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ZIF-67 Derived Nanoreactors for Controlling Product Selectivity in CO2 Hydrogenation Guowu Zhan, and Hua Chun Zeng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01827 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017
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ZIF-67 Derived Nanoreactors for Controlling Product Selectivity in CO2 Hydrogenation Guowu Zhan and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260. Cambridge Centre for Advanced Research in Energy Efficiency in Singapore, 1 Create Way, Singapore 138602. ABSTRACT: CO2 hydrogenation to produce useful C1 chemicals (such as CO, CH4 and CH3OH) plays a pivotal role in future energy conversion and storage, in which catalysts lie at the heart. However, our fundamental understanding of the correlation between catalyst structures and product selectivity is still limited, due to that in most cases the catalyst structures in nanoscale are not well-defined. Herein, we report the design and synthesis of nanoreactors by phase transformations of sandwich-structured ZIF-67@Pt@mSiO2 nanocubes via a simple water soaking method, where ZIF-67 serves not only as a morphological template but also as a sacrificial cobalt source. The resultant porous maze-like nanoreactors are highly active in gas phase CO2 hydrogenation, in which the reaction pathway involves (i) dissociation of CO 2 to form CO over Pt site via reverse water-gas shift reaction and then (ii) methanation of CO catalyzed by the nearby cobalt site. It was found that the overall “long retention time” for feed gases on catalysts significantly affected the product distribution. Thereby, the specific activity (in the form of turnover frequency) of the nanoreactor having prolonged diffusion paths was around six times as much as that of other comparative catalysts with shorter diffusion paths. This work contributes insights to the CO2 hydrogenation to methane over bifunctional nanoreactors with designed structures. KEYWORDS: metal–organic frameworks, CO2 hydrogenation, catalysis, diffusion pathway, nanoreactors
■ INTRODUCTION Utilization of carbon dioxide (CO2) via catalytic hydrogenation processes to produce useful fuels and chemicals (e.g., CO, CH4, olefin, methanol, formic acid, dimethyl ether, etc.) has been extensively studied in recent years due to both economic benefits and environmental concerns (fixation of the “greenhouse” gas).1-2 An effective catalyst is essential to activate and split the thermodynamically stable CO2 molecule, and exploring new catalyst with aimed activity, selectivity, and prolonged lifetime is still necessary to industrialize the process. Regarding the kinetics and mechanisms (e.g., intermediates, elementary steps, active sites etc.), however, there is still no consensus, and some existing mechanisms are even highly debatable. For instance, there are two totally different reaction mechanisms proposed for CO2 methanation (CO2 + 4H2 ⇆ CH4 + 2H2O, ∆Ĥo298K = −165 kJ/mol, also named as Sabatier reaction)3-6: (i) the dissociation of CO2 to CO (or carbonyl (COad)) via forming carboxylate (HOCO) species prior to methanation, and the subsequent reaction follows the same route as CO methanation (CO + 3H2 ⇆ CH4 + H2O, ∆Ĥo298K = −206 kJ/mol); and (ii) the direct associative adsorption of CO 2 and H2 to form CH4 without the formation of CO (or COad) as an intermediate, but it forms formate (HCOO) species. The former proposed methanation mechanism was named as dissociative scheme while the latter was called as associative scheme (refer to Scheme S1).7 Similarly, for CO methanation, two types of methanation mechanisms were proposed, the dissociative scheme involves surface-carbon (Cad) as intermediate, while CHxOad intermediate was present in the associative scheme.6
It is well known that Pt tends to selectively produce CO from CO2/H2 mixture via the reverse water–gas shift reaction (RWGS, CO2 + H2 ⇆ CO + H2O, ∆Ĥo298K = 41.2 kJ/mol).8-9 Therefore, very few studies use Pt catalysts alone for CO2 hydrogenation towards CH4, where the formation of CH4 in some reports is probably related to synergetic effects of reducible oxide carriers and the Pt crystals.10-11 Even for the bimetallic Co@Pt core-shell catalyst, CO is produced almost exclusively (selectivity to CH4 was Pt0.5Cu0.5 (27.6%) > Considering that Pt@CSN is the best catalyst in the present Au0.5Cu0.5 (16.0%) > Cu (8.5%) > Au (6.0%) > Ag (4.4%). investigation, in the following we concentrate on the Pt catalysts to Regarding product distribution, under all the tested conditions, investigate the effects of catalyst structures (viz., diffusion pathway), CO and CH4 were the only two products found (no other higher
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operating conditions (200−320°C and 1−30 bar), and gas flow rate (6−240 mL/min) on the product distribution. Firstly, the pressure dependent CO2 hydrogenation was measured (Figure 6c). Along increasing the pressure, the conversion of CO2 drastically increases, and the CH4 selectivity becomes more prominent. For instance, CH4 selectivity increases from 6.6% to 93.8%, as raising the pressure from 1 to 30 bar. Higher pressure thermodynamically facilitates the formation of CH4, but without affecting the equilibrium of CO formation. When the reaction was run under 1 bar, the major product was CO (93%−97%) regardless the change of temperature (see Figure S23). However, in Figure 6d (at 30 bar), temperature changes not only the activity, but also the product distribution. As expected, the CO2 conversion progressively increased with temperature. In particular, the conversion of CO2 increases from 1.1% to 41.8% upon the temperature change from 200 to 320 oC. The selectivity towards CH4 increases dramatically but that towards CO decreases with increasing the temperature. Maximum CH4 selectivity was achieved at 280°C (94%), but further increasing temperature (300 and 320oC) did not lead to any further changes of product selectivity, due to thermodynamically limited conversion of CO to CH4 at high temperatures. It is known that CO formation is favored at high temperature due to the endothermic character of RWGS reaction,44 while the exothermic character of both CO and CO2 methanation reactions are unfavorable as temperature increases. In contrast, clearly in our case, the selectivity towards CH4 increases with increasing temperature, although the theoretical values are decreased. Moreover, for all of the conditions studied Figure 7. Characterizations of comparison catalysts. (a-h) TEM images of Pt (Figure 6a,c,d), CH4 selectivity increased with increase in nanoparticles loading on different supports. (a, d) CoSi@Pt, (b, e) MnSi@Pt, (c, f) the conversion of CO2. As reported, CO2 cannot directly SiO2@Pt, and (g, h) SiO2@Pt@mSiO2. Insets in (d-f) show the statistics of Pt proceed with methanation in the presence of CO, nanoparticle sizes (horizontal axis: particle size (nm); vertical axis: relative frequency because CO interacts more strongly with the catalyst (%)). (i) N2 physisorption isotherms of different catalysts (insets: structural models). surface, and thus CO has a much higher reactivity for the methanation compared to CO2.3, 8, 45 Therefore, it is inferred that reactions. This plausible mechanism for the overall CO2 CH4 was not directly obtained from CO2, but solo methanation of methanation process over our designed nanoreactors is consistent CO (from RWGS) occurred in our system. In other words, CO was with other observations.46-48 produced from RWGS reaction (primary pathway) and CO was consumed via the methanation reaction (sequential pathway), and Since CO2 methanation consists of a sequential formation and thereby the overall CO yield reflects the rate of the two consecutive methanation of CO over our designed catalysts, the kinetic factor
Figure 8. Comparison of catalytic performance of Pt catalysts with different structures at 320 oC, 30 bar. (a) CoSi@Pt, CoSi = cobalt silicate, (b) MnSi@Pt, MnSi = manganese silicate. (c) The comparison of TOF calculated from data achieved at gas flow rate of 6 mL/min. TOF values were defined as mole of CO2 converted per mole of Pt metal per hour.
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(e.g., retention time) should have a significant impact on CO (an intermediate product) yields. In this work, the influence of retention time was investigated by varying the gas flow rate (model ii in Figure 1d) and catalyst structures (model iv in Figure 1d), respectively. The effect of gas flow rate was summarized in Figure 6e. Apparently, longer retention time (that is, lower gas flow rate) led to both higher CH4 selectivity and higher CO2 conversion. When the retention time was reduced, an increased CO selectivity with decreased CO2 conversion was found, due to the lower consumption of CO to CH4. Besides Pt catalyst, similar trends were found over other five metal catalysts: the formation of methane was consecutively increased as increasing retention time (Figure 6f). The observation on the change of retention time on the product selectivity is also consistent with previous studies.49-51 Catalyst performance of our designed nanoreactors (namely, Pt@CSN) was further compared with other two Pt catalysts of different structures (namely, CoSi@Pt, and MnSi@Pt). Morphology and porosity properties of the comparison catalysts are summarized in Figure 7, Table 1 and Scheme 1b. As shown in Figure 8a,b, when the CoSi@Pt and MnSi@Pt catalysts were used, both CO and CH4 were produced, and the product distribution is also highly dependent on the retention time. Similar to the above investigation, again, we found that the decreasing gas flow rate (viz., increasing retention time) favours high selectivity towards CH4. The surface intermediates during the reactions on Pt@CSN, CoSi@Pt, SiO2@Pt@mSiO2, and pure CSN were further analysed by in situ diffuse reflectance infrared Fourier Transform (DRIFT) spectroscopy, as shown in Figure S24. In all the samples, the IR band at 1460 cm−1 is characteristic of chemisorbed CO2 on catalyst surface.52 It is indicated that both chemisorbed CO (COad) and formate species were found on Pt@CSN and CoSi@Pt samples, and the COad was mainly on Pt sites (Pt−CO species adsorbed in linear form, IR band at 2036 cm−1).52-53 No COad was seen in the absence of Pt (e.g., pure CSN sample) and no formate species was seen on SiO2@Pt@mSiO2 sample. Negligible CO2 conversion over pure CSN catalyst indicates that the formation of COad is critical for CO2 hydrogenation to produce the final product of CH4. Based on these results, it is known that the Pt is highly active for the conversion of CO2 to CO; all COad leads to the gaseous CO product over pure SiO2 support (SiO2@Pt@mSiO2). While both COad and formate are pivotal intermediates to CH4, hydrogenation of COad is much kinetically faster than hydrogenation of formate.54 In our bifunctional catalysts, further hydrogenation of the intermediates (particularly, COad) would perform over transition metal active sites (e.g., Co or Mn). Therefore, we can deduce that the formation of methane over our bifunctional catalysts (Pt@CSN) proceeds along the RWGS and CO hydrogenation pathways (viz., the dissociative scheme). Moreover, as shown in Figure 8c, it was found that the specific activity in the form of turnover frequency (TOF) value of the Pt@CSN is 5.8−6.5 times as much as that of other comparative catalysts. For the five catalysts, the average sizes of Pt nanoparticles are almost the same, thus excluding the effect of Pt size. In addition, there was no significant size change of Pt nanoparticles after the reactions (Figure S25), which rules out the effect of active site agglomeration on the comparison results of catalytic performance. Therefore, it is clear that the diffusion pathway appears to be significant for both activity and product distribution via enhancing the retention time. Long-term stability of catalyst is also a critically important issue in industrial application. As shown in Figure S26, no noticeable change in the catalytic activity and product selectivity of Pt@CSN was observed over a 24 h duration, suggesting a pronounced stability. Furthermore, the enduring catalyst can
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produce major CO (>93.3%) or major CH4 (>92.6%) by adjusting the reaction operation parameters. The used catalysts were further examined by TEM, XRD and N2 physisorption (Figures S27-29), showing that the catalysts maintained their morphological and structural integrities and Pt nanoparticles were against sintering due to the physical barriers of three dimensionally assembled cobalt silicate nanosheets confined inside the maze-like nanoreactors.
Table 1. Textural properties of the studied samples. Sample
SBET * (m2/g)
Vt † (cm3/g)
Pore size § (nm)
ZIF-67
1172
0.496
