發(fā)布時(shí)間：2018-05-23 16:08

  本文選題：耐高溫 + 貴金屬　； 參考：《山東大學(xué)》2017年博士論文

【摘要】：開(kāi)采頁(yè)巖氣技術(shù)不斷提升,天然氣將逐漸取代煤炭成為第二大化石資源。作為一種儲(chǔ)量豐富的清潔資源,天然氣除直接作為燃料提供熱能和電能外,其作為發(fā)動(dòng)機(jī)燃料可替代石油或作為C1化工原料可替代煤制合成氣,這對(duì)降低氮氧化物及粉塵等污染物及CO2排放具有重要意義。目前,天然氣在以上利用過(guò)程中存在的核心問(wèn)題是:1)天然氣發(fā)動(dòng)機(jī)尾氣中殘留的低濃度甲烷和一氧化碳需催化燃燒消除;2)甲烷間接轉(zhuǎn)化制合成氣尚缺乏更加環(huán)保而經(jīng)濟(jì)的路線。解決以上兩問(wèn)題依賴于催化一氧化碳及甲烷燃燒和甲烷干重整反應(yīng),燃燒產(chǎn)生的高溫?zé)狳c(diǎn)和重整反應(yīng)所需的長(zhǎng)時(shí)間高溫可導(dǎo)致催化劑不可逆燒結(jié)失活,這是這兩個(gè)過(guò)程存在的共性科學(xué)問(wèn)題,開(kāi)發(fā)高活性耐高溫催化劑是該領(lǐng)域的最具挑戰(zhàn)性課題。基于反應(yīng)高溫條件對(duì)催化劑高的活性尤其熱穩(wěn)定性要求,本論文通過(guò)合理選擇和設(shè)計(jì)氧化物載體組成與形貌,構(gòu)建氧化物載體與貴金屬納米物種外延生長(zhǎng)界面結(jié)構(gòu),增強(qiáng)二者之間相互作用及高溫穩(wěn)定性,并進(jìn)一步通過(guò)優(yōu)化貴金屬種類或具有高氧化還原性的氧化物助劑以針對(duì)不同反應(yīng)提高其催化活性,研制了高活性長(zhǎng)壽命的甲烷和一氧化碳催化燃燒及甲烷二氧化碳干重整催化劑,并對(duì)該類催化劑的穩(wěn)定機(jī)制進(jìn)行了研究。主要研究?jī)?nèi)容如下:(1)介孔氧化鈰形貌對(duì)其直接作為催化劑或催化劑載體對(duì)甲烷和一氧化碳催化燃燒性能影響。采用溫和的溶劑熱方法,通過(guò)控制表面活性劑、沉淀劑、氧化劑和溶劑熱處理時(shí)間獲得了空心納米錐、納米片和納米線型介孔氧化鈰,發(fā)現(xiàn)該具有多級(jí)介孔結(jié)構(gòu)的氧化鈰納米催化劑催化CO燃燒活性均明顯高于商品氧化鈰,其中介孔空心納米錐型形氧化鈰催化CO燃燒活性最高,起燃溫度(T50)約為200 ℃;向制備介孔氧化鈽空心納米錐體系中加入過(guò)渡金屬(Cu、Mn、Co、Ni)離子,所得銅摻雜氧化鈰樣品催化CO燃燒活性大幅提高,起燃溫度可降低到127℃,這與其優(yōu)異的氧化還原性能相關(guān);對(duì)于催化甲烷燃燒反應(yīng),不同氧化鈰負(fù)載貴金屬Pd后,起燃溫度都在550-580℃之間,反而低于商品氧化鈰負(fù)載的Pd催化劑,其原因可能在于氧化鈰的特殊形貌結(jié)構(gòu)在高溫下坍塌后導(dǎo)致Pd更易燒結(jié)失活。(2)MgAl204負(fù)載的耐高溫Pt(Pd)催化劑及Ce02助劑對(duì)催化一氧化碳和甲烷燃燒性能影響。基于MgAl204對(duì)尖晶石通過(guò)外延生長(zhǎng)結(jié)構(gòu)對(duì)Pt和Pd優(yōu)異的穩(wěn)定作用,我們通過(guò)傳統(tǒng)等體積浸漬過(guò)程制備了高溫穩(wěn)定單Pt和Pt-Pd雙金屬催化劑(質(zhì)量分?jǐn)?shù)為1wt.%),并進(jìn)一步通過(guò)浸漬法引入氧化鈰獲得氧化鈰修飾的上述催化劑。在800 ℃空氣氣氛中老化7天后,A1203負(fù)載的貴金屬Pt發(fā)生嚴(yán)重?zé)Y(jié),而Pt/MgAl2O4和Pt-Ce/MgAl204分別實(shí)現(xiàn)了貴金屬納米粒子(2-3 nm)的部分和全部穩(wěn)定,尤其是后者,在氧化鈰的輔助下,Pt甚至可穩(wěn)定在單原子分散狀態(tài)!相應(yīng)的,Pt/MgAl204和Pt-Ce/MgAl204在老化處理后仍保持較高CO氧化低溫活性,在空速高達(dá)500000 mL/g-s時(shí),CO的起燃溫度分別低至約190 ℃和160 ℃,表現(xiàn)出耐高溫高分散貴金屬Pt對(duì)CO燃燒的優(yōu)異催化性能。然而,這些具有高熱穩(wěn)定性的高分散Pt催化劑催化甲烷燃燒反應(yīng)時(shí)(空速50000 mL/g-s),其起燃溫度分別為645 ℃和526 ℃,遠(yuǎn)高于常規(guī)大尺寸Pt催化劑,表明小尺寸Pt不利于CH4催化燃燒反應(yīng)。雙金屬催化劑Pt-Pd/MgAl204老化后催化甲烷燃燒起燃溫度可降低至495 ℃,經(jīng)氧化鈰修飾后催化劑催化甲烷燃燒起燃溫度可進(jìn)一步降低到475 ℃,且低溫段活性顯著提高。(3)低載量高分散耐高溫Ru/MgA1204催化甲烷二氧化碳干重整反應(yīng)研究�；贛gAl204對(duì)Ru納米粒子的高穩(wěn)定作用,我們利用氧化釕物種在高溫氧化氣氛中的高揮發(fā)特性,通過(guò)物理氣相沉積(PVD)過(guò)程,在900 ℃流動(dòng)空氣中,沉積氧化釕制備了原子級(jí)分散Ru/MgAl204催化劑,經(jīng)900 ℃H2高溫還原后,金屬Ru納米顆粒平均尺寸約為1.1nm,完全避免了通過(guò)傳統(tǒng)浸漬法制備的Ru催化劑樣品中尺寸大于100 nm大顆粒Ru的形成。在重整反應(yīng)生成氫氣和一氧化碳的還原氣氛下,金屬Ru不再揮發(fā)流失。在850 ℃反應(yīng)溫度下,PVD法制備的Ru/MgAl204催化劑(質(zhì)量分?jǐn)?shù)0.15%)催化甲烷干重整反應(yīng)速率可達(dá)279.50 mol/molRu-s,約是浸漬法Ru/MgAl204催化劑(質(zhì)量分?jǐn)?shù)1%)反應(yīng)速率(28.61 mol/molRu-s)的10倍,是目前文獻(xiàn)中高活性Ni催化劑NiCoMg/Al203(0.61 mol/molNi-s)的457倍;該催化劑在850℃,空速高達(dá)400000 mL/g-h(CH4:C02 =1:1)條件下對(duì)催化甲烷干重整反應(yīng)表現(xiàn)出及其優(yōu)異的高溫穩(wěn)定性和抗積碳性能,在長(zhǎng)達(dá)600 h的穩(wěn)定性實(shí)驗(yàn)中,甲烷和二氧化碳轉(zhuǎn)化率(96.7%和98.6)均達(dá)到熱力學(xué)平衡轉(zhuǎn)化率,H2/CO比值接近理論值1;反應(yīng)后Ru平均尺寸增大為2.7 nm,反應(yīng)速率反而提高到反應(yīng)前的1.5倍,催化劑上無(wú)可測(cè)的積碳量。與此相反的,浸漬法制備的Ru/MgAl204催化劑(質(zhì)量分?jǐn)?shù)1%)在此反應(yīng)條件下10小時(shí)內(nèi)迅速失活,同時(shí)伴隨大量積碳。若降低PVD法制備的Ru/MgAl204催化劑的Ru質(zhì)量分?jǐn)?shù)到0.07 wt.%,得到的金屬Ru粒子尺寸更小,其在同等條件下的反應(yīng)速率為35.72mol/molRu-s。這些結(jié)果表明,至少在金屬Ru尺寸低于2.7 nm的范圍內(nèi),增大Ru納米粒子尺寸可顯著增加其表觀活性,且在此尺寸范圍內(nèi),催化劑上基本不產(chǎn)生積碳;但存在于浸漬法制備催化劑中的大顆粒Ru似乎是導(dǎo)致催化劑嚴(yán)重積碳的重要原因。這種極低金屬釕用量的高活性長(zhǎng)壽命催化劑有顯著的工業(yè)應(yīng)用潛力。綜上,該論文針對(duì)高溫反應(yīng)對(duì)催化劑活性及高溫穩(wěn)定性的要求,利用MgAl204尖晶石作為載體成功制備了在納米、亞納米乃至單原子尺度穩(wěn)定的Pt,Pd和Ru基催化劑,并選擇性的利用具有高氧化還原性的氧化鈰作為助劑,進(jìn)一步增強(qiáng)了其催化性能。通過(guò)研究耐高溫Pt,Pd和Ru催化劑在催化CO和CH4催化燃燒及CH4/C02干重整反應(yīng)中的行為,發(fā)現(xiàn)在被穩(wěn)定在不同尺寸的貴金屬物種中,較大顆粒尺寸的貴金屬具有更好的表觀活性。因此,適當(dāng)增大高熱穩(wěn)定貴金屬納米粒子的尺寸有望進(jìn)一步增強(qiáng)其催化活性,尤其是催化CO和CH4燃燒反應(yīng)的低溫活性。
[Abstract]:With the continuous improvement of shale gas technology, natural gas will gradually replace coal and become the second largest fossil resource. As a rich and abundant clean resource, natural gas is used as fuel to replace petroleum or substitute for coal synthetic gas as C1 chemical raw material. And dust and other pollutants and CO2 emissions are of great significance. At present, the core problems in the utilization of natural gas are as follows: 1) the residual low concentration methane and carbon monoxide in the gas engine exhaust gas need catalytic combustion; 2) the indirect conversion of methane to synthetic gas is still lack of a more environmental and economical route. The above two questions are solved. The problem depends on the catalytic reaction of carbon monoxide and methane combustion and methane dry reforming. The hot hot spots and the long time high temperature required by the combustion can lead to the irreversible inactivation of the catalyst. This is the common scientific problem in these two processes. The development of high activity and high temperature resistant catalysts is the most challenging topic in this field. In this paper, the interfacial structure of oxide carrier and noble metal nanoscale is constructed by rational selection and design of the composition and morphology of oxide carrier, and the interaction between the two and the high temperature stability is enhanced by the rational selection and design of the composition and morphology of the oxide carrier. In order to improve its catalytic activity with high oxidation-reducibility, the catalytic combustion of methane and carbon monoxide and carbon dioxide dry reforming catalyst for high active and long life are developed, and the stability mechanism of this kind of catalyst has been studied. The main contents are as follows: (1) the morphology of mesoporous cerium oxide is straight. The effect of catalyst or catalyst carrier on the catalytic combustion performance of methane and carbon monoxide was obtained. The hollow nanoscale, nanoscale and nanoscale mesoporous cerium oxide were obtained by controlling the surface active agent, precipitant, oxidant and solvent heat treatment by mild solvent thermal method. The catalytic activity of CO is obviously higher than that of commercial cerium oxide, in which the mesoporous hollow nano cone shaped cerium oxide catalyzes the highest combustion activity of CO, and the ignition temperature (T50) is about 200 c, and the transition metal (Cu, Mn, Co, Ni) ions are added to the mesoporous hollow plutonium oxide nanocone system, and the copper doped cerium oxide sample catalyzes the combustion of CO. The ignition temperature can be reduced to 127 degrees centigrade, which is related to the excellent oxidation and reduction performance. For the catalytic methane combustion reaction, the ignition temperature is between 550-580 centigrade and less than the commercial cerium oxide supported by different cerium oxide Pd. The reason may be that the special morphology of cerium oxide is high. Pd is more prone to sinter inactivation after the temperature collapse. (2) the effect of high temperature Pt (Pd) catalyst and Ce02 promoter on the catalytic performance of carbon monoxide and methane combustion with MgAl204 load. Based on the excellent stabilizing effect of spinel on Pt and Pd by the epitaxial growth structure of spinel, we prepared high temperature stable single Pt and Pt- through the uniform impregnation process. Pd bimetallic catalyst (mass fraction is 1wt.%), and cerium oxide was further introduced by impregnation to obtain cerium oxide modified catalyst. After 7 days of aging in air atmosphere at 800 C, A1203 loaded precious metal Pt was seriously sintered, and Pt/MgAl2O4 and Pt-Ce/MgAl204 showed part and all of the noble metal nanoparticles (2-3 nm) respectively. Stability, especially the latter, with the aid of cerium oxide, Pt can even be stable in the state of single atom dispersion! Corresponding, Pt/MgAl204 and Pt-Ce/MgAl204 still maintain high CO oxidation activity after aging, and the ignition temperature of CO is lower to about 190 and 160 degrees, respectively, when the air velocity is up to 500000 mL/g-s, showing high temperature and high dispersive noble metal P T has excellent catalytic performance for CO combustion. However, these high thermal stability highly dispersed Pt catalysts catalyze methane combustion (air velocity 50000 mL/g-s), and the ignition temperature is 645 C and 526 C, which is far higher than the conventional large Pt catalyst, indicating that small size Pt is not conducive to the catalytic combustion reaction of CH4. Bimetallic catalyst is old and old. The combustion temperature of methane combustion can be reduced to 495 C after the catalytic combustion of methane, and the catalytic combustion temperature of methane combustion can be further reduced to 475 degrees C, and the activity of low temperature section increases significantly. (3) study on the dry reforming reaction of Methane Carbon Dioxide Catalyzed by low load and high dispersing high temperature Ru/MgA1204. Based on the height of MgAl204 to Ru nanoparticles We use the high volatilization characteristic of ruthenium oxide in the high temperature oxidation atmosphere. Through the physical vapor deposition (PVD) process, the atomic level dispersed Ru/MgAl204 catalyst is prepared by the deposition of ruthenium oxide at 900 centigrade. The average size of the metal Ru nanoparticles is about 1.1nm after the high temperature reduction at 900 C H2. In the Ru catalyst samples prepared by traditional impregnation, the size of the large particles larger than 100 nm was formed. Under the reduction atmosphere of hydrogen and carbon monoxide in the reforming reaction, the metal Ru no longer volatilized and lost. At the reaction temperature of 850 C, the Ru/MgAl204 catalyst prepared by PVD method (mass fraction 0.15%) catalyzed the methane dry reforming reaction rate of 279.50 mol /molRu-s, about 10 times the reaction rate (28.61 mol/molRu-s) of the impregnation method Ru/MgAl204 catalyst (mass fraction 1%), is 457 times of the present high active Ni catalyst NiCoMg/Al203 (0.61 mol/molNi-s) in the literature. The catalyst shows its excellent catalytic performance on methane dry reforming under the condition of 850 centigrade and high air speed up to 400000 mL/g-h (CH4:C02 =1:1). In the 600 h stability experiments, the conversion rate of methane and carbon dioxide (96.7% and 98.6) reached the thermodynamic equilibrium conversion rate, and the H2/CO ratio was close to the theoretical value of 1. The average size of Ru increased to 2.7 nm after the reaction, and the reaction rate was 1.5 times higher than that before the reaction. There was no measurable carbon accumulation on the catalyst. On the contrary, the Ru/MgAl204 catalyst prepared by the impregnation method (mass fraction 1%) quickly deactivated in 10 hours under the reaction conditions, and accompanied by a large amount of carbon. If the Ru mass fraction of the Ru/MgAl204 catalyst prepared by the PVD method was 0.07 wt.%, the obtained metal Ru particle size was smaller, and the reaction rate under the same condition was 35.72mol/mol Ru-s. results show that, at least in the range of metal Ru size below 2.7 nm, increasing the size of Ru nanoparticles can significantly increase its apparent activity, and in this size range, there is no carbon deposition on the catalyst, but the large particle Ru in the catalyst prepared by impregnation seems to be an important cause of the serious carbon deposition of the catalyst. The highly active and long-life long-life catalysts with extremely low metal ruthenium have significant industrial potential. To sum up, this paper successfully prepared Pt, Pd and Ru based catalysts with MgAl204 spinel as a carrier for the catalytic activity and high temperature stability of the catalyst. The catalytic performance of the high temperature resistant Pt, Pd and Ru catalysts in the catalytic combustion of CO and CH4 and in the CH4/C02 dry reforming reaction was further enhanced by using high oxidizing and reductive cerium oxide as a promoter. It was found that the larger size of noble metals in different sizes of precious metals have a better table in the noble metal species that are stable in different sizes. Therefore, it is expected that the proper increase in the size of high heat and stable noble metal nanoparticles is expected to further enhance its catalytic activity, especially at the low temperature activity of the CO and CH4 combustion reactions.
【學(xué)位授予單位】：山東大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：O643.36
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本文編號(hào)：1925445