NING Hong-Yan YANG Qi-Lei YANG Xiao LI Ying-Xia SONG Zhao-Yu LU Yi-Ren ZHANG Li-Hong ,* LIU Yuan,*

?

Carbon Fiber-supported Rh-Mn in Close Contact with Each Other and Its Catalytic Performance for Ethanol Synthesis from Syngas

NING Hong-Yan1,2YANG Qi-Lei1,2YANG Xiao1,2LI Ying-Xia1,2SONG Zhao-Yu1,2LU Yi-Ren3ZHANG Li-Hong1,2,*LIU Yuan1,2,*

(1;2;3)

Rh-Mn-based catalysts are promising for ethanol synthesis from syngas. In this work, a carbon fiber (CF)-supported Rh-Mn catalyst, with highly dispersed Rh and close contact Rh-Mn species, has been prepared by a newpolymerization route using citric acid and ethylene glycol interaction. The structure and physicochemical properties of both the calcined and reduced samples have been characterized by TPR, XRD, TEM, EDS, CO-TPD, H2chemisorption, and XPS techniques and the catalytic performance for ethanol synthesis from syngas has been evaluated. The results show that the new method is beneficial in forming close contact Rh-Mn species than that obtained with the conventional impregnation method. This can enhance the dispersion and sintering resistance of Rh and effectively improve the activity of CO hydrogenation and the selectivity to ethanol in ethanol synthesis from syngas.

Close contact Rh-Mn; Syngas; Ethanol synthesis; Carbon fiber; CO hydrogenation

1 Introduction

The sustainable ethanol has been attracting a great deal of interest due to its potential application as clean fuel or gasoline additive1,2, which can simultaneously reduce the air pollution and delay the depletion of petroleum resources3,4. It is considered to be one of the best technologies to synthesize ethanol from syngas derived from coal, natural gas or renewable biomass. This route can avoid the negative effect of fermentation technology on global food supplies5and instead of the costly ethylene hydration6,7.

The supported Rh-based catalysts have been widely studied in ethanol synthesis8–14. However, the poor selectivity to ethanol is still an urgent problem15–17. To select proper support and additive is the usual way to solve the problem.

The carbon materials, such as mesoporous carbon, carbon nanotubes (CNTs) and active carbon (AC), have been widely used as supports of Rh-based catalysts in ethanol synthesis18, due to their abundant surface functional groups after modification, high mechanical strength and excellent thermal stability19. Particularly, the good thermal conductivity is beneficial to the exothermic reaction of ethanol synthesis from syngas. In fact, the carbon fiber (CF) is more suitable for practical use because the micron CF can be directly used in fluidized bed reactor without shaping. Furthermore, the large pore channel of CF is beneficial to improving the catalytic20.

For Rh-based catalyst, Fe, Mn, Zr, V or La-based oxides are usually applied as additives. The aim of adding additives is to improve the catalytic performance and inhibit sintering of Rh species21–23. Rh-Mn based catalysts used for ethanol synthesis have been extensively studied24. van den Berg.25and Ojeda.26suggested that the MnOmight act as an electron-withdrawing promoter, which partially oxidizes the Rh atoms at the interface of Rh-MnOto enhance the CO insertion in the higher alcohol synthesis. Furthermore, some results indicate that the Mn additive can increase the CO conversion and improve the selectivity of ethanol due to the formation of a tilted mode of CO adsorption at the Rh-Mn interface for CO dissociation27and the new active species (Rh0–Rh+)-O-Mn2+for CO insertion28,29. It means that the Rh-Mn interaction by close contact with each other is important to improve the catalytic performance of syngas conversion into ethanol. The problem is how to obtain the close contact Rh-Mn without any isolated MnO.

Some researcher30have tried to obtain the close contact Rh-Mn catalyst by using of the strong electrostatic adsorption method, which utilizes pH control to steer the promoter precursor onto Rh oxide supported on SiO2. And the results suggest that intimate interaction between the promoter and the metal is a critical factor for improving selectivity to ethanol.

The most widely used method for the preparation of the supported Rh-Mn catalyst is the conventional impregnation method31. It only can make metal ions physically adsorb on the support. So it is difficult to disperse the Rh and Mn species on the surface of support uniformly.

To overcome the drawback of impregnation methods, in the present study, we proposed a newpolymerization route utilizing citric acid and ethylene glycol interaction to obtain close contact Rh-Mn species on the surface of CF support. The structure and physico-chemical properties of the Rh-Mn/CF catalysts prepared bypolymerization and impregnation method were studied by Temperature-programmed reduction (TPR), X-Ray Diffraction (XRD), Transmission electron microscopy and energy dispersive spectroscopy (TEM-EDS), Temperature-programmed desorption experiment (TPD), H2chemisorption and X-ray photoelectron spectroscopy (XPS) techniques in detail. Owing to the close contact of Rh-Mn on CF, the resulting catalyst prepared by new method inhibits the sintering of Rh and promotes the CO hydrogenation activity and selectivity towards ethanol compounds in the ethanol synthesis from syngas.

2 Experimental section

2.1 Raw materials and catalyst preparation

2.1.1 Raw materials

5%Rh(NO3)3and 50%Mn(NO3)2were purchased from Aladdin (Shanghai, China) and used without further purification. Concentrated sulfuric acid and concentrated nitric acid were purchased from Tianjin Guangfu Technology Development Co. Ltd. Carbon fiber which was used for supports were purchased from Shenzhen Jingzhiyuan of Carbon Materials Co. Ltd. The specific surface area, pore volume and pore diameter of the CF are 43.8 m2?g?1, 0.3 cm3?g?1and 20.8 nm respectively. After loading of Rh and Mn onto CF, there are no obvious changes for these values.

2.1.2 Pretreatment of CF

The introduction of oxygen functional groups onto the surface of CF were carried out according to the method in the literature32. The concrete steps are as follows: Firstly, CF was immersed into mixed acids (included 45 mL 98%H2SO4and 15 mL 69%HNO3) and then transferred into an ultrasonic bath for thorough dispersion. Next, the solution was stirred and refluxed in a three-necked flask at 100 °C for 4 h followed by rinsing with deionized water until pH = 7. Finally, the collected sample was dried at 65 °C for 24 h.

2.1.3 Catalyst preparation

A series of CF supported Rh-Mn composites were prepared by the newpolymerization route utilizing citric acid and ethylene glycol interaction. Specific amounts of Rh(NO3)3, Mn(NO3)2and citric acid and ethylene glycol were dissolved in specific volume of distilled water at ambient temperature. Citrate acid with the molar amount of 1.2 times of the total cations and the glycol with a molar amount of 20% of the citrate acid were added into the above solution. And then CF was added to form the suspension. After impregnation for 24 h, all samples were subsequently dried in air at 110 °C for 12 h and calcined at 300 °C for 4 h in air. The synthesized samples were denoted Rh-%Mn/CF (means the mass percentage of Mn, such as 0, 1, 2, 5). The contrast sample was synthesized by conventional incipient wetness method without citric acid and ethylene glycol, marked as Rh-2%Mn/CF-A. The preparation process is the same as described above.

2.2 Catalyst characterization

2.2.1 Temperature-programmed reduction (TPR)

Temperature programmed reduction (TPR) tests were carried out in a quartz tube reactor. In each run, 50 mg of the sample was pretreated at 100 °C in 5% N2for 1 h. And then the sample was reduced in a stream of 5% H2/Ar with a flow of 30 mL?min?1and the temperature was ramped from room temperature to 800 °C at a heating speed of 10 °C?min?1while the effluent gas was analyzed with a thermal conductivity detector (TCD).

2.2.2 X-Ray Diffraction (XRD)

X-ray diffraction (XRD) patterns were collected on a Bruker D8-Focus X-ray diffractometer (BRUKER AXS GMBH, Germany) with Ni-filtered CuKradiation (= 0.15406 nm). The scanning range (2) was between range 15° to 70° at a scanning speed of 5°?min?1.

2.2.3 Transmission electron microscopy and energy dispersive spectroscopy (TEM-EDS)

Transmission electron microscopy and energy dispersive spectroscopy (TEM-EDS) experiments were conducted on a FEI (PHILIPS) TecnaiG2F20 instrument equipped with Field Electronic Emitter (JEOL, Japan). Samples were finely ground to fine particles in an agate mortar, and then dispersed into ethanol ultrasonically. Finally the well dispersed samples were deposited onto a Mo grid with a layer of the holey carbon film.

2.2.4 Temperature-programmed desorption experiment (TPD)

Temperature-programmed desorption experiment (CO-TPD) was conducted on a Thermo Finnigan TPDRO 1100 SERIES instrument (THERMO, Shanghai). 200 mg of sample was loaded into the reactor and reduced by H2at 350 °C, and then purged by He at the same temperature for 0.5 h. After cooled down to 50 °C, CO was pre-adsorbed on the catalyst at 50 °C for 30 min to ensure saturated adsorption. After that, the catalyst was swept with He for 30 min in order to purge the gaseous and physisorbed CO from the catalyst surface, and then the sample was heated to 900 °C at a heating rate of 10 °C?min?1. The released CO was detected by a TCD.

2.2.5 X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) of the reduced samples was performed using Al K (= 1253.6 eV) radiation with PHI-1600 photoelectron spectrometer. The spectra were obtained at an X-ray power of 250 W and an energy step of 0.1 eV. Before the test, all the samples were vacuumed at room temperature and sealed to prevent oxidation. The residual pressure in the analysis chamber was approximately 2.65×10-8Pa during the analysis. The C 1peak (284.6 eV) was used to correct all XPS spectra.

2.2.6 H2Chemisorption

The dispersion of Rh particles was determined by H2chemisorption using a Thermo Finnigan TPDRO 1100 SERIES instrument (THERMO, Shanghai). Prior to the measurement, 0.1 g of sample was reduced at 350 °C in a H2flow (30 mL?min?1) for 2 h, followed by purging with Ar (30 mL?min?1) at 350 °C for 2 h to remove any physically or chemically adsorbed impurities. After cooling down to 40 °C in the same Ar flow, H2uptake was measured by introducing pulses of H2with a quantitative loop into the Ar flow by a six-way valve. The pulses were continued until no further uptake of H2was detected. Rh dispersion (Rh/Rhtotal) was calculated subsequently with assuming H/Rh(mol)= 1.

2.2.7 Activity test

Catalytic performance tests were carried out in a stainless-steel continuous fixed-bed micro-reactor (stainless steel, 300 mm length and 8 mm internal diameter). In each run, 500 mg sample was sandwiched with quartz sand and loaded into the reactor. The gas hourly space velocity was set to 3900 mL?gcat?1·h?1. Prior to exposure to the reaction gas, 500 mg sample with 40–60 mesh grain size in the reactor was pre-reduced in H2with a flow rate of 22.2 mL?min?1at 350 °C for 4 h. After reduction, the CO hydrogenation reaction was measured at 280 °C, 3 MPa and gas hourly space velocity of 3900 mL?gcat?1·h?1in H2/CO/N2= 8/4/1. In order to obtain stationary activity and selectivity, the reaction was maintained for 12 h at each reaction temperature. The gas products of CO, H2, CH4, CO2and N2were separated online using a TDX-01 packed column (2 m) connected to a TCD. The condensed liquid products of alcohols and hydrocarbons were analyzed off-line using a Porapak Q column (3 m) connected to a FID detector.

The CO conversion (co) and product selectivity (S) were calculated based on the following equations:

co(%) = (COin? COout)/COin× 100%

S(%) =C/ΣC× 100%

where COin, COoutis the moles of CO in the feed-gases and off gases, respectively;, Crepresents the number of carbon atoms and moles of a carbon-containing product, respectively.

3 Results and discussions

3.1 TPR

In order to verify the chemical state and reducibility of metal species, the hydrogen temperature programmed reduction (H2-TPR) tests were performed on fresh samples. The corresponding spectra are drawn in Fig.1, and qualitative data are listed in Table 1.

Two H2consumption peaks, namelypeak at low temperature andpeak at high temperature, can be observed in the TPR profile of Rh/CF sample (Fig.1a). Thepeak can be attributed to the reduction of highly-dispersed Rh3+to Rh+and the shoulder peak ofcan be ascribed to the reduction of Rh+to Rh033,34. While thepeak indicates that some Rh species are much more difficult to reduce, probably due to larger particle size and too strong metal-support interaction. The high dispersion of Rh species should be attributed to the interaction between Rh and CF support by means of surface hydroxyls formed by oxidative pre-treatment and the high specific surface area of CF support.

Fig.1 TPR profiles of Mn-promoted Rh/CF samples.

(a) Rh/CF; (b) Rh-1%Mn/CF; (c) Rh-2%Mn/CF; (d) Rh-5%Mn/CF; (e) Rh-2%Mn/CF-A; (f) 2%Mn/CF.

Table 1 TPR data for Rh-x%Mn/CF (x = 0, 1, 2, 5) and Rh-2%Mn/CF-A and 2%Mn/CF.

aRelative peak area calculated by that ofpeak in Rh/CF as reference peak;bRelative peak area calculated by that of2peak of Rh-1%Mn/CF as reference peak;cPeak temperature.

It is worth noting that thepeak shifts towards higher temperature with the increase of Mn. This implies that there is surface interaction between Rh and Mn, and the strength of interaction can be enhanced by increasing Mn species35. In other words, the Mn is closing to Rh. The formation of this close contact Rh-Mn species is related to thepolymerization reaction. In the process of citric acid and ethylene glycol interaction, the carboxyl groups (–COOH) in the citric acid and the two hydroxyl groups (–OH) in the glycol are easily complexed with the metal ions to form homogeneous stable macromolecule polymers36. Although the calcination can make the macromolecule polymers disappear, the highly dispersed metal ions can be anchored on the surface of CF support. In addition, thepeak temperature of contrast Rh-2%Mn/CF-A is slightly lowerthan that of Rh-2%Mn/CF. It indicates that thepolymerization method is more beneficial to enhancing the Rh-Mn interaction and improve the dispersion of Rh species.

Additionally, the data in Table 1 shows that the area ofpeak increases in the following order Rh/CF < Rh-1%Mn/CF < Rh-2%Mn/CF, indicating the amount of well-dispersed Rh species follows an increasing trend in accordance with the same order. This indicates that thepolymerization method is of great benefit to dispersing Rh under the help of Mn by forming moderate Rh-Mn interaction.

However, it can be found that thepeak areas of Rh-5%Mn/CF (Fig.1d) and Rh-2%Mn/CF-A (Fig.1e) are nearly the same as that of sample Rh/CF (Fig.1a). Nevertheless the reasons are fully different. For Rh-5%Mn/CF obtained bypolymerization method, it is mainly due to the excessive Mn species make the interaction of partial Rh-Mn become too strong to reduce. Compared with Rh-2%Mn/CF, the relative smallpeak area of Rh-2%Mn/CF-A obtained by the conventional impregnation method should be mainly explained by the lack of Rh-Mn interaction in Rh-2%Mn/CF-A. Actually, it clearly demonstrates that the role of Mn additive cannot be exerted sufficiently for the sample prepared by conventional impregnation method35. On the other point of view, it also implies that there must be abundant isolated MnOon the CF support. Just the partial coverage of isolated Mn oxides without any Rh-Mn interaction make the reduction of Rh species decrease slightly37.

After incorporation of Mn, thepeak completely disappears. It also can be deduced that thepolymerization method is beneficial to dispersing Rh by Mn while the impregnation method leads to the partial coverage of Rh by isolated Mn oxides. In Fig.1(b–e), the samples containing Rh and Mn exhibits new1and2peaks instead of the weakpeak. While in Fig.1f, 2%Mn/CF sample only has a2peak and the2peak temperature is higher than that of other samples. Therefore, the broad1peak can be assigned to the step reduction of Mn4+to Mn3+and to Mn2+38, which is promoted by Rh species in light of the low reducing temperature. The weak2peak should be ascribed to the reduction of free Mn ions in isolated Mn oxides, which interacted with Rh and the support only by physical bonds39.

As for these samples obtained bypolymerization method, it can be noted that the peak areas of1are larger than that of2. It implies that the Mn species are easier to interact with Rh than to form isolated Mn oxides under the condition of citric acid and ethylene glycol. With the increase of Mn from 0 to 5% (see Fig.1(a–d)), the broad1peak shows a trend towards slightly higher temperature and H2consumption, while there are no obvious shift for the2peaks except a slight increase of H2consumption for Rh-5%Mn/CF. These results further prove that the Mn species are mainly interacted with active Rh by chemical bonds and the strength of Rh-Mn interaction has an increase trend. In fact, the influence of tiny isolated Mn oxides on the reduction of Rh can be ignored for Rh-5%Mn/CF. It can be clearly observed that the sample Rh-2%Mn/CF-A has a negligible1peak and a huge2peak in Fig.1e. This further reveals that it is easy to form isolated Mn oxides instead of building Rh-Mn interaction by conventional method.

Fig.2 XRD patterns of reduced Rh-x%Mn/CF and reduced contrast sample Rh-2%Mn/CF-A.

(a) Rh/CF; (b) Rh-1%Mn/CF; (c) Rh-2%Mn/CF; (d) Rh-5%Mn/CF; (e) Rh-2%Mn/CF-A.

3.2 XRD

In order to ascertain the crystal phase structure and dispersion of samples in details, the XRD patterns of various samples after reduction at 350 °C are shown in Fig.2. It could be seen that the XRD pattern of all samples exhibits clear characteristic diffraction peaks of graphite carbon (PDF# 41-1487) at 2of around 27°, 43°, 54° respectively. In Fig.2(b–d), the weak diffraction peak at 2of around 30° can be ascribed to another carbon phase (PDF#46-0944). It is difficult to find the Rh diffraction peak. The absence of the Rh diffraction peak only can be ascribed to low loading and high dispersion40.

3.3 TEM-EDS

Fig.3 displays TEM images of the representative CF-supported catalysts. The Rh particle size distribution (PSD) was obtained by the statistical calculation from the corresponding TEM image. From Fig.3(a, c, e), it can be obtained that the mean particle size is 3.6, 2.2 and 3.0 nm for reduced Rh/CF and reduced Rh-2%Mn/CF and reduced Rh-2%Mn/CF-A respectively, and the PSD of reduced Rh-2%Mn/CF is narrower than those of reduced Rh/CF and reduced Rh-2%Mn/CF-A. According to reports, the relative small Rh particle is beneficial to the CO insertion and therefore favorable to ethanol synthesis41,42. The mean particle size of reduced Rh-2%Mn/CF is just in the optimal Rh particle size range of 2.0 to 3.5 nm43. Furthermore, the particles are distributed most uniformly for the reduced Rh-2%Mn/CF in Fig.3(c), while in comparison, the particles are agglomerate for the reduced Rh-2%Mn/CF-A in Fig.3(e). Additionally, the TEM image of Rh-2%Mn/CF-A also confirms that the partial Rh particles can be inevitably covered by the isolated MnO particles.

A lattice distance of 2.19 ? corresponding to Rh (111) can be seen in Fig.3(b), while in Fig.3(d, f), another lattice distance of 1.76 ? corresponding to MnO (201) is observed besides that of Rh (111). Furthermore, it appears that the Rh and MnO particles are close contact with each otherin the reduced Rh-2%Mn/CF,while, there is no contact Rh-Mn particles except the clustered Rh and isolated MnO particles in the reduced Rh-2%Mn/CF-A (Fig.3(f)).

The corresponding TEM-EDS results of reduced Rh-2%Mn/CF and reduced Rh-2%Mn/CF-A are compared in Fig.4. The left diagrams are back scattered electron images of the whole chemical elements and right diagrams are the mapping images of Rh and Mn elements with green and blue bright spots respectively. These mapping images represent distributions of Rh and Mn elements. Obviously, the distributions of Rh and Mn elements in reduced Rh-2%Mn/CF are homogeneous, while the clustered Rh and Mn can be found in reduced Rh-2%Mn/CF-A. The result verified above deductions, which is the high dispersion of Rh particles are mainly attributed to thepolymerization method which can drive the Mn to disperse and interact with Rh.

3.4 CO-TPD

To investigate the chemical adsorption mechanism of reactant CO, the CO-TPD profiles of the support and catalysts were tested which is shown in Fig.5. It can be seen apparently that there is no any desorption peak in the CO-TPD curve of the support CF (curve a). After oxidative pretreatment, a wide desorption peak can be found in Fig.5b. The low temperature range can be assigned to the desorption of surface OH species and the high temperature region might correspond to the release of CO2formed by reaction between the adsorbed CO and the surface OH species40,44.

Fig.3 TEM images of (a, b) reduced Rh/CF, (c, d) reduced Rh-2%Mn/CF and (e, f) reduced Rh-2%Mn/CF-A.

Fig.4 EDS mapping images of (a) reduced Rh-2%Mn/CF, (b) reduced Rh-2%Mn/CF-A.

For the reduced Rh/CF sample in Fig.5c, the peak at ca. 500 °C could be related to bridge-adsorbed CO, and the peak at700 °C also can be attributed to the CO2desorption temperature40,44.

As for the reduced Rh-1%Mn/CF, the two peaks shift towards low temperature besides the increasing strength. The decrease of peak temperature is mainly attributed to the Rh-Mn interaction, which can weaken the interaction of Rh atom with C atom and further decrease the desorption temperature of bridge-adsorbed CO andformed CO2. The increase of the low temperature peak area should be arising from the increase surface Rh species. As for the high temperature one, the increase area should be related to the formation of tilt-adsorbed CO species on the Rh-MnOinterface44, whose carbon end is bonded to the Rh and oxygen end bonded to the Mn. In fact, the bridge- or tilt-adsorbed CO species plays an important role in the ethanol synthesis from syngas40,45.

Fig.5 CO-TPD profiles of reduced Rh-x%Mn/CF, pretreated CF and CF.

(a) CF; (b) pretreated CF; (c) Rh/CF; (d) Rh-1%Mn/CF; (e) Rh-5%Mn/CF; (f) Rh-2%Mn/CF-A.

However, the excess Mn incorporated in reduced Rh-5%Mn/CF will make the Rh bonded with Mn too strongly to interact with CO well and then decrease the desorption peak area associated with the bridged form of CO species. Additionally, the high temperature peak of reduced Rh-5%Mn/CF become stronger and even split into two peaks, the lower one is also can be explained by the increase tilt-adsorbed CO species, while the higher one should be due to the tilt-adsorbed CO species reacting with the surface isolated MnO species to form CO2.

For the reduced Rh-2%Mn/CF-A, the low temperature peak area is smaller than Rh-2%Mn/CF and the peak temperature is higher than Rh-2%Mn/CF obviously. It proves that the Rh species are partially covered by the isolated MnO. The coverage decreases the bridge-adsorbed CO species. In addition, the increased peak temperature is ascribed to the weak interaction between Rh and CF support. This leads to the strengthening adsorption between Rh and CO. And the result is consistence with TPR and XPS. As for the split peaks shifting towards higher temperature, it can be explained that the isolated MnO species are more likely to adsorb CO and react with it. The absent of the bridge-adsorbed CO species must bring negative effects on the catalytic properties.

3.5 XPS

In order to obtain the chemical states of the Rh and Mn elements, the Rh 3and Mn 2XPS spectra of reduced Rh-%Mn/CF (= 0, 1, 2, 5) and reduced Rh-2%Mn/CF-A are presented in Fig.6 and the corresponding XPS data are collected in Table 2.

As shown in Fig.6(A), the Rh 35/2peak of all samples could be deconvoluted into three components. The peak at about 309.5, 308.5 and 307.5 eV can be attributed to Rh3+, Rh+and Rh0species, respectively46. Some researchers have suggested that Rh0is available to CO dissociation and Rh+is beneficial to CO insertion13,26,28. And Bao11pointed that the ratio of Rh+/Rh0is an crucial factor to improve CO insertion. The ratio of Rh+/Rh0increases from Rh/CF of 1.15 to Rh-1%Mn/CF of 1.43 to Rh-2%Mn/CF of 1.63 and then deceases to Rh-5%Mn/CF of 1.26 and Rh-2%Mn/CF-A of 1.25. It can be seen that the binding energy (BE) of Rh 3in these samples has the similar shift trend.

According to the TPR analysis, it suggests that Rh0and Rh+should be arised from the highly dispersed Rh species on the surface of support, which can be easily reduced. The higher BE values of Rh 3after introduction of Mn, indicates that the electronic density on Rh particles decreased47. This also suggests that some positively charged Rh+species also co-exist with Rh0on the surface of catalysts containing Mn. The presence of such oxidized Rh species might be indicative of a strong interaction between Rh and Mn, due to the electron withdrawing property of Mn. Correspondingly, the residual Rh3+ions are mainly arising from the large Rh2O3particles strongly interacted with support for Rh/CF, and the small Rh2O3particles strongly bonded by excessive MnOfor Rh-5%Mn/CF, and Rh2O3particles buried under the much isolated MnOfor Rh-2%Mn/CF-A, respectively.

Fig.6 Rh 3d (A) and Mn 2p (B) XPS spectra of (a–d) reduced Rh-x%Mn/CF (x = 0, 1, 2, 5 in sequence) and (e) reduced Rh-2%Mn/CF-A.

According to the TPR and TEM results above, the peak at ca. 641.5 eV in the Mn 23/2region (Fig.6(B)) can be assigned to Mn2+ion on the CF support48–50, which confirms that the existence of MnO in the reduced samples. On the other hand, the higher the Mn amount, the higher the peak area of Mn 23/2region is. It is worth mentioning that the BE value of the Mn 23/2region of reduced Rh-%Mn/CF (= 1, 2, 5) is higher than that of reduced Rh-2%Mn/CF-A. It just indicates that the different coordination environment of Mn2+ions which arising from the surface interaction between Mn and Rh species49.

3.6 H2 chemisorption

The Rh dispersion was determined by H2chemisorption and the qualitative data are shown in Table 3. It is observed that the Rh dispersion increases from 48% of reduced Rh/CF to 69% of reduced Rh-2%Mn/CF as H2chemisorption amount increase from 64.32 to 98.60 μmol·gcat?1and then decreases to a low value for high Mn-loading sample and contrast sample. The variation of Rh dispersion with Mn loading is practically identical to that ofpeak area in H2-TPR. Certainly, the reason is the same as that described in TPR section.

3.7 Catalytic activity

The result of CO hydrogenation activity tests at 280°C over the various Rh-based catalysts are listed in Table 4. From Table 4, we can observe that Rh/CF catalyst has good performance for ethanol synthesis, and the selectivity of higher alcohols arrives nearly 30%. This is due to the co-existence of two kinds of Rh active species. The results of XPS have stated that the Rh-based catalyst contains Rh0, Rh+and Rh3+species. According to the reaction mechanism of ethanol synthesis from syngas12, the metal Rh0species are CO dissociation active sites, the Rh+ions are responsible for CO insertion to form intermediates of C2oxygenates13,26.

What the most important is that there is an obvious increase of CO conversion when the amount of Mn promoter goes from 0 to 1% and 2%, and then there appears an opposite trend for catalyst Rh-5%Mn/CF. The contrast catalyst Rh-2%Mn/CF-A gives the lowest CO conversion. The variety is consistent with the change of TPRpeak area and Rh dispersion, and CO conversion is improved from Rh/CF to Rh-1%Mn/CF and Rh-2%Mn/CF due to the increasing well-dispersed surface metal Rh. Therefore, the low activity of Rh-5%Mn/CF and Rh-2%Mn/CF-A is arising from fewer metal Rh species on surface which is also supported by the XPS result13. According to the TPR, TEM, TPD and H2chemisorption analysis, two factors need to be considered to obtain much well-dispersed surface metal Rh species on the surface of these catalysts. One is to construct the uniform Rh-Mn interaction by using thepolymerization method to prepare, and the other is to obtain the moderate Rh-Mn interaction strength by loading appropriate Mn.

Table 2 XPS data for reduced Rh-x%Mn/CF (x = 0, 1, 2, 5) and Rh-2%Mn/CF-A.

Table 3 H2 Chemisorption of reduced Rh-x%Mn/CF (x = 0, 1, 2, 5) and Rh-2%Mn/CF-A.

aMeasured by ICP.bBased on the H2chemisorption and assuming a stoichiometry of H/Rh(mol)= 1.

Table 4 Catalytic activity and selectivity of CO hydrogenation over the various reduced Rh-based catalystsa.

aReaction conditions:= 280 °C,= 3 MPa, GHSV = 3900 mL·gcat?1·h?1, H2/CO/N2= 8/4/1.

Another important observation is that the selectivity of ethanol and higher alcohols has the same trend with the addition of Mn bypolymerization method. As for the contrast sample Rh-2%Mn/CF-A, the selectivity of ethanol is between that of Rh-1%Mn/CF and Rh-2%Mn/CF. This is in keeping with the trend of TPRpeak temperature, that is to say, the strength of Rh-Mn interaction is the main factor to determine the ethanol selectivity as the Mn less than 2% ().

As is described in CO-TPD, the CO can be activated into bridged form adsorbed CO at the Rh-Rh interface and tilt-adsorbed CO at the Rh-Mn interface. The oxophilic MnOhas been proposed as a good promoter for the CO hydrogenation27,28,51which can help CO dissociation, weaken the C–O bond and then enhance CO conversion by forming tilted mode of CO adsorbed at the Rh-Mn interface. On the other hand, the incorporation of Mn is favor to maintain partial Rh+ions and increase Rh-Mn interface which is CO insertion center to form ethanol13,14,52. Therefore, the ethanol formation can be promoted with the increase of Mn in a definite range35. As the excess Mn loading, the activity of sample drops. The drop of activity can be attributed to the serious interaction of Rh-Mn interface which can inhibit the reduction of catalysts and cut down the surface dissociation sites Rh0(see Table 2 in XPS section), and therefore decrease the CO conversion. The stronger the Rh-Mn interaction, the easier the CO dissociation is, in other word, the faster the rate of CO dissociation is. Additionally, the appropriate Mn is of benefit to disperse Rh and increase Rh-Mn interface, but the excess Mn will give rise to the shrinkage of the Rh-Mn interface which restricts the CO insertion to form ethanol. It can be deduced that the sharply decreased ethanol involving high alcohols of Rh-5%Mn/CF is due to the CO dissociation is faster than its insertion. It implies that CO insertion is the rate-controlling step of ethanol formation. Anyway, the ethanol selectivity of Rh-2%Mn/CF-A is still lower than that of Rh-2%Mn/CF due to the side-effect of isolated MnOon the CO insertion rate. This further reflects the advantage ofpolymerization method.

It is necessary to say that the ethanol selectivity and CO conversion of Rh-2%Mn/CF (25.6%, 20.9%) are also higher than that of CNTs supported Rh-based catalyst (22.3%, 16.1%)40.

Fig.7 Conversion of CO, selectivity to ethanol, hydrocarbons, CO2 and distribution of alcohols over reduced Rh-2%Mn/CF vs reaction time on stream at 280 °C, 3 MPa and GHSV of 3900 mL·gcat?1·h?1 in H2/CO/N2 = 8/4/1.

Fig.8 TEM images of reduced Rh-2%Mn/CF after reaction 200 h (a) TEM (b) HAADF-STEM.

Fig.9 XRD patterns of reduced Rh-2%Mn/CF (a) before and (b) after 200 h of reaction.

In one word, this is indicative of an optimal activity and selectivity for Rh-2%Mn/CF. It is necessary to emphasize that preparation method play a crucial role in promoting CO hydrogenation activity and selectivity towards ethanol compounds.

3.8 Stability test

In order to investigate its resistance to deactivation, the stability test of the reduced Rh-2%Mn/CF catalyst was examined at 280 °C for 200 h. The variations of gas products composition with time-on-stream are shown in Fig.7. It shows that the CO conversion and ethanol selectivity of the catalyst have a slight rise in the initial reaction 20 h, and then maintain at 21.4% for the CO conversion and 26.8% for the ethanol selectivity. After 130 h, the CO conversion begins to decrease and the ethanol selectivity has fluctuation in the range from 26.5% to 24.6%. Until up to 200 h, the CO conversion only drops by 10%. It implies that the catalyst only has a slightly deactivation within reaction 200 h.

In order to find out the reason of deactivation, the TEM image of reduced Rh-2%Mn/CF after 200 h of reaction is presented in Fig.8. And the XRD pattern of Rh-2%Mn/CF after 200 h of reaction is also obtained to compare with fresh Rh-2%Mn/CF, and the result is shown in Fig.9.

From Fig.8, the reduced Rh-2%Mn/CF gives an increase particle size after reaction 200 h and the PSD becomes wide. This means the catalyst shows a trend towards sintering to some extent during reaction, depending on the reaction mechanism. Fortunately, the close-contact Rh-Mn still can be found in Fig.8(b), which indicates the catalyst Rh-2%Mn/CF still has a relatively good stability compared with some other Rh-based catalyst17,31.

Compared with the XRD pattern of catalyst before reaction, it can be observed that the XRD pattern of catalyst after 200 h of reaction exhibits characteristic diffraction peaks of Rh (111) (PDF# 05-0685) at 2of ca. 40° (Fig.9b). In combination with the result of XRD, TEM, we can conclude that the main reason for the slightly deactivation of Rh-2%Mn/CF is the sintering of Rh particles. No matter what, the most important is that thepolymerization route utilizing citric acid and ethylene glycol interaction is really an effective method to improve the activity, selectivity and stability of Rh-%Mn/CF series catalysts.

4 Conclusions

A series of carbon fiber (CF) supported Rh-Mn composites with different mass percentage of Mn can be successfully prepared by thepolymerization route utilizing citric acid and ethylene glycol interaction. The characteristic results demonstrate that the highly dispersed Rh mainly exists in the close contact Rh-Mn species. Thepolymerization method can drive the Mn to disperse and interact with Rh. The higher activity is attributed to the high dispersion of Rh and the higher selectivity to ethanol is owing to the close contact of Rh and Mn on the surface of CF. The novel catalyst shows the excellent stability, owing to the good thermal conductivity of CF support and the interaction among Rh, Mn and CF. Thepolymerization route reported here can be extended to development of other supported bi-component catalysts for a broad spectrum of green chemical processes.

(1) Rankovic, N.; Bourhis, G.; Loos, M.; Dauphin, R.2015,, 41. doi:10.1016/j.fuel.2015.02.005

(2) Rapp, V. H.; Mack, J. H.; Tschann, P.2014,, 3185. doi:10.1021/ef5001453

(3) Spivey, J. J.; Egbebi, A.2007,, 1514. doi:10.1039/B414039G

(4) Olah, G. A.2013,, 104.doi:10.1002/anie.201204995

(5) Balat, M.; Balat, H.2009,, 2273. doi:10.1016/j.apenergy.2009.03.015

(6) Himmel, M. E.; Ding, S. Y.; Foust, T. D.2007,, 804. doi:10.1126/science.1137016

(7) Kumar, N.; Smith, M. L.; Spivey, J. J.2012,, 218. doi:10.1016/j.jcat.2012.02.011

(8) Yu, J.; Mao, D.; Han, L.; Guo, Q.2013,, 344. doi:10.1016/j.fuproc.2012.08.020

(9) Kim, M. J.; Chae, H. J.; Ha, K. S.2014,, 365. doi:10.1007/s10934-014-9782-y

(10) Choi, Y.; Liu, P.2009,, 13054. doi:10.1021/ja903013x

(11) Pan, X.; Fan, Z.; Chen, W.2007,, 507. doi:10.1038/nmat1916

(12) Chuang, S. S. C.; Stevens, R. W.; Khatri, R.2005,, 225. doi: 10.1007/s11244-005-2897-2

(13) Han, L.; Mao, D.; Yu, J.2012,, 20. doi:10.1016/j.catcom.2012.02.032

(14) Jiang, D.; Ding, Y.; Pan, Z.2007,, 241. doi:10.1007/s10562-007-9322-3

(15) Subramani, V.; Gangwal, S. K.2008,, 814. doi:10.1021/ef700411x

(16) Song, X.; Ding, Y.; Chen, W.2012,, 100. doi:10.1016/j.catcom.2011.12.015

(17) Hu, J.; Wang, Y.; Cao, C.2007,, 90. doi:10.1016/j.cattod.2006.07.006

(18) Fan, Z.; Chen, W.; Pan, X.; Bao, X.,2009,, 86. doi:10.1016/j.cattod.2009.03.004

(19) Shiba, K.; Tagaya, M.; Samitsu, S.2014,, 197. doi:10.1002/cplu.201300356

(20) De Jong, K. P.; Geus, J. W.2007,, 481. doi:10.1081/CR-100101954

(21) Subramanian, N. D.; Gao, J.; Mo, X.; Goodwin, J. G., Jr.2010,, 204. doi:10.1016/j.jcat.2010.03.019

(22) Palomino, R. M.; Magee, J. W.; Llorca, J.2015,, 87. doi:10.1016/j.jcat.2015.04.033

(23) Haider, M.; Gogate, M.; Davis, R.2009,, 9. doi:10.1016/j.jcat.2008.10.013

(24) Chen, W. M.; Ding, Y. J.; Xue, F.; Song, X. G.2015,, 1.[陳維苗, 丁云杰, 薛 飛, 宋憲根. 物理化學(xué)學(xué)報(bào), 2015,, 1.] doi:10.3866/PKU.WHXB201411054

(25) Van den Berg, F.; Glezer, J.; Sachtler, W.1985,, 340. doi:10.1016/0021-9517(85)90181-2

(26) Ojeda, M.; Granados, M. L.; Rojas, S.2004,, 47. doi:10.1016/j.apcata.2003.10.033

(27) Ichikawa, M.; Fukushima, T.1985,, 1564. doi:10.1021/j100255a003

(28) Wang, Y.; Luo, H.; Liang, D.2000,, 46. doi:10.1006/jcat.2000.3026

(29) Trevi?o, H.; Hyeon, T.; Sachtler, W. M. H.1997,, 236. doi:10.1006/jcat.1997.1756

(30) Liu, J.; Tao, R.; Guo, Z.2013,, 3665. doi:10.1002/cctc.201300479

(31) Mao, W.; Su, J.; Zhang, Z.2015,, 301. doi:10.1016/j.ces.2015.03.023

(32) Zhang, X.; Fan, X.; Yan, C.2012,, 1543. doi:10.1021/am201757v

(33) Egbebi, A.; Schwartz, V.; Overbury, S. H.2010,, 91. doi:10.1016/j.cattod.2009.07.104

(34) Yin, H.; Ding, Y.; Luo, H.2003,, 155. doi:10.1016/S0926-860X(02)00560-4

(35) Ojeda, M.; Granados, M. L.; Rojas, S.2004,, 47. doi:10.1016/j.apcata.2003.10.033

(36) Arima, M.; Kakihana, M.; Nakamura, Y.1996,, 2847.doi:10.1111/j.1151-2916.1996.tb08718.x

(37) Schwartz, V.; Campos, A.; Egbebi, A.2011,, 1298. doi: 10.1021/cs200281g

(38) Trevino, H.; Lei, G. D.; Sachtler, W. M. H.1995,, 245. doi:10.1006/jcat.1995.1166

(39) Bastein, A. G. T. M.; Van Der Boogert, W. J.; Van Der Lee, G.1987,, 243. doi:10.1016/S0166-9834(00)82896-1

(40) Wang, S. R.; Guo, W. W.; Wang, H. X.2014,, 1305. doi:10.1007/s10562-014-1248-y

(41) Yu, J.; Mao, D.; Han, L.2013,, 38. doi:10.1016/j.molcata.2012.10.022

(42) Ojeda, M.; Rojas, S.; Boutonnet, M.2004,, 33. doi:10.1016/j.apcata.2004.05.014

(43) Hanaoka, T.; Arakawa, H.; Matsuzaki, T.2000,, 271. doi:10.1016/S0920-5861(00)00261-3

(44) Ma, H.; Yuan, Z.; Wang, Y.2001,, 224. doi:10.1002/sia.1042

(45) Yu, J.; Mao, D. S.; Lu, G. Z.2012,, 25. doi:10.1016/j.catcom.2012.03.015

(46) Jin, C.; Xia, W.; Nagaiah, T. C.2009,, 7186. doi:10.1016/j.electacta.2009.06.095

(47) Raskó, J.; Bontovics, J.1999,, 27. doi:10.1023/A:1019053228217

(48) Yu, J.; Mao, D.; Ding, D.2016,, 151. doi:10.1016/j.fuel.2015.02.005

(49) Tang, L.; Song, C.; Yang, X.2013,, 826. doi:10.1002/cjoc.201300248

(50) Li, F.; Zhang, L.; Duan, X.2004,, 169. doi:10.1016/j.colsurfa.2004.06.022

(51) Yu, J.; Mao, D. S.; Han, L. P.2013,, 806. doi:10.1016/j.jiec.2012.10.021

(52) Liu, J.; Guo, Z.; Childers, D.2014,, 149. doi:10.1016/j.jcat.2014.03.002.

碳纖維負(fù)載Rh-Mn緊密接觸的催化劑及其合成氣制乙醇催化性能

寧紅巖1,2楊其磊1,2楊 曉1,2李鷹霞1,2宋兆鈺1,2魯逸人3張立紅1,2,*劉 源1,2,*

(1天津大學(xué)，天津市應(yīng)用催化科學(xué)與工程重點(diǎn)實(shí)驗(yàn)室，天津 300072;2化學(xué)工程與技術(shù)(天津)協(xié)同創(chuàng)新中心，天津 300072;3天津大學(xué)環(huán)境科學(xué)與工程學(xué)院，天津 300072)

Rh-Mn基催化劑用于合成氣制乙醇反應(yīng)頗具潛力。本文采用檸檬酸乙二醇原位聚合的方法在碳纖維(CF)表面負(fù)載Rh-Mn催化劑。采用TPR, XRD, TEM, EDS, CO-TPD, H2脈沖吸附和XPS技術(shù)對(duì)焙燒和還原后樣品的結(jié)構(gòu)和物化性能進(jìn)行了表征，并考察了催化劑催化合成氣制乙醇的反應(yīng)性能。結(jié)果表明，原位聚合法相比傳統(tǒng)浸漬法更利于形成緊密接觸的Rh-Mn物種，可有效提高Rh的分散度和抗燒結(jié)性，顯著改善合成氣轉(zhuǎn)化反應(yīng)中CO加氫活性和乙醇選擇性。

Rh-Mn相互作用；合成氣；乙醇合成；碳纖維；CO加氫

O643

10.3866/PKU.WHXB201704285

March 23, 2017;

April 19, 2017;

April 28,2017.

ZHANG Li-Hong, Email: zlh_224@tju.edu.cn; Tel: +86-15022255828. LIU Yuan, Email: yuanliu@tju.edu.cn.

The project was supported by State Key Laboratory of Chemical Resource Engineering and National Natural Science Foundation of China (21576192, 21376170).

化學(xué)資源有效利用國家重點(diǎn)實(shí)驗(yàn)室基金和國家自然科學(xué)基金(21576192, 21376170)