Full Length ArticleTheoretical investigations of HCOOH decomposition on ordered Cu-Pd alloy surfaces
Graphical abstract
Introduction
Formic acid (FA) is considered as one of the most promising materials for hydrogen storage in direct liquid fuel cells (DLFC) [1], [2], [3]. Despite the gravimetric and volumetric hydrogen capacities of formic acid (HCOOH) are 4.4 wt.% and 53.4 g/L, respectively [4], [5], it shows several merits comparing to the other hydrogen-containing liquid fuels (e.g., methanol). For instance, formic acid is nontoxic, and H can be produced through the selective dehydrogenation of formic acid on electrode catalyst at low temperature. Generally, the oxidation reaction of formic acid on anodic catalysts of fuel cells follows the so-called dual pathways [6], i.e., direct pathway (HCOOH → CO2 + 2H+ + 2e−) and indirected pathway (HCOOH → CO + H2O). Moreover, formic acid can be regenerated by hydrogenation of the by-product CO2 formed in dehydrogenation pathway [7]. Therefore, direct formic acid fuel cells (DFAFC) is a safe and renewable energy carriers, which can be equipped in the portable devices, vehicles and other energy-related appliances.
It is crucial to design a novel anodic catalyst with highly catalytic activity and selectivity toward HCOOH electrooxidation in the development of DFAFC. To understand the intrinsic chemical properties of catalysts, theoretical and experimental investigations have been carried out about the HCOOH decomposition on metal surfaces [8], [9], [10], [11], [12], [13], [14], [15], [16]. This decomposition process follows two reaction pathways of HCOOH → CO2 + H2 (dehydrogenation pathway) and HCOOH → CO + H2O (dehydration pathway). On Pt and Pd catalyst, HCOOH is decomposed via both dehydrogenation and dehydration pathways [11], [12], [13], [14]. Currently, the rational design of multicomponent Pt- and Pd-based alloy catalysts arounds great interests through adjusting the composition and atomic arrangements of reactive sites [17], [18], [19], [20], [21].
According to the nature of atomic ordering, the alloy catalysts can be classified into two categories: disordered solid-solution alloys and ordered intermetallic compounds. The substitutional solid-solution alloys by incorporating of minor Pt or Pd metals into less active metal (e.g., Au, Ag, and Cu) can significantly enhance the catalytic performance of Pt and Pd catalysts by “ligand effect” [22], [23] and “ensemble effects” [24], [25]. It was reported that the small Pt or Pd ensembles containing several metal atoms incorporated into Au surface show optimum catalytic selectivity for the direct oxidation of formic acid [15], [16], [17], [18]. The Pt or Pd atomic ensembles neighbouring Au atoms can efficiently inhibit CO production in HCOOH dehydration pathway. In particular, single isolated Pt atom incorporated into Ag(1 1 1) and Cu(1 1 1) surfaces can promote the selectivity of OH bond breaking in HCOOH dehydrogenation comparing to pure Pt(1 1 1) [20], [21]. However, it is a challenging task to experimentally control the atomic composition and geometric arrangements of random alloy catalysts on surface.
Recently, the ordered intermetallic alloy nanocrystals with a specific crystal phase and electronic structure are considered as highly effective catalysts [26], [27], [28], [29]. Great efforts have been performed to screen the suitable ordered Pd- or Pt-based intermetallic alloy catalysts, such as PtX (X = Bi, Pb, Pd, Ru) [30], [31], Pt3Pb [32], PdSn [33], PtTe [34], Pt3Ni [35], and PtZn [36], toward semihydrogenation of alkynes, dehydrogenation of saturated hydrocarbons and oxygenates, steam reforming of methanol, and FA oxidation. The atomic composition and geometric distribution of surface can be changed by forming different crystal phases of intermetallic alloys. Hafner et al. predicted that an isolated Pt/Pd atom shows highly catalytic activity for hydrogenation of ethylene on ordered Ga-Pd alloys by DFT calculations [37], [38]. Also, Pt-Bi intermetallic alloys possess superior electrocatalytic activity toward the oxidation of FA as well as the high tolerance against CO, and the selectivity of direct pathway in FA electrooxidation can be improved by decreasing the number of the continuous neighbouring Pt atoms [31]. On Ni-rich surface of Pt3Ni catalyst, HCOOH electrooxidation follows the direct pathway mechanism (HCOOH → CO2 + 2H+ + 2e−) [35]. Remarkably, Matsumoto et al. found that the catalytic activity of Pt3Pb with Cu3Au-type structure is approximately twofold higher than that of PtPb with NiAs-type structure for FA oxidation [39]. Recently, Norkskøv et al. predicted that Cu3Pt alloy is a potential catalysts for FA decomposition by theoretical calculations [10]. However, comparing to the disordered alloys, it is not clear that the relationship between catalytic activity/selectivity and structures (including crystal structure and atomic arrangement on surface) of intermetallic alloys for FA decomposition. It is imperative to screen an optimum geometry and composition of a specific intermetallic alloy for the development of more active and inexpensive electrocatalysts for DFAFC.
On elemental Pd catalyst, the dual pathway reaction mechanism has been proposed for HCOOH decomposition, and the dehydration product (i.e., ) inevitably leads to poisoning of catalytic sites, thereby suppressing the dehydrogenation efficiency of formic acid on Pd catalyst [11], [12], [13], [14]. Cu surfaces are known to be highly selective to the competition between OH and CH bond scission in HCOOH dehydrogenation [40]. However, the high barrier to the dehydrogenation of the intermediate HCOO limits the hydrogen productivity of HCOOH on Cu surfaces, and then requires a high operating temperature of DFAFC [41]. Recently, some Cu-Pd nanocatalysts were synthesized as promising catalysts for formic acid oxidation including Cu-Pd nanoparticle [42], [43], Cu core/Pd shell structure [44], and porous network structure [45]. Additionally, the ordered CuPd nanoparticles with B2 phase exhibit superior activity and durability for the oxygen reduction reaction [46], and the ordered Pd2Cu nanoparticles show the high C2H5OH production and selectivity for CO2 hydrogenation [47]. To the best of our knowledge, few studies were performed about the decomposition of formic acid on Cu-Pd intermetallic alloys. Herein, we present that the ordered Cu-Pd compounds are highly active and selective catalysts toward hydrogen production from formic acid.
Section snippets
Methods
Our calculations were performed in the framework of the density functional theory (DFT), implemented in the plane-wave based Vienna ab initio Simulation Package (VASP) [48], [49]. The interaction between ion and core electrons is described by the projector augmented wave (PAW) method [50], and plane waves with an energy cutoff of 500 eV are used to expand the Kohn-Sham (KS) wave functions. The generalized gradient approximation (GGA) for the exchange and correlation functional is employed with
Adsorption of HCOOH and intermediates
The adsorption energy is defined as , where and are the total energies of CuPd surface with or without adsorbate, respectively, while is the total energy of gas-phase molecule. The adsorption energies and optimized geometries of molecules on surfaces are presented in Table 1 and Fig. S1–6, respectively.
For trans-HCOOH, there are four possible adsorption configurations, i.e., O/OH-down, HCOOH-flat, CH-down,
Summary
In summary, we studied HCOOH’s decomposition on the ordered CuPd intermetallic catalysts by first-principles calculations. Changing crystal structures and atomic composition of CuPd compounds leads to high catalytic selectivity toward HCOOH decomposition. The OH bond dissociation of HCOOH becomes energetically favorable in comparison with CH bond scission. However, the catalytic activity for HCOO’s dehydrogenation depends on crystal structures of CuPd alloys with the sequence of L10-CuPd(1 1
Acknowledgment
This work was supported by the National Natural Science Foundation of China (Grant No. 11674091) and the Natural Science Foundation of Hunan province (Grant No. 2017JJ2046). The calculations were performed using the National Supercomputing Center in Changsha, China.
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