Accepted Manuscript Title: Selective hydrogenation of acetylene on SiO2 supported Ni-In bimetallic catalysts: Promotional effect of In Author: Yanjun Chen Jixiang Chen PII: DOI: Reference:
S0169-4332(16)31294-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.06.067 APSUSC 33437
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-3-2016 6-6-2016 11-6-2016
Please cite this article as: Yanjun Chen, Jixiang Chen, Selective hydrogenation of acetylene on SiO2 supported Ni-In bimetallic catalysts: Promotional effect of In, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.06.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective hydrogenation of acetylene on SiO2 supported Ni-In bimetallic catalysts: Promotional effect of In
Yanjun Chen, Jixiang Chen*
[email protected]
Tianjin Key Laboratory of Applied Catalysis Science and Technology, Department of Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
*
Corresponding author. Postal address: Department of
Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel.: +86-22-27890865; fax: +86-22-87894301.
1
Selective hydrogenation of acetylene on SiO2 supported Ni-In bimetallic catalysts: Promotional effect of In
A suitable Ni/In ratio remarkably enhanced the acetylene conversion, the selectivity to ethylene and the catalyst stability. 70
100
Selectivity to ethylene/ %
Acetylene conversion/ %
60
80
60
40
50 40
Ni/SiO2 Ni10In/SiO2 Ni6In/SiO2 Ni4In/SiO2 Ni3In/SiO2 Ni2In/SiO2 NiIn/SiO2
30 20 10
20
0
0
5
10
25
30
Time on stream/ h
35
0
5
10
25
30
Time on stream/ h
35
Highlights
There was a promotional effect of In on the performance of Ni/SiO2. A suitable Ni/In ratio was required for good performance of NixIn/SiO2. Both geometrical and electronic effects of In contributed to good performance. Ni/SiO2 deactivation is mainly owing to phase change from Ni to nickel carbide. The carbonaceous deposit was the main reason for NixIn/SiO2 deactivation.
2
Abstract: Ni/SiO2 and the bimetallic NixIn/SiO2 catalysts with different Ni/In ratios were tested for the selective hydrogenation of acetylene, and their physicochemical properties before and after the reaction were characterized by means of N2-sorption, H2-TPR, XRD, TEM, XPS, H2 chemisorption, C2H4-TPD, NH3-TPD, FT-IR of adsorbed pyridine, and TG/DTA and Raman. A promotional effect of In on the performance of Ni/SiO2 was found, and NixIn/SiO2 with a suitable Ni/In ratio gave much higher acetylene conversion, ethylene selectivity and catalyst stability than Ni/SiO2. This is ascribed to the geometrical isolation of the reactive Ni atoms with the inert In ones and the charge transfer from the In atoms to Ni ones, both of which are favorable for reducing the adsorption strength of ethylene and restraining the C-C hydrogenolysis and the polymerizations of acetylene and the intermediate compounds. On the whole, Ni6In/SiO2 and Ni10In/SiO2 had better performance. Nevertheless, with increasing the In content, the selectivity to the C4+ hydrocarbons tended to increase due to the enhanced catalyst acidity because of the charge transfer from the In atoms to Ni ones. As the Lewis acid ones, the In sites could promote the polymerization. The catalyst deactivation was also analyzed. We propose that the Ni/SiO2 deactivation is mainly attributed to the phase change from metallic Ni to nickel carbide. The introduction of In inhibited the formation of nickel carbide. However, as 3
the In content increased, the carbonaceous deposit became the main reason for the NixIn/SiO2 deactivation due to the enhanced catalyst acidity. Keywords: Selective hydrogenation, Acetylene, Ni-In bimetallic catalyst, Geometrical and electronic effects, Catalyst deactivation. 1. Introduction Ethylene is an important raw material for industrial products, particularly for polyethylene production. Typically, the ethylene stream derived from a naphtha cracker unit contains about 0.1–1% acetylene impurity, which must be reduced to lower than 5 ppm because it poisons the catalyst for the following ethylene polymerization process and eventually degrades the polyethylene quality[1,2]. The most efficient way to remove acetylene is selective hydrogenation [ 3 , 4 , 5 ], during which acetylene is selectively hydrogenated to ethylene while ethylene is not expected to be further hydrogenated. Also, the polymerization of acetylene as well as the C-C bond hydrogenolysis should be avoided. Thus, there is a high demand on the catalyst for the selective hydrogenation of acetylene. Some transition metals (such as Pd and Ni) have been investigated
for
the
selective 4
hydrogenation
of
acetylene.
Particularly, palladium has been identified to have both high activity and high selectivity and Pd-based catalysts have played a leading role in the industry. To improve the performance of mono-metallic Pd catalyst, a second metal (such as Ag [6], Zn [7], Cu [8], Au[9,10] and Ga[11,12]) is added to form alloy or intermetallic compound. In the alloy (such as Pd-Ag[6]) and intermetallic compound (such as PdGa[11,12]), the second metal atoms can dilute the surface Pd ones and subsequently to form more isolated Pd atoms, while the isolated Pd atoms can restrain the adsorption of acetylene or ethylene via triply-bond that can easily be hydrogenated to ethane. Again, the charge transfer from the second metal to Pd enriches the electron density of the Pd atom, reducing the adsorption strength of acetylene and ethylene and subsequently inhibiting the further hydrogenation of ethylene. Although the Pd-based catalysts are currently preferential for the selective hydrogenation of acetylene, they are also of costly. Therefore, it is highly expected to develop low cost catalysts [13]. Ni-based catalysts are widely used for hydrogenation. However, the mono-metallic Ni catalyst usually gives the poor selectivity to ethylene in the selective hydrogenation of acetylene [14,15], while Ni-based bimetallic and trimetallic catalysts have attracted attention [16,17,18,19]. For instance, the density functional 5
theory (DFT) calculations indicate that the Ni-Zn bimetallic catalysts would be practical, and their higher selectivity to ethylene was experimentally verified compared with the pure Ni catalysts [20]. Furthermore, using DFT and Langmuir-Hinshelood kinetics analysis, Spanjers et al. [21] found that there was a charge transfer from Zn (electronegativity of 1.65) to Ni (electronegativity of 1.91) in the Ni-Zn bimetallic catalyst, which is responsible for the increase of the ethylene selectivity because the adsorption energy of acetylene was lower on the intermetallic compound NiZn than on the metallic Ni. Subsequently, the coverage of acetylene on the catalyst surface was reduced and the propensity for the formation of oligomeric species was inhibited due to the formation of the intermetallic compound. Moreover, the isolation of the Ni atoms due to the Zn ones reduced the multi-σ-bonded surface intermediate species, impeding the C-C dissociation and over-hydrogenation and ultimately enhancing the ethylene selectivity. As mentioned above, compared with the mono-metal, alloy and intermetallic compound are better alternatives for the selective hydrogenation of acetylene. Inspired by this, we speculate that the Ni-In bimetallic catalyst is also potentially promising on the base of the following reasons. The electronegativity of In (1.78) lower than that of Ni(1.91) may lead to the charge transfer from In to Ni, while 6
the formation of the Ni-In alloy and intermetallic compound may make the Ni atoms isolate, which can reduce the multi-σ-bonded sites and enhance the ethylene selectivity. Indeed, Li et al.[22] have experimentally and theoretically revealed the charge transfer from the In atom to the Ni one as well as the Ni-site isolation, which promote the nucleophilic addition process of the C=O group instead of the electrophilic addition of the C=C bond, accounting for largely enhanced
activity
and selectivity
to
the
hydrogenation
of
α,β-unsaturated aldehydes on the Ni−In intermetallic compound. Additionally, the Ni-In alloy and intermetallic compound have been proved to be good catalysts for the hydrogenation of the carboxylic acid to alcohol [23,24,25,26,27]. However, to the best of our knowledge, there have been no reports about the hydrogenation of acetylene on the Ni-In bimetallic catalysts. Here, we aimed at investigating the effect of In on the performance of Ni-based catalyst for the selective hydrogenation of acetylene and the relationship between the catalyst structure and performance. To reduce the support effect, silica was selected as support. The supported catalysts with different Ni/In ratios (between 10 and 1.0) were prepared and systematically characterized. We found that the Ni-In bimetallic catalysts with suitable Ni/In ratios possessed much better performance than Ni/SiO2. Additionally, the 7
catalyst deactivation took place during the reaction and was also analyzed.
2. Experimental 2.1 Catalyst preparation The SiO2-supported Ni and bimetallic Ni-In catalysts were prepared by the incipient wetness method. A commercial B-type silica was purchased from Qingdao Haiyang Chemicals Co. Ltd., Qingdao, China. First, SiO2 (150-300 µm in diameter) was incipiently impregnated with an aqueous solution of Ni(NO3)2 or a mixing aqueous solution Ni(NO3)2 and In(NO3)3. After drying at 120 oC for 12 h and calcination at 500 oC for 4 h, the catalyst precursor in the oxidation state was obtained. Second, the precursor was reduced by a hydrogen flow on a quartz fixed-bed reactor (12 mm in inner diameter) at 450 °C for 2 h to produce the catalyst. The H2 flow was set at 200 mL·min-1 per gram of the precursor. Here, the bimetallic Ni-In catalysts (denoted as NixIn/SiO2) with different Ni/In molar ratios (x=10, 6, 4, 3, 2 and 1) were prepared. In Ni (Ni/SiO2) and NixIn/SiO2, mNi/(mNi+mSiO2) maintained to be 8.0 wt.%. That is, the Ni content in NixIn/SiO2 decreased with decreasing Ni/In molar ratio. 2.2 Catalyst characterization 8
Hydrogen temperature-programmed reduction (H2-TPR) was carried out on a homemade instrument to investigate the reducibility of the catalyst precursor. 50 mg precursor was loaded into a quartz U-tube reactor (4 mm in inner diameter) and heated at a heating rate of 10 oC·min-1 in a 10% H2/N2 flow (60 mL·min-1). The hydrogen consumption was determined by a thermal conductivity detector (TCD). X-ray diffraction (XRD) patterns were obtained on a D8 Focus powder diffractometer operated at 40 kV and 40 mA using Cu Kα radiation (λ = 0.15406 nm). The nickel-containing crystallite size was calculated using Scherrer equation. The STEM image was obtained on a JEOL JEM-2100F high resolution transmission electron microscopes equipped with energy dispersive X-ray spectroscopy (EDS) at 200 kV. The powder sample was ultrasonically dispersed in ethanol and then deposited on a carbon film supported on a copper grid. H2 chemisorption was conducted on the same apparatus as H2-TPR. The catalyst precursor (100 mg) was in situ reduced at 450 o
C for 1 h in a H2 flow (60 mL·min-1), and then purged by a N2 flow
(60 mL·min-1) at 450 oC for 1 h. After the temperature cooling to 30 o
C and TCD was stable, H2 (50 mL) pulses were injected into the N2
stream until the effluent areas of consecutive pulses were constant. 9
The X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000VersaProbe instrument with Al Kα radiation (1486.6 eV) and the binding energy was corrected using adventitious carbon (C1s at 284.6 eV). Since the passivation can mask the electron states of the fresh catalysts, the passivated catalysts were sputter-cleaned with an Ar+ ion beam. C2H4-TPD was used to investigate the interaction between C2H4 and catalysts on the laboratory-made instrument. The catalyst precursor (100 mg) was loaded into a quartz reactor (4 mm in inner diameter) and reduced at 450 oC for 1 h in a H2 flow (60 mL·min-1), and then purged by a He flow (40 mL·min-1) at 450 oC for 1 h. After the temperature cooling to 30 oC, the reduced catalyst was exposed to a mixture of ethylene and He for about 10 min, and then purged with the He flow at 30 oC to remove physically adsorbed ethylene. Afterward, C2H4-TPD was performed at a heating rate of 10 o
C·min-1 in the He flow (40 mL·min-1). The desorbed C2H4 was
detected by a TCD. NH3-TPD
was
also
measured
on
the
laboratory-made
instrument. 150 mg catalyst precursor was loaded into a quartz reactor (4 mm in inner diameter) and reduced with a H2 flow (60 mL·min-1) at 450 °C for 1 h and then cooled to 100 °C. After NH3 adsorption for 30 min, the sample was purged by a He flow to 10
remove the physically adsorbed NH3. NH3-TPD was performed in a He flow (60 mL·min-1) at a heating rate of 15 oC·min-1. The desorbed NH3 was detected by a TCD. Transmission infrared spectra were in situ collected on a CSY-5 reactor cell placed in a FTIR spectrometer (Bruker Tensor 27) at a resolution of 4 cm-1 and using 64 scans. The finely powdered catalyst precursor was mixed with KBr powder (mass ratio of 1/4) to obtain a good pressed wafer. The wafer was placed on the sample holder and reduced in a H2 flow at 450 oC for 60 min, followed by the evacuation at 1.0 × 10-3 Pa for 120 min. The sample was then cooled to room temperature and a background spectrum was recorded. After pyridine adsorption for 20 min, the cell was evacuated at 1.0 × 10-3 Pa for 30 min and a spectrum was recorded. Afterward, the sample was increased to 100 oC and the cell was evacuated for 30 min, followed by recording a spectrum. Thermogravimetric (TG) analysis and differential thermal analysis (DTA) were carried out on a Mettler-Toledo TGA 1/SF instrument at a heating rate of 10 °C·min-1 in an air flow (100 mL·min-1). N2 adsorption-desorption isotherms were measured on a Micromeritics
ASAP
2020
apparatus
at
-196
°C.
The
Brunauer–Emmett–Teller (BET) equation was used to calculate the 11
specific surface area (SBET). The total pore volume (Vp) was estimated at a relative pressure of 0.99. The mean pore diameter (d) was calculated using d = 4Vp/SBET. 2.3 Catalytic test The selective hydrogenation of acetylene was performed on an atmospheric fixed-bed quartz tube reactor. From the viewpoint of the practical application, it is very valuable to perform the selective hydrogenation in the ethylene-rich environment. Here, to avoid the interference of the excess ethylene in the selective hydrogenation of acetylene via the competitive adsorption followed by some reactions (such as hydrogenation and polymerization), a feed mixture without ethylene was adopted. 0.5 g precursor was loaded into the reactor and in situ reduced at 450 °C in a hydrogen flow (100 mL·min-1) for 2 h. After cooling to the reaction temperature (180 °C), the reactant stream containing 1.0 vol % acetylene and 5.0 vol % hydrogen balanced with argon was introduced into the reactor with a gas hourly space velocity of 36,000 mL·h-1·g-1. All gas flows were controlled by the mass flow controllers. The gaseous products were on-line analyzed on a SP-3420 gas chromatography equipped with a flame ionization detector and a HP-AL/S capillary column (50 m × 0.535 mm×15 µm).
12
The conversion (X) of acetylene and the selectivity to product i (Si) were calculated as the following formulas: X=(1- n/n0) ×100% Si=(αni/2)/(n0 - n)×100% where n0 and n denote the moles of acetylene in the feed and product, respectively. ni and α denote the mole of product i (including methane, ethane, ethylene, propane) and the number of carbon atoms in the product i (e.g. α=2 for ethylene and ethane, and α=1 for methane), respectively. The selectivity to C4+ hydrocarbons (SC4+) (including C4 and C6 hydrocarbons and green oil) was calculated according to the formula: SC4+=1-∑Si (i<4)
3 Results and discussion 3.1 Catalyst reactivity In the present work, during the selective hydrogenation of acetylene, apart from ethylene, other products (methane, ethane, propane and C4 and C6 hydrocarbons) were also detected in the gas stream. In addition, there was liquid green oil found in the gas pipeline.
13
The possible three pathways during the selective hydrogenation of acetylene are presented in scheme 1 [ 28 ]. (1) The partial hydrogenation of acetylene produces ethylene, which either desorbs from the catalyst surface or is further hydrogenated to form ethane. (2) Acetylene is directly hydrogenated to ethane via ethylidyne intermediate species [29]. (3) The strongly adsorbed acetylene is converted to the C4 and C6 hydrocarbons as well as liquid green oils via polymerization. Here, the C4+ hydrocarbons denote the ones with carbon number ≥ 4. Additionally, methane and propane are derived from the hydrogenolysis of the C-C bond in even-numbered carbon species [30]. The conversion of acetylene and the selectivities to different products are presented in Fig.1. Fig.1(A) shows the acetylene conversions on Ni/SiO2 and NixIn/SiO2 with time on stream. The conversion on In/SiO2 was about 0.1% (that is, almost no activity) and not presented in Fig.1(A). For all catalysts, the conversions were 100% at the initial stage and then gradually decreased as the reaction proceeded. Compared with Ni/SiO2 on which the conversion maintained at 100% only for about 2 hours, NixIn/SiO2 (x>1) gave a longer time at the conversion of 100 %, especially for the catalysts with larger Ni/In ratios. The conversion on Ni10In/SiO2 was always 100% during 36 hours. However, with decreasing the Ni/In ratio from 6 to 2, the time maintained at the conversion of 100% became short. The decrease of 14
the conversion indicates the catalyst deactivation. The less the Ni/In ratio was (i.e., the larger the In content was), the more remarkable the catalyst deactivation. On NiIn/SiO2, the conversion sharply decreased from about 100% to 37% at the second hour. Fig.1(B) shows the selectivity to ethylene with time on stream. Obviously, the ethylene selectivity on each catalyst reached a relative steady state after a time. The ethylene selectivity on Ni/SiO2 was almost zero at the first hour, and quickly increased to about 50% after 5 h and maintained at about 45% after about 24 h. Interestingly, it took only about 1 h to achieve relatively stable ethylene selectivity on NixIn/SiO2 (except Ni10In/SiO2 on which the selectivity became stable after about 5 hours, similar to the case on Ni/SiO2). Also, the ethylene selectivities on NixIn/SiO2 (x>2) were much higher than that on Ni/SiO2 during the steady stage. Moreover, they tended to increase with increasing the Ni/In ratio, and maintained above 60% on Ni10In/SiO2 and Ni6In/SiO2. Exceptionally, NiIn/SiO2 gave lower ethylene selectivity than Ni/SiO2. On the whole, during the relatively steady stage, the ethylene selectivity decreased on the catalysts in the order: Ni10In/SiO2 ≈ Ni6In/SiO2 > Ni4In/SiO2> Ni3In/SiO2 > Ni2In/SiO2 > Ni/SiO2 > NiIn/SiO2.
15
Fig.1(C) shows the selectivity to ethane along with time. On the whole, the ethane selectivities decreased with time on stream. The ethane selectivity on Ni/SiO2 was as high as about 55% at the first hour and decreased quickly to about 10% after 5 h and then slowly to about 5% after 10 h. The initial ethane selectivities on NixIn/SiO2 were lower than that on Ni/SiO2 and decreased with decreasing the Ni/In ratio. Apart from Ni10In/SiO2, the other NixIn/SiO2 catalysts gave the ethane selectivity lower than 2.0% after 5 h. Particularly, the ethane selectivity on NiIn/SiO2 was always lower than 0.5% during 10 h. Clearly, the addition of In to Ni/SiO2 efficiently suppressed the further hydrogenation of ethylene to ethane, which was more remarkable with increasing the In content. As shown in Fig.1(D) and (E), Ni/SiO2 gave the initial propane selectivity of 5% and the initial methane selectivity of 2%, both of which rapidly decreased to nearly zero along with the reaction. However, the selectivities to propane and methane on all Ni-In bimetallic catalysts were negligible. That is, the NixIn/SiO2 catalysts had much lower activities for the C-C bond hydrogenolysis than Ni/SiO2. Fig.1(F) shows the selectivity to the C4+ hydrocarbons. Apart from NiIn/SiO2 and Ni2In/SiO2, the other Ni-In bimetallic catalysts 16
always gave lower selectivity to the C4+ hydrocarbons than Ni/SiO2 during 36 hours. The selectivity to the C4+ hydrocarbons on Ni10In/SiO2 was the lowest, while it increased gradually as the Ni/In ratio
decreased.
In
addition, the selectivities
to the
C4+
hydrocarbons on all of catalysts tended to increase with time on stream, which was harmful for the catalyst stability. As indicated above, the addition of In to Ni/SiO2 is favorable for enhancing the conversion of acetylene and the selectivity to ethylene and inhibiting the over-hydrogenation of ethylene to ethane, the hydrogenolysis of C-C bond as well as the polymerization. Also, although the deactivation took place for both Ni/SiO2 and NixIn/SiO2, the NixIn/SiO2 catalysts with higher Ni/In ratios (especially x=10 and 6) had much better stability than Ni/SiO2. To obtain the insight into these results, the catalysts were systematically characterized. 3.2 Catalyst characterization 3.2.1 H2-TPR Fig.2 demonstrates the H2-TPR profiles of the catalyst precursors. The Ni/SiO2 precursor gave a main peak at about 400 oC with a shoulder at high temperature, which are ascribed to the reductions of bulk NiO and nickel silicate[31], respectively. The In/SiO2 precursor exhibited a broad reduction peak started from 200 17
to 600 °C with the relatively obvious peaks at about 440 and 530 °C. Clearly, the In/SiO2 precursor was more difficultly reduced than the Ni/SiO2 one. Apart from the Ni10In/SiO2 one, the other NixIn/SiO2 precursors gave only one reduction peak with the temperature between those of the Ni/SiO2 and In/SiO2 precursors, implying the simultaneous reductions of the indium and nickel species. In other words, the Ni species promoted the reduction of the indium oxides. This is ascribed to the easy H2 dissociation on the Ni species to form more reactive hydrogen atoms, followed by spill-over to the indium oxide. In addition, the peaks for the NixIn/SiO2 precursors were narrower than those for the Ni/SiO2 and In/SiO2 precursors. Also, they became narrower and narrower with increasing indium content, further indicating that there was a strong interaction between the indium and nickel species which might form a solid solution. This interaction led to forming Ni-In alloy and Ni2In (an intermetallic compound) as indicated by the XRD results of the reduced catalysts. 3.2.2 XRD Fig. 3 shows the XRD patterns of the reduced catalysts. In the pattern of In/SiO2 (not shown here), only the peak due to metallic In was found at 2θ =32.8o. For Ni/SiO2, three diffraction peaks at 2θ=44.5o, 51.8o and 76.4o correspond to the (111), (200) and (220) 18
reflections of the fcc Ni metal phase (PDF 04-0850). In the patterns of the NixIn/SiO2(x≥6) catalysts, the peak due to the Ni(111) reflection gradually shifted to low angle with the increase of In content, and the more the In content was, the more obvious the shift. This indicates the incorporation of the In atoms into the metallic Ni lattice and subsequently the formation of Ni-In alloy because the In atom has larger radius (1.66Å) than the Ni one (1.24Å). In Ni3In/SiO2, apart from the metallic Ni, the intermetallic compound Ni2In was detected at 2θ=30.2o, 34.9 o and 43.2o (PDF 42-1033). There was only the Ni2In phase found in Ni2In/SiO2 and NiIn/SiO2. Compared with those for Ni2In/SiO2, the peaks due to Ni2In located at slightly lower angles for NiIn/SiO2. We speculate that this might be related to more In atoms in the Ni2In phase. Clearly, there was a strong interaction between Ni and In formed during the catalyst preparation, leading to formation of the Ni-In alloy at high Ni/In ratio (≥6) and the intermetallic compound Ni2In at low Ni/In ratio (≤3). As a result, the geometrical property of metallic Ni was modified by the introduction of In into Ni/SiO2. Table 1 lists the average sizes of metallic Ni and Ni-containing crystallites. The metallic Ni crystallite size was about 8.7 nm in Ni/SiO2. However, the Ni crystallite size decreased with increasing In content, and it was 4.3 nm in Ni3In/SiO2, indicating that the introduction of In reduced 19
the Ni-containing crystalline size. Additionally, the average Ni2In crystallite sizes in Ni2In/SiO2 and NiIn/SiO2 were 5.2 and 8.3 nm 3.2.3 TEM The XRD result indicates the formation of Ni-In alloy in NixIn/SiO2(x≥6). To obtain the information about the In and Ni distribution in the bimetallic catalysts, Ni6In/SiO2 was selected to be characterized by STEM-EDS (see Fig. 4). Fig. 4(a) shows the EDS spectrum of a random area. The obtained Ni/In atomic ratio of about 6 was very close to the nominal one, implying a homogeneous dispersion of In and Ni. The In and Ni distributions within an individual particle are also shown in the HAADF-STEM-EDS line-scanning spectra (see Fig.4(b) and (c)). Clearly, Ni and In uniformly distributed in the single particle. Thus, the Ni-In alloy with uniform distribution formed in Ni6In/SiO2. Combined with the XRD result, it is reasonably speculated that Ni and In uniformly distributed in other NixIn/SiO2 catalysts, especially for the ones where the Ni2In phase formed. It is well known that alloy and intermetallic compound have different geometrical structure from metallic Ni, subsequently exhibiting different performance. 3.2.4 H2 chemisorption H2 uptakes of the reduced catalysts were measured. H2 uptake of
Ni/SiO2
was
12.8
µmol·g-1. 20
Interestingly,
although the
introduction of In to Ni/SiO2 reduced the Ni-containing crystallite sizes (apart from NiIn/SiO2), it indeed made the H2 uptake decrease. Apart from Ni10In/SiO2 that gave the H2 uptake of 2.1 µmol·g-1, there was no apparent H2 chemisorption for other NixIn/SiO2 catalysts (x>10). That is, the addition of In blocked the H2 adsorption. This is possibly due to the In atoms that diluted Ni surface atoms, while In has a lower activity for the H2 adsorption and activation. The dilution of the surface Ni atoms made the H2 adsorption difficult, and the H2 adsorption and activation might require a longer time, and subsequently the H2 chemisorption could not be measured by the pulse technique adopted here. This interesting issue is very worth further investigating in the future. Even so, the H2 chemisorption result indicates that the H2 adsorption and activation was restrained by the introduction of In to Ni/SiO2, which is beneficial to inhibit the over-hydrogenation of ethylene and subsequently increase the selectivity to ethylene. Additionally, as the Ni/In molar ratio decreased, the decreased Ni content also contributed to no H2 chemisorption on NixIn/SiO2 catalysts (x>10). However, the role of In should be dominating. 3.2.5 XPS XPS was used to explore the effect of the In species on the electronic structure of Ni. Fig. 5(A) shows the XPS spectra in the Ni 21
2p3/2 region for Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2. Ni/SiO2 gave the electron binding energy (BE) at around 852.8 eV that is in accordance with that of Ni0 [32]. Compared with that for Ni/SiO2, the Ni 2p3/2 peak shifted slightly to 852.7 eV for Ni6In/SiO2 and obviously to 852.3 eV for Ni2In/SiO2. The decrease of Ni2p3/2 BE in Ni6In/SiO2 and Ni2In/SiO2 indicates the charge transfer from In to Ni, and the higher the In content was, the more remarkable the charge transfer. This is reasonable because Ni has larger electronegativity (1.9) than In(1.7). Also, Li et al. have confirmed a charge transfer from In to the Ni in the Ni–In Intermetallic compound by means of EXAFS [22]. Fig.5(B) shows the In 3d XPS spectra of Ni6In/SiO2 and Ni2In/SiO2. The BEs of In 3d5/2 and 3d3/2 in Ni6In/SiO2 and Ni2In/SiO2 were higher than those (444eV and 451.3 eV) of metallic In0 [33,34]. This further confirms the charge transfer from In to Ni. Such a charge transfer can contribute to increasing the ethylene selectivity via weakening the adsorption strength of ethylene on the Ni sites. Additionally, Ni2In/SiO2 gave lower In 3d BEs than Ni6In/SiO2. This might be ascribed to its relatively more In amount and less Ni amount. There was indeed more charge transferred from In to Ni, while the charge transfer of a single In atom became less, leading to lower In 3d BE as the In content increased. 22
3.2.6 C2H4-TPD Ethylene is the desired product in the selective hydrogenation of acetylene. During the reaction, it is expected that the formed ethylene quickly desorbs from the catalyst surface without further hydrogenation. That is, the ethylene adsorption on the catalyst surface should be weak to achieve a high selectivity. Here, C2H4-TPD was performed to elucidate the effect of In on the ethylene adsorption property of Ni/SiO2. As shown in Fig. 6, Ni/SiO2 gave two desorption peaks at about 85 and 265 oC, indicative of two adsorption modes with different strength. Referring to the opinion about C2H4-TPD on Pd-based catalysts [35], we temporarily assign the peak at 85 oC to the desorption of π-bonded ethylene and the peak at 265 oC to triply bound species (i.e., ethylidyne). In/SiO2 also gave two main desorption peaks with higher temperatures than Ni/SiO2, indicating that the ethylene adsorption on In/SiO2 was stronger than that on Ni/SiO2. Interestingly, in the profiles of NixIn/SiO2, with decreasing the Ni/In ratio to 4.0, the peak at about 85 oC was still observed and became large, while the peak at about 265 oC became small and nearly disappeared. Surprisingly, as the Ni/In ratio further decreased to 1.0, the peaks at about 265 oC is again visible although it was still small, and there was a new peak at about 430 oC for NiIn/SiO2. In 23
any case, ethylene had weaker adsorption strength on NixIn/SiO2 than on Ni/SiO2. This can be explained by the effect of In on the electronic and geometrical properties of Ni/SiO2 as the discussion in Section 3.3. 3.2.7 NH3-TPD and FT-IR of adsorbed pyridine As shown in Section 3.1, Ni/SiO2 and NixIn/SiO2 catalyst deactivation took place. Usually, the catalyst acidity contributes to carbonaceous
deposit,
subsequently
leading
to
the
catalyst
deactivation. Fig.7 shows the NH3-TPD profiles of Ni/SiO2, NixIn/SiO2 and In/SiO2. Here, to conveniently describe the change tendency of catalyst acidity, the NH3 desorptions are temporarily divided to two zones (i.e., (I) and (II)), and the acid amounts in two zones and the total acid amounts are listed in Table 1. Ni/SiO2 gave two peaks at about 180 and 320 °C, respectively corresponding to the weak and medium strength acid sites. NixIn/SiO2(x>1) still had low temperature peak, and gave more acid sites than Ni/SiO2. However, they gave the second peaks higher than 320 °C, and the less the Ni/In ratio was, the higher the peak temperature, indicating that the catalyst acidity was enhanced due to the introduction of In into Ni/SiO2. Interestingly, the peak at high temperature disappeared for NiIn/SiO2, similar to the case for In/SiO2 that gave one main peak at about 200 °C with a shoulder peak at about 380 °C. In short, 24
the addition of In not only enhanced the acidity but also increased the acid amount. To identify the acid kinds on the catalysts, Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2 were characterized by FT-IR of adsorbed pyridine. As indicated in Fig. 8, after the adsorption and the evacuation at room temperature, there were physically adsorbed pyridine (characterized by the absorption at 1592~1599 cm-1) and chemically adsorbed pyridine (characterized by the absorption at about 1446, 1494, 1577 and 1610 cm-1) [36]. After the evacuation at 100 oC for 30 min, the bands due to the physical adsorption disappeared. According to the literature [36], the bands at about 1446, 1577 and 1610 cm−1 correspond to the vibrations of pyridine adsorbed on the Lewis acid sites, the peaks at about 1540 and 1638 cm−1 correspond to the vibrations of pyridine adsorbed on the Brønsted acid sites, and the band at about 1494 cm−1 is related to both Brønsted and Lewis acid sites. Clearly, there were only the Lewis acid sites on Ni/SiO2 and Ni-In bimetallic catalysts. Some researchers have only found Lewis acid sites on Ni/SiO2 and attributed them to the unreduced Ni species in the form of nickel silicate [37,38]. We agree this explanation because nickel silicate is difficultly reduced. The amount of nickel silicate in Ni/SiO2 was too small to be detected by XRD. For Ni6In/SiO2 and 25
Ni2In/SiO2, the Lewis acid sites are also related to the In species because there was a charge transfer from In to Ni (indicated by XPS), and the In species with positive charge can be considered as Lewis acid sites.
3.3 Relationship between catalyst structure and performance To better understand the factors that influence the catalyst performance for the selective hydrogenation of acetylene, it is necessary to consider the adsorption modes of acetylene and ethylene on the catalyst surface. It is suggested that there are similar adsorption modes of acetylene and ethylene [39,40]. Here, the possible adsorption modes of acetylene on a Pd surface (some in the presence of pre-adsorbed H) are presented in Scheme 2 [41,42,43]. It is usually accepted that there are three main adsorption modes, that is, π-bonded acetylene (π complex), di-σ-adsorbed acetylene and multi-σ-bonded ethylidyne. As summarized by Molnár et al. [28], the π-adsorbed acetylene is transformed to the di-σ-adsorbed acetylene, and then to the vinyl species, which are the precursors to form ethylene. The multi-σ-bonded ethylidyne and ethylidene are converted to ethane via a sequential hydrogenation process (i.e., ethylidyne → ethylidene → ethyl → ethane). In addition, the dissociatively adsorbed acetylene and vinylidene were suggested to 26
participate in forming oligomers and benzene. Thus, to improve the selectivity
to
ethylene,
the
desired
adsorption
modes
are
π-adsorbed/di-σ-adsorbed acetylene rather than multi-σ-bonded ethylidyne and the dissociatively adsorbed acetylene and vinylidene. Since the formation of the multi-σ-bonded surface intermediates requires the Pd ensembles with three contiguous adsorption sites, a common approach to improve the ethylene selectivity is to block these ensembles through the modification with an inert component, so-called “active site isolation concept” [44,45,46]. According to this concept, the isolation of the surface Pd atoms inhibits the formation of multi-σ-adsorbed species and subsequently the formations of ethane, oligomers as well as carbonaceous deposit, while the selectivity to ethylene is enhanced. The DFT calculations [21] have also demonstrated that the additives has lower activation energy for the ethylene desorption than for further hydrogenation, making the catalyst more selective. Again, the charge transfer from the additives to Pd can reduce the adsorption strength of acetylene and ethylene [6,11,12]. Therefore, the geometrical and electronic properties remarkably influence the catalyst performance for the selective hydrogenation of acetylene [12]. Here, as demonstrated by the XRD and TEM results, there were the Ni-In alloy and intermetallic compound (i.e.,Ni2In) formed 27
in the NixIn/SiO2 catalysts; moreover, the Ni and In species uniformly distributed. Also, as indicated in Section 3.1, the activity of Ni/SiO2 was much higher than that of In/SiO2 for the selective hydrogenation of acetylene. Thus, In can be deemed as an inert component in NixIn/SiO2. The isolation of the reactive Ni atoms with the inert In ones reduces the amount of the contiguous Ni sites and subsequently suppresses the formation of multi-σ-adsorbed hydrocarbon species. As to the electronic property, as demonstrated by the XPS results, there was the charge transfer from the In atoms to the Ni ones in the Ni-In alloy and the intermetallic compound Ni2In. This helpfully reduces the adsorption strength of the electronegative acetylene and ethylene on the NixIn/SiO2 catalysts. That is, both geometrical and electronic effects due to In suppress the formation of strong multi-σ-adsorbed ethylene and facilitate the desorption of ethylene from the NixIn/SiO2 catalysts, confirmed by the C2H4-TPD profiles (Fig.6). This can account for the increased ethylene selectivity (Fig.1(B)) and the reduced selectivity to ethane (Fig.1(C)) due to the introduction of In to Ni/SiO2. Moreover, suppressing the formation of the strong adsorption mode of multi-σ-adsorbed intermediates (like ethylidene and ethylidyne in Scheme 2) inhibited the C-C bond dissociation and the selectivity to the products with odd-numbered carbon atoms (i.e., 28
propane and methane) was reduced (Fig.1(D), Fig.1(E)). Similarly, it has also been found that the isolation of Ni atoms due to In inhibits the C-C hydrogenolysis in the hydroconversion of propylamine and acetic acid [27]. Compared to that on Ni/SiO2, the reduced adsorption strength of acetylene and ethylene on NixIn/SiO2 catalyst may accordingly restrain the polymerizations of acetylene and ethylene. Indeed, as shown in Fig.1(F), NixIn/SiO2(x>2) gave lower selectivity to the C4+ hydrocarbons than Ni/SiO2. However, as the In content increased, the selectivity to the C4+ hydrocarbons tended to increase, and Ni2In/SiO2 and NiIn/SiO2 had higher ones than Ni/SiO2 during the initial phase. This might be attributed to the following reasons. Firstly, as indicated by the NH3-TPD results, as the In content increased, the catalyst acidity became strong. Acting as the Lewis ones, the In sites could catalyze the polymerization of acetylene and ethylene. Secondly, with increasing In content, the hydrogen
chemisorption
was
significantly
inhibited,
which
suppressed the hydrogenation of acetylene while the polymerization was promoted. In summary, the isolation and the enriched electron density of the Ni atoms due to the introduction of In enhanced the selectivity to ethylene while reduced the activities for the C-C bond cleavage 29
and the polymerization. However, a suitable In content was required, and the high In content favoured the polymerization reaction. 3.4 Catalyst deactivation As shown in Fig.1(A), NixIn/SiO2 with the suitable Ni/In ratio (such as 10 and 6) had much higher stability than Ni/SiO2, while the low Ni/In ratio (i.e., the high In content) was harmful for the NixIn/SiO2 stability. Usually, the catalyst deactivation is attributed to poisoning, sintering and carbonaceous deposition [47]. Here, no poisons were introduced into the reactant, and so the poisoning was impossible. Because the reaction took place at 180 oC, such a low reaction
temperature
might
not
lead
to
the
sintering
of
nickel-containing particles (shown in Section 3.4.2). To analyze the catalyst deactivation, the used Ni/SiO2, NixIn/SiO2 (especially Ni6In/SiO2 and Ni2In/SiO2) catalysts after the reaction were characterized by the N2 sorption, TG/DTA, Raman and XRD. As indicated in Table 1, compared with the fresh ones, the used Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2 catalysts had lower BET specific surface areas and pore volumes(especially the used Ni2In/SiO2). This is mostly related to the formation of the carbonaceous deposit. 3.4.1 Carbonaceous deposit 30
TG-DTA was used to measure the amount of carbonaceous deposit on the used catalysts. The profiles of the used Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2 catalysts are presented in Fig.9. For all samples,
the
weight
loss
below
100oC
accompanied
with
endothermic process is primarily ascribed to the water desorption. The weight loss accompanied with two exothermic peaks between 300 and 600 oC corresponds to the combustion of carbonaceous deposit. Obviously, the weight loss increased with increasing the In content, and the used Ni2In/SiO2 catalyst had the most amount of carbonaceous deposit. Also, the exothermic peaks shifted to high temperature with increasing the In content. Two exothermic peaks may indicate that there were two kinds of carbonaceous deposits with different activities, and the higher exothermic peak temperature was, the more inactive the carbonaceous deposit. Therefore, the activity of the carbonaceous deposit decreased with increasing the In content, which is detrimental to the catalyst stability. To identify the nature of the carbonaceous deposit, the used catalysts were characterized by Raman, a powerful technique for characterizing the structure of carbonaceous deposits [48]. Fig.10 shows the Raman spectra of the used Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2 in range of 1000-1800 cm-1. According to the results from the references[49,50,51,52], the G band (about 1590 cm-1) is 31
characteristic of condensed, ordered or graphitic aromatic structures, and the D band (1300-1490 cm-1 ) is associated with vibrations of carbon atoms with dangling bonds in the defective and disordered graphite planes. It is well known that the typical graphite is more inactive than the disordered one. Here, compared with Ni/SiO2 and Ni6In/SiO2, Ni2In/SiO2 gave the stronger G band than the D one, indicating that it contained more amounts of inactive carbonaceous species, well consistent with the TG-DTA analysis. The more amounts of carbonaceous deposit on NixIn/SiO2 is ascribed to their stronger Lewis acid sites that catalyze the polymerization. There are different opinions on the effect of polymers (such as green oil) on the catalyst stability. Ahn et al. [53] and Liu et al. [54] have reported that the green oil on the catalyst surface inhibit the diffusions of acetylene and hydrogen and subsequently drastically reduce the catalyst activity. However, Kim et al. [55] consider that the relatively light green oil on or in the vicinity of the Pd surface only slightly decreased the catalytic activity. They attributed this to the hydrogen transfer mechanism, that is, the carbonaceous overlayer worked as a medium to allow the hydrogen transfer from the Pd surface to acetylene associated with the overlayer [56,57,58,59]. Accordingly, the acetylene hydrogenation rate was unaffected by the large amounts of relative light green oil. Here, as 32
the In content increased, green oil gradually became heavier and eventually inactive carbonaceous deposit due to the enhanced acidity. The carbonaceous deposit covered the active sites and subsequently resulted in the catalyst deactivation. Indeed, the NixIn/SiO2 stability became inferior with increasing the In content ( Fig.1(A)). 3.4.2 Phase change from metallic Ni to nickel carbide According to the results of TG-DTA and Raman, Ni/SiO2 had less amounts of carbonaceous deposit than Ni6In/SiO2, but its deactivation was more remarkable. This contradiction can be explained by the formation of nickel carbide on Ni/SiO2. To clearly reveal the phase change during the reaction, the used catalysts after reaction for 10 h were also prepared. Fig.11 shows the XRD patterns of the used Ni/SiO2 and NixIn/SiO2 catalysts after reaction for 10 and 36 h. For the used Ni/SiO2, the peaks at 2θ=43o, 50 o and 73o due to the NiCx solution are visible, [60] while the peaks corresponding to metallic Ni are not obvious even after reaction for 10 h. This indicates that metallic Ni phase was easily converted to nickel carbide during the reaction. Bartholomew [61] also found that nickel carbide formed at the temperature range of 150~250 oC. For the used Ni10In/SiO2, a part of the metallic Ni was transformed to nickel carbide after reaction for 10 h, and a small diffraction peak 33
at 44.5o corresponding to metallic Ni phase is still visible after reaction for 36 h. Because the peaks at about 43o due to Ni2In and NiCx are close, the peak at about 30.2 o and the one at about 50 o are more valuable for identifying the existence of Ni2In and NiCx, respectively. As the In content increased, the peaks due to nickel carbide tended to be weak. For the used Ni3In/SiO2 and Ni2In/SiO2, the peaks due to the Ni2In phase are clearly observed and the peaks due to nickel carbide almost disappear. That is, the Ni2In phase was not converted to nickel carbide. Therefore, the introduction of In suppresses the formation of nickel carbide, which is more obvious with higher In content. As mentioned in Section 3.3, the introduction of In inhibited the adsorption of multi-adsorbed intermediates (like ethylidene and ethylidyne) that are primarily responsible for the C-C bond dissociation. That is, the introduction of In restrained the formation of dissociative carbon atoms that are prone to enter into the Ni phase to form the nickel carbide. It has been reported that nickel carbide has a significantly negative effect on the catalytic behavior in the selective hydrogenation of acetylene. For example, Ko and Madix [62] found that a carburized (by cracking ethylene) surface of Ni(110) could significantly reduce the binding energy of H2 from 92 kJ/mol to about 42 kJ/mol, subsequently lowering sticking 34
possibility and restraining the activation of H2. As a result, the hydrogenation reaction was hindered. Again, the chemical property of nickel carbide is thought to be similar to metallic copper that has weaker adsorption ability for the unsaturated hydrocarbon (i.e. acetylene, butadiene) than metallic nickel, decreasing the acetylene conversion and leading to the catalyst deactivation. Additionally, as indicated in Table 1, there was no obvious change in the nickel-containing crystalline sizes before and after reaction, indicating that no sintering took place, confirming the above prediction. From the above, we suggest that the Ni/SiO2 catalyst deactivation is mainly ascribed to the formation of nickel carbide, while the NixIn/SiO2 deactivation is related to the formation of nickel carbide and the carbonaceous deposit. The introduction of In inhibited the formation of nickel carbide, and the dominating reason for the catalyst deactivation changed from the formation of nickel carbide to the carbonaceous deposit with increasing the In content. Additionally, the change of the catalyst property in bulk and surface can account for the dramatic change of the product selectivity during the first few hours of reaction (Fig.1). For all catalysts, during the first few hours, the selectivity to ethylene sharply increased, while the selectivity to ethane dramatically decreased. Also, the selectivities to methane and 35
propane obviously decreased on Ni/SiO2. This is ascribed to the change of the catalyst property. The gradual transformation of metallic Ni to nickel carbide for Ni/SiO2 and NixIn/SiO2 (with high Ni/In ratios) may reduce the catalyst activities for the over-hydrogenaiton and the C-C bond hydrogenolysis. Also, the carbonaceous deposit could cover the nickel sites, subsequently inhibiting the over-hydrogenation of ethylene to ethane. After a certain time, when the catalyst property in bulk and surface reached relatively stable, the selectivities to the products did not changed obviously.
Conclusions The addition of a suitable amount of In into Ni/SiO2 geometrically
isolated
the
reactive
Ni
atoms,
subsequently
decreasing the amount of the contiguous Ni sites and inhibiting the formation of strongly adsorbed multi-σ-bonded hydrocarbon species. Moreover, the charge transfer from the In atoms to the Ni ones weakened the adsorption of acetylene and ethylene. Both geometrical and electronic effects contribute to enhancing the selectivity to ethylene and restraining the C-C bond dissociation and the polymerization. However, with increasing the In content, the acidity of NixIn/SiO2 was enhanced due to the charge transfer between the In and Ni sites, and so the polymerization of acetylene
36
and ethylene became serious. As for the catalyst stability, the phase change from metallic Ni to nickel carbide is the dominating reason for the Ni/SiO2 deactivation. The introduction of In inhibited the formation of nickel carbide. As the In content increased, the dominating reason for the NixIn/SiO2 catalyst deactivation changed from the formation of nickel carbide to that of carbonaceous deposit. On the whole, Ni6In/SiO2 and Ni10In/SiO2 had better stability than Ni/SiO2 and other NixIn/SiO2. Our finding may provide valuable information for the rational design of the Ni-based catalysts for the selective hydrogenation of acetylene.
Acknowledgements The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 21576193).
37
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palladium/alumina
hydrogenation
catalysts,
Appl.
Catal.,
A.
282(2005)111-121. [55] W.J. Kim, E.W. Shin, J.H. Kang, S.H. Moon, Performance of Si-modified Pd catalyst in acetylene hydrogenation: catalyst deactivation behaviour, Appl. Catal., A. 251(2003)305-313. [56] S.J. Thomson, G. Webb, Catalytic hydrogenation of olefins on metals: a new interpretation, J. Chem. Soc., Chem. Commun. 13(1976)526-527. [57] A. Borodzinshi, A. Golebiowski, Surface heterogeneity of supported palladium catalyst for the hydrogenation of acetylene-ethylene mixtures, Langmuir. 13(1997)883-887. [58] A. Borodzinshi, Hydrogenation of acetylene-ethylene mixtures on a commercial palladium catalyst, Catal. Lett. 63(1999)35-42. [59] A. Borodzinshi, The effect of palladium particle size on the kinetics of hydrogenation of acetylene-ethylene mixtures over Pd/SiO2 catalysts, Catal. Lett. 71(2001)169-175. [60] A. Śrebowata, W. Juszczyk, Z. Kaszkur, J.W. Sobczak, L. Kepiński, Z.
Karpiński,
Hydrodechlorination
of
1,2-dichloroethane
and
dichlorodifluoromethane over Ni/C catalysts: The effect of catalyst carbiding, Appl. Catal., A. 319(2007)181-192.
47
[61] C.H. Bartholomew, Carbon deposition in steam reforming and methanation, Cataly. Rev. 24(1982)67-112. [ 62 ] E.I. Ko, R.J. Madix, Decomposition of formic acid on Ni((100)-p(2×2)C, Appl. Surf. Sci. 3(1979)236-250.
48
Figure captions
(A)
Acetylene conversion/ %
100
80
60
40
20
0 0
5
10
25
Time on stream/h
49
30
35
70
(B)
Selectivity to ethylene/ %
60
50
40
Ni/SiO2 Ni10In/SiO2 Ni6In/SiO2 Ni4In/SiO2 Ni3In/SiO2 Ni2In/SiO2 NiIn/SiO2
30
20
10
0 0
5
10
25
30
35
Time on stream/ h
60
(C)
Selectivity to ethane/ %
50
40
30
20
10
0 0
5
10
25
Time on stream/ h
50
30
35
6
(D)
Selectivity to propane/ %
5
4
3
2
1
0 0
5
10
25
30
35
Time on stream/ h
(E)
Selectivity to methane/ %
2.0
1.5
1.0
0.5
0.0 0
5
10
25
Time on tream/ h
51
30
35
70
Selectivity to C4+ hydrocarbons/ %
(F)
60
50
40
30 0
5
10
25
30
35
Time on stream/ h
Fig. 1 Acetylene conversion and product selectivity on Ni/SiO2 and NixIn/SiO2 with time on stream. The symbols denoting the catalysts are shown in (B).
52
(h)
(g)
H2 consumption/a.u.
(f) (e) (d) (c) (b) (a)
0
100
200
300
400
500
600
700
o
Temperature/ C
Fig. 2 H2-TPR profiles of catalyst precursors of (a) Ni/SiO2; (b) Ni10In/SiO2; (c) Ni6In/SiO2; (d) Ni4In/SiO2; (e) Ni3In/SiO2; (f) Ni2In/SiO2; (g) NiIn/SiO2; (h) In/SiO2
53
▼: Ni
▽
(A)
▽: Ni2In
▽ ▼
(g)
▼
▽
▼ Relative intensity/a.u.
(f) (e) (d) (c) (b) (a)
20
30
40
50
60
70
80
2Theta/degree
(B) ▼: Ni : Ni In ▽ 2
▽
(g)
▼
Relative intensity/a.u.
(f) (e) (d) (c) (b) (a)
40
42
44
46
48
2Theta/degree
Fig. 3 XRD patterns of (a) Ni/SiO2; (b) Ni10In/SiO2; (c) Ni6In/SiO2 (d) Ni4In/SiO2; (e) Ni3In/SiO2; (f) Ni2In/SiO2; (g) NiIn/SiO2. (A) 2θ between 20~80o; (B) enlarged (A) between 40~48o 54
Fig. 4 (a) EDS spectrum; (b) and (c) EDS line scanning spectra of Ni6In/SiO2.
55
(A)
Ni 2p3/2
Relative intensity/a.u.
852.8
Ni/SiO2 852.7 Ni6In/SiO2 852.3 Ni2In/SiO2
864
862
860
858
856
854
852
850
Binding energy/eV
In 3d5/2
In 3d3/2
(B)
Relative intensity/a.u.
444.8 452.5
Ni6In/SiO2 444.3 451.8 Ni2In/SiO2
456
454
452
450
448
446
444
442
Binding energy/eV
Fig. 5 XPS spectra of Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2 in (A)Ni 2p3/2 and (B) In 3d regions.
56
C2H4 desorption/a.u.
(h) (g) (f) (e) (d) (c) (b) (a) 0
100
200
300
400
500
o
Temperature/ C
Fig. 6 C2H4-TPD profiles of (a) Ni/SiO2; (b) Ni10In/SiO2; (c) Ni6In/SiO2; (d) Ni4In/SiO2 ;(e) Ni3In/SiO2; (f) Ni2In/SiO2; (g) NiIn/SiO2; (h) In/SiO2.
57
(II)
(I) NH3 desorption/a.u.
(h) (g) (f) (e) (d) (c) (b) (a) 100
200
300
400
500
600
700
800
o
Temperature/ C
Fig. 7 NH3-TPD profiles of (a) Ni/SiO2; (b) Ni10In/SiO2; (c) Ni6In/SiO2; (d) Ni4In/SiO2; (e) Ni3In/SiO2; (f) Ni2In/SiO2; (g) NiIn/SiO2; (h) In/SiO2.
58
0.1
1577
1610
Absorbance
(A)
1446
1599
1540
1494
(c)
1592
(b) (a)
Absorbance
0.05
1608
(B) (c)
1612
(b) 1606
(a) 1650
1600
1550
1500
Wavenumber/ cm
1450
1400
1350
-1
Fig. 8 FT-IR spectra of adsorbed pyridine on(a) Ni/SiO2; (b) Ni6In/SiO2 and (c) Ni2In/SiO2. (A) After adsorption and evacuation at room temperature; (B) Evacuation at 100 oC.
59
used Ni/SiO2 used Ni6In/SiO2 used Ni2In/SiO2
Weight loss/%
100
90
80
70 9
△T/ oC
6 3 0 -3 0
100
200
300
400
500
600
700
800
o
Temperature/ C
Fig. 9 TG-DTA curves of the used Ni/SiO2, Ni6In/SiO2 and Ni2In/SiO2 catalysts after reaction for 36 h.
60
G band
Relative intensity/a.u.
D band
(c) (b) (a)
1000
1200
1400
Raman shift/cm
1600
1800
-1
Fig.10 Raman spectra of (a) used Ni/SiO2; (b) used Ni6In/SiO2 and (c) used Ni2In/SiO2 after reaction for 36 h.
61
○ ▽▼
▼ : Ni ▽ : Ni2In ○ : NiCx
○▼
▽ Relative intensity/ a.u.
▽
(A)
(g) (f) (e) (d) (c) (b) (a)
20
30
40
50
60
70
80
2 Theta/ degree
▼ : Ni ▽ : Ni2In
○
▽ ▼
○▼
(B)
○ : NiCx
▽ Relative intensity/ a.u.
(f) (e) (d) (c) (b) (a)
20
30
40
50
60
70
80
2 Theta/ degree
Fig.11 XRD patterns of the used catalysts after reaction for (A) 10 h and (B) 36 h. (a) Ni/SiO2; (b) Ni10In/SiO2; (c) Ni6In/SiO2; (d) Ni4In/SiO2; (e) Ni3In/SiO2; (f) Ni2In/SiO2; (g) NiIn/SiO2 62
(2) HC
CH
(1) H2C CH2
H3C
CH3
C4+ hydrocarbons
(3)
Scheme 1 Possible pathways during selective hydrogenation of acetylene
HC
CH
H2C
CH2
H
CH3 C
*
π -complex
*
*
di-σ-adsorbed
*
*
ethylidene
CH2
CH3
CH2
CH
C
C
* vinyl
* * *
ethylidyne
Scheme 2 Adsorption modes of acetylene on Pd surface
63
*
vinylidene
Table
Table 1 Properties of the Ni/SiO2 and NixIn/SiO2 catalysts before and after reaction. Relative acid amount e
Crystallite
SBET
Pore volume
Average pore
size (nm)
(m2·g-1)
(cm3·g-1)
(nm)
(I)
(II)
Total
Ni/SiO2
8.7a
453
0.687
6.06
1.00
1.08
2.08
Ni10In/SiO2
8.7a
-
-
-
1.71
0.61
2.32
Ni6In/SiO2
8.1a
460
0.694
6.04
1.55
0.60
2.15
Ni4In/SiO2
5.9a
-
-
-
1.55
0.63
2.18
Ni3In/SiO2
4.3a
-
-
-
1.48
0.66
2.15
Ni2In/SiO2
5.2b
415
0.640
6.16
2.03
0.78
2.81
NiIn/SiO2
8.3b
-
-
-
3.06
-
3.06
5.87
-
5.87
Catalyst sample
In/SiO2 used Ni/SiO2 c
8.8 d
377
0.631
6.70
-
--
-
used Ni6In/SiO2 c
6.0 d
307
0.522
6.80
-
-
-
used Ni2In/SiO2 c
5.7 b
95
0.202
8.46
-
-
-
a
Calculated with Scherrer formula using (111) reflection of fcc Ni. b Calculated with Scherrer formula using (110) reflection of Ni2In. c After reaction for 36 h. d Calculated with Scherrer formula using peak at 43 o for NiCx. e The acid amount of the first peak for Ni/SiO2 was designed as 1.00.
64