Efficient Electro-Oxidation of 2-Propanol at Platinum-and Gold-Modified Palladium Nanocatalysts

Tis study aims at investigating the catalytic performance of Pd, Pd/Pt, and Pd/Au nanocatalysts toward the 2-propanol electro-oxidation reaction (2POR) in an alkaline medium. Te catalyst components (Pd, Pt, and Au) were sequentially electrodeposited onto the glassy carbon (GC) electrode surface and further characterized using electrochemical (cyclic voltammetry (CV)) and materials (feld-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray (EDX)) characterization methods. Te Pd/Au/GC catalyst showed the highest catalytic activity in terms of the highest oxidation current (0.386mA) and the highest stability in terms of the highest obtained current after 1800s of continuous electrolysis. Tis behaviour was attributed to the enhancement in the charge transfer kinetics where the Pd/Au/GC catalysts acquired the lowest charge transfer resistance ( R ct , 1.85k Ω ) during the 2POR.


Introduction
Te energy dilemma is undeniably one of the most pressing concerns confronting modern society, as it plays a critical role in global and domestic economic development and security [1,2].Due to the overwhelming increase in global population, coupled with the continuous economic growth seen in multiple developed and developing nations, an increase in global energy demand and consumption is unavoidable [3,4].To address these concerns, cost-efective, clean, and renewable energy sources that could be used as viable substitutes for fossil fuels are urgently needed [5][6][7][8][9].Energy storage and conversion devices have received worldwide interest as a key step toward using renewable energy resources as primary energy sources, and it has become necessary to thoroughly examine the various promising energy conversion and storage solutions, such as fuel cells (FCs) [10][11][12][13][14][15][16][17][18][19], and embark on their implementation [3,12].In this regard, the direct alcohol fuel cells, such as the direct 2-propanol FCs (D2PFCs), appeared as excellent candidates [20][21][22][23][24][25][26].
In fact, 2-propanol is an excellent solvent and is involved in many industry processes such as cosmetics and pharmaceutical industries.Being twice as efective as ethanol, 2-propanol is the broadly used disinfectant in pharmaceutical, medical care, cleanrooms, and electronics industries [27].It is also used as a gasoline additive to keep carburetors from freezing up in internal combustion vehicles [28].Te higher gravimetric energy density (8.6 kWh/kg) compared with that of methanol (6.1 kWh/kg) and ethanol (8.0 kWh/kg) is one more advantage for 2-propanol as a liquid fuel [29].
Recently, interests have been more dedicated to alkaline than acidic D2PFCs because of their higher performance which is likely resulted from the faster kinetics of the alcohol electrooxidation and the oxygen electroreduction in alkaline medium [30][31][32][33][34][35].As the alkaline 2POR proceeds at low overpotential via the acetone production pathway (CH 3 CH OHCH [30,36,37], several noble metals, including Pt, Pd, and Au, that exhibited previously excellent catalytic properties for several vital processes, have been recommended for 2POR [38][39][40][41].However, the obtained oxidation currents (I p ) decayed dramatically at low overpotentials which implies the rapid poisoning of the catalyst surface from the oxidation products [30,42,43].Acetone was the suspicious poisoning species, as previously proposed for 2POR at Pd surfaces in alkaline medium [44].
Tis research addresses the impact of modifying a nanoparticle-based Pd surface with Pt and Au nanoparticles to test their potential to mitigate the activity deterioration of the Pd substrates during 2POR due to acetone poisoning.Tree diferent catalysts (Pd/GC, Pd/Pt/GC, and Pd/Au/GC) will be prepared, characterized, and catalytically evaluated toward 2POR.Te relationship between the concentration of electrolyte medium (NaOH) and the potential scan rate on the I p will be obtained and the mechanism of enhancement toward the 2POR will be discussed.

Experimental Procedures
A cleaned GC (d � 5.0 mm), Ag/AgCl/NaCl (3 M), a spiral Pt wire were served, respectively, as the working, reference, and counter electrodes.Te cleaning of the GC electrode was carried out through the conventional cleaning procedures described previously [5].
All chemicals in the current study were of high purity and were used without any prior purifcation.Te electrodepositions of Pd, Pt, and Au were all carried out in 0.1 M Na 2 SO 4 solutions containing, respectively, 2.0 mM Pd (CH 3 COO) 2 , 2.0 mM H 2 PtCl 6 .6H 2 O, 2.0 mM HAuCl 4 .3H 2 O at 0.1 V, allowing the passage of 10 mC.
Electrochemical tests were conducted at room temperature (25 ± 1 °C) in a two compartments three electrodes glass cell.Te electrochemical workstation, SP-150 Bio-Logic SAS potentiostat operated with EC-Lab software was employed.
Te surface morphology and composition of the proposed catalysts were obtained using a Zeiss Ultra 60 feld emission scanning electron microscope (FE-SEM) equipped with energy-dispersive X-ray spectroscopy (EDX).

Electrochemical and Material Characterization.
Figure 1 shows the typical behaviour of Pd-based catalysts in alkaline media where the Pd oxidation to PdO which extended from −0.2 to 0.6 V was coupled with the subsequent reduction of PdO to Pd again at −0.4 V. Along with that, the hydrogen adsorption/desorption (H ads/des ) region was observed between −0.6 and −0.9 V. Tis behaviour was observed for all proposed catalysts (Figures 1(a Te SA was calculated to be 0.08, 0.52, 0.59, and 0.83 cm 2 for the Pd, Pd/GC, Pd/Pt/GC, and Pd/Au/GC catalysts, respectively, based on the charge associated with the PdO reduction peak using a reference value of 420 μC•cm −2 [5].Tis trend appeared again in the H ads/des region because of the large surface area ofered by nanoparticles.(ii) Te Pd/Pt/GC (Figure 1 To further confrm the successful deposition of all ingredients of the proposed catalysts, EDX analysis was obtained.Figure 3 shows the EDX analyses of the Pd/GC (Figure 3

Electrocatalytic Activities of the Catalysts toward 2POR.
Figure 4 shows the alkaline 2POR at the proposed catalysts.All catalysts showed the typical behaviour of the 2POR where two anodic peaks were observed at ca. −0.15 and −0.28 V, respectively, in the positive and the negative scans, which agreed with previous literature [37,41,43].Te low oxidation current (I p , ca. 0.029 mA) obtained at the Pd catalyst (Figure 4(a)) was attributed to the surface poisoning with the strongly adsorbed oxidation product, acetone, at the Pd surface [44].Te poisoning with acetone was assumed to block most of the Pd active sites and, therefore, diminish the overall activity of the 2POR.A better scenario was observed at the Pd/GC (Figure 4(b)) catalyst at which the I p reached ca.0.279 mA (ca. 10 times higher than that obtained at the Pd catalyst).
Te consecutive modifcation of the Pd/GC catalyst with Pt and Au could further diminish such a poisoning impact, where the I p value reached 0.299 and 0.386 mA, respectively, at the Pd/Pt/GC (Figure 4(c)) and the Pd/Au/GC (Figure 4(d)) catalysts.Te inset of Figure 4 shows the current values normalized to the mass of deposited Pd (specifc currents) for the Pd/GC, Pd/Pt/GC, and the Pd/Au/GC catalysts.Te obtained values were 51, 55, and 71 mA•mg Pd −1 , respectively.Te enhancement behaviour in the catalytic activity was also refected in measuring the catalytic stability.Figure 5 shows the current transients (i-t) of the proposed catalysts.Te enhancment in the catalytic stability was refected from achieving higher currents after 1800 s of contineous electrolysis.As observed, the current trend was in the order of Pd (Figure 5 To understand the mechanism of enhancement toward the 2POR, in terms of the charge transfer, Nyquist plots were obtained and analyzed (see Figure 6).As well-known, Nyquist plots assess the charge transfer resistance (R ct ) of the catalyst, which is obtained from the semicircle diameter, evaluating the reaction kinetics [45].In agreement with the data of Figures 4(a) and 5(a) depicting, respectively, the poor activity and stability of the Pd catalyst, Figure 6(a) assigned the highest R ct value (45.39 kΩ) for the same "pristine" Pd catalyst toward the 2POR.Tis indicates a sluggish kinetics toward the 2POR at the Pd catalyst.4

Journal of Chemistry
On the other hand, the Pd/GC (Figure 6(b)), Pd/Pt/GC (Figure 6(c)), and the Pd/Au/GC catalysts ofered much lower R ct values (2.79, 2.18, and 1.85 kΩ, respectively), indicating faster charge transfer kinetics toward the 2POR.It is thought that such an enhancement came from the weak adsorption of acetone and/or its faster removal at the modifed catalysts [30].

Parameters Afecting 2POR.
To achieve a better electrocatalytic 2POR, the efect of changing the NaOH concentration and scan rate were examined.Figures 7(a  Journal of Chemistry that increasing the OH − concentration facilitated the removal of the adsorbed intermediates that increased the availability of more active sites for the 2POR [46].Additionally, Figures 8(a)-8(d) shows the efect of changing the scan rate on the I p value of the 2POR at all proposed catalysts.It was clear that a slight shift of the I p to more positive values was observed with increasing the potential scan rates, suggesting a kinetic limitation [47].Moreover, as the scan rate increases, the I p value increases and a linear dependence of the square root of the scan rate and the I p was observed with a correlation coefcient ranging from 0.95 to 0.99, which confrmed the difusion-controlled process [48].

Conclusion
Tis research aimed at examining the electrocatalytic performance toward 2POR in an alkaline medium at several proposed (Pd/GC, Pd/Pt/GC, and Pd/Pt/GC) nanocatalysts.Te Pd/Au/GC catalyst showed the highest catalytic activity and stability in terms of the highest current values.Nyquist plots assessed the charge transfer enhancement where the Pd/Au/GC catalyst showed the fastest charge transfer kinetics (the lowest R ct value) toward the 2POR.From another perspective, the efects of changing the potential scan rate and the NaOH concentration on the oxidation currents were monitored and several mechanistic correlations were deduced in view of the recommended modifcations at all the proposed catalysts.
(c)) and the Pd/Au/GC (Figure1(d)) catalysts acquired a broader PdO reduction peak compared to that obtained at the Pd/ GC catalyst (Figure1(b)).Tis highlighted the role of adding the Pt and Au surface modifers in providing diverse Pd-Pd and Pd-O bonding and/or facets' reconstruction for the Pd surface.(iii) Te large H ads/des peaks at the Pd/Pt/GC catalyst referred to the participation of both Pd and Pt in this reaction[45].(iv) Te disappearance of the Au characteristic peaks at the Pd/Au/GC catalyst (Figure1(d)) might refer to the formation of a "core (Au)-shell (Pd)" structure[45].Additionally, by using the state-of-art technologies, the surface morphology (Figure2) of the proposed catalysts; Pd/C (Figure2(a)), Pd/Pt/GC (Figure 2(b)), and Pd/Au/GC (Figure 2(c)) was obtained using the FE-SEM.Figure 2(a) confrmed the successful deposition of Pd in the Pd/GC catalyst as well-distributed spherical nanoparticles having an average diameter of ca. 30 nm.Too larger sizes (100 and 120 nm) of spherical Pd particles were obtained, respectively, at the Pd/Pt/GC (Figure 2(b)) and the Pd/Au/GC (Figure 2(c)) catalysts.It seems that Pd was deposited onto Pt and Au in single bigger particles.
(a)), Pd/Pt/GC (Figure 3(b)), and the Pd/Au/GC (Figure 3(c)) catalysts.As clearly shown, all catalysts' components (C, O, Pd) existed in the three proposed catalysts additionally with Pt and Au that were observed, respectively, at the Pd/Pt/GC and the Pd/Au/GC catalysts.