甲醇水蒸气重整制氢Pd/ZnO催化剂的研究

采用并流共沉淀法制备Pd／ZnO甲醇水蒸气重整制氢催化剂,考察了Pd的质量分数和还原温度对催化剂性能的影响。结果表明,当Pd质量分数为15．9％,还原温度为573K时,催化剂有较好的甲醇转化率及二氧化碳选择性。TPR结果表明,PdO在室温下被还原为金属Pd,在440K开始有部分ZnO被还原。XRD分析结果表明,PdZn合金是甲醇水蒸气重整反应的活性中心;在21．9％Pd／ZnO催化剂上出现了Pd2Zn合金相,导致催化剂的活性下降;反应过程中还原催化剂形成PdZn合金,其活性不如相同条件下纯氢还原的。15．9％Pd／ZnO催化剂及工业铜基催化剂的初始稳定性结果显示,在8h内,15．9％Pd／ZnO催化剂上甲醇转化率保持在66％以上,而铜基催化剂的活性下降了14．4％。     

甲醇水蒸气重整制氢Pd/ZnO催化剂还原过程中Pd与ZnO的相互作用

 采用程序升温还原、程序升温脱附、程序升温电导及X射线衍射研究了并流共沉淀法制备的15.9%Pd/ZnO催化剂还原过程中结构和物种的变化及电荷的传递,考察了还原温度对该催化剂催化甲醇水蒸气重整制氢的影响. 结果表明,还原过程中Pd与ZnO间存在明显的相互作用,导致氢溢流,溢流氢促进了ZnO还原并使ZnO与Pd形成活性中心PdZn合金. 催化剂的还原历程为 PdO/ZnO → Pd/ZnO → PdZnO1-x/ZnO → 无定形PdZn合金/ZnO → 晶型PdZn合金/ZnO. 并流共沉淀法制备的Pd/ZnO催化剂Pd分散度高, PdZn合金形成温度低. 在523～573 K还原后,催化剂的PdZn合金粒径为5～14 nm, 此时催化剂对甲醇的转化率及二氧化碳选择性均达到最大值.     

Pd/ZnO催化剂的还原及其催化甲醇水蒸气重整制氢

考察了共沉淀法制备的15.9%Pd/ZnO催化剂的还原对甲醇水蒸气重整制氢反应的影响.结果显示,当催化剂的还原温度为523~573K时,523K下反应的甲醇转化率达到了41.6%,CO2选择性为94.6%,出口CO浓度为1.26%.X射线衍射结果显示,当催化剂的还原温度为523K时PdZn合金开始形成.还原温度为523~573K范围内催化剂活性的提高归因于5~14nmPdZn合金粒子的存在.用程序升温还原及X射线衍射表征手段探究了还原过程中Pd与ZnO之间的相互作用.结果表明,Pd/ZnO可能经历了PdO/ZnO→Pd/ZnO→PdZnO1-x/ZnO→PdZn合金/ZnO的还原过程,而部分PdZn合金在反应过程中可重新被氧化成PdZnO1-x.对反应的活性物种进行了初步探讨.     

水热合成介孔ZnO及Pd／m－ZnO催化剂甲醇水蒸气重整制氢性能研究

以三嵌段高聚物F-127为模板剂采用水热法合成了介孔ZnO（m—ZnO）,以甲醇水蒸气重整制氢为探针反应,在连续流动反应条件下考察了Pd／m．ZnO催化剂的性能,并利用XRD、FT—IR、H2-TPR、TEM和BET等手段对载体及催化剂进行了表征．结果表明,该法所制m-ZnO具有较大的比表面积（124．7m^2／g）,其比表面积不但高于非介孔ZnO,而且大于文献值（103．6m^2／g）．与Pd／ZnO催化剂相比,Pd／m—ZnO催化剂中活性组份Pd的分散度较高以及与m—ZnO间的相互作用较强,因此该催化剂对甲醇水蒸气重整制氢反应具有较高的催化活性、氢气产率、CO2选择性和稳定性．250℃时,Pd／m—ZnO催化剂的催化活性、氢气产率和C02选择性分别比Pd／ZnO催化剂提高了38．4％、44．4％和30．0％,170oC连续反应100h后,Pd／m-ZnO催化剂的活性仍为95％,仅下降了4．9％．     

利用ZnO@ZIF-8核壳结构构建高选择性、高稳定性的Pd/ZnO催化剂用于CO_2加氢制甲醇（英文）

近年来由于环境问题CO_2加氢制甲醇催化反应重新回归为研究热点。对于Pd/ZnO催化剂,研究表明PdZn合金相是制甲醇反应的活性中心,而单独Pd利于CO生成。为了实现Pd和ZnO的充分接触,本工作以一种ZnO@ZIF-8核壳型结构为载体负载Pd纳米颗粒后经由高温煅烧制得PZZ8-T催化剂(T为不同煅烧温度),同时制备了ZnO纳米棒负载Pd的PZ催化剂作为对比。在随后的CO_2加氢反应中,相比于PZ,PZZ8-T展现出极高的甲醇选择性。之后我们通过一系列表征探究了催化剂的构效关系,发现催化剂的甲醇选择性与表面Pd的化学态有关,更多的Pd以PdZn合金的形式存在将会带来更高的甲醇选择性。XPS O 1s谱图和EPR分析表明CO_2的活化与催化剂表面的氧空穴和ZnO极性面含量直接相关。而化学吸附手段进一步对Pd-ZnO界面进行了表征,揭示了其与CO_2转化速率的关联。本工作的意义在于,一是展现了利用新材料制备更优的传统催化剂的方法,二是通过表面分析手段加深了对催化剂构效关系的理解。     

草酸盐固相化学法制备高性能Cu/ZnO甲醇水蒸气重整催化剂

采用草酸盐前驱物固相化学法制备了用于甲醇水蒸气重整制氢反应的Cu/ZnO催化剂, 并与传统液相共沉淀方法制备的Cu/ZnO催化剂在相同条件下的催化性能进行了比较. 结果表明, 通过该“干法”合成的Cu/ZnO催化剂具有比传统液相共沉淀法所制备的催化剂更高的催化活性和制氢选择性, 以及更好的稳定性. N₂O吸附和原位XRD分析结果证实固相反应时间对Cu/ZnO催化剂的金属铜表面及晶格微应力等微结构性质可产生重要的调控作用, 从而大大改善其催化活性和制氢选择性.     

氧化锌纳米线外延生长PdZn纳米粒子的制备及在甲醇水蒸气重整反应中的催化性能

以采用改进的气相沉积法制备的具有规整{1010}晶面的氧化锌纳米线为载体,合成了氧化锌纳米线负载钯催化剂,考察了还原温度和负载量对催化剂表面形成Pd Zn合金过程的影响,并通过适当的后处理过程制备了氧化锌纳米线外延生长Pd Zn纳米粒子催化体系.结果表明,当金属钯负载量较低(质量分数约为2%)时,经400℃还原后的催化剂表面会形成PdxZny(xy)合金,从而影响催化剂的CO选择性;提高钯负载量或还原温度有利于将PdxZny(xy)合金转化为Pd Zn合金,降低CO选择性.负载Pd Zn合金纳米粒子与氧化锌纳米线载体之间外延生长的界面关系使其在甲醇水蒸气重整反应中显示出优异的反应稳定性.     

陈化时间对CuO/ZnO/CeO2/ZrO2甲醇水蒸气重整制氢催化剂性能的影响

采用共沉淀法制备了CuO/ZnO/CeO₂/ZrO₂甲醇水蒸气重整催化剂,探讨了陈化时间对催化剂性能的影响.结果发现,延长陈化时间能增加催化剂的表面铜原子数和改善催化剂的还原性能,但与此同时也降低了催化剂的储放氧性能.延长陈化时间,CuO/ZnO/CeO₂/ZrO₂催化剂的氢产率随表面铜原子数的增加而成线性增长.另一方面,重整尾气中的CO含量也随着储放氧能力的下降而增加.综合考虑产氢率和重整尾气中CO含量,最佳陈化时间为2h,此时,CuO/ZnO/CeO₂/ZrO₂催化剂表现出了最佳性能.     

ZnO对Au-Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响

采用沉积沉淀法制备了Au-Pd双金属催化剂, 研究了ZnO对Au-Pd/CeO_2催化剂甲醇部分氧化性能的影响, 并运用N_2吸附、 XRD、 UV-Vis、 TPR、 H2-TPD和CO-IR等手段对催化剂进行了表征. 结果表明, ZnO的引入减少了Pd活性中心, 降低了催化剂的活性, 但提高了催化剂H2选择性和降低了CO选择性. Au-Pd/ZnO-CeO_2催化剂的TPR表明, 在约200℃时开始有部分ZnO被还原, CO-IR显示CO吸收峰移向低频, 这些结果表明Au-Pd/ZnO-CeO_2催化剂中Pd和Zn之间发生了相互作用. Pd和Zn之间相互作用抑制了Pd的甲醇分解活性, 有利于H2和CO_2的生成, 使Au-Pd/ZnO-CeO_2催化剂表现出较高的H2选择性和较低的CO选择性.     

焙烧温度对甲醇水蒸气重整制氢Ce/Cu/Zn-Al水滑石衍生催化剂的影响

采用原位合成法在γ-Al_2O_3载体表面上合成了Zn-Al水滑石,再采用顺序浸渍法制备了一系列Ce/Cu/Zn-Al催化剂,并采用XRD、BET、H_2-TPR和XPS等手段对催化剂进行了表征,考察了焙烧温度对Ce/Cu/Zn-Al催化剂表面结构及其催化甲醇水蒸气重整制氢性能的影响。结果表明,焙烧温度主要影响了催化剂的Cu比表面积、表面氧空穴含量和Cu-Ce间相互作用。当焙烧温度为500℃时,催化剂Cu的比表面积较大,表面氧空穴含量较多,Cu-Ce间相互作用较强,因此,催化甲醇水蒸气重整制氢活性较好。当焙烧温度升高到700℃时,Cu物种主要以稳定的CuAl_2O_4尖晶石形式存在,不利于甲醇水蒸气重整制氢反应的进行,因此,催化活性较差。     

Controlling ZnO morphology for improved methanol steam reforming reactivity

The selectivity towards CO2 during steam reforming of methanol on Pd increases in the order Al2O3 < ZrO2 < ZnO. However, conventional catalyst preparation can damage the ZnO surface, even causing complete dissolution. The faceted, prismatic ZnO crystals in the support (Aldrich) get easily destroyed during catalyst preparation. We show in this work that, by using organic precursors, the faceted ZnO particles can be preserved. The role of ZnO morphology on reactivity for methanol steam reforming (MSR) is explored. Since the MSR reactivity and selectivity is also a function of the particle size of the nanoparticles as well as the presence of the PdZn ordered alloy phase, we have controlled for both these parameters to derive the true influence of the support. We find that the catalyst prepared from an organic precursor is more active than one prepared from acidic precursors, despite having similar particle size and extent of bulk PdZn ordered alloy formation. The results suggest that preserving certain ZnO surfaces is beneficial, and the ZnO support may play an important role in the overall reaction of methanol steam reforming.     

CH3O decomposition on PdZn(111), Pd(111), and Cu(111). A theoretical study

Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH(3)O) have been carried out, namely, dehydrogenation to formaldehyde and C-O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd-Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd-Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C-H scission of CH(3)O on various substrates under study was demonstrated to proceed easier than C-O bond breaking. The energy barrier for the dehydrogenation of CH(3)O on PdZn(111) (113 kJ mol(-)(1)) and Cu(111) (112 kJ mol(-)(1)) is about 4 times as high as that on Pd(111), due to the fact that CH(3)O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces.     

First-principles study towards the reactivity of the Pd(111) surface with low Zn deposition

Methanol steam reforming (MSR) is an important means to produce hydrogen. While metal Pd shows no selectivity to MSR, PdZn alloy exhibits both high selectivity and activity towards this process. Recently a high temperature desorption peak of formaldehyde is observed when methanol is dosed onto Pd(111) surfaces on which 0.03-0.06 monolayer Zn is deposited. Strikingly such surface which is predominated by Pd atoms was suspected to be active for MSR. To determine the structure on which the high desorption peak is observed and its performance to MSR, we studied adsorption and dehydrogenation of formaldehyde on various models. It is demonstrated that the high desorption peak of CH(2)O may originate from the supported surface clusters. The calculated energy barriers of CH(2)O dehydrogenation show that while formaldehyde can decompose easily into formyl on the supported PdZn and Pd(2) clusters, this process is kinetically difficult on the surface Zn(3) clusters. It is further revealed that formation of dioxymethylene, the proposed precursor for CO(2) production, from formaldehyde and oxygen is feasible on the surface Zn cluster. Based on these calculations we predict that compared with 1:1 PdZn alloy, the activity of the Zn clusters to MSR is lower, though its selectivity may be higher.     

Comparative theoretical study of formaldehyde decomposition on PdZn, Cu, and Pd surfaces

Methanol steam reforming, catalyzed by Pd/ZnO (PdZn alloy), is a potential source of hydrogen for on-board fuel cells. CO has been reported to be a minor side product of methanol decomposition that occurs in parallel to methanol steam reforming on PdZn catalysts. However, fuel cells currently used in vehicles are very sensitive to CO poisoning. To contribute to the understanding of pertinent reaction mechanisms, we employed density functional slab model calculations to study the decomposition of formaldehyde, a key intermediate in methanol decomposition and steam reforming reactions, on planar surfaces of Pd, Cu, and PdZn as well as on a stepped surface of PdZn. The calculated activation energies indicate that dehydrogenation of formaldehyde is favorable on Pd(111), but unfavorable on Cu(111) and PdZn(111). On the stepped PdZn(221) surface, the dehydrogenation process was calculated to be more competitive to formaldehyde desorption than on PdZn(111). Thus, we ascribe the experimentally observed small amount of CO, formed during steam reforming of methanol on the Pd/ZnO catalyst, to occur at metallic Pd species of the catalyst or at defect sites of PdZn alloy.     

Effect of steps on the decomposition of CH3O at PdZn alloy surfaces

The decomposition of methoxide (CH(3)O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH(3)O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH(3)O decomposition on PdZn(221) surfaces that expose Pd, (221)(Pd), and Zn, (221)(Zn), steps. C-H bond cleavage of CH(3)O is greatly facilitated on (221)(Pd): the calculated activation energy is dramatically reduced, to approximately 50 kJ mol(-1) from approximately 90 kJ mol(-1) on flat PdZn surfaces, increasing the rate constant by a factor of 10(8). The lower barrier is mainly due to a weaker interaction of the reactant CH(3)O and an enhanced interaction of the product CH(2)O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)(Pd) step. On the (221)(Zn) step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH(3)O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)(Pd) is a realistic model for studying the reactivity of this catalyst.     

无表面活性剂条件下一锅法制备金属/氧化锌复合材料用于催化二氧化碳加氢制甲醇反应

Catalytic hydrogenation of CO₂ to methanol is an important chemical process owing to its contribution in alleviating the impacts of the greenhouse effect and in realizing the requirement for renewable energy sources. Owing to their excellent synergic functionalities and unique optoelectronic as well as catalytic properties, transition metal/ZnO (M/ZnO) nanocomposites have been widely used as catalysts for this reaction in recent years. Development of size-controlled synthesis of metal/oxide complexes is highly desirable. Further, because it is extremely difficult to achieve the strong-metal-support-interaction (SMSI) effect when the M/ZnO nanocomposites are prepared via physical methods, the use of chemical methods is more favorable for the fabrication of multi-component catalysts. However, because of the requirement for an extra H₂ reduction step to obtain the active metallic phase (M) and surfactants to control the size of nanoparticles, most M/ZnO nanocomposites undergo two- or multi-step synthesis, which is disadvantageous for the stable catalytic performance of the M/ZnO nanocomposites. In this work, we demonstrate facile one-pot synthesis of M/ZnO (M = Pd, Au, Ag, and Cu) nanocomposites in refluxed ethylene glycol as a solvent, without using any surfactants. During the synthesis process, Pd and ZnO species can stabilize each other from further aggregation by reducing their individual surface energies, thereby achieving size control of particles. Besides, NaHCO₃ serves as a size-control tool for Pd nanoparticles by adjusting the alkaline conditions. Ethylene glycol serves as a mild reducing agent and solvent owing to its capacity to reduce Pd ions to generate Pd crystals. The nucleation and growth of Pd particles are achieved by thermal reduction, while the ZnO nanocrystals are formed by thermal decomposition of Zn(OAc)₂. X-ray diffraction patterns of the M/ZnO and ZnO were analyzed to study the phase of the nanocomposites, and the results show that no impurity phase was detected. Transmission electron microscopy (TEM) was used to study the morphology and structural properties. In addition, X-ray photoelectron spectroscopy analysis was performed to further confirm the formation of M/ZnO hybrid materials, and the results confirm SMSI between Pd and ZnO. Inductively coupled plasma mass spectrometry was used to check the actual elemental compositions, and the results show that the detected atomic ratios of Pd/Zn were consistent with the values in the theoretical recipe. To investigate the effects of the Pd/Zn molar ratios and the added amount of NaHCO₃ on Pd size, the average sizes of Pd particles were calculated, and the results were confirmed by TEM observation. The Cu/ZnO/Al₂O₃ composite is a widely known catalyst for hydrogenation of CO₂ to methanol, and other M/ZnO composites are also catalytic for this reaction. Therefore, different M/ZnO hybrids were further studied as catalysts for hydrogenation of CO₂ to methanol, among which Pd/ZnO (1 : 9) demonstrated the best performance (30% CO₂ conversion, 69% methanol selectivity, and 421.9 g_methanol·(kg catalyst·h)^-1 at 240 ℃ and 5 MPa. The outstanding catalytic performance may be explained by the following two factors: first, Pd is a good catalyst for the dissociation of H₂ to give active H atoms, and second, SMSI between Pd and ZnO favors the formation of surface oxygen vacancies on ZnO. Moreover, most M/ZnO composites exhibit excellent performance in methanol selectivity, especially the Au/ZnO catalyst, which has the highest methanol selectivity (82%) despite having the lowest CO₂ conversion. Hopefully, this work would provide a simple route for synthesis of M/ZnO nanocomposites with clean surfaces for catalysis.     

Zn adsorption on Pd(111): ZnO and PdZn alloy formation

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.     

Zn modification of the reactivity of Pd(111) toward methanol and formaldehyde

The adsorption and reaction of methanol and formaldehyde on two-dimensional PdZn alloys on a Pd(111) surface were studied as a function of the Zn content in the alloy in order to understand the role of Zn in Pd/ZnO catalysts for the steam reforming of methanol (SRM). Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) data show that Zn atoms incorporated into the Pd(111) surface dramatically decrease the dehydrogenation activity and alter the preferred bonding sites for adsorbed CO, CH3O, and CH2O intermediates. The experimental results obtained in this study are consistent with previous theoretical studies of this system and provide new insight into how Zn alters the reactivity of Pd.     

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

Palladium monolayer and palladium alloy electrocatalysts for oxygen reduction

We investigated the oxygen-reduction reaction (ORR) on Pd monolayers on various surfaces and on Pd alloys to obtain a substitute for Pt and to elucidate the origin of their activity. The activity of Pd monolayers supported on Ru(0001), Rh(111), Ir(111), Pt(111), and Au(111) increased in the following order: Pd/Ru(0001) < Pd/Ir(111) < Pd/Rh(111) < Pd/Au(111) < Pd/Pt(111). Their activity was correlated with their d-band centers, which were calculated using density functional theory (DFT). We found a volcano-type dependence of activity on the energy of the d-band center of Pd monolayers, with Pd/Pt(111) at the top of the curve. The activity of the non-Pt Pd2Co/C alloy electrocatalyst nanoparticles that we synthesized was comparable to that of commercial Pt-containing catalysts. The kinetics of the ORR on this electrocatalyst predominantly involves a four-electron step reduction with the first electron transfer being the rate-determining step. The downshift of the d-band center of the Pd "skin", which constitutes the alloy surface due to the strong surface segregation of Pd at elevated temperatures, determined its high ORR activity. Additionally, it showed very high methanol tolerance, retaining very high catalytic activity for the ORR at high concentrations of methanol. Provided its stability is satisfactory, this catalyst might possibly replace Pt in fuel-cell cathodes, especially those of direct methanol oxidation fuel cells (DMFCs).