王虹力, 王男, 王麗瑩, 宋二紅, 趙占奎

功能化石墨烯擔載型AuPd納米催化劑增強甲酸制氫反應(yīng)

王虹力1, 王男1, 王麗瑩1, 宋二紅2, 趙占奎1

(1. 長春工業(yè)大學 材料科學與工程學院, 先進結(jié)構(gòu)材料教育部重點實驗室, 長春 130012; 2. 中國科學院 上海硅酸鹽研究所, 高性能陶瓷和超微結(jié)構(gòu)國家重點實驗室, 上海 200050)

甲酸(FA)因具有儲氫量高、易加注等優(yōu)點而成為極具應(yīng)用前景的新型儲氫材料, 尋求高效率催化劑對于解決甲酸制氫反應(yīng)動力學緩慢的問題尤為重要。本工作以聚乙烯亞胺修飾石墨烯(PEI-rGO)作為催化劑襯底, 通過濕化學法制備PEI-rGO擔載型AuPd納米復(fù)合材料(Au0.3Pd0.7/PEI-rGO)。Au0.3Pd0.7/PEI-rGO催化劑在催化FA制氫的反應(yīng)中表現(xiàn)出極其優(yōu)異的活性, 在無添加劑輔助下的轉(zhuǎn)化頻率(TOF)為2357.5 molH2?molcatalyst–1?h–1, 高于大多數(shù)相同反應(yīng)條件下的異相催化劑。這歸因于PEI-rGO襯底與AuPd納米顆粒之間的強相互作用對金屬活性組分的尺寸、分散度和電子結(jié)構(gòu)的調(diào)控。此外, 循環(huán)測試結(jié)果表明該催化劑的穩(wěn)定性良好。

功能化石墨烯; 納米金屬催化劑; 甲酸; 制氫反應(yīng)

化石能源的廣泛使用帶來的能源枯竭和環(huán)境污染等問題引起了越來越多研究者的關(guān)注和思考[1-2]。氫能作為高效清潔能源具有燃燒性能好、產(chǎn)熱值高和無毒無污染等獨特的性質(zhì), 在各種新能源中脫穎而出[3]。與傳統(tǒng)的高壓氣態(tài)儲氫和低溫液態(tài)儲氫相比, 儲氫材料兼具高存儲密度和高安全性等優(yōu)點[4]。甲酸(HCO2H, FA)來源豐富, 含氫量高(43 g·kg–1), 且易于填裝液體燃料電池, 是一種具有巨大應(yīng)用潛力的儲氫材料[5]。在催化劑的作用下, FA可以通過脫氫反應(yīng)(反應(yīng)(1))生成氫氣(H2)和二氧化碳(CO2), 也可以通過脫水反應(yīng)(反應(yīng)(2))生成水(H2O)和一氧化碳(CO)[5], 其中, 反應(yīng)(1)是人們期望的路徑, 而反應(yīng)(2)生成的CO會使催化劑中毒而失去活性, 需要抑制。

HCO2H (l)→H2(g) + CO2(g)(1)

HCO2H (l)→H2O (l) + CO (g)(2)

催化劑是決定FA分解路徑的關(guān)鍵因素, 在目前報道的催化劑中, 單組元Pd、Au或Pd與其他貴金屬(如Au、Ag等)[6]結(jié)合的二元或三元納米材料對于FA分解表現(xiàn)出良好的催化活性。因此本研究選用Au來調(diào)節(jié)Pd周圍的電子結(jié)構(gòu)使其整體作為活性主體。然而單獨使用金屬納米粒子會由于表面能過大而在液相中產(chǎn)生團聚現(xiàn)象, 使用適合的襯底來負載金屬納米顆粒(NPs)是改善分散性及獲取細小、均勻粒徑尺寸的一種行之有效的方法[7-8]。在此基礎(chǔ)上, 對襯底進行功能化修飾, 可以有效促進襯底與金屬活性組分之間的相互作用, 從而提升擔載型催化劑整體的催化性能[9]。

石墨烯(rGO)是一種以碳原子sp2雜化形成的近似蜂窩狀的結(jié)構(gòu)為基礎(chǔ), 在表面缺陷位置和邊緣處存在羥基、羧基和環(huán)氧基等官能團的二維材料[10-12]。正是因為這些表面缺陷以及官能團, 使得rGO易于進行功能化修飾。由于氮和金屬之間的配位作用, 外來的氨基官能團可為附著金屬前驅(qū)體提供幫助, 使得金屬NPs在襯底上高度分散[13], 可以有效地改善金屬納米顆粒在液相中的團聚現(xiàn)象, 同時增強金屬納米顆粒與襯底之間的相互作用, 進一步增強催化性能?；诖? Rana等[14]使用-(2-氨基乙基)-3-氨基丙基三甲氧基硅烷修飾rGO, 再將金屬納米粒子負載到其表面, 使得該催化劑具有優(yōu)異的催化活性及良好的循環(huán)穩(wěn)定性。Imani等[15]在rGO表面修飾八精氨酸(R8), 得到了負載率高且分散性和生物相容性較好的襯底材料。

鑒于聚乙烯亞胺(PEI)中的胺基能夠與rGO的羧基等含氧基團以酰胺鍵等形式結(jié)合[16], 本研究通過濕化學法成功制備PEI功能化rGO擔載AuPd NPs (Au0.3Pd0.7/PEI-rGO), 并將其作為催化劑應(yīng)用在FA分解制氫反應(yīng)中。在323 K且無添加劑的情況下, 對Au0.3Pd0.7/PEI-rGO催化劑催化FA分解制氫反應(yīng)速率和循環(huán)穩(wěn)定性進行探究; 計算反應(yīng)激活能(a); 初步分析催化活性增強機制及反應(yīng)機理。

1 實驗方法

1.1 催化劑制備

實驗中所用藥品均購自國藥集團化學試劑公司。首先用Hummers法制備氧化石墨烯(GO)[17], 然后將200 mg PEI與35 mL GO (1.43 g/L)充分混合, 并在80 ℃下攪拌2 h, 得到PEI-rGO溶液。隨后取5 mL PEI-rGO分散液, 加入1.5 mL HAuCl4(0.02 mol/L)和2.8 mL Na2PdCl4(0.025 mol/L), 混合均勻后加入40 mg NaBH4繼續(xù)磁力攪拌30 min, 離心后得到Au0.3Pd0.7/PEI-rGO催化劑。此外, 采用與Au0.3Pd0.7/PEI-rGO類似的方法制備。無PEI修飾的Au0.3Pd0.7/rGO和無PEI-rGO襯底的Au0.3Pd0.7NPs AuPd1–x/PEI-rGO (=0, 0.1, 0.7, 0.9, 1.0)的制備方法與Au0.3Pd0.7/PEI-rGO (即=0.3)相似, 通過調(diào)整HAuCl4和Na2PdCl4溶液用量調(diào)節(jié)。

1.2 催化活性測試

將制備好的催化劑放置于圓底燒瓶中, 加入5.0 mL FA溶液(1.0 mol/L), 在磁力攪拌下, 用氣體滴定管測量反應(yīng)過程中的氣體產(chǎn)量。在不同溫度(303, 313, 323和333 K)下進行催化反應(yīng)。在323 K下FA充分反應(yīng)后, 再向燒瓶中加入5.0 mL FA溶液進行循環(huán)實驗。

1.3 催化劑表征

采用X射線衍射儀(XRD, Rigaku RINT-2000)對樣品進行物相分析; 采用傅里葉變換紅外光譜儀(Nicolet IS50)檢測樣品的官能團; 采用透射電子顯微鏡(TEM, FEI Talos F200S)觀察樣品形貌、分布及結(jié)構(gòu); 采用X射線光電子能譜儀(XPS, ESCALABMKLL)分析樣品的電子結(jié)構(gòu); 采用電感耦合等離子體原子發(fā)射光譜儀(ICP, Thermo TJA 6000)對樣品成分進行定量分析。

1.4 計算方法

FA分解制氫反應(yīng)在催化劑作用下的轉(zhuǎn)化頻率(TOF)及激活能(a)計算方法如下:

其中, TOF單位為molH2?molcatalyst–1?h–1,atm是標準大氣壓(105Pa),H2是轉(zhuǎn)化率達到50%時產(chǎn)生H2的體積(mL),是通用氣體常數(shù)(8.314 J·mol–1·K–1),是室溫(298 K),catalyst是使用ICP測得的AuPd的摩爾數(shù)(mmol),是轉(zhuǎn)化率達到50%的反應(yīng)時間(h)。溫度與TOF值之間的關(guān)系遵循Arrhenius特性, Arrhenius方程式如下:

其中是指前因子。

2 結(jié)果與討論

2.1 表征結(jié)果分析

圖1(a)的XRD圖譜中PEI修飾后GO在2=10°的(002)衍射峰消失, 而在2=15°～25°之間出現(xiàn)一個無定型峰, 表明經(jīng)PEI修飾之后, GO被還原為rGO[18]。圖1(b)的FT-IR譜圖中, 與GO相比, PEI-rGO保留了GO基本峰型, 在～1240、～1620和～3400 cm–1處新增峰分別對應(yīng)C–N的伸縮振動和N–H的彎曲和伸縮振動, 表明氨基成功修飾到rGO上[19]。

從圖2的TEM照片中可以觀察Au0.3Pd0.7/PEI-rGO的形貌、結(jié)構(gòu)、顆粒的尺寸及分散度。如圖2(a)所示, PEI-rGO為褶皺的薄膜形態(tài), 說明氨基修飾不會改變rGO的基本形貌。從圖2(b)中可以看到 AuPd NPs均勻地分散在PEI-rGO上, 平均粒徑尺寸為3.88 nm。高分辨TEM(HRTEM)照片中NPs的晶面間距為0.230 nm (圖2(c)), 該數(shù)值介于面心立方(fcc) Au (111)晶面間距與fcc Pd (111)晶面間距之間, 說明AuPd NPs在Au0.3Pd0.7/PEI-rGO中以合金結(jié)構(gòu)形式存在[20]。從圖2(d)的XRD圖譜可見, 2=15°～25°之間的峰依舊存在, 說明PEI-rGO襯底的結(jié)構(gòu)穩(wěn)定。此外, 相對于fcc Au (111), Au0.3Pd0.7/PEI-rGO的峰位向fcc Pd (111)偏移, 進一步證明AuPd NPs具有合金結(jié)構(gòu)且附著在PEI-rGO襯底上, 與HRTEM的結(jié)果高度吻合。能譜(EDX)檢測所得的結(jié)果證明樣品中除了Au、Pd以外, 還存在N, 這進一步證實N成功修飾到Au0.3Pd0.7/PEI-rGO中(圖2(e))。ICP測定結(jié)果中, Au0.3Pd0.7/PEI-rGO催化劑中Au與Pd的原子比為0.298:0.701, 這與3 : 7的理論值相符。

圖1 GO和PEI-rGO的(a)XRD圖譜和(b)FT-IR光譜圖

圖2 Au0.3Pd0.7/PEI-rGO的TEM照片、XRD及EDX圖譜

Fig. 2 TEM images, XRD pattern and EDX spectrum of Au0.3Pd0.7/PEI-rGO

(a-b) TEM images and (c)HRTEM image for Au0.3Pd0.7/PEI-rGO with inset in (b) showing corresponding histogram of particle size distribution, (d) XRD pattern and (e) EDX pattern for Au0.3Pd0.7/PEI-rGO

為調(diào)查Au0.3Pd0.7/PEI-rGO催化劑的電子結(jié)構(gòu), 對催化劑進行XPS分析。如圖3(a)所示, 與Au0.3Pd0.7/rGO相比, Au0.3Pd0.7/PEI-rGO在399 eV處新增了1個N峰。通過進一步分析可知, N的化學狀態(tài)包括C=N (398.6 eV)和C–NH2(399.4 eV)[21](圖3(b)), 這表明PEI成功修飾到Au0.3Pd0.7/PEI-rGO中。從圖3(c～d)可見, Pd、Au以Pd0、Au0價態(tài)存在, 少量的Pd2+、Au+是由于XPS測試樣品在處理過程中暴露于空氣中所致。此外, 相比于Au0.3Pd0.7/rGO, Au0.3Pd0.7/ PEI-rGO中的Pd3d5/2峰出現(xiàn)偏移, 類似的情況也出現(xiàn)于Au4f7/2XPS分譜中, 這表明Au0.3Pd0.7NPs和rGO襯底之間的相互作用在PEI修飾襯底以后顯著增強, 從而導致PEI-rGO和AuPd NPs之間發(fā)生電子轉(zhuǎn)移, 改變了AuPd電子結(jié)構(gòu), 有助于提升其催化性能。

2.2 催化性能分析

圖4(a)對比了Au0.3Pd0.7/PEI-rGO、Au0.3Pd0.7/rGO與Au0.3Pd0.7這三種催化劑催化FA分解制氫的催化活性, 由圖中可見, Au0.3Pd0.7/PEI-rGO催化劑的活性最高, 在323 K且無添加劑的條件下, 3 min內(nèi)產(chǎn)生224 mL氣體(此氣體產(chǎn)量與FA分解反應(yīng)(1)的理論產(chǎn)量吻合), 說明Au0.3Pd0.7/PEI-rGO通過反應(yīng)(1)高效地將FA完全分解為H2和CO2, 并抑制了反應(yīng)(2)。

相比而言, 在相同外界條件下, Au0.3Pd0.7/rGO與Au0.3Pd0.7催化的反應(yīng)僅產(chǎn)生了84和30 mL氣體, 分別用時82和13 min。經(jīng)計算, Au0.3Pd0.7/PEI-rGO在323 K下的TOF值為2357.5 molH2?molcatalyst–1?h–1, 不僅明顯優(yōu)于Au0.3Pd0.7/ rGO和Au0.3Pd0.7(圖4(b)), 而且超過了目前報道的大多數(shù)FA分解制氫異相催化劑[7,10,22-28](表1), 這說明氨基功能化的rGO擔載型AuPd NPs具有非常優(yōu)越的催化性能。這一方面歸因于PEI-rGO對于AuPd NPs的錨定作用和限域作用使得小尺寸金屬NPs在PEI-rGO襯底上彌散分布, 從而增加了反應(yīng)活性位點。另一方面氨基化的PEI-rGO襯底與金屬活性組元之間的電子轉(zhuǎn)移有利于調(diào)控FA在催化劑表面的吸附能[29], 進而增強Au0.3Pd0.7/PEI-rGO催化劑對FA分解制氫反應(yīng)的催化活性。

圖3 Au0.3Pd0.7/PEI-rGO與Au0.3Pd0.7/rGO的XPS圖譜

(a) XPS total spectra for (1) Au0.3Pd0.7/rGO and (2) Au0.3Pd0.7/PEI-rGO; (b) High resolution XPS spectra of N1s for Au0.3Pd0.7/PEI-rGO; (c) Au4f, (d) Pd3d XPS spectra for (1) Au0.3Pd0.7/PEI-rGO and (2) Au0.3Pd0.7/rGO

圖4 Au0.3Pd0.7/PEI-rGO、Au0.3Pd0.7/rGO與Au0.3Pd0.7催化劑在FA分解制氫反應(yīng)中的催化性能對比

(a) Volume of gastime for the dehydrogenation of FA (1 mol/L, 5 mL) catalyzed by (1) Au0.3Pd0.7/PEI-rGO, (2) Au0.3Pd0.7/rGO and (3) Au0.3Pd0.7; (b) Corresponding TOF values

表1 不同F(xiàn)A脫氫催化劑的TOF

catalyst/FArepresents the molar ratio of catalyst to FA; a: Initial TOF values calculated based on total metal; b: Initial TOF values calculated based on total Pd atoms.

為了進一步明確Au0.3Pd0.7/PEI-rGO的動力學性能, 進行了一系列變溫催化FA制氫實驗。圖5(a)顯示了Au0.3Pd0.7/PEI-rGO在不同溫度下的催化活性, 其結(jié)果表明, 隨著反應(yīng)溫度上升, 反應(yīng)速率提高。根據(jù)此結(jié)果計算出Au0.3Pd0.7/PEI-rGO催化劑的a為35.93 kJ/mol (圖5(b)), 小于相同或相似反應(yīng)條件下大多數(shù)FA分解制氫異相催化劑的a[7,10,25], 這進一步表明 Au0.3Pd0.7/PEI-rGO催化劑在無添加劑存在的溫和條件下具有突出的催化性能。此外, 測試了催化劑活性組元的成分配比對于其催化活性的影響。圖5(c)顯示了不同Au/Pd比例的AuPd1-x/PEI-rGO催化劑對于FA分解效率的影響, 經(jīng)對比得到Au與Pd的最佳原子比為Au : Pd = 3 : 7。此催化活性的增強機制為采用合金化引入比Pd功函低的Au, 通過合金中AuPd組元間的電子協(xié)同作用, 調(diào)控FA在催化劑表面的吸附能, 活化C–H鍵, 從而抑制反 應(yīng)(2)[30], 進而提升催化活性。根據(jù)上述分析和其他相關(guān)研究結(jié)果[31], Au0.3Pd0.7/PEI-rGO催化FA分解制氫反應(yīng)機理可以解釋如下: 來自PEI的氨基作為電子供體, 在反應(yīng)中起到質(zhì)子吸收劑的作用, 促進FA分子中的O–H鍵斷裂, 形成[H2NH]+。同時, 富電子的金屬活性組元調(diào)控HCOO*的橋位吸附能, 活化C–H, 以生成CO2和金屬氫化物。最后, 金屬氫化物與[H2NH]+反應(yīng)生成了H2。

圖5 (a) Au0.3Pd0.7/PEI-rGO催化劑在不同溫度下催化FA脫氫的氣體體積與時間的關(guān)系曲線, (b) Au0.3Pd0.7/PEI-rGO催化劑的lnTOF與1/T的關(guān)系擬合直線, (c) 在323 K下,不同比例金屬組分的AuxPd1–x/PEI-rGO(x=0, 0.1, 0.3, 0.7, 0.9, 1.0)催化FA脫氫的氣體體積與時間的關(guān)系曲線, (d) Au0.3Pd0.7/PEI-rGO催化FA(1.0 mol/L, 5.0 mL)分解制氫的循環(huán)穩(wěn)定性測試

催化劑的循環(huán)穩(wěn)定性對于其在實際生產(chǎn)生活中的推廣應(yīng)用具有重要影響。如圖5(d)所示, 在經(jīng)過5次循環(huán)后, Au0.3Pd0.7/PEI-rGO催化劑催化FA分解制氫反應(yīng)仍保持224 mL的氣體產(chǎn)量, 完成反應(yīng)時間從第一輪循環(huán)反應(yīng)的3 min略延長至第五輪反應(yīng)的4 min, 這說明Au0.3Pd0.7/PEI-rGO催化劑在溫和的反應(yīng)過程中可以保持較為穩(wěn)定的狀態(tài), 具有良好的循環(huán)穩(wěn)定性。

3 結(jié)論

本研究采用濕化學法制備PEI-rGO襯底擔載型AuPd 納米催化劑, 并應(yīng)用在催化FA分解制氫領(lǐng)域。結(jié)果表明, 制備得到的AuPd NPs顆粒細小均勻(3.88 nm), 分散性良好。在323 K且無添加劑的情況下, Au0.3Pd0.7/PEI-rGO催化劑具有優(yōu)異的催化活性和良好的循環(huán)穩(wěn)定性, 其催化反應(yīng)的TOF高達2357.5 molH2?molcatalyst–1?h–1,a低至35.93 kJ/mol, 催化活性優(yōu)于目前報道的大多數(shù)相似反應(yīng)條件下的FA分解制氫異相催化劑。這歸因于PEI-rGO襯底與AuPd NPs之間的強相互作用對NPs的尺寸、分散度和電子結(jié)構(gòu)的調(diào)控。此工作為甲酸制氫用高效率催化劑的設(shè)計開拓了新的思路, 同時為異相催化劑在能源與環(huán)境領(lǐng)域的應(yīng)用提供了更多機會。

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Hydrogen Generation from Formic Acid Boosted by Functionalized Graphene Supported AuPd Nanocatalysts

WANG Hongli1, WANG Nan1, WANG Liying1, SONG Erhong2, ZHAO Zhankui1

(1. Key Laboratory of Advanced Structural Materials, Ministry of Education, School of Materials Science and Engineering, Changchun University of Technology, Changchun 130012, China; 2. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China)

Formic acid (FA) is considered as a new type of hydrogen storage material with great application prospect due to its high hydrogen content and easy recharging as a liquid. Seeking high efficiency catalysts to solve the problem of slow reaction kinetics of hydrogen evolution from FA is vital. In this work, polyethyleneimine modified graphene (PEI-rGO) was used as the catalyst substrate, and PEI-rGO supported AuPd nanocomposite material (Au0.3Pd0.7/PEI-rGO) was prepared by wet chemical method. The Au0.3Pd0.7/PEI-rGO catalyst exhibits remarkable activity for the hydrogen generation from FA, affording an unprecedented turnover frequency (TOF) of 2357.5 molH2?molcatalyst–1?h–1without any additives, which is superior to most heterogeneous catalysts under similar reaction conditions. Its excellent catalytic performance is attributed to the strong interaction between PEI-rGO substrate and AuPd nanoparticles, which regulates the size, dispersion and electronic structure of metal active components. Furthermore, the recycle test result shows that the catalyst has good stability.

functionalized graphene; nano metal catalyst; formic acid; hydrogen generation reaction

1000-324X(2022)05-0547-07

10.15541/jim20210311

TQ174

A

2021-05-17;

2021-07-12;

2021-07-12

國家自然科學基金(51601018, 51671035); 上海市自然科學基金面上項目(21ZR1472900); 吉林省教育廳“十三五”科學技術(shù)項目(JJKH20200660KJ)

National Natural Science Foundation of China (51601018, 51671035); Shanghai Natural Science Foundation of China (21ZR1472900); Science and Technology Research Project of the Education Department of Jilin Province (JJKH20200660KJ)

王虹力(1989–), 女, 副教授. E-mail: wanghongli@ccut.edu.cn

WANG Hongli (1989–), female, associate professor. E-mail: wanghongli@ccut.edu.cn

宋二紅, 副研究員. E-mail: ehsong@mail.sic.ac.cn; 趙占奎, 教授. E-mail: zhaozk@ccut.edu.cn

SONG Erhong, associate professor. E-mail: ehsong@mail.sic.ac.cn;

ZHAO Zhankui, professor. E-mail: zhaozk@ccut. edu.cn