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Kinetics, Catalysis, and Reaction Engineering
SiO2-modified Pt/Al2O3 for oxidative dehydrogenation of ethane: a preparation method for improved catalytic stability, ethylene selectivity and coking-resistance Yanyan Xi, Jianmei Xiao, Xufeng Lin, Weining Yan, Chuangye Wang, and Chenguang Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01163 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018
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SiO2-modified Pt/Al2O3 for oxidative dehydrogenation of ethane: a preparation method for improved catalytic stability, ethylene selectivity and coking-resistance Yanyan Xi1,2, Jianmei Xiao2, Xufeng Lin1,3,4*, Weining Yan,2 Chuangye Wang,1,3 Chenguang Liu1,2,4
1
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),
Qingdao, P. R. China, 266580 2
College of Chemical Engineering, China University of Petroleum (East China), Qingdao, P. R.
China, 266580 3
College of Science, China University of Petroleum (East China), Qingdao, P. R. China, 266580
4
Key Laboratory of Catalysis of China National Petroleum Corporation, China University of
Petroleum (East China), Qingdao, P. R. China, 266580
*Authors to whom correspondence should be addressed. Email:
[email protected]; Tel: +86-532-86981579
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Abstracts: SiO2-modified Pt/Al2O3 catalyst (SiO2/Pt/Al2O3) was synthesized with a solution-based method using Pt/Al2O3 as the starting catalytic material and tetraethoxysilane (TEOS) as the Si precursor. The modification process can be conducted with several cycles. Each of the modification cycle included sequentially an evacuation step at an elevated temperature, an impregnation step followed by a solution-treatment step. The introduced SiO2 layer showed an obvious sintering-resisting effect for the Pt nanoparticles at a high temperature of 600 oC. When used for catalytic oxidative dehydrogenation of ethane (ODHE), SiO2/Pt/Al2O3 presented a significantly improved catalytic stability and coking depression compared to the unmodified Pt/Al2O3. Furthermore, the introduced SiO2 layer helped improve the ethane conversion slightly and the selectivity to ethylene mildly. The improved selectivity to ethylene could be associable with the Brönsted acid sites brought by the SiO2 layer, as well as site distribution of the exposed Pt surface.
Keywords:
Surface
modification;
catalyst
preparation;
dehydrogenation of ethane; catalytic performance improvement
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platinum
catalyst;
oxidative
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1. Introduction Platinum(Pt)-based catalysts belong to one of the most frequently used types of catalyst in chemical industry as well as in fundamental research. Especially they are widely involved in the catalytic processes like hydrogenation of C=C1,2 or C=O bond,3,4 dehydrogenation5,6 of alkanes, partial7,8 and complete9 oxidation of alkanes, oxidation of CO,10,11 hydrogenolysis12,13 and aqueous phase reforming14-16 of bio-deritivated alcohols or polyols. Parts of these processes are of great economic significance. Pt-based catalysts can be divided into homogeneous and heterogeneous types, and presently the latter one are much more often used in the industry. The heterogeneous Pt catalysts are usually prepared and used in the form of supported Pt catalysts. Supported metal catalysts in general and Pt catalysts in special have highly dispersed metal/metal oxide clusters resided on a catalytic support having large surface area.17,18 In principle, the catalytic properties of a supported catalyst highly depend on the size and the chemical status of its active metal component, as well as how the active metal component interacts with the catalytic support and the surrounding species. This principle results in the case that the way of catalyst preparation always significantly influences on the catalytic performance of a certain catalyst, and consequently the competitiveness of a production process using this catalyst. Residing of the metal component(s) onto the catalytic support is always the key and final step in most cases of preparing supported metal catalysts in general17 and supported Pt catalysts in special. Classic examples of preparation method include wetness impregnationm,19 metal compound vapor deposition.8 Further modification with non-metal substances, e.g, Al2O3,20,21 SiO222,23 can potentially has several interesting advantages for catalytic applications, especially when used at high temperatures. However, the reports for further modification with non-metal substances of the supported metal catalysts are sparse in the literature. For example, Lu et al.20 showed that a second layer of Al2O3 can be introduced to the Pd/Al2O3 catalyst by atomic layer deposition (ALD). This second layer of Al2O3 was highly effective in preventing sintering of the Pd nanoparticles on the catalyst when used for the oxidative dehydrogenation (ODH) of ethane (ODHE) at 675 oC. Page 3
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Furthermore, this second layer can also effectively prevent formation of cokes during the catalytic test. The coking- and sintering-resisting properties of the introduced Al2O3 layer resulted in a significant improvement in the catalytic stability. Later, they found that21 the introduced Al2O3 layer was able to enhance the selectivity to butenes, especially 1-butene, for Pd/Al2O3 catalyzed hydrogenation of 1,3-butadiene. However, as is known, the ALD technique requires very high cost of apparatus and operation. From the aspect of practice it seems unlikely to use the ALD-based methods to produce enough amount of catalyst for a large-scale test than a lab-scale test, which may limit their further application. Nakagawa et al. modified Pt/C catalyst with several Si precursors.22 They showed that the microporous silica coated Pt/C catalyst had a remarkable thermal stability compared to the uncoated Pt/C. However, they tested their catalyst for a non-oxdiative dehydrogenation reaction only at low temperatures like 250 oC. Therefore, the catalytic properties of the coated Pt/C at higher temperatures like 600 oC are unknown. In principle, the catalytic support of carbon used in their catalyst may limit the application of such catalysts when exposed to an oxidative reactant feed. Similar results of coated Pd/C were also reported by Nakagawa et al.23 Nishina et al.24 coated Pt/GO (GO = grapheme oxide) catalyst with hydrosilanes (R3SiH) to afford Pt/SiGO catalyst, and the durability of Pt/SiGO was improved to 400 oC when exposed to the oxidative reactant feed. To our best knowledge, none of the reported Si-modified catalyst showed catalytic stability when in situ used at high temperatures like 600 oC in the presence of oxidative reactants. However, in practice catalysts which are durable under such reaction conditions is of great importance to many applications like ODH of light alkanes. On the other hand, the ODH of light alkanes25,26 is a hopeful alternative method for the present production of light alkenes like steam reforming. The competitiveness of the ODH method is highly subject to the performance of the catalyst. In particular for ODHE, most of the efforts in the catalyst searching in this several decades are related with V-27,28 and Mo-based28 catalytic systems. The supported catalysts containing VIII metals as the active component were found to be hopeful Page 4
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catalysts for ODH from late 1990s.7,29-33 For example, Cimino et al.30 showed that the Pt-LaMnO3 can present a C2H4 yields exceeding 65% while preserving selectivities above 80% under self-sustained operation. In a high temperature between 900 ~950 oC,7 the Pt catalyst supported on an alumina foam monolith can give a C2H4 selectivity of 64% at the C2H6 conversion of 69%. When the Pt-Sn alloy was used instead of Pt as the supported active component, a C2H4 selectivity of 83% at the C2H6 conversion of 73% was achieved at the presence of H2. Lower temperatures of 400 ~ 600 oC were often utilized when Ni-based catalysts were applied to ODHE.31-33 In this context it is interesting to know whether that modification of the supported VIII metal catalysts can be an effective way for improving their catalytic performance for the ODH of light alkanes in general and for the ODHE in particular. In this paper, we show a solution-based method for modification of the Pt/Al2O3 catalyst with a porous SiO2 layer. The obtained catalyst, denoted as SiO2/Pt/Al2O3, had some interesting properties such as sintering-resisting, capability to present improved C2H4 selectivity for ODHE, etc. Several characterization tools like transmission electron microscopy (TEM), diffuse-reflectance infrared Fourier transformation (DRIFT) were used to find and explain the above-mentioned properties. This work provided a new method for modification of a supported metal catalyst in one hand, and on the other hand new insights for improving the catalytic performance of supported metal catalyst for selective conversion of alkanes especially in high temperatures. 2. Experimental section 2.1 Catalyst preparation All liquid and solid materials used in this work were of analytical pure, and all gaseous materials used were of ultra high pure. The preparation method for the Al2O3 support can be seen elsewhere.34 Al(OH)3 power (Henghui Chemicals Co. Ltd., Yantai, China) of 200 g, 6.0 g sesbania power, 9.2 g HNO3 (w%=65%) and 140 g deionized water were uniformly mixed. Then the mixture was extruded into strips with a diameter of 15 mm by an extruding machine. The obtained strips were dried in air at Page 5
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room temperature for 24 h and then at 120 oC for another 24 h. The dried strips were calcined in air at 500 oC for 4 h to form γ-Al2O3, denoted simply as Al2O3 in this paper. Then the Al2O3 strips were ground into 20~40 mesh particles. The Pt/Al2O3 catalyst prepared with a conventional equal-volume wetness impregnation method in general can be seen elsewhere.17,19 In particular in this work, the above-described Al2O3 particles were impregnated with the H2PtCl6 aqueous solution for 12 h. The volume of this solution was equal to the volume of water absorbed by a known amount of the calcined Al2O3 particles. The concentration of this solution was explicitly controlled so that finally the Pt loading of the Pt/Al2O3 catalyst was 2.0 wt.%. After impregnation, the catalyst was dried at 110 oC in air for 12 h, and reduced with a H2 flow at 300 oC for 4 h. The SiO2 modified Pt/Al2O3 catalyst was prepared by treating the above-described Pt/Al2O3 catalyst with different numbers of modification cycles. Each of the modification cycle contained three steps. First, the Pt/Al2O3 catalyst was loaded to a glass chamber which was evacuated to a pressure of < 1 kPa, and heated at 110 oC for 1 h. Second, the catalyst in the chamber was cooled down to room temperature. Keeping the above vacuum condition unchanged, a mixture of tetraethoxysilane (TEOS) and ethanol (v/v = 1/1) was rapidly added to the chamber to wholly immerse the catalyst. The pressure of this chamber was recovered to ambient pressure, and the immersion process was kept for 1 h. Then the catalyst was filtered, and was wholly immersed in an aqueous solution of acetic acid (0.1 mol/L) for 2 h. The obtained catalyst was filtered again and the whole cycle of modification ended. After several cycles of modification (up to totally 9 cycles in this work), the obtained catalyst was dried in air 110 oC for 4 h. The above modification process with any number of cycles can be conducted with a home-made apparatus without moving the catalyst, making the modification process facile. Before further use, all catalysts were calcined at 600 oC for 2 h in air unless specially specified. For the purpose of easy description, the catalyst Pt/Al2O3 modified with the above-mentioned method was denoted as nc-SiO2/Pt/Al2O3, where n is the number of cycles of modification. When a Page 6
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catalytic sample was not calcined before use, the prefix of “un-” was added to the above notation of catalyst, representing “uncalcined”. 2.2 Catalyst characterization The Pt loading of the prepared catalysts was measured by UV-Vis spectroscopy36 using SnCl2 as the chromogenic agent. Before the spectrocopic measurement, a Pt/Al2O3 catalyst of known mass (~ 60 mg) was dissolved in a solution mixture of HCl, H3PO4 and H2O2 to obtain an aqueous solution of H2PtCl6. Then a 250 g/L SnCl2 solution of 2 mL was added to above mixture, and the obtained solution was diluted to 100.0 mL. The UV-vis absorbance at 403 nm was measured for this diluted solution as well as of the standard solutions containing a known concentration of Pt, and thus the Pt amount of the measured sample can be calculated. For a SiO2/Pt/Al2O3 catalyst, HF was firstly used to dissolve the SiO2 component, followed by the other steps same to those for a Pt/Al2O3 catalyst. The prepared catalysts were ground to >200 mesh particles and then X-ray fluorescence (XRF, PANalytical, Netherlands ) spectroscopy was used to measure the Si and Al contents of the catalysts. During the modification process the mass ratio of Pt over Al2O3 was kept unchanged. Given that the introduced Si component existed in the status of SiO2, in principle SiO2/AlO2 ratio can be calculated according to the Pt loading on the modified catalysts. The calculated ratios of the modified catalysts were consistent with the Si/Al ratios obtained from the XRF measurement. The amount of carbon of a certain catalyst was measured with an elemental analyzer (Vario ELIII, Elementa). The TEM image of the catalysts was taken using a JEOL JEM-2100 UHR microscope. The 77K-N2 BET surface area and the size distribution of the pores of the calcined catalysts were measured with the static BET method using a Micromeritcs ASAP2010 instrument. The X-ray diffraction (XRD) patterns of the prepared catalysts were recorded by a X’pert PRO MPD diffractometer (PANalytical, Netherlands) with Cu-K radiation (40 kV, 40 mA) at a speed of 5 K/min. Page 7
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The NH3-TPD (temperature programmed desorption of NH3) experiment was conducted with a Finesorb-3010C analyzer (Zhengjiang Fantai Co.). About 500 mg of catalyst, sandwiched by quartz wool, was loaded into a quartz tube, and heated at 600 ◦C in a He flow (30 mL/min) for 1 h to remove moisture and other impurities. Then the quartz tube was cooled down to 100 ◦C and saturated with NH3 in a 10% NH3/He gas mixture for 1 h, and then it was flushed by the above helium flow for another 1 h to remove physically adsorbed NH3. Then the quartz tube was heated to 650 ◦C at a heating rate of 10 ◦C/min with the above helium flow as carrier flow. The thermal conductivity detector (TCD) signal of this carrier flow compared to that of another reference He flow (also 30 mL/min) was recorded as a function of temperature. The amount of desorbed NH3 was determined by titration of a 0.01 mol/L HCl solution which absorbed NH3 from the exhaust of the carrier flow. The thermogravimetry (TG) and differential scanning calorimetry (DSC) experiments were performed using a HCT-1 thermogravimetry and differential thermal analyzer (Beijing Henven Scientific Instrument). Before a temperature scan, a catalyst exposed in air was put onto the sample holder, and its temperature was controlled at 45 oC for 40 min. The scan rate was 10 oC/min and the temperature range was 45 ~ 800 oC. All DRIFT spectroscopic measurements were carried out using a Thermo Nicolet Nexus FTIR spectrometer equipped with a mercury cadmium telluride detector cooled with liquid N2. Each DRIFT spectrum was collected using 128 scans with a resolution of 4 cm-1. All of the spectrum-collection processes were carried out with samples exposed to a N2 atmosphere (steady flow rate of 60 mL/min) at room temperature. The DRIFT measurement of a certain catalyst was carried out with CO (CO-DRIFT) or pyridine (Py-DRIFT) as the probe molecule. For the CO-DRIFT experiment, before CO was introduced into the infrared (IR) spectra cell, the catalyst was in situ reduced by H2 (steady flow rate of 60 mL/min) at 300 ˚C for 1 h. After the sample was cooled down to room temperature at the N2 atmosphere, the background spectrum of this sample was collected. Then a CO flow of 60 mL/min was introduced to the cell for 30 min, and then the Page 8
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cell was purged with the above N2 flow continuously. Until the IR signal of CO in the gas-phase completely disappeared, the CO-DRIFT spectrum was collected. Before the Py-DRIFT experiment, a procedure of pyridine absorption on a certain catalyst at room temperature was kept for 12 h, and desorption of the physically adsorbed pyridine was carried out at 120 oC in vacuum for 24 h. The same catalyst without pyridine adsorption was used for collecting the background spectrum. The Pt dispersion of a certain catalyst was calculated from the amount of chemisorbed CO on this catalyst relative to the amount of Pt. The amount of chemisorbed CO was measured by a pulse CO-chemisorption experiment, which was operated with an automated chemisorption analyzer (AutoChem II 2920, Micromeritics). 2.3 Catalytic oxidative dehydrogenation of ethane Catalytic ODHE was carried out in a tubular quartz fixed-bed reactor (i.d. of ~10 mm) operating at atmospheric pressure. The as-prepared Pt/Al2O3 and SiO2/Pt/Al2O3 catalysts were further sieved to the particles with the size of 420~840 µm. One of these two catalysts of 0.5 g was diluted with 1 g catalytically inert quartz particles having the same size range. The mixed particles were packed in the fixed-bed reactor. The reactant feed flow was a mixture of 9.0 sccm C2H6, 4.3 sccm O2, 62.5 sccm He and 6.7 sccm N2 (internal reference). The temperature of the center of the catalyst bed was controlled by a temperature controller. After the temperature reached 600 oC and kept at 600 oC for an interested time-on-stream, the product flow was analyzed by an online gas chromatograph, equipped with a thermal conductivity detector for analyzing O2, N2, CO and CO2 and a flame ionization detector for analyzing hydrocarbons. The carbon balance was 100 ± 3%. The catalytic performance of a certain catalyst for ODHE was calculated by the following equations: Conversion of C2H6 or X(C2H6) = 1 - ([C2H6]/[C2H6]0), Conversion of O2 or X(O2) = 1 - ([O2]/[O2]0), Selectivity to C2H4 or S(C2H4) = [C2H4]/([C2H6]0 - [C2H6]), Selectivity to COx (where x = 1 or 2) or S(COx) = 0.5[COx]/([C2H6]0 - [C2H6]), and Page 9
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Selectivity to CH4 or S(CH4) = 0.5[CH4]/([C2H6]0 - [C2H6]). where “[ ]” and “[ ]0” represent the concentrations of a certain substance in the product flow and in the reactant feed flow, respectively. 3. Results and discussion 3.1 Elemental and thermal analysis Table 1. The composition of SiO2-modified Pt/Al2O3 catalysts with different number (n) of modification cycles. number of cycles
Catalyst notation
Pt content In wt.% of Pt
Si content In wt.% of SiO2 n=0 Pt/Al2O3 2.00 0.00 n=1 1c-SiO2/Pt/Al2O3 1.83 8.90 n=3 3c-SiO2/Pt/Al2O3 1.58 21.3 n=5 5c-SiO2/Pt/Al2O3 1.43 28.4 n=7 7c-SiO2/Pt/Al2O3 1.36 32.3 n=9 9c-SiO2/Pt/Al2O3 1.32 34.0 *No carbon content was detected for the SiO2 modified catalysts.
Al content In wt.% of Al2O3 98.0 89.3 77.1 70.2 66.3 64.7
Table 1 shows the amount (in weight percent) of Pt, SiO2 and Al2O3 of the Pt/Al2O3 catalyst without and with different numbers of modification cycle. The Si content did not increase linearly with the number of cycle (n) of modification. This can be due to the decreased pore volume of catalyst after each cycle of modification. The acetic acid solution in the second step of modification (see Section 2.1) can catalyze the hydrolysis of TEOS to remove its ethyl groups,35 and the product formed from the hydrolysis of TEOS was silicic acid or its oligomers. Since only the ethyl group has the carbon content in the TEOS molecule, the complete hydrolysis of TEOS should in principle result in no carbon content on the catalyst. In order to assure a complete hydrolysis of TEOS as well as a complete removal of introduced ethanol (as solvent in the second step of modification) during impregnation, the carbon content of catalysts with different cycles of modification was monitored, with the value lower than the detection limit ( < 0.05 wt% of carbon) given by the instrument (see Section 2.2). Page 10
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Since the modified catalysts would be used at high temperatures in the following studies, it is necessary to select an appropriate temperature for calcinating these catalysts to a stable status. For this purpose, thermal analyses were done for the unmodified and modified Pt/Al2O3 catalysts without calcination. Figure 1 shows the TG and the DSC curves of un-Pt/Al2O3 and un-9c-SiO2/Pt/Al2O3. Other un-nc-SiO2/Pt/Al2O3 catalysts showed a same qualitative thermal behavior as un-9c-SiO2/Pt/Al2O3, and thus their results are not presented here. Both catalysts showed a weight loss at the temperature lower than 200 oC, mainly accounting for loss of the physically adsorbed water. The unmodified un-Pt/Al2O3 did not show noticeable weight loss after 200 oC, and in contrast to that, significant weight loss can still be observed before 450 oC for the modified un-9c-SiO2/Pt/Al2O3 catalyst. This weight loss in the range of 200 ~450 oC can be accounted for the dehydration process of silicic acid or its oligomers. Correspondingly, the DSC curve of un-9c-SiO2/Pt/Al2O3 (see in Figure 1b) showed an exothermic feature at the temperature range of ca. 150 ~450 oC with the peak of ~ 260 oC, which was absent for the case of un-Pt/Al2O3. This feature can be accounted for the oxidation of the small amount of ethyl group by air, as well as in part due to the dehydration of the products formed from the hydrolysis of TEOS.
(a) (b) Figure 1. The TG (a) and DSC (b) curves of the un-Pt/Al2O3 and un-9c-SiO2/Pt/Al2O3 catalysts.
At the temperature range of > 450 oC, the DSC curve of both samples was quite similar to each other. At the same time in this temperature range, no observable weight loss can be seen in the TG Page 11
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curve of both samples. These reinforced that the temperature of 600 oC is appropriate for calcination for further use as the catalysts for ODHE (vide infra). 3.2 The sintering resisting effect of the modified Pt catalyst 3.2.1 Morphology characterization
Figure 2. TEM images of (a, upper left) un-Pt/Al2O3, (b, upper right) un-9c-SiO2/Pt/Al2O3, (c, lower left) Pt/Al2O3 (after calcination) and (d, lower right) 9c-SiO2/Pt/Al2O3 (after calcination), and (the insets in panels a-d) the Pt particle size distributions of these four samples.
The TEM images of the un-Pt/Al2O3 and un-9c-SiO2/Pt/Al2O3 catalysts are shown in the Page 12
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panels of a and b in Figure 2. The size distribution of the Pt nanoparticles (see the insets in these two pictorial panels) shows that these two uncalcined catalysts had similar particle sizes in the range of 1~3 nm, comparable to the sizes reported by Haneda et al.19 This similarity showed that the modification of a SiO2 layer did not have a noticeable influence on the Pt nanoparticle sizes. However, calcination of these two catalysts in air at 600 ˚C for 2 h led to a significant difference in their morphology change. A significant sintering was observed on the Pt/Al2O3 surface (Figure 2c and its inset) since a great portion of the Pt nanoparticles had the size near 10 nm. In contrast to that, the Pt nanoparticles in the 9c-SiO2/Pt/Al2O3 catalyst (Figure 2d and its inset) had much better thermal stability with a same calcination process. Only a slight sintering was observed and most of the Pt nanoparticles had the size in the range of 1~5 nm. In short, the results from the morphology characterization showed that the introduced SiO2 layer can prevent the aggregation of Pt nanoparticles on the Pt/Al2O3 surface, thus significantly increased the thermal stability of this Pt catalyst at a high temperature. 3.2.2 DRIFT spectroscopic study with CO adsorption IR spectroscopy using CO as a probe is a frequently used technique for characterizing the surface of transition metal materials,37 especially the noble metals.38,39 The geometrical information of the underlying metallic substrate can be qualitatively or quantitatively deduced from the vibrational feature of linear (and/or bridge) adsorbed CO. Because of the surface heterogeneity of the supported metal particles, the above information is always qualitative. Figure 3 shows the CO-DRIFT spectra of the Pt/Al2O3 and 9c-SiO2/Pt/Al2O3 catalysts before and after calcination in air at 600 oC for 2 h. In the case of CO adsorbed on the un-Pt/Al2O3 catalyst (curve a in Figure 3), only one asymmetric broad band was observed from 1946 to 2100 cm-1 with the peak located at ~2066 cm-1. Kappers and van der Maas38 found that the
CO
increased linearly with the coordination number
(Cn) for the EUROPT-3 catalyst. Since the defect sites including edges, corners and steps have lower Cn than the sites in planes,39 the defect sites typically have lower Page 13
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CO
than the plane sites.
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For examples, for CO adsorption on EUROPT-3, the 2096 cm-1 band was assigned to the Pt(111) plane.38 In contrast, the 2063 and 2034 cm-1 bands were assigned to the edge sites having the coordination numbers of 7 and 5, respectively, and the ~2000 cm-1 band was assigned to the step sites. In this work, it is beyond our scope to assign the vibrational features to explicit sites, however, the qualitative rules were still quite obvious. The broad band in the uncalcined Pt/Al2O3 in Figure 3 reflected that the Pt nanoparticles may have a broad distribution of different Cn, and the overall IR signals had contributions from various lower Cn sites and plane sites.
Figure 3. In situ DRIFT spectra of the linear adsorbed CO on different catalysts including (a) un-Pt/Al2O3, (b) Pt/Al2O3, (c) un-9c-SiO2/Pt/Al2O3 and (d) 9c-SiO2/Pt/Al2O3. See the experimental details of DRIFT and notation of catalysts in the experimental section. “K.M.” represents KublkaMukk transformation.
Compared to the case of un-Pt/Al2O3 catalyst, the overall intensity of the νCO band for the case of calcined Pt/Al2O3 catalyst (curve b in Figure 3) had a dramatic decrease. The decrease in the band intensity is not surprising because of the sintering effect at high temperatures that led to the decrease of the number of the exposed Pt atoms. This reinforced the conclusion from the TEM results shown in Figure 1. Two separate bands were observed for CO adsorbed on the calcined Pt/Al2O3 catalyst, whose peaks located at 2061 and 2093 cm-1, respectively. The 2093 cm-1 peak Page 14
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showed the Pt(111) plane or similar planes had a significant contribution to the Pt sites after calcination, according to the assignment of Kappers and Van der Maas.38 The edge sites also had a significant contribution as reflected by the ~2061 cm-1 peak. The vibrational feature lower than ~2030 cm-1 was nearly disappeared, reflecting that the lower Cn sites only have slight contribution. These spectral changes suggested that high temperature calcination led to the aggregation as well as surface-reconstruction of the Pt nanoparticles. The surface-reconstruction tended to decrease the Pt sites having lower Cn over those having higher Cn, like plane sites. In contrast to the dramatic difference between the IR bands from the uncalcined and calcined Pt/Al2O3 catalysts, the difference between those from the uncalcined and calcined 9c-SiO2/Pt/Al2O3 catalysts was slight. For the un-9c-SiO2/Pt/Al2O3 catalyst (curve c in Figure 3), the peak of the CO-IR band had a moderate blue shift by ~ 10 cm-1 compared to the case of un-Pt/Al2O3 without SiO2 modification. Accompanied with the blue shift, the IR band looked more symmetric after modification. On the other hand, the peak area of this IR band was smaller than that for the un-Pt/Al2O3 catalyst. The decrease of the peak area can be accounted for part of the Pt surface being covered by the modified SiO2 layer, leading to the decrease of the number of the exposed Pt atoms. However, the decrease in the lower decrease in the higher
CO
CO
region (< 2066 cm-1) was much more obvious than the
region (> 2066 cm-1), accounting for the change in the peak shape. This
reflected that the introduced SiO2 layer tended to cover the lower Cn sites like edge ones rather than higher Cn sites like plane ones. More importantly, the DRIFT spectrum of the 9c-SiO2/Pt/Al2O3 catalyst (curve d in Figure 3) showed only a slight change compared to that of the un-9c-SiO2/Pt/Al2O3 catalyst in shape and in position of the CO-IR band. The integrated area of this band also only had a slight decrease. Those slight differences reflected that the introduced SiO2 layer prevented the aggregation of the Pt nanoparticle, and thus prevented significant surface reconstruction of the Pt surface as occurred in the unmodified Pt/Al2O3 catalyst. There were significant differences between the CO-DRIFT spectra of the two calcinated Page 15
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catalysts, Pt/Al2O3 and 9c-SiO2/Pt/Al2O3 (see curve b vs. d in Figure 3). Curve b had two peaks (2061 and 2093 cm-1) and curve d had only one peak (2079 cm-1). The difference in the number of peaks and in peak positions showed the Pt-site distribution was quite different between Pt/Al2O3 and 9c-SiO2/Pt/Al2O3. As mentioned above, the unmodified catalyst had two main features of Pt sites (like plane sites and edge sites) after a high-temperature use. As a comparison, the SiO2 modified catalyst had one main feature (sites with an averaged Cn lower than plane sites and higher than the edge sites) after a high temperature use. In addition, the integrated area of the whole CO-IR band was much smaller for curve b than d. This difference implicated that, when catalysts were used in a high temperature like 600 oC, the introduced SiO2 layer can make the Pt sites more exposed through its sintering-resisting effect. This hypothesis was further supported by the Pt dispersion data from the CO-chemisportion experiment. As can be seen in Table 2, the modified catalyst had a larger Pt dispersion than the unmodified catalyst.
Table 2. The Pt dispersion of the calcinated Pt/Al2O3 and 9c-SiO2/Pt/Al2O3 catalysts (see the experimental section for detail, and see the Pt loading in Table 1 as well). Catalyst
Amount of chemisoprtion CO μmol CO/gcat
Pt dispersion (Percentage of Pt exposed)
Pt/Al2O3 9c-SiO2/Pt/Al2O3
10.5 11.5
10.2% 17.0%
3.3 Other physiochemical properties 3.3.1 Py-DRIFT study of the acid sites Acid sites on the surface may significantly influence the performance of the active-metal components on a catalyst.40,41 Pyridine is often used as the probe molecule in investigating the surface acid sites on a catalyst by IR spectroscopy. Chemisorption of pyridine on Brönsted acid site leads to an IR peak at 1540 cm-1 which is assigned to the ring deformation vibration of pyridine.42,43 Meanwhile, chemisorptions of pyridine on Lewis acid sites shows a peak at 1450 cm-1 which is assigned to the deformation vibration of C-H bonds. DRIFT spectra of the Pt/Al2O3 and Page 16
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9c-SiO2/Pt/Al2O3 catalysts after pyridine adsorption are shown in Figure 4. For the Pt/Al2O3 catalyst, only the feature assigning to Lewis acid site was observed. For the 9c-SiO2/Pt/Al2O3 catalyst, both of the features assigning to Lewis and Brönsted acid site respectively were observed. The new Brönsted acid sites were brought by the introduced SiO2 layer. This is reinforced by the results reported by Jo et al.43 where new Brönsted acid site generation was identified when introduced SiO2 onto the pure η-Al2O3 support. Meanwhile, in the wavenumber range between 3500 and 4000 cm-1, a new inverted peak was observed at ca. ~3750 cm-1 for the 9c-SiO2/Pt/Al2O3 catalyst. This new feature can be attributed to the OH group on the SiO2 surface.41,44 The Si-OH groups could be generated during the hydrolysis of TEOS especially at the presence of acid catalyst,45,46 and on the other hand, this OH group may lead to new Brönsted acid sites on the surface of the 9c-SiO2/Pt/Al2O3 catalyst.
Figure 4. The Py-DRIFT spectra of 9c-SiO2/Pt/Al2O3 (upper) and Pt/Al2O3 (lower). “K.M.” represents for Kublka-Mukk transformation.
3.3.2 NH3-TPD study Figure 5 shows the NH3-TPD profiles of the unmodified and SiO2-modified Pt/Al2O3. For the total amount of acid sites (from titration, see Section 2.2 for experimental details), the introduced SiO2 layer resulted in a slight decrease of this value. However, the shape of the TPD profiles showed the modified catalyst had a lager contribution of mild strong acid sites (where NH3 Page 17
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desorbed in the temperature range of 200 ~400 oC47,48) compared to the unmodified one. Correspondingly, the contribution of the weak acid sites (where NH3 desorbed in the temperature range of < 200 oC) on the modified catalyst decreased. With a quantitative analysis of the contributions of different temperature region/acidic strength (Table 3), the above changes were confirmed. For instance, the contribution of mild strong acid sites had a ~6% increase for the modified catalyst compared to the unmodified one. The amount of mild strong acid sites was also slightly increased accompanied with the decrease of the other two types of acid sites. Combined with the Py-DRIFT results (Section 3.3.1) it can be seen that the Brönsted acid sites brought by the introduced SiO2 layer partly replaced the original Lewis acid sites on the Al2O3 surface. The Brönsted acid sites had stronger acidity, but a slightly lower density compared to the original Lewis acid sites.
Figure 5. The NH3-TPD profiles of Pt/Al2O3 (lower) and 9c-SiO2/Pt/Al2O3 (upper). Table 3. Quantitative analysis of the contributions from different temperature ranges in the NH3-TPD profiles of the Pt/Al2O3 and 9c-SiO2/Pt/Al2O3 catalysts.
Temperature range of NH3 desorption o
Low: 80-200 C Middle: 200-400 oC High: 400-650 oC
Type of acid site depicted as Weak Mild strong Strong
Area contribution to total in percentage Pt/Al2O3 9c-SiO2/Pt/Al2O3 28.4% 48.9% 22.7%
24.5% 55.3% 20.2%
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Amount of acid sites in mmol NH3/g Pt/Al2O3 0.142 0.245 0.113
9c-SiO2/Pt/Al2O3 0.113 0.254 0.093
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3.3.3 The N2 adsorption-desorption isotherm study
Figure 6. (a) The N2 adsorption–desorption isotherms of the Pt/Al2O3 and SiO2/Pt/Al2O3 catalysts at 77 K, and (b) the pore size distribution calculated from the adsorption branch of the isotherms.
Figure 6 shows the 77K-N2 adsorption-desorption isotherm and the pore size distribution of the Pt/Al2O3 and 9c-SiO2/Pt/Al2O3 catalysts. For both catalysts, the isotherms can be identified as a type-IV one, which was characteristic of mesoporous materials. The Pt/Al2O3 catalyst had an H1 type hysteresis loop according to the IUPAC classification, which indicated that the pores in this catalyst were rather uniform. The isotherm of the 9c-SiO2/Pt/Al2O3 catalyst showed no significant saturated adsorption platform and can be classified as the H4 type hysteresis loop, indicating that there were micropores and crack spaces in the sample. The pore-size distribution curves showed Pt/Al2O3 had a most probable pore size at ~ 7 nm, while 9c-SiO2/Pt/Al2O3 did present a most probable aperture in the region of the mesoporous size. The aperture sizes of < 4 nm had a major contribution to the overall pore volume. The surface areas and the t-plot micropore area and external surface area calculated from the isotherms for both catalysts are shown in Table 4. The surface area of Pt/Al2O3 was mildly decreased after SiO2 the modification, however the ratio of the micopore area has a significant increase. From these trends it can be hypothesized that the SiO2 layer grew in the pores of Al2O3 and mainly existed in micropores. In addition, the XRD patterns of nc-SiO2/Pt/Al2O3 and Pt/Al2O3 were both similar to that of the catalytic support of Al2O3 (therefore Page 19
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not shown here). The absence of the SiO2 signal43 on nc-SiO2/Pt/Al2O3 suggested that the SiO2 components did not exist in a large bulk phase, supporting the above hypothesis.
Table 4. The BET surface area, t-plot micropore (< 2 nm) and external surface area of the Pt/Al2O3 and 9c-SiO2/Pt/Al2O3 catalysts calculated from the 77K-N2 adsorption-desorption isotherms. Catalyst
SBET (m2g-1)
Pt/Al2O3 9c-SiO2/Pt/Al2O3
286 195
micropore area (m2g-1) 6 101
external surface area (m2g-1) 280 94
3.3.4 Discussion on the catalyst models for surface modification process Putting the adsorption-desorption isotherm, TEM and CO-DRIFT (Section 3.2) results together, one can see a clearer picture for how the surface modification undergo from un-Pt/Al2O3 to SiO2/Pt/Al2O3. The pictorial models shown in Figure 7 can also help better understand how the SiO2 layer prevented aggregation of the Pt nanoparticles on the Al2O3 surface, as well the results of catalytic test shown in the following section. Figure 7a depicts a suggested model for the unmodified un-Pt/Al2O3 catalyst. Most of pores in the Al2O3 support have the size of ~ 7 nm (see Figure 6b), far larger than the size of the Pt nanoparticles of the un-Pt/Al2O3 catalyst (see Figure 2a) There was little spatial hindrance for the Pt nanoparticles to aggregate at high temperatures for such catalyst, as reflected by the TEM results in Figures 2a and 2c, and by the CO-DRIFT results in curves a and b in Figure 3. Figure 7b depicts a model for un-Pt/Al2O3 coated with the layer of products formed from the hydrolysis of TEOS, affording un-nc-SiO2/Pt/Al2O3. This layer may most of even all of the exposed Pt particles considered the amount of the introduced SiO2 in 9c-SiO2/Pt/Al2O3 (see Table 1 for the SiO2 content).
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Figure 7. Schematic diagrams for the suggested models of (a) unmodified un-Pt/Al2O3, (b) un-Pt/Al2O3 modified with the TEOS/ethanol and HAc solutions affording un-nc-SiO2/Pt/Al2O3, and (c) the un-nc-SiO2/Pt/Al2O3 catalyst calcined at 600 oC affording nc-SiO2/Pt/Al2O3. See surface modification steps described in Section 2.1 as well. . When un-nc-SiO2/Pt/Al2O3 was calcined at a high temperature, the coated layer in Figure 7b became cracked and shrank, since water loss was inevitable during calcinations (see thermal analysis results in Figure 1). Consequently the formation of meso/micropores (Figure 7c) of the coating layer made a large portion of the Pt surface re-exposed, which was supported by the N2-adsorption isotherm and the CO-DRIFT results (curves c and d in Figure 3). From this model it can be imagined that there should be an appropriate range for the amount of SiO2 introduced to un-Pt/Al2O3. This “appropriate amount” should match the requirements that it is large enough to effectively hinder the aggregation of Pt nanoparticles on one hand, and it is not too much so as to allow Pt sites re-exposed during calcinations on the other hand. This was the reason why we focused on the 9c-SiO2/Pt/Al2O3 catalyst instead of other nc-SiO2/Pt/Al2O3 ones.
3.4 Catalytic oxidative dehydrogenation of ethane of the SiO2 modified Pt/Al2O3 3.4.1 Improved catalytic stability and coking resistance at a high temperature The results shown in the previous sections intrigued us to further investigate the catalytic Page 21
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properties of nc-SiO2/Pt/Al2O3 when used for reactions at high temperatures, especially the results about the sintering-resisting effect (Sections 3.2) and the surface Brönsted acid sites (Section 3.3.1) introduced by the SiO2 layer. On the other hand, Pt is one of the most frequently studied active-metal component for the ODHE7,30,31 from late 1990s. The ODHE usually requires high temperatures (typically 400 ~900 oC), making the thermal stability of the active-metal components very important in the related catalyst development. In this context it is interesting to compare the catalytic performances of the Pt/Al2O3 and nc-Si/Pt/Al2O3 catalysts for ODHE. Figure 8 compares the reactant conversions and the selectivities to all observed carbon-containing compounds for the ODHE catalyzed by Pt/Al2O3 and by 9c-SiO2/Pt/Al2O3, respectively. From the aspect of catalytic stability, the C2H6 conversion, X(C2H6), given by 9c-SiO2/Pt/Al2O3 within 10 h of reaction was rather stable, and that given by Pt/Al2O3 had a slight decrease from 3 to 10 h. The selectivity to the main product of ODHE, S(C2H4), had a slight decrease of ~2% at the same time given by 9c-SiO2/Pt/Al2O3, while it had an obvious decrease of ~8% given by Pt/Al2O3. These comparisons showed that 9c-SiO2/Pt/Al2O3 had a better catalytic stability than Pt/Al2O3 for ODHE at a high temperature. In principle the better catalytic stability can be in part accounted for the sinter-resisting effect introduced by the SiO2 layer. Another reason accounted for the better catalytic stability was less coke formation during the period of catalytic test. The carbon content of used catalyst after 10-h reaction was measured for both of the 9c-SiO2/Pt/Al2O3 and Pt/Al2O3 catalysts, which was 1.5% and 7.5%, respectively. In another word, we show here that the introduced SiO2 layer significantly suppressed the coke formation during the high temperature reaction. It is well known that coke formation is one of the main problems in high-temperature catalysis. Considered that the reaction condition in this study was a O2-lean [X(O2) of 100%, see Figure 8] situation, the coking resistance brought by the SiO2 layer could be useful for a wider catalytic application.
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Figure 8. Catalytic performance of the unmodified Pt/Al2O3 catalyst and the 9c-SiO2/Pt/Al2O3 catalyst used for oxidative dehydrogenation of ethane at the time of reactions of (a) 3 h and (b) 10 h. Mass of catalyst: 0.5 g. Reaction feed flow: 9.0 sccm C2H6 + 4.3 sccm O2 + 62.5 sccm He + 6.7 sccm N2. Reaction temperature: 600 oC.
The above results also encompass other interesting findings for the metal-catalyzed ODHE in special and for the field of metal-catalyzed oxidation of light alkanes in general. Firstly, as is known Pt is a highly precious catalytic metal, and in many cases, the Pt cost composes most of the whole cost of a Pt-based catalyst. The increased stability in the catalytic performance for a long-time reaction during the industrial application directly means the decreased cost of the Pt catalyst. Page 23
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Compared to the ALD-based method,20 our work provided an easy-operating and low-cost method for increasing catalytic stability and coking-depression at high temperatures. Secondly, our method does not sacrifice the recyclability of Pt in the supported Pt catalyst. The supported Pt-Sn catalyst7,30 was found to have a higher catalytic stability for oxidative and non-oxidative dehydrogenation of light alkanes than the supported Pt catalyst. In preparation of the supported Pt-Sn catalyst the two metal sources are always resided onto the catalytic support simultaneously and finally these two metal components form Pt-Sn alloy(s). In this case, the difficulty in separation of Pt from the Pt-Sn alloys adds more difficulty in recycling Pt from the used catalyst. In contrast to that, our catalytic material of nc-SiO2/Pt/Al2O3 does not appear to have the problem of recycling Pt from an alloy. 3.4.2 Improved selectivity to C2H4 and the underlying reason Data shown in the 3rd group of columns in Figure 8 indicated that 9c-SiO2/Pt/Al2O3 presented a noticeable higher selectivity to C2H4 compared to Pt/Al2O3 by ~12% at 3 h of reaction and by ~ 19 % at 10 h. Correspondingly the selectivity to COx (CO2 and CO) was lower for the case of 9c-SiO2/Pt/Al2O3 than that of Pt/Al2O3. C2H4 was the major product given by both catalysts. Selectivity to methane was minor. Other carbon-containing compound was not detected in this work except small amount of coke on the used catalyst. As is known that X(C2H6) and S(C2H4) often have a trade-off relation [higher X(C2H6) corresponds to lower S(C2H4) and vice versa, e.g. see reference 49] for a certain catalyst. However interestingly, in this work our modified catalyst helped improve S(C2H4) and at the same time helped improving X(C2H6), although slightly (1st group of column in Figure 8). The previous results reinforced that the development of a new method for catalyst preparation can provide a good chance to improve the catalytic performance for important reactions in the chemical industry. In the present stage, it still requires a substantial investigation to gain a deeper understanding of the underlying reason for the improved S(C2H4) as well as the coking-resistance shown in the case of 9c-SiO2/Pt/Al2O3 compared to that of Pt/Al2O3. Such an investigation is Page 24
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presently out of the scope of this paper. However, the characterization results shown above still provided some implicit clues associable with the improved S(C2H4). Firstly, the improved S(C2H4) can be associable with the Brönsted acid sites of the SiO2 layer (see Section 3.3.1). In a previous work33 we reported that the Ni/H-Y zeolite gave higher X(C2H6) and S(C2H4) values for ODHE compared to the Ni/Na-Y zeolite for a same Ni loading on zeolite Y. It has been well established that for catalytic ODH of light alkanes in general and ODHE in special, the catalytic reaction always starts with the activation of a C-H bond on a metal surface,26 forming a metal-alkyl or metal-alkoxide intermediate. For the Ni/H-Y catalyst a mechanism was proposed which involves the formation of metal ethoxides and a subsequent β-H abstraction to form C2H4. This mechanistic scheme allows a Brönsted acid site catalyzes a β-H abstraction process undergoes in its vicinity. The Brönsted acid site-aided S(C2H4) for both of the Ni/H-Y and 9c-SiO2/Pt/Al2O3 cases may share a similar mechanism through comparison. Interestingly, the NH3-TPD results showed that Ni/H-Y had more Brönsted acid sites than Ni/Na-Y mainly with the mild strong ones,33 and similar results can be seen in this study for 9c-SiO2/Pt/Al2O3 compared to Pt/Al2O3. The catalyst model shown in Figure 7c also suggests that part of the Brönsted acid sites on the SiO2 surface can be quite close to the Pt sites. Secondly, the improved S(C2H4) may be associable with site distribution of the exposed Pt surface. In general there has been a noticeable amount of reportse.g.,50,51 showing that the product selectivity of a certain heterogeneously catalyzed reaction may be influenced by the type of metal surface of the supported metal nanoparticles. In this study the CO-DRIDT results implicitly showed that the modified SiO2 layer prevented surface reconstruction of the Pt nanoparticles simultaneously with aggregation when the catalyst was used at high temperatures like 600 oC. As discussed in Section 3.2.2, this may lead to a situation that the Pt site distribution of the used/calcined 9c-SiO2/Pt/Al2O3 was rather close to that of un-9c-SiO2/Pt/Al2O3 and also that of un-Pt/Al2O3, while quite different to that of used/calcined Pt/Al2O3. Although presently it is not explicitly clear about how the Pt site distribution influences on the product selectivity of ODHE, it is in principle Page 25
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reasonable to anticipate that different Pt site distribution can present different selectivity to C2H4 for ODHE.
4. Conclusions SiO2-modified Pt/Al2O3 catalysts were prepared, characterized and used for catalytic oxidative dehydrogenation of ethane at a high temperature. Following conclusions can be drawn from the above-described results. (1) This work provided a new method for surface modification of a supported Pt catalyst by a SiO2 layer. The modification process can be conducted with several cycles for different amount of SiO2 introduced, and each cycle contained three sequential steps. This method is easy-operating and low-cost compared to the ALD-based methods. (2) The introduced SiO2 layer had a sintering-resisting effect, that is, it prevented the aggregation of Pt nanoparticles on the Al2O3 support when used at a high temperature. The introduced SiO2 layer tended to cover the Pt sites having lower Cn rather than those having higher Cn. As a consequence, this layer may prevent surface reconstruction of the Pt nanoparticles. (3) The introduced SiO2 layer led to an increased catalytic stability of Pt/Al2O3 for ODHE at a high temperature, especially from the aspect of selectivity to C2H4. Simultaneously, this layer also led to a mildly increased selectivity to C2H4 with a slight increase of C2H6 conversion. Furthermore, this layer suppressed coke formation during the ODHE test. (4) The modifying SiO2 layer introduced new Brönsted acid sites on the catalyst, which may be associable with the selectivity improvement of C2H4 for the 9c-SiO2/Pt/Al2O3 catalyst.
Acknowledgement Supports from the National Natural Science Foundation of China (21576291, 21306230, 21203250), Shandong Province Natural Science Foundation (ZR2014BM002, ZR2012BQ020), and the Fundamental Research Funds for the Central Universities (15CX05052A, 17CX02067, Page 26
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15CX08010A) are gratefully acknowledged.
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n-butane over platinum-tin catalysts supported on alumina. J. Nanosci. Nanotechno. 2016, 16, 10816-10822. (7) Bodke, A. S.; Olschki, D. A.; Schmidt, L. D.; Ranzi, E. High selectivities to ethylene by partial oxidation of ethane. Science 1999, 285, 712-715. (8) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; Zapol, P. Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane. Nature Materials 2009, 8, 213-216. (9) Zheng, X.; Mantzaras, J.; Bombach, R. Hetero-/homogeneous combustion of ethane/air mixtures Page 27
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