Interconversion between CO2 and HCOOH under Basic Conditions

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Interconversion Between CO2 and HCOOH Under Basic Conditions Catalyzed by PdAu Nanoparticles Supported by Amine Functionalized Reduced Graphene Oxide as a Dual Catalyst Heng Zhong, Masayuki Iguchi, Maya Chatterjee, Takayuki Ishizaka, Mitsunori Kitta, Qiang Xu, and Hajime Kawanami ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00294 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Catalysis

Interconversion Between CO2 and HCOOH Under Basic Conditions Catalyzed by PdAu Nanoparticles Supported by Amine Functionalized Reduced Graphene Oxide as a Dual Catalyst Heng Zhong,† Masayuki Iguchi,† Maya Chatterjee,† Takayuki Ishizaka,† Mitsunori Kitta,‡ Qiang Xu,¶ Hajime Kawanami*,†

†

Research Institute for Chemical Process Technology, National Institute of Advanced

Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan

‡

Research Institute of Electrochemical energy, National Institute of Advanced

Industrial Science and Technology, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan

1

ACS Paragon Plus Environment

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¶

AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory

(ChEM-OIL), National Institute of Advanced Industrial Science and Technology, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

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ABSTRACT

Recently, the utilization of formic acid (FA) or formate as promising hydrogen carriers through the interconversion between CO2 and HCOOH or HCO3– and HCOO–, respectively has attracted increasing research interest. In this work, a PdAu bimetallic catalyst

supported

on

phenylenediamine-alkalized

reduced

graphene

oxide

(Pd0.50Au0.50/PDA-rGO) was developed for catalyzing bicarbonate hydrogenation under basic conditions as well as FA/formate dehydrogenation under acidic and basic conditions. Without any additives, a very high yield (94%) of potassium formate (PF) can be achieved from the hydrogenation of potassium bicarbonate at 50 ºC for 16 h. On the other hand, initial TOFs of 1.63×103 and 6.98×103 h–1 were accomplished in the dehydrogenations of 6 mol/L PF and 8 mol/L FA, respectively at 80 ºC. This work successfully demonstrates highly efficient CO2 hydrogenation and is the first report of a Pd-based heterogeneous catalyst for the additive-free dehydrogenation of concentrated (>6 mol/L) PF or FA solution. It significantly enhanced the hydrogen capacity and is of great interest for practical applications. The good performance of this catalyst is probably attributed to (i) the nanosized (1.8 ± 0.5 nm) metal particles, (ii) the presence 3


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of an amine group on the support, which can act as a proton scavenger, and (iii) the additional Au component prohibiting CO formation and enhancing the durability of the catalyst even in high concentration FA/formate solutions.

Keywords: liquid organic hydrogen carrier; hydrogen storage; formic acid dehydrogenation; CO2 reduction; heterogeneous catalyst; renewable energy

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1. INTRODUCTION Due to the growing concern over the rapidly increasing demand for energy in our society and the depletion of fossil fuels, seeking renewable energy sources to replace fossil fuels has become one of the most urgent and critical targets in the 21st century. Although our planet has plenty of renewable energy resources, including solar power, wind, ocean energy (wave, tide), hydrothermal, and geothermal power, they are generally unstable and unevenly distributed. Thus, renewable energies require proper processing in terms of conversion, storage and transportation systems for practical utilization. Conversion of renewable energy sources into electricity followed by water splitting to generate H2 is a promising way to store the renewable energies in the form of chemical energy.1-5 However, gaseous H2 has a very low volumetric energy density (ca. 10 kJ/L at atmospheric conditions), which makes it unsuitable for massive energy storage and transportation.6 As a result, seeking a promising hydrogen carrier which is not only able to physically or chemically store the hydrogen with a high concentration

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to increase its volumetric energy density but which can also discharge the hydrogen readily has becomes another important research topic. Typically, hydrogen carriers can be divided into two series – solid and liquid. Although solid hydrogen carriers such as metal hydrides (NaAlH4, MgH2 etc.) and porous materials (zeolites, metal-organic frameworks (MOFs), porous carbon, etc.) have a relatively high theoretical hydrogen capacity (e.g., 18.5 wt% for LiBH4), they usually require high desorption temperatures and/or low adsorption and storage temperatures (e.g., –196 ºC).7-10 On the other hand, liquid organic hydrogen carriers (LOHCs) such as hydrazine, ammonia, and methyl cyclohexane are often toxic, flammable or explosive, which limits their practical applications.11-14 Recently, formic acid (FA) with a 4.4 wt% hydrogen content has been regarded as a promising LOHC, since it is stable, has a low-toxicity, is bio-degradable, and is easy to store and transport.14-16 More importantly, CO2 is the only by-product after the FA dehydrogenation, which can be readily recovered

and

reduced

to

FA

again

to

achieve

a

complete

hydrogenation-dehydrogenation cycle for the continuous utilization of H2 (Scheme 1).15 If this interconversion cycle is operated under basic conditions, it becomes a conversion 6


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between HCO3– and HCOO–, either as solids or in aqueous solution, which generates only H2 gas, and the need to separate H2 from CO2 in the generated gas can therefore be avoided.

Scheme 1. Interconversion between CO2 and formic acid H2 carrier.

Highly efficient heterogeneous catalysis is one of the preferred choices to achieve the CO2-HCOOH or HCO3–/CO32–-HCOO– cycles for continuously discharging the H2 and for H2 storage.17-18 Although various heterogeneous catalysts have been developed for CO2 hydrogenation19 and FA dehydrogenation,20-21 reports of a single heterogeneous catalyst that is able to catalyze both reactions are limited. According to the existing literature, Pd based nanocatalysts exhibited good activity for catalyzing the interconversion of CO2 and HCOOH.22-25 However, most of the strategies rely on harsh reaction conditions, have long reaction times, and exhibit low activity (turnover number and TOF). Importantly, in most of the cases, substrates with low concentrations (< 2

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mol/L) are used for the FA/formate dehydrogenation.26-33 Thus, they are limited to the laboratory-scale experiments or primitive applications but are unsuitable for scaling-up since the hydrogen capacity decreases greatly in diluted FA/formate solutions. In our previous research, we have found that a PdAu bimetallic catalyst supported on a phenylenediamine-alkalized reduced graphene oxide (PdAu/PDA-rGO) can effectively decompose a mixed solution of 6.7 mol/L FA and 6.7 mol/L sodium formate to high pressure H2 and CO2.34 In this work, we further investigated the application of the PdAu bimetallic catalyst for the high concentration FA/formate dehydrogenation and its catalytic ability for the CO2/bicarbonate hydrogenation.

2. EXPERIMENTAL SECTION The synthesis of the PdAu/PDA-rGO catalyst is based on a simple wetness impregnation and reduction method, the details of which can be found in the Supporting Information section. A glass-lined stainless steel (SUS316) tubular reactor was used for the hydrogenation reaction and a lined-glass tube (1.25 cm i.d., 18.5 mL inner volume) was also used separately as the reactor for the dehydrogenation reaction. The schematic

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diagrams and the details of specific reaction processes are in the Supporting Information section (Figure S1). After the reactions, gas samples were collected and analyzed by gas-chromatography with a thermal conductivity detector (GC-TCD). Liquid samples were separated from solid precipitate by centrifugation and analyzed by a Shimadzu Nexera X2 HPLC system after being filtered through a 0.45 µm PTFE syringe filter. Solid samples were collected and vacuum dried for further analysis by X-ray diffraction (XRD, Rigaku Smartlab), scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscope (TEM, Tecnai G2 20, FEI ), transmission electron microscope Energy dispersive X-ray spectroscopy (TEM-EDS, TITAN3 G2 60-300, FEI), Fourier transform infrared spectroscopy (FT-IR, Horiba FT-720), and inductively coupled plasma atomic emission spectroscopy (ICP-AES, SPS-3100, SII Nano Technology Inc.). More details can be found in the Supporting Information.

3. RESULTS AND DISCUSSION

3.1. Catalyst Characterization. PdAu bimetallic catalysts with various Pd:Au molar ratios (Pd:Au = 2, 1 and 0.5) were synthesized and their actual metal ratios were 9


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analyzed by ICP-AES. Results showed that the obtained Pd/Au molar ratio is in good accordance with the initial stoichiometric ratio added during the synthesis (Table S1). The XRD patterns of the as-prepared PdAu bimetallic catalysts are shown in Figure 1 (a). A strong peak at 2θ of 9.9º was observed for the sample of the graphene oxide (GO), which is attributed to the interlayer diffraction of the GO.35-36 However, this peak disappeared after the PDA treatment and metal loading, suggesting the disordered re-stacking of the PDA-rGO layers.34 Two peaks at 38.14º and 44.50º can be observed in the XRD pattern of the Pd0.50Au0.50/PDA-rGO catalyst. The 2θ positions of these two peaks are slightly larger than the reference diffraction peaks of the Au(111) (38.08º) and Au(200) (44.26º), respectively according to the international center for diffraction data (ICDD: 00-004-0784). However, no Pd diffraction peaks corresponding to Pd(111) (40.12º) and Pd(200) (46.66º) (ICDD: 00-005-0681) can be found in the XRD pattern. Similar XRD patterns were also observed for the Pd0.66Au0.34/PDA-rGO and Pd0.32Au0.68/PDA-rGO catalysts. For comparison, a monometallic Pd/PDA-rGO catalyst was also synthesized and analyzed by XRD. No obvious Pd diffraction peak can be found from the XRD pattern of Pd/PDA-rGO, suggesting that the Pd particles were well 10


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dispersed on the support. From the SEM image of the Pd0.50Au0.50/PDA-rGO catalyst (Figure 1 (b)), a silk wave-like morphology of the catalyst was found, which is contributed by the PDA-rGO support. According to the TEM images of the catalysts (Figure 1 (c) and Figure S2), the presence of two different kinds of nanoparticles can be suggested for the PdAu catalysts, one with a relatively larger size of ca. 5~8 nm, and another with a relatively smaller size of ca. 1.8 nm (Figure S2). The sizes of the smaller nanoparticles were determined to be 1.6±0.5, 1.8±0.5, and 1.8±0.7 nm for the Pd0.66Au0.34/PDA-rGO (Figure S2(b)), Pd0.50Au0.50/PDA-rGO (Figure 1(c)), and Pd0.32Au0.68/PDA-rGO (Figure S2(c)) respectively, which suggests that the change in the Pd/Au ratio does not obviously affect to the size of the nanoparticles. However, the Pd/PDA-rGO mono-metallic catalyst had a slightly larger particle size of 2.3 ± 0.5 nm (Figure S2 (a)). TEM-EDS of Pd0.5Au0.5/PDA-rGO shows that the nanoparticles are bimetallic composed of Au and Pd, and no mono-metallic nanoparticles were observed (Figure S3). XPS analyses (Figure S4) of Pd/PDA-rGO, Pd0.5Au0.5/PdA-rGO and Au/PDA-rGO shows that Au signals of the PdAu/PDA-rGO catalyst were slightly shifted to a lower energy band compared to that of the Au/PDA-rGO catalyst (Figure 11


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S4(b)). In addition, Pd(0) signals of PdAu/PDA-rGO catalyst were also slightly shifted to lower energy compared to that of the Pd/PDA-rGO catalyst (Figure S4(c), which implies that the bond energy would be changed by the addition of Au to form PdAu nanoparticles. The amount of Pd(0) and Pd(II) of the Pd/PDA-rGO and Pd0.5Au0.5/PDA-rGO catalyst were also checked. In Pd0.5Au0.5/PDA-rGO, the Pd(0) amount was higher (54.3%) compared to that of Pd/PDA-rGO (39.4%), and the Pd(II) amount of Pd0.5Au0.5/PDA-rGO was 45.7% (Figure S5). From the FT-IR spectra of the Pd0.50Au0.50/PDA-rGO (Figure S6), the ring vibration (833 cm-1), antisymmetric C-N stretching (1224 cm-1), aromatic C-C stretching (1508 cm-1), N-H bending (1575 cm-1), and N-H stretch vibration (3434 cm-1) peaks of the PDA-rGO support were clearly observed.34 3.2. Hydrogenation of Bicarbonate. Compared to the CO2-HCOOH cycle, the HCO3–-HCOO– cycle has more practical importance related to the easy separation of the targeted H2 gas, since H2 is the only gaseous product observed during the decomposition of formate, in contrast to the decomposition of FA which also generates CO2 gas. Thus, we mainly focused on the hydrogenation of bicarbonate into formate for 12


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developing the HCO3–-HCOO– cycle.37 Optimization of different reaction parameters such as reaction time, temperature, H2 pressure, catalyst amount, and potassium bicarbonate (KHCO3) concentration for the hydrogenation of KHCO3 using the Pd0.50Au0.50/PDA-rGO catalyst are shown in Figure 2. Results show that the temperature and reaction time can significantly influence the PF yield (Figure 2 (a)). At 30 ºC, the PF yield increased quickly to 73% within the first 4 h and finally reached a maximum of 90% in 16 h, and then remained constant even after extending the reaction time to 24 h, suggesting an equilibrium for the reaction. These results reflect an exceptional activity of the Pd0.50Au0.50/PDA-rGO catalyst in the conversion of KHCO3 into PF even at room temperature. Similar trends for the PF yield were also detected at 50 and 80 ºC. Although a 91% yield of PF was obtained within 6 h at 80 ºC, which apparently indicates an accelerated reaction rate at higher temperature, the optimum yields of PF at 30, 50, and 80 ºC were very close (90%, 92%, and 91% respectively), indicating a negligible effect of temperature on the reaction equilibrium. Figure 2 (b) shows the influence of H2 pressure on the KHCO3 hydrogenation. An 84% PF yield was obtained when only 1 MPa H2 was used. However, with an increase 13


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in H2 pressure from 3 to 7 MPa, the PF yield increased slightly from 92 to 94%, revealing that the reaction equilibrium can hardly be further promoted for the PF formation by increasing the H2 pressure. The effect of the catalyst amount, denoted as the (Pd+Au)/KHCO3 molar ratio, on the PF yield is depicted in Figure 2 (c). It can be seen that, when the (Pd+Au)/KHCO3 ratio exceeded 1.83×10-2, a plateau was reached, indicating that additional amounts of the PdAu catalyst would not promote the PF yield. The effect of the KHCO3 concentration on the PF yield was also tested at a constant (Pd+Au)/KHCO3 molar ratio of 1.83×10-2, which means that the absolute amount of the Pd0.50Au0.50/PDA-rGO catalyst was increased simultaneously with the concentration of the KHCO3. From Figure 2 (d), the concentration of the KHCO3 aqueous solution had a significant effect on the PF yield. The PF yield was first increased from 84 to 88% as the KHCO3 concentration increased from 0.25 to 1.0 mol/L at 80 ºC for 2 h, then it dropped drastically to 59% after a further increase in the KHCO3 concentration to 2.0 mol/L, probably because of the insufficient reaction time. Therefore, the reaction time was prolonged from 2 to 6 h. Although the PF yield was largely enhanced from 59% (2h) to 83% (6h) using 2 mol/L KHCO3 at 80 ºC, it was still 14


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lower than the yield of 91% obtained in the 0.5 mol/L KHCO3 under the same conditions (6 h, 80 ºC), indicating an adverse effect of high concentration of KHCO3 on the PF yield. It should be noted that the generated PF concentration proportionally increased to 1.2 mol/L upon increasing the concentration of KHCO3 up to 1.5 mol/L, and then it remained unchanged even using the higher concentration (2.0 mol/L) of KHCO3. Hence, the decrease in the PF yield with the increase in the KHCO3 concentration is probably related to the reaction equilibrium. Nevertheless, a PF concentration as high as 1.66 mol/L could be finally achieved at 80 ºC for 6 h even though the PF yield was 83% under these conditions. The effects of the Pd/Au ratio and metal loading on the PF yield were also examined (Figure S7). The PDA-GO without any metal loading remained inactive for the hydrogenation of KHCO3 to PF, whereas, the monometallic Au catalyst (Au/PDA-rGO) showed poor catalytic activity for PF production (8.1% PF yield). On the contrary, a significant activity and a 93% PF yield was obtained on the monometallic Pd catalyst (Pd/PDA-rGO). Interestingly, all PdAu bimetallic catalysts (Pd0.32Au0.68/PDA-rGO, Pd0.50Au0.50/PDA-rGO, and Pd0.66Au0.34/PDA-rGO) exhibited 15


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excellent activity for PF production compare to the Pd monometallic catalyst. These results indicate that the outstanding performance of the PdAu bimetallic catalysts for the hydrogenation of KHCO3 into PF probably originates from the Pd component and the Pd/Au ratio does not apparently affect the activity in the tested range. However, it should be noted that although the Pd monometallic catalyst exhibited an excellent activity for the KHCO3 hydrogenation, its performance for the PF and FA dehydrogenation was rather limited compared to the PdAu bimetallic catalyst, which will be discussed in the following section. As can be seen from Table S1, the total metal loadings (Pd + Au) of the bimetallic catalysts were relatively high (17.8–27.6 wt%). Thus, a low metal loading (5.0 wt%) catalyst (Pd0.57(Low)Au0.43(Low)/PDA-rGO) was synthesized and the size of the nanoparticles was determined to be 1.7±0.5 nm from the TEM image (Figure S8), which is similar to the other PdAu bimetallic catalysts. With the Pd0.57(Low)Au0.43(Low)/PDA-rGO catalyst, the obtained PF yields were 91% and 93% at the (Pd+Au)/KHCO3 ratios of 7.9×10-3 and 1.42×10-2, respectively (Figure S9). This indicates that the low metal loading catalyst also has a comparable catalytic activity for

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the hydrogenation of KHCO3 to PF and the amount of metal loading is not the critical factor to achieve a high PF yield. The stability and recyclability of the Pd0.50Au0.50/PDA-rGO catalyst were also tested by recovering and reusing the catalyst two times. Unfortunately, the PF yield decreased with the number of recycling times (Figure S10). The PF yield reduced from 74% to 32% after two cycles in the hydrogenation of 0.5 mol/L KHCO3 at 30 ºC for 4 h. However, the TEM image (Figure S11(a)) and XRD pattern (Figure S11(b)) of the Pd0.50Au0.50/PDA-rGO catalyst after two cycles were almost the same as those of the fresh catalyst (Figure 1(a) and (c)). Our previous research has shown that the PDA in the support material is not stable under high pressure conditions.34 Thus, the structures of the fresh and used catalyst were further analyzed by FT-IR (Figure S12). The results revealed that obvious decreases in the intensities and slight negative shifts in the wavenumbers of the aromatic C-C stretching (1506 cm–1), C-N stretching (1211 cm-1), ring vibration (809 cm-1), N-H bending (1558 cm–1), and N-H stretching (3428 cm–1) absorption peaks were observed. These peaks are attributed to the PDA in the PDA-rGO support, and thus the PDA in the PDA-rGO was transformed and removed during the 17


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reaction under high pressure H2 conditions, as already discussed in our previous report.34 We further synthesized a Pd0.49Au0.51/rGO catalyst (Table S1, Entry 7) without the PDA component to examine the effect of the PDA on the hydrogenation of KHCO3 into PF. The particle size of the Pd0.49Au0.51/rGO catalyst was determined to be 2.7±0.8 nm from the TEM image (Figure S13), which is comparatively higher than those of the PdAu bimetallic catalysts with the PDA in the support (Figure 1(c), Figure S2 (b) and (c), Figure S8). This suggests that the PDA can act as a stabilizer to reduce the size of the nanoparticles during the synthesis process.35 The PF yield obtained with the Pd0.49Au0.51/rGO was 85%, which is slightly lower than that obtained with the Pd0.50Au0.50/PDA-rGO under the same conditions (Figure S14). It can be related to the reduced particle size in the presence of PDA. However, the PF yield was drastically decreased over the used Pd0.50Au0.50/PDA-rGO catalysts (91% in Figure S7) compared to the presence and the absence of PDA in the support material. We also checked the recyclability of the catalyst without PDA (Pd0.49Au0.51/rGO) and confirmed that the PF yields are maintained until the 3rd cycle (Figure S15). Thus, the damaged PDA structure 18


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in the Pd0.50Au0.50/PDA-rGO catalyst after the reaction probably has other adverse effects on the hydrogenation of KHCO3 into PF. However, the detailed mechanism is still not clear yet. 3.3. Dehydrogenation of FA and PF. In the next step, the dehydrogenation of PF and FA with the Pd0.50Au0.50/PDA-rGO catalyst under atmospheric pressure was studied. As shown in Figure 3, the Pd0.50Au0.50/PDA-rGO was able to catalyze the dehydrogenations of both PF and FA. The H2:CO2 molar ratios in the final gas phase were determined to be 5.45, 4.68 and 1.05 in the case of 2 mol/L PF, 6 mol/L PF, and 2 mol/L FA, respectively. No CO was detected in all cases. In the dehydrogenation of PF, CO2 present in the gas phase was obtained through the decomposition of HCO3– at elevated temperature based on eq. 1, which has been studied in detail previously.38 2HCO3– ⇌

CO32- + CO2 + H2O

(1)

In 2 mol/L PF, the H2 generation rate was as fast as 11.8 mL/min at 20% PF conversion and then gradually slowed down. The initial TOF (at 20% PF conversion) for the PF dehydrogenation at 80 ºC was 9.18×102 h–1. When the PF concentration increased to 6 mol/L, a significant enhancement in the H2 generation rate of 21.0 mL/min (at 20% PF 19


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conversion) was observed, and the initial TOF of the PF dehydrogenation increased to 1.63×103 h–1. To the best of our knowledge, using a heterogeneous catalyst, this is the highest TOF obtained for the decomposition of formate salts based on the overall catalyst amount (Table S2) and the concentration of the formate solution is also the highest compared to the prior report (Table S2),39 which leads to a better hydrogen capacity. Interestingly, when PF was replaced by FA, the H2 generation was drastically promoted, and an initial TOF of 4.87×103 h–1 was obtained for the FA dehydrogenation at 80 ºC. This is probably because the decomposition of PF requires a simultaneous dissociation of water in the basic conditions (eq. 2), which might be more difficult than the direct dehydrogenation of FA (eq. 3). HCOO– + H2O ⇌ H2 + HCO3–

(2)

HCOOH ⇌ H2 + CO2

(3)

It has to be mentioned that in the FA dehydrogenation with heterogeneous catalysts, additives such as PF or other formate salts are commonly used,40-43 but the direct dehydrogenation of formate (Table S2) is rarely reported. To the best of our knowledge, the presented research is the first report of a single heterogeneous catalyst that can 20


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efficiently catalyze both reactions of bicarbonate hydrogenation and PF/FA dehydrogenation without using any additives. Since the Pd0.50Au0.50/PDA-rGO catalyst exhibited a better performance for the FA dehydrogenation than the PF dehydrogenation, the activity of the catalyst for FA dehydrogenation was further examined at various reaction conditions (Figure S16 and Table S3). The initial TOF increased considerably from 4.87×103 to 7.18×103 h–1 when the ratio of n(Pd+Au)/nFA decreased from 7.9×10-3 to 3.9×10-3 in the dehydrogenation of 2 mol/L of FA (Table S3, Entries (a) and (b)), although the gas generation rate slightly dropped due to the absolute reduction in the amount of the catalyst (Figure S16). On the other hand, increasing the FA concentration from 2 to 8 mol/L did not obviously change the initial TOF of the FA dehydrogenation (6.98×103 h–1 for 8 mol/L FA, Table S3). However, the gas generation rate dropped quickly after 5 min in the case of 8 mol/L FA. Nevertheless, a FA conversion of 96% without any detectable CO generation could be finally achieved in the case of 8 mol/L FA. Such high FA concentration has importance in the large-scale application because of the enhanced hydrogen capacity. In this context, monometallic Au catalysts have been reported for the high concentration FA (> 6 mol/L) 21


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dehydrogenation.44-46

However,

for

Pd-based

Page 22 of 47

mono-,

bi-,

and

tri-metallic

heterogeneously catalyzed FA dehydrogenation, the concentration of the FA is generally limited to 0.2 ~ 2.0 mol/L.26-31,

39, 47

Reports of the decomposition of

concentrated FA solutions are limited.48 According to the reported literature, Au is more durable than the Pd in high concentration FA solutions. Although the details are still unclear, this is probably because of the stability of Au in acid conditions due to the higher redox potential of Au (Au3+/Au: 1.498 V) compared to that of Pd (Pd2+/Pd: 0.951 V).49 Therefore, the good performance of the Pd0.50Au0.50/PDA-rGO catalyst for the high concentration FA dehydrogenation (96% conversion of 8 mol/L FA with an initial TOF of 6.98×103 h–1) probably benefited from the addition of the Au component, but this requires further studies. It should be noted that the excellent performance of the Pd0.50Au0.50/PDA-rGO catalyst for the FA dehydrogenation should also be attributed to the combined effect of the Pd and the amine group contained in the PDA-rGO support, which acted as a proton scavenger for promoting the O-H cleavage. The details have already been reported in previous research.34-35

22


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ACS Catalysis

The reaction kinetics of the Pd0.50Au0.50/PDA-rGO catalyzed FA dehydrogenation was also studied. As can be seen in Figure 4 (a), a decrease in the reaction temperature considerably inhibited the FA decomposition. While using 2 mol/L of FA, the initial TOF (7.18×103 h–1) of the reaction obtained at 80 ºC decreased to 2.36×103 h–1 and 6.84×102 h–1 at 50 ºC and 25 ºC, respectively. To study the reaction kinetics, the linear fitting of the ln(cFA) (cFA: concentration of FA) versus reaction time in the initial state (open symbols in Figure 4 (b)) of the FA dehydrogenation reactions are plotted. Based on these results, an Arrhenius plot of the Pd0.50Au0.50/PDA-rGO catalyzed FA dehydrogenation was obtained (Figure 4 (c)) and the activation energy for the FA dehydrogenation was determined to be 46.6 kJ/mol. We also compared the activity of the Pd0.50Au0.50/PDA-rGO catalyst with the Pd/PDA-rGO catalyst (Table S1, Entry 1) for the FA dehydrogenation (Figure S17). The initial TOF value was 2.46×103 h-1 at 80 ºC for the Pd/PDA-rGO catalyst, which is much lower than that obtained with the Pd0.50Au0.50/PDA-rGO (7.18×103 h-1). Furthermore, no gas generation was observed at 25 ºC when the Pd/PDA-rGO was used, while an initial TOF of 6.84×102 h–1 at 25 ºC can be obtained with the 23


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Pd0.50Au0.50/PDA-rGO catalyst as already discussed. These results indicate that the Pd0.50Au0.50/PDA-rGO is more active for the FA dehydrogenation than the Pd/PDA-rGO catalyst. This is probably because the Au can (i) affect the electron state of the Pd and increase the intrinsic catalytic activity of Pd, and (ii) prohibit the formation of CO which can poison the Pd catalyst. The detailed effect of the Au in the PdAu bimetallic catalyst systems has been reviewed in our recent report.39

4. CONCLUSIONS In this work, a PdAu bimetallic catalyst on a p-phenylenediamine functionalized reduced graphene oxide support (Pd0.50Au0.50/PDA-rGO) was developed for the catalytic interconversion of HCO3– and HCOO– for a H2 storage system under basic conditions. This Pd0.50Au0.50/PDA-rGO catalyst exhibited excellent catalytic activity not only for the hydrogenation of HCO3– but also for the efficient dehydrogenation of FA and PF. For the hydrogenation of KHCO3, an optimum PF yield of 94% was obtained at 50 ºC in 16 h. However, an impressive yield (90%) was also obtained even at 30 ºC. For the dehydrogenation of PF or FA, initial TOFs of 1.63×103 h–1 and 6.98×103 h–1 were

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ACS Catalysis

achieved in 6 mol/L PF and 8 mol/L FA, respectively at 80 ºC. Notably, the concentrations of FA and PF were high compared to the reported work on FA/formate dehydrogenation systems based on a heterogeneous Pd catalyst, which is one of the critical parameters for practical application since the hydrogen capacity is greatly enhanced as the FA/formate concentration is increased considerably. The extraordinary performance of this catalyst at high concentrations of PF and FA solutions is probably due, in part, to the addition of the Au to the Pd catalyst, although this requires further investigation.

ASSOCIATED CONTENT Supporting Information. Details of catalyst preparation, reaction processes and analytical methods; TEM, TEM-EDS, XRD, XPS and FT-IR of the catalyst; data for PF yield with different Pd/Au ratios, catalyst stability, and FA dehydrogenation; TOF comparison with other reported data.

AUTHOR INFORMATION 25


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Corresponding Author *E-mail for H.K.: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS HK thanks Dr. David C. Grills in Brookhaven National Laboratory, USA for the English corrections. HK also thanks Dr. Koichi Sato in AIST, Japan for the XPS analysis. The authors acknowledge the financial support of Japan Science and Technology Agency (JST), CREST, and the International Joint Research Program for Innovative Energy Technology of the Ministry of Economy, Trade, and Industry (METI), Japan.

REFERNCES

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Figure captions

Figure 1. (a) XRD patterns of the as synthesized PdAu catalysts; (b) SEM image of the Pd0.50Au0.50/PDA-rGO catalyst; (c) TEM image and size distribution of the Pd0.50Au0.50/PDA-rGO catalyst. Figure 2. Effect of various reaction conditions on the potassium formate (PF) yield and concentration from the hydrogenation of KHCO3 catalyzed by Pd0.50Au0.50/PDA-rGO (a) KHCO3: 0.5 mol/L, 2 mL, (Pd+Au)/KHCO3= 1.83×10-2, H2: 5 MPa, temp.: 30, 50 and 80 oC, time: ~24 h; (b) KHCO3: 0.5 mol/L, 2 mL, (Pd+Au)/KHCO3= 1.83×10-2, temp.: 50 ºC, time: 16 h; (c) KHCO3: 0.5 mol/L, 2 mL, H2: 5 MPa, temp.: 80 ºC, time: 2 h; (d) KHCO3: 2 mL, (Pd+Au)/KHCO3= 1.83×10-2, H2: 5 MPa, temperature: 80 ºC). Figure 3 Gas generation as a function of time obtained from the decomposition of 2 mL of (a) 2 mol/L PF, pH = 8.45 (b) 6 mol/L PF, pH = 9.07 and (c) 2 mol/L FA, pH = 1.57 with the Pd0.50Au0.50/PDA-rGO catalyst at 80 ºC ((Pd+Au) = 31.6 µmol). Figure 4 Gas generation (a), variations of ln(cFA) versus time and their linear fittings (b), and related Arrhenius plot (c) obtained from the FA dehydrogenation catalyzed by the Pd0.50Au0.50/PDA-rGO at various temperatures (FA: 2 mol/L, 2 mL; n(Pd+Au)/nFA = 37


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3.9×10-3; cFA: concentration of FA, calculated from the initial FA concentration and generated gas volume; in Figure 4 (b), the linear fitting was only performed in the initial relatively linear state (open square, open circle, and open triangle) of the reaction).

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(a) Intensity (Counts)

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Au (111) Pd (111) Au (200) Pd (200)

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GO Pd/PDA-rGO

Pd 0.66Au0.34/PDA-rGO Pd 0.50Au0.50/PDA-rGO Pd0.32Au 0.68/PDA-rGO

10

20

30

40

50

60

2θ (degree)

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70

80

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(C) dTEM = 1.8 ± 0.5

0

1

2

3

4

5

6

Particle size (nm) Figure 1. (a) XRD patterns of the as synthesized PdAu catalysts; (b) SEM image of the Pd0.50Au0.50/PDA-rGO catalyst; (c) TEM image and size distribution of the Pd0.50Au0.50/PDA-rGO catalyst.

40


100 90 (a) 80 70 60 50 40 30 20 10 0 0 4

80 oC 50 oC 30 oC 8

12

16

20

24

28

Time (h)

PF yield (%)

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PF yield (%)

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100 90 (b) 80 70 60 50 40 30 20 10 0 0 1

2

3

4

5

6

7

H2 pressure (MPa)

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8

100 90 (c) 80 70 60 50 40 30 20 10 0 0.00

0.02

0.04

0.06

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0.08

(Pd+Au)/KHCO3 (mol/mol) 100 90 80 70 60 50 40 30 20 10 0

1.75

(d)

0.0

1.50 1.25 1.00 0.75 0.50 PF yield @ 2 h PF yield @ 6 h PF concentration @ 2 h PF concentration @ 6 h

0.5

1.0

1.5

2.0

0.25

PF concentration (mol/L)

PF Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PF yield (%)

ACS Catalysis

0.00

2.5

KHCO3 (mol/L) Figure 2. Effect of various reaction conditions on the potassium formate (PF) yield and concentration from the hydrogenation of KHCO3 catalyzed by Pd0.50Au0.50/PDA-rGO (a) KHCO3: 0.5 mol/L, 2 mL, (Pd+Au)/KHCO3= 1.83×10-2, H2: 5 MPa, temp.: 30, 50 and

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80 oC, time: ~24 h; (b) KHCO3: 0.5 mol/L, 2 mL, (Pd+Au)/KHCO3= 1.83×10-2, temp.: 50 ºC, time: 16 h; (c) KHCO3: 0.5 mol/L, 2 mL, H2: 5 MPa, temp.: 80 ºC, time: 2 h; (d) KHCO3: 2 mL, (Pd+Au)/KHCO3= 1.83×10-2, H2: 5 MPa, temperature: 80 ºC).

350

(b)

300

V (mL)

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250 (c) 200 150

(a)

100 50 0

0

5

10

15

20

25

30

Time (min) Figure 3 Gas generation as a function of time obtained from the decomposition of 2 mL of (a) 2 mol/L PF, pH = 8.45 (b) 6 mol/L PF, pH = 9.07 and (c) 2 mol/L FA, pH = 1.57 with the Pd0.50Au0.50/PDA-rGO catalyst at 80 ºC ((Pd+Au) = 31.6 µmol).

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(a)

200

50 oC

80 oC

25 oC

V (mL)

150 100 50 0

0

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Time (min) 1

ln(cFA) (mol/L)

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(b)

0 -1 -2 25 oC

-3 -4 -5

80 oC 0

50 oC

500 1000 1500 2000 2500 3000 3500

Time (s)

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-4

(c) 3 (1/T) lnk11.5 = 11.5 - 5608.2 (1/T) lnk = – 5.61×10

-5

lnk (s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

-6 -7 -8 0.0028 2.8

0.0030 3.0

0.0032 3.2

0.0034 3.4

-1

-3 1/T (K (K) -1) ×101/T

Figure 4 Gas generation (a), variations of ln(cFA) versus time and their linear fittings (b), and related Arrhenius plot (c) obtained from the FA dehydrogenation catalyzed by the Pd0.50Au0.50/PDA-rGO at various temperatures (FA: 2 mol/L, 2 mL; n(Pd+Au)/nFA = 3.9×10-3; cFA: concentration of FA, calculated from the initial FA concentration and generated gas volume; in Figure 4 (b), the linear fitting was only performed in the initial relatively linear state (open square, open circle, and open triangle) of the reaction).

45


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents (TOC) HCOOH HCOOK PdAu H

N

H

NH

rG O

H2

CO2 KHCO2 K2CO2

H2

46


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Page 47 of 47

HCOOH

ACSHCOOK Catalysis PdAu

1 2 3 4 5 H2 6 7

H

N

H

NH

rGO

ACS Paragon Plus CO2 Environment KHCO2 K2CO2

H2

Interconversion between CO2 and HCOOH under Basic Conditions

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