On the Stability of Pt‐Based Catalysts in HBr/Br
 2
 Solution

a Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering, ETH Zurich, Vladimir Prelog Weg 1, CH-8093 Zurich, Switzerland, e-mail: jeroen.vanbokhoven@chem.ethz.ch b TFP Hydrogen Products Ltd., Unit 5 & 6, Merchants Quay, Pennygillam Industrial Estate, UK-Launceston, PL15 7QA, United Kingdom c Operando spectroscopy group, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland d Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland e Laboratory of Environmental Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland


Introduction
Heterogeneous catalysts, consisting of metal nanoparticles (NPs) and a solid support, are widely applied in energy-related fields. [1][2][3] Precious metal-based catalysts such as Pt/C (platinum NPs supported carbon material) hold great promise for a number of catalytic transformations. Challenges associated with unstable performance and easy deactivation limit the practical application of these catalysts. [4][5][6] Deactivation tends to become more severe for supported metal-based catalysts at high temperatures as well as in an oxidizing atmosphere and in condensed corrosive solvents. An in-depth understanding of catalyst deactivation is, therefore, crucial for the rational design and optimization of (electro)catalysts with high stability. [7][8][9] During cell operations, the direct contact with liquid electrolytes and continuous potential fluctuations usually put a greater demand on the durability of electrocatalysts, because electrocatalytic properties can be critically hindered by catalyst degradation. [10][11][12][13][14][15][16] The degradation of supported metal NPs is generally reported to originate from dissolution, detachment from the support, coalescence and sintering. [17][18][19][20] In particular, metal NPs dissolution usually occurs through direct chemical reactions with reactants and/or electrolyte components accompanied by (electro)catalytic oxidation and reduction processes. [9,11,13,21,22] As well as being determined by the intrinsic structural properties of the catalyst, the intensity of the dissolution of supported metal NPs tends to vary, depending on the applied operating conditions in energy conversion and storage devices. [23][24][25] For instance, hydrogen-bromine redox flow batteries (H 2 -Br 2 RFBs) involve highly reversible reactions at the anode and cathode side by flowing with H 2 gas and acid electrolyte composed of protons (H + ) and oxidizing Br x species. Platinum-based catalysts still serve as the most active H 2 -related electrode materials. Considerable degradation in performance often occurs due to the strong and irreversible sorption/oxidation property of Br x on Pt metal surface. [26][27][28] Long-term practical durability of Pt-based catalysts is hence desirable when contacting with highly concentrated HBr/Br 2 solutions in H 2 -Br 2 RFBs. In this regard, the studies on how the carbon-supported Pt-based NPs dissolve in Br xcontaining solutions and the factors that influence this can aid in optimizing and improving catalyst dissolution resistance. Up to now, however, only a few such stability studies have been published; their focus is mainly on the dipping of the Pt-based catalyst into an acidic HBr solution without Br 2 before carrying out electrical measurements. [29,30] Here, we report on the systematic evaluation of the stability of Pt/C with a regulated particle size and PtIr/ C consisting of PtIr alloy NPs in HBr/Br 2 solution with different concentration regimes. Polybromide anions (i. e., Br 3 À ) in the HBr/Br 2 solution accelerated the dissolution of metal NPs. Metal dissolution rates depend to a great extent on the size of the NPs and the surface property rather than on the concentration of Br 2 in the investigated HBr/Br 2 solutions. Sterically limited accessible sites on the PtIr/C-based catalysts after surface organic modification were responsible for the observed marked resistance towards dissolution. The findings in this work serve to provide proper guides on the design of durable (electro)catalysts, which are applied especially in HBr/Br 2 solutioninvolved catalytic system, such as membraneless H 2 -Br 2 RFBs.

Structural and Dissolution Properties of the Pt/C Catalysts
The studies on the dissolution of carbon-supported metal NPs began with the selection of the commercial Pt/C catalyst (Alfa Aesar), the catalyst which is used most frequently in electrocatalytic transformations. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis demonstrated that 40 wt-% of Pt was loaded on carbon. Figure 1,a is the TEM image of Pt NPs with a diameter of 1.0 -2.0 nm, uniformly dispersed on the support. In order to investigate the effect of particle size on metal dissolution, Pt/C was annealed at 350°C and 700°C in a N 2 atmosphere. Carbon-supported Pt NPs with a mean diameter of 3.5 nm and 5.0 nm were obtained on the Pt/C-350 and Pt/C-700, respectively ( Figure 1,b and 1,c). When Pt/C is not heated, it is referred to as Pt/C-uncal.   Figure 2,a presents Pt NPs dissolution profiles for the Pt/C catalysts after 16 h in HBr solution with a concentration ranging from 8.2 M to 2.4 M. The highest degree of dissolution (66 %) was found for the Pt/Cuncal. in 8.2 M HBr, while that were much lower for Pt/C-350 and Pt/C-700 having larger metal particle size. With a decrease in HBr concentration, the degree of Pt dissolution also tends to decrease. In comparison, however, as shown in Figure 2,b, severe dissolution (more than 80 %) of Pt NPs from Pt/C-uncal. and Pt/C-700 had taken place in HBr/Br 2 solutions after 2 h. Even when the reaction time was reduced to 40 min, the extent of Pt dissolution was still high (over 60 %). All the investigated Pt/C-uncal., Pt/C-350 and Pt/C-700 exhibited almost complete Pt dissolution in HBr/Br 2 solutions when immersing for 16 h ( Figure S1). The above results indicate that nanoparticle size played an important role in the dissolution of Pt NPs in acidic HBr solution, because the surface amount of lowcoordinated atoms and the chemical potential for dissolution vary with the size of nanoparticles. [11,31] Besides, the addition of Br 2 into the HBr solution can significantly accelerate the dissolution of Pt NPs. As shown in Figure S2, the enhancement of Pt dissolution by adding Br 2 to the HBr solution was more remarkable for 2.4 M HBr with the highest Br 2 concentration (2.1 M).
In bromide/bromine-containing solution, polybromide complexes such as tribromide (Br 3 À ) and pentabromide (Br 5 À ) anions are formed through the Eqns. 1 and 2: The equilibrium constant for the formation of Br 3 À and Br 5 À anions are expressed by Eqns. 3 and 4. Earlier literatures have reported the K 3 value in the range of 16 to 20 L mol À 1 (at 25.0°C), [32 -35] while that of K 5 is between 19 and 41 L 2 mol À 2 . [33 -36]  For the series of HBr/Br 2 solution employed for the dissolution of Pt/C, the equilibrium concentration of Br À , Br 2 and polybromide (Br 3 À and Br 5 À ) anions were simulated based on the initial concentration of HBr and Br 2 , as well as the K 3 (16 L mol À 1 ) and K 5 (37 L 2 mol À 2 ). [37] As shown in Figure S3, Br 2 was almost dissolved in the highly concentrated HBr solution by forming Br 3 À as the dominant species (more than 95 %). Then we performed ultraviolet visible spectrometry (UV-Vis) measurements on the solution before and after Pt/C dissolution. Typically, from the spectra of 5.6 M HBr/0.6 M Br 2 ( Figure S4,a), the adsorption peak at 267 nm was likely assigned to the Br 3 À species, [38,39] and the contributions by other polybromide anions such as Br 5 À and Br 7 À could not be ruled out. While that observed at 390 nm, with a much lower intensity, was the adsorption of Br 2 . [40] The UV-Vis analysis indicated that it is the polybromide anions formed through Br À /Br 2 equilibrium reactions rather than the bromide (Br À ) that play a crucial role in enhancing the dissolution of carbon-supported Pt NPs. Furthermore, the bromoplatinic acid complex was distinguished from the two broad peaks centered at 315 and 260 nm in the UV-Vis spectra. [41,42] This metal complex was also observable in pure HBr solution (comprising protons and Br À anions) after the dissolution of Pt/C, which suggests that both Br À and polybromide anions participated in the formation of Pt complexes in the HBr/Br 2 solution.

Structural and Dissolution Properties of the PtIr/C Catalysts
After studying the dissolution of Pt/C, PtIr alloys became the focus of the investigation on metal nanoparticle dissolution in HBr/Br 2 solutions. It has already been shown that PtIr alloyed catalysts are active in various electrocatalytic reactions. [43 -46] We obtained a PtIr/C catalyst with 40 wt-% metal loading on carbon material from TFP Hydrogen Products Ltd.. This catalyst was developed to exhibit much higher activity than the commercial Pt/C towards the hydrogen evolution reaction (HER), which is the half-cell reaction on the anode side of H 2 -Br 2 RFBs. Carbonsupported PtIr NPs were characterized to be polycrystalline nature with a particle size distribution between 30 and 40 nm from the TEM images (Figure 3,a and  3,b). PtIr NPs were additionally observed to have porous nano-dendritic structures. It is worth noting that the particle size (6.6 � 0.2 nm) of PtIr calculated based on XRD measuring is in essence not in agreement with that from TEM. It points out the PtIr nanodendritics are formed by the aggregations of small nanoparticles during the synthesis. The bulk composition of PtIr NPs was found to have a Pt/Ir atomic ratio of 80 : 20, based on the analysis of energy-dispersive Xray (EDX) spectra ( Figure S4,c). HAADF-STEM mapping images (Figure 3,c) suggested that Pt and Ir were welldispersed on the PtIr NPs, but they did not display the characteristic core-shell structure. The collected XRD pattern shown in Figure 3,d revealed the reflections at 39.93°, 46.42°, 67.83°, 81.57°2θ angles, which were indexed as (111), (200), (220) and (311) planes of typical face-centered cubic (fcc) structure of Pt. [47] The absence of any phase of Ir (Ir 0 JCDPS# 87-0715; IrO x JCDPS# 88-0288) stands in line with the formation of a PtIr alloy phase in the resulting PtIr/C catalyst. [44,46] The lattice constant for PtIr/C was refined in the space group Fm � 3m to be 3.9076 Å and PtIr NPs had a nearly 100 % of alloying degree, [45] as compared with that (3.9038 Å) calculated from Vegard's law. These findings proved the formation of well-alloyed PtIr NPs on the prepared PtIr/C catalyst. The extent of PtIr/C dissolution was evaluated under the similar conditions as for Pt/C. There was a gradual decrease in the amount of dissolved Pt from PtIr/C in the concentrated HBr as well as in the HBr/Br 2 solutions. In contrast to Pt/C, the enhancement of Pt dissolution as a result of adding Br 2 to the HBr was not as strong in the case of 2.4 M HBr/2.1 M Br 2 , compared to that of 8.2 M HBr/0.3 M Br 2 (Figure 4). A similar tendency was found for Ir on PtIr/C ( Figure S5). The lower degree of metal dissolution compared with Pt/C was probably a consequence of the distinct surface structure and the self-stability of PtIr nano-dendritics on PtIr/C. Furthermore, negligible metal dissolution occurred on PtIr/C when HBr was replaced by KBr in the case of 4.0 M KBr/1.3 M Br 2 ( Figure S6). This confirmed that the synergy between highly concentrated H + and the as-formed polybromide anions was linked to the rapid dissolution of carbon-supported Ptbased NPs.
The respective dependence of HBr and Br 2 on concentration with regard to the dissolution of PtIr NPs in a variety of HBr/Br 2 media was checked. Table S1 also shows the concentrations of HBr/Br 2 solution employed in this concentration-dependent study. The simulated concentrations of polybromide equilibria and Br 2 in these HBr/Br 2 solutions are plotted into Figure 5,a and Figure 5,c. Increasing the concentration of the initially added Br 2 could induce the formation of Br 5 À anions, but still maintain Br 3 À as the dominant species. The formation of heptabromide  Helv. Chim. Acta 2021, 104, e2100082 www.helv.wiley.com (4 of 10) e2100082 © 2021 The Authors. Helvetica Chimica Acta Published by Wiley-VHCA AG (Br 7 À ) species at higher Br 2 concentrations should not be ruled out. [37] Their exact amount, however, cannot be determined due to the lack of the equilibrium constant of Br 7 À from the literature. Figure 5,b shows the rate of Pt dissolution (Ln(mmol/mg cat. /h); Ln is the natural logarithm function. There is a positive trend as a function of Ln([HBr]), showing a reaction order of ca. 4. However, a near-zero reaction order was determined from the dependence on Br 2 concentration ( Figure 5,d). The dissolution of Ir from the PtIr/C also exhibited similar concentration-dependent properties ( Figure S7). These findings demonstrated that polybromide anions in HBr/Br 2 solution could easily get adsorbed on the surface of metal without transport/diffusion limitation. [48] The dissolution of metal was supposed to be started through the dissociative reaction of adsorbed polybromide anions through the BrÀ Br bond cleavage on the metal sites and then eventually form the metal-bromide complexes. The representative UV-Vis spectra of the HBr/Br 2 solution after PtIr/C dissolution showed a decreased intensity of the adsorption peak of polybromide species (Figure S8,a), which was most likely due to changes of the equilibrious state of HBr/Br 2 solution after the formation of bromoplatinic acid and Ir-bromide complexes.
The representative XRD patterns in Figure S8,b with decreasing intensities of the diffraction peaks corre-sponding to the (111), (200) and (220) planes of the alloy structure verified the different degrees of dissolution of PtIr NPs following ICP-OES analysis. The TEM images of PtIr/C ( Figure S9) illustrated that the evolution of the morphological structure of PtIr NPs or their absence from the carbon support are indicative of severe dissolution. TEM images did not show detectable re-deposition of metal cations onto the carbon support by forming smaller clusters or nanoparticles. This re-deposition can be ruled out, because the PtIr/C catalyst was exposed to highly oxidizing conditions, the formed metal-bromide complexes are usually stable in HBr/Br 2 solution and, more importantly, thorough rinsing with deionized water was performed during the catalyst separation process. It is noteworthy that PtIr NPs exhibited a hollow-like nanostructure after dissolution in the HBr/Br 2 solution ( Figure S9), whereas it retained its spherical solid structure on the catalyst after dissolution in HBr without Br 2 . One explanation may be that polybromide anions, such as Br 3 À , when adsorbed on the surface of metal, form a driving force for the outward diffusion of the inner metal atoms during the dissolution of PtIr NPs, identical to the Kirkendall effect in the synthesis of hollow-structured nanocatalysts. [49] X-ray photoelectron spectroscopy (XPS) measurements were carried out to detect the surface composition and oxidation state of PtIr/C before and after dissolution. In the high-resolution Pt 4f spectra of fresh PtIr/C (Figure 6,a), the doublet peaks with a binding energy (BE) of 71.8 and 75.1 eV represented metallic Pt, while the second doublet pairs at 73.0 and 76.3 eV were assigned to oxidized Pt 2 + . [45] In addition, from the Ir 4f XPS spectra, peaks ascribed to metallic Ir (61.3 and 64.3 eV) and oxidized Ir (62.4 and 65.4 eV) were observed. [50] Surface analysis indicated that Ir was relatively more abundant on the surface of PtIr NPs than in the alloyed phase of the inner bulk; the Pt/Ir atomic ratio was 59 : 41. Metallic Pt 0 (77 %) and Ir 0 (64 %) were dominant on PtIr/C together with the appearance of surface cationic metal species (higher BEs), which was confirmed by X-ray absorption spectroscopy (vide infra). Surface adsorbed Br À and covalently coordinated Br species were also detected on the surface of dissolved materials. [51,52] The deconvolution analysis of core-level spectra indicated that both Pt and Ir atoms with a metallic and oxidized state tended to dissolve in HBr/Br 2 mixtures. There was no shift in BEs for Pt, while the Ir doublet peaks showed a positive shift after different levels of dissolution. Moreover, the surface atomic ratio of Pt/Ir decreased as dissolution processed ( Table S2), suggesting that sur- Helv. Chim. Acta 2021, 104, e2100082 face-exposed Pt on PtIr NPs dissolved more easily than Ir. The observed positive shift of BEs for Ir and the high dissolution extent of Ir from PtIr/C was probably due to the disruptive effect of Pt dissolution, which might increase the amount of less stable low-coordinated surface Ir atoms. [53] Structural and Dissolution Properties of the Surface Organic-Patterned PtIr/C Molecular patterning of the surface of metal by organics with -NH 2 functional groups is one of the effective strategies for enhancing the electrocatalytic properties of Pt-based electrocatalysts. [54][55][56][57] The fundamental basis of this strategy relies on the surfaceengineered modifications by regulating the accessibility of metal sites to corrosive and/or poisoning adsorbates such as PO 4 3À , SO 4 2À and crossoverobtained methanol molecule, etc. Inspired by this strategy, we fabricated several organic molecules (Fig-ure 7), including cyclohexylamine (L1), aniline (L2), ethylenediamine (L3) and benzene-1,4-diamine (L4), onto the surface of PtIr NPs by using a facile dipping approach, followed by rinsing and drying steps (detailed in the Supporting Information). 5.6 M HBr/0.6 M Br 2 was selected for the evaluation of the stability of surface organic-modified catalysts, because it can induce a high degree of dissolution of PtIr/C after a certain time (Figure 4). From the dissolution in 5.6 M HBr/0.6 M Br 2 for 16 h, all the surface organic modified catalysts, with the exception of PtIr/C-L1, showed a marked decrease in the amount of dissolved Pt. In particular, as shown in Figure 7, 7.8 % and below 1.0 % of Pt dissolved from PtIr/C-L3 and PtIr/C-L4, respectively, which was significantly lower than the unmodified PtIr/C (80 %). As expected, the dissolution of Ir species in PtIr/C-L3 and PtIr/C-L4 was undetectable under the given conditions.
A negligible amount of Pt and Ir dissolved from the PtIr/C catalyst during the surface organic modification, as indicated by the ICP-OES analysis of the L4 diaminecontaining ethanol solution. Elemental mapping images of PtIr/C-L4 (Figure 8,a and Figure S10) illustrated that PtIr NPs had retained their geometric properties in terms of particle size, morphology or shape following surface organic modification. Due to the strong interactions between metal atoms and -NH 2 groups, [58,59] organic diamine molecules were still   Figure S11).
The improvement in stability was studied by X-ray absorption spectroscopy (XAS) characterization of PtIr/ C and PtIr/C-L4. As displayed in Figure 8,b, the white line of the Pt L 3 -edge decreased, possibly due to surface reduction of PtIr NPs by diamine molecules possessing electron-donating -NH 2 groups. It was estimated from Pt L 3 -edge extended X-ray absorption fine structure (EXAFS) analysis (Figure 8,c, Table S3) that the first shell coordination number of both samples is about 10, which is characteristic of Pt nanoparticles. [60] Since there is almost no difference between Pt and Ir absorbers, which have similar backscattering amplitudes and lattice parameters, it is impossible to distinguish between Pt or Ir atoms. Linear combination fitting of the Pt L 3 -edge X-ray absorption near edge structure (XANES) for PtIr/C ( Figure S12) suggested that Pt may be present in an oxidized form, which accounts for only less than 5.5 % of the total amount of Pt. Oxidized Pt species are, thus, ascribed to those species that are distributed on the surface of PtIr NPs, as shown by XPS ( Figure 6). The lower amount (2.8 %) of oxidized Pt after surface organic modification confirmed the formation of interactions between metal atoms and diamine molecules (L4) on PtIr NPs.
An evaluation of the accessible number of metal sites on PtIr NPs after surface organic modification by amine or diamine molecules was carried out. Pulse CO-chemisorption measurement (at 35°C) was performed, a commonly employed technology to determine the dispersion of metal NPs on metal-based catalysts. Metal dispersion was found to decrease due to surface occupation by the organic molecules through interactions between metal atoms and -NH 2 groups of amine molecules. [58,59] In particular, among the surface organic-modified samples, at least 85 % of the surface sites on PtIr/C-L4 was lost ( Figure S13). It is noteworthy that the synergy between highly concentrated H + and polybromide anions accelerates the dissolution of metal NPs in acidic and oxidizing HBr/ Br 2 solutions. The marked stability of PtIr/C-L4 can, therefore, be explained by the steric limitation of accessibility of the metal surface to these dissolution enhancers, in particular, the polybromide anions.
The surface patterned molecules are beneficial for regulating the adsorption of spectator species on metal particle at the expense of blocking certain number of active sites, which is well documented in the literature in electrocatalysis field. [54,55,57] Regarding this, we further collected the cyclic voltammograms of PtIr/C and surface organic-modified PtIr/C at a scan rate of 20 mV s À 1 in 0.5 M H 2 SO 4 solution under Ar. The electrochemical surface area (ECSA), a critical parameter for electrocatalytic property, was calculated based on the hydrogen desorption charge as 0.21 μC mg Pt À 2 . [55] The modification with L1, L2 and L4 organic was observed to decrease 22 -25 % of ECSA, while that of L3-modified PtIr/C showed 13 % of loss ( Figure S14). These results indicate that the surface organic-modified PtIr/C catalysts might possess the potential activities for the H 2 -related oxidation/evolution reactions, in particular, in the H 2 -Br 2 RFBs. Evaluations on the performance of these catalysts in terms of electrocatalytic activity and stability are undergoing and will be covered in the following report.

Conclusions
In summary, the results of the present study show that the synergy between acidic H + and polybromide anions (i. e., Br 3 À and Br 5 À ) in HBr/Br 2 solution contributed to the high level of dissolution of Pt-based nanoparticles from the carbon support. PtIr/C material, which was characterized to feature well-alloyed and nanodendritic structural property, was more stable than Pt/C under similar dissolution conditions. Surface organic modification on PtIr nanoparticles by means of diamine molecules was effective in improving the dissolution resistance in acidic and oxidizing HBr/Br 2 solutions. Further studies on the electrocatalytic properties of the surface organic-modified PtIr/C catalysts are necessary to gain an in-depth understanding of the (electro)catalyst degradation mechanism and improvement of electrocatalytic efficiencies.

Notes
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