at 595 cm−1 for the Pd/CeO2 catalyst, suggesting the existence of a strong interaction between Pd and CeO2, enabling changes in the Ce−O bond. This shift is most likely due to the lattice expansion induced by oxygen vacancy generation.41 Generally, Raman band ceria-based samples are observed in the range of 580− 600 cm−1, corresponding to the presence of oxygen vacancy defects in CeO2.24 The results here suggested that the sample of synthesized Pd/CeO2 catalyst had defect sites and was rich in oxygen vacancies, corresponding to the XRD results. The N2 adsorption−desorption measurements of the two synthesized samples were applied to attain Brunauer−Emmett− Teller (BET) surface areas, pore size, and pore volume. The obtained BET surface areas were 15.34 and 18.06 m2 g−1 for CeO2 and Pd/CeO2 samples, respectively (Table 1), indicating 10105


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Figure 10. Total ion charomatograms of thermochemolysis products: (a) RL, (b) AL, (c) ACL, and (d) APL.

As shown in Table 4, O contents of APL were moderately higher than those of RL, indicating that the activation process increased the O contents of the function groups or compounds of lignite. This was probably due to the catalystreleased oxygen and water in the process of activation and oxidation. Previous studies have shown that the ratio of H/C is inversely proportional to the content of the aromatic structure.44 In this work, the H/C ratios were 0.07 for RL, 0.07 for AL, 0.05 for ACL, and 0.04 for APL, showing a decreasing trend due to the activation process. This result suggests that the treatment with the Pd/CeO2 catalyst (APL) produced more aromatic structures and thus a more stable structure of humic acid.

Pd/CeO2, they were applied to activate lignite with four types of treatments. The elemental analysis results of RL, AL, ACL, and APL are shown in Table 4. Generally, the commercial oxidation processes that produce humic acid from lignite are mainly through liquid phase oxidation using oxidants such as hydrogen peroxide (H2O2), nitric acid (HNO3), and potassium permanganate (KMnO4).43 Some studies have suggested that carbon content has no significant difference for nitric acid and hydrogene peroxide oxidation for the humic acid of lignite.9 Table 4 shows no significant difference in carbon content for RL, AL, ACL, and APL, which indicates that the results are consistent with previous results. This phenomenon indicated that the total content of humic acid was the same for different treatments. 10106


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indicating all treatments contained oxygen and nitrogen, consistent with the elemental analysis data (Table 4). A weak signal at 100 ppm for APL was also observed, indicating there was triple bond in aliphatic carbon region (0−110 ppm). In the aromatic region (110−160 ppm), four treatments had strong peaks at 128 ppm, which belong to aromatic units in ligninite.32 Similarly, the peak at 173 ppm was observed in all four treatments, indicating there was carboxyl C (160−190 ppm). The relative intensities of the different carbon shifts differed for the four samples. The spectra data were analyzed quantitatively, according to the literature,48,49 which divided the spectra into three regions as in Table 6. The results indicate that APL contained more carboxyl carbon and less aliphatic carbon than other treatments. The results suggest that the Pd/CeO2 catalyst and lignite produced an oxidation reduction reaction at the interface. The Carom/Calip values for different treatments revealed that the change occurred during in the process of activation: the spectrum of RL was dominated by aromatic carbon, while in the spectrum of AL, the percentage of aromatic carbon decreased and the percentage of aliphatic and carboxyl carbon increased. The GC-MS spectrometry data are shown in Figure 10 and Table 7. A few peaks were observed for RL, including

The contents of total humic acid, water humic acid, E4/E6 ratio, and different average molecular mass (including low, medium, and high average molecular mass) are listed in Table 5. The contents of total humic acid and water humic acid for RL, AL, ACL, and APL increased with increasing O content, suggesting that Pd/CeO2 showed excellent catalytic activity for lignite activation. It is evident that the contents of total humic acid and water humic acid of APL increased by 30% and 29%, respectively, compared with RL. Previous studies have suggested that the E4/E6 ratio decreases with increasing content of condensed aromatic rings.27 As shown in Table 5, the E4/E6 ratio of RL, AL, ACL, and APL showed an increasing trend, suggesting that APL contained more low average molecular mass than the other treatments.1 The humic acid was further fractionated into three sizes by a continuous flow analytical system: low (50,000 Da) average molecular mass. APL contained more low and medium average molecular mass than the other three (Table 5), in agreement with the E4/E6 ratio analysis. Figure 7 shows the static classification (settling) of RL, AL, ACL, and APL. On the first day, four treatment solutions were all black without notable separation phenomenon. After one year, the four treatment solutions were very different: RL showed obvious separation phenomenon. AL had partial separation, and the solution color became brown. ACL and APL showed no obvious separation phenomenon, but the color of the APL solution was darker than that of ACL (Figure 7b). The results indicate that APL was more stable than the other treatments, confirming that it contained more small molecules and water humic than the others. These results further suggest that Pd/CeO2 was an excellent catalyst for the activation of lignite. The SEM images and the corresponding particle size distribution of lignite also supported thet good catalytic ability of Pd/CeO2 (Figure 8). The SEM images showed that coarse and loose surfaces appeared on the four different treated samples. In addition, the particles of lignite became smaller and the surface of lignite became smoother after the activation, particularly for the APL treatment (Figure 8d). The particle sizes of RL and HL were mainly in the ranges of 800−1200 and 150−300 μm, respectively. However, the particle sizes of ACL and APL were mainly in the range of 50−150 μm. These results confirmed that the Pd/CeO2 catalyst promoted activation of lignite to produce smaller humic molecules. Mechanism of Catalytic Activation. To further explore the mechanisms of synthesized CeO2 and Pd/CeO2 catalysts in the process of activation of lignite, solid-state 13C NMR and the GC-MS analyses were conducted to analyze the reaction intermediates.36 The solid-state 13C NMR spectra of different treatments are shown in Figure 9, and the relative distribution of signal areas for different treatments (which revealed related molecular composition of lignite) are reported in Table 6. The results show that the humic acid of the lignite included mainly aliphatic carbon, aromatic carbon, and carbonyl carbon because all the samples showed peaks in the resonance areas of aliphatic carbon (0−110 ppm), aromatic C (110−160 ppm), and carbonyl C (160−220 ppm). The spectra also had peaks at 30 ppm in the alkyl C (0−50 ppm) range, which was likely due to aliphatic carbons in alkyl chains.45,46 In the O-alkyl C (50−110 ppm) region, signals for aliphatic carbon substituted by oxygen and nitrogen are usually observed.47 In this region, a weak signal at 73−75 ppm for all the samples were observed,

Table 7. Thmermochemolysis Products Released from Different Treated Lignite RT

Compound

7.16 7.27 8.2 8.96 9.27 9.37 9.94 10.55 11.26 11.3 11.45 12.65 13.08 17.61 19.58 21.65 25.54 27.41

Cyclopropane, pentyl Cyclopropane, pentyl Cyclohexyldimethoxymethyl 1-Methoxydecane 1-Decanol Pentadecane Cyclooctasiloxane Dodecanal Cycloheptasiloxane, tetradecamethyl Phenol, 2,4-bis(1,1-dimethylethyl) 2,4-bis(1,1-Dimethylethyl) Cyclooctasiloxane, hexadecamethyl Hexadecane, 2,6,11,15-tetramethy Hexasiloxane Cyclononasiloxane, octadecamethyl Phenol Octasiloxane Octasiloxane

cyclopropane, 1-methoxydecane, 1-decanol, cyclooctasil-oxane, and phenol (Table 7). Similar peaks were found for AL treatment. However, a considerable number of small molecule structures were found for ACL and APL, including pentadecane, dodecanal, octasiloxane, and hexasiloxane (Figure 10). The GC-MS spectrometry further confirmed that the Pd/CeO2 catalyst was highly effective in lignite activation, oxidizing it into a number of small molecular compounds. The water-soluble calcium and magnesium contents of different treatments are shown in Figure 11. The soluble calcium and magnesium contents of RL are the lowest because most of them are bound to humic acid. Since K+ can replace Ca2+and Mg2+ through KOH activation, the contents of soluble calcium and magnesium were higher for the other three treatments. It is notable that the water-soluble calcium and magnesium of ACL and APL treatments were significantly higher than that of AL, 10107


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lignite to yield the total humic acid and water humic acid product. Generally, the oxidation of humic acid of lignite usually results in the increase in acidic functionality. Based on all the analyses, the governing mechanism can be summarized as follows (Figure 12): Calcium- and magnesium-combined humic acid in the lignite was turned into water-soluble humic acid by the alkaline conditon under the grinding mill process. Therefore, the content of total humic acid and water-soluble humic acid for RL were the lowest compared with other treatments (Table 5). The oxidation processes were related to cleavage of the condensation linkages, leading to the formation of humic acids with abundant functionality and smaller fragments.43 However, the oxidation processes of catalysts indicate the formation of lower molecular weight during the solid-phase activation condition (Figure 10). Usually, catalytic efficiency will be improved effectively when the catalyst particles are in the nanometer range. HRTEM (Figure 1) demonstrates a high dispersion of Pd along the edges of CeO2 with a nanometer range.12 This enhanced the humic acid content of the activated lignite. The catalysts, especially Pd/CeO2, promoted the oxidation processes at the interfaces and thus produced more complex compounds and small molecules from the activated lignite. When the lignite is in contact with a catalyst, adsorbed oxygen could react with complex compounds from the activated lignite. In this study, the result of XPS (Figure 5) indicated the existence of Ce3+, but the amount of oxygen vacancy increased after Pd nanoparticles dispersed on the surface of a CeO2 support (Table 3). Therefore, the vacancies in Pd/CeO2 composites improve the adsorption of oxygen, leading to an increase in the oxidation efficiency of humic acid of lignite. The complex compounds from the activated lignite were mainly aliphatic aromatic compounds and some carboxyl. The small molecules under alkaline conditions were further converted into watersoluble humic acid, thereby increasing the stability of the system.

Figure 11. Water-soluble Ca2+ (a) and Mg2+ (b) in RL, AL, ACL, and APL treatments.

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CONCLUSIONS In the condition of solid-state activation, the Pd/CeO2 catalyst oxidized the lignite, and total humic acid and water humic acid from APL increased by 30% and 29%, respectively, compared with RL. The Pd/CeO2 catalyst showed excellent catalytic ability even at room temperature. The excellent performance of the catalyst arises from its high concentration of oxygen vacancies, which play an important role in the activation of lignite. It is important that the whole process of oxidation of humic acid of lignite by solid-state activation methods does not produce air pollution. Moreover, the solid-state activation method consumes less energy compared with the traditional liquid activation methods. Therefore, compared with the traditional activation of the humic acid process, the present study showed promise. From a perspective of application, this study suggests that the Pd/CeO2 catalyst is a promising candidate for the synthesis of value-added humic acids from lignite via solid-state activation,

indicating the effectiveness of the catalysts. For APL, the content of water-soluble magnesium was greater than in ACL, suggesting the Pd/CeO2 was a better catalyst for lignite activation. Overall, the Pd/CeO2 catalyst significantly improved the water solubility of calcium ion and magnesium (Figure 11). This was probably because the Pd/CeO2 catalyst promoted the lignite activation to produce more small particles and small molecular groups and thus released more calcium and magnesium. All the findings from this work indicated that the catalyst significantly improved the water-soluble humic acid content of the activated lignite. The oxidation capacity of CeO2 increased after the addition of Pd. Therefore, the oxidation efficiency of Pd/CeO2 to lignite was significantly improved. Most importantly, a high selectivity toward the water-soluble humic acid product (34.7%) was found for the APL sample at the KOH solid-phase activation condition. A possible mechanism was found in the literature43 for the oxidation of humic acid of

Figure 12. Schematic representation of the catalytic activation mechanism. 10108


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which has great potentials to reduce pollution and improve environmental sustainability.

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AUTHOR INFORMATION

Corresponding Authors

*Yuechao Yang. Phone: 86-538-824 2900. E-mail: [email protected]. *Dongdong Cheng. Phone: 86-538-824 2900. E-mail: [email protected]. ORCID

Yuechao Yang: 0000-0003-4045-0252 Bin Gao: 0000-0003-3769-0191 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by Shandong Province Key R&D Program (2017CXGC0306), Shandong Agricultural Innovation Team (SDAIT-17-04), Taishan Industrial Experts Programme (LJNY201609), Great Innovation Projects in Agriculture of Shandong Province (Grant [2013] 136), and National Key Research and Development Program of China (2016 YFD0201105).

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1 Title Page Value-Added Humic Acid Derived from Lignite Using

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