本文選題：過渡金屬 + 復(fù)合材料��； 參考：《山東大學(xué)》2016年博士論文

【摘要】：隨著全球經(jīng)濟的高速發(fā)展,對化石燃料的過度開發(fā)使得傳統(tǒng)燃料資源愈加顯得捉襟見肘。在眾多新能源中,氫被認(rèn)為是有史以來最為清潔的能源,具有安全、高效、零污染等諸多優(yōu)點。迄今為止,雖然人們在氫能的開發(fā)利用方面取得了一系列重要進展,但在氫燃料電池的應(yīng)用方面還面臨許多難題,其中,如何為燃料電池的發(fā)電裝置提供高質(zhì)量而廉價的氫氣是主要問題之一。直接以氫為燃料無論在運輸方面還是儲存方面都存在諸多問題,而氨分解在線制氫技術(shù)可以有效的解決這兩大難題。氨是一種富氫化合物,能量密度遠遠高于甲醇、汽油等燃料,常溫下易于以液體形式存在,而液氨的儲存安全可靠,成本低廉。此外,由于氨分子中不含碳元素,因而從源頭上杜絕了COr等有毒氣體的生成。將氨分解反應(yīng)用于在線制氫領(lǐng)域是目前解決燃料電池氫能來源的有效途徑。作為氨分解反應(yīng)的研究內(nèi)容之一,氨分解催化劑的設(shè)計和制備至關(guān)重要。目前對于氨分解催化劑的研究多數(shù)集中在負載型材料上,主要包含以釕為代表的貴金屬型催化劑和以鐵、鎳等為代表的廉價過渡金屬型催化劑。貴金屬催化劑的催化氨分解反應(yīng)活性一般較高,但其價格非常昂貴。而對于用于催化氨分解反應(yīng)的負載型過渡金屬催化劑,較低的活性組分含量與高溫下易燒結(jié)的缺點是其存在的主要問題。此外,在已有的文獻中,人們較多關(guān)注的是催化劑體系的選擇和制備,而對催化劑在反應(yīng)條件下活性物相的指認(rèn)工作則涉及較少。本論文擬以過渡金屬為主體,利用優(yōu)良的熱穩(wěn)定性物質(zhì)作為結(jié)構(gòu)穩(wěn)定組分,通過廉價、易行的合成方法,設(shè)計制備具有高活性組分含量的過渡金屬基復(fù)合材料,研究其作為催化劑在氨分解反應(yīng)中的催化行為,考察穩(wěn)定劑和活性組分的相對含量、介觀分布以及微觀結(jié)構(gòu)對催化劑結(jié)構(gòu)穩(wěn)定性和催化活性的影響。同時使用原位x射線粉末衍射表征技術(shù)追蹤催化劑的活性組分在反應(yīng)中的物相變化,揭示物相以及結(jié)構(gòu)變化與其催化活性的內(nèi)在聯(lián)系,指認(rèn)催化活性物相,從而進一步指導(dǎo)氨分解催化劑的設(shè)計合成,挖掘過渡金屬基納米復(fù)合材料在工業(yè)催化領(lǐng)域的應(yīng)用價值。論文主要分為以下三部分工作：1.利用共沉淀方法合成了Fe、Co、Ni分別與氧化鋁復(fù)合的三種高活性組分含量的過渡金屬催化劑體系,通過XRD、氮氣吸脫附、TEM、SEM等一系列手段表征了反應(yīng)前后催化劑的物相和結(jié)構(gòu)特點。研究表明,過渡金屬的含量較低時,催化劑主要由大量的無定形的氧化鋁和過渡金屬氧化物組成,隨著含量的提高,過渡金屬以結(jié)晶態(tài)的氧化物形式存在。所得具有高活性組分含量的催化劑表現(xiàn)出優(yōu)異的催化活性和穩(wěn)定性,其中鈷含量為90 at%的樣品催化活性最高,在空速高達72000 cm3 goat-1 h-1的條件下,600℃時,可以實現(xiàn)88%的高轉(zhuǎn)化率,并且在72小時的測試時間內(nèi)沒有任何降低的趨勢。催化氨分解反應(yīng)后,氧化鐵被氨氣氮化,生成氮化鐵,而氧化鈷和氧化鎳均被還原成金屬單質(zhì)。相應(yīng)的元素分布結(jié)果顯示,摻入過渡金屬化合物中的少量鋁,分布在催化劑表面,有效抑制了活性組分在反應(yīng)過程中的燒結(jié)現(xiàn)象,從而提高了氨分解反應(yīng)的催化活性和穩(wěn)定性。H2-TPR結(jié)合相應(yīng)的原位XRD實驗用于探究三種催化劑體系在氫氣條件下的還原行為,結(jié)果表明,鋁的含量越高,與過渡金屬的相互作用越強,其中鋁的加入使得氧化鐵更容易被還原,而對于氧化鈷和氧化鎳來說,與氧化鋁之間的強相互作用導(dǎo)致二者更難被還原。2.使用溶膠凝膠一步合成法,得到鈷含量不等的Co-Al系列氨分解反應(yīng)催化劑,并通過XRD、XAFS、、XPS,氮氣吸脫附、TEM、SEM等一系列手段進行了表征。對樣品的XRD衍射譜Rietveld結(jié)構(gòu)精修結(jié)果證明,在含有鋁的催化劑中,鈷并不是以單純的C0304相存在,而是Co和Al同時占據(jù)四面體和八面體配位位置的混合尖晶石相(Co,Al)(Co,Al)204。XAFS結(jié)果表明,反應(yīng)前的樣品中Co原子與O原子配位,反應(yīng)后呈Co-Co配位。少量鋁的加入,能夠有效的阻止活性組分在反應(yīng)過程中的聚集,從而大大提高鈷催化劑的氨分解反應(yīng)催化穩(wěn)定性。在36000 cm3 gcat-1 h-1的高空速下,該系列催化劑表現(xiàn)出良好的催化氨解活性和穩(wěn)定性,600℃時,鈷含量為90 at%的催化劑產(chǎn)氫速率達到37 mmol gcat-1 min-1,并在120 h測試時間內(nèi)沒有絲毫降低。研究發(fā)現(xiàn),影響Co-Al催化劑催化活性的因素有多種,包括鈷的含量、相態(tài)、晶粒尺寸、比表面積和鈷鋁之間的相互作用等等。原位XRD用于追蹤Co-Al催化劑在氨分解反應(yīng)中的物相變化,結(jié)果證明,由混合尖晶石相(Co,Al)(Co,Al)2O4到CoO、CoO進一步到金屬Co的還原過程受起始樣品中鈷含量的影響,鈷的含量越高,(Co,Al)(Co,Al)2O4到CoO的還原溫度越低。將原位XRD結(jié)果與催化活性數(shù)據(jù)結(jié)合在一起,首次指認(rèn)了立方相的金屬Co是鈷基氨分解催化劑最可能的活性相,此外CoO也具有一定的催化活性。3.表面活性劑在溶液中可以組裝成為有序分子聚集體,通過進一步的溶劑蒸發(fā)形成層狀液晶,以層狀液晶為模板,經(jīng)過高溫碳化、強堿剝離等過程,合成了碳納米片基三元復(fù)合材料。通過TEM、SEM、XPS、氮氣吸脫附、拉曼光譜、原位XRD等一系列手段對該碳納米片基復(fù)合催化劑進行了研究,發(fā)現(xiàn)碳納米片上同時分散著鈷基化合物和無定形的鋁,其中鈷基化合物在碳納米片上呈交聯(lián)狀分布。拉曼光譜顯示,雖然碳化溫度只有400℃,但是得到的碳納米片具有非常高的石墨化結(jié)晶度。氨分解催化反應(yīng)后,鈷氧化物被還原成金屬鈷單質(zhì),而經(jīng)過相變重結(jié)晶的過程,原先處于交聯(lián)分布的鈷物種變成了尺寸均一的鈷顆粒,同時均勻而密集地嵌入在鋁和碳的復(fù)合納米片中,這樣的結(jié)構(gòu)特點可以有效的阻止鈷顆粒在氨分解反應(yīng)中的聚集,從而保證這種復(fù)合材料高效穩(wěn)定的催化氨分解反應(yīng)。催化測試結(jié)果顯示,空速為12000 cm3 gcat-1 h-1時,鈷與鋁投料比為3/7的樣品在500℃的低溫下就可以實現(xiàn)氨的完全轉(zhuǎn)化,即使將反應(yīng)氣體空速提高至76 000 cm3 gcat-1 h-1,600℃時,該催化劑仍然能達到96%的氨轉(zhuǎn)化率。其催化穩(wěn)定性更為突出,在長達144 h的穩(wěn)定性測試中,催化劑沒有發(fā)生任何失活現(xiàn)象。
[Abstract]:With the rapid development of the global economy, the overexploitation of fossil fuels has made traditional fuel resources more and more difficult. In many new energy sources, hydrogen is considered as the most clean energy in history, with many advantages, such as safety, efficiency, zero pollution and so on. So far, people have made a line in the development and utilization of hydrogen energy. However, there are many difficulties in the application of hydrogen fuel cells, among which, how to provide high quality and cheap hydrogen for the power plant of fuel cells is one of the main problems. There are many problems in both transport and storage for direct hydrogen as fuel, and the on-line hydrogen production technology of ammonia decomposition can be effective. To solve these two difficult problems. Ammonia is a kind of hydrogen rich compound. The energy density is far higher than that of methanol and gasoline. It is easy to exist in liquid form at normal temperature, while the storage of liquid ammonia is safe and reliable, and the cost is low. In addition, the formation of toxic gases such as COr is eliminated from the source because of no carbon element in the ammonia molecule. The field of on-line hydrogen production is an effective way to solve the hydrogen energy sources of fuel cells. As one of the research contents of the ammonia decomposition reaction, the design and preparation of the ammonia decomposition catalyst are very important. At present, most of the research on the ammonia decomposition catalyst is concentrated on the loaded materials, mainly including the precious metal catalysts and iron, represented by ruthenium. The catalytic ammonia decomposition reaction of the noble metal catalyst is generally high, but its price is very expensive. For the supported transition metal catalyst used to catalyze the ammonia decomposition reaction, the shortcomings of the lower active component content and the easy firing at high temperature are the main problems. In the existing literature, more attention is paid to the selection and preparation of the catalyst system, while the identification of the active phase of the catalyst is less than that of the catalyst. This paper is designed to use the transition metal as the main body, and use the excellent thermal stability material as the structural stability component, and design the preparation by the cheap and easy synthesis method. The transition metal matrix composite with high active component content was studied as a catalyst in the ammonia decomposition reaction, and the effect of the relative content of stabilizers and active components, mesoscopic distribution and microstructure on the structure stability and catalytic activity of the catalyst were investigated. At the same time, the technology of in situ X ray powder diffraction was used to characterize the technology. The active component of the tracer catalyst changes in the phase of the reaction, reveals the intrinsic relationship between the phase and the structural change and its catalytic activity, and identifies the catalytic active phase, thus further directing the design and synthesis of the ammonia decomposition catalyst and excavating the application value of the transition metal matrix nanocomposites in the industrial catalysis field. The thesis is mainly divided into the following The three part: 1. using coprecipitation method to synthesize the transition metal catalyst system with three kinds of high active component content of Fe, Co, Ni and alumina respectively. Through a series of means such as XRD, nitrogen desorption, TEM, SEM and so on, the phase and structure characteristics of the catalyst before and after the reaction are characterized. The catalyst is mainly composed of a large number of amorphous alumina and transition metal oxide. With the increase of the content, the transition metal exists in the form of crystalline oxide. The catalyst with high active component content shows excellent catalytic activity and stability. The catalytic activity of the sample with a cobalt content of 90 at% is the highest, and the velocity is high at the air velocity. Under the condition of 72000 cm3 goat-1 H-1, the high conversion rate of 88% can be achieved at 600 C, and there is no downward trend in the test time of 72 hours. After the catalytic ammonia decomposition reaction, the iron oxide is nitriding by ammonia to produce iron nitride, while cobalt oxide and nickel oxide are reduced to metal monomer. A small amount of aluminum in the metal compound is distributed on the surface of the catalyst, which effectively inhibits the sintering of the active components during the reaction process, thus improving the catalytic activity and stability of the ammonia decomposition reaction..H2-TPR binding in situ XRD experiment is used to explore the reduction behavior of the three catalyst systems under hydrogen conditions. The higher the content, the stronger the interaction with the transition metal, in which the addition of aluminum makes the iron oxide easier to be reduced, and for the cobalt oxide and the nickel oxide, the strong interaction with the alumina leads to the more difficult to be reduced by the reduced.2. using the sol-gel one step synthesis method, and the Co-Al series ammonia decomposition reaction with different cobalt content has been obtained. XRD, XAFS, XPS, XPS, nitrogen desorption, TEM, SEM and so on. The XRD diffraction spectrum Rietveld structure refinement results show that in the catalyst containing aluminum, cobalt is not a pure C0304 phase, but a mixed spinel phase (Co, Al) at the same time that Co and Al occupy the position of tetrahedron and eight sides. L) 204.XAFS results show that Co atoms are coordinated with O atoms in the samples before reaction and Co-Co coordination after reaction. The addition of a small amount of aluminum can effectively prevent the accumulation of active components during the reaction process, thus greatly improving the catalytic stability of the ammonia decomposition reaction of the cobalt catalyst. At the altitude of 36000 cm3 gcat-1 H-1, the series of catalyst tables The catalytic activity and stability of ammonia solution are obtained. At 600 C, the hydrogen production rate of the catalyst with a cobalt content of 90 at% reaches 37 mmol gcat-1 min-1 and has no reduction in the 120 h test time. It is found that there are a variety of factors affecting the catalytic activity of the Co-Al catalyst, including the content of cobalt, phase state, grain size, specific surface area and cobalt aluminum. In situ XRD is used to trace the phase change of the Co-Al catalyst in the ammonia decomposition reaction. The results show that the reduction process of the mixed spinel phase (Co, Al) (Co, Al) to CoO, CoO further to the reduction of metal Co is influenced by the cobalt content in the starting sample, the higher the cobalt content, the lower the reduction temperature of the (Co, Al). In situ XRD results are combined with catalytic activity data. For the first time, the metal Co of the cubic phase is the most likely active phase of the cobalt based ammonia decomposition catalyst. In addition, CoO has a certain catalytic activity of.3. surfactants which can be assembled into ordered molecular aggregates in the solution and through further solvent evaporation to form layered liquid crystals. A carbon nanoscale three element composite was synthesized by high temperature carbonization and strong alkali stripping. The carbon nanoscale composite catalyst was studied by means of TEM, SEM, XPS, nitrogen desorption, Raman spectroscopy, and in situ XRD. The Co based compound and amorphous carbon nanocomposite were simultaneously dispersed on the carbon nanoscale. The cobalt based compounds are crosslinked on carbon nanoscale. Raman spectra show that, although the carbonization temperature is only 400 C, the obtained carbon nanoscale has very high graphitization crystallinity. After the ammonia decomposition catalytic reaction, cobalt oxide is reduced to metal cobalt, and the process of phase change recrystallization is originally in the crosslinking distribution. The cobalt species becomes a homogeneous cobalt particle, which is uniformly and densely embedded in the composite nanoscale of aluminum and carbon. The structural characteristics can effectively prevent the aggregation of cobalt particles in the ammonia decomposition reaction, thus ensuring the efficient and stable catalytic ammonia decomposition reaction of the composite. The catalytic test results show that the velocity of air is 12000. At cm3 gcat-1 H-1, the total conversion of ammonia can be achieved at a low temperature of 500 C with the sample of CO and Al with a ratio of 3/7 to 3/7. Even if the reaction gas velocity is increased to 76000 cm3 gcat-1 h-1600, the catalyst can still reach 96% of the ammonia conversion rate. The catalytic stability is more prominent. In the stability test of up to 144 h, the catalyst has not been tested. Any inactivation occurs.
【學(xué)位授予單位】：山東大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：O643.36;TQ116.2
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本文編號：1977639