Heterogeneous Catalytic Generation of Hydrogen from Formic Acid

29 Feb 2012 - Theoretical Study on the Mechanism of Aqueous Synthesis of Formic Acid Catalyzed by [Ru]-EDTA Complex. Zhe-Ning Chen , Kwong-Yu Chan ...
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Heterogeneous Catalytic Generation of Hydrogen from Formic Acid under Pressurized Aqueous Conditions Siu-Wa Ting,† Chaoquan Hu,† Jayasree K. Pulleri,† and Kwong-Yu Chan*,† †

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, P.R. China ABSTRACT: PtRuBiOx/C catalyst has shown the promise for catalyzing CO-free hydrogen generation from formic acid in aqueous solution at room temperature and atmospheric pressure. In order to produce hydrogen at moderate-pressure to feed into a fuel cell stack, postgeneration compression is needed to overcome the flow resistance. In the present study, liquid formic acid decomposition over the PtRuBiOx/C catalyst was investigated at temperatures ranging from 80 to 140 °C and pressure up to 350 psi. It was found that the selectivity of the catalyst for formic acid decomposition remained almost 100%, and a complete conversion of formic acid could be achieved in several hours, which is significantly shorter than that at ambient conditions. The overall activation energy was also found to be 78 kJ·mol−1 under present conditions. The increase from the previously determined value of 37 kJ/mol at open atmosphere pressure was due to carbon dioxide release beyond saturation at elevated pressures. Furthermore, the stability of the catalyst was confirmed by performing a series of repeated runs.

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also been proposed.3−6 Similarly, electricity can also be stored directly in chemical form via electrochemical reduction of carbon dioxide to formic acid in aqueous media.7−10 Apart from combating environmental issues on a large scale, small-scale usage of formic acid for powering portable electrical devices can be commercially attractive. The use of liquid fuel has the convenience of rapid refueling in place of the lengthy electrical recharge in conventional batteries. Indirect formic acid fuel cells have shown more promise than direct formic acid fuel cells since the generated hydrogen can deliver higher electrochemical power than direct electrochemical oxidation of formic acid due to better hydrogen anode catalysis

INTRODUCTION To reduce carbon emission and mitigate global warming, there have been worldwide government pledges to increase fractions of renewable energy sources, e.g. biomass, wind, and solar energy, in national energy supply profiles. Two critical issues in adopting these renewable energies are 1) storing the excessive and “peak-hour generated” energy sources that are daily and weekly fluctuating in nature and 2) converting raw and centrally generated energy to clean and amenable forms of fuels for distributing and consumption, e.g. in transport or other applications. Formic acid, if decomposes easily to hydrogen, has been proposed to be a medium for solving these two critical issues, as illustrated schematically in Figure 1. As a natural compound in insects, formic acid can also be produced from biomass via biochemical or catalytic processes.1,2 Storing centrally generated hydrogen (from wind, solar, or nuclear) as formic acid with simultaneous capture of carbon dioxide has

HCOOH(l) → CO2 (g) + H2(g) Δr G° = −33.0 kJ/mol HCOOH(l) → CO(g) + H2O(l) Δr G° = −13.0 kJ/mol

Δr H° = 31.2 kJ/mol (1)

Δr H° = 28.4 kJ/mol (2)

As shown in reactions 1 and 2, formic acid decomposes in two competing paths to (1) hydrogen and carbon dioxide or (2) water and carbon monoxide, with similar energy changes. To facilitate the above-mentioned goals of increased adoption of renewable energy, efficient, complete, and selective decomposition of formic acid to hydrogen is a necessity. The use of a specific and efficient catalyst is needed. Various homogeneous and heterogeneous catalysts have been proposed recently with high selectivity and different activities.4,5,11−26 A series of ruthenium5,21,28,29 and iron15 complexes as catalysts for formic acid dehydrogenation were studied. Iridium20 and rhodium25,26 complexes were also used as catalysts for dehydrogenating formic acid. A few studies Received: Revised: Accepted: Published:

Figure 1. The role of formic acid in deploying renewable energy sources. © 2012 American Chemical Society

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December 22, 2011 February 27, 2012 February 29, 2012 February 29, 2012 dx.doi.org/10.1021/ie2030079 | Ind. Eng. Chem. Res. 2012, 51, 4861−4867

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using heterogeneous catalysts like palladium,30 bimetallic palladium,23,31 supported gold cluster,19 Ru-loaded CdS/AlHMS,16 and Cu2O32 have been reported. Recently, the PtRuBiOx/C catalyst17 has been reported to have high reactivity and selectivity at near ambient temperature for the formic acid dehydrogenation. Its activity has been among the highest reported for heterogeneous catalysts with the same reaction conditions. It has also one of the lowest activation energies reported and performs at temperatures lower than previously reported. The use of heterogeneous catalyst enables easy separation of catalysts, simpler reaction control, and the use of a fixed bed reactor. The previous study of PtRuBiOx/C catalyst for formic acid dehydrogenation was performed at atmospheric pressure and temperatures up to 90 °C. It is desirable to have the catalytic conversion conducted efficiently under a wide range of conditions such as temperature and pressure. Feeding hydrogen to a fuel cell stack will require moderate pressure to overcome flow resistance and achieve high power. Catalytic decomposition of aqueous formic acid with moderate heating in a closed vessel is a simple way to create pressure without the need of a hydrogen compressor to achieve pressurized hydrogen. Formic acid can be the downstream product of a biomass or CO2 conversion process in which high temperature or high pressure is a default reaction condition. Formic acid dehydrogenation may also be part of the water gas shift reaction which normally proceeds at higher pressure and elevated temperature. Stability and reusability of the catalyst are also needed. In this paper, we report performance of the hetereogeneous PtRuBiOx/C catalyst for a wider range of temperature and pressure. We also report the complete conversion of formic acid, which has not been mentioned in the literature. The repeated use of the catalyst is also demonstrated with consistent performance.

Figure 2. A schematic of Parr reactor.

heating was commenced). It was observed that pressure increased steadily with evolution of product gases during the reaction. Once the pressure reading stabilized at the end of a run, the reactor was cooled down to room temperature by water. The product gas was collected in a gas sampling bag for composition analysis by a gas chromatograph (GC) from Perkin-Elmer equipped with a packed column and a thermal conductivity detector (TCD). The end solution was filtered and titrated with dilute NaOH solution to determine the concentration of unreacted formic acid.

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EXPERIMENTAL SECTION The catalysts were prepared by the citrate gel method as previously described.17 Bi(NO3)3·5H2O (Merck) was dissolved in nitric acid. H2PtCl6·6H2O and RuCl3·xH2O, (Fluorochem) were dissolved in separate portions of water. The three metalcontaining solutions were mixed according to the designed composition of end products. Citric acid and ammonium citrate tribasic (Sigma Aldrich) were added as protecting agents. After the mixed solution was stirred for 1 h, Vulcan XC-72 carbon was added and stirring continued for another 24 h. The mixture was heated in an oil bath at 90 °C until most of the water vaporized, resulting in a semidry black powder. The gel-like powder was heated under argon atmosphere at a rate of 2 °C/ min and kept at 800 °C for 4 h. The synthesized carbon supported PtRuBiOx catalyst was characterized by transmission electron microscopy (TEM, Philips Tecnai G2 20 S-TWIN) at 200 kV. The metal content on carbon was determined by thermal gravimetric analysis (TGA, Perkin-Elmer thermal gravimetric analyzer 7) in which carbon was combusted in air. The formic acid decomposition reaction was carried out in a 100 mL Parr reactor (which schematic is shown as Figure 2) with temperature control and allowance of elevated pressure. In a typical experimental procedure, 10 mL of 15%v/v formic acid and 0.2 g of PtRuBiOx/C were added at room temperature to the autoclave of the reactor which was closed immediately with air trapped in the gas phase. The autoclave was heated externally to the target temperature with a feedback control by a thermocouple. The pressure inside the autoclave was measured by a gauge (monitored by a camera since time

RESULTS AND DISCUSSION Figure 3 shows the TEM images the PtRuBiOx catalyst supported on carbon in low magnification (Figure 3 left) and

Figure 3. TEM images of Pt/Ru/BiOx on C with atomic ratio 1:3:1 of Pt:Ru:Bi and metal loading 34%w/w.

high magnification (Figure 3 right). The metal loading on carbon was determined by TGA to be 34%w/w. The PtRuBiOx nanoparticles are uniform in size and well dispersed inside or on the surface of the porous carbon support. No free metal particles were observed outside the support. Figure 4 shows the particle size distribution of the PtRuBiOx on carbon. In the synthesis with citric acid and ammonium citrate tribasic as protecting agents, sizes of the particles were controlled to be less than 20 nm with a mean particle size of 4.2 nm. The metal 4862

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Without terminating the reaction, it is not possible to monitor the intermediate progress of the reaction via titration. It can be, however, monitored by the pressure reading.21,27 Figure 5 shows the pressure profile of the reaction at a fixed temperature ranging from 80 to 140 °C. At the end of each run, there is a final maximum pressure reading. Since 100% conversion was observed by titration in all runs, the final pressure reading corresponds to the full extent of the reaction. The initial pressure reading corresponds to zero conversion. While the initial and final pressure readings represent 0 and 100% conversion, an intermediate pressure reading can be used as a good estimation of the extent of the reaction via linear interpolation given as

Figure 4. Particle size distribution of Pt/Ru/BiOx on carbon (total no. of particles = 300).

conversion(t) =

confined on the carbon support can also prevent their growth to larger particles. The metal ratio of Ru:Pt:Bi in the catalyst measured from TEM-EDS was 3.3 ± 1.1: 1:1.7 ± 0.6. The volume area mean diameter of this sample was 7.36 nm. The calculated average metal dispersion is 16.4%. The 0.2 g PtRuBiOx/C catalyst used in the experiments corresponds to 72.6 μmol of surface metal. (Details of calculations can be found in ref 17.) Once the heating process started, the reaction pressure was increased. Small amounts of the evaporated solution and expanded air may also contribute to this pressure increment. The formic acid starts decomposing when mixing with PtRuBiOx/C. As the temperature increases, the decomposition rate increases and the gas produced results in a higher reactor pressure. The blank experiment which ran without catalyst was conducted, and blank correction was applied to all the pressure readings. The pressure reading changes throughout a reaction at a fixed temperature ranging from 80 to 140 °C are shown as Figure 5. The final conversion of formic acid was determined by

pressure(t) − pressure(t = 0) pressure(t = ∞) − pressure(t = 0) × conversion(t = ∞)

(4)

The conversion profiles at different temperatures are shown in Figure 6. High conversion can be achieved in a short period

Figure 6. Percentage of conversion were calculated from the pressure profile for the reaction runs at 80−140 °C.

of time at temperatures higher than 110 °C. Contribution to pressure rise in the reactor comes almost from gases evolved via formic acid decomposition. Calculations show that a small pressure increases can result from thermal expansion of initial gases and evaporation of liquid species. Control experiment of the same solution in the absence of catalyst confirmed a negligible increase of pressure. The final pressure containing this small pressure contribution represents 100% conversion which was determined by titration. This small pressure contribution only affects, to a very minor extent, the accuracies in determination of the intermediate conversion values. If the fraction of this pressure contribution remains the same throughout the reaction, i.e., also varies linearly, then there is no error at all in the values of intermediate conversions. The overall rate of reaction can be judged by tabulating in Table 1 the durations required to reach 95% and 100% conversions. It is shown that the time required for the last 5% conversion of formic acid is close to the time required for the first 95%. The highest average rate to reach 95% conversion is 15.9 mL H2/min at 140 °C corresponding to 234 mL H2/min per g metals and 795 mL H2/min per g Pt. The highest TOF can be achieved 548 h−1. This compares favorably with other reported rates at high temperature.31

Figure 5. Pressure profiles of the reaction runs at 80−140 °C.

titrating the solution in the reactor at the end of a run. Percentage of conversion is calculated according to the equation %conversion ⎞ ⎛ amount of NaOH titrating 1 mL of end solution = ⎜1 − ⎟ amount of NaOH titrating 1 mL of initial HCOOH solution ⎠ ⎝ × 100%

(3)

The titration results indicated >99.9% formic acid conversion in all runs. 4863

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Table 1. Summary of Reactions Ran at 80−140 °Ca temperature (°C)

time100% (h)

time95% (h)

RH2/100 (mL/min)

RH2/95 (mL/min)

dP/dt (psi/h)

TOF95% (h−1)

80 90 100 110 120 130 140

44.0 18.0 13.7 6.7 4.7 4.1 2.0

27.0 13.5 9.1 3.6 2.5 2.0 1.0

0.4 0.9 1.2 2.4 3.4 3.9 8.1

0.6 1.2 1.7 4.4 6.4 8.0 15.9

11.1 ± 0.2 23.5 ± 0.5 37.7 ± 0.9 99 ± 1 166 ± 3 306 ± 4 602 ± 7

21 41 59 152 221 276 548

a Time100% is time required for 100% formic acid decomposition. Time95% (h) is time required for 95% formic acid decomposition. RH2/100 is average hydrogen generation rate to 100% conversion. RH2/95 is the average hydrogen generation rate to 95% conversion.

In this study, we applied nonlinear regression method to minimize the objective function for the residual sum of squares (RSS)

In the present study, the increase in total pressure to a minor from steam pressure is caused by generation of CO2 and H2 from HCOOH dehydrogenation. At a constant reaction temperature, the variation of the pressure can be calculated as follows

Vg dP = RTdn

⎞2 dP dP ⎟ RSS = ∑ ⎜⎜ − ⎟ dt dt exp, n cal , n ⎠ n=1 ⎝ N ⎛

(5)

where (dP)/(dtexp,n) is the experimental value shown in Table 1, (dP)/(dtcal,n) is the value calculated in the optimization according to eq 10, and N is the total number of the experiments. The experimental dP/dt was taken from at the same percentage of conversion (20−70%) for each temperature run. In the regression, most of the terms in eq 10 are constant, Ea is determined to give the best fit or min RSS. The reaction order m is assumed to be zero as a result of insensitivity of rate to concentration as seen in Figure 6 with conversion between 20 and 70%. The activation energy was determined to be 78 kJ/mol here compared to 37 kJ/mol reported earlier under open atmosphere.17 In the previous report,17 activation energy was determined in an inverse temperature plot of initial rates in open atmosphere when the solution was not saturated with carbon dioxide. Addition activation energy is expected under conditions of present study for liberation of saturated carbon dioxide against elevated pressure in a closed vessel. More precise determination of activation energy requires knowledge of the rate law and determination of CFA during the course of reaction. It is well-known that a catalytic reaction always consists of three stages, e.g. reactant adsorption, chemical reaction, and product desorption. As discussed above, the reaction rate did not depnd on the HCOOH conversion in the range of 20− 70%, suggesting the chemical reaction is not the ratedetermining step in the present study. The product desorption can be assumed to be the key step controlling the overall reaction rate. Indeed, pressure can be supposed to only have effect on the step involving gas-phase reactant and product. In the present study, the desorption of CO2 should experience the following equilibrium

where R is the gas constant, T is the absolute temperature, and n is the number of moles of gas. If a power-law rate is assumed, the reaction rate can be described as

−

dCFA = kCFA m dt

(6)

where CFA is the concentration of formic acid, t is the reaction time, k is the reaction constant, and m is reaction order with respect to formic acid concentration. Note that the dn in eq 5 is equal to −αdnFA (1 ≤ α ≤ 2 where α = 1 means all CO2 was dissolved and α = 2 means no CO2 was dissolved) by taking the stoichiometry into account

dn = −αdnFA

(7)

−αdnFA dn = = −αdCFA Vl Vl

(8)

Thus, combining the eqs 5 to 8, we can obtain Vg dP = αRTkCFA m dt Vl

(9)

(dP)/(dt) of each run was obtained from the linear range (20− 70% conversion) of the pressure-time curve in Figure 5, and the results are summarized in Table 1. Taking the natural logarithm on both sides of eq 9 gives ln

Vg dP = ln + ln α + ln R + ln T + m ln CFA dt Vl ⎛ E ⎞ + ⎜ − a + ln A⎟ ⎝ RT ⎠

(11)

H2CO3(aq) ↔ H2O(l) + CO2 (g)

(12)

The CO2 is formed from formic acid as carbonic acid (dissolved carbon dioxide) and released by shifting the reaction 12 to the right. Under high pressure, the above equilibrium shifts to the left-hand side and CO2 is more difficult to enter the gas phase. The change in enthalpy of solution for CO2 due to pressure change at a fixed temperature is less than 0.5 kJ/mol between 1 to 10 bar.33 The enthalpy change per mole of CO2(g) liberated in eq 12 is not very sensitive to pressure since partial volume of

(10)

where Ea is the activation energy, and A is the pre-exponential factor. According to the absolute rate theory, the preexponential factor can be further expressed as A = (kBT/h) exp(ΔS ±/R) where kB is the Boltzmann constant, h is Planck’s constant, and ΔS ± is the entropy of activation. 4864

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CO2 is proportionally smaller at high pressure. Pressure may affect rate indirectly through the entropy term in the preexponential factor A of eq 10. Thermodynamics calculations show that the temperature dependent enthalpy change of liberating CO2(g) from carbonic acid has the value 27 to 34 kJ/ mol between 80 to 140 °C.34 Adding this to the initial activation energy obtained at atmospheric pressure, the overall energy is determined to be in the range of 64−71 kJ/mol, which is lower than the apparent activation energy of 78 kJ/mol determined via regression of data according to eqs 10 and 11. The discrepancy between the two values may arise from the effect of formic acid on CO2 dissolution and the ideal assumptions in eqs 7 to 10. With the PtRuBiOx/C catalyst, 100% conversion is observed even at low temperature. This complete conversion has not been reported in the literature. In some catalyst systems, formate was added to optimize reaction conditions but will limit the final conversion due to equilibrium between formic acid and formate. Complete conversion is highly desirable for formic acid to play the role of distributing renewable energy sources to convenient and clean consumption. At the end of a run, the product gas was released into a gas sampling bag for analysis. Five gas samples were collected and injected into a GC-TCD analyzer. Figure 7 shows the analyzed

Figure 8. GC analyzed CO concentrations in product gas of different reaction temperatures.

80 °C. The Parr reactor used in this work was made of stainless steel whose minor components nickel and iron may catalyze the formic acid dehydration reaction35 with generation of CO. To test the lifetime and robustness of the PtRuBiOx/C catalyst, repeated use of the catalyst was attempted for the reaction at 140 °C. At the end of a typical experiment, the product gases were released. Without taking out the catalyst for rinsing or regeneration, 1.5 mL of pure formic acid was replenished and reaction conditions return to the same initial state. Identical procedures of closing reactor, heating, pressure monitoring, and product gas analysis, were performed. The same catalyst was used six times in these experiment cycles. The conversion profiles in these runs with repeated use of the same catalyst are shown in Figure 9. There are some reductions in

Figure 7. Compositions of the end product gases from the reaction runs at 80−140 °C.

compositions of product gases for all the runs at different temperatures. The error bars indicate statistical variations of the five samples. The concentrations of hydrogen and carbon dioxide are almost identical and together constitute most of the gas contents. Only a trace amount of carbon monoxide was observed in a few runs, as shown in Figure 8. In Figure 8, the CO level varies between 0.2% to 0.7% in the temperature range of 90−140 °C. This level of CO level is closed to the experimental error in detection. Selectivity of reaction 1 over reaction 2 can be calculated as the ratio of the amount of carbon dioxide to the total amount of carbon dioxide and carbon monoxide as shown in the following equation selectivity =

amount of CO2 total amount of (CO2 + CO)

Figure 9. Conversion of formic acid at 140 °C in successive runs with repeated use of catalyst.

reaction rates and reduced performance of catalyst in the second and third runs. Subsequent runs up to the sixth, however, showed very steady rates and consistent performance of the catalyst, indicating its robustness for continuous usage. The slight deactivation may be due to the formation of equilibrated fraction of bismuth oxide, bismuth bicarbonate, and bismuth hydroxyl carbonate and loss of fresh active site. One distinct advantage of the heterogeneous PtRuBiOx/C catalyst is that the reaction experiment can be repeated simply by replenishing formic acid. On the other hand, there are needs of catalyst extraction and separation from end solutions in many cases of homogeneous catalysts. The durability and

(13)

The PtRuBiOx/C shows high selectivity (>0.981) toward dehydrogenation. In the earlier reported work17 using the same PtRuBiOx/C catalyst, CO was not detected by GC-TCD which generally has a CO detection limit of 15 ppm. Previous experiments17 were performed in an open glass flask reactor at 4865

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performance of the PtRuBiOx/C catalyst under extended reaction conditions have been well demonstrated. The catalyst was used repeatedly at elevated temperatures up to 140 °C and pressures up to 350 psi.

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CONCLUSIONS Formic acid decomposition on PtRuBiOx/C in aqueous solution was studied from 80 to 140 °C in a closed reactor. Compared to operating temperatures below the boiling point of formic acid (∼100 °C) in open pressure, a significantly improved hydrogen generation rate was exhibited at high temperature and pressure, while maintaining a similar selectivity without dehydration decomposition of formic acid. The observed activation energy of 78 kJ/mol was higher than the previous value of 37 kJ/mol due to additional enthalpy for liberating dissolved carbon dioxide. Repeated runs of the used catalyst indicated the robustness and durability of the catalyst at elevated temperatures and pressures. The results in this study would ensure a rational design of moderate-pressure hydrogen feed for a fuel-cell stack without additional compressor.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +852 28597917. Fax: +852 28571586. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from The Hong Kong Research Grants Council GRF HKU 700208P, “Initiative on Clean Energy and Environment” of The University of Hong Kong Development Fund, and the HKU Strategic Research Theme on “Clean Energy Research” are acknowledged.

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