Low CO2 hydrogen streams production from formic acid through control of the reaction pH

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in Chemical Engineering Journal on 1 January 2023, available at: https://doi.org/10.1016/j.cej.2022.140645 .” 1 Low CO2 hydrogen streams production from formic acid through control of the reaction pH José Luis Santos*,a, Estela Ruiz Lópeza, Svetlana Ivanovaa, Antonio Monzónb, Miguel Ángel Centenoa and José Antonio Odriozolaa. aDepartamento de Química Inorgánica e Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, 41092 Sevilla, Spain bInstituto de Nanociencia de Aragón (INA) e Instituto de Ciencia de Materiales de Aragón (ICMA), Universidad de Zaragoza-CSIC, 50018 Zaragoza, Spain *corresponding author: [email protected] Abstract There are multiple factors that influence the catalyst performance in the reaction of formic acid dehydrogenation: the nature of catalyst and/or support, the used solvent and reaction variables such as temperature, time, formic acid concentration, presence/absence of formates and pH of the solution. This work evaluates a series of important parameters like the influence of the pH by itself, the influence of the nature of used alkali agents and the effect of direct formate addition as reactive on hydrogen production via formic acid dehydrogenation over a commercially available catalyst. The catalytic performance appears to depend on the ionic radius of the cations of the used base which reflects consequently on the hydrogen selectivity. The best base to be used must have lower hydrated cationic radii and a starting pH around 4 to achieve important hydrogen selectivity for medium term formic acid conversion. Keywords: Formic Acid, pH control, cleaner hydrogen streams, formate, CO2 retention and Pd catalyst. 2 1. Introduction One of the major challenges to establish an up-to-date hydrogen economy is the difficulties encountered in its storage and transportation. Recently, as a safer option for chemical storage appears the use of liquid organic hydrogen carriers (LOHCs) [1,2]. Within this group of compounds, the formic acid (FA) is one of the most promising due to numerous features such as, low toxicity, non-flammability, stability at room temperature, high volumetric H2 capacity (53 gH2L-1) and possibility to be produced through biomass-based processes [1]. Hydrogen generation from formic acid is based on its decomposition and lead either to H2 and CO2 production via dehydrogenation process (Eq. 1) or to H2O and CO production via dehydration reaction (Eq. 2) [3]. HCOOH → CO2 + H2 (1) HCOOH → H2O + CO (2) An optimal process for hydrogen generation must be selective to dehydrogenation and inhibit the dehydration. Using aqueous media at mild temperatures (around 50 ºC) or a selective catalyst in gas phase can control the reaction products by suppressing the dehydration reaction [4–8]. One of the most reported systems for liquid or gas phase formic acid decomposition are the palladium based systems [9–14]. Nevertheless, this metal suffers deactivation and its activity depends greatly on the nature of the used support [15-17] on the presence of secondary active metal such as Ag [18], Au [19, 20], Co [21] or Ni [22], and on support functionalization [15,18,20,21,23]. An important improvement can be also achieved by replacing the formic acid with mixed formic acid/formate salts solutions [2,15,24–28]. 3 Kim et al. [26] observed this positive effect and attributed it to a synergetic common ion effect. This effect consists in formate intermediates stabilization on Pd surface via H- bonding in presence of formic acid. In addition, a conjugated acid-base pair buffer system is formed, where the common ions help the protons refilling via formic acid dissociation thus assuring the pH control via Henderson-Hasselbalch equation (Eq. 3). HCOOHaq ⇄ HCOO-aq + H+aq (3) HCOO-aq → CO2 + H+aq (4) There are two different routes for formic acid dehydrogenation, the direct or formate route, where the formic acid dissociates to formate via eq. 3 and then to CO2 via eq. 4 and indirect formic acid dehydrogenation. The latter consists in a first step formic acid dehydration to CO (eq. 2) and its hydration to CO2 and H2 via water gas shift reaction Eq. 5 [29]. CO+H2O ⇄ CO2 + H2 (5) Nevertheless, it could be considered that in aqueous media the dehydration route is completely unfavorable. On the other hand, if we consider the formate route as the preferred one, it must be influenced by the changes in pH and formate concentration [2,26,27]. What is more, any control of the pH should be able, in principle, to orient the reaction via one or another pathway. Léval et al. [30] reported that in aqueous solution, formic acid consumption leads to pH increase and drop in activity (following the eq. 2 equilibrium) evited by varying the initial pH to a value where the catalyst remains active. Jeon et al. [8] found that the surface charge of catalyst also has important influence on the reaction pH. They conclude that the presence of different functional groups on the catalyst surface and their behavior in aqueous solution determine the reaction pH and activity in a volcano-type like dependence. 4 The pH variation could also influence the CO2 solubility independently on formic acid dehydrogenation pathway via bicarbonate/carbonate equilibrium [31]. One could imagine that, an optimal pH exists where the H2/CO2 ratio reach a maximum to produce greener hydrogen from formic acid. And this is the motivation to study the influence of the reaction pH over hydrogen production via formic acid dehydrogenation evaluating the effect of i) the pH by itself; ii) the nature of the alkaline solutions used to change it and iii) the influence of the formate salt addition to H2 generation. The study also includes the pH effect on the CO2 distribution. As a way to minimize as much as possible, the catalysts surface contribution to the reaction pH, the whole study is carried out over a commercial sample of carbon supported palladium. 2. Experimental 2.1. Catalysts and chemicals Commercial palladium on activated carbon catalyst, purchased by Sigma Aldrich (10wt.% Pd/AC), was used in this study. Prior activity measurements, the sample was dried at 100 ºC overnight and activated in reducing atmosphere, (N2/H2, 1:1) during 2 h at 300 ºC. For the dehydrogenation experiments, 100 mL of 1M formic acid aqueous solution (HCOOH, Sigma Aldrich, ACS reagent > 96%) was used. The reaction pH was set initially at 2.2 (average pH of 1 M formic acid solution) and then changed to 2.8, 3.5, 4.5 and 6.5 values obtained after addition of a few drops of highly concentrated alkaline solutions (10 M). With this, the total volume of HCOOH remain unchanged, keeping the initial formic acid concentration practically constant. Lithium hydroxide (LiOH, ACS reagent, >98%, Sigma Aldrich), sodium hydroxide (NaOH, ACS reagent, >97%, Sigma 5 Aldrich), potassium hydroxide (KOH, ACS reagent, >85%, Sigma Aldrich) and ammonium hydroxide (NH4OH, ACS reagent, 28-30% NH3 basis, Sigma Aldrich) were used for the study. 2.2. Characterization methods XRD measurements were performed on X’Pert Pro PANalytical diffractometer equipped with Cu anode working at 45 kV and 40 mA. The diffractograms were recorded from 10 to 90° 2θ with 0.05° step size and 300 seconds step time. The structure/phase determination was performed by comparison with PDF2 ICDD2000 (Powder Diffraction File 2 International Center for Diffraction Data, 2000) database. The graphitization degree of the sample was estimated by calculating the stacking height parameter (Lc) using the empirical Scherrer equation [32,33] (Eq. 6) and the R-value defined graphically according to Li et al. [34]. Lc = Kc·λ β002·cos𝛳002 (6) where λ is the X-ray wavelength (1.5406 Å for Cu Kα radiation); ϴ002 is the position of the (002) diffraction; β002 is the full width at half maximum (FWHM) of the (002) diffraction and Kc is a constant depending on the X-ray reflection plane (0.89 for (002)). The empirical R-value is defined as the ratio of the (002) relative intensity to the background at the same scattering angle. Lower R-values indicate higher concentrations of single layers and R = 1 the absence of (002) diffraction in the XRD pattern. The textural properties were determined by N2 physisorption at -196 ºC in Micromeritics Tristar II equipment. Prior to N2 adsorption, the samples were degassed at 350 ºC for 12 hours. The BET method was used to obtain the specific surface area from adsorption isotherm while pore sizes and distribution were calculated from the desorption part using BJH method. 6 The real metal loading was measured by inductively coupled plasma optical emission spectroscopy (ICP-MS) using a Spectro ICP spectrometer (Model: ULTIMA 2). TEM micrographs were acquired in a FEI Talos electron microscope equipped with a Field Emission filament operating at 200 kV. Digital images were taken with a side mounted Ceta 16M camera. A few milligrams of the sample have been deposited directly on 300 mesh holey carbon coated copper TEM-grid and introduced in the microscope. The mean particles size (Dp) was estimated from the HR-TEM micrographs by single particle measurements over at least 200 particles and considering the surface distribution calculations (Eq. 7). Dp= ∑ni di 3 ∑ni di 2 (7) where di is the geometric diameter of the ith particle, and ni the number of particles with this diameter. Metal dispersion was calculated from the average metal particle size determined by TEM and the metal loading by using a mathematical model for cuboctahedra particles [35]. 2.3. Catalytic set up The formic acid dehydrogenation reaction was carried out in a four-neck round glass semibatch reactor (250 mL) equipped with gas inlet/outlet (100 mL min-1 N2 flow, used as carrier), pH meter, automatic burette and cooling system, Figure 1. 7 Figure 1. 3D schematic diagram of formic acid dehydrogenation set up. In a typical experiment, the reactor was first charged with distilled water (100 mL) and catalyst (0.1 g) under continuous stirring (1036 rpm) until the temperature rise up to 50 ºC under flowing nitrogen (100 mL min-1). Once the temperature reached the selected value, 4.7 g of formic acid (purity 98%) was added to fix the initial concentration to 1 M at zero time of reaction. The pH of the reaction was adjusted by using a 10 M aqueous solution of different bases as pH regulating agents. The working pH values (indicated with red stars) have been selected according to the CO2-HCO3--CO32- species distribution diagram, Figure 2, in order to obtain different coexisting species in the solution. This pH also reflects the distinct formic acid/formate distribution obtained after the addition of the base. Figure 2. Distribution diagram of CO2 species in water [31]. 8 The high concentration of base solution (10 M) was selected on purpose to maintain the initial total reaction volume (100 mL) unaltered by needing only few mL of the solution to achieve the desired pH. The pH was followed-up with the help of pH meter as shown in Figure 1. The resulting gas mixture was analyzed online using Varian CP-4900 gas chromatograph (H2 and CO using N2 as internal standard) coupled to an external CO2 analyzer, Vaisala MI70 (0.2 vol.% accuracy). The overall reaction time was fixed to 150 min. Formic acid conversion was calculated considering the experimental and theoretical moles of produced hydrogen (Eq. 8), the first being calculated taking the total cumulative hydrogen flow and assuming the generation of one per one mole of H2 and HCOOH. In addition, the catalytic activity was compared with the literature and discussed in terms of turnover frequency (TOF, Eq. 9) where the moles of exposed Pd were calculated based on the real Pd moles multiplied by the metallic dispersion obtained after TEM analysis and calculation. HCOOH conversion (%) = moles of produced H2 theoretical moles H2 ·100 (8) TOF (h-1) = moles of produced H2 (moles of exposed Pd)·h (9) 3. Results and Discussion 3.1. Catalyst characterization The commercial Pd catalyst showed a metal loading slightly inferior to the nominally stated (Table 1) and the X-Ray diffractogram of the reduced catalyst (Figure s1) shows the presence of amorphous carbon (2ϴ = 25°, 44° and 80°) and palladium metal (ICDD # 00-004-0784) with broad diffractions at 2θ ≈ 40.1°, 46.7°, 68.1°, 82.1° and 86.6°. The XRD data allows the calculation of Pd average size using Debye-Scherrer equation over 15 The ammonium behavior, although fitting perfectly into the trend, must be treated carefully; ammonium ion can participate directly in the reaction increasing the hydrogen production and the H2/CO2 ratio, by ammonia decomposition. It has been reported, that amine group presence could facilitate formic acid dehydrogenation through O-H bond dissociation and proton elimination creating basic environment around the metal catalyst. Based on the proposal reaction pathway, the latter would lead to faster formate C-H bond cleavage over the metal catalyst and higher H2 release [43,44]. In addition, the formation of the formate/formic acid complexes would become more difficult thus decreasing the rate of catalyst inhibition. As mentioned previously, highly alkaline pH becomes undesirable due to the competitive catalyst inhibition. Nevertheless, the alkaline pH could affect interestingly the volumetric H2 and CO2 production and their corresponding ratio, presented in Figure 6. In the case of working pH of 3.5, the volumetric ratio values are close to one, as it should be if only the dehydrogenation reaction takes place. For the other working pH (6.5) after the first 30 min a fast increase of pH around 9 deviates the H2/CO2 ratio from the stoichiometric value to 15 in steady state. On the other hand, at alkaline pH the hydrogen production diminishes in comparison to the flow observed at pH 3.5. Considering above, it is worth to represent the hydrogen selectivity versus HCOOH conversion for all samples (Figure 6b). The highest selectivity to H2 is produced at lower conversions and mainly at higher pH, while the opposite is true for the lowest measured pH. As an optimum condition, the one measured at 6.5 and using KOH as a base can be selected. The presence of K+ ions promote not only the conversion but also the hydrogen selectivity. 16 Figure 6. a. H2/CO2 volumetric ratio at initial pH of 3.5 and 6.5 and b. HCOOH conversion vs. H2 selectivity after 2.5 h as a function of the used bases (same color code as for a). Another way to compare is the plot of H2 or CO2 cumulative volume (curves as a function of time included as Figure S3.) versus the experimental H2/CO2 ratio, Figure 7. It can be seen, that at working pH close to 3.5 the H2/CO2 ratios remain close to unity. However, the increase of the pH leads to a positive deviation due to i) a possible failure in our CO2 measuring equipment at low concentrations; to ii) an overproduction of H2, or to iii) CO2 retention in the solution media. a. b. 17 Figure 7. a. H2 cumulative volume and b. CO2 cumulative volume vs. H2/CO2 ratio. Although a hydrogen overproduction might be expected for the use of ammonium, it is shown that the H2/CO2 ratios for this experiment at both pH 3.5 and 6.5 are between 1 and 2. This would indicate that an overproduction of H2 by ammonia decomposition can not occur. For the rest of the species measured at pH 6.5, no extra hydrogen production would be expected, due to the lack of hydrogen donor groups (NaOH, LiOH and KOH). In addition, no clear linear dependence can be found at pH 6.5 for the H2 production indicating the absence of hydrogen overestimation in these conditions (Figure 7a). On the other hand, a decreasing trend in CO2 production with the H2/CO2 ratio is found (Figure 7b). Apparently, the use of bases such as NaOH, KOH or LiOH almost a. b. 18 completely decreases the CO2 production. The supposed problems caused by low concentration CO2 detection can be also ruled out, since similar H2 yields (KOH at pH 6.5 and NaOH at pH 3.5) lead to a very different CO2 yields. Thus, the deviation of the H2/CO2 ratio from the unity could be only due to a CO2 adsorption in the reaction media and its adsorption capacity at different pH is further evaluated. 3.2.3. pH effect on CO2 adsorption For these experiments, the pH of the formic acid solutions is adjusted by adding concentrated ammonia to 3.5 and 7 and once the pH reached, a continuous CO2 gas flow of 5 mL min-1 is passed through (without catalyst) the reaction media monitoring the pH (Figure 8). Figure 8. pH effect on CO2 adsorption. While the CO2 gas flow did not perturb the reaction solution at pH 3.5, the 7-pH solution suffered some changes. During the first 10 minutes, in absence of CO2 flow the pH of the solution remain constant (≈ 7.1) and decreased thereafter to 6.4 with the CO2 introduction. A variation of the CO2 concentration at the inlet and outlet were also observed (lower 19 outlet that inlet values). The solubility of CO2 in aqueous phase at acid pH obeys the Eq. 12 and two species can be found, dissolved CO2 and carbonic acid H2CO3 (Figure 2). CO2 + H2O ⇄ H2CO3 (12) However, at moderate pH (above 6.4), carbonic acid decomposes to bicarbonate as follows (Eq. 13), thus decreasing the pH by liberation of protons H2CO3 ⇄ H+ + HCO3 - (13) This reaction suggests that at pH close to neutral the dissolved CO2 as carbonic acid deprotonates and the liberation of protons decreases the pH, as in our case. These experiments prove that a tuning of the initial reaction pH could affect the CO2 solubility and as a consequence the obtained H2/CO2 ratio. In that sense, the use of alkaline media would allow CO2 retention and the production of green H2 directly from formic acid. 3.2.4. Influence of the nature of the formate: stability of sodium and ammonium formates Since the catalytic behavior is associated to the formate ion generation and relation to the formic acid the reaction was studied starting with formates directly. As a reaction solution, 1 M sodium and ammonium formate (100 mL) solutions were used in the same conditions as formic acid. The obtained results are shown in Figure 9. Clearly, the use of formates improves the hydrogen production. The reaction mechanism suggests that the addition of formate (considered as intermediates) improves the rate of formic acid dehydrogenation and it is reported in several studies of formic acid –sodium formate solutions (FA-SF) [2,12,15,16,24-28,45]. Kim et al. [2] concluded that sodium ion has no effect on catalytic activity, suggesting that only the increase of HCOO- concentration affects positively catalyst behavior. This statement was checked by measuring the 20 response of the generated hydrogen to an increase in FA and SF concentrations. Indeed, the hydrogen flow increases with the formate concentration, confirming that its dehydrogenation is improved, and that the continuous generation of hydrogen passes preferentially through formate as intermediate. Moreover, it was found that the total amount of hydrogen produced from these buffer solutions was higher than expected considering the moles of the used formic acid, i.e. the surplus of hydrogen proceeded naturally from the formate contribution [15]. What is also interesting is the fact that the influence of the initial concentration of formic acid on hydrogen generation was far lower than that observed for formate ions [2,12]. In addition, some other authors suggest the formation of hydrogen-bonded complexes of formic acid and formate that help to stabilization of the latter in a monodentate configuration facilitating the dehydrogenation reaction [12,26]. Nevertheless, their stabilization in bidentate mode results in the opposite effect, the reaction inhibition [38,39]. Thus, the importance of the HCOO- role in the reaction of formic acid dehydrogenation relies on four aspects: (i) formate ion has been described as the active intermediate [12,16] and its presence forces the active adsorption of HCOO- and formic acid on the catalyst surface in a special configuration which facilitates the dehydrogenation process; (ii) the presence of HCOO- favors the total reduction of palladium into metallic palladium being the formate stronger reducing agent than formic acid; (iii) the ammonium formate (AF) addition decreases formic acid acidity and improves Pd catalyst stability and (iv) HCOO- ion is more active species than formic acid decomposing rapidly in H2 and CO2, as observed in Figure 9. 21 Figure 9. Catalytic stability for a. Hydrogen (circles) and CO2 (squares) production in sodium formate, ammonium formate and formic acid solutions and b. H2/CO2 volumetric ratio. Both, sodium and ammonium formate dehydrogenations present higher hydrogen generation especially at longer reaction times (above 1.5 h). The TOF values obtained for the two formates (Figure S4), after 7.5 h. of reaction were close to 120 h-1, trice the obtained value for formic acid (≈ 40 h-1). Comparing the two formates, sodium formate showed a slightly lower activity being the only difference the presence of NH4+ instead of Na+. Therefore, the higher activity must be attributed to the role of NH4+. Although some authors declared that the presence of sodium had no effect on activity, other authors pointed out that NH4+ could facilitate the hydrogen production as a good H+ proton donor a. b. 22 agent. This could justify our observation of catalytic activity improvement in presence of ammonium instead of sodium formate. The H2/CO2 volumetric ratio is also shown in Figure 9b. While for ammonium formate and formic acid the ratio is close to 1 a higher H2/CO2 volumetric ratio is observed for sodium formate. Although less active the sodium formate results in the production of greener hydrogen being the H2/CO2 ratio higher to that obtained by the other reactive. We can consider that the pH of an aqueous solution of sodium formate is above 7, and that the sodium cations dissociated in the medium according to Eq. (14) lead to a further hydrolysis of the formate ions and pH increase like in Eq. (15): NaCOOH (aq) Na+ (aq) + HCOO- (aq) (14) HCOO- (aq) + H2O (aq) HCOOH (aq) + OH-(aq) (15) So, on this stage, it would be possible that the CO2 produced by decomposition of HCOOH became retained in the aqueous solution forming sodium bicarbonate or carbonate species as a function of the operating pH, Figure 2. Consequently, an increase in the H2/CO2 ratio occurs, making our hydrogen stream much cleaner. 4. Conclusions Different alkaline solutions were used to adjust the reaction pH (KOH, NaOH, LiOH and NH3) in the reaction of formic acid dehydrogenation over commercial catalyst. An inversely proportional dependence on the hydrated cationic radius over the activity is observed for the series of alkali metals. The higher polarization potential of potassium reflected in higher ionic and smaller hydrated ion radii resulting in stronger Coulomb interactions with the catalyst surface and therefore highest activity. What is more, the formation of formates appears easier for the potassium bases and results more effective 23 H2 formation. In the case of NH4OH solution, the observed enhancement of the catalytic activity could be due to the ability of the amine groups to eliminate the possible protons present in the support. It is also observed that the control of the pH reaction can shift the gas production selectivity. The use of alkaline pH allows a partial retention of the produced CO2 as bicarbonate generating greener hydrogen stream. When using NH4+ the overall production of hydrogen increases due to the H+ proton donor characteristics of the base. On the other hand, the use of potassium base, results in slightly lower hydrogen production but appears to establish better formic acid/formate relation maintaining the solution potential optimal for a continuous hydrogen formation showing high H2 selectivity and formic acid conversion. 5. Acknowledgements Financial support was obtained from Spanish Ministerio de Ciencia e Innovación (MCIN/AEI /10.13039/501100011033/) and for FEDER Funds, Projects ENE2017- 82451-C3-3-R and PID2020-113809RB-C32. Also the financial support from Junta de Andalucía via Consejería de Transformación Económica, Industria, Conocimiento y Universidades and its PAIDI 2020 program (Grant P18-RT-3405) co-financed by FEDER funds from the European Union is welcomed. 6. 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