Insitu FTIR studies for the enhanced activity of Pt(HY) and Pt-Ru(HY) zeolite catalysts for electrooxidation of methanol in fuel cells

Journal Pre-proofs Research paper Insitu FTIR studies for the enhanced activity of Pt(HY) and Pt-Ru(HY) zeolite catalysts for electrooxidation of methanol in fuel cells Purnakala Samant, J.B. Fernandes PII: DOI: Reference:

S0009-2614(20)30192-5 https://doi.org/10.1016/j.cplett.2020.137277 CPLETT 137277

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

8 January 2020 22 February 2020 24 February 2020

Please cite this article as: P. Samant, J.B. Fernandes, Insitu FTIR studies for the enhanced activity of Pt(HY) and Pt-Ru(HY) zeolite catalysts for electrooxidation of methanol in fuel cells, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137277

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Insitu FTIR studies for the enhanced activity of Pt(HY) and Pt-Ru(HY) zeolite catalysts for electrooxidation of methanol in fuel cells . Purnakala Samanta and J. B. Fernandesb aGovernment bSchool

College of Arts, Science and Commerce, Khandola, Marcela Goa, 403107, India. of Chemical Sciences, Goa University, Taleigao Plateau, Goa 403206, India.

Corresponding author E-mail: [email protected];

9326142001

Abstract: insitu FTIR (Fourier transform infrared spectroscopy) technique has been employed to study the oxidation of methanol on Platinum and Platinum - Ruthenium catalysts supported on HY zeolites [Pt-Ru (HY)/C] . The reaction has been studied by recording the spectra at different temperature ranges from room temperature till 170oC. The catalytic activity has been studies in presence and absence of oxygen supply. The mechanism for oxidation of methanol has also been interpreted from the data obtained. The decomposition of methanol on Platinum (HY)/C catalyst occurs via carbonate/ formate pathway in presence of oxygen and carbonate/carboxylate in absence of oxygen. The optimum efficiency of the Pt (HY) catalyst occurs at 1200C. But on the surface of Pt- Ru (HY)/C catalyst, methanol oxidation occurs at room temperatures, thus emphasizing higher catalytic efficiency.The enhanced activity of the Platinum–Ruthenium / HY has been explained on the basis of intermediates formed during the reaction. . Key words: Methanol oxidation, Platinum–Ruthenium(HY) catalyst, carbonate/formate intermediates, fuel cells.

Insitu FTIR studies for the enhanced activity of P(HY) and Pt-Ru(HY) zeolite catalysts for electrooxidation of methanol in fuel cells Abstract: insitu FTIR (Fourier transform infrared spectroscopy) technique has been employed to study the oxidation of methanol on Platinum and Platinum - Ruthenium catalysts supported on HY zeolites [Pt-Ru (HY)/C] . The reaction has been studied by recording the spectra at different temperature ranges from room temperature till 170oC. The catalytic activity has been studies in presence and absence of oxygen supply. The mechanism for oxidation of methanol has also been interpreted from the data obtained. The decomposition of methanol on Platinum (HY) /C catalyst occurs via carbonate / formate pathway in presence of oxygen and carbonate / carboxylate in absence of oxygen. The optimum efficiency of the Pt (HY) catalyst occurs at 1200C. But on the surface of Pt- Ru (HY)/C catalyst, methanol oxidation occurs at room temperatures, thus emphasizing higher catalytic efficiency.The enhanced activity of the Platinum–Ruthenium / HY has been explained on the basis of intermediates formed during the reaction. . Key words: methanol oxidation, Platinum–Ruthenium(HY) catalyst, carbonate/formate intermediates, fuel cells.

* e-mail: [email protected] Mob:9823556759 [email protected] , Mob:7507164506

1. Introduction:

Direct methanol fuel cells are attractive as an important portable energy source for transportation since aqueous methanol can be electrochemically oxidized when fed to the anode [1,2]. Rate of methanol oxidation reaction (MOR) can be enhanced by improving activity and stability of Platinum catalysts when alloyed with other transition metals and metal oxides [35]. Role of supporting materials such as carbons for the active components have great importance in the recipe of catalyst preparation since they impart special characteristics such as mesoporosity, shape selectivity and surface functionalities which reduce diffusional constraints and improve metal - support interactions required for conventional heterogeneous catalysis as well as electrocatalysis [6-8]. Enhanced activity of Pt-Ru supported on zeolites has been already reported by present authors as well as recently by Z. Mojović [9, 10]. Synthesis method employed for catalysts can directly influence its activity. Different methods have been developed to synthesize active noble metal-based nanocomposites with high surface area and enhanced activity for electrocatalysis of methanol oxidation reaction [11]. Additionally one of the important research in Fuel cell technology also includes developing polymeric materials for PEMFC and different polymer nanocomposite membranes for DMFC applications are studied [12]. The oxidation of methanol is limited by the formation of CO and/or C-OH type of intermediates at the electrocatalyst surface [13-14]. In recent years, significant development has been made in understanding the mechanism of methanol oxidation. The insitu FTIR technique is being widely used for CO or pyridine adsorption studies to determine the type of acid sites present on the surface and also for the intermediates formed during the several gas phase reactions [15-17]. The mechanism of selective oxidation of methanol over stannic oxidemolybdenum oxide catalyst is studied by FT-IR spectroscopy and the behaviour of surface species with catalyst requirement has been discussed [18].

ATR- FTIR spectroscopy

employed by Watanabe et al. [19] had clearly identified COL, COB and COO- as adsorbed species during methanol oxidation in presence of HClO4 on the surface of Pt-Ru catalyst obtained by sputtering technique. Methanol oxidation can follow special pathways on various electro catalysts such as Ru/OMS-2 etc. [4,8]. The enhanced activity of the Pt–Ru catalyst when compared with Pt for methanol oxidation has been attributed to both a bi-functional mechanism and a ligand (electronic) effect.We have also shown earlier the superiority of Pt-Ru (HY zeolite ) /C catalysts over Pt-Ru / C. Zeolite can provide guest- host type of interaction in their super cages to promote facile oxidation of methanol to CO2. The porous material can provide improved permeability and minimize the disadvantage associated with restricted gas diffusion. The present investigation is to throw further light on the mechanism of electrocatalytic oxidation of methanol over Pt-Ru catalysts in zeolite matrix.

2. Experimental 2.1: Preparation of Pt (HY) and Pt-Ru(HY) zeolite catalysts The technique of synthesis involved during the preparation of catalysts was reported earlier [9], and briefly illustrated in the following schemeI.The amount of the Pt complex and ruthenium salt were taken so as to have combined metal loading within the zeolite was 1%.

2.1: insitu FTIR studies insitu FTIR studies following methanol adsorption was carried out on Pt-Ru(HY) and Pt(HY) in the temperature range 180oC and at pressures 10-100 torr. The self-supporting sample wafer of 25 mm diameter and weighing about 80 mg were mounted in a high temperature high pressure IR cell fitted with water cooled CaF2 window. A Mattson model Cygnus 100 FTIR equipped with DTGS detector is used in this study. The samples were heated insitu with or without oxygen atmosphere to remove adsorbed hydrogen. Samples were cooled under vacuum up to room temperature. For reference, background spectra were recorded with pellet as reference. Various scans of the spectrum at different time intervals were recorded. The overlapping bands were separated using computer software of FTIR in order to determine the number and position of the individual peaks.

Samples were evacuated and heated at 300oC and methanol absorption studies were carried out with or without passing oxygen into the reactor. Each sample was exposed by injecting 10 µL of distilled methanol followed by 4 mL of oxygen.

3. Results and Discussion 3.1Effect of CH3OH + O2 on Pt (HY) catalyst The well accepted mechanism during electrooxidation of methanol is a four step dehydrogenation, forming adsorbed CO as a precursor to CO2 which is presented as follows

CH3OH  CH2OH + H++ ex

CH2OH  CHOH+ H+ + ex

xx

xxx

xx

CHOH

 COH + H++ e-

COH

 CO + H+ + e-

xxx

x

where, x stands for a Pt site. The intermediates CH2OH and CHOH can further react to give specieslike formaldehyde and formic acid [20].

The various species namely Pt(CO)ad, Ru(CO)ad, Ru(OH)ad and Pt(OH)adcan be involved in the oxidation process at the surface of Pt-Ru catalyst [21].The Ru atoms brings about the facile adsorption of oxygen containing species at lower potentials thereby promoting the oxidation of CO to CO2, which can be summarized in following steps. Pt + CH3OH → PtCOads+ 4H+ +4e−

Ru+ H2O → Ru(OH)ads+ H+ + e−

(0.2V)

Pt + H2O →Pt OH ads + H+ + e-

(0.7V)

Finally, a Langmuir–Hinshelwood type mechanism is envisaged on carbon supported Pt-Ru catalysts to produce CO2 Pt-COads+Ru(OH)ads→ CO2+Pt+ Ru+ H+ + e−

It was previously reported through insituFTIR studies of adsorbed CO that oxidation of CO to CO2 is greatly facilitated on zeolite supported Pt – Ru catalyst. Further investigation revealed that these catalysts showed proportionately high activity for electro oxidation of methanol, as evident in the observed current densities [9] (presented here in Table 1 for ready reference). The present investigation describes insitu FTIR studies on theses catalysts to understand the possible intermediate pathways during methanol oxidation.Thus methanol adsorption / oxidation studies were carried out on Pt(HY) and Pt–Ru(HY) catalysts by using methanol & O2 feed in a 1:1 molar ratio. The catalysts were exposed to the reactant mixture for 30 minutes, followed by evacuation at different temperatures. The ir spectra of adsorbed methanol over the Pt(HY) catalyst are presented in Fig 1 in the relevant ir region of 1300cm-1 – 3000cm-1. It is clear from Fig 1 that methanol adsorbed at ambient temperature (270C) showed absorptions at 2943, 2819, 1480 and 1384 cm-1 respectively which are characteristic of CH3 stretching and deformation vibrations as also specified in Table 2, [22-25]. Peaks due to carbonates observed at 1659 cm-1 and 1454 cm-1 suggests that methanol oxidation could follow via carbonate intermediate. The peak at 1659 cm-1 is not attributed to hydroxyl or moisture as it grows in intensity at higher temperatures. Moreover carbonate peaks around this value has been reported following methanol adsorption over alkali metal ion exchanged X zeolites [24].

However following evacuation at 80oC, two additional peaks began to appear at 1599 cm-1 and 1350 cm-1 suggesting that at elevated temperatures catalysis would involve simultaneous additional formation of formate intermediate. After further evacuation at 120oC, both the carbonate and formate peaks greatly increased in intensity as shown in Fig 1 (b) and (c). This effect was also associated with complete disappearance of all four methanol absorption peaks at 2943, 2819, 1480 and 1384 cm-1. Methanol gets apparently converted into the above intermediates simultaneously. At the same time peaks due to formation of CO and CO2 at 2358 cm-1 appeared at these temperatures. When,the evacuation was carried out at 180oC, the absorptions of both the intermediates particularly that of the formate decreased in intensity, as they decomposed further to CO2. The intensity decrease was anticipated, as there was no further supply of methanol to the in situ catalyst.It can thus be concluded that Pt(HY) catalyst decomposes methanol via simultaneous formation of carbonate/ formate type intermediates. The spectra in Fig. 2 were recorded before evacuation, other conditions being same as in Fig. 1. It is evident that carbonate peaks around 1650 cm-1 appear almost at the same rate but the formate peaks particularly at 120oC , appeared at significantly lower intensity probably indicative of its facile dissociation to give CO2 , as intense absorption due to formation of CO2 is seen at this stage . However the carbonate peak intensity was almost same before and after evacuation Also the spectra at 120oC and 180oC, (Fig 1 C & D,) show small but broad absorption due to CO formation between 2100 cm-1to 2250 cm-1and also peaks due to CO2formation are seen at 2337 cm-1and 2258 cm-1. It may be noted that these peaks are better evident in the spectra taken prior to evacuation (Fig 2 C & D). Since CO or CO2 formation appears at or beyond 120oC, one may conclude that platinum electrocatalyst would presumably show optimum efficiency for methanol oxidation, when operated around 120oC.

The corresponding spectra were also recorded for Pt (HY) catalyst when exposed to methanol alone without oxygen(Fig 3).The noticeable difference was that the formate peaks were not seen but a peak at 1574 cm-1due to carboxylate appeared instead of 1599 cm-1while the carbonate peaks continued to be prominent at around 1650 cm-1at all temperatures studied even though they were also reduced in intensities. Thus one may conclude that decomposition of methanol to CO2follows carbonate/ formate pathways in presence of oxygen and carbonate /carboxylate in absence of oxygen. However the carbonate peak intensity was almost same before and after evacuation. Therefore it appears that better electrocatalysis should follow via formate rather than carbonate formation and the catalyst composition should be such that it will prevent carbonate formation and or enhance its decomposition. Unland (22) suggested generation of three types of formates structures which involve covalent bonding with the metal in zeolites.Formation of Type b (bidentate) and Type c (bridged) metal formate structures can also be predicted inX zeolite during methanol oxidation compared to Type a ( monodentate) as shown in (Fig 4). Thus the present insitu FTIR investigation of methanol oxidation on Pt(HY) can provide direct evidence for formation of metal formates intermediate within zeolites.

Thus

one may conclude that decomposition of methanol to CO2 on Pt(HY) catalyst follows carbonate/ formate pathway in presence of oxygen.The higher activity of the catalyst could also be attributed to the synthesis route, where it is expected that Pt can be deposited both external and internal surface of zeolite when prepared from [Pt (NH3)4+3] precursor [20]. Further, in the crystallographic structure of Y zeolite, each supercage possesses four nearest neighbours with crystallographic channel diameter of size of 7.1 A0. Hence molecular diffusion of methanol can occur freely in three dimensionsof the Y zeolite to approach the platinum sites which are located internally. It is known from previous studies [1,2] that Pt2CO obtained from

deprotonation of methanol, is a precursor for the carboxylate type intermediate. In light of the observations made in the present studies vis a vis the existing concepts, the mechanism for methanol decomposition on Pt(HY) catalyst may be summarised as per the scheme illustrated in Fig. 5

3.2 Effect of CH3OH + O2 on Pt-Ru (HY) catalyst The room temperature ir spectra (Fig.6) following methanol adsorption on the Pt-Ru(HY) catalyst after evacuation is similar to Pt( HY) catalyst ( Fig 1). However the spectra obtained following evacuation at 80 oC is markedly different for methanol adsorption on the two catalysts. Thus the formate bands at 1559 cm-1 and 1350 cm-1 seen in Pt(HY) (Fig 1) are conspicuous by their absence in the Pt-Ru(HY) catalyst. Further additional peaks at 1720 cm-1 and 1330 cm-1 appeared. These were attributed to bent CO2 molecules lying on the surface with its carbon attached to the surface. At this stage there is simultaneous appearance of bands at 2330 cm-1 and 1352 cm-1 corresponding to CO2 absorption, CO2 being desorbed following evacuation. The carbonate ir absorption frequency is greatly enhanced with simultaneous diminishing of methanol absorptions. CO2 formation at this temperature was not significant at 80oC on Pt(HY), but showed intense CO2 band on Pt-Ru(HY) at this temperature. This reemphasizes higher efficiency of the Pt-Ru(HY) over Pt zeolite catalyst for methanol decomposition. Further enhanced effect (more intense CO2 band) was seen following evacuation at 120oC was observed. Also no formate peaks were observed on Pt-Ru(HY) catalyst unlike the case with Pt(HY) catalyst suggesting different methanol decomposition pathways on the two catalysts.. Thus Pt-Ru (HY) decomposes methanol to CO2 through intermediate formation of carbonate and CO2. Also as carbonate phase persist substantially without decomposition even at higher temperature it seems to be one of the factors responsible for poor catalytic efficiency reported for methanol oxidation.

It is observed that the carbonate band around 1650 cm-1 which was smooth in Pt(HY), tend to show multiple splitting in Pt-Ru(HY) both at room temperatures and at 80oC, more prominently at 80oC. This could be due to lower stability of the carbonate showing tendency to dissociate into CO2 via intermediate formation of CO type species. This hypothesis could be justified when one looks at the spectrum ‘C’ at 120oC. The carbonate band has greatly decreased in intensity with simultaneous appearance of a clear band due to C – O stretch of carbonyls in the region 1880 – 2100 cm-1 and could be associated with the formation and growth of (CO)x clusters [26 - 27]. The cluster can be trapped in zeolite cages. The formation of Pt (CO) clusters in the form of either [Pt12(CO)24] 2- or [Pt9(CO)18] 2- in the cages of NaY zeolite has been reported [28]. Roduner [29] discussed the reconstruction of bigger size Pt13 clusters into smaller sized Pt2(CO)m (m= 2-3) on exposure of CO gas onNaY zeolite. Hence formation of [PtX(CO)Y ] (X= 2-3 , Y= 2-3) types of clusters within the cages of zeolite on the Pt -Ru be predicated .

The [Pt-RuX(CO)Y] clusters get constrained due to entrapment in zeolite cages and have a facile tendency to dissociate into CO2. In fact an intense band due to CO2 formation is simultaneously seen at around 2352 cm-1

Conclusions: 1. (i)

The decomposition of methanol on Pt (HY) catalyst occurs via carbonate / formate pathway in presence of oxygen and carbonate / carboxylate in absence of oxygen.

(ii)

Considering decomposition of the intermediates into CO or CO2, the optimum efficiency of the Pt(HY) catalyst occurs at 1200C

(iii)

CO2 formation on Pt- Ru (HY) catalyst occurs even at room temperatures, thus re-emphasizing its higher catalytic efficiency.

(iv)

The decomposition of methanol on Pt- Ru (HY) catalyst occurs predominantly through intermediates carbonate / formate.

2. The carbonate intermediate is unstable on the Pt- Ru (HY) catalyst and shows facile tendency to dissociate into CO2.

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Table 1: Electrochemical activity of catalysts expressed in terms of current densities at a potential of 250 mV [9]. Catalyst

HY

Pt/C

Pt(HY)

Pt-Ru/C

Pt-Ru(HY)

Current density

3

45

60

75

140

-

130

118

125

95

(mAcm-2) Tafel slopes (mV per decade)

Table 2 : Assignment of ir absorption in cm-1 of Pt(HY) and Pt-Ru(HY) catalyst at various

ir absorption ,cm-1

temperatures following adsorption [20--23]. 270C Pt (HY) 2943

PtRu(HY) 2951

800C

1200C

Pt (HY) PtRu(HY)

Pt (HY) PtRu(HY)

2943

2951

-

Band Assignment

-

CH3asymmetric stretching in CHO-

2819

2842

2819

2842

-

-

CH3

symmetric

stretching in CHO1480

1480

1480

1480

-

-

C-H

deformation

vibrations of CH3 1384

1384

1384

1384

C-H

deformation

vibrations of CH3 -

1760

-

1750

Bent CO2 molecule

1670 1649

1648

1650

1648

1650

1648

Bidentate carbonates

1454

1425

1454

1425

-

1425

Symmetric carbonates

1599

-

Asymmetric OCO in

1330 -

-

1599

-

stretching in formate HCOO-

-

1350

-

1350

-

Symmetric OCO in stretching in formate HCOO-

-

2352

-

2352

2351

2352

Formation of CO2

-

-

-

2330

2351

2336

Formation of CO2

-

-

-

-

2180

2180

Formation of CO

Credit author statement J. B. Fernandes: Conceptualization, Methodology, Validation and Supervision. Purnakala V. Samant: Software, Data Curation, Writing- Original draft preparation, Visualization, Investigation, Software, Validation, Writing- Reviewing and Editing,

Declaration of interests ✔The authors declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.

Figure 1: IR spectra of methanol+ Oxygen adsorption on Pt (HY) after evacuation.

Figure 2: IR spectra of methanol + O2 on Pt (HY) before evacuation

Figure 3: IR spectra of methanol adsorption on Pt (HY) after 30 minutes in absence of oxygen

Figure 4: Different structures of metal formates formed during methanol oxidation on NaX zeolite [22].

Figure 5: Proposed mechanism of decomposition of methanol to CO2 on a Pt (HY) catalysts.

Figure 6: IR spectra of methanol adsorption on Pt-Ru(HY) after evacuation.

The highlights of the present investigations are 1. insituFTIR technique has been employed for the evaluation of electrocatalytic activity Pt/HY and Pt-Ru/HY. 2. Mechanistic pathways are been interpreted for the facile oxidation of methanol to carbondioxide. 3. The decomposition of methanol on Platinum (HY) /C catalyst occurs via carbonate/formate pathway in the presence of oxygen. 4. Methanol oxidation to CO2 occurs at room temperatures in case Pt- Ru (HY)/C catalyst as compared to Pt/HY catalyst