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• Consumption of fossil fuel brings about environmental issues, e. g., pollution and emission of greenhouse gas^[1], and alternative energies thus attract extensive attentions for decades. Among the alternative energies, hydrogen is a promising candidate for its cleanness and high energy density^[2]. Biomass, as an abundant renewable resource, can be converted into bio-oil via fast pyrolysis, and can be processed for hydrogen production^[3]. Acetic acid (HAc), as a main component in bio -oil with content up to mass fraction of 33%, has been selected as a feasible hydrogen resource via reforming processes^[4-5]. Within reforming processes, there are steam reforming (SR), partial oxidation (CPOX) and auto-thermal reforming (ATR), while ATR shows potential for its selfheat sustainability^[6-7].

  In ATR of HAc (CH₃COOH+1.44H₂O+0.28O₂→2CO₂+3.44H₂, ΔH=0 kJ·mol^-1), nickel -based catalysts are effective for their reactivity in breaking C—C and C—H bond within acetic acid^[8], but deactivation issues, like coking and sintering, hinder the process; for example, in Ca-Al layered double hydroxides-derived Ni-based catalysts for ATR reaction, as reported by Wang et al.^[7], the layered double hydroxides structure promotes stability of the nickel-based catalyst, but severe coking is still observed with time-on-stream. To address these issues, additives are then introduced in Ni-based catalysts to modify their structures and electronic properties. Manganese, as a transition metal, exhibits redox ability to activate oxygen species for its multi-valances and can be effective to gasify coke precursors in ATR^[9-10]. As reported by Liu et al., within a spinel Co-Mn-Ni-O catalyst, the redox cycle of Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ promotes the transfer of oxygen species in zinc-air batteries^[10]. However, the MnO_x is easy to be sintered which results in poor thermostability in NiMn_4.78O_7.39±δ catalysts, as reported by An et al^[11-12]. To improve thermostability, yttrium can be a promising candidate^[13-14]; for example, nickel-based catalysts with yttrium oxide presents stable reactivity in partial oxidation of methane (POM) at a high temperature (850 ℃) ^[13-14]. Besides, with the addition of manganese into perovskite structure of ABO₃ with nickel and yttrium (NiYO₃), a perovskite-like (Ni, Mn)YO₃ phase can be formed, which is promising to restrain sintering and oxidation within nickel-based catalysts.

  In light of above reports, a series of NiMnY catalysts were synthesized by hydrothermal method in the current work and the calcined catalysts were then tested in ATR of HAc. The characterization techniques, such as X-ray diffraction (XRD), N₂ adsorption-desorption test and H₂-temperature -programmed reduction (H₂-TPR), were carried out to explore the internal relationship within those NiMnY catalysts. To the best of the author's acknowledge, there is no report on the perovskite-like (Ni, Mn)YO₃ catalysts for hydrogen production in ATR of HAc.

  1.   Experimental

  1.1   Catalyst preparation

  A series of NiMnY catalysts with mass fraction of 15% NiO were prepared by hydrothermal method. Chemicals of Y(NO₃)₃·6H₂O, Mn(NO₃)₂ and Ni(NO₃)₂· 6H₂O with stoichiometric ratios as listed in Table 1 were mixed with ethylene glycol monomethyl ether under vigorous stirring, then transferred into a teflon bottle in an oven and remained at 240 ℃ for 12 h. The precipitate was then collected via centrifugation, washed with deionized water and dried at 105 ℃ for 12 h, followed by calcined in air at 850 ℃ for 4 h. The obtained catalysts were named as NY, NMY1-1, NMY6 -1 and NM, respectively, as listed in Table 1.

  Table 1

  

  Table 1.  Composition, BET (Brunauer-Emmett-Teller) specific surface areas (S_BET), BJH (Barrette-Joyner-Halenda) pore volume (V_BJH), average pore size (D_BJH) and particle sizes of the Ni-based catalysts as prepared

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  Catalyst	Molar composition	SBET/(m2·g-1)	VBJH/(cm3·g-1)	DBJH/nm	Particle size measured by XRDa/nm
  					Reduced	Spent
  NY	Ni0.27YO1.77±δ	13	0.024	7.4	30.6	37.1
  NMY1-1	Ni0.39Mn0.61YO3.11±δ	13	0.028	8.2	22.8	27.6
  NMY6-1	Ni1.25Mn4.75YO12.25±δ	12	0.033	10.7	39.9	40.7
  NM	Ni0.21Mn1.02O2.25±δ	5	0.016	12.2	31.2	32.9
  a Calculated by Ni0 at 44.6°

  1.2   Catalytic performance evaluation

  150 mg of catalyst was loaded in a continuous-flow fixedbed quartz tubing reactor and reduced in hydrogen at 700 ℃ for 1 h, then a mixture of HAc/H₂O/O₂/N₂ (molar ratio of 1: 4: 0.28: 3) with GHSV (gas hourly space velocity) of 37 260 mL·g^-1·h^-1 was introduced into the reactor for ATR test. TCD (thermal conduction detector) and FID (flame ionization detector) detectors within gas chromatography (SC-3000B, Chuanyi Instrument) were used to monitor products. The selectivity (S_i) of carbon-containing products (i= CO, CO₂, CH₄, CH₃COCH₃), HAc conversion (X_HAc) and hydrogen yield (Y_H2) were calculated by Eq. (1~3), respectively, where F_{i, in} or F_{i, out} is the molar flow of i species at the inlet or outlet of the reactor, F_{HAc, in} or F_{HAc, out} is the molar flow of HAc at the inlet or outlet of the reactor, n_i is the carbon stoichiometric factor between HAc and carbon-containing products. Besides, the F_{H2, product} represents the molar ratio of hydrogen in the product.

  $S_{\mathrm{i}}=\frac{F_{\mathrm{i}}}{n_{\mathrm{i}}\left(F_{\mathrm{HAc}, \mathrm{in}}-F_{\mathrm{HAc}, \mathrm{out}}\right)} \times 100 \%$

  (1)

  $X_{\mathrm{HAc}}=\frac{F_{\mathrm{HAc}, \mathrm{in}}-F_{\mathrm{HAc}, \mathrm{out}}}{F_{\mathrm{HAc}, \mathrm{in}}} \times 100 \%$

  (2)

  $Y_{\mathrm{H}_{2}}=\frac{F_{\mathrm{H}_{2}, \mathrm{product}}}{F_{\mathrm{HAc}, \mathrm{in}}} \times 100 \%$

  (3)

  1.3   Characterizations

  XRD patterns were recorded via an X-ray diffractometer (Rigaku, Ultima Ⅳ) with a Cu Kα radiation source (λ =0.154 18 nm, 200 mA, 40 kV) from 5° to 80°. N₂ physisorption isotherms were recorded by using a JW-BK112 apparatus at -196 ℃. 50 mg catalyst loaded within a TP -5076 apparatus (Xianquan Instrument) was used to perform H₂-TPR experiments in a volume fraction of 5.0% H₂/N₂ gas flow, while the signal of H₂ was collected by a TCD.

  2.   Results and discussion

  2.1   Characterizations of calcinated catalysts

  The XRD patterns for calcinated catalysts were recorded and showed in Fig. 1. For NY catalyst without Mn, strong peaks of Y₂O₃ appeared and the peaks of NiO were detected as well^[15]. For NMY1 -1 with Mn (Ni_0.39Mn_0.61YO_3.11±δ), Y₂O₃ phase disappeared; meanwhile, there was no Mn-containing species, and perovskite peaks of NiYO₃ were found with peaks shifting to lower angles, as compared with standard NiYO₃ phase, indicating that addition of manganese stabilized the NiYO₃ phase during calcination by partly replacing nickel and could form a perovskite -like (Ni, Mn)YO₃ phase, while nickel species were highly dispersed within the perovskite structure^[16]. For the NMY6-1 with more Mn, the main phase was YMn₂O₅, along with trace of NiO, Mn₂O₃ and NiMn₂O₄. Over the NM catalyst without Y, there were spinel phases NiMn₂O₄/Mn₃O₄ and trace of Mn₂O₃, while no obvious peak of NiO was found.

  Figure 1

  

  Figure 1.  XRD patterns for the calcined catalysts

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  The calcined catalysts were further screened by nitrogen physisorption, as shown in Fig. 2. For the NMY1 -1 catalyst, there was type Ⅱ isotherm, while other three catalysts presented type Ⅲ isotherms. Meanwhile, NMY1 -1 showed a high specific surface area of 13 m² ·g^-1 with a concentrated pore distribution near 2 nm, as shown in Fig. 2B.

  Figure 2

  

  Figure 2.  Adsorption-desorption equilibrium curves (A) and pore size distribution of the catalysts (B)

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  2.2   Characterizations of reduced catalysts

  XRD patterns of the reduced catalysts were shown in Fig. 3. All catalysts presented peaks of Ni^{0 [17]}. Strong peaks of Y₂O₃ were still remained within NY catalyst. The perovskite -like (Ni, Mn)YO₃ species in NMY1 -1 was transformed into Y₂O₃ and MnO, while highly dispersed Ni species with particle size near 22.8 nm was obtained, as shown in Table 1. Similar species were found in NMY6 -1. For the NM catalyst, Mn₃O₄ and Mn₂O₃ were converted to MnO, and the appearance of Ni⁰ can be ascribed to reduction of NiMn₂O₄ species.

  Figure 3

  

  Figure 3.  XRD patterns for the reduced catalysts

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  Figure 4

  

  Figure 4.  H₂-TPR profile for the calcinated catalysts

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  Over H₂-TPR profile of NY, the peak around 382 ℃ was ascribed to surface NiO species, while the other one was related to NiO contacted with Y₂O₃^[15]. For NMY1-1, besides the weak reduction peak of surface Ni species near 374 ℃, a strong peak near 658 ℃ can be attributed to reduction of (Ni, Mn)YO₃, which is consistent with species of Ni, MnO and Y₂O₃ found by XRD^[18]. For NMY6 -1, the broad peak around 490 ℃ can be attributed to the continuous reduction of NiO, Mn₂O₃ and Mn₃O₄ (Mn₂O₃→Mn₃O₄→MnO) and the reduction of Mn⁴⁺ in Y₂MnO₅^{[12, 19]}, while the peak near 657 ℃ can be ascribed to the reduction of spinel (NiMn₂O₄→NiO+MnO→Ni+MnO)^[20]. Over the NM catalyst, similar peaks were found for reduction of NiMn₂O₄ near 672 ℃ and species of NiO, Mn₂O₃ and Mn₃O₄ near 441 ℃.

  2.3   Reactivity in ATR of HAc

  The catalysts were then tested in ATR of HAc at 650 ℃ for 20 h, as shown in Fig. 5. For the NY catalyst, both HAc conversion and hydrogen yield decreased overtime, and finally reached 69.6% and 1.56 mol_H2· mol_HAc^-1, respectively. For the NMY1-1, the HAc conversion remained stable near 100% with the hydro -gen yield at 2.68 mol_H2·mol_HAc^-1, while only trace by -products of methane/acetone were detected and the selectivity to hydrogen was as high as 99.1%. Over the NMY6-1 catalyst, the conversion of HAc was stable at near 100%, but the hydrogen yield was near 2.50 mol_H2·mol_HAc^-1 with by -product of methane near 3.5%. In contrast, for the NM catalyst, the HAc conversion and hydrogen yield decreased to 92.3% and 2.40 mol_H2·mol_HAc^-1, respectively.

  Figure 5

  

  Figure 5.  Reactivity of (a) NY, (b) NMY1-1, (c) NMY6-1 and (d) NM catalysts in ATR of HAc

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  Effect of temperature was further estimated over the NMY1-1 catalyst, as seen in Fig. 6. At 400 ℃, the HAc conversion was only 48.6% with a low hydrogen yield (0.39 mol_H2·mol _HAc^-1), while the selectivity to acetone was as high as 34.1%, indicating ketonization of HAc happened via CH₃CO*+CH₃*→CH₃COCH₃^[15]. As the temperature went up, the hydrogen yield and HAc conversion both increased. The conversion of acetic acid reached near 100% with a high hydrogen production near 2.7 mol _H2·mol _HAc^-1 was recorded at 650 ℃. However, as the temperature further increasing to 700 ℃, the hydrogen production slightly decreased because of the increase of CO/CO₂ via reverse water -gas shift reaction. Therefore, 650 ℃ can be a suitable temperature for ATR of HAc within the NMY1-1 catalyst.

  Figure 6

  

  Figure 6.  Effect of temperature over NMY1-1

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  2.4   Characterizations of spent catalysts

  To investigate the structure variation during ATR test, XRD was carried out over the spent catalysts, as shown in Fig. 7. For the spent NY catalyst, strong peaks of Y₂O₃ remained with a Ni⁰ particle size at 37.1 nm, which was slightly increased from 30.6 nm in the fresh catalyst and can be due to the weak interaction within NY. For NMY1-1, the structure phases of Ni/MnO/Y₂O₃ remained stable, and the smallest particle size at 27.6 nm for metallic nickel among these spent catalysts was observed, as listed in Table 1. As compared to NMY1 -1, the strength of MnO became stronger over NMY6-1, and a Ni⁰ particle size at 40.7 nm was recorded. Besides, the main phase was still MnO over spent NM catalyst, with a Ni⁰ particle size at 32.9 nm.

  Figure 7

  

  Figure 7.  XRD patterns for the spent catalysts

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  2.5   Discussion

  In case of the NY catalyst without Mn, there was weak interaction between Ni and Y₂O₃, and during the ATR reaction, the particle size of metallic nickel increased from 30.6 to 37.1 nm, resulted in sintering and deactivation for hydrogen production. For the NMY1-1 catalyst, perovskite-like (Ni, Mn)YO₃ structure was formed and promoted dispersion of nickel species. With the addition of yttrium, the thermostability of catalyst was enhanced, while the reduction characteristic and interaction of nickel and support were tuned. After reduced in hydrogen at 700 ℃, the (Ni, Mn)YO₃ transformed into thermostable species of Ni-Y-Mn-O with strong interaction among Ni, Y₂O₃ and MnO, while metallic nickel with the smallest particle size within the four catalysts (27.6 nm) was recorded; therefore, a high hydrogen yield near 2.7 mol _H2·mol_HAc^-1 was obtained and remained stable without deactivation, suggesting that the Mn and Y species in (Ni, Mn)YO₃ constrained sintering and oxidation. As comparison, over NMY6-1 catalyst, a large Ni⁰ particle size near 39.9 nm was found, and a hydrogen yield near 2.5 mol_H2·mol _HAc^-1 was recorded with a high selectivity to methane near 3.5% in ATR, as shown in Fig. 5c. For the NM catalyst, Ni species mainly existed in spinel NiMn₂O₄ phase and were partly reduced in hydrogen at 700 ℃, while a low surface area near 5 m²·g^-1 was found as well, resulting in less active sites to convert acetic acid and produce hydrogen, and a low hydrogen yield was then recorded near 2.4 mol_H2·mol_HAc^-1.

  3.   Conclusions

  A series of NiMnY catalysts were prepared by hydrothermal method, and tested in ATR of HAc for hydrogen production. Over NMY1-1 catalyst, the incorporation of manganese led to the formation of perovskite -like (Ni, Mn)YO₃ species, which modify the interaction of (Ni, Mn)YO₃ and inhibit the sintering of nickel. Meanwhile, the addition of manganese accelerates the conversion of carbon precursor while the thermostability of catalyst is enhanced with the incorporation of yttrium. After reduction in hydrogen, the (Ni, Mn)YO₃ phase converted into MnO and Y₂O₃, along with the highly dispersed nickel nanoparticles, providing more active sites to convert HAc and generate hydrogen. Thus, a high and stable catalytic performance in ATR of HAc was obtained with HAc conversion near 100% and hydrogen yield of 2.68 mol_H2·mol_HAc^-1.

  
  
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• 

  Figure 1  XRD patterns for the calcined catalysts

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  Figure 2  Adsorption-desorption equilibrium curves (A) and pore size distribution of the catalysts (B)

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  Figure 3  XRD patterns for the reduced catalysts

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  Figure 4  H₂-TPR profile for the calcinated catalysts

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  Figure 5  Reactivity of (a) NY, (b) NMY1-1, (c) NMY6-1 and (d) NM catalysts in ATR of HAc

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  Figure 6  Effect of temperature over NMY1-1

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  Figure 7  XRD patterns for the spent catalysts

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  Table 1.  Composition, BET (Brunauer-Emmett-Teller) specific surface areas (S_BET), BJH (Barrette-Joyner-Halenda) pore volume (V_BJH), average pore size (D_BJH) and particle sizes of the Ni-based catalysts as prepared

  Catalyst	Molar composition	SBET/(m2·g-1)	VBJH/(cm3·g-1)	DBJH/nm	Particle size measured by XRDa/nm
  					Reduced	Spent
  NY	Ni0.27YO1.77±δ	13	0.024	7.4	30.6	37.1
  NMY1-1	Ni0.39Mn0.61YO3.11±δ	13	0.028	8.2	22.8	27.6
  NMY6-1	Ni1.25Mn4.75YO12.25±δ	12	0.033	10.7	39.9	40.7
  NM	Ni0.21Mn1.02O2.25±δ	5	0.016	12.2	31.2	32.9
  a Calculated by Ni0 at 44.6°

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