Effect of Gd promoter on the structure and catalytic performance of

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Kinetics, Catalysis, and Reaction Engineering

Effect of Gd promoter on the structure and catalytic performance of mesoporous Ni/Al2O3-CeO2 in dry reforming of methane Guo xia Zhang, Yuqi Wang, Xin kai Li, Yu kun Bai, Lan Zheng, Le Wu, and Xiao long Han Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03612 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 23, 2018

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Industrial & Engineering Chemistry Research

Effect of Gd promoter on the structure and catalytic performance of mesoporous Ni/Al2O3–CeO2 in dry reforming of methane

Guoxia Zhang 1, 2, Yuqi Wang 1, 2*, Xinkai Li 1, 2, Yukun Bai 1, 2, Lan Zheng 1, 2, Le Wu 1, 2, Xiaolong Han 1, 2

1

School of Chemical Engineering, Northwest University, Xi ’an 710069, China

2

Shaanxi Provincial Institute of Energy Resources & Chemical Engineering, Xi’an 710069,

China

*Corresponding Author Tel: 13119159985 E-mail: [email protected]

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A few Gd promoted Ni/Al2O3–CeO2 catalysts were synthesized by one pot Pechini method and were employed in dry reforming of methane (DRM). The physicochemical characteristics of the catalyst samples were detected by N2 adsorption–desorption (BET), etc., which confirmed the successful synthesis of Ni/Al2O3–CeO2–x%Gd2O3 with mesoporous structure. The effects of Gd loading amount and calcined temperature on the catalytic ability were investigated and evaluated. Experimental results indicated that the optimal Gd loading amount and calcined temperature for the desired catalysts were 1.2% and 800 ℃ , respectively. Meanwhile the Ni/Al2O3–CeO2–1.2%Gd2O3 achieved its peak CH4 & CO2 conversions of 86% and 94%, respectively. It can be concluded that the Gd promoted catalysts showed an outstanding catalytic performance and stability in DRM, and the proper amount Gd addition tended to weaken NiAl2O4 formation and improved the catalytic activity, which would present a great potential for industrial utilization in the future.

Keywords: Mesoporous structure, Ni/Al2O3–CeO2–x%Gd2O3, Calcined temperature, Dry reforming of methane

2


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1. Introduction Global warming has becoming one of the most challenging environmental issues, which is caused by the greenhouse gases (CO2, CH4 and N2O) from the use of fossil fuels 1; meanwhile, methane and carbon dioxide have the key contributions for greenhouse effect, and gas capture & conversion utilization technology can be used to eliminate their impact on environment 2, 3.From the aspect of the environment and energy, the most effective way is to convert CO2 & CH4 to chemical raw materials through catalytic reforming 4, and dry reforming of methane (DRM) always draws much attention among different methane reforming technologies, which can transform two greenhouse gases into valuable syngas effectively (Eq. (1)). Since the ratio of H2 to CO (H2/CO) for DRM is around 1:1, the syngas is more appropriate for Fischer-Tropsch synthesis of various hydrocarbons products 5, 6. DRM reaction: CH4 + CO2↔2CO + 2H2 (ΔH298 = 247 kJ/mol)

(1)

Methane decomposition: CH4 ↔2H2 + C (ΔH298 = 75 kJ/mol)

(2)

Reverse water gas shift reaction: CO2 + H2↔CO + H2O (ΔH298 = 41 kJ/mol)

(3)

Boudouard reaction: 2CO↔CO2 +C (ΔH298 = –173 kJ/mol)

(4)

Due to the strong endothermic feature of DRM reaction based on the thermodynamics calculations, high reaction temperatures tend to achieve high conversion levels of CH4 and CO2. Whereas, a few side reactions would be accompanied such as methane decomposition (Eq. (2)), Reverse water gas shift reaction (Eq. (3)) and Boudouard reaction (Eq. (4)), leading to the carbon deposition on the surface of catalysts and sintering of the active metals

7, 8.

Hence the DRM catalyst would lose parts of its activity and stability, resulting in the decrease of H2 or CO selectivity and catalytic performance in the long run 9. Therefore, there is necessary to develop and prepare a catalyst with high activity and stability. In recent decades, both Ni-based and noble metal catalysts have been used for DRM. 3



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Although noble metal (Pt, Ru and Rh) based catalysts can provide high activity and anti-coking ability

10, 11.

Ni-based active phase is still the most profitable choice for methane

reforming reactions characterized by its extensive availability and low cost 12. However, the use and development of Ni-based catalysts suffered from the major drawback of highly sensitive to coke deposition, and various methods were employed to reduce carbon deposition and improve activity of catalysts. A small amount of noble metal (Rh, Ru and Pt) or transition metal (Fe, Co) was added to Ni based catalysts

13, 14,

and high specific surface area

mesoporous oxygen supports & additional promoters could be selected to improve the interaction between active metals and supports, resulting in a high dispersion of active metals and favorable anti-coking ability 15, 16. Al2O3 is widely used as support due to its high specific surface area and low cost for Ni-based catalyst, but its poor thermal stability becomes more prone to carbon deposition, thereby causing the deactivation of catalyst during reforming reaction

17.

A lot of scholars

attempted to advance the performance of Ni/Al2O3 catalyst, and introduced serval promoters (Mg, Ca, Ce, Zr, and La) to further improve its catalytic activity during DRM

18, 19.

Meanwhile, CeO2 exhibited an outstanding ability to store/release oxygen and reduce carbon formation

20, 21,

and the prominent performance of Ni/Al2O3–CeO2 catalyst was proved by

many previous studies during DRM. Aghamohammadi et al.

21

studied the effect of CeO2

addition on the reaction activity of Ni/Al2O3, and the results confirmed the performance improvement of DRM catalysts. Han et al.

22

prepared Ni/CeO2–Al2O3 catalyst through

co-precipitation method and used it in DRM, which demonstrated its superior catalytic activity and stability compare to Ni/Al2O3 catalyst. Unfortunately, the NiAl2O4 phase existed in the catalyst behaved negative effect on DRM owning to its own difficult reduction process of Ni, and thus cause a decrease both active sites and deactivation of the catalyst accordingly 4


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23.

Hence, some effective measures should be carried out to avoid NiAl2O4 formation, and the

activity & anti-coking ability of the catalysts still need to be further enhanced. A few studies had reported that adding other metal promoter based on Ni/Al2O3–CeO2 catalyst to synthesis ternary metal supports. Cai et al.

17

investigated Ni/Al2O3–CeO2–ZrO2

catalyst in auto thermal reforming of methane, and it can be concluded that the NiAl2O4 phase was inhibited which may touch off an improvement of catalytic activity relative to Ni/Al2O3 catalyst. Miletic et al. 24 prepared Ni/Al2O3–CeO2–La2O3 catalyst through sol-gel method and evaluated by oxidative steam reforming of methane, confirming the performance promotion compared with the other synthesized nickel catalysts. In our previous study, Yuan et al.

20

prepared different ratios of mesoporous Ce/Zr catalyst via a one-pot surfactant assisted Pechini method, which exhibited highly catalytic activity and anti-coking ability in DRM; Hou et al. 25 introduced La2O3/ZrO2 into Al2O3–MgO to produce mesoporous Ni/Mg0.4Al0.4– La0.1Zr0.1 (O) catalyst, which presented remarkable catalytic performance and stability during DRM reaction. It was also reported that Gd could be employed to improve the Ni dispersion and catalytic stability during DRM

26, 27.

However, few reports were found on the catalytic

reaction about Ni/Al2O3–CeO2 using Gd as promoter. In this work, we introduced a new Gd promoted Ni/Al2O3–CeO2 catalyst, and attempted to investigate the promotion effect on both structure and catalytic performance of Ni/Al2O3–CeO2–x%Gd2O3 in DRM.

2. Experimental 2.1 Catalyst preparation A series of Ni/Al2O3–CeO2–x%Gd2O3 (x=0, 0.8, 1, 1.2, 1.4, 2) catalysts were prepared by one pot Pechini method. Firstly, 1 g CTAB was dissolved in deionized water, then 11 g Al(NO3)3·6·H2O, 0.465 g Ce(NO3)3·6·H2O, 0.814 g Ni(NO3)2·6·H2O and the required amount of Gd(NO3)3·6·H2O were added into the above solution, and the solution was stirred for 0.5 h. 5



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Next, adding citric acid and ethylene glycol into the above solution gradually, and maintained the dosage proportion of metal ions, citric acid and ethylene glycol at 1:2:2 (detailed data displayed in supporting information); the final mixture solution was stirred at 353 K till a gel appeared in a water bath. Secondly, the gel was kept in an oven overnight at 383 K and calcined at 1073 K for 4 h, and then the Ni/Al2O3–CeO2–x%Gd2O3 mixed oxides were obtained. For convenience, Ni/Al2O3–CeO2 catalyst was designated as NAC, and the Gd promoted catalyst was expressed as NAC–x%Gd2O3. The chemical reagents used for the catalysts

preparation

were

listed

as

follows:

Al(NO3)3·6·H2O,

Ce(NO3)3·6·H2O,

Gd(NO3)3·6·H2O, Ni(NO3)2·6·H2O, citric acid, ethylene glycol and metal cation dispersant CTAB (cetyl trimethyl ammonium bromide) were all analytical grade, which were provided by Sinopharm Chemical Reagent Co., Ltd.

2.2 Catalytic activity evaluation The catalytic performance of CH4–CO2 reforming were measured and evaluated in a fixed-bed quartz reactor with 10 mm internal diameter, which was assembled in an electric heating furnace at atmospheric pressure. Typically, 200 mg catalyst (20–40 mesh) was mixed with same size and amount of quartz sand particles to eliminate the bed temperature gradients. Before the reaction, the catalyst was first reduced with 30 mL/min combined flow of H2 (10 mL/min) and N2 (20 mL/min) for 60 min at 700 ℃; next the catalyst bed was increased to its reaction temperature (800 ℃) with the heating rate of 10 ℃/min, and the gaseous mixture of CH4 (35 mL/min), CO2 (35 mL/min), and N2 (20 mL/min) were introduced into the reaction system; Afterwards a cold trap was used to remove the water in the reaction gases, and an online GC system equipped with TCD & 5A molecular sieves was used to analyze the gas product and recorded every 30 min. CH4 & CO2 mainly converted into CO and H2 during the DRM, and the side reactions 6


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may produce a small amount of C2-C4 hydrocarbon and carbon deposition, which could be ignored and the carbon balance (Bcarbon (%)) of DRM was defined as follows: Bcarbon (%) 

[CH4]out + [CO2]out + [CO]out [CH4]in + [CO2]in

× 100

(5)

The CH4 & CO2 conversions (XCH4 & XCO2) and yields of H2 & CO (YH2&YCO) were calculated by the following expressions: XCH4 = XCO2 = YH2 =

[CH4]in–[CH4]out [CH4]in

[CO2]in–[CO2]out [CO2]in [H2]out

2 × [CH4]in (CO)out YCO = [CO2]in + [CH4]in

(6) (7) (8) (9)

2.3 Catalyst characterization Inductively Coupled Plasma Optical Emission Spectrometer (ICP–OES) was performed on an IRIS Advantage equipment to determine the elemental compositions of catalysts. X-ray diffraction analysis (XRD) patterns were detected by a D8 ADVAHCL equipped with Cu, K radiation source (λ=1.54056 Å) at 40 kV voltage and 40 mA current. The 2θ ranged from 20° to 80° at a step size and scan rate of 0.02° and 17.8°/min, respectively. Compared with the Joint Committee of Powder Diffraction Standards database (JCPDS), the crystal structures for both fresh and used catalysts were identified. Transmission Electron Microscopy (TEM) images of the samples were obtained using the JEM-2100F microscope operated at 200 kV. The hydrogen temperature programmed reduction (H2–TPR) profiles of fresh catalysts were collected using Chembet Pulsar TPR/TPD; 10 mg sample was loaded and pretreated at 200 ◦C for 30 min under He (80 mL/min) to remove H2O and other gas impurities, after cooling to 55 ◦C, the reducing gas H2 was introduced at a flow rate of 80 mL/min, and the 7



temperature climbed to 900 ℃ at a heating rate of 10 ℃/min. NH3–TPD was performed on Chembet Pulsar TPR/TPD. 10 mg sample was loaded into the quartz tube and pretreated at 200 ◦C for 30 min under He (rate: 80 mL/min), then the sample was cooled down to 50 ◦C and introduced NH3 (80 mL/min) into the tube for 30 min. Next the sample was purged with He (rate: 80 mL/min) to remove the physically adsorbed NH3 for 30 min, and the temperature eventually climbed to 800 ℃ at a heating rate of 10 ℃ /min. N2-adsorption-desorption (BET) were measured at 77 K using a Micrometrics TriStar II 3020, and the catalysts were degassed using a vacuum at 300 ℃ for 24 h before test. X-ray photoelectron spectroscopy (XPS) analysis of both calcined and reduced catalysts were detected on a Thermo-ESCALAB 250XI spectrometer. Scanning Electron Microscopy (SEM) was conducted under an electron microscope (ZEISS SIGMA) to observe both surface morphology and coke formation for fresh and spent catalysts, respectively. Thermo-gravimetric (TG) analysis was recorded on Discovery TGA to observe the coking deposition on the spent catalysts, and the operation temperature ranged from room temperature to 800 ℃ at a heating rate of 10 ℃/min.

3. Results and Discussion 3.1 Catalyst characterization 3.1. 1 ICP analysis The concentrations of Ni, Al, Ce and Gd inside the samples were measured by ICP– OES, and the detail amount was shown in Table 1. Experimental results confirmed that the actual element concentration was in accord with calculation values. 8


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Table 1. The compositions of the prepared catalysts. Measured by ICP (wt %)

catalyst

Calculation values (wt %)

Ni/Al+Ce+Gd

Ce/Al

Gd/Al+Ce

NAC

12.1

11.3

–

NAC–0.8%Gd2O3

11.9

10.8

NAC–1%Gd2O3

–

NAC–1.2%Gd2O3

Ni/Al+Ce+Gd

Ce/Al

Gd/Al+Ce

12

11

0

0.8

12

11

0.8

–

–

12

11

1

12

11

1.1

12

11

1.2

NAC–1.4%Gd2O3

11.7

11

1.4

12

11

1.4

NAC–2%Gd2O3

12.3

11.2

1.8

12

11

2

3.1.2 XRD analysis The calcined temperature is the key factor which has great impact on the properties of catalyst. It can not only affect the interaction between the active component and support, but also may affect the grain size of nickel crystallite and the lattice structure of catalyst, leading to the deviation of activity and stability of catalysts 28, 29. The XRD was chosen to characterize the phase compositions of catalyst samples. Figure 1 (a) showed the XRD pattern of the NAC–1.2%Gd2O3 catalyst calcined at different temperatures. Based on the JCPDS cards, the diffraction patterns at 2θ=43.6° and 63.6° belonged to AlNi3 (PDF No. 09–0097); the peaks at 2θ=37.3°, 43.4 , 63° and 75.6 assigned to NiO (PDF No. 73–1519); other peaks observed at 2θ=45.6° and 66.6° can be identified as –Al2O3 (PDF No. 50–0741). The result showed that alumina exhibited an amorphous structure and the appeared diffraction peak intensity (NiO &AlNi3) were sharp when calcined temperature ranged within 600 ℃–700 ℃. Moreover, it is worth noting that the crystal transformation occurred when the calcined temperature exceeded 800 ℃, the mesoporous –Al2O3 diffraction peaks appeared while the characteristic peaks of AlNi3 vanished. The catalyst samples displayed relatively large diffraction peaks of NiAl2O4 at 900 ℃ , whereas low peaks could be observed at 800 ℃ , suggesting the less formation of NiAl2O4 phase. It can be concluded from the figure that high calcined

9



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temperature may enhance the formation of NiAl2O4 phase, which was difficult for the reduction process of Ni, leading to the decrease of catalytic activity as a result

30.

For the

spent catalyst, as shown in Figure 1 (c), the crystal sizes of Ni (111) were estimated by Scherrer equation and the calculation results were listed in Table 2. From the table we can see the crystal size would obey the order: NAC–1.2%Gd2O3 600 ℃ >NAC–1.2%Gd2O3 700 ℃ >NAC–1.2%Gd2O3 800 ℃ . It is obvious that the catalyst calcined at 800 ℃ exhibited the smallest and the most favorable Ni crystal size. Figure 1 (b) revealed the XRD profiles for different dosage of Gd promoted catalysts. It is evident from the figure that all samples exhibited characteristic peaks of –Al2O3 (45.5°, 66°, PDF No. 50–0741), AlNi3 (35.2°, 63.6°, PDF No. 09–0097), NiAl2O4 (45°, 65.5°, PDF No. 10–0339) and NiO (37.2°, 43.4°, PDF No.73–1519) species. Moreover, no obvious peak corresponding to Gd2O3 or CeO2 was found from all catalyst samples, which confirmed the well dispersion of Gd2O3 and CeO2 as small particle size of within the XRD resolution limit. From the overview of XRD patterns in Figure 1 (b), the peak intensity of NAC with different Gd2O3 loading had a change compared with that of NAC (without Gd loading). As shown in Figure 1 (b), when the doping amount of Gd was less than 1.2%, the peak intensity of NiO (111) & NiAl2O4 (400) dropped obviously, and the peak of NiO (200) arose with increasing Gd content. This could probably cause the high dispersion of active metals, which inclined to create NiO (200) or non-stoichiometric spinel phase, weakening the formation of NiAl2O4 phase. However, it is apparent that the peak of NiAl2O4 phase became sharper when Gd content exceeded 1.4%, which would form Gd doped ceria solid solution (CexGd1–xO2). Although either CeO2 or Gd2O3 can inhibit the production of NiAl2O4 spinel, the cerium and gadolinium would lose their suppress ability to spinel phase after the formation of CexGd1– xO2,

leading to the formation of large size NiAl2O4 under excessive Gd dosage 10


17, 31.

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Therefore, it can be inferred that proper amount of Gd addition might restrain the NiAl2O4 production and promote the reduction ability of active metals, and thus the prepared catalyst possessed more active sites and its catalytic performance could be increased essentially. For the spent catalysts, as shown in Figure 1 (d), the diffraction peaks of Ni (PDF No. 04–0850) appeared at 44.8° (111), 51.8° (200) and 76.7° (220). Although the three Ni crystal planes could supply active sites for DRM reaction, Ni (111) played a significant role in adsorbing and dissociating of CH4 32. The crystal sizes of Ni (111) were estimated by Scherrer equation and TEM analysis, the calculation results were also listed in Table 2, and the crystal size

would

obey

the

order:

NAC>NAC–2%Gd2O3>NAC–0.8%Gd2O3>NAC–

1.4%Gd2O3>NAC–1%Gd2O3>NAC–1.2%Gd2O3. It is noteworthy that all the catalysts with Gd addition exhibited the relatively small crystal sizes of Ni than NAC, and NAC– 1.2%Gd2O3 presented the smallest Ni crystal size of 11.7 nm, which can promote the catalytic performance remarkably 28. Furthermore, from Figure 1 (d) we can see a relatively broad peak at 2θ=26.1°, corresponding to the graphitic carbon (PDF No. 41–1487), suggesting the formation of carbon deposition during DRM. Meanwhile, the carbon deposition peak tended to be indistinct gradually with increasing Gd loading amount, confirming the favorable effect of Gd on the inhabitation ability of carbon deposition.

11



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Figure 1. The XRD patterns of NAC–x%Gd2O3 catalysts (a) fresh catalyst at different calcined temperature, (b) fresh catalyst with different Gd loading amount, (c) spent catalyst at different calcined temperature, (d) spent catalyst with different Gd loading amount. Table 2. Ni crystal sizes and carbon deposition amount of NAC–x%Gd2O3 catalysts catalyst

Ni crystal

Total amount of carbon

size(nm) a

deposition (mg · g–1 · h–1) b

NAC–1.2%Gd2O3 600

℃

14.8

–

NAC–1.2%Gd2O3 700

℃

12.8

–

NAC–1.2%Gd2O3 900

℃

16

–

NAC

14.7

15

NAC–0.8%Gd2O3

13.3

13.5

NAC–1%Gd2O3

11.8

11.2

NAC–1.2%Gd2O3

11.7

9.5

NAC–1.4%Gd2O3

13.5

13

NAC–2%Gd2O3

14.7

11

a: Estimated by the Scherrer equation. b: Calculated by TGA.

3.1.3 TEM analysis The TEM images and Ni particle size distribution of the reduced catalysts including NAC, NAC–0.8%Gd2O3, NAC–1%Gd2O3, NAC–1.2%Gd2O3 and NAC–2%Gd2O3 were shown in Figure 2 (a)–(i). As shown in the figure, the average particle size of reduced catalyst with Gd addition was between 8.6 nm and 10.5 nm, which was smaller than that of NAC (12 nm), and the XRD patterns (Table 2) also presented the similar result. It can be observed from the figure that NAC–1.2%Gd2O3 behaved the smallest particle size and the most uniform distribution (7 nm–10 nm) than other samples. The TEM results confirmed the evident enhancement effects after Gd addition, which may prevent the agglomeration of active metals and improve particle size distribution as a result 19, 25.

12


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Figure 2. The TEM image and Ni particle size distribution of reduced NAC–x%Gd2O3 catalysts: (a)–(b) NAC, (c)–(d) NAC–0.8%Gd2O3, (e)–(f) NAC–1%Gd2O3, (g)–(h) NAC–1.2%Gd2O3, (i)–(j) NAC– 2%Gd2O3.

3.1.4 H2–TPR analysis To determine the interaction between Ni and the support, H2–TPR analysis was implement subsequently. The H2–TPR analysis of the NCA–x%Gd2O3 samples was carried 13



Page 14 of 33

out, which can present the reduction ability of the catalyst. Different peaks corresponding to H2 reduction processes were observed, and the results were shown in Figure 3. From the figure we can witness the first H2 consumption peak arose at the region of 350 ℃–400 ℃, which can be assigned to the reduction of NiO and small amount of CeO2 33; Moreover, a peak at 500 ℃ –550 ℃ was revealed in the TPR curve of Gd promoted catalyst, which related to the reduction of more nickel oxides species may strongly interact with the support 34.

The peak appeared at 700 ℃ may explained by the reduction of complex NiO or the

non-stoichiometric spinel

17.

In addition, the peak near 820 ℃ could be assigned to the

reduction of NiAl2O4 spinel formed by Ni2+ incorporated in the Al2O3 structure Damyanova et al.

36

35.

reported that the reduction temperature of NiO was lower than 750 ℃,

meanwhile the reduction of NiAl2O4 was above 800 ℃, which was in good agreement with experimental results in current research. It was worth noting that Gd addition amount could change the reduction temperature of the catalyst obviously, indicating the interaction difference between Ni and support. As can be seen from Figure 3, when the Gd doping amount800 ℃ ) may cause sintering or low 21



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reducibility of active metals, leading to the decrease in stability & activity of the catalysts 29. Thus, 800 ℃ was the most suitable calcined temperature for NAC–1.2%Gd2O3 preparation.

3.2.2 Effect of Gd2O3 loading amount on the catalytic activity of NAC Different Gd loading amount of NAC–x%Gd2O3 catalysts were experimentally investigated in DRM reaction at CH4/CO2=1 and 800 ℃ (reaction temperature) ,and their catalytic performances were shown in Figure 10 (c)–(g). It was evident from the figure that CO2 conversation was higher than the corresponding CH4 conversion for each catalyst. The syngas ratio of H2/CO

Effect of Gd promoter on the structure and catalytic performance of

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