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  . Compared with methanol, dimethyl ether (DME) is more suitable for being a H2 carrier, since DME has higher energy density, non-corrosive and non-carcinogenic properties. Steam reforming of dimethyl ether (SRD) is proposed as a promising Hydrogen source for cell vehicles. The overall SRD proceeds via two successive steps: hydration of DME to methanol (MeOH) (Equation (1)), and methanol steam reforming to hydrogen (SRM) (Equation (2)).
MeOH steam reforming (SRM):
DME steam reforming (SRD):
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  -  . Amongst them, γ-Al2O3 shows a higher catalytic stability along with the inhibition of side-reactions in DME hydrolysis, but its active temperature for DME hydrolysis ranges from 300˚C to 400˚C  . Meanwhile, SRM to produce hydrogen takes place over metal catalysts, in which Cu-based catalyst is the most widely applied due to its high activity and low price  . But the sintering of metallic Cu will lead to a poor thermal stability above 300˚C, which is a shortcoming for combining metallic Cu and γ-Al2O3 as the bi-functional catalysts for SRD. To solve this problem, copper-based spinel type of catalysts have been investigated as alternative metallic components, since they have a high thermal stability in SRD above 325˚C, but a very high temperature (up to 1000˚C) was required to prepare the spinel    . Another promising method is to improve the dispersion of metal Cu on the support by adding an extra metal. Fan et al.  prepared the Cu-based catalysts with a high thermal stability of 180 h at 400˚C by adding nickel. But the concentration of by-product CO is very high (above 25%), which will deteriorate the performance of proton exchange membrane fuel cells (PEMFC), so reducing the CO content in the hydrogen fuel is essential. Until now, there are two solutions proposed to address this problem. The first solution is the preferential oxidation (CO-PrOx) reaction with Fe-based catalyst follows the SRD stage to remove CO from hydrogen-rich gas. Yan et al.  decreased the CO concentration to 3% - 5% in the new-type SRD-COPrOx system by using a Pt/Fe-γ-Al2O3 catalyst, and the high temperature water gas shift reaction (HT-WGSR) was performed at 350˚C - 450˚C. However, the addition of extra reaction device will lead to the problem of the technical complexity and multiple stages. Second, the rare earth elements (lanthanum, La, etc.) have been extensively applied to Ce-based materials to eliminate CO   . Lu et al.  found that the addition of lanthanum into the Ni-based catalyst only decreased the CO selectivity to 23% from 35%. Therefore, the improvement of a multi-functional catalyst for SRD and the CO in situ removal is a new challenge, which can inhibit CO formation without any extra CO removal device.
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   . Differ to the conventional coating method, the anodic γ-Al2O3 layer on-situ generates from the surface of the aluminum substrate by anodization method. In our previous work, the generation mechanism and properties of the anodic alumina by different treatments were investigated    .
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.
2.1. Catalyst Preparation
The flow scheme of the preparation of multi-catalysts was shown in Figure 1. The structured γ-Al2O3/Al monolith was prepared by the anodization method. The Al mesh (1060, 99.6%) was pretreated in a 10 wt% sodium hydroxide solution for 4 min and a 10 wt% nitric acid solution for 2 min, followed by a flush with deionized water. Then the pretreated mesh was anodized in a 0.4 mol/L oxalic acid solution for 10 h with a current density of 25 A/m2 at 20˚C. The resulting mesh with porous alumina layers was calcined in air at 350˚C for 1 h to remove the residual oxalic acid. Afterward, it was subjected to hot water treatment (HWT) in deionized water at 80˚C for 1 h. Finally, the obtained meshes were dried and calcined in air at 500˚C for 4 h.
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.
2.2. Catalyst Characterization
The surface morphology of catalysts was measured by a scanning electron microscope (JSM-6360LV, JEOL).
Figure 1. The flow schematic diagrams of the preparation of anodic alumina supported catalysts and photos (left) and SEM spectra (right) of catalysts with different steps.
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.
2.3. Catalytic Activity and Durability Evaluation
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. Figure 2 shows the diagram of the testing system for SRD. The water was injected with a constant flux pump and evaporated at 150˚C, then was mixed with DME and fed into the reforming chamber (fixed-bed reactor or microreactor). The compositions of reformate gases were determined by an on-line gas chromatograph equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The steam in the feed and reformate was trapped by a condenser before the gas analysis. A PORAPAK-Q column was used to separate the DME and CH4 gas, and a TDX-01 column was used to separate the
Figure 2. Schematic diagrams of the activity testing system for the DME reforming fixed-bed reactor to produce hydrogen.
CO, CO2, and N2. DME conversion and selectivity of products are defined as follows:
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).
3. Results and Discussion
3.1. Fe-, Zn- and La-Promoted Cu/Ni/γ-Al2O3/Al Catalysts Characterization
The chemical composition, surface area and pore properties of different catalysts were summarized in Table 1. It can be seen that the supporting material, γ-Al2O3/Al catalyst presented the highest BET surface area and pore volume. The addition of active metals decreased the specific surface and pore volume. This could be attributed to the deposition of the active components in pore and the block of micropore in the substrate. Besides, the Cu dispersion of different catalysts decreased in the order of: Cu/Ni/Fe/γ-Al2O3/Al > Cu/Ni/γ-Al2O3/Al > Cu/Ni/Zn/γ-Al2O3/Al > Cu/Ni/La/γ-Al2O3/Al, indicating that the Cu dispersion was improved by the incorporation of Fe promoter, while decreased by the addition of Zn or La.
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 Figure 3(a). It can be seen that all four types of catalysts had strong peaks of CuO, which appeared at 2θ = 35.6˚ and 38.5˚. Obviously, the addition of La and Zn promoters made the Cu diffraction peak sharper and stronger to varying degrees than Cu/Ni/γ-Al2O3/Al catalyst while the Cu/Ni/Fe/γ-Al2O3/Al catalyst exhibited the weakest peak of CuO. This further confirmed that the addition of Fe promoted the dispersion of Cu and reduced the crystallite size of Cu and Fe. The formation of smaller crystallite size of Cu provided more active sites, leading to more active catalyst at low temperature. There was no peak assigned to NiO, indicating that NiO existed in a high dispersion or a microcrystalline state on the gamma alumina substrate  .
H2-TPR was used to analyze the reduction properties of catalysts loaded with different metals. From Figure 3(b), the two reduction peaks located at 425˚C and 750˚C for Fe/γ-Al2O3 were ascribed to the reduction of Fe2O3 to Fe3O4 and Fe3O4 to FeO or Fe, respectively   . An interesting observation from this study was that the addition of Fe to Cu/Ni/γ-Al2O3/Al catalyst decreased the reduction temperature of hematite to magnetite considerably, to 259˚C. In general, the reduction temperature of CuO to Cu was around 300˚C  . For
Table 1. Physicochemical properties of the catalysts with different promoters.
Figure 3. (a) XRD patterns and (b) H2-TPR profiles of γ-Al2O3 catalysts loaded with different metals.
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 Table 1 and XRD analysis, which indicated that compared with the Cu/Ni/γ-Al2O3 catalyst, a higher metallic dispersion and a decrease of the crystallite size lead to an improvement of reducibility for Cu/Ni/Fe/γ-Al2O3 catalyst.
3.2. Catalytic Behaviors of Fe-, Zn- and La-Promoted Cu/Ni/γ-Al2O3/Al Catalysts for CO in Suit Removal
Figure 4 shows the influences of promoters and temperatures on the values of the DME conversion and the effect of CO in situ removal in SRD system. It can be seen that the DME conversion increased while CO selectivity decreased over all the catalysts with the temperature increasing, because SRD is endothermic. The Cu/Ni/La/γ-Al2O3/Al catalyst had the lowest DME conversion and H2 selectivity. It was due to the generation of CuLaO3 co-oxide that occupied and
(a) (b) (c) (d)
Figure 4. Catalytic performance of Cu/Ni/γ-Al2O3/Al catalysts with different metal loadings: (a) DME conversion; (b) H2 selectivity; (c) CO selectivity; (d) CO2 selectivity and (e) CH4 selectivity; Reaction conditions: n(DME): n(H2O) = 1:4, Total flow rate = 4000 ml/(g・h).
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 Table 1). As reported, Zn and La could promote WGSR (Equation (6)) and decrease the amount of CO     . Nonetheless, the best temperature range is 250˚C - 300˚C for Cu/Zn/Al oxides  , and 200˚C - 250˚C for La2O3/CuO/CeO2 catalyst  . As such, the addition of Zn and La could not exhibit a better performance at the SRD reaction temperature at 350˚C - 450˚C. It also can be seen that for the Cu/Ni/Fe/γ-Al2O3/Al catalyst performed the highest H2 selectivity and best effect of CO removal. This was mainly attributed to that Fe-based catalysts have a high catalytic performance on WGSR (Equation (6)) above 330˚C   . It was obvious that for Cu/Ni/γ-Al2O3/Al catalyst, the H2/CO2 molar ratio of ca. 6 was larger than that produced by the SRM process, indicating the step of methanol decomposition (Equation (7)) took place to produce H2, and CO2 methanation (Equation (9)) to consume CO2. The fact that the selectivity of CH4 increased with the decrease of CO selectivity indicated the occurrence of CO methanation (Equation (10)).
3.3. The Synergetic Effect Behavior between Active Metals on the Cu/Ni/Fe/γ-Al2O3/Al Catalyst in the SRD
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 Figure 5. It was found that DME conversion was 100% over the four catalysts above 375˚C. However, there was a large difference in the selectivity of H2 and CO on different catalyst. The order of CO selectivity is: Cu/Ni > layered Cu/Ni+Fe > mixed Cu/Ni*Fe > supported Cu/Ni/Fe, while the H2 selectivity had a contrary order. The minimum CO removal over layered Fe+Cu/Ni catalyst is mainly due to that WGSR over Fe/γ-Al2O3/Al can’t proceed effectively after SRD over Cu/Ni/γ-Al2O3/Al, which results from the non-contact between the active components of Cu/Ni/γ-Al2O3 and Fe at the unit level. On the contrary, CO can be reformed to CO2 and H2 instantly over the supported Cu/Ni/Fe catalyst. This is probably because of the good synergetic effect of the active components of Cu/Ni/γ-Al2O3 and Fe at the atomic level.
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 Figure 6(a). It was found that the A-catalyst exhibited Cu+/Cu0 and Cu2+ at near 932.4 and 934.4 eV, accompanied
Figure 5. Catalytic performance of catalysts with different packing methods in SRD: H2 selectivity and CO selectivity; Reaction conditions: n(DME): n(H2O) = 1:4, Total flow rate = 4000 ml/(g・h).
Figure 6. XPS spectra of Cu/Ni/Fe/γ-Al2O3/Al catalyst: (a) Cu 2p3/2 and Cu LMM (inset) XP spectra; (b) Fe 2p.
by the characteristic Cu2+ shakeup satellite peaks (938 - 945 eV)    . B-catalyst exhibited the presence of Cu+/Cu0 at near 932.4 eV, along with a very slight amount of Cu2+ species (satellite) at ca. 943 eV. Cu 2p3/2 XPS cannot be used to differentiate the peak at 932.4 eV between Cu+ and Cu0 species because they are separated by only 0.1 eV and neither would exhibit satellite peaks  . Auger Cu LMM spectra were used to verify the presence of Cu+ at BE ∼570 eV   . Both the XPS and Cu LMM Auger spectra the Cu+/Cu0 and Cu2+ species co-existed in the surface of A-catalyst. The surface compositions of the catalysts were calculated by measuring the areas under the curve, corresponding results were shown in Table 2. After evaluation, a significant increase from
Table 2. Surface composition estimated from XPS spectra.
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 Figure 6(b). The peaks of Fe 2p spectra were due to magnetite with contributions from both Fe2+ and Fe3+ ions. Specifically, Fe2+ ions can be assigned to peaks around 710 ~ 711 eV with a satellite around 723 ~ 724 eV, while the peaks around 712 ~ 713 eV with a satellite at 725 ~ 726 eV are assigned to Fe3+ ions   . In was magnetite, 2/3 of the sites are occupied by Fe3+ ions and 1/3 of the sites by Fe2+ ions leading to a Fe2+/Fe3+ ratio in the bulk  . The results from Table 2 showed that the Fe2+/Fe3+ fraction increased from 0.45 on the surface of the A-catalyst to 1.12 over B-catalyst, indicating that Fe2O3 and Fe3O4 species co-existed in the surface of A-catalyst, and Fe3O4 species existed in the surface of B-catalyst. The results further confirmed Fe3O4 supported active sites for WGS.
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 Figure 7. In the initial stage (Figure 7(a)), DME hydrolysis reaction took place on the γ-Al2O3/Al substrate to produce MeOH. Both DME and MeOH were decomposed to CO2 and H2 on Cu2O species. Meanwhile, the produced CO and reactant H2O were shifted to CO2 and H2 on Fe3O4 species with the WGSR. The produced H2 initiated the reduction of the CuO and Fe2O3 to Cu2O/Cu and Fe3O4 in turn. And the processes were recycled all the time until the CuO and Fe2O3 species on the catalyst could be completely reduced by hydrogen from SRD reaction. In the stabilization phase (Figure 7(b)), SRD was catalyzed on the γ-Al2O3/Al substrate and the reduced Cu2O/Cu species, and then CO was directly transformed to H2 and CO2 on Fe3O4 specie.
3.4. Stability Evaluation of the Fe/Cu/Ni/γ-Al2O3/Al Catalyst
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 Figure 8. It can be seen that Cu/Ni/Fe/γ-Al2O3/Al catalyst exhibited a good stability and no deactivation was observed during a 400 h test at 400˚C. And during the whole test process, DME conversion remained at 100%. The selectivity of H2, CO2 and CH4 maintained at ca. 71.2%, 21.5% and 4.3%, while the CO selectivity remained at ca. 3.0%, which met the requirement of inlet CO concentration (3% - 5%) for HT-WGSR. In addition, the addition of
Figure 7. The synergetic mechanism of the active components over Cu/Ni/Fe/γ-Al2O3 for SRD and CO in suit removal: (a) initial stage; (b) stabilization stage.
Figure 8. Stability test over the Cu/Ni/Fe/γ-Al2O3/Al catalyst, reaction conditions: n(DME): n(H2O) = 1:4, Total flow rate = 4000 ml/(g・h), T = 400˚C.
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).