A mesh-type structured anodic alumina supported Cu/Ni bi-functional catalyst was developed for steam reforming of dimethyl ether (SRD). It was found that the Cu/Ni/γ-Al 2 O 3 /Al catalyst had remarkable catalytic activity and stability, but a high CO selectivity. Therefore, a multi-functional catalyst was proposed by metals (Fe, Zn, or La) addition to inhibit CO formation during the SRD process. The results show that promoter Fe can improve the Cu dispersion and decrease the reduction temperature of catalyst, and CO selectivity was minimized from 27% to around 3%. However, the addition of Zn and La only can decrease the CO selectivity to 12%. Furthermore, there was an excellent synergetic effect between Cu/Ni/γ-Al2O3 and Fe over the Cu/Ni/Fe/γ-Al 2 O 3 /Al catalyst by evaluating catalytic performance of catalysts with different packing structures. And the synergetic mechanism of the active components (γ-Al 2 O 3 , Cu or Cu 2 O, and Fe 3 O 4 ) for SRD and CO in suit removal was proposed. Finally, a 400-h durability test was carried out and the results show that the Cu/Ni/Fe/γ-Al2O3/Al catalyst had an excellent stability with a 100% DME conversion and low CO selectivity.
As an alternative energy source, hydrogen has received much attention due to its higher combustion efficiency, no-polluting characteristics and potential applications in several conversion processes. Moreover, it can be applied directly in transport and stationary power generation or via a hydrogenated intermediate (H2 carrier) that can be transformed on-spot into H2 by reforming for use in hydrogen fuel cells [
DME hydrolysis:
CH 3 OCH 3 + H 2 O → 2 CH 3 OHΔHr = + 37 kJ ⋅ mol − 1 (1)
MeOH steam reforming (SRM):
CH 3 OH + H 2 O → 3 H 2 + CO 2 Δ Hr = + 49 kJ ⋅ mol − 1 (2)
DME steam reforming (SRD):
CH 3 OCH 3 + 3 H 2 O → 6 H 2 + 2 CO 2 Δ Hr = + 135 kJ ⋅ mol − 1 (3)
It is generally acknowledged that DME hydrolysis to methanol proceeds over a solid acid catalyst, such as HZSM-5, H-mordenite, ZrO2, and γ-Al2O3 [
Meanwhile, a structured anodic alumina catalyst was proposed due to its excellent flexibility and heat endurance, which is very promising in the application of micro-reactor [
In this study, a mesh-type Cu/Ni/γ-Al2O3/Al catalyst was prepared and applied in SRD. Different composition (Fe, Zn, or La) was evaluated by adding to the Cu/Ni/γ-Al2O3/Al catalyst. The series of catalysts were characterized by using N2O pulse chemisorption, XRD, H2-TPR and BET to analysis the effects of different promoters on the catalysts’ metallic dispersion and crystallite size. Meanwhile, the catalytic activity evaluations were carried out to compare CO in situ removal performance over different catalysts. Furthermore, the effects of packing structures on the catalytic performance were investigated. Finally, a 400-h stability evaluation was carried out over the optimized Cu/Ni/Fe/γ-Al2O3/Al catalyst.
The flow scheme of the preparation of multi-catalysts was shown in
With the anodic alumina substrate, a series of Cu/X/γ-Al2O3/Al (X = Ni, Zn, La or Fe) catalysts were prepared through impregnation method. The monolithic γ-Al2O3/Al substrates were impregnated in an aqueous solution of Cu (II) nitrate or X nitrate (X = Ni, Zn, La or Fe) under ambient conditions. The prepared catalyst was then dried naturally, and calcined in air at 500˚C for 4 h.
The surface morphology of catalysts was measured by a scanning electron microscope (JSM-6360LV, JEOL).
The specific surface area and pore structure of catalysts was examined using adsorption method by a physisorption analyzer (ASAP 2020-M, Micromeritics). The specific surface area was calculated by the BET method, and the BJH method was used to determine the pore volume and average pore diameter.
The metal loading was analyzed by an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, 725ES, Agilent) and is reported here based on the quantity of the surface alumina layers.
An X-ray diffraction (D/max 2500 VB/PC, Rigaku) was applied to characterize the crystal structure of catalysts, and the average grain size of metal species was calculated through Scherrer’s equation. The tested powder was scratched from the surface of the catalysts.
H2 temperature-programmed reduction (H2-TPR) analysis was performed in a Micromeritics ChemiSorb 2720 apparatus. Each sample including 100 mg catalyst was outgassed at 160˚C for 40 min under He flow (25 ml/min), then cooled to room temperature. Afterward, the temperature was raised to 920˚C with a ramping rate of 10˚C/min under a 10 vol% H2/Ar flow atmosphere.
The dispersion of Cu and exposed copper surface area (SCu) were measured by selective N2O chemisorption method in a chemisorption analyzer (ChemiSorb 2720, Micromeritics). First, catalysts were reduced in a 10 vol% H2/Ar stream at 400˚C for 2 h. Subsequently, the samples were purged with He at 400˚C for 20 min to remove hydrogen species adsorbed on the surface, and subsequently cooled to 50˚C. A flow of 20% N2O/N2 (30 mL/min) was used to oxidize surface copper atoms to Cu2O at 50˚C for 0.5 h. The reactor was flushed with He to remove the oxidant. Finally, another TPR experiment was performed in 10% H2/Ar at a flow rate of 30 mL/min. The copper surface density is 1.46 × 1019 copper atoms per square meter. The copper dispersion (DCu) was defined as the ratio of copper atoms on the surface of the catalysts to the total amount of copper atoms in the catalyst.
X-ray photoelectron spectroscopy (XPS) was obtained with a Thermo ESCALAB 250 spectrometer with an Al Kα radiation. The binding energies scale (BEs) of the spectrometer was calibrated using the carbonaceous C 1s line at 284.6 eV.
Two different reactors were applied to quantify the catalytic performances of the catalysts under atmospheric pressure. The mesh-type catalysts were cut into small pieces (2 mm*2 mm), mixed with Raschig rings (20 - 40 mesh), then packed into a fixed-bed reactor (I.D. 12 mm). The catalyst charge was 3 g. And no H2 pre-reduction treatment was conducted prior to the evaluation of catalysts.
CO, CO2, and N2. DME conversion and selectivity of products are defined as follows:
DMEconversion = F D M E , i n − F D M E , o u t F D M E , o u t × 100 % (4)
Selectivityofproducts = F i , o u t ∑ F i , o u t × 100 % (5)
where FDME,in and FDME,out are the influent and effluent molar flow rates of DME, respectively, Fi,out are the molar flow rates of gaseous products (H2, CO, CO2, CH4).
The chemical composition, surface area and pore properties of different catalysts were summarized in
In order to study the impacts of promoters on the crystal phases, the diffraction patterns for the Cu/Ni/γ-Al2O3/Al catalysts loaded with different metals were presented in
H2-TPR was used to analyze the reduction properties of catalysts loaded with different metals. From
Catalyst | X loadinga (wt%) | Cu loadinga (wt%) | Ni loadinga (wt%) | SBET (m2/g) | Vp (mL/g) | Dp (nm) | S Cu b (m2/g) | S Cu b (%) |
---|---|---|---|---|---|---|---|---|
γ-Al2O3/Al | - | - | - | 85 | 0.16 | 5.6 | - | - |
Fe/γ-Al2O3/Al | Fe12.5 | - | - | 79 | 0.15 | 5.9 | - | - |
Cu/Ni/γ-Al2O3/Al | - | 12.8 | 2.6 | 73 | 0.14 | 6.1 | 17.6 | 7.8 |
Cu/Ni/Fe/γ-Al2O3/Al | Fe12.5 | 12.9 | 2.7 | 72 | 0.13 | 6.3 | 19.9 | 8.9 |
Cu/Ni/Zn/γ-Al2O3/Al | Zn12.5 | 12.7 | 2.5 | 68 | 0.11 | 6.4 | 17.3 | 7.6 |
Cu/Ni/La/γ-Al2O3/Al | La12.5 | 12.8 | 2.6 | 55 | 0.06 | 6.2 | 16.1 | 6.8 |
Cu/Ni/Fe/γ-Al2O3/Al catalyst, the reduction peak of CuO decreased to 225˚C from 251˚C over the Cu/Ni/γ-Al2O3/Al catalyst. On the contrary, the addition of Zn and La to Cu/Ni/γ-Al2O3 made the reduction temperature increase to 287˚C and 324˚C, respectively. The results were consistent with
decreased the active sites of Cu, and increased the reduction temperature of CuO. The Cu/Ni/Zn/γ-Al2O3/Al catalyst had a lower DME conversion than the Cu/Ni/γ-Al2O3/Al catalyst due to a lower dispersion of Cu (as shown in
CO + H 2 O ↔ CO 2 + H 2 (6)
CH 3 OH → CO + 2 H 2 (7)
CH 3 OCH 3 → CH 4 + CO + H 2 (8)
CO 2 + 4 H 2 ↔ CH 4 + 2 H 2 O (9)
CO + 3 H 2 ↔ CH 4 + H 2 O (10)
The catalytic performance of different packing structures of Fe/γ-Al2O3/Al and Cu/Ni/γ-Al2O3/Al was evaluated to investigate the synergetic effect between Cu/Ni/γ-Al2O3 and Fe on the Cu/Ni/Fe/γ-Al2O3/Al catalyst. The three packing structures included supported Cu/Ni/Fe/γ-Al2O3/Al catalyst, layered Cu/Ni/γ-Al2O3/Al (former) and Fe/γ-Al2O3/Al (later) and mechanical mixture of the two kinds of catalysts. For convenience, “supported Cu/Ni/Fe”, “layered Cu/Ni+Fe” and “mixed Cu/Ni*Fe” represents above three packing structures, respectively. And “supported Cu/Ni” represents Cu/Ni/γ-Al2O3/Al. The results were showed in the
To verify the change that occur in the oxidation of active components during SRD, XPS measurements were further performed on the Cu/Ni/Fe/γ-Al2O3 catalyst before and after above activity evaluation. “A-catalyst” and “B-catalyst” are used to represent the fresh and spent catalyst, respectively. The photoelectron peaks of the Cu 2p were presented in
by the characteristic Cu2+ shakeup satellite peaks (938 - 945 eV) [
Catalyst | Cu0/Cu+ (%) | Cu2+ (%) | Fe2+/Fe3+ |
---|---|---|---|
A-catalyst | 34.5 | 65.5 | 0.45 |
B-catalyst | 91.1 | 8.9 | 1.12 |
34.5% to 91.1% in the amount corresponding to Cu+/Cu0 species was found. This observation confirms that the CuO species were reduced to metallic Cu+ and Cu0 species such as Cu2O and Cu, which further speculated the Cu+ and Cu0 species all are the active phase for the SRM process [
The photoelectron peaks of the Fe 2p were presented in
From the above XPS and catalytic performance results of Cu/Ni/Fe/γ-Al2O3 catalysts, the synergetic mechanism of the active components (γ-Al2O3, Cu or Cu2O, Fe3O4) for SRD and CO in suit removal can be obtained in
Since Cu/Ni/Fe/γ-Al2O3/Al catalyst had excellent catalytic performance, high H2 selectivity, and low CO selectivity, it was selected to evaluate the thermal stability and the results were shown in
Fe to Cu/Ni/γ-Al2O3/Al catalysts not only promoted the in situ CO removal, but also reduced the crystallite size of Cu, thus improved the thermal stability of Cu/Ni/γ-Al2O3/Al catalyst.
The multifunctional Cu/Ni/γ-Al2O3/Al catalysts loaded with different metals were prepared to inhibit CO formation in SRD system. The conclusions based on the above experiments can be summarized as follows:
1) The Cu/Ni/γ-Al2O3/Al catalysts loaded different promoters (Fe, Zn or La) were investigated. It was found that Cu/Ni/Fe/γ-Al2O3 catalyst had the highest H2 selectivity and the best CO in situ removal (from 27% to 3% CO selectivity). The characterization analysis showed that the Cu/Ni/Fe/γ-Al2O3 catalyst had the largest surface area and dispersion of Cu and the smallest Cu crystallite size. The WGSR was carried out over the Fe3O4 that had the similar optimized reaction range with SRD, which ensured the best reaction coupling effects.
2) The synergetic mechanism of the active components in SRD was obtained from the catalytic performance of catalysts with different packing structures and XPS analyses. It was evidenced that CO produced in SRD on Cu2O or Cu species can be reformed instantly to CO2 and H2 on Fe3O4 species. The Cu2O or Cu and the Fe3O4 species over Cu/Ni/Fe/γ-Al2O3/Al performed an excellent synergistic effect to enhance the CO in situ removal during the SRD.
3) The Cu/Ni/Fe/γ-Al2O3/Al catalyst exhibited an excellent stability in a 400-h durability test at 400˚C with a 100% DME conversion, a 71% H2 selectivity and a 3% CO selectivity.
This work was financially supported by the Fundamental Research Funds for the Central Universities (Grant No. 222201717013) and the Natural Science Foundation of Shanghai (Grant No. 16ZR1408200).
Zhang, Q., Chu, Y.M., Deng, X.Q., Zhang, L. and Chu, H.L. (2018) Improvement of a Mesh-Type Cu/Ni/γ-Al2O3/Al Catalyst for Steam Reforming of Dimethyl Ether by Metal (Fe, Zn or La) Addition for CO in Situ Removal. Modern Research in Catalysis, 7, 1-16. https://doi.org/10.4236/mrc.2018.71001