Highly selective synthesis of methanol from methane over carbon materials supported Pd-Au nanoparticles under mild conditions

Journal Pre-proof Highly selective synthesis of methanol from methane over carbon materials supported Pd-Au nanoparticles under mild conditions Yingluo He, Jiaming Liang, Yusuke Imai, Koki Ueda, Hangjie Li, Xiaoyu guo, Guohui Yang, Yoshiharu Yoneyama, Noritatsu Tsubaki

PII:

S0920-5861(19)30552-8

DOI:

https://doi.org/10.1016/j.cattod.2019.10.017

Reference:

CATTOD 12522

To appear in:

Catalysis Today

Received Date:

26 June 2019

Revised Date:

28 August 2019

Accepted Date:

1 October 2019

Please cite this article as: He Y, Liang J, Imai Y, Ueda K, Li H, guo X, Yang G, Yoneyama Y, Tsubaki N, Highly selective synthesis of methanol from methane over carbon materials supported Pd-Au nanoparticles under mild conditions, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.10.017

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Catalysis Today 17th KJSC special issue (YO-A14)

Highly selective synthesis of methanol from methane over carbon materials supported Pd-Au nanoparticles under mild conditions Yingluo He, Jiaming Liang, Yusuke Imai, Koki Ueda, Hangjie Li, Xiaoyu guo, Guohui Yang, Yoshiharu Yoneyama, Noritatsu Tsubaki*

Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku

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3190, Toyama 930-8555, Japan

*Corresponding author E-mail address:

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[email protected]

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Graphical abstract

Highlights 

The supported Pd-Au nanoparticles catalysts were prepared by incipient wetness impregnation method.



Au addition into Pd/CNTs catalyst increases the productivity of methanol.



The relatively larger nano-sized particles enable great methanol production ability.



Pd bivalent states are proved to be the most active content in Pd-Au nanoparticles.

Abstract As one of the Holy Grail reactions in C1 chemistry, direct selective oxidation of methane to methanol under mild and non-harsh conditions remains a big challenge. Hydroperoxide (H2O2), as a primary oxidant, applied widely in the low-temperature direct selective conversion of methane to methanol. Moreover, the hydrogen and oxygen mixture gas achieves better reaction activity and higher methanol selectivity than H2O2 when using palladium-gold

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(Pd-Au) bimetallic nanoparticles as the catalyst. In this paper, we studied the key roles of the physical and chemical characteristics of Pd-Au nanoparticles in this direct selective oxidation reaction with hydrogen and oxygen as the oxidant at 50 oC. Au metal shows an indispensable role in direct methane to methanol process in this study. High methanol productivity and selectivity are achieved when Au of 0.5-2.5 % is added into the carbon nanotubes (CNTs)

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supported Pd catalyst. Moreover, the loading amount of Pd-Au nanoparticles also affect the methane activation ability and the methanol selectivity obviously. Much more active and

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stable Pd-Au nanoparticles are generated in CNTs supported 2.5 % Pd-2.5 % Au nanoparticles catalyst than those in lower metal content catalysts. In addition, the Pd bivalent state is proved

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to be the most active content in the Pd-Au nanoparticles catalyst, achieving much higher methanol selectivity and productivity when compared to Pd metallic state. For further clarifying the physical and chemical characteristics of catalysts, X-ray powder diffraction

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(XRD), X-ray photoelectron spectroscopy (XPS), Hydrogen-Temperature-programmed reduction (H2-TPR), and CO pulse adsorption measurement (CO-PULSE) analysis methods

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are also measured.

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Keywords: Methane, methanol, oxidation, palladium, gold.

1. Introduction Methane, the main constituent of natural gas, is mainly used as a gaseous fuel, and an important raw material to produce methanol, hydrogen, acetylene and so on in the chemical industry. Methane is also known as one of most important resources as an inexpensive and abundant alternative to petroleum in the world wild. On the other hand, as a classified greenhouse-effect gas, the release of methane into the atmosphere is harmful. Methane’s impact on global warming is 25 times greater than that of carbon dioxide [1]. As a result of taking advantage of such a potential resource, focus on producing valuable chemicals from methane, for example, methanol, has captivated not only the interest of the scientific

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community for more than a century, but also from industry [2–6]. Methanol, the simplest alcohol which can be syntheized from synthesis gas or biomass resources, is widely used as feedstock in chemical and energy industries and also as one of the most important liquid fuels that can be supplied to the engines or fuel cells [7–9].

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In industry, there exists a two-steps process for converting methane to methanol indirectly [10–12]. Firstly, synthesis gas (CO + H2) is generated via the methane reforming reaction with

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the presence of steam (Equation 1), and then methanol will be formed through the synthesis gas to methanol reaction process, which is called syngas to methanol process (STM, see

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Equation 3) [13]. Both two steps need to be carried out under high temperatures and pressures, being high-cost and energy-intensive process. Moreover, a considerable amount of CO2, one of the essential greenhouse-effect gases is produced at the same time. Thus, a direct route

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involving the conversion of methane to methanol (Equation 4), especially under mild reaction conditions is a topic of interest for worldwide researchers [5,14–16]. ΔH0298 K = +206.2 kJ mol-1 (1)

CO + H2O → CO2 + H2

ΔH0298 K = -41.2 kJ mol-1 (2)

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CH4 + H2O → CO + 3H2

ΔH0298 K = -90.7 kJ mol-1 (3)

CH4 + 0.5O2 → CH3OH

ΔH0298 K = -126.2 kJ mol-1 (4)

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CO + 2H2 → CH3OH

However, the direct conversion of methane to high value-added chemical intermediates

such as methanol is a big challenge for decades [17–25]. Methane molecule is a tetrahedral structure with four high dissociation energy equivalent C-H bonds, which realizes its excellent stability both in thermodynamics and kinetics. Also, methanol, the target product, is readily to be activated under reaction conditions and further transform to other over-oxidation products. In recent decades, direct conversion of methane to methanol under high temperature and pressure at gas phase is proven to be a viable route, while the selectivity and yield of

methanol are low. Several groups have studied this direct oxidation reaction in the liquid phase and at very mild temperature, using H2O2 as the benign oxidant [26–29]. H2O2 is a green, environmentally, and highly effective oxidant in direct methane to methanol process with Pd based catalyst system (Scheme 1a). However, its high price and low oxygen utilization efficiency hinder its large-scale commercialization and industrialization. Therefore, discovering and developing new oxidants with excellent performance but low cost under very

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mild reaction conditions is necessary and urgent.

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Scheme 1. A possible reaction pathway of direct methane to methanol using H 2O2 (a) or O2/H2 mixture gas (b) as oxidant.

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Recently, it has been reported that carbon materials supported Pd-Au nanoparticles catalyst enable high methanol selectivity using oxygen and hydrogen mixture gas as the green and

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inexpensive oxidant (Scheme 1b) [30,31]. Compared with H2O2, oxygen and hydrogen mixture gas not only enhances the methane conversion but also increases the methanol selectivity in all oxidation products. Especially the CNTs (carbon nanotubes) supported Pd-Au nanoparticles, give excellent catalytic performance on converting methane to methanol at 50 oC in aqueous phase [31]. As many H2O2 direct synthesis system, H2O2 also be produced as a by-product in this H2-O2 mixture gas oxiditon reaction [32–34]. Herein, we further demonstrate the effect of the physical and chemical characteristics of the catalytically-active center, of the Pd-Au nanoparticles, in our oxygen/hydrogen oxidation system of methane

partial oxidation to methanol. 2. Experimental 2.1 Catalyst preparation 2.1.1 Synthesis of supported Pd-Au catalysts The supported Pd-Au catalysts were prepared by an incipient wetness impregnation method. In detail, an aqueous solution of HAuCl4·3H2O and PdCl2 with varied amount of metal salt was slowly added into carbon nanotubes (CNTs, inner diameter: 20–30 nm; length: 1–10 μm; Chengdu, China) under ultrasonic environment, keeping the ultrasound for about 30 minutes.

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Then the mixture was treated by vacuum drying for 1 h, followed by drying at 60 °C for overnight. After this, the sample was calcined in a muffle furnace in static air at 400°C after being increased by 2 oC/min. Via these treatments, we obtained a Pd-Au bimetallic catalyst and denoted it as A% Pd-B% Au/CNTs. A and B mean the weight percent of metallic

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palladium or gold in the catalyst. Both A and B varied between 0 to 2.5.

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2.2 Catalyst characterization

XRD measurements were carried out with a Rigaku RINT 2400 system diffractometer by

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employing Cu-Kα radiation at room temperature. Data points were acquired by scanning at a rate of 0.02o s-1 from 2θ=10o to 80o.

XPS was conducted with a Thermo Fisher Scientific ESCALAB 250Xi instrument

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equipped with an AlKa X-ray radiation source (hv=1486.6 eV). Binding energies of all samples were referenced to the C 1s binding energy of adventitious carbon contamination at 284.8 eV.

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H2-TPR was performed by using a BELCAT-B-TT catalyst analyzer equipped with a TCD detector. Before measurement, the sample was pretreated with helium gas at 150 oC for

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1 h to remove traces of water. After cooling to 40 oC, H2-TPR was conducted in a stream of 5% H2/Ar with a heating rate of 10 oC/min. CO-PULSE method was performed by using a BELCAT-B-TT catalyst analyzer

equipped with a TCD detector. The sample was first reduced with H2 at 300 °C for 2 h and then cooled to 70 oC in flowing He. Several pulses of CO were injected until saturated chemisorption was achieved. 2.3 Catalytic tests

Catalyst tests for direct synthesis of methanol from methane were accomplished in a stainless-steel autoclave as in Fig. 1. Typically, 10 mL of deionized water mixed with the catalyst of 30 mg was added into the autoclave. After sealing, the reactor was fed with a mixture gas (CH4:H2:O2:Ar=47%:4%:8%:41%, mixed from 90 % CH4 in Ar, 25 % H2 in Ar, and 25% O2 in Ar), keep the O2 / H2 ratio at 2/1, CH4 / (O2 + H2) ratio at 4/1 if without special explain. All reactions were carried out under the explosion limit. The motor was vigorously stirred at 1200 rpm, the temperature was raised to 50 °C to start the reaction at the same time. After the 30 min reaction, vessel was cooled by ice (< 10 °C), to avoid volatilization of the

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products.

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Fig. 1. Reaction Scheme of batch liquid-phase reactor used in this study.

2.4 Products analysis and calculation methods

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The oxygenate products such as methyl hydroperoxide (MeOOH), methanol (MeOH) and formic acid (HCOOH) in the liquid phase were analyzed by NMR spectroscopy after filtering

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off catalyst of the product solution. 1H-NMR spectra were acquired on a JNM-ECX 400 spectrometer. The products were calculated by the standard curve method with D2O/DSS solution as the internal standard. A trace amount of H2O2 generated in our reaction system was determined via titration of the filtered solution with 0.01 mol/L acidified Ce(SO4)2 solution, by using Ferroin as an indicator. The gas products were collected by the gas bag and measured by gas chromatography (GC) with a Porapak Q column and TCD detector. Products productivity and selectivity calculation as well as formula is as follows:

MeOH productivity

MeOH yield (mmol)

=

(1)

Catalyst weight (kg)

MeOOH productivity

=

MeOOH yield (mmol) Catalyst weight (kg)

(2)

HCOOH productivity

=

HCOOH yield (mmol) Catalyst weight (kg)

(3)

MeOH selectivity

=

MeOH yield (mmol) ∑Product yield (mmol)

×100 %

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3. Results and discussion

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3.1 Effects of Au loading on methanol productivity

In our previous study, we proved that CNTs supported 2.5 wt% Pd-2.5 wt% Au

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nanoparticles to be an effective catalyst for direct converting methane to methanol reaction process. Especially, when using the nitric acid treated CNTs as support, the Pd-Au

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nanoparticles catalyst resulted in an increase of methanol selectivity up to 90 % in oxidation products [31]. Herein, in this paper, we considered the important roles of composition, physical and chemical characteristics of the Pd-Au alloy nanoparticles in this reaction. In

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Table 1, the results of direct conversion of methane to methanol with various Au loading amount on 2.5 % Pd/CNTs catalysts are summarized. When no Au exists in the catalyst, the

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methanol productivity and selectivity of 2.5 % Pd/CNTs are maintained at a low level. The reaction performance is improved significantly when the Au loading increases above 0.5 % to 2.5 %, and the highest methanol productivity is achieved at about 2.5 % Au added in the

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catalyst. The H2O2 formation firstly decreases with the increasing of Au amount within 0 to 1.5 % in the catalyst and increases slightly after Au amount addition over 1.5 % to 2.5 %.

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These findings disclose that the Au loading amount mainly affects the catalytic performance of Pd-Au/CNTs in methane to methanol process. The addition of 2.5 % Au into the 2.5 % Pd/CNTs catalyst, the best methanol productivity is achieved.

Table 1. Catalytic performance of different carbon supported Pd-Au nanoparticles catalysts a Different

Product amount

Total

MeOH

Au loading

(mmol/kgcat.)

product

Selectivity of

wt%

MeOH

MeOOH

HCOOH

(mmol/kgcat.)

(%)

MeOH b

0.0

23.7

0.0

17.7

41.4

57.3

100.9

19.2

0.5

92.3

4.0

9.3

105.7

87.4

392.9

6.4

1.0

101.2

3.0

12.0

116.2

87.0

430.8

6.7

1.5

113.7

3.3

19.7

136.7

83.2

484.0

5.4

2.0

130.1

7.3

38.8

176.2

73.8

553.8

5.8

2.5

139.0

8.7

42.4

190.1

73.2

591.7

6.2

TON H2O2

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(μmol)

a: Reaction conditions: Time 30 min; Temp. 50 oC; Solvent H2O 10 mL; Catalyst weight 30 mg; Feed gas CH4/O2/H2/Ar; Total pressure 3.3 MPa. b: mmoles of MeOH formed by per mole Pd metal.

The characteristics of CNTs supported Pd-Au nanoparticles catalysts with different Au

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loading are investigated by XRD, XPS, CO-PULSE, and H2-TPR. In Fig. 2, the XRD characteristic diffraction peaks at 26.3o, 34o, and between 38.2o - 40.0o correspond to CNTs,

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PdO, and Pd-Au alloy nanoparticles, respectively [35,36]. The strength of PdO particles diffraction peaks decreases and the Pd-Au alloy peaks become strong with the increasing of

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Au loading amount from 0 to 2.5 %. This means that low-amount addition of Au in the CNTs supported Pd catalyst generates a part of Pd-Au alloy nanoparticles while PdO still co-exists. This situation changes until impregnating Au of 1.5 % into the catalyst. It is clear that when

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the ratio of Au/Pd exceeds 3/5, the loaded Pd and Au mainly form Pd-Au alloy nanoparticles structure. Moreover, the characteristic diffraction peaks of Pd-Au alloy nanoparticles shift slightly to a lower degree when increasing Au loading. According to Vegard’s law, the mole

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compositions of Pd and Au in the alloy structure can be calculated from the peak shifts of the angular position [37,38]. As shown in Table S1 and Fig. S1 in Supplementary Information,

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The mole fractions of Au/Pd ratio of 2.5 % Pd-0.5 % Au/CNTs, 2.5 % Pd-1.0 % Au/CNTs, 2.5 % Pd-1.5 % Au/CNTs, 2.5 % Pd-2.0 % Au/CNTs, 2.5 % Pd-2.5 % Au/CNTs are estimated to be 0.37, 0.60, 0.80, 0.83, 0.93, respectively. The Au/Pd ratios of the alloy nanoparticles calculated by XRD are higher than the actual added amount Au/Pd ratio, which indicates that the Pd-Au alloy structure changes from Pd rich particles to Au rich and/or Au surface-rich particles, when the Au loading amount increases from 0.5 to 2.5 wt% [39,40].

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Fig. 2. XRD patterns for different Au loading amount catalysts.

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XPS spectra of Pd 3d and Au 4f of four catalysts are recorded in Fig. 3. The Pd and Au in all catalysts have a similar chemical state. The palladium mainly appears in the form of Pd

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bivalent state, while Au exists in metallic state at the same time. In addition, the electronic transfer phenomenon intensifies with the addition of Au, leads to the lower binding energy of Pd 3d and the higher binding energy of Au 4f. The signal of Au also becomes stronger when

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the loading of Au increases. The dispersions of Pd metal in different Au loading catalysts are shown in Table S1 in Supplementary Information. With the increasing of Au loading amount, Pd dispersion and metal surface decrease from 26.3% to 14.3%, and from 117.3 to 63.5 m2/g,

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respectively. The particles size of Pd-Au alloy nanoparticles of 2.5 % Pd-2.5 % Au/CNTs (7.9 nm) becomes larger when compared to that of 2.5 % Pd-0.5 % Au/CNTs (4.2 nm). In the

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H2-TPR profiles (Fig. 4), 2.5 % Pd-0.5 %Au/CNTs, 2.5 % Pd-1.5 %Au/CNTs, and 2.5 % Pd-2.5 %Au/CNTs catalysts display a similar reduction behavior. A main reduction peak of the Pd-Au nanoparticles catalysts appears below 200 oC, and a small broad reduction peak appears at around 200 to 300 oC, which belongs to the palladium hydride decomposition and the reduction of PdO in Pd-Au nanoparticles, respectively [30,41]. The addition of small amount Au enhances the difficulty in reducing oxidized Pd due to the strong interaction of Pd-Au alloy nanoparticles, coursing the stability of oxidized Pd in the Pd-Au alloy structure. Low Au loading suppresses the formation of uniform alloy nanoparticles, which further

XPS

spectra

in

different

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inhibits the methane activation ability .

Fig. 4. H2-TPR profiles for different Au loading amount catalysts.

loading

amount

catalysts.

3.2 Effects of Pd-Au bimetal loading on methanol productivity After confirming the effects of Au amount in the CNTs supported Pd-Au nanoparticles catalyst, the reaction performance of different Pd-Au bimetal loadings were also tested. The addition amounts of Pd-Au bimetal were changed between 0 to 5 % and kept the Pd/Au weight ratio at 1/1. The catalytic results are shown in Table 2. Pure CNTs without noble metal loading has no catalytic activity of this direct methane to methanol process. Higher metal loading amount courses the increasing of methanol yield. 2.5 % Pd-2.5 % Au/CNTs catalyst enables methanol productivity nearly 10 times higher than 0.5 % Pd-0.5 % Au/CNTs, and 6 times higher than 1.0 % Pd-1.0 % Au/CNTs. However, a similar methanol selectivity as about

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70-73 % is observed for all supported Pd-Au nanoparticles catalysts with different metal loading amount. What’s more, the TON of high Pd-Au bimetal loading catalyst, for example, 2.5 % Pd-2.5 % Au/CNTs, is higher than all the other low Pd-Au bimetal loading catalysts. This result indicates that larger content of Pd-Au nanoparticles show better methanol produce

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ability.

Table 2. Catalytic performance with different reaction conditions of Pd-Au/CNTs catalyst a Different

Product amount

Pd-Au loading

(mmol/kgcat.)

wt%

MeOH

MeOOH

0.0

0

0

0.5, 0.5

13.7

0.0

1.0, 1.0

20.7

0.0

2.5, 2,5

139.0

8.7

MeOH

TON

product

Selectivity

of

(mmol/kgcat.)

(%)

MeOH b

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Total

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HCOOH

H2O2 (μmol)

0

-

-

0

5.7

19.4

70.7

291.6

15.8

7.7

28.4

72.9

220.3

13.4

42.4

190.1

73.2

591.7

6.2

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0

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a: Reaction conditions: Time 30 min; Temp. 50 oC; Solvent H2O 10 mL; Catalyst weight 30 mg; Feed gas CH4/O2/H2/Ar; Total pressure 3.3 MPa. b: mmoles of MeOH formed by per mole Pd metal.

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On the other hand, the H2O2 productivity of these various Pd-Au loading nanoparticles

shows a thoroughly opposite rule. 0.5 % Pd-0.5 % Au/CNTs achieves the highest H2O2 productivity compared to the other catalysts, and the H2O2 productivity decreases obviously with the increasing of Pd-Au loading on the CNTs support. This interesting finding suggests that the most active sites in our direct converting methane to methanol reaction and the direct synthesis of H2O2 from H2-O2 mixed system are unequal [42,43]. Although both reactions can take place in our single CNTs supported Pd-Au nanoparticles catalyst, they mainly happen on the different active parts or with different reaction pathway on the catalyst.

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Fig. 5. XPS spectra in different Pd-Au bimetallic loading catalysts.

To further prove our speculation, the XPS and XRD measurements were performed. The XPS spectra uncover that the chemical states of Pd-Au nanoparticles are similar in all three

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catalysts (see in Fig. 5). The binding energy of Pd 3d of 2.5 % Pd-2.5 % Au/CNTs is lower than that of 0.5 % Pd-0.5 % Au/CNTs, indicating that there is a stronger interaction and effect

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between Pd and Au atoms of high loading metal on the surface of CNTs. Furthermore, the stronger interaction and effect between Pd and Au atoms maybe one of the evidences of the presence of relatively large Pd-Au alloy nanoparticles as in Fig. 6. Pd-Au alloy nanoparticles

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structure are ascertained in both low and high metal loading catalysts. The strength of characteristic diffraction peaks of Pd-Au alloy is weaker in 0.5 % Pd-0.5 % Au/CNTs

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compared to that in 2.5 % Pd-2.5 % Au/CNTs, suggesting that there are less Pd-Au nanoparticles in 0.5 % Pd-0.5 % Au/CNTs detected by XRD analysis. The crystal size of Pd-Au nanoparticles in 0.5 % Pd-0.5 % Au/CNTs is also smaller than the high metal loading catalyst as calculated through Scherrer formula from XRD analysis. The Pd metal particles size caiculated by CO-PULSE method also indicates the results of XRD (Table S2 in Supplementary Information).

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Fig. 6. XRD patterns for different Pd-Au bimetallic loading catalysts.

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The metal ion concentration in impregnation solution maybe the main reason for the different metal nanoparticle sizes of 0.5 % Pd-0.5 % Au/CNTs, 1.0 % Pd-1.0 % Au/CNTs and

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2.5 % Pd-2.5 % Au/CNTs. In the preparation of Pd-Ad/CNTs catalysts, we used the same solvent volume to dissolve the different amount of PdCl2 and HAuCl4, which made the various concentration of PdCl2 and HAuCl4 in mixture solution. The impregnation of the

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concentrated metal salt solution results in two situations: one is forming more similar Pd-Au nano-sized particles as the dilute solution; or some relatively larger nano-sized particles will take shape, causing it easier to gather with each other in both impregnation and calcination

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steps. Based on the first situation, if only the difference of various Pd-Au content catalysts is their munber of Pd-Au nanoparticles, not the particles size, the reaction performance of

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Pd-Au/CNTs should be proportional to the loading amount of Pd-Au nanoparticles. According to the reaction results, 2.5 % Pd-2.5 % Au/CNTs has over 10 times methanol productivity ability when compared to 0.5 % Pd-0.5 % Au/CNTs. Moreover, the TON of 2.5 % Pd-2.5 % Au/CNTs is about twice that of 0.5 % Pd-0.5 % Au/CNTs. This result suggests that those relatively large and stable Pd-Au alloy structure nanoparticles in 2.5 % Pd-2.5 % Au/CNTs are the most active sites to this direct selective oxidation reaction.

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Fig. 7. XRD patterns for fresh and spent Pd-Au catalysts.

The XRD diffraction patterns of fresh and spent 0.5 % Pd-0.5 % Au/CNTs, 2.5 % Pd-2.5 % Au/CNTs are shown in Fig. 7. Comparing XRD patterns of fresh and spent 0.5 % Pd-0.5 %

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Au/CNTs and 2.5 % Pd-2.5 % Au/CNTs catalysts, it is found that both catalysts both kept a stable state during reaction test, and there is no significant difference between the fresh and

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spent catalysts (Fig. 7). This phenomenon indicates that the Pd-Au nanoparticles prepared in this study retain good physical and chemical stability, ensuring performance repeatability.

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3.3 Effects of Pd valence on methanol productivity Table 3. Catalytic performance with different reaction conditions of Pd-Au/CNTs catalyst a Total

MeOH

TON

(mmol/kgcat.)

product

Selectivity

of

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Product amount

Methods

2.5%Pd-2.5%Au /CNTs

H2O2 (μmol)

MeOH

MeOOH

HCOOH

(mmol/kgcat.)

(%)

MeOH b

139.0

8.7

42.4

190.1

73.2

591.7

6.2

7.7

0.0

5.7

13.4

57.5

32.8

0.0

Reduced 2.5%Pd-2.5%Au /CNTs

a: Reaction conditions: Time 30 min; Temp. 50 oC; Solvent H2O 10 mL; Catalyst weight 30 mg; Feed gas CH4/O2/H2/Ar; Total pressure 3.3 MPa. b: mmoles of MeOH formed by per mole Pd metal.

It is proved that the reduced graphene oxide (rGO) supported Pd-Au nanoparticles have low reaction activity in this direct conversion of methane to methanol process in our previous study [31]. The possible reason for this phenomenon is the low Pd2+/Pd0 ratio of Pd on the catalyst or the too strong intereaction between metal particles and support. Therefore, in order to figure out whether the Pd valence truely plays a role in direct converting methane to methanol reaction, the reduced the 2.5 % Pd- 2.5 % Au/CNTs catalysts under pure H2 atmosphere at 400 oC for 4h, was tested for its reaction performance. The reaction

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performance of the two catalysts is compared in Table 3. The reduced 2.5 % Pd- 2.5 % Au/CNTs enables deficient reaction activity for direct conversion of methane to methanol. CNTs has been investigated as adequate support in this reaction process, the low reactivity of reduced 2.5 % Pd- 2.5 % Au/CNTs should be attributed to the chemical state of Pd atoms

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[44,45]. According to XPS spectra results (Fig. 8), the difference between reduced 2.5 % Pd2.5 % Au/CNTs and 2.5 % Pd- 2.5 % Au/CNTs is the valence of Pd atoms. Most Pd remains

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metallic state on the reduced 2.5 % Pd- 2.5 % Au/CNTs, while there is only bivalent Pd on the 2.5 % Pd- 2.5 % Au/CNTs at the same time. These finding indicate that the bivalent Pd in the Pd-Au alloy nanoparticles plays a vital role in direct converting methane to methanol with

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high methanol productivity and selectivity. The Pd metallic sites in the Pd-Au particles are not active site in both H2O2 synthesis and methane selective oxidation reactions in our

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conditions.

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Fig. 8. XPS spectra in reduced and unreduced Pd-Au nanoparticles catalysts.

4. Conclusion

In conclusion, we studied the roles of the Pa-Au nanoparticles in this direct methane

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oxidation reaction using hydrogen and oxygen as the oxidant gas under mild conditions. The methanol productivity increases with the increasing of Au loading amount in the Pd-Au/CNts

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catalyst. Highest methanol productivity is achieved for the addition of 2.5 % Au into the CNTs supported 2.5 % Pd catalyst. The high reactivity may be due to the formation of active Au rich and/or Au surface-rich Pd-Au alloy nanoparticles. Low Au loading in the CNTs

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supported Pd-Au catalyst benefits neither the methanol production or the H2O2 synthesis reaction. The effects of the loading amount of the equal-content Pd-Au nanoparticles are also

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tested in this study. The high Pd-Au loading catalyst having the relatively larger nano-sized particles enables great methanol production ability, while the reaction activity of low Pd-Au loading amount catalyst is severely limited. Furthermore, it is proposed that the Pd bivalent state should to be the most active content in the Pd-Au nanoparticles catalyst to achieve high methanol selectivity and productivity in this study. More in-depth investigation of this Pd-Au nanoparticles catalyst may facilitate the catalyst design and preparation for this direct methane to methanol reaction or other catalytic processes in the future.

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