Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy

Catalysis Communications 39 (2013) 60–64

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Short Communication

Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy Jung Ho Choi a, Yoon Jae Lee a, Tae Hun Kang a, Yongju Bang a, Ji Hwan Song a, Ji Chul Jung b,⁎, In Kyu Song a,⁎⁎ a b

School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea Department of Chemical Engineering, Myongji University, Yongin, Gyeonggi-do 449-728, South Korea

a r t i c l e

i n f o

Article history: Received 3 January 2013 Received in revised form 11 April 2013 Accepted 2 May 2013 Available online 18 May 2013 Keywords: Scanning tunneling microscopy Tunneling spectroscopy Heteropolyacid catalyst Negative differential resistance Redox property

a b s t r a c t Scanning tunneling microscopy (STM) and tunneling spectroscopy investigations of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson heteropolyacids (HPAs) were conducted. HPAs formed self-assembled and well-ordered arrays on graphite surface. Negative differential resistance (NDR) phenomena were observed in the tunneling spectra of the HPAs. NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV–visible absorption edge energy. HPAs were then applied to the electro-oxidation of methanol as a redox mediator. The oxidation activity for residual intermediates in the reaction increased as NDR peak voltage of HPA appeared at less negative voltage. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Heteropolyacids (HPAs) are polymeric oxoanion clusters that have been widely employed as catalysts in several acid–base and oxidation reactions [1,2]. Catalytic properties such as thermal stability, hydrolytic stability, unique redox nature, and excellent acid–base property make HPAs well suitable for various catalytic reactions. One of the great advantages of HPAs is that their catalytic properties can be easily tuned by changing constituent elements (charge-compensating counter-cations, central atoms, and framework polyatoms) [3]. Because various metals, semi-metals, or even non-metals can be incorporated into HPA framework, many types of HPAs can be designed as catalysts [4]. Among these HPAs, transition metal-substituted HPAs have attracted much attention as an oxidation catalyst in several oxidation reactions [5,6]. Comprehensive understanding about redox properties of HPAs would be of great importance in designing HPAs as an oxidation catalyst [7–10]. The most conventional method to probe redox properties of HPAs is to examine reduction potential (oxidizing power) of HPAs by electrochemical method in solution. However, reduction potential determined by electrochemical method is significantly altered by a number of experimental conditions such as pH, electrolyte, and electrode. Electrochemical measurement under consistent conditions ⁎ Corresponding author. Tel.: +82 31 330 6390; fax: +82 31 337 1920. ⁎⁎ Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail addresses: [email protected] (J.C. Jung), [email protected] (I.K. Song). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.05.008

only gives comparable results, and therefore, direct comparison of results in literatures is not a simple task. It is obvious that there is a need for systematic investigation about redox properties of HPAs with various theoretical and experimental methods. In this work, scanning tunneling microscopy (STM) and tunneling spectroscopy studies of transition metal-substituted α2-K8P2W17 (M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson HPA catalysts were carried out to elucidate their redox property and oxidation catalysis. STM images were acquired by depositing HPA catalysts on highly oriented pyrolytic graphite (HOPG) surface. Negative differential resistance (NDR) peak voltages of HPA catalysts were determined by tunneling spectroscopy. The measured NDR peak voltage was correlated with the reduction potential and the absorption edge energy determined by electrochemical method and UV–visible spectroscopy, respectively. HPAs were applied to the electro-oxidation of methanol as a redox mediator. A correlation between oxidizing ability of HPAs determined by tunneling spectroscopy measurements and catalytic behavior of HPA-impregnated Pt/C catalysts in the electrooxidation of methanol was then established. 2. Experimental 2.1. Catalyst preparation α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson HPA catalysts were prepared according to the methods reported in a previous work [11]. Successful formation of

J.H. Choi et al. / Catalysis Communications 39 (2013) 60–64

α2-K8P2W17(M·OH2)O61 Wells–Dawson HPA catalysts was confirmed by FT-IR (Nicolet, Nicolet 6700), 31P-NMR (Bruker, AVANCE 600), and ICP-AES (Shimadzu, ICPS-1000IV) analyses as reported in our previous work [12]. In this work, α2-K8P2W17(M·OH2)O61 Wells–Dawson HPA catalysts with M = MnII, ZnII, FeII, CoII, and NiII were denoted as WD-MnII, WD-ZnII, WD-FeII, WD-CoII, and WD-NiII, respectively. 2.2. Scanning tunneling microscopy and tunneling spectroscopy measurements For STM investigation, 1 mM aqueous solutions of α2-K8P2W17 (M·OH2)O61 Wells–Dawson HPA catalysts were prepared. A drop of each HPA solution was then deposited on the surface of cleaved HOPG and allowed to dry in air for 1 h. STM measurements were conducted using a XE-100E instrument (Park Systems) equipped with mechanically formed Pt/Ir (90/10) tips. Pt/Ir tips were calibrated by imaging HOPG surface and confirming its standard periodicity (2.46 Å). Scanning was performed in the constant-current mode at a tunneling current of 1–2 nA and a sample bias of + 100 mV. STM image presented in this work was not filtered. In the tunneling spectroscopy measurements, tunneling currents were monitored as ramping the sample bias from − 2 to + 2 V. In tunneling spectroscopy measurements, base current and voltage were fixed at 1 nA and + 100 mV, respectively. The voltage axis in the tunneling spectra represented the potential applied to the sample relative to that of the tip. Tunneling spectra were measured several times with three different tips to obtain more accurate and reproducible results, and to provide a basis for statistical analysis. 2.3. Electro-oxidation of methanol over HPA-impregnated Pt/C (Pt/C-HPA) catalysts For electro-oxidation of methanol, electro-catalysts were prepared as follows. Commercially available carbon-supported Pt catalyst (E-TEK, 20 wt.% Pt on Vulcan XC-72) was dried at 80 °C overnight under nitrogen atmosphere. 0.1 g of HPA was loaded on 0.3 g of Pt/C catalyst by a conventional incipient wetness impregnation method. HPA-impregnated Pt/C catalyst (Pt/C-HPA) was then dried at 100 °C overnight under nitrogen atmosphere. The prepared Pt/C-HPA catalyst was suspended on deionized water and was sonicated for 2 min at 28 kHz. 5% Nafion solution (Sigma-Aldrich) and 2-propanol (Sigma-Aldrich) were then added to the catalyst suspension. The weight ratio of catalyst with respect to 5% Nafion solution was 100/8.3. The suspended mixture was sonicated for 30 min at 28 kHz. The well-blended resulting suspension was then sprayed on 2 cm × 2 cm gas diffusion layer (SIGRACET, GDL 10BC) electrode and dried under atmosphere. The amount of catalyst loading on GDL electrode was 1 mg/cm2. Electro-oxidation of methanol was carried out using Potentiostat/Galvanostat (Princeton Applied Research, VSP) with a three-electrode cell system at room temperature. Threeelectrode cell system was purged with nitrogen for 20 min prior to electrochemical measurements. Platinum electrode and Ag/AgCl electrode were used as a counter electrode and as a reference electrode, respectively. The prepared GDL electrode containing HPA-impregnated Pt/C catalyst was used as a working electrode. A solution of 2 M CH3OH in 0.5 M H2SO4 was employed as an electrolyte solution, and cyclovoltagram was obtained within the range of −0.1 to +1.3 V at a scan rate of 50 mV/s. Electrochemically active surface areas were calculated by hydrogen adsorption method, based on cyclovoltagrams recorded within the range of −0.2 to +1.0 V at a scan rate of 50 mV/s in 0.5 M H2SO4 solution [13]. 3. Results and discussion 3.1. Self-assembled HPA arrays on graphite Fig. 1(a) shows the typical molecular structure of transition-metal substituted α2-P2W17(M·L)O8− 61 (M = transition metal, L = ligand

61

molecule) Wells–Dawson heteropolyanion. In this work, α2-P2W17 II II II II II (M·OH2)O8− 61 (M = Mn , Zn , Fe , Co , and Ni ) Wells–Dawson 6− heteropolyanions were prepared from α-P2W18O62 heteropolyanion via mono-lacunary α2-P2W17O10− heteropolyanion. α-P2W18O6− 61 62 heteropolyanion consists of two central tetrahedral PO4 units sharing corners with eighteen octahedral WO6 units. A cap WO6 unit could be selectively removed by pH control, resulting in mono-lacunary − α2-P2W17O10 heteropolyanion. Due to the presence of five oxygens 61 in the vacant site of mono-lacunary heteropolyanion, it can act as a pentadentate ligand for transition metal [14]. Water, halides, or other organic molecules can be attached to sixth location of transition metal, resulting in rugby-ball shaped P2W17(M·L)O8− 61 Wells–Dawson heteropolyanion. Fig. 1(b) shows the STM image and unit cell structure of α2-K8P2W17 (MnII·OH2)O61 Wells–Dawson HPA catalyst deposited on a graphite surface. Each bright corrugation represents individual heteropolyanion. STM image clearly showed the formation of self-assembled and well-ordered HPA array on an HOPG surface. The periodicity and included angle of unit cell constructed on the basis of lattice constants determined from two-dimensional fast Fourier transformation are shown in Fig. 1(b). Periodicity of unit cell was well consistent with lattice constants of Wells–Dawson HPAs determined by X-ray crystallography [15]. Because STM image is based on contours of surface local density of electronic states (LDOS) at the Fermi level, STM image reflects convolution of geometric and electronic structures rather than purely geometric structure of the surface molecule [16]. For adsorbate–substrate system, STM imaging would be more complex due to interaction between adsorbate and substrate. Interaction may lead to the modification in the LDOS, and corrugation in the STM image may contain electronic contributions from substrate. This implies that STM image of adsorbate-covered surface may not reflect the actual topography of adsorbate. In this work, therefore, chemically inert graphite was employed as a substrate for HPA deposition in order to minimize the chemical interaction.

3.2. Negative differential resistance behavior Fig. 2(a) shows the typical tunneling spectra of α2-K8P2W17 (MnII·OH2)O61 Wells–Dawson HPA catalyst taken at two different sites, denoted as Site A and Site B in Fig. 1(b). The tunneling spectrum taken at Site A showed a distinctive current–voltage behavior referred to as negative differential resistance (NDR), which was known to be a characteristic feature of HPAs. This indicates that each bright corrugation corresponds to individual heteropolyanion. On the other hands, Site B showed almost the same current–voltage behavior as bare HOPG. This indicates that the self-assembled and well-ordered HPA array shown in Fig. 1(b) is a monolayer. Fig. 2(b) shows the distribution of NDR peak voltages of α2-K8P2W17 (MnII·OH2)O61 Wells–Dawson HPA catalyst. NDR peak voltage was defined as the voltage where the maximum current was observed. Average value of NDR peak voltage of α2-K8P2W17(MnII·OH2)O61 Wells–Dawson HPA catalyst was found to be − 0.99 V with a standard deviation of 0.07 V. In this work, average value of NDR peak voltage was taken as the representative NDR peak voltage of the HPA catalyst. NDR peak voltages of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson HPA catalysts are summarized in Table 1. NDR peak voltage was quite qualitative, but clearly showed a Gaussian distribution. Under atmospheric conditions, tunneling gap could be shortened by water bridges. Trajectories of tunneling electrons can be altered by effect of polarization of water which can modify the force field. This effect depends on tunneling current which is closely related to tunneling gap (z), and tunneling spectra would be different with tunneling current [17]. Uncontrollable z variations could also result in quite large variation of NDR peak voltage [18].

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a

b

Ligand molecule

11.3 Å

Transition metal

5.35 nm

10.4 Å 86 o

57 o

Site A Site B

8.0 Å

5.35 nm

Fig. 1. (a) Typical molecular structure of transition-metal substituted α2-P2W17(M·L)O8− 61 (M = transition metal, L = ligand molecule) Wells–Dawson heteropolyanion and (b) STM image of α2-K8P2W17(MnII·OH2)O61 Wells–Dawson HPA catalyst deposited on graphite surface.

3.3. Correlations between NDR peak voltage and redox property Fig. 3 shows the reduction potential and UV–visible absorption edge energy of α2-K8P2W17(M·OH2)O61 Wells–Dawson HPA catalysts, plotted as a function of NDR peak voltage. Reduction potential and UV–

a Tunneling current (nA)

10

− − −

− Site A − Site B − Bare HOPG

5

0

-5 NDR peak

-10 -2.0 -1.5 -1.0 -0.5

0

0.5

1.0

1.5

2.0

Sample bias voltage (volts)

b Frequency (A.U.)

-0.99 ± 0.07 V

visible absorption edge energy data taken from a literature [12] are summarized in Table 1. It is noteworthy that NDR peak voltage of HPA catalysts appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. The occurrence of NDR phenomenon has been conventionally explained in terms of resonant tunneling through a double-barrier quantum well structure; when energy state of incident electron is matched with a virtual empty state in the molecular well, electron can pass through the molecular well without any attenuation [19]. A recent study [17] has reported that NDR can be generated by tunneling transport of electrons via single HPA molecule that proceeds under Wannier–Stark localization condition in strong electric field of STM measurement. This resonance electron transition from negative potential to positive potential contains sequential electron transfers; from negative potential to oxygen ion, from oxygen ion to metal ion, and from metal ion to positive potential. Several previous STM works [3,20] have suggested that NDR phenomenon of HPA catalyst occurs via resonant tunneling through the lowest unoccupied molecular orbital (LUMO). A previous quantum chemical molecular orbital study [8] showed that energy gap between the highest occupied molecular orbital (HOMO) and the LUMO was well correlated with the reduction potential; higher reduction potential corresponds to smaller energy gap. It is well known that the LUMO of HPA catalysts comprises a combination of d-orbitals on the framework metal center and 2p-orbitals on the oxygens. However, the HOMO of HPA catalysts mostly comprises 2p-orbitals on the oxygens. Therefore, the HOMO is unchanged in the series of polyatom-substituted HPA catalysts and energy gap only depends on the energy state of the LUMO. Thus, it can be inferred that less negative NDR voltage of HPA catalysts corresponds to lower energy state of the LUMO, leading to smaller energy gap between the HOMO and the LUMO (leading to higher reduction potential).

Table 1 NDR peak voltage, reduction potential, and absorption edge energy of K8P2W17 (M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson HPA catalysts determined by tunneling spectroscopy, electrochemical method, and UV–visible spectroscopy, respectively. Catalyst

NDR peak voltage (volts)

First electron reduction potential (SCE, volts)a

Absorption edge energy (eV)a

WD-MnII WD-ZnII WD-FeII WD-CoII WD-NiII

−0.99 −1.05 −1.07 −1.07 −1.08

−0.354 −0.417 −0.437 −0.446 −0.480

3.513 3.519 3.524 3.525 3.548

-0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5

NDR peak voltage (volts) Fig. 2. (a) Typical tunneling spectra of α2-K8P2W17(MnII·OH2)O61 Wells–Dawson HPA catalyst taken at two different sites, denoted as Site A and Site B in Fig. 1(b), and (b) distribution of NDR peak voltages of α2-K8P2W17(MnII·OH2)O61 Wells–Dawson HPA catalyst.

a

± ± ± ± ±

0.07 0.08 0.06 0.07 0.08

Data were taken from a literature [12].

a

-0.34

First electron reduction potential (SCE, volts)

J.H. Choi et al. / Catalysis Communications 39 (2013) 60–64

-0.36

a WD-MnII

63

250 200

-0.38

mA/mgPt

150 -0.40 WD-ZnII

-0.42 WD-FeII

Pt/C-(WD-MnII) Pt/C-(WD-NiII)

100 50

-0.44 WD-CoII

0

-0.46 -0.48

-50 -0.2

WD-NiII

-0.50

0.0

0.2

-1.16 -1.12 -1.08 -1.04 -1.00 -0.96 -0.92

NDR peak voltage (volts)

b

0.6

0.8

Pt/C-(WD-MnII)

WD-FeII

3.525

WD-CoII

1.4

Pt/C-(WD-NiII)

Pt/C-(WD-FeII)

WD-ZnII

3.520

1.2

Pt/C-(WD-ZnII)

100

WD-MnII

3.515

1.0

150

3.510

mA/mgPt

Absorption edge energy (eV)

b

0.4

E/V vs. Ag/AgCl

Pt/C-(WD-CoII)

50

3.530 0

3.535 3.540

-50 3.545 3.550

0.1

NDR peak voltage (volts) Fig. 3. (a) Reduction potential and (b) absorption edge energy of α2-K8P2W17(M·OH2) O61 Wells–Dawson HPA catalysts, plotted as a function of NDR peak voltage.

Absorption edge energy of HPA catalysts determined by UV–visible spectroscopy reflects the energy required for ligand-to-metal charge transfer (LMCT) [21]. By irradiation, electrons are promoted from 2porbitals on the oxygens to the low-lying high-energy states which comprise d-orbitals on the framework metal center. Therefore, absorption edge energy of HPA catalysts reflects the energy required for electron transfer from the HOMO to the LUMO. Because energy gap only depends on energy state of LUMO in the series of polyatom-substituted HPA catalysts, smaller absorption edge energy corresponds to lower energy state of the LUMO. Thus, the correlation between NDR peak voltage and absorption edge energy may be understood in the same manner described above. It can be summarized that NDR peak voltage of α2-K8P2W17 (M·OH2)O61 Wells–Dawson HPA catalysts determined by tunneling spectroscopy could be utilized as a correlating parameter for reduction potential and absorption edge energy determined by electrochemical method and UV–visible spectroscopy, respectively. 3.4. HPA application to the electro-oxidation of methanol as a redox mediator

0.4

0.5

0.6

0.7

0.8

Fig. 4. (a) Typical cyclovoltagrams for α2-K8P2W17(Mn·OH2)O61- and α2-K8P2W17 (Ni·OH2)O61-impregnated Pt/C catalysts in 0.5 M H2SO4-containing 2 M CH3OH solution at a scan rate of 50 mV/s, and (b) magnified cyclovoltagrams for Pt/C-HPA catalysts during the backward scan.

The magnified cyclovoltagrams for Pt/C-HPA catalysts during the backward scan are presented in Fig. 4(b). Two different current peaks were clearly observed in the cyclovoltagrams. According to the previous works [22–24], one current peak appeared during the forward scan is attributed to the methanol oxidation while the other current peak appeared during the backward scan is attributed to the electrochemical oxidation of intermediate species (residual intermediates) adsorbed on Pt active sites. HPA-impregnated Pt/C catalyst exhibited a better catalytic activity in the electro-oxidation of methanol than HPA-free Pt/C catalyst (not shown here). Regardless of the identity of HPA, values of electrochemically active surface areas of HPA-impregnated Pt/C catalysts determined by hydrogen adsorption method were similar as summarized in Table 2. These results indicate that textural properties of Pt particles are not the major factor for the enhancement of catalytic activity. It has been reported that oxidizing ability of HPA can facilitate oxidative removal of intermediate species such as CO, resulting in Table 2 Electrochemically active surface area of Pt/C-HPA catalysts calculated by hydrogen adsorption method. Electrochemically active surface area (m2/g)

Catalyst II

HPAs were applied to the electro-oxidation of methanol as a redox mediator. Fig. 4(a) shows the typical cyclovoltagrams for α2-K8P2W17 (Mn·OH2)O61- and α2-K8P2W17(Ni·OH2)O61-impregnated Pt/C catalysts in 0.5 M H2SO4-containing 2 M CH3OH solution at a scan rate of 50 mV/s.

0.3

E/V vs. Ag/AgCl

WD-NiII

-1.16 -1.12 -1.08 -1.04 -1.00 -0.96 -0.92

0.2

Pt/C–(WD-Mn ) Pt/C–(WD-ZnII) Pt/C–(WD-FeII) Pt/C–(WD-CoII) Pt/C–(WD-NiII)

106.1 102.1 109.1 107.4 105.4

64

J.H. Choi et al. / Catalysis Communications 39 (2013) 60–64

140 Pt/C-(WD-MnII)

135

Ib(mA)

130 125 120

Pt/C-(WD-ZnII)

oxidizing ability of HPA facilitated electrochemical oxidation reaction of intermediate species (residual intermediates) such as CO, resulting in enhancement of catalytic activity. The performance for oxidative removal of intermediate species estimated from backward peak current increased in the order of Pt/C–(WD-NiII) b Pt/C–(WD-CoII) b Pt/C– (WD-FeII) b Pt/C–(WD-ZnII) b Pt/C–(WD-MnII), in good agreement with the trend of NDR peak voltage of HPA.

Pt/C-(WD-FeII)

115 110

Acknowledgments Pt/C-(WD-CoII) Pt/C-(WD-NiII)

-1.16 -1.12 -1.08 -1.04 -1.00 -0.96 -0.92

NDR peak voltage of HPA (volts) Fig. 5. A correlation between NDR peak voltage of HPAs and backward current peak (Ib) of cyclovoltagrams for Pt/C-HPA catalysts.

enhancement of catalytic activity. The amount of oxidative removal of intermediate species can be estimated from the backward peak current (Ib) appeared at around +0.4 V [22–24]. It is noteworthy that the backward peak current increased in the order of Pt/C–(WD-NiII) b Pt/C– (WD-CoII) b Pt/C–(WD-FeII) b Pt/C–(WD-ZnII) b Pt/C–(WD-MnII). The trend of backward current peak is well consistent with the trend of oxidizing ability of incorporated HPA determined from NDR peak voltage (Fig. 5). These results support that the enhancement of catalytic activity was mainly due to oxidizing ability of impregnated HPA. It can also be inferred that NDR peak voltage of HPA can be utilized as a probe of oxidizing ability and as a major parameter to determine the performance for oxidative removal of intermediate species in the electro-oxidation of methanol.

This work was supported by the Mid-career Researcher Program of National Research Foundation (NRF) grant funded by the Korea government (MEST) (No. 2012-R1A2A4A01001146). This work was also supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-10013). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

4. Conclusions STM studies of transition metal-substituted α2-K8P2W17(M·OH2) O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson HPA catalysts were carried out to elucidate their redox property. STM images clearly showed the formation of self-assembled and well-ordered HPA arrays on an HOPG surface. NDR phenomena were observed in the tunneling spectra of all HPA catalysts. The measured NDR peak voltage of HPA catalysts was then correlated with the reduction potential and the absorption edge energy determined by electrochemical method and UV–visible spectroscopy, respectively. NDR peak voltage appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. The prepared HPAs were then applied to the electro-oxidation of methanol as a redox mediator. HPA-impregnated Pt/C catalyst exhibited a better catalytic activity in the electro-oxidation of methanol than HPA-free Pt/C catalyst. The

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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