Improvement of a Mesh-Type Cu / Ni / γ-Al 2 O 3 / Al Catalyst for Steam Reforming of Dimethyl Ether by Metal ( Fe , Zn or La ) Addition for CO in Situ Removal

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-Al2O3/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-Al2O3/Al catalyst by evaluating catalytic performance of catalysts with different packing structures. And the synergetic mechanism of the active components (γ-Al2O3, Cu or Cu2O, and Fe3O4) 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.


Introduction
As an alternative energy source, hydrogen has received much attention due to its MeOH steam reforming (SRM): CH OH H O 3H CO Hr 49 kJ mol 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, ZrO 2 , and γ-Al 2 O 3 [2]- [6].Amongst them, γ-Al 2 O 3 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 [7].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 [8].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-Al 2 O 3 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 [9] [10] [11].Another promising method is to improve the dispersion of metal Cu on the support by adding an extra metal.
Fan et al. [12] 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. [13] decreased the CO concentration to 3% -5% in the new-type SRD-COPrOx system by using a Pt/Fe-γ-Al 2 O 3 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 Modern Research in Catalysis been extensively applied to Ce-based materials to eliminate CO [14] [15].Lu et al. [16] found that the addition of lanthanum into the Ni-based catalyst only de- creased 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 [17] [18].Differ to the conventional coating method, the anodic γ-Al 2 O 3 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 [5] [6] [19].
In this study, a mesh-type Cu/Ni/γ-Al 2 O 3 /Al catalyst was prepared and applied in SRD.Different composition (Fe, Zn, or La) was evaluated by adding to the Cu/Ni/γ-Al 2 O 3 /Al catalyst.The series of catalysts were characterized by using N 2 O pulse chemisorption, XRD, H 2 -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-Al 2 O 3 /Al catalyst.

Catalyst Preparation
The flow scheme of the preparation of multi-catalysts was shown in Figure 1.The structured γ-Al 2 O 3 /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/m 2 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-Al 2 O 3 /Al (X = Ni, Zn, La or Fe) catalysts were prepared through impregnation method.The monolithic γ-Al 2 O 3 /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.

Catalyst Characterization
The surface morphology of catalysts was measured by a scanning electron microscope (JSM-6360LV, JEOL).Modern Research in Catalysis 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.
H 2 temperature-programmed reduction (H 2 -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% H 2 /Ar flow atmosphere.
The dispersion of Cu and exposed copper surface area (S Cu ) were measured by selective N 2 O chemisorption method in a chemisorption analyzer (ChemiSorb 2720, Micromeritics).First, catalysts were reduced in a 10 vol% H 2 /Ar stream at Modern Research in Catalysis 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% N 2 O/N 2 (30 mL/min) was used to oxidize surface copper atoms to Cu 2 O 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% H 2 /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.

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 H 2 pre-reduction treatment was conducted prior to the evaluation of catalysts.
, , Selectivity of products 100% where F DME,in and F DME,out are the influent and effluent molar flow rates of DME, respectively, F i,out are the molar flow rates of gaseous products (H 2 , CO, CO 2 , CH 4 ).There was no peak assigned to NiO, indicating that NiO existed in a high dispersion or a microcrystalline state on the gamma alumina substrate [20].and 324˚C, respectively.The results were consistent with Table 1 and XRD analysis, which indicated that compared with the Cu/Ni/γ-Al 2 O 3 catalyst, a higher metallic dispersion and a decrease of the crystallite size lead to an improvement of reducibility for Cu/Ni/Fe/γ-Al 2 O 3 catalyst.

Catalytic Behaviors of Fe-, Zn-and La-Promoted
Cu/Ni/γ-Al2O3/Al Catalysts for CO in Suit Removal        34.5% to 91.1% in the amount corresponding to Cu + /Cu 0 species was found.This observation confirms that the CuO species were reduced to metallic Cu + and Cu 0 species such as Cu 2 O and Cu, which further speculated the Cu + and Cu 0 species all are the active phase for the SRM process [35] [36].

The Synergetic Effect Behavior between Active Metals on the
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 Fe 2+   and Fe 3+ ions.Specifically, Fe 2+ 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 Fe 3+ ions [37] [38].In was magnetite, 2/3 of the sites are occupied by Fe 3+ ions and 1/3 of the sites by Fe 2+ ions leading to a Fe 2+ /Fe 3+ ratio in the bulk [37].The results from Table 2 showed that the Fe 2+ /Fe 3+ fraction increased from 0.45 on the surface of the A-catalyst to

Conclusions
The multifunctional Cu/Ni/γ-Al 2 O 3 /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-Al 2 O 3 /Al catalysts loaded different promoters (Fe, Zn or La) were investigated.It was found that Cu/Ni/Fe/γ-Al 2 O 3 catalyst had the highest H 2 selectivity and the best CO in situ removal (from 27% to 3% CO selectivity).
The characterization analysis showed that the Cu/Ni/Fe/γ-Al 2 O 3 catalyst had the largest surface area and dispersion of Cu and the smallest Cu crystallite size.The WGSR was carried out over the Fe 3 O 4 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 Cu 2 O or Cu species can be reformed instantly to CO How to cite this paper: 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,

Figure 1 .
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.

Figure 2
Figure 2. Schematic diagrams of the activity testing system for the DME reforming fixed-bed reactor to produce hydrogen.
γ-Al 2 O 3 /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-Al 2 O 3 /Al > Cu/Ni/γ-Al 2 O 3 /Al > Cu/Ni/Zn/γ-Al 2 O 3 /Al > Cu/Ni/La/γ-Al 2 O 3 /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-Al 2 O 3 /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-Al 2 O 3 /Al catalyst while the Cu/Ni/Fe/γ-Al 2 O 3 /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.

H 2 -
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-Al 2 O 3 were ascribed to the reduction of Fe 2 O 3 to Fe 3 O 4 and Fe 3 O 4 to FeO or Fe, respectively [21] [22].An interesting observation from this study was that the addition of Fe to Cu/Ni/γ-Al 2 O 3 /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 [23].For Q. Zhang et al.DOI: 10.4236/mrc.2018.710017 Modern Research in Catalysis

Figure 4
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-Al 2 O 3 /Al catalyst had the lowest DME conversion and H 2 selectivity.It was due to the generation of CuLaO 3 co-oxide that occupied and
Cu/Ni/Fe/γ-Al2O3/Al Catalyst in the SRD The catalytic performance of different packing structures of Fe/γ-Al 2 O 3 /Al and Cu/Ni/γ-Al 2 O 3 /Al was evaluated to investigate the synergetic effect between Cu/Ni/γ-Al 2 O 3 and Fe on the Cu/Ni/Fe/γ-Al 2 O 3 /Al catalyst.The three packing structures included supported Cu/Ni/Fe/γ-Al 2 O 3 /Al catalyst, layered Cu/Ni/γ-Al 2 O 3 /Al (former) and Fe/γ-Al 2 O 3 /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-Al 2 O 3 /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 H 2 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 H 2 selectivity had a contrary order.The minimum CO removal over layered Fe+Cu/Ni catalyst is mainly due to that WGSR over Fe/γ-Al 2 O 3 /Al can't proceed effectively after SRD over Cu/Ni/γ-Al 2 O 3 /Al, which results from the non-contact between the active components of Cu/Ni/γ-Al 2 O 3 and Fe at the unit level.On the contrary, CO can be reformed to CO 2 and H 2 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-Al 2 O 3 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-Al 2 O 3 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 + /Cu 0 and Cu 2+ at near 932.4 and 934.4 eV, accompanied DOI: 10.4236/mrc.2018.71001 10 Modern Research in Catalysis

Figure 7 .
Figure 7.The synergetic mechanism of the active components over Cu/Ni/Fe/γ-Al 2 O 3 for SRD and CO in suit removal: (a) initial stage; (b) stabilization stage.
CO, CO 2 , and N 2 .DME conversion and selectivity of products are defined as follows: DOI: 10.4236/mrc.2018.710016 Modern Research in Catalysis

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 Table1.It can be seen that the supporting material,

Table 1 .
Physicochemical properties of the catalysts with different promoters.

Table
[29] for the Cu/Ni/Fe/γ-Al 2 O 3 /Al catalyst performed the highest H 2 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[28][29].It was obvious that for Cu/Ni/γ-Al 2 O 3 /Al catalyst, the H 2 /CO 2 molar [26]C -300˚C for Cu/Zn/Al oxides[24], and 200˚C -250˚C for La 2 O 3 /CuO/CeO 2 catalyst[26].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 Q.Zhanget al. DOI: 10.4236/mrc.2018.710019 Modern Research in Catalysis

Table 2 .
After evaluation, a significant increase from Q.Zhang et al.

Table 2 .
Surface composition estimated from XPS spectra.

4. Stability Evaluation of the Fe/Cu/Ni/γ-Al2O3/Al Catalyst
1.12 over B-catalyst, indicating that Fe 2 O 3 and Fe 3 O 4 species co-existed in the surface of A-catalyst, and Fe 3 O 4 species existed in the surface of B-catalyst.The results further confirmed Fe 3 O 4 supported active sites for WGS.From the above XPS and catalytic performance results of Cu/Ni/Fe/γ-Al 2 O 3 catalysts, the synergetic mechanism of the active components (γ-Al 2 O 3 , Cu or Cu 2 O, Fe 3 O 4 ) 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-Al 2 O 3 /Al substrate to produce MeOH.Both DME and MeOH were decomposed to CO 2 and H 2 on Cu 2 O species.Meanwhile, the produced CO and reactant H 2 O were shifted to CO 2 and H 2 on Fe 3 O 4 species with the WGSR.The produced H 2 initiated the reduction of the CuO and Fe 2 O 3 to Cu 2 O/Cu and Fe 3 O 4 in turn.And the processes were recycled all the time until the CuO and Fe 2 O 3 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-Al 2 O 3 /Al substrate and the reduced Cu 2 O/Cu species, and then CO was directly transformed to H 2 and CO 2 on Fe 3 O 4 specie.
3.2 , CO 2 and CH 4 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 Modern Research in Catalysis 2 and H 2 on Fe 3 O 4 species.The Cu 2 O or Cu and the Fe 3 O 4 species over Cu/Ni/Fe/γ-Al 2 O 3 /Al performed an excellent synergistic effect to enhance the CO in situ removal during the SRD.Q. Zhang et al.DOI: 10.4236/mrc.2018.7100113 Modern Research in Catalysis 3) The Cu/Ni/Fe/γ-Al 2 O 3 /Al catalyst exhibited an excellent stability in a 400-h durability test at 400˚C with a 100% DME conversion, a 71% H 2 selectivity and a 3% CO selectivity.