3.1. Monometallic Heterogeneous Catalyst
Several studies have been published recently on monometallic noble metal nanoparticles (NPs) based on different catalysts for FA dehydrogenation in an aqueous medium
[62,74,75,76][48][49][50][51]. Bi et al.
[77][52] used hyper-dispersed subnanometric gold NPs on ZrO
2 as catalysts to show the moderate and selective dehydrogenation of an FA/amine mixture. Under ambient conditions, the catalytic processes happen effectively and selectively (100%), with high TOFs/catalyst turnover numbers (TONs), and without generating any undesirable by-products such as CO. At ambient temperature, very efficient hydrogen was obtained from the production, from aqueous solution, of formic acid/sodium formate catalyzed via in situ-produced Pd/C with citric acid. Surprisingly, the addition of citric acid in the middle of the synthesis and development of Pd NPs on carbon improves the catalytically activity of the resultant Pd/C, on which the greatest conversion and turnover frequency for the breakdown of formic acid/sodium formate system can be achieved at ambient temperature
[78][53].
3.2. Bimetallic Heterogeneous Catalyst
It is generally known that adding a secondary metal to the active phase can change the electrical characteristics and adsorption behavior and the metal dispersion/particle size. Presently, supported Pd-based nanocatalysts are shown to be active for the dehydrogenation of aqueous FA
[62,63,64,65,78][48][54][55][56][53]. Adding Au or Ag to Pd NPs in an aqueous medium significantly enhances their stability and catalytic activity
[35,66,67,68,69][35][57][58][59][60]. The enhanced catalytic activity of bimetallic Pd-Au/C and Pd-Ag/C catalysts was attributable to the greater tolerance to CO poisoning of Ag and Au. The addition of CeO
2(H
2O)x increased catalytic activity even more because CeO
2 forms cationic palladium species with strong activity in CO oxidation
[80][61] and methanol decomposition
[81][62]. An alternative justification is that CeO
2(H
2O)x on the Pd surface can trigger FA breakdown via a more effective mechanism, resulting in less poisoning intermediates
[82][63].
3.3. Trimetallic Heterogeneous Catalyst
Trimetallic NPs have lately acquired increased attention, particularly in catalytic systems, because of their novel physicochemical features (e.g., catalytic, electrical, optical, and magnetic) which are caused by their monometallic counterparts’ synergistic effects
[89][64]. Yurderi et al. used a simple and repeatable wet impregnation followed by a simultaneous reduction method at room temperature to synthesize Pd-Ni-Ag (trimetallic nanoparticles) with various metal ratios, as well as their Pd-Ni, Ni-Ag, and Pd-Ag (bimetallic) and Pd, Ni, and Ag (monometallic) counterparts, loaded on active carbon
[72][65]. Under mild reaction conditions, all composites produced were used as nanoheterogeneous catalysts to break down FA.
4. Formic Acid Fuel Cells (DFAFCs)
FA’s hyper-gravimetric capability was recognized, and using FA as a secondary fuel in direct FA fuel cells (DFAFCs) was suggested and investigated
[90][66]. While DFAFCs suffer from significant problems, hydrogen fuel cells perform a role in a well-established technology that has been commercialized in fuel cell vehicles (FCVs) with outputs of over 140 kW and ranges of over 600 km. As a result, producing H
2 selectively from FA to power hydrogen fuel cells is a potential strategy with a speedy time-to-market. As a traditional fuel, energy discharge includes FA consumption, resulting in a massive release of CO
2 (
Figure 73).
Figure 7. Basics of formic acid fuel cell and hydrogen energy.
Direct formic acid fuel cells work in the same way as other fuel cells. They create electricity by oxidizing FA and reducing O
2. FA and O
2 (or air) are supplied to the anode and cathode, respectively, in the electrochemical cell. Protons can pass across an electrolyte membrane
[91][67].
5. Formic Acid Production
Formic acid is a crucial component created from a variety of chemical molecules. FA is found in the venom of ants in nature
[92][68] and is emitted into the atmosphere as a result of forest emissions. Various chemical techniques can be used to prepare it. The most frequent industrial procedure is the synthesis of methyl formate from a mixture of carbon monoxide and methanol in the existence of a strong base at 80 °C and 40 atm, followed by hydrolysis of the methyl formate to yield FA
[114][69]. FA can also be synthesized as a by-product of acetic acid production, biomass oxidation, CO
2 hydrogenation and biosynthesis via carbon dioxide reduction mediated by the enzyme formate dehydrogenase
[115,116][70][71]. Despite the enormous progress that has been made in nano-chemistry and nanotechnology over the last two decades, and that forming FA from carbonates (primarily Pd-based) has been a subject of research for some time, few instances of supported metal catalysts at the nanometer scale for direct hydrogenation of carbon dioxide have been reported. These primarily involve Au, Pd, and Ru.
6. Conclusions
The benefits of a hydrogen economy are obvious, even if significant research is required to accomplish the essential technological advancements. Formic acid is an environmentally-benign hydrogen storage substance because of its easy storage and lack of poisonousness. Its production through dehydrogenation releases only gaseous products (H
2/CO
2). Interestingly, CO
2 can be converted back to formic acid using catalysts under moderate conditions, resulting in a CO
2-neutral hydrogen storage cycle. Noble metals, such as Au and Pd, can serve as nanoheterogeneous catalysts that work in aqueous formic acid solutions and ambient temperature (20–50 °C). The Pd nanoparticles are employed in most nanoheterogeneous catalysts used in the formic acid dehydrogenation process. However, chemical intermediates adsorb on the nanoparticle surfaces and deactivate Pd monometallic systems. The situation with heterogeneous formic acid decomposition catalysts is identical to that with homogeneous systems. While the activity and H
2 selectivity have not yet been achieved homogeneous system levels and most heterogeneous systems tested still have some degree of decarbonylation activity, this gap is narrowing. The recent use of state-of-the-art nanoparticle synthesis techniques has resulted in a variety of high-performance catalysts, including bimetallic and trimetallic Pd and Au combinations that produce high-quality H
2 with minimal CO concentration. The direct formic acid fuel cell (DFAFC) example marks significant progress toward prototype development, scale-up, and commercialization. Furthermore, CO
2 may be converted back to formic acid using catalysts under moderate conditions, resulting in a CO
2-neutral hydrogen storage cycle.