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Comparative study of CO and CO2 hydrogenation over supported Rh–Fe catalysts
Catalysis Communications 11 (2010) 901–906
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Comparative study of CO and CO2 hydrogenation over supported Rh–Fe catalysts Makarand R. Gogate, Robert J. Davis ⁎ University of Virginia, Department of Chemical Engineering, 102 Engineers' Way, PO Box 400741, Charlottesville, VA 22904-4741, United States
a r t i c l e
i n f o
Article history: Received 20 January 2010 Received in revised form 18 March 2010 Accepted 26 March 2010 Available online 8 April 2010 Keywords: CO hydrogenation CO2 hydrogenation Rh/TiO2 Rh–Fe/TiO2 Methane Ethanol
a b s t r a c t The hydrogenation of CO, CO + CO2, and CO2 over titania-supported Rh, Rh–Fe, and Fe catalysts was carried out in a fixed-bed micro-reactor system nominally operating at 543 K, 20 atm, 20 cm3 min− 1 gas flow 1 −1 ), with a H2:(CO + CO2) ratio (corresponding to a weight hourly space velocity (WHSV) of 8000 cm3 g− cat h of 1:1. A comparative study of CO and CO2 hydrogenation shows that while Rh and Rh–Fe/TiO2 catalysts exhibited appreciable selectivity to ethanol during CO hydrogenation, they functioned primarily as methanation catalysts during CO2 hydrogenation. The Fe/TiO2 sample was primarily a reverse water gas shift catalyst. Higher reaction temperatures favored methane formation over alcohol synthesis and reverse water gas shift. The effect of pressure was not significant over the range of 10 to 20 atm. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The fixation of CO2 into valuable chemicals is potentially an important strategy for CO2 utilization. While CO hydrogenation over transition metal catalysts in general, and Rh in particular, has been studied quite extensively, there have been fewer studies of CO2 hydrogenation. Our group has recently studied CO hydrogenation over supported and promoted Rh catalysts with a goal to produce ethanol and higher alcohols, forming methane and other light paraffins as less desirable co-products [1]. One possible reaction network for CO hydrogenation includes the elementary steps of (a) dissociation of the adsorbed CO to form adsorbed carbon and oxygen, (b) hydrogenation of the adsorbed carbon to form an adsorbed methyl species, (c) insertion of non-dissociated CO into the adsorbed methyl species to form an adsorbed acyl species, and (d) hydrogenation of the adsorbed acyl species to form the ethanol product [1]. Thus, a properly promoted metal catalyst performs many different elementary reactions. One goal of the current work was to investigate how CO2 interacts with the reaction network for CO hydrogenation, so we have studied the hydrogenation of CO2 in the presence and absence of CO. Prior work on CO2 hydrogenation over Rh-based catalysts includes two relevant reports by Inoue et al. [2] and Trovarelli et al. [3]. Inoue et al. [2] examined the activity of supported Rh catalysts on different supports such as MgO, Nb2O5, ZrO2, and TiO2 at 473–573 K, 10 atm, 1 −1 H2:CO2 ratio of 3:1, and a WHSV of 2400 cm3 g− . The ZrO2- and cat h
⁎ Corresponding author. Tel.: +1 434 924 6284; fax: +1 434 982 2658. E-mail address: [email protected] (R.J. Davis). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.020
Nb2O5-supported catalysts were the most active as measured by the rate of formation of product hydrocarbons and alcohols, however, the selectivity to CH4 was nearly 100%. The Rh/TiO2 catalyst was not as active in terms of the rate of hydrocarbon and alcohol formation, but it was a good reverse water gas shift catalyst. The rate of CO formation on Rh/TiO2 was nearly an order of magnitude greater than that of the hydrocarbon and alcohol formation [2]. Trovarelli et al. [3] studied the effect of reduction temperature of Rh/TiO2 on the rates of CO and CH4 formation during CO2 hydrogenation reactions at 523 K with a CO2:H2 ratio of 1:1. For a catalyst prepared with RhCl3 as a precursor and reduced at a low temperature (473 K), methane was the primary product, however, for samples reduced at 623 and 723 K, the rate of CO formation increased significantly. For the catalyst reduced at 723 K, the rate of CO formation was about 2 orders of magnitude higher than that of CH4 formation. At least on Rh/TiO2, the extent of CO formation during CO2 hydrogenation reactions appears to be a function of many different factors. Some reports suggest however, that CO formation during CO2 hydrogenation is not significant on Rh/TiO2. For example, Szailer et al. [4] report that CH4 is the main product (99%+) at 548 K with a H2:CO2 ratio of 4:1 on Rh/TiO2 prepared from RhCl3 precursor (reduced at 673 K for 1 h). In another contribution from the same group [5], the rate of CH4 formation alone is reported for CO2 hydrogenation on a 1% Rh/TiO2 at 473 K, 1 atm, H2:CO2 ratio of 4:1, and with a SV of 6000– 9000 h− 1, so it was not clear if CO was detected. Solymosi et al. [6] suggest that on Rh/Al2O3 and Rh/TiO2, the rate of CO formation is less than 1% of the rate of CH4 formation for catalysts prepared using RhCl3 as the precursor and at the reaction conditions of 473 and 548 K, 1 atm, space velocities of 3000–6000 h− 1, with a H2:CO2 ratio of 4:1. Solymosi and Erdöhelyi [7] also report that the hydrogenation of CO2
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on alumina-supported metal catalysts (including Ru, Rh, Pt, Ir, and Pd, nominal 5 wt.% metal, 548 K, 1 atm, GHSV of 3000–6000 h− 1, and H2: CO2 ratio of 4:1) produces CH4 with a much higher selectivity than that of CO. Another objective of the current work is thus to ascertain the extent of CO formation during CO2 hydrogenation reactions over titania-supported Rh catalysts that have demonstrated activity for higher alcohol synthesis. Several reports by Kusama et al. [8–12] cover in detail the CO2 hydrogenation reaction over Rh/SiO2. In particular, the effect of Fe as a promoter for the reaction has been reported [8]. At 533 K, 50 atm, and a H2:CO2 ratio of 3:1, the results show that increasing the Fe/Rh atomic ratio from 0 to 3 on a 5 wt.% Rh/SiO2 catalyst increases the CO2 conversion and increases the selectivity to methanol and ethanol, at the expense of CO and CH4. On an Fe/SiO2 catalyst, the conversion of CO2 was appreciable, however the CO selectivity was high and very little CH4 was formed. More than 30 additives to Rh were also examined in another report [9], and under the conditions of 513 K, 50 atm, and H2:CO2 of 3, a Rh–Li/SiO2 catalyst (5 wt.% Rh, atomic ratio of 1:1 Rh:Li) exhibited a moderate CO2 conversion of 7% and a selectivity to ethanol of 15.5%. Other non-Rh catalysts for CO2 hydrogenation have also been studied. These include novel Fischer–Tropsch type composite catalysts [13], different metal carbides such as Mo2C, Fe3C, and WC [14], and other composite catalysts such as K/Fe, K/Cu–Fe, and K/Cu–Zn–Fe [15]. The reader is referred to Refs. [2] and [3], and the references therein, for additional background on catalysts for CO2 hydrogenation. Moreover, the mechanistic details of the synthesis of methanol and higher alcohols from CO/CO2/H2 on Cu catalysts have also been covered earlier in some representative references [16–19]. The specific objectives of the current work were to clarify the extent of CO formation in CO2 hydrogenation reactions over titaniasupported Rh, Rh–Fe, and Fe, to compare the reactivity results of CO hydrogenation to CO2 hydrogenation at identical conditions, and to examine the influences of process variables including temperature, pressure, and WHSV on the CO2 conversion and product selectivities during CO2 hydrogenation. 2. Experimental methods 2.1. Catalyst synthesis The supported Rh and Fe catalysts were synthesized by incipient wetness impregnation. Granular rhodium nitrate (Pfaltz and Bauer (Waterbury, CT)) was used as the rhodium precursor for all catalysts. As an example, a 2 wt.% Rh–2.5 wt.% Fe/TiO2 catalyst was prepared by dissolving 0.315 g Rh(NO3)3·2H2O and 0.90 g Fe(NO3)3·9H2O in 7.5 mL of distilled deionized water and adding this solution dropwise with proper kneading to 5 g of TiO2 (Degussa P-25) to the point of incipient wetness. The resulting paste was dried overnight in air at 413 K and subsequently calcined in air at 723 K for 4 h. 2.2. Catalyst characterization The metal loadings of the catalysts were determined by Galbraith Laboratories (Knoxville, TN) using ICP-OES analysis. Additionally, the Rh dispersion was evaluated by H2 chemisorption at 308 K performed using a Micromeritics ASAP 2020 automated adsorption system. Calcined catalysts were reduced in flowing H2 at 473 K for 2.5 h and evacuated at that temperature for 2 h prior to cooling in vacuo to 308 K. An H/Rhsurf stoichiometry equal to unity was assumed in the analysis. 2.3. Reactions of CO and H2 The catalysts were evaluated in a fixed-bed micro-reactor system (Autoclave Engineers' BTRS Jr., Erie, PA). The nominal dimensions of
the reactor tube were an ID of 8 mm, OD of 14.3 mm, and a total length of 275 mm. Calcined catalysts were first pressed into pellets, crushed, and size-separated (+40/−80 mesh). Approximately 0.150 g catalyst was intimately mixed with 2.5 g of silicon carbide (Universal Photonics, Inc., Hicksville, NY) and loaded into the reactor. The catalyst was then reduced in flowing dihydrogen (20 cm3 min− 1) at 573 K for 2.5 h under atmospheric pressure. Following reduction, the catalysts were tested at nominally identical conditions of 543 K, 20 atm total pressure, syngas (H2 + CO) flow of 20 cm3 min− 1, H2:CO ratio of 1:1, and a WHSV of 8000 cm3 g cat− 1 h− 1. After CO hydrogenation, the CO flow to the reactor was cut off and CO2 was introduced at a H2:CO2 ratio of 1:1, with a total flow of 20 cm3 min− 1. The catalyst bed temperature was monitored by a thermocouple inserted within the catalyst bed. All gases (CO (GT&S), CO2 (GT&S), and H2 (GT&S)) were UHP grade (99.999%). The results reported here are generally obtained after 1–1.5 h on-stream. Two HP 5890 Series II gas chromatographs were integrated downstream of the reactor for the analysis of reactants and products. The first one was equipped with a flame ionization detector and a 50 m-long HP-1 cross-linked methyl silicone gum capillary column to monitor hydrocarbons, alcohols, acetaldehyde, methyl acetate (not observed) and ethyl acetate. The second one was equipped with a thermal conductivity detector and a 6-ft long Alltech CTR packed column and was used to monitor the possible formation of CO via the reverse water gas shift reaction during CO2 hydrogenation. The conversion of CO was based on the fraction of CO that formed carbon-containing products according to: % Conversion = ðΣni ·Mi = MCO Þ·100% where ni is the number of carbon atoms in product i, Mi is the percentage of product i detected, and MCO is the percentage of carbon monoxide in the syngas feed. This equation was valid since differential conversion was maintained throughout this study. An analogous equation was used for CO2 conversion for the CO2 hydrogenation runs, with MCO replaced by MCO2 in the denominator. The extent of reverse water gas shift reaction was accounted for by CO formation in the products for CO2 hydrogenation. The selectivity to product i is based on the total number of carbon atoms in the product and is therefore defined as: Si = ðni ·Mi Þ = ð∑ni ·Mi Þ: 3. Results and discussion The reactivity results from CO hydrogenation and CO2 hydrogenation over 2% Rh/TiO2, 2%Rh–2.5%Fe/TiO2, and 2.5%Fe/TiO2 at nominally identical conditions of 543 K, 20 atm, a WHSV of 8000 cm3 gcat − 1 h− 1, with a H2:CO2 ratio of 1, are summarized in Tables 1 and 2. The promotional role of Fe for CO hydrogenation to ethanol (and light alcohols) as reported in our earlier work on CO hydrogenation is reproduced here [1]. For example, the CO conversion over 2% Rh/TiO2 was 4.74% and the ethanol selectivity was 11.3%. Addition of 2.5 wt.% Fe caused the CO conversion and ethanol selectivity to increase to 12.2% and 27.9%, respectively (Table 1). The 2.5% Fe/TiO2 catalyst was not very active for CO conversion since there was no Rh present on the surface. Based on our earlier analytical TEM studies and the results from H2 chemisorption [1], we concluded that for the Rh–Fe/TiO2 catalyst system, the Fe atoms were in close proximity to the Rh metal cluster of 1–2 nm in diameter. Moreover, a subsequent study of the Fe-promoted Rh/TiO2 system by EXAFS and XANES showed that most of the Fe promoter was partially reduced from Fe(III) to Fe(II) under reaction conditions, and thus was highly oxophilic in nature [20]. The results of CO hydrogenation for two distinct cases are reported in Table 1. Entry 1 in Table 1 presents the reactivity results for CO
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Table 1 Hydrogenation of CO over titania-supported Rh, Rh–Fe, and Fe catalystsa. Catalyst
Conv. (%)
% Sel. (CH4)
% Sel. (C2H6)
% Sel. (C3H8)
% Sel. (C4H10)
% Sel. (CH3CHO)
% Sel. (CH3OH)
% Sel. (EtOH)
% Sel. (EthAc)
2%Rh/TiO2 2% Rh/TiO2b 2%Rh–2.5%Fe/TiO2 2.5%Fe/TiO2
4.74 4.77 12.2 1.85
34.6 48.1 35.1 25.7
11.4 5.21 4.75 19.0
21.7 20.3 7.35 23.0
9.30 8.08 1.91 11.3
3.80 4.59 13.3 0.0
2.40 3.41 0.00 10.5
11.3 10.4 27.9 10.6
5.50 0.00 9.80 0.00
CO Conversion (%) = ΣniMi × 100 / MCO and Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO is the mole percent of carbon monoxide in the feed. a 1 −1 Nominal conditions are T = 543 K, P = 20 atm, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute the bed, H2:CO 1:1, syngas flow = 20 cm3 min− 1 or a WHSV of 8000 cm3 g− . cat h b CO for this run was purified by passing it through a silica trap immersed in a dewar containing a dry ice–acetone mixture.
hydrogenation where CO was not pre-purified before introduction into the reactor (it was used directly from the CO gas cylinder). Entry 2 in Table 1 presents the reactivity results for CO hydrogenation at nominally identical conditions, except that CO was pre-purified by passing it through a silica trap immersed in a dewar containing dry ice–acetone mixture. The reactivity results show a good experimental reproducibility, as the CO conversions are 4.74% and 4.77% for the two cases, and the product selectivities are also similar. The product selectivities during CO hydrogenation in Table 1 are similar to those in our earlier work [1] and are also comparable to those in a recent report involving CO hydrogenation over silicasupported Rh–La–V catalysts [21] under somewhat different condi1 −1 tions of 503 K, 1.8 atm, and a WHSV of 9000 cm3 g− . cat h In the present study, CO was observed as a reaction product during CO2 hydrogenation, and the product selectivity was a strong function of catalyst composition (Table 2). The first two entries in Table 2 are results of CO2 hydrogenation studies carried out to test the effect of removing Fe carbonyls from the CO feedstream. Although the CO hydrogenation results did not depend on the presence of the Fe carbonyl trap (see Entries 1 and 2, Table 1), the reactivity results for CO2 hydrogenation showed significant differences in conversion and selectivity. For example, the CO2 conversion for the second entry in Table 2 (after purification of CO) was 19.2%, compared to 7.89% for the first entry. Moreover, the selectivity for CO production decreased when an Fe carbonyl trap was used. These results suggest that there is a role of the Fe impurity in the CO feedstream on the CO2 hydrogenation reactions. Although the difference in the CO2 conversion was minor for 2% Rh/TiO2 (without a CO purifier) and 2%Rh–2.5%Fe/TiO2, there were significant differences in the product selectivities. Whereas the 2%Rh/ TiO2 catalyst had a poor ethanol selectivity of 1.93%, the selectivity to ethanol improved over the 2%Rh–2.5%Fe/TiO2 sample to 6.41%. The 2.5%Fe/TiO2 catalyst functioned primarily as a reverse water gas shift catalyst since there was a high selectivity of 73.0% to CO at a low conversion of CO2 (2.65%). For all three catalysts tested here under the reported reaction conditions, CO was formed in appreciable quantities. These results indicate that all catalysts have a measurable reverse water gas shift activity, and these results, while in partial agreement with the results reported in prior art [2,3], are not in
accordance with the works of Szailer et al. [4], Novák et al. [5], Solymosi et al. [6], and Solymosi and Erdöhelyi [7], which report that CO is a trace product over titania-supported Rh catalysts. It should be noted that the differences in metal precursors (used to prepare the titania-supported Rh catalyst) and/or the reaction conditions (the H2/CO2 ratio and temperature, in particular) may affect the relative rates of CO and CH4 formation. Kusama et al. reported [11] on the reactivity of 1% Rh/SiO2 for CO2 hydrogenation at 473 K, 50 atm, H2: CO2 ratio of 3:1, for three different catalysts where the precursor of Rh was acetate, chloride, and nitrate, respectively. The results showed that CO is the principal product for the catalysts prepared using acetate and nitrate precursors, while CH4 is the principal product for the 1% Rh/SiO2 prepared using chloride as the precursor. The reactivity results in Table 2 for CO2 hydrogenation are always reported for runs that were preceded by CO hydrogenation. To test the possible role of trace impurities or catalyst modifications accomplished by CO hydrogenation, a separate run of CO2 hydrogenation was performed without pretreating the catalyst by CO hydrogenation. In this case, the CO formed by reverse water gas shift was found to be only a trace product by chromatography (the peak for CO, although clearly present on the chromatogram, was not integrable). The CO2 conversion at 543 K, 20 atm, and 20 cm3 min−1 gas flow (corresponding to a weight hourly space velocity (WHSV) of 1 −1 8000 cm3 g− ) was 12.99%. Methane was the principal product, cat h with a selectivity of 91.6%, and the only other products detected were ethane and propane, with a selectivity of 4.8% and 3.6%, respectively. These results suggest that preceding CO2 hydrogenation with CO hydrogenation altered the catalyst in some fashion. Since purifying the CO with an Fe carbonyl trap (silica at dry ice/acetone temperature) prior to the reactor lowered the extent of reverse water gas shift (entries 1 and 2 in Table 2), we conclude that most of the CO produced during CO2 hydrogenation on Rh/TiO2 originated from the RWGS over the deposited Fe impurities from the CO cylinder. The 4.16% selectivity to CO (see entry 2, Table 2) for CO2 hydrogenation after using purified CO confirmed the minor role of Rh/TiO2 as a RWGS catalyst under our conditions. The results of reactivity studies with titania- and silica-supported Rh–Fe catalysts for CO hydrogenation and (CO + CO2) hydrogenation are given in Table 3. When compared to CO hydrogenation without
Table 2 Hydrogenation of CO2 over titania-supported Rh, Rh–Fe, and Fe catalystsa. Catalyst
Conv. (%)
% Sel. (CH4)
% Sel. (C2H6)
% Sel. (C3H8)
% Sel. (C4H10)
% Sel. (CH3CHO)
% Sel. (CH3OH)
% Sel. (EtOH)
% Sel. (CO)
2%Rh/TiO2 2%Rh/TiO2b 2%Rh–2.5%Fe/TiO2 2.5%Fe/TiO2
7.89 19.2 9.16 2.65
72.7 93.3 57.2 11.6
4.30 1.75 2.60 5.10
4.10 0.84 2.40 4.90
0.98 0.00 1.18 0.79
0.67 0.00 0.55 1.73
0.80 0.00 1.26 0.00
1.93 0.00 6.41 2.83
14.5 4.16 28.4 73.0
CO2 Conversion (%) = ΣniMi × 100 / MCO2 and Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO2 is the mole percent of carbon dioxide in the feed. 1 −1 a Nominal conditions are T = 543 K, P = 20 atm, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute the bed, H2:CO2 1:1, syngas flow= 20 cm3 min− 1 or a WHSV of 8000 cm3 g− . cat h b Prior to CO2 hydrogenation, a CO hydrogenation run was carried out (see entry 2, Table 1 for reactivity data) where CO was purified by passing it though a silica trap immersed in a dewar containing a dry ice–acetone mixture.
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Table 3 Hydrogenation of CO and (CO + CO2) mixture over supported Rh–Fe catalystsa. Catalyst
Synthesis gas composition
Conv. (%)
% Sel. (CH4)
% Sel. (C2H6)
% Sel. (C3H8)
% Sel. (C4H10)
% Sel. (CH3CHO)
% Sel. (CH3OH)
% Sel. (EtOH)
% Sel. (n-PrOH)
% Sel. (EthAc)
2%Rh–2.5%Fe/TiO2
H2:CO 10:10 H2:CO:CO2 10:5:5 H2:CO 10:10 H2:CO:CO210:5:5 H2:CO 10:10 H2:CO:CO2 10:5:5
13.8 10.5 6.23 5.00 5.62 4.42
38.5 45.2 33.9 38.3 32.2 38.3
3.50 3.83 7.04 8.16 6.88 9.45
6.03 5.34 8.75 9.51 10.2 13.9
1.50 0.93 2.85 2.35 2.58 2.87
10.7 6.21 3.73 2.37 3.13 2.20
2.05 2.71 9.68 9.11 9.70 8.29
27.2 29.0 30.3 26.4 28.5 20.2
0.00 0.00 0.00 0.00 6.78 4.80
10.6 6.78 3.78 3.77 0.00 0.00
2%Rh–10%Fe/TiO2 2%Rh–1%Fe/SiO2
Conversion (%) = ΣniMi × 100 / MCO for (H2 + CO) syngas; MCO is the mole percent of CO in the feed. Conversion (%) = ΣniMi × 100 / MCO + CO2 for (H2 + CO + CO2) syngas; MCO + CO2 is the mole percent of CO + CO2 in the feed. Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i and Mi is the mole percent of product i. a 1 −1 Nominal conditions are T = 543 K, P = 20 atm, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute the bed, syngas flow = 20 cm3 min− 1 or a WHSV of 8000 cm3 g− . cat h
CO2, the overall conversion decreased by about 15–20% for the mixed feed (CO + CO2), under otherwise identical conditions. For all three catalysts, there was an increase in methane selectivity for the mixed feed (CO + CO2) when compared to CO hydrogenation without CO2. This increase in methane selectivity, although modest, appeared to be above the experimental error, and is likely attributed to the increase in H2/CO ratio when CO2 is present in the syngas feed and the hydrogenation of CO2 to methane. A similar increase in methane (and other light paraffin) selectivity of the CO hydrogenation reactions with increasing H2/CO ratios was observed in our prior work [1]. The selectivity to ethanol appears to be relatively invariant with cofed CO2. The influence of temperature on the CO2 conversion and the product selectivities for CH4, CO, and ethanol over a 2% Rh/TiO2, at the 1 −1 nominal experimental conditions of 20 atm, 8000 cm3 g− , and a cat h H2/CO2 of 1, is summarized in Fig. 1. The CO2 conversion increased with temperature, as expected, but the CH4 selectivity increased at the expense of CO selectivity. The rates of CO and CH4 formation were a strong function of temperature with CO formation being favored at lower temperatures. The selectivity to methane approached nearly 85% at the highest temperature of 573 K. A similar finding has also been reported earlier by Kusama et al. [9] for the effect of reaction temperature on CO2 hydrogenation over 5 wt.% Rh–Li/SiO2 catalyst (with Rh/Li = 1) albeit at different reaction conditions of 50 atm, 1 −1 H2/CO2 = 3, and a WHSV of 6000 cm3 g− . In their study, the CH4 cat h selectivity increased from 32.2% to 81.3% as the temperature increased from 473 to 533 K, while the CO selectivity decreased from 55.2% to 6.2% over the same temperature range. The selectivity to ethanol also
appeared to go through a maximum in the previous study by Kusama et al. [9], a feature that is also evident from Fig. 1. The influence of WHSV on the CO2 conversion and selectivities to CH4, CO, and ethanol is shown in Fig. 2, as a plot of CO2 conversion and product selectivities vs. 1/WHSV at the nominal conditions of 20 atm, 543 K, and a H2/CO2 ratio of 1. The CO2 conversion increases with the inverse of WHSV at low conversion values, and that the product selectivities were fairly constant over the range of WHSV studied. These results are in general accordance with those of Takagawa et al. [15]. The influence of pressure on the reactivity of 2%Rh–2.5%Fe/TiO2 catalyst is summarized in Table 4 for a reaction performed at 543 K, H2/CO2 of 1:1, and a gas flow of 20 cm3 min− 1. The results show a decrease in CO2 conversion from 9.16% to 6.48% when the pressure was reduced from 20 atm to 10 atm, without any appreciable change in product selectivities. These results also appear to be consistent with the previous findings of Kusama et al. [9] for the effect of pressure on CO2 hydrogenation over 5 wt.% Rh–Li/SiO2 catalyst (with Rh/Li = 1) albeit at different reaction conditions of 50 atm, H2/CO2 = 3, and a 1 −1 WHSV of 6000 cm3 g− . cat h The results of the elemental analysis, H2 chemisorption, and the computed turnover frequencies are summarized in Table 5 for CO and CO2 hydrogenation. The results from the elemental analysis indicate a very good agreement between the theoretical and experimental metal loadings, for both Rh and Fe. The results from H2 chemisorption on the Rh-containing samples confirmed that the Rh was highly dispersed. Indeed, the H/Rh ratio for the 2% Rh/TiO2 and the 2% Rh–2.5%Fe/TiO2 were 54% and 66%, respectively, which corresponded to a metal particle size of about 2 nm. The turnover frequencies reported in
Fig. 1. Influence of temperature on CO2 conversion and product selectivities for CO2 1 −1 hydrogenation over 2% Rh/TiO2 (20 atm, 8000 cm3 g− , H2/CO2 = 1). It should be cat h noted that unpurified CO was hydrogenated prior to this test.
Fig. 2. Influence of WHSV on CO2 conversion and product selectivities for CO2 hydrogenation over 2% Rh/TiO2 (543 K, 20 atm, H2/CO2 = 1). It should be noted that unpurified CO was hydrogenated prior to this test.
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Table 4 Influence of pressure on CO2 hydrogenation over 2%Rh–2.5%Fe/TiO2a. Conditions
1
2
T (K) P (atm) H2 + CO2 (cm3 min–1) H2/CO2 Conversion (%)
543 20 20 1 9.16
543 10 20 1 6.48
57.2 2.61 2.38 1.18 0.55 1.26 6.41 0.00 28.4
58.7 2.57 2.26 1.05 0.44 1.28 4.37 0.00 29.3
Selectivities (%) CH4 C2H6 C3H8 C4H10 CH3CHO CH3OH C2H5OH Ethyl acetate CO
CO2 Conversion (%) = ΣniMi × 100 / MCO2 and Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO2 is the mole percent of carbon dioxide in the feed. a Nominal conditions are T = 543 K, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute −1 −1 the bed, H2:CO2 1:1, syngas flow = 20 cm3 min− 1 or a WHSV of 8000 cm3 gcat h .
Table 5 were calculated in two different ways, the first based on the total metal content and the second based on the number of metal active sites counted by H2 chemisorption. Adsorption of H2 could not be properly used to count the surface metal sites of the Fe catalyst and perhaps the Fe component of the bimetallic catalyst. The turnover frequencies for CO hydrogenation (based on H2 adsorption) are consistent with our previous study [1], demonstrating a good reproducibility of the reaction performance as well as the catalyst physicochemical properties. The turnover frequencies for CO2 hydrogenation also appear to be consistent with those reported for Rh–Co/SiO2 [12] and Rh/Y [22]. As an example, Kusama et al. [12] evaluated the CO2 hydrogenation in a fixed-bed flow reactor, at nominal conditions of 533 K, 50 atm, H2/CO2 ratio of 3, and a total gas flow rate of 100 cm3 min− 1 (corresponding to a WHSV of 1 −1 6000 cm3 g− based on a catalyst charge of 1 g). The CO2 cat h conversion and the turnover frequency (of total CO2 transformation per unit active site based on H2 chemisorption per second) both increased with Co/Rh atomic ratio in the range of 0–2. The TOF values for this range were 12–218 h− 1, or 0.0033–0.0606 s− 1. The values in Table 5 for CO2 hydrogenation over the 2%Rh/TiO2 and 2%Rh–2.5%Fe/ TiO2 are 0.038 and 0.041 s− 1, and are thus consistent with those of Kusama et al. [12]. As an additional example, for a 5 wt.% Rh ionexchanged Y zeolite (Rh/Y), Bando et al. [22] reported a TOF value of close to 60 h− 1 (0.0167 s− 1) for the total transformation of CO2 under the reaction conditions of 523 K, 30 atm, H2/CO2 of 3, and a WHSV of 1 −1 6000 cm3 g− , which is also similar to the values reported in cat h Table 5 of this work. A schematic representation of the reaction network for the formation of CH4, CO, and ethanol during CO2 hydrogenation is presented in Fig. 3 [8]. In the first step, CO2 is adsorbed on the catalyst surface
Fig. 3. A possible reaction network for CO2 hydrogenation. Adapted from [8].
as a CO* via the reverse water gas shift reaction. The adsorbed CO intermediates undergoes successive insertions of H to form an adsorbed CH*3 (methyl) and an adsorbed CH3CO* (acyl) groups. While the hydrogenation of CH*3 leads to the formation of CH4, the acyl group hydrogenation leads to the formation of ethanol. Based on the reactivity results presented here, it thus appears that CO is a suggested intermediate in the reaction network.
4. Conclusions The titania-supported Rh, Rh–Fe, and Fe catalysts studied here were active for the formation of CH4 and CO via CO2 hydrogenation at the 1 −1 nominal conditions of 20 atm, 543 K, and a WHSV of 8000 cm3 g− . cat h A direct comparison of turnover frequency based on H2 chemisorption on a 2% Rh–2.5% Fe/TiO2 revealed that the two reactions were catalyzed at about the same rate at 543 K. The promotional role of Fe on CO2 hydrogenation in terms of activity and selectivity to alcohol formation was not as straightforward as it was in the case of CO hydrogenation. The addition of Fe increased the extent of reverse water gas shift to produce CO from CO2, which also correlated to greater selectivity to ethanol. Higher temperature and a higher WHSV during CO2 hydrogenation favored CH4 formation at the expense of primarily CO. The blending of CO2 into a syngas stream did not significantly impact the rate or selectivity of ethanol formation. Removing iron carbonyl impurities from the CO feedstream was found to be important for catalysts that do not have reverse water gas shift activity. Otherwise, RWGS activity could be catalyzed by impurity Fe deposits on the catalyst surface.
Acknowledgments The authors acknowledge the seed funding from the University of Virginia and partial support from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02-95ER14549. The authors also acknowledge the help of Mr. Mohammad A. Haider for the H2 chemisorption analysis.
Table 5 Results from elemental analysis and H2 chemisorption used to determine turnover frequency for CO and CO2 hydrogenation. Catalyst composition (nominal)
Rh and/or Fe loadings (wt.%)
H/Rh (%)
CO TOF (total metal)a (s− 1)
CO TOF (H2 ads.)b (s− 1)
CO2 TOF (total metal)a (s− 1)
CO2 TOF (H2 ads.)b (s− 1)
2%Rh/TiO2 2%Rh/TiO2c 2%Rh–2.5%Fe/TiO2 2.5%Fe/TiO2
1.94% 1.94% 1.73% Rh, 1.79% Fe 2.11%
54 54 66 –
0.012 0.012 0.036 0.0022
0.023 0.023 0.055 –
0.021 0.051 0.027 0.0035
0.038 0.093 0.041 –
a b c
Molecules of CO or CO2 converted per total metal atom per second. Molecules of CO or CO2 converted per active site counted by H2 chemisorption per second. CO was purified by passing it through a silica trap immersed in a dewar containing a dry ice–acetone mixture.
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References [1] M.A. Haider, M.R. Gogate, R.J. Davis, J. Catal. 261 (2009) 9–16. [2] T. Inoue, T. Iizuka, K. Tanabe, Appl. Catal. 46 (1989) 1–9. [3] A. Trovarelli, C. Mustazza, G. Dolcetti, J. Kaspar, M. Graziani, Appl. Catal. 65 (1990) 129–142. [4] T. Szailer, É. Novák, A. Oszkó, A. Erdöhelyi, Top. Catal. 46 (2007) 79–86. [5] É. Novák, K. Fodor, T. Szailer, A. Oszkó, A. Erdöhelyi, Top. Catal. 20 (2002) 107–117. [6] F. Solymosi, A. Erdöhelyi, T. Bánsági, J. Catal. 68 (1981) 371–382. [7] F. Solymosi, A. Erdöhelyi, J. Mol. Catal. 8 (1980) 471–474. [8] H. Kusama, K. Okabe, K. Sayama, H. Arakawa, Energy 22 (1997) 343–348. [9] H. Kusama, K. Okabe, K. Sayama, H. Arakawa, Catal. Today 28 (1996) 261–266. [10] H. Kusama, K.K. Bando, K. Okabe, H. Arakawa, Appl. Catal. A 197 (2000) 255–268. [11] H. Kusama, K.K. Bando, K. Okabe, H. Arakawa, Appl. Catal. A 205 (2001) 285–294. [12] H. Kusama, K. Okabe, K. Sayama, H. Arakawa, Appl. Organometal. Chem. 14 (2000) 836–840.
[13] T. Inui, T. Yamamoto, M. Inoue, H. Hara, T. Takeguchi, J.B. Kim, Appl. Catal. A 186 (1999) 395–406. [14] J.L. Dubois, K. Sayama, H. Arakawa, Chem. Lett. (1992) 5–8. [15] M. Takagawa, A. Okamoto, H. Fujimura, Y. Izawa, H. Arakawa, Stud. Surf. Sci. Catal. 114 (1998) 525–528. [16] K.G. Chanchlani, R.R. Hudgins, P.L. Silveston, J. Catal. 136 (1992) 59–75. [17] K.J. Smith, R.G. Herman, K. Klier, Chem. Eng. Sci. 45 (1990) 2639–2646. [18] A.-M. Hilmen, M. Xu, M.J.L. Gines, E. Igelsia, Appl. Catal. A 169 (1998) 355–372. [19] K. Klier, R.G. Herman, J.G. Nunan, K.J. Smith, C.E. Bogdan, C.-W. Young, J.G. Santiesteban, Stud. Surf. Sci. Catal. 36 (1988) 109–125. [20] M.R. Gogate, R.J. Davis, Chem. Cat. Chem. 1 (2009) 295–303. [21] J. Gao, X. Mo, A.C.Y. Chien, W. Torres, J.G. Goodwin, J. Catal. 262 (2009) 119–126. [22] K.K. Bando, K. Soga, K. Kunimori, H. Arakawa, Appl. Catal. A: General 175 (1998) 67–81.
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Comparative study of CO and CO2 hydrogenation over supported Rh–Fe catalysts Makarand R. Gogate, Robert J. Davis ⁎ University of Virginia, Department of Chemical Engineering, 102 Engineers' Way, PO Box 400741, Charlottesville, VA 22904-4741, United States
a r t i c l e
i n f o
Article history: Received 20 January 2010 Received in revised form 18 March 2010 Accepted 26 March 2010 Available online 8 April 2010 Keywords: CO hydrogenation CO2 hydrogenation Rh/TiO2 Rh–Fe/TiO2 Methane Ethanol
a b s t r a c t The hydrogenation of CO, CO + CO2, and CO2 over titania-supported Rh, Rh–Fe, and Fe catalysts was carried out in a fixed-bed micro-reactor system nominally operating at 543 K, 20 atm, 20 cm3 min− 1 gas flow 1 −1 ), with a H2:(CO + CO2) ratio (corresponding to a weight hourly space velocity (WHSV) of 8000 cm3 g− cat h of 1:1. A comparative study of CO and CO2 hydrogenation shows that while Rh and Rh–Fe/TiO2 catalysts exhibited appreciable selectivity to ethanol during CO hydrogenation, they functioned primarily as methanation catalysts during CO2 hydrogenation. The Fe/TiO2 sample was primarily a reverse water gas shift catalyst. Higher reaction temperatures favored methane formation over alcohol synthesis and reverse water gas shift. The effect of pressure was not significant over the range of 10 to 20 atm. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The fixation of CO2 into valuable chemicals is potentially an important strategy for CO2 utilization. While CO hydrogenation over transition metal catalysts in general, and Rh in particular, has been studied quite extensively, there have been fewer studies of CO2 hydrogenation. Our group has recently studied CO hydrogenation over supported and promoted Rh catalysts with a goal to produce ethanol and higher alcohols, forming methane and other light paraffins as less desirable co-products [1]. One possible reaction network for CO hydrogenation includes the elementary steps of (a) dissociation of the adsorbed CO to form adsorbed carbon and oxygen, (b) hydrogenation of the adsorbed carbon to form an adsorbed methyl species, (c) insertion of non-dissociated CO into the adsorbed methyl species to form an adsorbed acyl species, and (d) hydrogenation of the adsorbed acyl species to form the ethanol product [1]. Thus, a properly promoted metal catalyst performs many different elementary reactions. One goal of the current work was to investigate how CO2 interacts with the reaction network for CO hydrogenation, so we have studied the hydrogenation of CO2 in the presence and absence of CO. Prior work on CO2 hydrogenation over Rh-based catalysts includes two relevant reports by Inoue et al. [2] and Trovarelli et al. [3]. Inoue et al. [2] examined the activity of supported Rh catalysts on different supports such as MgO, Nb2O5, ZrO2, and TiO2 at 473–573 K, 10 atm, 1 −1 H2:CO2 ratio of 3:1, and a WHSV of 2400 cm3 g− . The ZrO2- and cat h
⁎ Corresponding author. Tel.: +1 434 924 6284; fax: +1 434 982 2658. E-mail address: [email protected] (R.J. Davis). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.020
Nb2O5-supported catalysts were the most active as measured by the rate of formation of product hydrocarbons and alcohols, however, the selectivity to CH4 was nearly 100%. The Rh/TiO2 catalyst was not as active in terms of the rate of hydrocarbon and alcohol formation, but it was a good reverse water gas shift catalyst. The rate of CO formation on Rh/TiO2 was nearly an order of magnitude greater than that of the hydrocarbon and alcohol formation [2]. Trovarelli et al. [3] studied the effect of reduction temperature of Rh/TiO2 on the rates of CO and CH4 formation during CO2 hydrogenation reactions at 523 K with a CO2:H2 ratio of 1:1. For a catalyst prepared with RhCl3 as a precursor and reduced at a low temperature (473 K), methane was the primary product, however, for samples reduced at 623 and 723 K, the rate of CO formation increased significantly. For the catalyst reduced at 723 K, the rate of CO formation was about 2 orders of magnitude higher than that of CH4 formation. At least on Rh/TiO2, the extent of CO formation during CO2 hydrogenation reactions appears to be a function of many different factors. Some reports suggest however, that CO formation during CO2 hydrogenation is not significant on Rh/TiO2. For example, Szailer et al. [4] report that CH4 is the main product (99%+) at 548 K with a H2:CO2 ratio of 4:1 on Rh/TiO2 prepared from RhCl3 precursor (reduced at 673 K for 1 h). In another contribution from the same group [5], the rate of CH4 formation alone is reported for CO2 hydrogenation on a 1% Rh/TiO2 at 473 K, 1 atm, H2:CO2 ratio of 4:1, and with a SV of 6000– 9000 h− 1, so it was not clear if CO was detected. Solymosi et al. [6] suggest that on Rh/Al2O3 and Rh/TiO2, the rate of CO formation is less than 1% of the rate of CH4 formation for catalysts prepared using RhCl3 as the precursor and at the reaction conditions of 473 and 548 K, 1 atm, space velocities of 3000–6000 h− 1, with a H2:CO2 ratio of 4:1. Solymosi and Erdöhelyi [7] also report that the hydrogenation of CO2
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on alumina-supported metal catalysts (including Ru, Rh, Pt, Ir, and Pd, nominal 5 wt.% metal, 548 K, 1 atm, GHSV of 3000–6000 h− 1, and H2: CO2 ratio of 4:1) produces CH4 with a much higher selectivity than that of CO. Another objective of the current work is thus to ascertain the extent of CO formation during CO2 hydrogenation reactions over titania-supported Rh catalysts that have demonstrated activity for higher alcohol synthesis. Several reports by Kusama et al. [8–12] cover in detail the CO2 hydrogenation reaction over Rh/SiO2. In particular, the effect of Fe as a promoter for the reaction has been reported [8]. At 533 K, 50 atm, and a H2:CO2 ratio of 3:1, the results show that increasing the Fe/Rh atomic ratio from 0 to 3 on a 5 wt.% Rh/SiO2 catalyst increases the CO2 conversion and increases the selectivity to methanol and ethanol, at the expense of CO and CH4. On an Fe/SiO2 catalyst, the conversion of CO2 was appreciable, however the CO selectivity was high and very little CH4 was formed. More than 30 additives to Rh were also examined in another report [9], and under the conditions of 513 K, 50 atm, and H2:CO2 of 3, a Rh–Li/SiO2 catalyst (5 wt.% Rh, atomic ratio of 1:1 Rh:Li) exhibited a moderate CO2 conversion of 7% and a selectivity to ethanol of 15.5%. Other non-Rh catalysts for CO2 hydrogenation have also been studied. These include novel Fischer–Tropsch type composite catalysts [13], different metal carbides such as Mo2C, Fe3C, and WC [14], and other composite catalysts such as K/Fe, K/Cu–Fe, and K/Cu–Zn–Fe [15]. The reader is referred to Refs. [2] and [3], and the references therein, for additional background on catalysts for CO2 hydrogenation. Moreover, the mechanistic details of the synthesis of methanol and higher alcohols from CO/CO2/H2 on Cu catalysts have also been covered earlier in some representative references [16–19]. The specific objectives of the current work were to clarify the extent of CO formation in CO2 hydrogenation reactions over titaniasupported Rh, Rh–Fe, and Fe, to compare the reactivity results of CO hydrogenation to CO2 hydrogenation at identical conditions, and to examine the influences of process variables including temperature, pressure, and WHSV on the CO2 conversion and product selectivities during CO2 hydrogenation. 2. Experimental methods 2.1. Catalyst synthesis The supported Rh and Fe catalysts were synthesized by incipient wetness impregnation. Granular rhodium nitrate (Pfaltz and Bauer (Waterbury, CT)) was used as the rhodium precursor for all catalysts. As an example, a 2 wt.% Rh–2.5 wt.% Fe/TiO2 catalyst was prepared by dissolving 0.315 g Rh(NO3)3·2H2O and 0.90 g Fe(NO3)3·9H2O in 7.5 mL of distilled deionized water and adding this solution dropwise with proper kneading to 5 g of TiO2 (Degussa P-25) to the point of incipient wetness. The resulting paste was dried overnight in air at 413 K and subsequently calcined in air at 723 K for 4 h. 2.2. Catalyst characterization The metal loadings of the catalysts were determined by Galbraith Laboratories (Knoxville, TN) using ICP-OES analysis. Additionally, the Rh dispersion was evaluated by H2 chemisorption at 308 K performed using a Micromeritics ASAP 2020 automated adsorption system. Calcined catalysts were reduced in flowing H2 at 473 K for 2.5 h and evacuated at that temperature for 2 h prior to cooling in vacuo to 308 K. An H/Rhsurf stoichiometry equal to unity was assumed in the analysis. 2.3. Reactions of CO and H2 The catalysts were evaluated in a fixed-bed micro-reactor system (Autoclave Engineers' BTRS Jr., Erie, PA). The nominal dimensions of
the reactor tube were an ID of 8 mm, OD of 14.3 mm, and a total length of 275 mm. Calcined catalysts were first pressed into pellets, crushed, and size-separated (+40/−80 mesh). Approximately 0.150 g catalyst was intimately mixed with 2.5 g of silicon carbide (Universal Photonics, Inc., Hicksville, NY) and loaded into the reactor. The catalyst was then reduced in flowing dihydrogen (20 cm3 min− 1) at 573 K for 2.5 h under atmospheric pressure. Following reduction, the catalysts were tested at nominally identical conditions of 543 K, 20 atm total pressure, syngas (H2 + CO) flow of 20 cm3 min− 1, H2:CO ratio of 1:1, and a WHSV of 8000 cm3 g cat− 1 h− 1. After CO hydrogenation, the CO flow to the reactor was cut off and CO2 was introduced at a H2:CO2 ratio of 1:1, with a total flow of 20 cm3 min− 1. The catalyst bed temperature was monitored by a thermocouple inserted within the catalyst bed. All gases (CO (GT&S), CO2 (GT&S), and H2 (GT&S)) were UHP grade (99.999%). The results reported here are generally obtained after 1–1.5 h on-stream. Two HP 5890 Series II gas chromatographs were integrated downstream of the reactor for the analysis of reactants and products. The first one was equipped with a flame ionization detector and a 50 m-long HP-1 cross-linked methyl silicone gum capillary column to monitor hydrocarbons, alcohols, acetaldehyde, methyl acetate (not observed) and ethyl acetate. The second one was equipped with a thermal conductivity detector and a 6-ft long Alltech CTR packed column and was used to monitor the possible formation of CO via the reverse water gas shift reaction during CO2 hydrogenation. The conversion of CO was based on the fraction of CO that formed carbon-containing products according to: % Conversion = ðΣni ·Mi = MCO Þ·100% where ni is the number of carbon atoms in product i, Mi is the percentage of product i detected, and MCO is the percentage of carbon monoxide in the syngas feed. This equation was valid since differential conversion was maintained throughout this study. An analogous equation was used for CO2 conversion for the CO2 hydrogenation runs, with MCO replaced by MCO2 in the denominator. The extent of reverse water gas shift reaction was accounted for by CO formation in the products for CO2 hydrogenation. The selectivity to product i is based on the total number of carbon atoms in the product and is therefore defined as: Si = ðni ·Mi Þ = ð∑ni ·Mi Þ: 3. Results and discussion The reactivity results from CO hydrogenation and CO2 hydrogenation over 2% Rh/TiO2, 2%Rh–2.5%Fe/TiO2, and 2.5%Fe/TiO2 at nominally identical conditions of 543 K, 20 atm, a WHSV of 8000 cm3 gcat − 1 h− 1, with a H2:CO2 ratio of 1, are summarized in Tables 1 and 2. The promotional role of Fe for CO hydrogenation to ethanol (and light alcohols) as reported in our earlier work on CO hydrogenation is reproduced here [1]. For example, the CO conversion over 2% Rh/TiO2 was 4.74% and the ethanol selectivity was 11.3%. Addition of 2.5 wt.% Fe caused the CO conversion and ethanol selectivity to increase to 12.2% and 27.9%, respectively (Table 1). The 2.5% Fe/TiO2 catalyst was not very active for CO conversion since there was no Rh present on the surface. Based on our earlier analytical TEM studies and the results from H2 chemisorption [1], we concluded that for the Rh–Fe/TiO2 catalyst system, the Fe atoms were in close proximity to the Rh metal cluster of 1–2 nm in diameter. Moreover, a subsequent study of the Fe-promoted Rh/TiO2 system by EXAFS and XANES showed that most of the Fe promoter was partially reduced from Fe(III) to Fe(II) under reaction conditions, and thus was highly oxophilic in nature [20]. The results of CO hydrogenation for two distinct cases are reported in Table 1. Entry 1 in Table 1 presents the reactivity results for CO
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Table 1 Hydrogenation of CO over titania-supported Rh, Rh–Fe, and Fe catalystsa. Catalyst
Conv. (%)
% Sel. (CH4)
% Sel. (C2H6)
% Sel. (C3H8)
% Sel. (C4H10)
% Sel. (CH3CHO)
% Sel. (CH3OH)
% Sel. (EtOH)
% Sel. (EthAc)
2%Rh/TiO2 2% Rh/TiO2b 2%Rh–2.5%Fe/TiO2 2.5%Fe/TiO2
4.74 4.77 12.2 1.85
34.6 48.1 35.1 25.7
11.4 5.21 4.75 19.0
21.7 20.3 7.35 23.0
9.30 8.08 1.91 11.3
3.80 4.59 13.3 0.0
2.40 3.41 0.00 10.5
11.3 10.4 27.9 10.6
5.50 0.00 9.80 0.00
CO Conversion (%) = ΣniMi × 100 / MCO and Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO is the mole percent of carbon monoxide in the feed. a 1 −1 Nominal conditions are T = 543 K, P = 20 atm, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute the bed, H2:CO 1:1, syngas flow = 20 cm3 min− 1 or a WHSV of 8000 cm3 g− . cat h b CO for this run was purified by passing it through a silica trap immersed in a dewar containing a dry ice–acetone mixture.
hydrogenation where CO was not pre-purified before introduction into the reactor (it was used directly from the CO gas cylinder). Entry 2 in Table 1 presents the reactivity results for CO hydrogenation at nominally identical conditions, except that CO was pre-purified by passing it through a silica trap immersed in a dewar containing dry ice–acetone mixture. The reactivity results show a good experimental reproducibility, as the CO conversions are 4.74% and 4.77% for the two cases, and the product selectivities are also similar. The product selectivities during CO hydrogenation in Table 1 are similar to those in our earlier work [1] and are also comparable to those in a recent report involving CO hydrogenation over silicasupported Rh–La–V catalysts [21] under somewhat different condi1 −1 tions of 503 K, 1.8 atm, and a WHSV of 9000 cm3 g− . cat h In the present study, CO was observed as a reaction product during CO2 hydrogenation, and the product selectivity was a strong function of catalyst composition (Table 2). The first two entries in Table 2 are results of CO2 hydrogenation studies carried out to test the effect of removing Fe carbonyls from the CO feedstream. Although the CO hydrogenation results did not depend on the presence of the Fe carbonyl trap (see Entries 1 and 2, Table 1), the reactivity results for CO2 hydrogenation showed significant differences in conversion and selectivity. For example, the CO2 conversion for the second entry in Table 2 (after purification of CO) was 19.2%, compared to 7.89% for the first entry. Moreover, the selectivity for CO production decreased when an Fe carbonyl trap was used. These results suggest that there is a role of the Fe impurity in the CO feedstream on the CO2 hydrogenation reactions. Although the difference in the CO2 conversion was minor for 2% Rh/TiO2 (without a CO purifier) and 2%Rh–2.5%Fe/TiO2, there were significant differences in the product selectivities. Whereas the 2%Rh/ TiO2 catalyst had a poor ethanol selectivity of 1.93%, the selectivity to ethanol improved over the 2%Rh–2.5%Fe/TiO2 sample to 6.41%. The 2.5%Fe/TiO2 catalyst functioned primarily as a reverse water gas shift catalyst since there was a high selectivity of 73.0% to CO at a low conversion of CO2 (2.65%). For all three catalysts tested here under the reported reaction conditions, CO was formed in appreciable quantities. These results indicate that all catalysts have a measurable reverse water gas shift activity, and these results, while in partial agreement with the results reported in prior art [2,3], are not in
accordance with the works of Szailer et al. [4], Novák et al. [5], Solymosi et al. [6], and Solymosi and Erdöhelyi [7], which report that CO is a trace product over titania-supported Rh catalysts. It should be noted that the differences in metal precursors (used to prepare the titania-supported Rh catalyst) and/or the reaction conditions (the H2/CO2 ratio and temperature, in particular) may affect the relative rates of CO and CH4 formation. Kusama et al. reported [11] on the reactivity of 1% Rh/SiO2 for CO2 hydrogenation at 473 K, 50 atm, H2: CO2 ratio of 3:1, for three different catalysts where the precursor of Rh was acetate, chloride, and nitrate, respectively. The results showed that CO is the principal product for the catalysts prepared using acetate and nitrate precursors, while CH4 is the principal product for the 1% Rh/SiO2 prepared using chloride as the precursor. The reactivity results in Table 2 for CO2 hydrogenation are always reported for runs that were preceded by CO hydrogenation. To test the possible role of trace impurities or catalyst modifications accomplished by CO hydrogenation, a separate run of CO2 hydrogenation was performed without pretreating the catalyst by CO hydrogenation. In this case, the CO formed by reverse water gas shift was found to be only a trace product by chromatography (the peak for CO, although clearly present on the chromatogram, was not integrable). The CO2 conversion at 543 K, 20 atm, and 20 cm3 min−1 gas flow (corresponding to a weight hourly space velocity (WHSV) of 1 −1 8000 cm3 g− ) was 12.99%. Methane was the principal product, cat h with a selectivity of 91.6%, and the only other products detected were ethane and propane, with a selectivity of 4.8% and 3.6%, respectively. These results suggest that preceding CO2 hydrogenation with CO hydrogenation altered the catalyst in some fashion. Since purifying the CO with an Fe carbonyl trap (silica at dry ice/acetone temperature) prior to the reactor lowered the extent of reverse water gas shift (entries 1 and 2 in Table 2), we conclude that most of the CO produced during CO2 hydrogenation on Rh/TiO2 originated from the RWGS over the deposited Fe impurities from the CO cylinder. The 4.16% selectivity to CO (see entry 2, Table 2) for CO2 hydrogenation after using purified CO confirmed the minor role of Rh/TiO2 as a RWGS catalyst under our conditions. The results of reactivity studies with titania- and silica-supported Rh–Fe catalysts for CO hydrogenation and (CO + CO2) hydrogenation are given in Table 3. When compared to CO hydrogenation without
Table 2 Hydrogenation of CO2 over titania-supported Rh, Rh–Fe, and Fe catalystsa. Catalyst
Conv. (%)
% Sel. (CH4)
% Sel. (C2H6)
% Sel. (C3H8)
% Sel. (C4H10)
% Sel. (CH3CHO)
% Sel. (CH3OH)
% Sel. (EtOH)
% Sel. (CO)
2%Rh/TiO2 2%Rh/TiO2b 2%Rh–2.5%Fe/TiO2 2.5%Fe/TiO2
7.89 19.2 9.16 2.65
72.7 93.3 57.2 11.6
4.30 1.75 2.60 5.10
4.10 0.84 2.40 4.90
0.98 0.00 1.18 0.79
0.67 0.00 0.55 1.73
0.80 0.00 1.26 0.00
1.93 0.00 6.41 2.83
14.5 4.16 28.4 73.0
CO2 Conversion (%) = ΣniMi × 100 / MCO2 and Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO2 is the mole percent of carbon dioxide in the feed. 1 −1 a Nominal conditions are T = 543 K, P = 20 atm, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute the bed, H2:CO2 1:1, syngas flow= 20 cm3 min− 1 or a WHSV of 8000 cm3 g− . cat h b Prior to CO2 hydrogenation, a CO hydrogenation run was carried out (see entry 2, Table 1 for reactivity data) where CO was purified by passing it though a silica trap immersed in a dewar containing a dry ice–acetone mixture.
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Table 3 Hydrogenation of CO and (CO + CO2) mixture over supported Rh–Fe catalystsa. Catalyst
Synthesis gas composition
Conv. (%)
% Sel. (CH4)
% Sel. (C2H6)
% Sel. (C3H8)
% Sel. (C4H10)
% Sel. (CH3CHO)
% Sel. (CH3OH)
% Sel. (EtOH)
% Sel. (n-PrOH)
% Sel. (EthAc)
2%Rh–2.5%Fe/TiO2
H2:CO 10:10 H2:CO:CO2 10:5:5 H2:CO 10:10 H2:CO:CO210:5:5 H2:CO 10:10 H2:CO:CO2 10:5:5
13.8 10.5 6.23 5.00 5.62 4.42
38.5 45.2 33.9 38.3 32.2 38.3
3.50 3.83 7.04 8.16 6.88 9.45
6.03 5.34 8.75 9.51 10.2 13.9
1.50 0.93 2.85 2.35 2.58 2.87
10.7 6.21 3.73 2.37 3.13 2.20
2.05 2.71 9.68 9.11 9.70 8.29
27.2 29.0 30.3 26.4 28.5 20.2
0.00 0.00 0.00 0.00 6.78 4.80
10.6 6.78 3.78 3.77 0.00 0.00
2%Rh–10%Fe/TiO2 2%Rh–1%Fe/SiO2
Conversion (%) = ΣniMi × 100 / MCO for (H2 + CO) syngas; MCO is the mole percent of CO in the feed. Conversion (%) = ΣniMi × 100 / MCO + CO2 for (H2 + CO + CO2) syngas; MCO + CO2 is the mole percent of CO + CO2 in the feed. Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i and Mi is the mole percent of product i. a 1 −1 Nominal conditions are T = 543 K, P = 20 atm, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute the bed, syngas flow = 20 cm3 min− 1 or a WHSV of 8000 cm3 g− . cat h
CO2, the overall conversion decreased by about 15–20% for the mixed feed (CO + CO2), under otherwise identical conditions. For all three catalysts, there was an increase in methane selectivity for the mixed feed (CO + CO2) when compared to CO hydrogenation without CO2. This increase in methane selectivity, although modest, appeared to be above the experimental error, and is likely attributed to the increase in H2/CO ratio when CO2 is present in the syngas feed and the hydrogenation of CO2 to methane. A similar increase in methane (and other light paraffin) selectivity of the CO hydrogenation reactions with increasing H2/CO ratios was observed in our prior work [1]. The selectivity to ethanol appears to be relatively invariant with cofed CO2. The influence of temperature on the CO2 conversion and the product selectivities for CH4, CO, and ethanol over a 2% Rh/TiO2, at the 1 −1 nominal experimental conditions of 20 atm, 8000 cm3 g− , and a cat h H2/CO2 of 1, is summarized in Fig. 1. The CO2 conversion increased with temperature, as expected, but the CH4 selectivity increased at the expense of CO selectivity. The rates of CO and CH4 formation were a strong function of temperature with CO formation being favored at lower temperatures. The selectivity to methane approached nearly 85% at the highest temperature of 573 K. A similar finding has also been reported earlier by Kusama et al. [9] for the effect of reaction temperature on CO2 hydrogenation over 5 wt.% Rh–Li/SiO2 catalyst (with Rh/Li = 1) albeit at different reaction conditions of 50 atm, 1 −1 H2/CO2 = 3, and a WHSV of 6000 cm3 g− . In their study, the CH4 cat h selectivity increased from 32.2% to 81.3% as the temperature increased from 473 to 533 K, while the CO selectivity decreased from 55.2% to 6.2% over the same temperature range. The selectivity to ethanol also
appeared to go through a maximum in the previous study by Kusama et al. [9], a feature that is also evident from Fig. 1. The influence of WHSV on the CO2 conversion and selectivities to CH4, CO, and ethanol is shown in Fig. 2, as a plot of CO2 conversion and product selectivities vs. 1/WHSV at the nominal conditions of 20 atm, 543 K, and a H2/CO2 ratio of 1. The CO2 conversion increases with the inverse of WHSV at low conversion values, and that the product selectivities were fairly constant over the range of WHSV studied. These results are in general accordance with those of Takagawa et al. [15]. The influence of pressure on the reactivity of 2%Rh–2.5%Fe/TiO2 catalyst is summarized in Table 4 for a reaction performed at 543 K, H2/CO2 of 1:1, and a gas flow of 20 cm3 min− 1. The results show a decrease in CO2 conversion from 9.16% to 6.48% when the pressure was reduced from 20 atm to 10 atm, without any appreciable change in product selectivities. These results also appear to be consistent with the previous findings of Kusama et al. [9] for the effect of pressure on CO2 hydrogenation over 5 wt.% Rh–Li/SiO2 catalyst (with Rh/Li = 1) albeit at different reaction conditions of 50 atm, H2/CO2 = 3, and a 1 −1 WHSV of 6000 cm3 g− . cat h The results of the elemental analysis, H2 chemisorption, and the computed turnover frequencies are summarized in Table 5 for CO and CO2 hydrogenation. The results from the elemental analysis indicate a very good agreement between the theoretical and experimental metal loadings, for both Rh and Fe. The results from H2 chemisorption on the Rh-containing samples confirmed that the Rh was highly dispersed. Indeed, the H/Rh ratio for the 2% Rh/TiO2 and the 2% Rh–2.5%Fe/TiO2 were 54% and 66%, respectively, which corresponded to a metal particle size of about 2 nm. The turnover frequencies reported in
Fig. 1. Influence of temperature on CO2 conversion and product selectivities for CO2 1 −1 hydrogenation over 2% Rh/TiO2 (20 atm, 8000 cm3 g− , H2/CO2 = 1). It should be cat h noted that unpurified CO was hydrogenated prior to this test.
Fig. 2. Influence of WHSV on CO2 conversion and product selectivities for CO2 hydrogenation over 2% Rh/TiO2 (543 K, 20 atm, H2/CO2 = 1). It should be noted that unpurified CO was hydrogenated prior to this test.
M.R. Gogate, R.J. Davis / Catalysis Communications 11 (2010) 901–906
905
Table 4 Influence of pressure on CO2 hydrogenation over 2%Rh–2.5%Fe/TiO2a. Conditions
1
2
T (K) P (atm) H2 + CO2 (cm3 min–1) H2/CO2 Conversion (%)
543 20 20 1 9.16
543 10 20 1 6.48
57.2 2.61 2.38 1.18 0.55 1.26 6.41 0.00 28.4
58.7 2.57 2.26 1.05 0.44 1.28 4.37 0.00 29.3
Selectivities (%) CH4 C2H6 C3H8 C4H10 CH3CHO CH3OH C2H5OH Ethyl acetate CO
CO2 Conversion (%) = ΣniMi × 100 / MCO2 and Selectivity = niMi / ΣniMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO2 is the mole percent of carbon dioxide in the feed. a Nominal conditions are T = 543 K, 0.150 g catalyst (40–80 mesh), 2.5 g SiC to dilute −1 −1 the bed, H2:CO2 1:1, syngas flow = 20 cm3 min− 1 or a WHSV of 8000 cm3 gcat h .
Table 5 were calculated in two different ways, the first based on the total metal content and the second based on the number of metal active sites counted by H2 chemisorption. Adsorption of H2 could not be properly used to count the surface metal sites of the Fe catalyst and perhaps the Fe component of the bimetallic catalyst. The turnover frequencies for CO hydrogenation (based on H2 adsorption) are consistent with our previous study [1], demonstrating a good reproducibility of the reaction performance as well as the catalyst physicochemical properties. The turnover frequencies for CO2 hydrogenation also appear to be consistent with those reported for Rh–Co/SiO2 [12] and Rh/Y [22]. As an example, Kusama et al. [12] evaluated the CO2 hydrogenation in a fixed-bed flow reactor, at nominal conditions of 533 K, 50 atm, H2/CO2 ratio of 3, and a total gas flow rate of 100 cm3 min− 1 (corresponding to a WHSV of 1 −1 6000 cm3 g− based on a catalyst charge of 1 g). The CO2 cat h conversion and the turnover frequency (of total CO2 transformation per unit active site based on H2 chemisorption per second) both increased with Co/Rh atomic ratio in the range of 0–2. The TOF values for this range were 12–218 h− 1, or 0.0033–0.0606 s− 1. The values in Table 5 for CO2 hydrogenation over the 2%Rh/TiO2 and 2%Rh–2.5%Fe/ TiO2 are 0.038 and 0.041 s− 1, and are thus consistent with those of Kusama et al. [12]. As an additional example, for a 5 wt.% Rh ionexchanged Y zeolite (Rh/Y), Bando et al. [22] reported a TOF value of close to 60 h− 1 (0.0167 s− 1) for the total transformation of CO2 under the reaction conditions of 523 K, 30 atm, H2/CO2 of 3, and a WHSV of 1 −1 6000 cm3 g− , which is also similar to the values reported in cat h Table 5 of this work. A schematic representation of the reaction network for the formation of CH4, CO, and ethanol during CO2 hydrogenation is presented in Fig. 3 [8]. In the first step, CO2 is adsorbed on the catalyst surface
Fig. 3. A possible reaction network for CO2 hydrogenation. Adapted from [8].
as a CO* via the reverse water gas shift reaction. The adsorbed CO intermediates undergoes successive insertions of H to form an adsorbed CH*3 (methyl) and an adsorbed CH3CO* (acyl) groups. While the hydrogenation of CH*3 leads to the formation of CH4, the acyl group hydrogenation leads to the formation of ethanol. Based on the reactivity results presented here, it thus appears that CO is a suggested intermediate in the reaction network.
4. Conclusions The titania-supported Rh, Rh–Fe, and Fe catalysts studied here were active for the formation of CH4 and CO via CO2 hydrogenation at the 1 −1 nominal conditions of 20 atm, 543 K, and a WHSV of 8000 cm3 g− . cat h A direct comparison of turnover frequency based on H2 chemisorption on a 2% Rh–2.5% Fe/TiO2 revealed that the two reactions were catalyzed at about the same rate at 543 K. The promotional role of Fe on CO2 hydrogenation in terms of activity and selectivity to alcohol formation was not as straightforward as it was in the case of CO hydrogenation. The addition of Fe increased the extent of reverse water gas shift to produce CO from CO2, which also correlated to greater selectivity to ethanol. Higher temperature and a higher WHSV during CO2 hydrogenation favored CH4 formation at the expense of primarily CO. The blending of CO2 into a syngas stream did not significantly impact the rate or selectivity of ethanol formation. Removing iron carbonyl impurities from the CO feedstream was found to be important for catalysts that do not have reverse water gas shift activity. Otherwise, RWGS activity could be catalyzed by impurity Fe deposits on the catalyst surface.
Acknowledgments The authors acknowledge the seed funding from the University of Virginia and partial support from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02-95ER14549. The authors also acknowledge the help of Mr. Mohammad A. Haider for the H2 chemisorption analysis.
Table 5 Results from elemental analysis and H2 chemisorption used to determine turnover frequency for CO and CO2 hydrogenation. Catalyst composition (nominal)
Rh and/or Fe loadings (wt.%)
H/Rh (%)
CO TOF (total metal)a (s− 1)
CO TOF (H2 ads.)b (s− 1)
CO2 TOF (total metal)a (s− 1)
CO2 TOF (H2 ads.)b (s− 1)
2%Rh/TiO2 2%Rh/TiO2c 2%Rh–2.5%Fe/TiO2 2.5%Fe/TiO2
1.94% 1.94% 1.73% Rh, 1.79% Fe 2.11%
54 54 66 –
0.012 0.012 0.036 0.0022
0.023 0.023 0.055 –
0.021 0.051 0.027 0.0035
0.038 0.093 0.041 –
a b c
Molecules of CO or CO2 converted per total metal atom per second. Molecules of CO or CO2 converted per active site counted by H2 chemisorption per second. CO was purified by passing it through a silica trap immersed in a dewar containing a dry ice–acetone mixture.
906
M.R. Gogate, R.J. Davis / Catalysis Communications 11 (2010) 901–906
References [1] M.A. Haider, M.R. Gogate, R.J. Davis, J. Catal. 261 (2009) 9–16. [2] T. Inoue, T. Iizuka, K. Tanabe, Appl. Catal. 46 (1989) 1–9. [3] A. Trovarelli, C. Mustazza, G. Dolcetti, J. Kaspar, M. Graziani, Appl. Catal. 65 (1990) 129–142. [4] T. Szailer, É. Novák, A. Oszkó, A. Erdöhelyi, Top. Catal. 46 (2007) 79–86. [5] É. Novák, K. Fodor, T. Szailer, A. Oszkó, A. Erdöhelyi, Top. Catal. 20 (2002) 107–117. [6] F. Solymosi, A. Erdöhelyi, T. Bánsági, J. Catal. 68 (1981) 371–382. [7] F. Solymosi, A. Erdöhelyi, J. Mol. Catal. 8 (1980) 471–474. [8] H. Kusama, K. Okabe, K. Sayama, H. Arakawa, Energy 22 (1997) 343–348. [9] H. Kusama, K. Okabe, K. Sayama, H. Arakawa, Catal. Today 28 (1996) 261–266. [10] H. Kusama, K.K. Bando, K. Okabe, H. Arakawa, Appl. Catal. A 197 (2000) 255–268. [11] H. Kusama, K.K. Bando, K. Okabe, H. Arakawa, Appl. Catal. A 205 (2001) 285–294. [12] H. Kusama, K. Okabe, K. Sayama, H. Arakawa, Appl. Organometal. Chem. 14 (2000) 836–840.
[13] T. Inui, T. Yamamoto, M. Inoue, H. Hara, T. Takeguchi, J.B. Kim, Appl. Catal. A 186 (1999) 395–406. [14] J.L. Dubois, K. Sayama, H. Arakawa, Chem. Lett. (1992) 5–8. [15] M. Takagawa, A. Okamoto, H. Fujimura, Y. Izawa, H. Arakawa, Stud. Surf. Sci. Catal. 114 (1998) 525–528. [16] K.G. Chanchlani, R.R. Hudgins, P.L. Silveston, J. Catal. 136 (1992) 59–75. [17] K.J. Smith, R.G. Herman, K. Klier, Chem. Eng. Sci. 45 (1990) 2639–2646. [18] A.-M. Hilmen, M. Xu, M.J.L. Gines, E. Igelsia, Appl. Catal. A 169 (1998) 355–372. [19] K. Klier, R.G. Herman, J.G. Nunan, K.J. Smith, C.E. Bogdan, C.-W. Young, J.G. Santiesteban, Stud. Surf. Sci. Catal. 36 (1988) 109–125. [20] M.R. Gogate, R.J. Davis, Chem. Cat. Chem. 1 (2009) 295–303. [21] J. Gao, X. Mo, A.C.Y. Chien, W. Torres, J.G. Goodwin, J. Catal. 262 (2009) 119–126. [22] K.K. Bando, K. Soga, K. Kunimori, H. Arakawa, Appl. Catal. A: General 175 (1998) 67–81.