Electrocatalytic Oxidation of Small Molecule Alcohols Over Pt

catalysts

Article Electrocatalytic Oxidation of Small Molecule Alcohols over Pt, Pd, and Au Catalysts: The Eﬀect of Alcohol’s Hydrogen Bond Donation Ability and Molecular Structure Properties

1, 1, 1 1 1 1, 2, Bei Wang †, Liu Tao †, Yu Cheng , Fang Yang , Yuguang Jin , Chunmei Zhou *, Hao Yu * and Yanhui Yang 1 1 School of Chemistry and Molecular Engineering (SCME), Institute of Advanced Synergetic Innovation Center for Advanced Synthesis (IAS), The Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816, China; [email protected] (B.W.); [email protected] (L.T.); [email protected] (Y.C.); [email protected] (F.Y.); [email protected] (Y.J.); [email protected] (Y.Y.) 2 The School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China * Correspondence: [email protected] (C.Z.); [email protected] (H.Y.); Tel.: +86-2558-13-0047 (C.Z.); +86-2087-11-4916 (H.Y.) Liu Tao and Bei Wang contributed equally to this work. † Received: 1 March 2019; Accepted: 16 April 2019; Published: 25 April 2019

Abstract: The direct alcohol fuel cell is a kind of power generation device that directly converts the chemical energy of small molecule alcohols into electric energy. In this paper, the electro-oxidation behaviors of some typical alcohols (methanol, ethanol, ethylene glycol, n-propanol, 2-propanol, and glycerol) over Pt, Pd, and Au electrodes were investigated in acidic, neutral, and alkaline media, respectively. By analyzing the activity information from a cyclic voltammetry (CV) method and some dynamic tests, several regularities were revealed in those electro-oxidation behaviors. Firstly, alkaline media is the best for the electro-oxidation of all these alcohols over Pt, Pd, and Au catalysts. Secondly, the hydrogen bond donation abilities (HBD) of different alcohols were found have a great relationship with the catalytic performance. In alkaline media, on Pt electrodes, the solute HBD is positively correlated with the ease of electrooxidation within the scope of this experiment. Contrarily, it is negatively correlated on Pd and Au electrodes. Additionally, for Pt catalysts in acidic and neutral media, the relationship becomes negative again as the HBD increases. Finally, the alcohol’s molecular structure properties were found to have a remarkably influence on the activity of different catalysts. Over the Pt electrode in alkaline media, the activation energy of methanol oxidation is 44.1 KJ/mol, and is obviously lower than the oxidation of other alcohols. Under similar conditions, the lowest activation energy was measured in the oxidation of n-propanol (14.4 KJ/mol) over the Pd electrode, and in the oxidation of glycerol (42.2 KJ/mol) over the Au electrode. Totally, among all these electrodes, Pt electrodes showed the best activities on the oxidation of C1 alcohol, Pd electrodes were more active on the oxidation of C2-3 monobasic alcohols, and Au electrodes were more active on the oxidation of polybasic alcohols.

Keywords: DAFCs; electro-oxidation; gold; palladium; platinum

1. Introduction Direct alcohol fuel cells (DAFCs) generate electric power by feeding liquid fuels directly to the anode, which have been recognized as green energy generators capable of converting renewable sources

Catalysts 2019, 9, 387; doi:10.3390/catal9040387 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 387 2 of 18 into electric power [1,2]. The thermodynamic efficiency of a DAFC is >90% because energy from the fuel is directly transformed into electric energy without the constraints of Carnot’s theorem [3,4]. There have also been recent studies that found that DAFCs selectively produce high-value products while obtaining electric energy [5,6]. So far, Pt, Pd, and Au catalysts are thought to be the most mature and efficient anode catalysts for DAFCs. Especially, Pt-based electrocatalysts are considered to be the best alcohol oxidation catalysts at low temperatures [7–9]. However, due to the high cost and limited resources of Pt, it is necessary to increase its activity and utilization rate. On the one hand, to increase dispersion, Pt nanoparticles are prepared and loaded on carbon materials with good electric conductivity and large specific surface areas. On the other hand, Pt-based alloy catalysts, such as Pt-Au [10], Pt-Ni [11], Pt-Co [12], and Pt-Pd [13], are introduced to obtain higher performance at lower Pt contents. As to the fuel of DAFCs, many small molecule alcohols are of great superiority, due to their high energy density and ease of thorough decomposition. Among them, methanol is the most typical for a fuel cell containing one carbon (C1) atom [14]. In the early days of electro-catalytic research, methanol was the first small molecule compound to be used to study the catalytic oxidation DAFCs. Compared to other small molecules, the complete oxidation of methanol to produce a unit volume of CO2 produces the highest energy. Although methanol was most studied due to various advantages, the main problem of using methanol as a fuel for DAFCs is its high toxicity to the catalyst by the intermediate product in the oxidation process. Therefore, some alcohols that can replace methanol have been studied in depth. C2-alcohols, such as ethanol and ethylene glycol, are one of the more intensive alcohols studied. They not only have a higher energy density, but also have one break of the C–C bond in the complete oxidation process. Since C2-alcohol also produces intermediates, such as CO or oxalic acid, during complete oxidation, the stability of electrocatalysts is a challenge. One of the motivations for studying glycerol with three carbons is that they can be used in direct alcohol fuel cells for electric energy generation, in addition to the symbiosis of heat and chemicals. In particular, glycerol is produced in large quantities as a by-product of biodiesel production and has a high energy content. Moreover, due to the unique structure of glycerol, a large number of chemicals with high economic value can be produced by chemical reactions. Electrochemical studies would be carried out using these representative alcohols as substrates. However, the complete oxidation of C–C bonds to CO2 is a problem. Pt is now considered to be the best oxidation catalyst, but its ability to break C–C bonds is limited [15]. Xu Changwei et al. [16,17] systematically studied the electrochemical oxidation activity of Pd on various alcohols in alkaline media and found that Pd/C had higher electrochemical oxidation activity for ethanol than Pt/C in alkaline solution. At the same time, the catalytic activity and the anti-poisoning ability of ethanol oxidation on Pd/C is much better than that of methanol oxidation. When oxides, like CeO2, NiO, Co3O4, or Mn3O4, are combined with Pd, the catalytic activity and stability can be significantly improved. It has been found that the Pd-NiO/C catalyst has unique properties for ethanol oxidation. The starting potential for oxidation is 200 mV earlier than that of the Pt/C catalyst. This result provides very valuable materials for the development of ethanol fuel cells. Claudio Bianchini and Pei Kang Shen [18] described and discussed many different types of anodes containing Pd-based electrocatalysts. They found that Pd nanocatalysts exhibit higher activity for the electrooxidation of monohydric alcohols and polyols in alkaline media. The remarkable feature of Pd catalysts for the electrooxidation of alcohols is that it is difficult to break the C–C bonds, even in the strong alkaline media. Au is inactive in acidic media for alcohol oxidation, while the surface becomes more active in alkaline media [19,20]. It has been reported [21] that no toxic intermediate products appeared and adsorbed on the surface of the Au electrode in alkaline solution. Au showed high electrocatalytic activity for electrochemical oxidation of ethanol, but excessive overpotential constrained its use. Some researchers [22] studied the gold-modified palladium as a catalyst for the electrochemical oxidation of isopropanol, and found that the catalytic activity of Pd was greatly improved. Catalysts 2019, 9, 387 3 of 18

It is well known that conventional direct alcohol fuel cells use alkaline aqueous electrolyte. However, CO2 produced by complete oxidation of alcohols is converted into carbonate/bicarbonate in alkaline medium. Although it is now possible to solve this problem by using an anion exchange membrane, the use of the membrane increases the cost of the battery and brings about safety issues that cannot be ignored [23]. Therefore, it is necessary to study the use of acidic and neutral electrolytes. Although previous researchers have conducted extensive research on anode reactions for DAFCs, there are relatively few lateral comparisons and correlations reported on the key factors, such as catalysts, fuels, and electrolytes, which is very important and has guiding significance for the study of DAFCs. However, most current research on precious metal catalysts is limited to one metal. Small molecule alcohols are used as an important substrate for fuel cells. The scope of its research is also relatively small. In addition, the comparison of electrolytes of different pH is also insufficient. Herein, in this paper, we attempted to investigate the electro-oxidation behaviors of some typical alcohols (methanol, ethanol, ethylene glycol, n-propanol, 2-propanol, and glycerol) over different electrodes in acidic, neutral, and alkaline media, respectively. The electrodes included are commercial Pt, Pd, and Au disk electrodes and self-synthesized Pt/CNTs, Pd/CNTs, and Au/CNTs coated electrodes. A cyclic voltammetry (CV) method was used to test the catalytic performance of these electrodes to discover some regularities of those electro-oxidation reactions.

2. Results and Discussion

2.1. Characterization of Pt/CNTs, Pd/CNTs, and Au/CNTs Catalysts The noble metal contents of the self-synthetized Pt/CNTs, Pd/CNTs, and Au/CNTs catalysts were firstly confirmed by ICP measurement and are listed in Table1. The Pt, Pd, and Au loading for these catalysts are well controlled at about 0.8 wt.%. TEM and XRD patterns (shown in Figure1) were introduced to observe the morphology and crystal structure of these catalysts. The relative results are also listed in Table1. From the TEM images, nanoparticles were observed well dispersed on the surface of the CNT for all the catalysts. Their average diameters were similar at around 3.2 to 3.9 nm. From the XRD patterns, the weak diffraction peaks of Pt, Pd, and Au were observed at 2θ of 38.2◦, 39.1◦, and 40◦ for Pt/CNTs, Pd/CNTs, and Au/CNT, respectively. Based on the diffraction peaks, the grain sizes of the metal crystals were estimated to be 2.2, 2.3, and 2.4 nm for Pt/CNTs, Pd/CNTs, and Au/CNT, respectively, by the Scherrer formula.

Table 1. Structural parameters for Pt/CNTs, Pd/CNTs, and Au/CNTs catalysts measured by ICP, TEM, XRD, and Cu-UPD.

a b c d e 2 Catalysts Metal Loading (wt%) Particle Size (nm) Grain Size (nm) 2θ (◦) EAS (m /g) Pt/CNTs 0.85 3.2 2.2 38.2 110.0 Pd/CNTs 0.75 3.6 2.3 39.1 123.0 Au/CNTs 0.80 3.9 2.4 40 100.0 a Metal loading amounts on the carbon support estimated from ICP analysis. b Particle diameter of the catalysts from TEM images. c Crystallite sizes of the catalysts calculated by line broadening of the powder XRD peak. d The XRD angular position of the corresponding metal. e Electro-chemical surface areas measured by the Cu-UPD method.

The XPS spectra of Pt/CNTs, Pd/CNTs, and Au/CNTs catalysts are shown in Figure2. We performed a deconvolution of the C 1s spectra of CNTs into four peaks centered at 284.5 0.2, ± 285.2 0.2, 286.2 0.2, and 288.9 0.2 eV. The peaks at 284.5 and 285.2 eV can be assigned to ± ± ± sp2-hybryidized graphite-like carbon atoms and sp3-hybridized carbon atoms, respectively [24]. The peaks at 286.2 and 288.9 eV are considered as C–O–C and O–C=O states, respectively [25]. Since CNTs have been oxidized by nitric acid, the O 1s spectra of the three catalysts (Figure2c,f,i) can be deconvoluted into the peak at 533.6 0.1 eV attributed to organic C=O and the peak at 532.2 0.1 eV ± ± attributed to organic C–O. In Figure2a, for Pt /CNTs, the larger two peaks at 74.8 eV and 71.5 eV represent metallic Pt, and the smaller two peaks at 76 eV and 72.78 eV represent oxidized Pt in the Catalysts 2019, 9, 387 4 of 18 divalent states. For the Pd 3d spectra of Pd/CNTs in Figure2d, the doublet at 336.02 eV and 341.28 eV is due to PdCatalysts metal. 2019 Similarly,, 9, x FOR PEER the REVIEW doublet at 83.99 eV and 87.71 eV in Figure2g is assignable4 of to 19 Au metal.

a b

3.9 ±1.2 nm 30 35 3.650 ±1.5nm nm

24 28

18 21

12 14

Partical Number Partical 6 50 nm 7 Partical Number 0 0 50 nm 2345678 12345678910 Size (nm) Size (nm)

c d

Pt/CNTs

Pd/CNTs

35 3.2 ±0.6 nm Au/CNTs Intensity (a.u.)

14 CNTs

Partical Number 30 35 40 45 50 55 60 0 50 nm 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Size (nm) 2 Theta (Degree) Catalysts 2019, 9, x FOR PEER REVIEW 5 of 19

Figure 1.FigureTEM 1. images TEM images of Au of/ CNTsAu/CNTs (a ),(a), Pd Pd/CNTs/CNTs ( bb),), Pt/CNTs Pt/CNTs (c), (andc), andXRD XRDpatterns patterns (d) for them. (d) for them.

Pd 3d a Pt 4f 4f Au 4f The4f XPS spectra of Pt/CNTs, Pd/CNTs, and Au/CNTs catalysts areg shown5/2 in Figure 2. We 5/2 d 3d3/2 performed a deconvolution4f of the C 1s spectra of CNTs into3d four peaks centered at 284.5 ± 0.2, 285.2 7/2 5/2 4f 7/22 ± 0.2, 286.2Pt (Ⅱ ±) 0.2, and 288.9 ± 0.2 eV. The peaks at 284.5 and 285.2 eV can be assigned to sp -hybryidized Pt (Ⅱ)

graphite-like carbon atoms and sp 3-hybridized carbon atoms, respectively [24]. The peaks at 286.2 and 288.9 eV are considered as C–O–C and O–C=O states, respectively [25]. Since CNTs have been Intensity (a.u.)Intensity Intensity (a.u.) Intensity oxidized by nitric acid, the O 1s spectra of the three catalysts (Figure (a.u.) Intensity 2c,f,i) can be deconvoluted into the peak at 533.6 ± 0.1 eV attributed to organic C=O and the peak at 532.2 ± 0.1 eV attributed to organic 344 342 340 338 336 334 C–O. In80 Figure 78 76 2a, 74 for 72 Pt/CNTs, 70 68 66 64the larger two peaks at 74.8 eV and 71.590 eV represent 88 86 metallic 84 Pt, 82 and Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) the smaller two peaks at 76 eV and 72.78 eV represent oxidized Pt in the divalent states. For the Pd 2 C 1s C 1s 2 C 1s 3d spectra of Pd/CNTssp in Figure 2d, the doublet at 336.02 eV and 341.28 eVsp is due to Pd metal. b e sp2 h Similarly, the doublet at 83.99 eV and 87.71 eV in Figure 2g is assignable to Au metal. sp3 sp3 sp3

C-O-C C-O-C C-O-C O-C=O O-C=O O-C=O Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity

292 290 288 286 284 282 280 292 290 288 286 284 282 280 292 290 288 286 284 282 280

Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) c O 1s f O 1s i O 1s C=O C=O C=O C-O C-O C-O

Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity

536 534 532 530 528 536 534 532 530 528 537.0 535.5 534.0 532.5 531.0 529.5 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) Figure 2. XPS spectra of Pt/CNTs (a–c), Pd/CNTs (d–f), and Au/CNTs (g–i) catalysts. Figure 2. XPS spectra of Pt/CNTs (a–c), Pd/CNTs (d–f), and Au/CNTs (g–i) catalysts.

2.2. Electrochemical Characterization of Pt, Pd, and Au Disk Electrodes Cyclic voltammograms of commercial Pt, Pd, and Au disk electrodes in alkaline, acid, and neutral supporting electrolyte were conducted and are shown in Figure 3. The voltammogram of a Pt disk electrode recorded in 0.1 M KOH under N2 saturation displays three regions according to the applied potential (Figure 3a). Region I at lower potentials is associated with the adsorption (reduction peaks)-desorption (oxidation peaks) of hydrogen, whereas region III at higher potentials reveals the adsorption (oxidation peaks)-desorption (reduction peak) of oxygenated species at the electrode surface. Between them is the double-layer region II, where no net electrochemical reaction occurs, and which is characterized by small currents associated with the charge and discharge of the double- layer capacity [26]. The pattern of a Pd electrode is similar to that of a platinum electrode. However, the onset potential of region III is much more positive than that of a platinum electrode. In the case of the Au electrode, the hydrogen region disappeared. When the same experiments were performed in acid medium, 0.1 M HClO4, and neutral medium, 0.05 M K2SO4, instead of alkaline electrolyte, the main features of both voltammograms remain unchanged (Figure 3b for acid and Figure 3c for neutral supporting electrolyte). For a Pt disk electrode, the onset potential of the oxygen region, which is related to earlier OH adsorption, gradually becomes positive as the electrolyte’s pH decreases. When the pH varies from 1 to 13, the range of the onset potential is about −0.2 to 0.4 V/vs. RHE. However, when it comes to Pd electrodes, except that the onset potential of the oxygen region is 0.1 V when the pH is 13, the onset potential in neutral medium is 0.6 V. When the pH is 1, the potential is as high as 0.75 V. The onset potential for an Au electrode in acidic and neutral media is more positive than Pd. When the pH is 13, the onset potential for Pd is 0.15 V and for Au it is about 0.35 V. Although the onset potential changes when other solutes are present in the solution, these phenomena may explain part of the results, which will be given below.

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The electrochemical active surface area (EAS) of Pt/CNTs, Pd/CNTs, and Au/CNTs were 2.2.measured Electrochemical by a Cu-UPD Characterization method and of Pt,calculated Pd, and Auby the Disk following Electrodes equation:

Cyclic voltammograms of commercial Pt, Pd, and Au disk electrodes in alkaline, acid, and (/) = (1) neutral supporting electrolyte were conducted and0.042 are shown ∙ () in Figure3. The voltammogram of a Pt disk electrode recorded in 0.1 M KOH under N2 saturation displays three regions according where QCu was the average electric quantity of copper stripping and w represents the content of noble to the applied potential (Figure3a). Region I at lower potentials is associated with the adsorption metal on the working electrode. Where QCu is calculated by the following formula: (reduction peaks)-desorption (oxidation peaks) of hydrogen, whereas region III at higher potentials s reveals the adsorption (oxidation peaks)-desorption () = (reduction peak) of oxygenated species at(2) the f electrode surface. Between them is the double-layer region II, where no net electrochemical reaction occurs,where s and is the which area is characterizedof the oxidation by smallpeak currentsof the alcohol associated after with subtracting the charge the and background discharge ofand the f double-layerrepresents the capacity scan rate [26 of]. the The cyclic pattern voltammetry. of a Pd electrode is similar to that of a platinum electrode. However,The CV the results onset potential are shown of regionin Figure III is 4. much As list moreed in positive Table than1, EAS that for of Pt/CNTs, a platinum Pd/CNTs, electrode. and In theAu/CNTs case of are the 110.0, Au electrode, 123.0, and the 100.0 hydrogen m2/g, respectively. region disappeared.

-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 -0.8 -0.4 0.0 0.4 0.8 1.2 1.0 0.6 0.6 0.5 0.3 Ⅱ Ⅲ Ⅱ Ⅲ 0.3 Ⅲ 0.0 Ⅱ 0.0 0.0

-0.3 -0.5 -0.3 -0.6 Au -1.0 Au Au -0.6 -0.9 -1.5 10 5.0 1.5 Ⅰ Ⅱ Ⅲ 5 2.5 Ⅰ Ⅱ Ⅲ Ⅰ Ⅱ Ⅲ A) A)

0.0

A) 0 0.0 -4 -4 -4

-1.5 -5 -2.5 I (10 I (10 I I (10 -3.0 Pd -10 Pd -5.0 Pd -15 -7.5 1 6 1.5 0 Ⅰ Ⅱ Ⅲ 3 0.0

Ⅰ Ⅲ -1 Ⅰ Ⅱ Ⅲ Ⅱ

0 -2 -1.5 -3 -3 Pt Pt -3.0 Pt -6 -4 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 -0.8 -0.4 0.0 0.4 0.8 1.2 (a)(b) E (V/vs.RHE) (c) E (V/vs.RHE) E (V/vs.RHE) 1 Figure 3. Cyclic voltammograms (100 mV s− ) of metal disk electrodes in diﬀerent electrolytes with N2 Figure 3. Cyclic voltammograms (100 mV s−1) of metal disk electrodes in different electrolytes with N2 saturated: 0.1 M KOH (a); 0.1 M HClO4 (b); and 0.05 M K2SO4 (c). saturated: 0.1 M KOH (a); 0.1 M HClO4 (b); and 0.05 M K2SO4 (c). When the same experiments were performed in acid medium, 0.1 M HClO4, and neutral medium, 0.05 M K2SO4, instead of alkaline electrolyte, the main features of both voltammograms remain 1.00 Au disk 1.0 Pd disk 2 Pt disk unchanged (Figure3b for acid and Figure3c for neutral supporting blank electrolyte). For a Pt disk electrode, blank blank with CuSO 0.75 ) 4 ) ) with CuSO 0.5 1 with CuSO the onset potential of the oxygen region,4 A which is related to earlier OH adsorption, gradually4 becomes A A -4 -4 -4 0.50 10 10 10 0.0

positive as the electrolyte’s pH decreases.( When the pH varies from 1 to 13, the range of the onset ( ( 0 0.25 potential is about 0.2 to 0.4 V/vs. RHE.-0.5 However, when it comes to Pd electrodes, except that the − 0.00 -1 Current Current onset potential Current of the oxygen region is 0.1-1.0 V when the pH is 13, the onset potential in neutral medium -0.25 is 0.6 V. When the pH is 1, the potential-1.5 is as high as 0.75 V. The onset-2 potential for an Au electrode in acidic and-0.2 neutral 0.0 0.2 media 0.4 0.6 is more0.8 positive-0.2 0.0 than 0.2 Pd. 0.4 When 0.6 0.8 the pH is-0.2 13, 0.0 the 0.2 onset 0.4 0.6 potential 0.8 for Pd Potential (V/vs.RHE) Potential (V/vs.RHE) Potential (V/vs.RHE) is 0.15 V and1.5 for Au it is about 0.35 V. Although1.2 the onset potential1.2 changes when other solutes are present in the solution,Au/CNT on GC these phenomena mayPd/CNT explain on GC part of the results,Pt/CNT which on GC will be given below. 1.0 blank 0.8 blank 0.8 ) ) blank ) A A

The electrochemical active with CuSO surfaceA area (EAS) of Pt/CNTs, with CuSO Pd/CNTs, and Au/CNTs were measured -4 4 4 -4 with CuSO -4 4 10 0.4 10 0.5 10 0.4 ( ( by a Cu-UPD method and calculated by( the following equation:

0.0 0.0 0.0 Current

Q Current Current Current 2 Cu EAS-0.4 m /g = (1) -0.5 0.042 w(g) -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6· 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 Potential (V/vs.RHE) where QCu was thePotential average (V/vs.RHE) electric quantity of copper stripping and w representsPotential (V/vs.RHE) the content of noble metal on the working electrode. Where QCu is calculated by the following formula: Figure 4. Cyclic voltammograms (100 mV s−1) of Cu-UPD in 0.5 M H2SO4 in the absence and presence s of 1 mM CuSO4. Q (C) = (2) Cu f

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The electrochemical active surface area (EAS) of Pt/CNTs, Pd/CNTs, and Au/CNTs were measured by a Cu-UPD method and calculated by the following equation:

(/) = (1) 0.042 ∙ () where QCu was the average electric quantity of copper stripping and w represents the content of noble metal on the working electrode. Where QCu is calculated by the following formula: s () = (2) f where s is the area of the oxidation peak of the alcohol after subtracting the background and f represents the scan rate of the cyclic voltammetry. The CV results are shown in Figure 4. As listed in Table 1, EAS for Pt/CNTs, Pd/CNTs, and Au/CNTs are 110.0, 123.0, and 100.0 m2/g, respectively.

-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 -0.8 -0.4 0.0 0.4 0.8 1.2 1.0 0.6 0.6 0.5 0.3 Ⅱ Ⅲ Ⅱ Ⅲ 0.3 Ⅲ 0.0 Ⅱ 0.0 0.0

-0.3 -0.5 -0.3 -0.6 Au -1.0 Au Au -0.6 -0.9 -1.5 10 5.0 1.5 Ⅰ Ⅱ Ⅲ 5 2.5 Ⅰ Ⅱ Ⅲ Ⅰ Ⅱ Ⅲ A) A)

0.0

A) 0 0.0 -4 -4 -4

-1.5 -5 -2.5 I (10 I (10 I I (10 -3.0 Pd -10 Pd -5.0 Pd -15 -7.5 1 6 1.5 0 Ⅰ Ⅱ Ⅲ 3 0.0

Ⅰ Ⅲ -1 Ⅰ Ⅱ Ⅲ Ⅱ

0 -2 -1.5 -3 -3 Pt Pt -3.0 Pt Catalysts 2019, 9, 387 -6 6 of 18 -4 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 -0.8 -0.4 0.0 0.4 0.8 1.2 (a)(b) E (V/vs.RHE) (c) E (V/vs.RHE) E (V/vs.RHE) where s is the area of the oxidation peak of the alcohol after subtracting the background and f represents the scanFigure rate 3. ofCyclic the cyclicvoltammograms voltammetry. (100 mV s−1) of metal disk electrodes in different electrolytes with N2

Thesaturated: CV results 0.1 M KOH are shown(a); 0.1 M in HClO Figure4 (b4);. and As 0.05 listed M K in2SO Table4 (c).1 , EAS for Pt /CNTs, Pd/CNTs, and Au/CNTs are 110.0, 123.0, and 100.0 m2/g, respectively.

1.00 Au disk 1.0 Pd disk 2 Pt disk blank blank blank with CuSO 0.75 ) 4 ) ) with CuSO 0.5 1 with CuSO 4 A 4 A A -4 -4 -4 0.50 10 10 10 0.0 ( ( ( 0 0.25 -0.5

0.00 -1 Current Current Current Current -1.0 -0.25 -1.5 -2 -0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 Potential (V/vs.RHE) Potential (V/vs.RHE) Potential (V/vs.RHE)

1.5 1.2 1.2 Au/CNT on GC Pd/CNT on GC Pt/CNT on GC

1.0 blank 0.8 blank 0.8 ) ) blank ) A A

with CuSO A with CuSO -4 4 4 -4 with CuSO -4 4 10 0.4 10 0.5 10 0.4 ( ( (

0.0 0.0 0.0 Current Current Current Current -0.4 -0.5 -0.4

-0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 Potential (V/vs.RHE) Potential (V/vs.RHE) Potential (V/vs.RHE) 1 Figure 4. Cyclic voltammograms (100 mV s− ) of Cu-UPD in 0.5 M H2SO4 in the absence and presence Figure 4. Cyclic voltammograms (100 mV s−1) of Cu-UPD in 0.5 M H2SO4 in the absence and presence of 1 mM CuSO4. of 1 mM CuSO4. 2.3. Electrocatalytic Oxidation Behaviors of Alcohols over Different Electrodes The electro-oxidation behaviors of different alcohols over different electrodes were researched by Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts CV curves. Figures5–7 shows the forward peaks of CV curves for the oxidation of methanol, ethanol, ethylene glycol, n-propanol, 2-propanol, and glycerol over Pt, Pd, and Au electrodes in alkaline, acidic, and neutral media, respectively. The acid-base property of the electrolyte media was shown as a dominating influencing factor for the catalytic activity of all these electrodes. As shown in Figure5, in alkaline media, alcohol oxidation peaks appeared in the corresponding potential range over all electrodes. However, in acidic and natural media, oxidation peaks emerged on only Pt electrodes (as shown in Figures6C and7C), but hardly any oxidation signals were observed on Pd and Au electrodes (as shown in Figure6A,B and Figure7A,B). It indicated that Pt, Pd, and Au have catalytic activities in alkaline media, while only Pt have catalytic activities in acidic and natural media. After comparing CV peaks for Pt electrodes in the three kinds of media (as shown in Figures5C,6C and7C), we found that the lowest onset potential and highest current density were mostly emerged in alkaline media, and the situation was contrary in neutral media. It demonstrated that alkaline media is best for the electro-oxidation of these alcohols, and then acidic media is better than neutral media. The onset potential and current density of the CV peaks can be used to quantitatively analyze the catalytic performance of electro-oxidation reactions. Especially, the onset potential of the oxidation peak usually stands for the energy barrier of the reaction. Peak current density represents the maximum reaction rate. At this point, the diffusion rate and the reaction rate are balanced [27]. The method used to find the onset potential in this paper is to find the point at which the derivative of the current density with respect to the potential is zero. In addition, the peak potential can also be found by this method. The corresponding one is also the point where the derivative is zero [28]. The current density corresponding to the peak potential is the peak current density. The potential corresponding to this point is the onset potential. As shown in Figure8 we present an example. In the end, the initial potential of the electro-oxidation of ethanol on the Pt electrode is 0.3995 V (vs. RHE). For − the convenience of correlation analysis, the onset potential and current density data of CV peaks are generalized in Figures9 and 10, respectively. Obviously, the lowest onset potential and highest Catalysts 2019, 9, x FOR PEER REVIEW 7 of 19

2.3. Electrocatalytic Oxidation Behaviors of Alcohols over Different Electrodes The electro-oxidation behaviors of different alcohols over different electrodes were researched by CV curves. Figures 5–7 shows the forward peaks of CV curves for the oxidation of methanol, ethanol, ethylene glycol, n-propanol, 2-propanol, and glycerol over Pt, Pd, and Au electrodes in alkaline, acidic, and neutral media, respectively. The acid-base property of the electrolyte media was shown as a dominating influencing factor for the catalytic activity of all these electrodes. As shown in Figure 5, in alkaline media, alcohol oxidation peaks appeared in the corresponding potential range Catalysts 2019, 9, 387 7 of 18 over all electrodes. However, in acidic and natural media, oxidation peaks emerged on only Pt electrodes (as shown in Figures 6C and 7C), but hardly any oxidation signals were observed on Pd currentand Au densityelectrodes mostly (as shown emerged in Figures over both 6A,B the and Pt disk7A,B). and It Ptindicated/CNT electrodes, that Pt, Pd, except and Au for have the oxidation catalytic ofactivities ethanol in and alkaline n-propanol. media, Fromwhile theonly overall Pt have perspective catalytic activities of Figure in 10 acidic, Pt’s activityand natural is the media. best for After the oxidationcomparing of CV most peaks alcohols, for Pt whileelectrodes Pd is in in the the three medium. kinds However,of media (as in alkalineshown in media, Figures this 5C, regularity 6C, 7C), iswe not found completely that the accordedlowest onset with potential in the oxidation and highes oft ethanol,current density n-propanol, were andmostly glycerol. emerged Additionally, in alkaline polylolsmedia, and (ethylene the situation glycol andwas glycerol)contrary in are neutral oxidized media. more It easily demonstrated and have that a resulting alkaline higher media current is best densityfor the electro-oxidation than other alcohols, of these especially alcohols, on Au and electrodes. then acidic media is better than neutral media.

6 10 (A1) Au disk (A2) Au/CNT-GC 5 blank 8 blank MeOH 4 MeOH )

) EtOH

-2 EtOH -2 6 n-PrOH n-PrOH

3 cm . cm EG .

EG GLY 2 4 GLY 2-PrOH 2-PrOH j (mA j (mA j 1 2 0 0 -0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 E (V vs. RHE) E (V vs. RHE) 6 (B1) Pd disk (B2) Pd/CNT-GC 6 blank 5 blank 5 MeOH MeOH EtOH ) ) 4 EtOH -2 4 n-PrOH -2 n-PrOH EG cm cm . . 3 EG

3 GLY GLY 2-PrOH 2-PrOH 2 j (mA j 2 j (mA

1 1

0 0 -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.6 -0.4 -0.2 0.0 0.2 0.4 E (V vs. RHE) E (V vs. RHE) 10 15 (C1) Pt disk (C2) Pt/CNT-GC 8 blank 12 MeOH blank ) ) MeOH -2

-2 EtOH 6 EtOH n-PrOH 9 cm cm . . EG n-PrOH

EG GLY 4 6 GLY 2-PrOH j(mA j (mA 2-PrOH 2 3

0 0 -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.6 -0.4 -0.2 0.0 0.2 0.4 E (V vs. RHE) E (V vs. RHE) Figure 5. Voltammetry curves (100 mV s 1) of alcohol electro-oxidation over diﬀerent electrodes in Figure 5. Voltammetry curves (100 mV s−1−) of alcohol electro-oxidation over different electrodes in 0.1 0.1 M KOH solution. M KOH solution. Combining the trend lines of Figure9A,B, we found that for the electrocatalytic alcohol reaction by the same catalyst, the rate of electrooxidation in an alkaline medium is much greater than that in acidicCatalysts and 2019, neutral 9, x; doi: media.FOR PEER Additionally, REVIEW in acidic and neutral media, wewww.mdpi.com/journ could not ﬁnd a clearal/catalysts rule. Alcohols are more incompletely oxidized in acidic and neutral media. Catalysts 20192019,, 99,, 387x FOR PEER REVIEW 8 of 1819

0.04 2.0 (A1) Au disk (A2) Au/CNT-GC 0.02 1.6 )

) 1.2 blank -2

-2 0.00 MeOH blank cm .

cm 0.8 . EtOH MeOH -0.02 n-PrOH EtOH 0.4 n-PrOH EG j (mA j(mA GLY EG -0.04 2-PrOH 0.0 GLY 2-PrOH -0.06 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 E(V/vs.RHE) E (V vs. RHE)

0.06 2.0 (B2) Pd/CNT-GC (B1) Pd disk blank MeOH 1.5 0.04

) EtOH ) -2

n-PrOH -2 1.0 cm EG blank . cm 0.02 GLY . MeOH 2-PrOH EtOH 0.5 j(mA n-PrOH j (mA EG 0.00 0.0 GLY 2-PrOH

-0.02 -0.5 0.00.20.40.60.81.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 E(V/vs.RHE) E (V vs. RHE)

0.15 (D1) Pt disk 0.21 (C2) Pt/CNT-GC blank 0.18 0.12 MeOH EtOH 0.15 ) ) 2 -

0.09 2

n-PrOH - 0.12

cm EG blank

. cm

0.06 GLY . 0.09 MeOH 2-PrOH EtOH j(mA 0.03 j(mA 0.06 n-PrOH EG 0.03 GLY 0.00 2-PrOH 0.00 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.40.50.60.70.80.91.0 E(V/vs.RHE) E(V/vs.RHE) Figure 6. Voltammetry curves of alcohol electro-oxidation over diﬀerent electrodes in 0.1 M Figure 6. Voltammetry curves of alcohol electro-oxidation over different electrodes in 0.1 M HClO4 HClO solution. solution.4 After comparing the onset potentials for the oxidation of normal monohydric alcohols (methanol, ethanol, and n-propanol) in Figure 10A, the alcohol with a longer carbon chain has more negative potential over the Pt electrodes. However, this regularity is the opposite over Pd and Au electrodes. It means that normal monohydric alcohols with longer carbon chains to start to be oxidized on Pt electrodes more easily, while those with shorter carbon chains start to be oxidized on Pd and Au electrodes more easily. When comparing the onset potentials for the oxidation of n-propanol, 2-propanol, ethylene glycol, and glycerol in Figure9A, we found that those alcohols (2-propanol, glycerol) with a secondary hydroxyl group are oxidized more easily on Au electrodes than similar alcohols (n-propanol and ethylene glycol) without a secondary hydroxyl group. This regularity is also the opposite over Pt electrodes. It is suspected that Pt is more favorable towards the attack of the primary hydroxyl group, while Au is more favorable to the attack of the secondary hydroxyl group in our reaction conditions. This conclusion is the same as that of Mario Simoes et al. [20] for the electrooxidation of glycerol. From the overall perspective of Figure 10B, alkaline medium is most beneﬁcial for the oxidation of most alcohols, while acid medium is in the medium. Moreover, the onset potential in acidic media is similar to that in neutral media.

Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts Catalysts 2019, 9, 387 9 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 9 of 19

1.5 0.06 (A1) Au disk (A2) Au/CNT-GC blank MeOH 1.0 ) -2 0.03 EtOH ) n-PrOH -2 blank cm .

cm MeOH EG . 0.5 EtOH 0.00 GLY

j(mA 2-PrOH n-PrOH

j (mA EG 0.0 GLY -0.03 2-PrOH

-0.5 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -0.8 -0.6 -0.4 -0.2 0.0 0.2

E(V/vs.RHE) E (V vs. RHE) 1.5 Pd disk (B2) Pd/CNT-GC 0.04 (B1) blank

MeOH ) 1.0 -2 )

-2 EtOH cm 0.02 n-PrOH . blank cm . EG 0.5 MeOH

GLY EtOH j (mA

j(mA 2-PrOH n-PrOH 0.00 0.0 EG GLY 2-PrOH

-0.02 -0.5 -0.4 -0.2 0.0 0.2 0.4 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 E(V/vs.RHE) E (V vs. RHE)

0.027 (C2) Pt/CNT-GC (C1) Pt disk 0.027 blank 0.024 blank MeOH 0.024 MeOH

EtOH )

) EtOH -2

2 0.021 - n-PrOH 0.021 n-PrOH cm

EG . cm

EG .

0.018 GLY 0.018 GLY 2-PrOH 2-PrOH j (mA j(mA 0.015 0.015

0.012 0.012

-0.2 0.0 0.2 0.4 0.6 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 E(V/vs.RHE) E (V vs. RHE) Catalysts 2019, 9, x FOR PEER REVIEW 10 of 19 Figure 7. Voltammetry curves of alcohol electro-oxidation over diﬀerent electrodes in 0.05 M Figure 7. Voltammetry curves of alcohol electro-oxidation over different electrodes in 0.05 M K2SO4 K2SO4 solution. solution.

The onset potential and current density of the CV peaks can be used to120 quantitatively analyze blank 4 the catalytic performance of electro-oxidation reactions. Especially, EtOH the onset potential of the oxidation peak usually stands foronset the potential energy barriepeak potentialr of the reaction. d(j)/d(E) Peak cu80rrent density represents ) the maximum reaction rate.3 At this point, the diffusion rate and the reaction rated(j)/d(E) are balanced [27]. -2 The method used to find the onset potential in this paper is to find the point at which the derivative of the current densitycm with2 respect to the potential is zero. In addition, the peak40 potential can also be · found by this method. The corresponding one is also the point where the derivative is zero [28]. The

current density correspondingmA 1 to the peak potential is the peak current density. The potential

( 0 corresponding to thisj point is the onset potential. As shown in Figure 8 we present an example. In the end, the initial potential0 of the electro-oxidation of ethanol on the Pt electrode is −0.3995 V (vs. RHE). For the convenience of correlation analysis, the onset potential and current-40 density data of CV peaks are generalized in-1 Figures 9 and 10, respectively. Obviously, the lowest onset potential and highest current density mostly emerged-0.6 -0.3 over both 0.0 the Pt 0.3 disk and 0.6 Pt/CNT 0.9 electrodes, except for the oxidation of ethanol and n-propanol. FromE (V/vs.RHE)the overal l perspective of Figure 10, Pt’s activity is the best for the oxidation of most alcohols, while Pd is in the medium. However, in alkaline media, this regularityFigure is 8. notLinear completely sweep voltammetry accorded ofwith ethanol in the oxidation oxidation on aof Pt ethanol, electrode n-propanol, in 0.1 M KOH and solution. glycerol. Figure 8. Linear sweep voltammetry of ethanol oxidation on a Pt electrode in 0.1 M KOH solution. Additionally,Blue line: polylols The current (ethylene density ofglycol ethanol and oxidation glycerol) takes are a oxidized derivative more with respect easily toand the have potential. a resulting Blue line: The current density of ethanol oxidation takes a derivative with respect to the potential. higher current density than other alcohols, especially on Au electrodes.

Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts

Figure 9. Comparison of the current density of electro-oxidation over different electrodes in 0.1 M KOH solution (A) and over a Pt electrode in different solutions (B). Hollow icons are for metal disk electrodes; solid icons are for nanocatalyst covered GC disk electrodes.

Figure 10. Comparison of the onset potential of electro-oxidation over different electrodes in 0.1 M KOH solution (A) and over a Pt electrode in different solutions (B). Hollow icons are for metal disk electrodes; solid icons are for nanocatalyst covered GC disk electrodes.

Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts Catalysts 2019, 9, x FOR PEER REVIEW 10 of 19 Catalysts 2019, 9, x FOR PEER REVIEW 10 of 19

120

blank 4 120 blankEtOH 4 d(j)/d(E) onset potential peak potential EtOH 80 )

3 d(j)/d(E) d(j)/d(E) -2 onset potential peak potential 80 )

3 d(j)/d(E) cm -2 2 40 · cm

2 40 ·

mA 1

( 0 j mA 1

( 0

j 0 0 -40 -1 -40 -0.6 -0.3 0.0 0.3 0.6 0.9 -1 -0.6 -0.3E (V/vs.RHE) 0.0 0.3 0.6 0.9 E (V/vs.RHE) Figure 8. Linear sweep voltammetry of ethanol oxidation on a Pt electrode in 0.1 M KOH solution. FigureBlue line: 8. LinearThe current sweep density voltammetry of ethanol of ethanol oxidation oxidation takes a derivativeon a Pt electrode with respect in 0.1 toM the KOH potential. solution. Blue line: The current density of ethanol oxidation takes a derivative with respect to the potential. Catalysts 2019, 9, 387 10 of 18

Figure 9. Comparison of the current density of electro-oxidation over different electrodes in 0.1 M Figure 9. Comparison of the current density of electro-oxidation over diﬀerent electrodes in 0.1 M KOH solution (A) and over a Pt electrode in different solutions (B). Hollow icons are for metal disk KOHFigure solution 9. Comparison (A) and overof the a Ptcurrent electrode density in di ofﬀ erentelectro- solutionsoxidation (B ).over Hollow different icons electrodes are for metal in 0.1 disk M electrodes; solid icons are for nanocatalyst covered GC disk electrodes. electrodes;KOH solution solid (A icons) and are over for a nanocatalyst Pt electrode coveredin different GC disksolutions electrodes. (B). Hollow icons are for metal disk electrodes; solid icons are for nanocatalyst covered GC disk electrodes.

Figure 10. Comparison of the onset potential of electro-oxidation over different electrodes in 0.1 M Figure 10. Comparison of the onset potential of electro-oxidation over different electrodes in 0.1 M KOH solution (A) and over a Pt electrode in different solutions (B). Hollow icons are for metal disk KOH solution (A) and over a Pt electrode in different solutions (B). Hollow icons are for metal disk electrodes;Figure 10. solidComparison icons are of for the nanocatalyst onset potential covered of electro- GC diskoxidation electrodes. over different electrodes in 0.1 M KOHelectrodes; solution solid (A icons) and areover for a nanocatalystPt electrode incovered different GC solutions disk electrodes. (B). Hollow icons are for metal disk Theelectrodes; hydrogen solid bondicons are donation for nanocatalyst ability (HBD) covered is GC a major disk electrodes. property parameter of organic solvents; theCatalysts larger 2019 the, 9, HBD,x; doi: FOR the PEER stronger REVIEW the proton donation ability [29]. This propertywww.mdpi.com/journ of different alcoholsal/catalysts mayCatalysts have 2019 a, great9, x; doi: influence FOR PEER on REVIEW their oxidization reactions. Thus, we attemptedwww.mdpi.com/journ to use HBD toal/catalysts analyze the electro-oxidation behaviors of different alcohols. The oxidation of glycerol is relatively specific and is related to the intrinsic properties of the catalyst [30]. Therefore, the oxidation of glycerol is not discussed here. The correlations between the onset potential and HBD of alcohols over different electrodes in alkaline media are shown in Figure 11A. The catalytic ability of different electrodes for alcohols’ oxidation was found to have a certain correlation with the HBD of various alcohols. On Pt electrodes, as the HBD increases, the onset potential decreases. The relationship is contrary on Au and Pd electrodes. The relationship over the Pt electrode in alkaline media is contrary to that in acidic and natural media. The main reason for the relationships might be that the oxidation of alcohols has two important elementary steps: Firstly, the adsorption of alcohols by dehydrogenation of the -OH group, and followed by the electrolysis of protons combined with the hydroxide ions provided by the adsorbed water [31]. The second step contributes the current at the onset of the electron oxidation reaction. The rate-limiting step of alcohols’ electro-oxidation on Pt catalysts is the removal of protons [14]:

CH OH + Pt Pt-CH OH 3 → 3 ads + Pt-CH OH Pt-CO + 4 H + 4 e− 3 ads → ads Catalysts 2019, 9, 387 11 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 13 of 19

76 84 86 90 98 76 84 86 90 98 -0.8 -0.8 EG EG EG

EtOH EtOH -0.6 EtOH EtOH n-PrOH MeOH MeOH 2-PrOH 1-PrOH -0.6 2-PrOH -0.4 -0.4 Pt -0.2 -0.2 in KOH (PH=13)

in HClO (PH=1) 4 0.0 Pd 0.0 (V/vs.RHE) in K SO (PH=7)

(V/vs.RHE) 2 4

0.2 onset 0.2 onset E E 0.4 (B) (A) Au 0.4 0.6 0.6 70 80 90 100 70 80 90 100 HBD / a.u. HBD / a.u.

Figure 11. Correlations between electro-oxidation performance and hydrogen bond donation ability Figure 11. Correlations between electro-oxidation performance and hydrogen bond donation ability (HBD) of alcohols over different electrodes in 0.1 M KOH solution (A) and over a Pt electrode in (HBD) of alcohols over different electrodes in 0.1 M KOH solution (A) and over a Pt electrode in different solutions (B). Hollow icons are for metal disk electrodes; solid icons are for nanocatalyst different solutions (B). Hollow icons are for metal disk electrodes; solid icons are for nanocatalyst covered GC disk electrodes. covered GC disk electrodes. So, as the HBD increases, the activity of alcohols’ electrooxidation increases. At low over potentials on PdThe catalysts, apparent hydroxyl activation adsorption energies is the(Ea rate-determining) for alcohol oxidation step [32 ]:were measured and used to characterize the activity of the catalyst on electo-oxidizing alcohols [44]. Figures 12–14 show the linear sweep voltammetry (LSV) at differentPd + temperaturesOH- Pd-OH and +their4 e− Arrhenius plots of alcohol eletro- → ads oxidation on electrodes at peak potentials. According to the slope of the Arrhenius plots, Ea values were Fromobtained the as results follows of Na[45]: Li et al. [33], the activity of the electrooxidation of alcohols as a surface reaction is determined by the coverage of OH− onlnj the Au surface. In short, the adsorption and =−R( ) (3) dissociation of water on Pd and Au catalysts at low∂(1/T) potentials becomes a rate-determining step. The larger the HBD of the alcohol, the harder the electrolysis of water, which results in the lower catalytic activity.The E Thea values correlations for alcohol between electrooxidation the onset potentialat peak potentials and HBD are of shown alcohols in overin Figures the Pt 12d,13d,14d. electrode in Overdifferent the Au medias electrode are shown in alkaline in Figure media 11 B.(Figure The adsorption 12d), compared and dissociation with blank oflinear water voltammetry, on Pd and Au it wascatalysts found becomes that Au quite had dialmostfficult no in electrooxidation acidic and neutral activity media. for It meansmethanol. that The Pd and activation Au catalysts energy have of glycerolno significant oxidation activity is 27.5 on alcoholKJ/mol, oxidationand is obviously in acidic lower and neutral than the media. oxidation The case of n-propanol of Pt catalysts (42.2 in KJ/mol).acidic and It indicated neutral media that Au is similaris more to favorable that of Pd of andattacking Au in the alkaline secondary medium. and Thepolybasic adsorption hydroxyl and groupdissociation in our ofreaction water becomesconditions, a rate-determining and it is in accordance step. The with larger the results the HBD of ofSimoes the alcohol, et al.’s thework lower on thethe electrochemical catalytic activity. valorisation of glycerol [20]. Under similar conditions, the lowest activation energyThe was electrooxidation measured in the of oxidation alcohols of is an-propanol surface reaction, over the its Pd adsorption electrode. processThe Ea values is very of important. glycerol oxidationThus, the (44.9 above KJ/mol) rules are and also methanol explained oxidation from the (67.7 viewpoint KJ/mol) of adsorptionare much higher energy. than Table that2 lists of then- propanoladsorption (14.4 energies KJ/mol) of over water, the methanol, Pd electrode. and ethanolBased on on previous Pt, Pd, and research, Au surfaces the characteristics calculated by of DFT Pd of all havingwithin strong the generalized C-C bonds gradient [18] and approximationstrong adsorpti proposedon of its active by Perdew–Burke–Ernzerhof site to the ethoxy group ((CH [34–3CO)40].ads As) [46]shown jointly in Tabledetermine2, the this adsorption rule. For energies the Pt electrod of watere, the on activation Pt and Pd energy are similar, of methanol while theoxidation adsorption (44.1 KJ/mol)energies is ofthe methanol smallest, and is ethanol much lower on Pd than are almostthe oxidation twice asof n-propanol high as those (50.7 on KJ/mol) Pt. It indicated and glycerol that (61.7both KJ/mol). water and This alcohols is due can to the be absorbedmechanism on of Pt the sites, oxidation thus the of proton alcohols from on alcohol the Pt dehydrogenationelectrode. In the alkalinecan be transferredmedium, the to rate-limiting water and generatestep of alcohol current. oxidation Thus, is over the removal the Pt electrode, of protons when from the alcohol alcohol adsorbedis more easily on the dehydrogenated Pt electrode. Methanol (with higherhas the HBD largest value), HBD theand reaction the strongest carries proton on more capacity, easily. so That the activationmight be whyenergy we is observed the lowest. the The onset kinetic potential data decreases was found as theto be HBD in agreement increased overwith thethe Ptdata electrode, of the currentas shown density in Figure and onset11. It potential is reasonable of alcohol that a electro-oxidation. positive correlation They was were found all betweenreleased theat a catalyticsimilar regularityactivity and of HBDthe catalytic over the activity, Pt electrode and affected with moderate by the alcohol’s adsorption molecular energies structure. for both thePt electrodes subtracted showedand solvent the best molecule. activities T. Iwasita on the et oxidation al. [41] synthesized of C1 alcohol, PtRu Pd alloy electrodes and used were in situ more FTIR active to study on the the oxidationenergy change of C2-3 of themonobasic surface on alco thehols, adsorption and Au of electrodes matter during were methanol more active oxidation, on the and oxidation proved thatof polybasicthe addition and of secondary Ru makes alcohols. the adsorption of Pt to methanol weaker, which is more conducive to the oxidation of methanol. However, over the Pd catalyst, as the adsorption energies for alcohols are much higher than that of water, alcohol molecules easily occupy most of the active sites, and block the proton transmission channel. Based on the results of Tian Sheng et al. [39], the adsorption energy of

Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts Catalysts 2019, 9, 387 12 of 18 methanol ( 0.51 eV) and ethanol ( 0.74 eV) on the catalytic center of Pd-OH is much larger than − − ads that of H O on the surface active site of Pd ( 0.33 eV). This causes the adsorption of a single layer of 2 − alcohol molecules on the surface of Pd, and even the formation of a structure of a two-layer alcohol molecule [36]. In this case, the higher HBD of alcohol means it is more easily adsorbed to the Pd surface. The active site of the adsorbed H2O is reduced, and it becomes more diﬃcult for the reaction to carry on. It is in accordance with the negative correlation between the catalytic activity and HBD over the Pd electrode. The adsorption of alcohols and water over the Au surface is similar to that of the Pd surface, and Au adsorbs alcohol more strongly than water. Thus, a negative correlation between the catalytic activity and HBD was also detected over the Au electrode. Rakesh K. Pandey and V. Lakshminarayanan analyzed Tafel plots and found that the plots of the gold loaded on conductive polymer material systems ﬁtted two slopes at lower and higher potentials [42]. The higher slope at higher potentials indicates that the adsorption of intermediate carbonaceous species became stronger, and these substances occupied the active site of Au, resulting in an inability to form new Au-OHads. Therefore, HBD is inversely related to the ease of the reaction. Chuancheng Jia et al. proved that the highly active photo-generated intermediates on the surface of TiO2 can freshen the inert Au surface oxides, thereby increasing the catalytic activity of the catalyst [43].

Table 2. Adsorption energies (Eads) for H2O, methanol, and ethanol on several metal surfaces.

Metal H O E (eV) [34] CH OH E (eV) C H OH E (eV) 2 ads − ads 3 ads − ads 2 5 ads − ads Pt 0.35 0.32 [35] 0.37 [38] − − − Pd 0.33 0.51 [36] 0.74 [39] − − − Au 0.13 1.18 [37] 0.56 [40] − − −

The apparent activation energies (Ea) for alcohol oxidation were measured and used to characterize the activity of the catalyst on electo-oxidizing alcohols [44]. Figures 12–14 show the linear sweep voltammetry (LSV) at diﬀerent temperatures and their Arrhenius plots of alcohol eletro-oxidation on electrodes at peak potentials. According to the slope of the Arrhenius plots, Ea values were obtained as follows [45]: ! ∂lnj Ea = R (3) − ∂(1/T) E

The Ea values for alcohol electrooxidation at peak potentials are shown in in Figures 12d, 13d and 14d. Over the Au electrode in alkaline media (Figure 12d), compared with blank linear voltammetry, it was found that Au had almost no electrooxidation activity for methanol. The activation energy of glycerol oxidation is 27.5 KJ/mol, and is obviously lower than the oxidation of n-propanol (42.2 KJ/mol). It indicated that Au is more favorable of attacking the secondary and polybasic hydroxyl group in our reaction conditions, and it is in accordance with the results of Simoes et al.’s work on the electrochemical valorisation of glycerol [20]. Under similar conditions, the lowest activation energy was measured in the oxidation of n-propanol over the Pd electrode. The Ea values of glycerol oxidation (44.9 KJ/mol) and methanol oxidation (67.7 KJ/mol) are much higher than that of n-propanol (14.4 KJ/mol) over the Pd electrode. Based on previous research, the characteristics of Pd of having strong C-C bonds [18] and strong adsorption of its active site to the ethoxy group ((CH3CO)ads)[46] jointly determine this rule. For the Pt electrode, the activation energy of methanol oxidation (44.1 KJ/mol) is the smallest, and is much lower than the oxidation of n-propanol (50.7 KJ/mol) and glycerol (61.7 KJ/mol). This is due to the mechanism of the oxidation of alcohols on the Pt electrode. In the alkaline medium, the rate-limiting step of alcohol oxidation is the removal of protons from the alcohol adsorbed on the Pt electrode. Methanol has the largest HBD and the strongest proton capacity, so the activation energy is the lowest. The kinetic data was found to be in agreement with the data of the current density and onset potential of alcohol electro-oxidation. They were all released at a similar regularity of the catalytic activity, and aﬀected by the alcohol’s molecular structure. Pt electrodes showed the best activities Catalysts 2019, 9, 387 13 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 14 of 19

0.3 0.6 on the oxidationMeOH oxidation of C1 alcohol, Pd electrodes were more activen-PrOH on theoxidation oxidation of C2–3 monobasic alcohols, and Au electrodes were more active on the oxidation of polybasic and secondary alcohols. Catalysts 2019, 9, x FOR PEER REVIEW 14 of 19 0.2 293 K 0.355 V

) 0.4 0.3 298 K -2 0.6 ) MeOH oxidation n-PrOH oxidation303 K -2 0.1 293 K 308 K

313 K 0.2 298 K 293 K 0.355 V 303 K 0.2 318 K

) 0.4

j(mA cm j(mA 298 K -2 ) 0.0 308 K 303 K j(mA cm j(mA -2 0.1 313293 KK 308 K

318298 KK 313 K 0.0 -0.1 blank303 K 0.2 318 K (a) cm j(mA 0.0 308 K (b) j(mA cm j(mA 0.0 0.2 0.4 0.6 313 K 0.8 0.00.10.20.30.40.50.6 318 K 0.0 -0.1 (a) E(V vs. RHE) blank E(V vs. RHE) (b) 0.0 0.2 0.4 0.6 0.8 0.00.10.20.30.40.50.6

12 GLY oxidation E(V vs.0.299 RHE) V 3 E(V vs. RHE)

293 K 2 9 298 K 12 GLY oxidation 0.299 V -1 ) 303 K 3 Ea/kJ mol -2 1 308 K n-PrOH 42.2 6 293 K 313 K 2 GLY 27.5

9 298 K 318 K 0 -1 ln j

) 303 K Ea/kJ mol -2 308 K 1 j(mA cm j(mA 3 n-PrOH 42.2 6 -1 313 K GLY 27.5

318 K 0 (c) ln j 0 -2 j(mA cm j(mA 3 -1 (d) -3 -0.6(c) -0.3 0.0 0.3 0.6 0.9 -20.0031 0.0032 0.0033 0.0034 0 (d) -1 -1 E(V vs. RHE) -3 T /K -0.6 -0.3 0.0 0.3 0.6 0.9 0.0031 0.0032 0.0033 0.0034 Figure 12. (a–c) LinearE(V scan vs. RHE)curves (LSVs) of Au electrode in 0.1 M KOH -1+ 0.025-1 M alcohol; (d) T /K Arrhenius plot of alcohol eletro-oxidation on Au at peak potentials. FigureFigure 12. 12.(a –(ca)– Linearc) Linear scan scan curves curves (LSVs) (LSVs) of Au of electrode Au electrode in 0.1 in M KOH0.1 M+ KOH0.025 + M 0.025 alcohol; M alcohol; (d) Arrhenius (d) plotArrhenius of alcohol plot eletro-oxidation of alcohol eletro-oxi on Audation at peak on Au potentials. at peak potentials.

1.2 1.0 MeOH oxidation -0.001 V n-PrOH oxidation

1.01.2 293K 1.00.8 293 K MeOH oxidation n-PrOH oxidation 298K -0.001 V 298 K 0.8

) 1.0 303K ) 303 K

-2 0.8 293 K

293K -2 -0.099 V 0.6 308 K 308K 298K 298 K 0.60.8 313 K

)

313K 303K ) 303 K -2 -2 0.6 -0.099 V 318 K 318K 308K 0.4 308 K 0.40.6 313 K

313K j (mA cm j (mA j (mA cm 318 K 318K 0.4 0.20.4(a) 0.2 j (mA cm j (mA j (mA cm 0.2 (a) 0.2 (b) 0.0 0.0 -0.2 -0.1 0.0 0.1 0.2 (b) -0.2 0.0 0.2 0.4 0.0 0.0 -0.2 -0.1E (V vs. 0.0 RHE) 0.1 0.2 -0.2E(V 0.0 vs. RHE) 0.2 0.4 E (V vs. RHE) E(V vs. RHE) E /kJ mol-1 GLY oxidation a 8 0.060 V 2 MeOH 67.7 -1 GLY oxidation E /kJ mol 8 293 K n-PrOHa 14.4 0.060 V 2 MeOH 67.7 298 K GLY 44.9 6 293 K 1 n-PrOH 14.4 ) 303 K -2 298 K GLY 44.9 6 308 K 1 ) 303 K -2 313 K

4 308 K

318 313 K K ln j

4 j(mA cm 318 K ln j 2 -1 j(mA cm 2 -1 (c) -2 0 (d) (c) -2 0 (d) -0.4 -0.2 0.0 0.2 0.4 0.0031 0.0032 0.0033 0.0034 -0.4 -0.2 0.0 0.2 0.4 0.0031 0.0032 0.0033 0.0034 E(V vs. RHE) T-1/K-1 E(V vs. RHE) T-1/K-1 Figure 13. (a–c) LSVs of Pd electrode in 0.1 M KOH + 0.025 M alcohol; (d) Arrhenius plot of alcohol eletro-oxidation on Pd at peak potentials. Figure 13. (a–c) LSVs of Pd electrode in 0.1 M KOH + 0.025 M alcohol; (d) Arrhenius plot of alcohol Figure 13. (a–c) LSVs of Pd electrode in 0.1 M KOH + 0.025 M alcohol; (d) Arrhenius plot of alcohol eletro-oxidationeletro-oxidation on on Pd Pd at at peak peak potentials. potentials.

CatalystsCatalysts 2019 2019, 9,, 9x;, x;doi: doi: FOR FOR PEER PEER REVIEW REVIEW www.mdpi.com/journwww.mdpi.com/journal/catalystsal/catalysts Catalysts 2019,, 99,, 387x FOR PEER REVIEW 1415 of 1819

0.3 0.3 MeOH oxidation n-PrOH oxidation -0.067 V -0.116 V 0.2 293K 0.2 298K ) )

303K -2 -2 308K 0.1 0.1 313K 293K 318K 298K 303K j (mA cm j (mA j (mA cm j (mA 0.0 0.0 308K 313K 318K (a) (b) -0.1 -0.1 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 -0.30 -0.15 0.00 0.15 E (V vs. RHE) E (V vs. RHE)

4 2 -1 GLY oxidation -0.040 V Ea/kJ mol MeOH 44.1 1 50.7 3 293 K n-PrOH 298 K GLY 61.7

) 303 K 0 -2 2 308 K

313 K -1

318 K j ln mA cm mA ( j 1 -2 (c) -3 0 (d)

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.0031 0.0032 0.0033 0.0034 E(V vs. RHE) T-1/K-1

Figure 14. (a–c) LSVs of Pt electrode in 0.1 M KOH + 0.025 M alcohol; (d) Arrhenius plot of alcohol eletro-oxidationFigure 14. (a–c) onLSVs Pt atof peakPt electrode potentials. in 0.1 M KOH + 0.025 M alcohol; (d) Arrhenius plot of alcohol eletro-oxidation on Pt at peak potentials. 3. Experimental 3. Experimental 3.1. Materials and Reagents 3.1. MaterialsCarbon-nanotubes and Reagents (CNTs) (>50 nm outer diameter, 5–15 nm inner diameter, 1–5 um length) were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences (Chengdu, Carbon-nanotubes (CNTs) (>50 nm outer diameter, 5–15 nm inner diameter, 1–5 um length) were China). Methanol ( 99.5%), ethanol ( 99.7%), and perchloric acid (70–72%) were obtained from purchased from Chengdu≥ Organic Chemicals≥ Co. Ltd., Chinese Academy of Sciences (Chengdu, Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Ethylene glycol (EG, >99%), n-propanol China). Methanol (≥99.5%), ethanol (≥99.7%), and perchloric acid (70%–72%) were obtained from ( 99.9%), 2-propanol ( 99.9%), glycerol (GLY, 99.5%), sodium borohydride (NaBH , 98%), cupric Sinopharm≥ Chemical Reagent≥ Co. Ltd (Shanghai,≥ China). Ethylene glycol (EG, >99%),4 n-propanol sulfate anhydrous (CuSO4, 99%), potassium hydroxide (KOH, 95%), and potassium sulfate (K2SO4, (≥99.9%), 2-propanol (≥99.9%), glycerol (GLY, ≥99.5%), sodium borohydride (NaBH4, 98%), cupric ACS) were obtained from Aladdin (Shanghai, China). Chloroplatinic acid hexahydrate (H2PtCl6 6H2O, sulfate anhydrous (CuSO4, 99%), potassium hydroxide (KOH, 95%), and potassium sulfate (K2SO4, ACS reagent, 37.50% Pt basis), palladium (II) chloride (PdCl2, anhydrous, 60% Pd basis), gold (III) ACS) were obtained≥ from Aladdin (Shanghai, China). Chloroplatinic acid hexahydrate (H2PtCl6 chloride trihydrate (HAuCl4 3H2O, ACS reagent, 49.0% Au basis), and poly (vinyl alcohol) (PVA, 6H2O, ACS reagent, ≥37.50% Pt· basis), palladium (II)≥ chloride (PdCl2, anhydrous, 60% Pd basis), gold MW = 10,000, 80% hydrolyzed) were obtained from Sigma-Aldrich (Shanghai, China). (III) chloride trihydrate (HAuCl4·3H2O, ACS reagent, ≥49.0% Au basis), and poly (vinyl alcohol) 3.2.(PVA, Catalyst MW = Synthesis 10,000, 80% and hydrolyzed) Physical Characterizations were obtained from Sigma-Aldrich (Shanghai, China).

3.2. CatalystPlatinum Synthesis was deposited and Physical on theCharacterizations carbon nanotubes (Nitric acid pretreated) by ethylene glycol reduction [47]. The amount of platinum precursor added was calculated to obtain a load of 1 wt.% Platinum was deposited on the carbon nanotubes (Nitric acid pretreated) by ethylene glycol Pt/CNTs in the catalyst. EG was both a solvent and a reducing agent in this preparation. A solution of reduction [47]. The amount1 of platinum precursor added was calculated to obtain a load of 1 wt.% H2PtCl6 3.2 mmol L− was prepared from high purity reagent and ethylene glycol. First, the precursor Pt/CNTs in the catalyst.· EG was both a solvent and a reducing agent in this preparation. A solution solution was slowly added to a dispersion of CNTs in EG under sonication and, afterwards, the pH of of H2PtCl6 3.2 mmol·L−1 was prepared from high purity reagent and ethylene glycol. First, the the dispersion was adjusted by adding an appropriate amount of KOH solution. The dispersion was precursor solution was slowly added to a dispersion of CNTs in EG under sonication and, afterwards, stirred under reﬂux at 140 ◦C for 2 h. Subsequently, the catalyst was ﬁltered and thoroughly washed the pH of the dispersion was adjusted by adding an appropriate amount of KOH solution. The with ultrapure water (with a resistivity of 18.2 MΩ cm at 20 ◦C), and then dried in vacuo for 12 h at dispersion was stirred under reflux at 140 °C for 2 h. Subsequently, the catalyst was filtered and 70 ◦C. thoroughly washed with ultrapure water (with a resistivity of 18.2 MΩ cm at 20 °C), and then dried Pd nanoparticles were deposited on CNTs by a colloid method for a total metal concentration of in vacuo for 12 h at 70 °C. 1 wt.% [48]. For the preparation of supported Pd colloidal materials, an aqueous solution of PdCl2 of

Catalysts 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts Catalysts 2019, 9, 387 15 of 18

the desired concentration was prepared. An aqueous solution of NaBH4 was used as the reduction agent. Fresh solutions of PVA were also prepared, as the stabilizing agent. First, to an aqueous PdCl2 solution of the desired concentration, the required amount of a PVA solution was added (vinyl alcohol/Pd (mol/mol) = 1.2). A freshly prepared solution of NaBH4 (NaBH4/Pd (mol/mol) = 20) was then added to form a colloid. After stirring for 30 min, the colloid was immobilized by adding CNTs under vigorous stirring conditions. Lastly, the catalyst was filtered and thoroughly washed with 95 ◦C hot ultrapure water, and then dried in vacuum for 12 h at 70 ◦C. The same steps were used to prepare Au/CNTs, but the amount of NaBH4 and PVA was changed and the corresponding precursor was used. X-ray diffraction (XRD) analyses of the catalysts were performed using a Bruker AXS D8 Advanced Focus diffractometric using an Ni filtered Cu Kα radiation under a 0.154 nm k-value, and operated at 40 kV and 40 mA. Transmission electron microscopy (TEM, JEM-2100Plus) analysis was carried out by dispersing the powder products as slurry in ethanol before drying them on a Cu grid. Inductively coupled plasma atomic emission spectroscopy (ICP–AES, AA6800, Shimadzu, Japan) was used to determine the metal loading amounts on the carbon support. X-ray photoelectron spectra were obtained from an ESCALAB MKII spectrometer (VG Scientific, Waltham, MA, USA) using Al KR radiation. Spectra correction was based on adventitious carbon, using a C 1s binding energy of 284.6 eV.

3.3. Fabrication and Electrochemical Characterization of Electrodes Electrochemical experiments were carried out in an electrocatalytic three-electrode reactor using an Autolab PGSTAT 302N Potentiostat-Galvanostat (Shanghai, China) at room temperature. Underpotential deposition (UPD) of copper was used to characterize high-surface-area electrocatalysts [49]. A Pt tablet and Ag/AgCl (in 3 M KCl) were used as the counter and reference electrodes, respectively. A rotating disk electrode (RDE, diameter 5 mm) coated with catalyst was used as the working electrode. A catalytic ink was prepared from the metal/CNTs powder (2 mg) added in a mixture of 925 µL of a water and ethanol (1:1) mixture and 75 µL of Nafion (0.5 wt.% from DuPont). Twenty µL of the catalytic ink was finally deposited onto each face of the carbon sheet and the solvent was evaporated at room temperature; this amount represents 0.0204 µg cm2 of the metal loading. The cyclic voltammetry experiments were performed using a disk electrode rotated at 2000 rpm. The electrolyte was saturated with N2 at room temperature. All copper upd experiments were carried out in a 0.5 M H2SO4 and 0.001 M CuSO4 solution unless otherwise stated. A cyclic voltammetry scan was then performed from the admission potential to a point at which all of the upd copper was oxidized at a scan rate of 0.1 V s 1. · − The electro-oxidation of alcohols was carried out as above unless otherwise stated. In an alkaline environment, cyclic voltammetrys of different alcohols’ oxidation were carried out in 50 mL 0.1M KOH solution containing 1.25 mmol alcohol at a sweep rate of 100 mV s 1. In an acidic environment, the · − supporting electrolyte was replaced with 0.1 M HClO4. In a neutral environment, it was replaced with 0.05 M K2SO4. The onset potential of a CV peak was determined by a derivative method. As shown in Figure8 a point was found at which the derivative of the current density with respect to the potential was positive. The potential corresponding to this point is the onset potential.

3.4. Apparent Activation Energy of Electrodes Electrochemical experiments were carried out in an electrocatalytic three-electrode reactor using an Autolab PGSTAT 302N Potentiostat-Galvanostat at room temperature. Solutions were freshly prepared and purged with high purity nitrogen before each experiment. A Pt tablet and a saturated calomel electrode were used as the counter and reference electrodes, respectively. A commercially available metal electrode was used as the working electrode, and linear scanning was performed at a series of temperatures. Catalysts 2019, 9, 387 16 of 18

4. Conclusions By investigating the electro-oxidation behaviors of some typical alcohols over Pt, Pd, and Au electrodes in acidic, neutral, and alkaline media, respectively, several regularities were discovered. Firstly, the acid-base property of the electrolyte media was a dominating influence for the catalytic activity of all these electrodes. Alkaline media as found to be the best, then acidic media. Alcohols were the most difficult to oxidize in neutral media, and thus had the lowest current density. Except for a few special cases, the onset potentials of the oxidation peak in alkaline media were about 0.6 V lower than that in acidic and neutral media. Secondly, Pt electrodes showed the best activities on the oxidation of most alcohols in all media, excepting for ethanol and n-propanol in alkaline media. Finally, the molecular structure distinction and hydrogen bond donation ability (HBD) of different alcohols were also found to have a great relationship with the catalytic performance. In alkaline media, on Pt electrodes, methanol with the highest HBD was more easily oxidized than other alcohols with low HBD. Contrarily, on Pd and Au electrodes, n-propanol with the lowest HBD was more easily oxidized compared to other alcohols. The case of Pt catalysts in acidic and neutral media was similar to that of Pd and Au in alkaline medium. The reasons of these regularities are still being explored. We hope that the results of this work will have guiding significance for the study of DAFCs in the future.

Author Contributions: Conceptualization, C.Z. and H.Y.; Methodology, B.W.; Software, Y.J.; Validation, Y.C., F.Y. and B.W.; Formal Analysis, B.W., and L.T.; Investigation, L.T.; Resources, Y.Y.; Data Curation, B.W.; Writing-Original Draft Preparation, B.W., and L.T.; Writing-Review & Editing, C.Z., and H.Y.; Visualization, Y.J.; Supervision, C.Z.; Project Administration, Y.Y.; Funding Acquisition, C.Z. Funding: This research was funded by [China National Natural Science Foundation] grant number [21503187]. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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