MWCNT-Supported Ni Mo K Catalyst for Higher Alcohol Synthesis from Syngas

Transcription

1 Catal Lett (2010) 137: DOI /s y MWCNT-Supported Ni Mo K Catalyst for Higher Alcohol Synthesis from Syngas Chun-Hui Ma Hai-Yan Li Guo-Dong Lin Hong-Bin Zhang Received: 12 February 2010 / Accepted: 6 April 2010 / Published online: 22 April 2010 Ó Springer Science+Business Media, LLC 2010 Abstract A type of novel MWCNT-supported Ni Mo K catalysts for higher alcohol synthesis (HAS) from syngas was developed, which displayed excellent performance for the selective formation of C 1 3 -alcohols and DME from syngas. Over the 15%Ni 1 Mo 1 K 0.05 /CNTs catalyst under the reaction conditions of 5.0 MPa and 538 K, the C-based selectivity of (C 1 3 -alc.? DME) reached 59.0%, with the corresponding STY of 205 mg h -1 g -1. This value was 1.16 times that of the 35%Ni 1 Mo 1 K 0.05 /AC catalyst with the optimal Ni 1 Mo 1 K 0.05 loading, and 1.76 times that of the non-supported co-precipitated Ni 1 Mo 1 K 0.05 host system, under the same reaction conditions. The results of the catalyst characterizations revealed that the CNT-support has strong influence on the chemical states of catalyst via its interaction with the supported Ni Mo components, leading to the increase of the molar percentage of NiO(OH) and Mo 4? /Mo 5? (the two kinds of catalytically active surface-species related closely to selective formation of C 1 3 -alcohols) and the dramatic decrease of the molar C.-H. Ma H.-Y. Li G.-D. Lin H.-B. Zhang (&) Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen , China hbzhang@xmu.edu.cn C.-H. Ma chma@xmu.edu.cn H.-Y. Li hyli@xmu.edu.cn G.-D. Lin gdlin@xmu.edu.cn C.-H. Ma H.-Y. Li G.-D. Lin H.-B. Zhang State Key Laboratory of Physical Chemistry for Solid Surfaces and National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, Xiamen University, Xiamen , China percentage of Ni 0 and Mo 0 (the two kinds of catalytically active surface-species related closely to selective formation of hydrocarbons, especially methanation of CO). Moreover, the CNT-support also provided the sp 2 -C surfacesites for adsorption-activation of H 2. All these factors contribute to an increase in selectivity of generating C alcohols. Keywords Multi-walled carbon nanotubes CNT-supported Ni Mo K catalyst Higher alcohol synthesis 1 Introduction The higher alcohols (C 2? -alcohols), together with methanol and dimethyl ether (DME), have been considered as the most important species among coal-based clean synthetic fuels and chemical feedstocks. These C 2? -oxygenates have been confirmed to be a better and cleaner automobile fuel with high octane number and low emissions of NO x, ozone, CO, and aromatic vapors. Higher alcohol synthesis (HAS) on the catalysts containing Mo, Group VIII metals and alkali have been extensively studied since the 1980s [1 6]. Progress in this field has contributed considerably to the growing understanding of the nature of those catalytic reaction systems. Nevertheless, the existing technology of HAS is still on a small scale. The single-pass-conversion of feed syngas and selectivity of forming C 2? -alcohols are both relatively low [7 9]. Development of catalysts with high efficiency and selectivity for formation of C 2? -alcohols has been one of the key objectives for R&D efforts. Multi-walled carbon-nanotubes (symbolized as MWCNTs and simplified as CNTs in later text, unless otherwise specified), as a type of novel nano-carbon

2 172 C.-H. Ma et al. support or promoter of catalyst, have drawn increasing attention [10 13] since their discovery [14]. From a chemical catalysis point of view, the excellent performance of CNTs in adsorption-activation of hydrogen and in promoting spillover of adsorbed H-species is very attractive, in addition to its high mechanical strength, nanosize channels, sp 2 -carbon-constructed surfaces, graphite-like tube-walls and high thermal/electrical conductivity, all of which render this kind of nanostructured carbon materials full of promise as a novel catalyst support and/or promoter. The catalytic studies conducted so far on CNT-based systems have shown encouraging results in terms of activity and selectivity [11 13]. In the present work, a type of novel CNT-supported Ni Mo K oxide-based catalysts was developed. The catalysts displayed excellent performance for selective formation of the C 1 3 -alcohols and DME from syngas, as compared to the AC-supported counterpart and the CNT-free co-precipitated host system. The catalysts were characterized by means of SEM/TEM/EDX, XRD, XPS, and H 2 -TPD, and the nature of promoting action by the CNTs as support was discussed. The results shed some light on understanding the mechanism of promoting action by the CNT-support and on the design of practical catalysts for the HAS. 2 Experimental 2.1 Preparation of CNTs The CNTs used in the present work were prepared from the catalytic decomposition of CH 4 following the method reported previously [15]. The freshly prepared CNTs were purified through treatment of boiling nitric acid (*8 mol/ L) for 12 h, followed by rinsing with the de-ionized water twice, and then drying at 383 K. In the purified CNTs, contents of the total element-carbon and the graphitized carbon were C99.5 and C90% (mass percentage), as evidenced by elemental analysis and H 2 -TPH (temperatureprogrammed hydrogenation) measurements, respectively. Previous characterization studies [15, 16] have demonstrated that those CNTs were Herringbone-type CNTs, with the outer diameters of nm, inner diameters of 3 7 nm, and N 2 -BET surface area of 130 m 2 g Preparation of Supported Ni Mo K Catalysts A series of CNT-supported Ni Mo K catalysts (noted as x%ni i Mo j K k /CNTs, where x% represents mass percentage) were prepared by a stepwise incipient wetness method. An aqueous solution containing a desired amount of (NH 4 ) 6 Mo 7 O 24 4H 2 O (AR grade) was impregnated onto the HNO 3 -treated CNT-support, followed by drying at 383 K for 6 h. An aqueous solution containing a desired amount of Ni(NO 3 ) 2 6H 2 O (AR grade) was impregnated onto the above Mo-impregnated intermediate, followed by drying at 383 K for 10 h and calcining at 848 K for 4 h under N 2 atmosphere. Finally, an aqueous solution containing a desired amount of K 2 CO 3 (AR grade) was impregnated onto the above Mo and Ni-impregnated intermediate, followed by drying at 383 K for 6 h, and calcining at 673 K for 4 h under N 2 atmosphere, thus yielding the precursor of x%ni i Mo j K k /CNTs catalyst. A reference catalyst supported on activated carbon (AC), noted as x%ni i Mo j K k /AC, was prepared in the similar way. Prior to being used, the AC support (Xiamen Chem. Reagent Co., with 830 m 2 g -1 of N 2 -BET-SSA) was pretreated first with 10% NaOH aqueous solution and then with 50% HNO 3 solution, followed by rinsing with de-ionized water and drying at 383 K for 8 h. Another reference catalyst, non-supported Ni i Mo j K k host system, was prepared by a combined coprecipitation-impregnation method [17]. All samples of catalyst precursors were pressed, crushed, and sieved to a size of mesh for the activity evaluation. 2.3 Evaluation of Catalyst Evaluation of the catalyst performance for HAS from syngas was carried out in a fixed-bed continuous-flow reactor and gas chromatograph (GC) combination system. 0.5 g of catalyst sample was used for each test. Prior to the reaction, the sample of the catalyst precursor was prereduced in situ under purified H 2 stream (0.1 MPa and 1,800 ml h -1 ). The reduction temperature was programmed to rise from room temperature to 623 K and to maintain at that temperature for 12 h, and then to lower down to the desired temperature for the catalyst test. The HAS reaction was conducted at a stationary state under reaction conditions of K, MPa, V(H 2 )/ V(CO)/V(N 2 ) = 45/45/10. Exit gas from the reactor was immediately brought down to atmospheric pressure and transported, while its temperature was maintained at 403 K, to the sampling valve of the GC (Model GC-2010 by Shimadzu GC Instruments, Inc), which was equipped with dual detectors (TCD and FID) and dual columns filled with carbon molecular sieve (TDX-01) and Porapak Q-S (USA), respectively, for online analysis. The former column (0.8-m length) was used for the analysis of N 2 (as internal standard), CO and CO 2, and the latter (2.0-m length) for C 1 4 -hydrocarbons, C 1 4 -alcohols and other oxygenates (mainly DME and acetaldehyde in a small amount). Conversion of CO (noted as X CO ) was determined through the internal standard (N 2 ), and the carbon-based selectivity for the carbon-containing products, including alcohols, hydrocarbons (HC), and other oxygenates (noted

3 CNT-Supported Ni Mo K Catalyst for HAS 173 as S alc., S HC, etc.) was calculated by an internal normalization method. 2.4 Characterization of Catalyst Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed with LEO electron microscope. XRD measurements were carried out on an X Pert PRO X-ray Diffractometer (PANalytical) with Cu K a (k = nm) radiation. A continuous scan mode was used to collect 2h data from 10 to 90. The voltage and current were 40 kv and 30 ma, respectively. X-ray photoelectron spectroscopy (XPS) measurements were done on a Quantum 2000 Scanning ESCA Microprobe instrument with Al K a radiation (15 kv, 25 W, hm = ev) under ultrahigh vacuum (10-7 Pa), calibrated internally by the carbon deposit C(1s) (E b = ev). The samples of the tested catalysts used for XPS-analysis were protected by nitrogen (of % purity), thus not exposed to air, during the transfer from the reaction tube to the XPS chamber. Specific surface area (SSA) was determined by N 2 adsorption using a Micromeritics Tristar-3000 (Carlo Erba) system. Tests of H 2 -temperature-programmed desorption (TPD) of catalyst were conducted on an adsorption/desorption system. 200 mg of catalyst precursor sample was used for each test. Prior to TPD test, the sample of catalyst was in situ pre-reduced by a flow of H 2 (of % purity) at 623 K for 6 h and then flushed by a flow of Ar (of % purity) at 623 K for 60 min to clean its surface, followed by cooling down to 433 K, switching to the flow of H 2 for hydrogen adsorption at 433 K for 60 min and subsequently at room temperature for 120 min. After that, the sample was flushed by the flow of Ar at room temperature till the stable baseline of GC appeared, followed by starting to conduct the TPD measurement from 298 to 873 K. The rate of temperature increase was 5 K min -1. The desorbed exitgases were analyzed by an on-line GC (Shimadzu GC-8A) with a thermal conductivity detector (TCD). 3 Results and Discussion 3.1 Optimization of Catalyst Composition Our previous investigation showed that, among a series of CNT-promoted co-precipitated Ni i Mo j K k oxide catalysts, the catalyst with the composition of Ni 1 Mo 1 K %CNTs displayed the highest catalytic activity for HAS from syngas. In the present work, with the composition of Ni 1 Mo 1 K 0.05 host, i.e. Ni/Mo/K = 1/1/0.05 (molar ratio), remaining unchanged and using the CNTs as the support of catalyst, the effect of Ni 1 Mo 1 K loading amount on the Fig. 1 HAS reactivity over the x%ni 1 Mo 1 K 0.05 /CNTs with varying (Ni 1 Mo 1 K 0.05 )-loading; reaction conditions: 2.0 MPa, 508 K, V(H 2 )/ V(CO)/V(N 2 ) = 45/45/10 and GHSV = 1,800 ml STP h -1 g -1 performance of the corresponding CNT-supported catalyst for HAS was investigated. The results showed that the observed carbon-containing products involved C 1 3 -alcohols and DME (noted as C 1 3 -oxygenates or total oxygenates ) and C 1 4 -hydrocarbons (noted as total HC ), as well as CO 2 resulting from the WGS side-reaction. The 15%Ni 1 Mo 1 K 0.05 /CNTs catalyst displayed the best catalytic performance. Under the reaction conditions of 2.0 MPa, 508 K, V(H 2 )/V(CO)/V(N 2 ) = 45/45/10 and GHSV = 1,800 ml STP h -1 g -1 ), the conversion of CO hydrogenation (noted as X CO-hydr. ) reached 8.5%, with the corresponding yield of total oxygenates (i.e., the product of X CO-hydr. and S Total-oxy., symbolized as Y Total-oxy. ) reaching 5.3% over this catalyst; while this value of the other four catalysts with varying Ni 1 Mo 1 K loading amounts was successively 3.7, 5.0, 4.8 and 4.3% (see Fig. 1). As a comparative study, the AC-supported counterpart, 15%Ni 1 Mo 1 K 0.05 /AC, was prepared and evaluated under the same reaction conditions. The result showed that over this catalyst, the X CO-hydr. reached 5.7%, with the corresponding space time-yield of total oxygenates (STY Total-oxy. ) reaching 34 mg h -1 g -1. This was merely 64% of the corresponding value (53 mg h -1 g -1 ) of the 15%Ni 1 Mo 1 K 0.05 /CNTs (see Table 1). Considering the marked difference in the specific surface area between the two types of C-supports (130 vs. 830 m 2 g -1 for CNTs vs. AC), the optimization of Ni 1 Mo 1 K loading amount on the AC-support was done. The result showed that the optimal Ni 1 Mo 1 K loading amount was 35% (mass percentage) for the AC. Over the 35%Ni 1 Mo 1 K 0.05 /AC catalyst, the X CO-hydr. reached 7.7% under the same reaction conditions, with the corresponding STY Total-oxy. reaching 46 mg h -1 g -1, which was 87% of the corresponding value (53 mg h -1 g -1 ) of the 15%Ni 1 Mo 1 K 0.05 /CNTs (see Table 1).

4 174 C.-H. Ma et al. Table 1 Reactivity of HAS over the CNT-supported Ni Mo K catalyst and the reference systems Catalyst React. conditions X CO (%) to Selectivity of CO-hydrogenation product (C%) STY of total oxy. (mg h -1 g -1 ) ROH? HC CO 2 Total oxy. Total HCs MeOH EtOH PrOH DME 15%Ni 1 Mo 1 K 0.05 /CNTs 15%Ni 1 Mo 1 K 0.05 /AC (a) (b) (a) %Ni 1 Mo 1 K 0.05 (a) /AC (b) Ni 1 Mo 1 K 0.05 (b) Reaction conditions: (a) 2.0 MPa, 508 K, V(H 2 )/V(CO)/V(N 2 ) = 45/45/10, GHSV = 1,800 ml STP h -1 g -1 (outlet); (b) 5.0 MPa, 538 K, V(H 2 )/ V(CO)/V(N 2 ) = 45/45/10, GHSV = 5,000 ml STP h -1 g -1 (outlet) 3.2 HAS Reactivity over the CNT-Supported Ni 1 Mo 1 K 0.05 Catalyst and the Reference Systems In order to better understand the performance of the CNTsupported catalyst under the working conditions with higher extent of reaction, the HAS from syngas was conducted under reaction conditions of higher pressure (5.0 MPa) and GHSV (5,000 ml STP h -1 g -1 ). Figure 2 showed the HAS reactivity over the 15%Ni 1 Mo 1 K 0.05 / CNTs catalyst at varying temperatures. The X CO-hydr. increased with increasing temperature, together with slowly descending S Total-oxy. and ascending S Total-HC. In order to obtain high yield of oxygenated products and low consumption of feed-gas, we set 538 K as the optimal operating temperature. Figure 3 showed the product distribution of HAS over the CNT-supported Ni Mo K catalyst and the two reference systems. The products of CO hydrogenation were the Fig. 2 HAS reactivity over the 15%Ni 1 Mo 1 K 0.05 /CNTs catalyst at varying temperatures; reaction conditions: 5.0 MPa, V(H 2 )/V(CO)/ V(N 2 ) = 45/45/10, GHSV = 5,000 ml STP h -1 g -1 mixtures consisting of C 1 3 -alcohols, C 1 4 -hydrocarbons and DME. The support could significantly influence the product distribution of CO hydrogenation. Under the reaction conditions of 5.0 MPa, 538 K, V(H 2 )/V(CO)/ V(N 2 ) = 45/45/10, GHSV = 5,000 ml STP h -1 g -1, S Totaloxy reached 59.0 and 56.2% over the 15%Ni 1 Mo 1 K 0.05 / CNTs and 35%Ni 1 Mo 1 K 0.05 /AC, respectively, while merely 46.6% over the non-supported co-precipitated Ni 1 Mo 1 K 0.05 host system. Under the aforementioned reaction conditions, the X COhydr. reached 11.9% over the 15%Ni 1 Mo 1 K 0.05 /CNTs catalyst, with the corresponding STY Total-oxy. being 205 mg h -1 g -1. This value was 1.16 times that (176 mg h -1 g -1 ) of the 35%Ni 1 Mo 1 K 0.05 /AC catalyst, and 1.76 times that (116 mg h -1 g -1 ) of the non-supported Ni 1 Mo 1 K 0.05 host system (see Table 1). Moreover, the WGS side-reaction was markedly inhibited, with the X CO- WGS being 2.3% for the former and 9.0 and 9.2% for the latter two. The apparent activation energy (E a ) of for the HAS reaction over the CNT-supported catalyst and the two reference systems was measured under reaction conditions of 2.0 MPa, K, V(H 2 )/V(CO)/V(N 2 ) = 45/45/10 and GHSV = 8,000 ml STP h -1 g -1, with mass transfer limitation ruled out. The results were shown in Fig. 4. The E a measured on the 15%Ni 1 Mo 1 K 0.05 /CNTs catalyst was 62.3 kj mol -1 for the reaction of CO hydrogenation. This value was very close to that (62.5 kj mol -1 ) of the 35%Ni 1 Mo 1 K 0.05 /AC, but lower than that (76.9 kj mol -1 ) of the Ni 1 Mo 1 K These results suggest that there was little difference between the two catalysts supported, respectively, on two carbon-supports (15%Ni 1 Mo 1 K 0.05 / CNTs and 35%Ni 1 Mo 1 K 0.05 /AC), but there was a little disparity between the supported catalysts and the nonsupported co-precipitated host system (Ni 1 Mo 1 K 0.05 ), in the main reaction pathway of the hydrogenation-conversion of syngas over these catalysts.

5 CNT-Supported Ni Mo K Catalyst for HAS 175 Fig. 4 Arrhenius plots of HAS over the catalysts: (a) 15%Ni 1 Mo 1 K 0.05 /CNTs; (b) 35%Ni 1 Mo 1 K 0.05 /AC; (c) Ni 1 Mo 1 K 0.05 ; taken under the reaction conditions of 2.0 MPa, K, V(H 2 )/ V(CO)/V(N 2 ) = 45/45/10 and GHSV = 8,000 ml STP h -1 g -1 CNT-support, as compared with those of the tested catalyst of 35%Ni 1 Mo 1 K 0.05 /AC (Fig. 5b). The EDX analysis results (Table 2) demonstrated that carbon was the dominant element at the surface of the two supported Ni 1 Mo 1 K 0.05 catalysts, with the atomic percentage at 90.10% and 85.25%, respectively. The relative content (molar ratio) of the three catalytically active elements, Ni, Mo and K, in each catalyst was in line with the formula for preparation of the respective catalyst on the whole XRD Analysis Fig. 3 Product distribution of HAS over the catalysts: (a) 15%Ni 1 Mo 1 K 0.05 /CNTs; (b) 35%Ni 1 Mo 1 K 0.05 /AC; (c) Ni 1 Mo 1 K 0.05 ; the reaction conditions were the same as in Fig. 2 except T = 538 K 3.3 Characterization of the Catalysts SEM and EDX Characterization Figure 5a showed the SEM image of the tested catalyst of 15%Ni 1 Mo 1 K 0.05 /CNTs. The Ni Mo K nanoparticles were smaller and more evenly dispersed on the surface of the The XRD analysis of the tested catalysts showed that the position and shape of the XRD pattern of the catalyst supported on the CNTs were very similar to those of the system supported on the AC, except the features at 2h = 26.1, 43.1 and 53.5, which was due to the diffraction of (002), (100) and (004) plane of graphite-like tube-wall of the CNTs [16, 18]. However, they were markedly different from those of non-supported Ni 1 Mo 1 K 0.05 system (see Fig. 6). On the CNT(or AC)- supported system, the observed Ni and Mo components existed mainly in the form of NiO (2h = 43.3 ) and MoO 2 (2h = 26.0, 37.0 and 53.6 ), although the presence of alloy phases of MoNi 4 (with the weak, even ambiguous, XRD features at 2h = 50.4 and 74.4 ) could not be excluded [19]. In contrast, on the non-supported Ni 1 Mo 1 K 0.05 host, the observed crystallite phases due to the Ni Mo components were mainly Mo 0.84 Ni 0.16 (2h = 36.8, 42.6, 61.3 ) and MoNi 4 (2h = 44.1, 50.4, 74.7 ) alloys, in addition to trace level of MoO 3 and NiO, of which the weak XRD features appeared at 2h = 39.2 and 43.3, respectively [19].

6 176 C.-H. Ma et al. Fig. 6 XRD patterns of the tested catalysts: (a) 15%Ni 1 Mo 1 K 0.05 / CNTs; (b) 35%Ni 1 Mo 1 K 0.05 /AC; (c) Ni 1 Mo 1 K 0.05 Fig. 5 SEM images of the tested catalysts: (a) 15%Ni 1 Mo 1 K 0.05 / CNTs; (b) 35%Ni 1 Mo 1 K 0.05 /AC Table 2 EDX-observed element composition of the surface of the tested catalysts (the average of the results taken on three samples of each type of catalyst) Element 15%Ni 1 Mo 1 K 0.05 /CNTs 35%Ni 1 Mo 1 K 0.05 /AC Weight% Atomic% Weight% Atomic% C O K Under EDX-observation limitation Ni Mo Total XPS-Analysis The XPS analysis of the tested catalysts revealed that the significant difference between the supported catalysts and the non-supported co-precipitated host system existed also in the valence-state or micro-environment of their surface Ni and Mo species. Referring to Ref. [20] and through computer-fitting, we found that each of the Ni(2p)-XPS spectra in Fig. 7A contained the contribution from four kinds of surface Ni-species with different valence-states or micro-environments (also see Table 3). At the surface of the CNT(or AC)-supported catalyst, the major Ni-species was NiO(OH) [Ni(2p 3/2 ) = ev(b.e.)], secondary was Ni(OH) 2 [Ni(2p 3/2 ) = ev(b.e.)], and both NiO [Ni(2p 3/2 ) = ev(b.e.)] and Ni 0 [Ni(2p 3/2 ) = ev(b.e.)] were in minority. At the surface of the non-supported Ni 1 Mo 1 K 0.05 host, the molar percentage of Ni 0 -species dramatically increased and that of NiO(OH) greatly decreased. Analysis and fitting of the Mo(3d)-XPS spectra (Fig. 7B) were also done following Ref. [21]. The results (see Table 3) showed that, at the surface of the CNT(or AC)-supported catalyst, a considerable part of the Mo 6? [Mo(3d 5/ 2) = ev(b.e.)] was reduced to lower valence: major portion to Mo 5? [Mo(3d 5/2 ) = ev(b.e.)] and minor to Mo 4? [Mo(3d 5/2 ) = ev(b.e.)]. In comparison, at the surface of the non-supported Ni 1 Mo 1 K 0.05 host, almost half of the Mo 6? was reduced to Mo 0, most probably in the form of Ni Mo alloys [Mo(3d 5/2 ) = ev(b.e.)], and the molar percentage of Mo 5? and Mo 4? species was considerably low. The aforementioned XPS results were in good agreement with the XRD results, and strongly implied that it was those surface NiO(OH) and Mo 4? /Mo 5? species that were closely related to the formation of the C 1 3 -oxygenated products. The sequence of relative content (mol%) of NiO(OH) and Mo 4? /Mo 5? species at the functioning surface of the three catalysts can be expected as: 15%Ni 1 Mo 1 K 0.05 /CNTs [ 35%Ni 1 Mo 1 K 0.05 /AC [ Ni 1 Mo 1 K This sequence was consistent with the observed sequence of Y Total-oxy. of the HAS over these catalysts.

7 CNT-Supported Ni Mo K Catalyst for HAS 177 Fig. 7 XPS spectra of Ni(2p) (A) and Mo(3d) (B) of the tested catalysts: (a) 15%Ni 1 Mo 1 K 0.05 /CNTs; (b) 35%Ni 1 Mo 1 K 0.05 /AC; (c) Ni 1 Mo 1 K H 2 -TPD Test H 2 -TPD measurements provided useful information about hydrogen ad-species and their relative concentrations at the surface of catalysts. Figure 8a showed the TPD profile of hydrogen adsorbed on the pre-reduced catalyst supported on the CNTs. It contained a lower-temperature feature (region I, K) and a higher-temperature feature (region II, K). The former was due to the desorption of weakly adsorbed H-species (probably including molecularly adsorbed hydrogen H 2 (a) and weakdissociatively adsorbed H-species), while the latter resulted from the desorption of H-species chemisorbed strongly, most probably dissociatively. The H 2 -TPD profile of the AC-supported system (Fig. 8b) was close to that of the CNT-supported catalyst in appearance but much weaker in area-intensity. It is conceivable that, at the operation temperatures ( K) for the HAS, the surface concentration of the weakly adsorbed H-species associated with region-i was considerably low, and most of H-adspecies at the surface of functioning catalysts were those associated Table 3 XPS binding energy and relative content of the Ni and Mo species at the surface of the tested catalysts Catalyst sample Relative content (mol%) Ni n? (2p 3/2 ) Mo n? (3d 5/2 ) Ni 0 NiO Ni(OH) 2 NiOOH Mo 0 Mo 4? Mo 5? Mo 6? B.E. & ev B.E. & ev B.E. & ev B.E. & ev B.E. & ev B.E. & ev B.E. & ev B.E. & ev 15%Ni 1 Mo 1 K 0.05 /CNTs %Ni 1 Mo 1 K 0.05 /AC Ni 1 Mo 1 K NiMo-alloy Reaction conditions: 5.0 MPa, 538 K, V(H 2 )/V(CO)/V(N 2 ) = 45/45/10, GHSV = 5,000 ml h -1 g -1 (outlet)

8 178 C.-H. Ma et al. Fig. 8 H 2 -TPD profiles of hydrogen adsorbed on the prereduced catalysts and related systems: (a) 15%Ni 1 Mo 1 K 0.05 /CNTs; (b) 35% Ni 1 Mo 1 K 0.05 /AC; (c) Ni 1 Mo 1 K 0.05 ;(d) CNTs; (e) AC with region-ii. An estimated ratio of the relative areaintensity of the H 2 -TPD profiles in region-ii was 50/35 for 15%Ni 1 Mo 1 K 0.05 /CNTs vs. 35%Ni 1 Mo 1 K 0.05 /AC. This result suggested that the sequence of increasing concentration of H-adspecies at the surface of functioning catalysts was: 15%Ni 1 Mo 1 K 0.05 /CNTs [ 35%Ni 1 Mo 1 K 0.05 /AC, in line with the observed sequence of reaction activity for the CO hydrogenation over the two catalysts. It is worth noting that the difference of the CNT-free coprecipitated Ni 1 Mo 1 K 0.05 host catalyst from the CNT(or AC)-supported system lies not simply in their capability of adsorbing hydrogen (with an estimated ratio of the relative area-intensity of the H 2 -TPD profiles in region-ii being 50/ 38 for 15%Ni 1 Mo 1 K 0.05 /CNTs vs. Ni 1 Mo 1 K 0.05 ) (Fig. 8a vs. 8c), but also in the property of H-adspecies on the two types of systems. Our laser Raman spectroscopic (LRS) investigation showed that the H-adspecies detected on the H 2 -prereduced Ni 1 Mo 1 K 0.05 host were the H(a) adsorbed on Ni 0 -sites and Mo 0 -sites (with the m Ni H and m Mo H at 1,983 and 1,878 cm -1, respectively), while those detected on the H 2 -prereduced 15%Ni 1 Mo 1 K 0.05 /CNTs were the H(a) adsorbed on sp 2 -C-sites (with the m C H at 2,935 and 3,233 cm -1 for CH 2 and CH, respectively). High hydrogenation activity of H-species adsorbed zero-valence metal (Ni 0 and Mo 0 ) sites enhanced the tendency of CO hydrogenation to generate hydrocarbon, thus leading to considerable increase of S Total-HC, together with marked decrease of S Total-oxy (see Table 1). 3.4 Nature of Promoter Action by the CNTs The aforementioned results showed that using the CNTs in place of AC as support of the Ni Mo K catalyst caused little change in the E a for the HAS reaction, yet it led to a marked increase, at the surface of the functioning catalyst, of the molar percentage of NiO(OH) and Mo 4? /Mo 5? species active catalytically in the respective total amounts of Ni and Mo. On the other hand, based on the results of H 2 -TPD and LRS, it could be inferred that there exists a considerably larger amount of H-adspecies in the form of (sp 2 -C) H at the surface of the Ni Mo K catalyst supported on the CNTs under the reaction conditions for the HAS. Those H-adspecies could be readily transferred to Ni i Mo j K k active sites via CNT-promoted hydrogen spillover and thus increase the rate of surface hydrogenation reactions in the HAS. Due to the lack of the graphene-like tube-wall structure and sp 2 -C-constructed surface, AC may simply act as the catalyst-support with low capability of adsorbing H 2 (see Fig. 8e) and, thus, displays rather limited promoter effect. In contrast to the case of the catalyst supported on the CNTs or AC, in which the supported Ni Mo components were highly dispersed (see Fig. 5), the Ni Mo components of the co-precipitated Ni Mo K host catalyst were easy to be deeply reduced and aggregated to generate Ni Mo alloys (see Fig. 6) under the reaction conditions of HAS. At the functioning surface of the co-precipitated Ni 1 Mo 1 K 0.05 catalyst, relatively larger amounts of the surface Ni and Mo species were Ni 0 and Mo 0 (the two kinds of surface-species active catalytically related closely to selective formation of hydrocarbons, especially methanation of CO), rather than NiO(OH) and Mo 4? /Mo 5? (see Fig. 7; Table 3). This, thus, resulted in the low selectivity of generating alcohols from CO hydrogenation (see Fig. 3c; Table 1). In conclusion, in the developed CNT-supported Ni Mo K catalyst for HAS from syngas, the CNTs played dual roles as a catalyst-support and a promoter. The CNT-supported Ni 1 Mo 1 K 0.05 catalyst achieves highly effective and selective formation of C 1 3 -alcohols and DME from CO hydrogenation. Acknowledgments The work is supported by National Basic Research ( 973 ) Project (2009CB939804) and Fujian Provincial Natural Science Foundation Project (E ) of China. References 1. Fujimoto K, Oba T (1985) Appl Catal 13: Inoue M, Miyake T, Takegami Y, Inui T (1987) Appl Catal 29: Tatsumi T, Muramatsu A, Fukunaga T, Tominaga H (1988) In: Phillips MJ, Ternan M (eds) Proceedings of the 9th international congress on catalysis, vol 2, pp Murchison CB, Conway MM, Stevens RR, Quarderer GJ (1988) In: Phillips MJ, Ternan M (eds) Proceedings of the 9th international congress on catalysis, vol 2, pp Li Z, Fu Y, Bao J, Jiang M, Hu T, Liu T, Xie Y (2001) Appl Catal A 220(1):21 30

9 CNT-Supported Ni Mo K Catalyst for HAS Li D, Yang C, Qi H, Zhang H, Li W, Sun Y, Zhong B (2004) Catal Commun 5: Forzatti P, Tronconi E, Pasquon I (1991) Catal Rev Sci Eng 33: Stiles AB, Chen F, Harrison JB, Hu X, Storm DA, Yang HX (1991) Ind Eng Chem Res 30: Slaa JC, van Ommen JG, Ross JRH (1992) Catal Today 15: de Jong KP, Geus JW (2000) Catal Rev Sci Eng 42(4): Serp P, Corrias M, Kalck P (2003) Appl Catal A 253(2): Zhang HB, Lin GD, Yuan YZ (2005) Curr Top Catal 4: Zhang HB, Liang XL, Dong X, Li HY, Lin GD (2009) Catal Surv Asia 13(1): Iijima S (1991) Nature 354: Chen P, Zhang HB, Lin GD, Hong Q, Tsai KR (1997) Carbon 35(10 11): Chen P, Zhang HB, Lin GD, Tsai KR (1998) Chem J Chin Univ 19(5): Ma CH, Dong X, Tang PP, Lin GD, Feng XT, Zhang HB (2008) Chem Ind Eng Prog (Chin) 27(suppl): Zhang HB, Lin GD, Zhou ZH, Dong X, Chen T (2002) Carbon 40(13): XRD data bank attached to X Pert PRO X-ray Diffractometer, PANalytical, The Netherlands (2003) 20. Moulder JF, Stickle WF, Sobol PE, Bomben KD (1995) Handbook of X-ray photoelectron spectroscopy a reference book of standard spectra for identification and interpretation of XPS data. Physical Electronics Inc., Eden Prairie 21. Abart J, Delgado E, Ertl G, Jeziorowshi H, Knözinger H, Thiele N, Wang XZ, Taglauer E (1982) Appl Catal 2: