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

https://doi.org/10.1016/j.cej.2022.140645Get rights and content

Highlights

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.

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 gH2 L−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].HCOOHCO2+H2HCOOHH2O+CO±

An optimal process for hydrogen generation must be selective to dehydrogenation and inhibit the dehydration. Using aqueous media at mild temperatures (around 50 °C) or a selective catalyst in gas phase can control the reaction products by suppressing the dehydration reaction [4], [5], [6], [7], [8].

One of the most reported systems for liquid or gas phase formic acid decomposition are the palladium based systems [9], [10], [11], [12], [13], [14]. Nevertheless, this metal suffers deactivation and its activity depends greatly on the nature of the used support [15], [16], [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], [25], [26], [27], [28]. 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)).HCOOHaqHCOOaq-+Haq+HCOOaq → CO2 + H+aq

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

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.

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.

Section snippets

Catalysts and chemicals

Commercial palladium on activated carbon catalyst, purchased by Sigma Aldrich (10 wt% Pd/AC), was used in this study. Prior activity measurements, the sample was dried at 100 °C overnight and activated in reducing atmosphere, (N2/H2, 1:1) during 2 h at 300 °C.

For the dehydrogenation experiments, 100 mL of 1 M 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,

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 the (1 1 1) diffraction as well as the evaluation of Lc parameter and R

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

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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.

References (43)

  • H. Takagi et al.

    XRD analysis of carbon stacking structure in coal during heat treatment

    Fuel

    (2004)
  • N. Iwashita et al.

    Specification for a standard procedure of X-ray diffraction measurements on carbon materials

    Carbon N Y

    (2004)
  • Q. Li et al.

    Investigation on the structure evolution of pre and post explosion of coal dust using X-ray diffraction

    Int. J. Heat Mass Transf.

    (2018)
  • S. Ivanova et al.

    Preparation of alumina supported gold catalysts: Influence of washing procedures, mechanism of particles size growth

    Appl. Catal. A

    (2006)
  • J.L. Santos et al.

    Structure-sensitivity of formic acid dehydrogenation reaction over additive-free Pd NPs supported on activated carbon

    Chem. Eng. J.

    (2021)
  • L. Jia et al.

    Hydrogen production from formic acid vapour over a Pd/C catalyst promoted by potassium salts: Evidence for participation of buffer-like solution in the pores of the catalyst

    Appl Catal B Environ

    (2014)
  • A. Bulut et al.

    Pd-MnOx nanoparticles dispersed on amine-grafted silica: Highly efficient nanocatalyst for hydrogen production from additive-free dehydrogenation of formic acid under mild conditions

    Appl Catal B Environ

    (2015)
  • M. Navlani-García et al.

    New approaches toward the hydrogen production from formic acid dehydrogenation over pd-based heterogeneous catalysts

    Front. Mater.

    (2019)
  • M. Grasemann et al.

    Formic acid as a hydrogen source – recent developments and future trends

    Energ. Environ. Sci.

    (2012)
  • K.V. Kordesch et al.

    Environmental Impact of Fuel Cell Technology

    Chem. Rev.

    (2002)
  • Z. Li et al.

    Tandem Nitrogen Functionalization of Porous Carbon: Toward Immobilizing Highly Active Palladium Nanoclusters for Dehydrogenation of Formic Acid

    ACS Catal.

    (2017)
  • Cited by (9)

    View all citing articles on Scopus
    View full text