Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro‐N‐ethylcarbazole on Pd(111)

Dodecahydro‐N‐ethylcarbazole (H₁₂‐NEC) has been proposed as a potential liquid organic hydrogen carrier (LOHC) for chemical energy storage, as it combines both favourable physicochemical and thermodynamic properties. The design of optimised dehydrogenation catalysts for LOHC technology requires a detailed understanding of the reaction pathways and the microkinetics. Here, we investigate the dehydrogenation mechanism of H₁₂‐NEC on Pd(111) by using a surface‐science approach under ultrahigh vacuum conditions. By combining infrared reflection–absorption spectroscopy, density functional theory calculations and X‐ray photoelectron spectroscopy, surface intermediates and their stability are identified. We show that H₁₂‐NEC adsorbs molecularly up to 173 K. Above this temperature (223 K), activation of C? H bonds is observed within the five‐membered ring. Rapid dehydrogenation occurs to octahydro‐N‐ethylcarbazole (H₈‐NEC), which is identified as a stable surface intermediate at 223 K. Above 273 K, further dehydrogenation of H₈‐NEC proceeds within the six‐membered rings. Starting from clean Pd(111), C? N bond scission, an undesired side reaction, is observed above 350 K. By complementing surface spectroscopy, we present a temperature‐programmed molecular beam experiment, which permits direct observation of dehydrogenation products in the gas phase during continuous dosing of the LOHC. We identify H₈‐NEC as the main product desorbing from Pd(111). The onset temperature for H₈‐NEC desorption is 330 K, the maximum reaction rate is reached around 550 K. The fact that preferential desorption of H₈‐NEC is observed even above the temperature threshold for H₈‐NEC dehydrogenation on the clean surface is attributed to the presence of surface dehydrogenation and decomposition products during continuous reactant exposure.     

UHV Studies of Methanol Decomposition on Mono‐ and Bimetallic CoPd Nanoparticles Supported on Thin Alumina Films

Bimetallic nanoparticles often turn out to be superior to the corresponding monometallic systems with respect to their catalytic properties. To study such effects for the methanol decomposition reaction, model catalysts were prepared by physical vapor deposition of Pd and Co under ultrahigh‐vacuum (UHV) conditions. Monometallic Pd and Co particles as well as CoPd core–shell particles were generated on an epitaxial alumina film grown on NiAl(110). The interaction with methanol is examined by temperature‐programmed desorption of methanol and carbon monoxide and by X‐ray photoelectron spectroscopy. The decomposition of methanol proceeds in two reaction pathways independent of the particle composition: complete dehydrogenation towards carbon monoxide and hydrogen, and C? O bond scission yielding carbon deposits. Pd is the most active material studied here. The relative importance of the two channels varies for the different particle systems: on Pd dehydrogenation is preferred, whereas the C? O bond cleavage is more pronounced on Co. The bimetallic clusters show a moderate performance for both pathways. Carbon deposition poisons the model catalysts by blocking the adsorption sites for methoxide, which is the first intermediate product during methanol decomposition. In particular on Co, large amounts of carbon deposits can also be caused by dissociation of the final product of the dehydrogenation pathway, carbon monoxide. A comparison with the results of methanol decomposition on Co, Pd, and CoPd catalysts in continuous‐flow reactors demonstrates that the findings of the present UHV study are relevant for catalytic performance under high‐pressure conditions.     

Pd/SWNTs负载型催化剂的制备及其催化性能

利用单壁碳纳米管(SWNTs)自身的还原性, 将PdCl2溶液中的Pd2+直接还原成金属Pd负载在SWNTs表面上, 制备了具有良好催化性能的Pd/SWNTs负载型催化剂. 通过透射电镜(TEM)、X射线衍射(XRD)、X射线光电子能谱(XPS)和热重分析(TG)对Pd/SWNTs 进行了表征, 并利用Suzuki反应对Pd/SWNTs的催化性能进行了测试. 实验结果表明, 用SWNTs与12 mmol·L-1的PdCl2的水溶液直接作用, 得到Pd/SWNTs催化材料的Pd负载量达到14.13%(w, 质量分数), 颗粒分散均匀, 粒径小(2 nm左右), 与SWNTs结合紧密; 用经过H2还原的Pd/SWNTs作催化剂, 在90 ℃下进行Suzuki反应, 30 min后反应就基本完成, 其联苯的产率达到98.10%, 催化活性较高, 可望广泛用于有机合成反应.     

Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism

Using scanning tunneling microscopy (STM), molecular‐beam (MB) methods and time‐resolved infrared reflection absorption spectroscopy (TR‐IRAS), we investigate the mechanism of initial NO_x uptake on a model nitrogen storage and reduction (NSR) catalyst. The model system is prepared by co‐deposition of Pd metal particles and Ba‐containing oxide particles onto an ordered alumina film on NiAl(110). We show that the metal–oxide interaction between the active noble metal particles and the NO_x storage compound in NSR model catalysts plays an important role in the reaction mechanism. We suggest that strong interaction facilitates reverse spillover of activated oxygen species from the NO_x storage compound to the metal. This process leads to partial oxidation of the metal nanoparticles and simultaneous stabilization of the surface nitrite intermediate.     

水滑石负载钯催化剂上的醇无氧脱氢反应

 研究了多种载体负载 Pd 催化剂上苯甲醇无氧脱氢反应. 结果发现, 以兼具较强酸性和碱性的水滑石 (HT) 为载体时, Pd 催化剂具有优异的苯甲醇转化活性和苯甲醛选择性, 当 Pd 含量为 0.32%~0.55% 时催化性能最佳. Pd/HT 催化剂可重复使用, 且对于含推电子取代基的芳香醇、2-噻吩甲醇、α,β-不饱和醇与环状脂肪醇等的直接脱氢反应均具有较好催化性能. HT 表面的 Pd(II) 物种反应后转变为平均粒径为 2.0~2.5 nm 的 Pd 纳米粒子或纳米簇. 具有较高分散度的 Pd(II) 物种易转变为较小的 Pd 纳米粒子, 从而具有较佳的催化性能. 本文推测, 催化剂表面的碱性位可促进苯甲醇 O–H 键的活化, 形成 Pd-苯甲氧基中间体, 该中间体进一步脱氢生成苯甲醛和 Pd-H 物种; 而催化剂表面的质子酸位可与 Pd-H 作用, 促进 H2 的脱除.     

制备方法对均相多相化钯催化剂活性物种的影响

近年来,多相化均相催化剂的研究,在有机合成、石油化工、精细化工等领域引起了广泛关注.和多相催化剂一样,负载化均相催化剂的性能也与制备技术有较大的关系.如在研究氯化钯、聚乙烯吡咯烷酮(PVP)和氧化铝所组成的双重负载催化剂时,虽然所用组分一样,比例相同,但由于负载顺序不同,所得催化剂的活性和重复使用性相差很大.国内外学者曾对类似催化剂的表征进行过讨论,认为催化剂的活性中心是在胶体保护的金属钯颗粒上.对于负载顺序的效应,则未见报道.我们以CO为探针     

金掺杂对碳负载钯催化还原性能的促进作用

在许多催化应用中双金属的PdAu催化剂性能优于单金属催化剂.科研人员对具有可控纳米结构和高活性的PdAu催化剂进行了广泛的研究,但该催化剂的制备需要多步且通常步骤复杂.本文仅通过浸渍和焙烧制得了Au掺杂的负载型Pd催化剂,所得PdAu/C催化剂用于室温水相三氯乙烯加氢脱氯反应.当Pd和Au负载量分别为1.0 wt%和1.1 wt%时,在经过干燥、空气处理和H2还原的过程后，所制得的PdAu/C催化剂活性最高,初始转化频率(TOF)为34.0×10–2 molTCEmolPd–1 s–1,是单金属1.0 wt%Pd/C催化剂TOF (2.2×10–2 molTCEmolPd–1 s–1)的15倍以上. X射线吸收光谱结果表明,金的加入避免了400oC焙烧时Pd的氧化.本文还提出了可能的催化剂纳米结构演变路径,以解释所观察到的催化现象.     

The state of the iron promoter in tungstated zirconia catalysts.

The activity and selectivity of tungstated zirconia (WZ) for the conversion of n- into isopentane are dramatically enhanced when the catalyst is modified with Pt and Fe. The state of iron in these catalysts was hitherto only poorly characterized. Therefore, in the present work we investigated the structural and electronic properties of iron in WZ catalysts containing 1 wt% Pt and 1 wt% Fe2O3, by a combination of spectroscopic techniques, namely X-ray absorption spectroscopy (XAS), in situ electron paramagnetic resonance (EPR), and M?ssbauer spectroscopy. In the oxidized catalyst, iron is present as Fe(III) and predominantly forms a surface solid solution in which the isolated Fe(III) ions are located in a distorted octahedral environment. A small amount of the total iron (around 10%) is present in the form of small iron oxide particles. Both iron species can be reduced in H2 and then easily reoxidized on exposure to air at room temperature. We infer that the promoter action of iron in these catalysts is intimately related to its redox properties and specifically affects the dehydrogenation activity of the materials.     

Selective Hydrogenation of Acrolein Over Pd Model Catalysts: Temperature and Particle‐Size Effects

The selectivity in the hydrogenation of acrolein over Fe₃O₄‐supported Pd nanoparticles has been investigated as a function of nanoparticle size in the 220–270 K temperature range. While Pd(111) shows nearly 100 % selectivity towards the desired hydrogenation of the C=O bond to produce propenol, Pd nanoparticles were found to be much less selective towards this product. In situ detection of surface species by using IR‐reflection absorption spectroscopy shows that the selectivity towards propenol critically depends on the formation of an oxopropyl spectator species. While an overlayer of oxopropyl species is effectively formed on Pd(111) turning the surface highly selective for propenol formation, this process is strongly hindered on Pd nanoparticles by acrolein decomposition resulting in CO formation. We show that the extent of acrolein decomposition can be tuned by varying the particle size and the reaction temperature. As a result, significant production of propenol is observed over 12 nm Pd nanoparticles at 250 K, while smaller (4 and 7 nm) nanoparticles did not produce propenol at any of the temperatures investigated. The possible origin of particle‐size dependence of propenol formation is discussed. This work demonstrates that the selectivity in the hydrogenation of acrolein is controlled by the relative rates of acrolein partial hydrogenation to oxopropyl surface species and of acrolein decomposition, which has significant implications for rational catalyst design.     

Ni的引入对Pd/Al2O3催化甲烷燃烧性能的影响

采用浸渍法制备了Ni掺杂的Pd/Al2O3催化剂,考察了其低浓度甲烷催化燃烧活性和水热稳定性.结果表明,随着Ni的引入及其含量的增加,Pd/Al2O3催化剂性能明显提高,Ni含量至20%时,在0.4%CH4,4%H2O和空气平衡的原料气组成,80000h-1空速和600oC条件下反应150h后,CH4转化率仍能保持在97.5%以上.X射线衍射、H2程序升温还原、NH3程序升温脱附和透射电镜等结果表明,NiAl2O4晶相的形成改善了载体酸性和活性组分的分散度,从而提高了催化剂性能.     

Hydrazine decomposition over Irn/Al2O3 model catalysts prepared by size-selected cluster deposition

Hydrazine decomposition chemistry was probed over a temperature range from 100 to 800 K for a series of model catalysts prepared by mass-selected Ir(n)(+) deposition on planar Al(2)O(3)/NiAl(110). Two sets of experiments are reported. Temperature-programmed desorption (TPD) was used to study hydrazine desorption and decomposition on Al(2)O(3)/NiAl(110) and on a model catalyst prepared by deposition of Ir(+) on Al(2)O(3)/NiAl(110) at a density large enough (5 x 10(14) cm(-2)) that formation of a distribution of small Ir(n) clusters on the surface is expected. This model catalyst was found to have hydrazine decomposition properties qualitatively similar to those observed on single-crystal Ir and polycrystalline Rh. This catalyst was also studied by X-ray photoelectron spectroscopy (XPS), to probe TPD-induced changes in the samples. A substantial decrease in the Ir XPS intensity suggests that considerable sintering takes place when the samples are heated to 800 K. In addition, a significant fraction of the nitrogen contained in the hydrazine is converted to an aluminum nitride (or mixed Al(x)O(y)N(z)) compound. Continuous flow experiments were used to probe relative reactivity at 300 and 400 K of samples prepared by depositing differently sized Ir(n)(+) clusters. At 300 K, samples prepared with preformed Ir(n)(+) (n = 5, 7, 10) are about twice as active, per Ir atom, as samples prepared with Ir(+) deposition, and there is a weaker trend to higher activity with increasing cluster size. At 400 K the trends are similar, but weaker, suggesting that thermal modification of the samples is already significant.     

Effect of Hydrohalogenation of Metal/Zeolite Catalysts for Cyclohexene Hydroconversion ‐Part 3‐ Pd/H‐ZSM‐5 Catalysts

Cyclohexene (CHE) hydroconversion was performed in a flow reactor at atmospheric pressure and temperatures of 50–400 °C using: Pd/H‐ZSM‐5, Pd/H‐ZSM‐5(HCl), and Pd/H‐ZSM‐5(HF) catalysts. These catalysts were characterized for acid site strength distribution via NH₃ TPD, Pd dispersion via H₂ chemisorption, TPR via reduction of the metal oxide in the catalysts and XRD for tracing crystallinity The hydroconversion steps proceeded as follows: CHE → Cyclohexane (CHA); CHE → Methylcyclopentenes (MCPEs) → Methylcyclopentane (MCPA); CHE → Cyclohexadienes (CHDEs) → Benzene → Alkylbenzenes; CHE and others → Hydrocrackedproducts. The overall hydroconversion of CHE was achieved in the catalyst order: Pd/H‐ZSM‐5 > Pd/H‐ZSM‐5(HF) > Pd/H‐ZSM‐5(HCl). CHE hydrogenation step was the major reaction at low temperatures which significantly inhibited via HCl treatment, but slightly enhanced via HF treatment. At medium temperatures, on all catalysts, isomerisation to MCPEs and MCPA increase to a maximum then a decline with a further increase of temperature. The overall isomerisation of CHE was highest on the untreated catalyst. During the higher temperature range, dehydrogenation, alkylation and hydrocracking were increased with temperature. Dehydrogenation of CHE always yielded larger amounts of 1,3‐CHDE than 1,4‐CHDE. These cyclohexadienes were produced in the catalyst order: Pd/H‐ZSM‐5(HF) > Pd/H‐ZSM‐5(HCl) > Pd/H‐ZSM‐5. In general, benzene alkylation to toluene exceeded that of xylenes, indicating that the second methylation is more difficult than the first. However, the catalytic activities for benzene and toluene production were in the order: Pd/H‐ZSM‐5 » Pd/H‐ZSM‐5(HCl) > Pd/H‐ZSM‐5(HF), whereas for xylenes production, Pd/H‐ZSM‐5 » Pd/H‐ZSM‐5(HF) > Pd/H‐ZSM‐5(HCl). Intrapore diffusion plays an important role during the dehydrogenation reactions as well as during the interconversion of individual aromatic hydrocarbons.     

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state‐of‐the‐art DFT methods and by a combination of supersonic molecular beams with high‐resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH₃ and CH₂ are specially stabilized at sites located at particles edges and corners by an amount of 50–80 kJ mol^?1. This stabilization is caused by an enhanced activity of low‐coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH₃ and CH₂ formation steps are endothermic on Pt(111) but markedly exothermic on Pt₇₉. An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol^?1 for first C? H bond scission and 40 kJ mol^?1 for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO₂ film on Cu(111). It is shown that CH₃ generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single‐crystal surfaces in which CH₃ was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.     

Palladium supported on structured and nonstructured carbon: a consideration of Pd particle size and the nature of reactive hydrogen

This study sets out a comprehensive characterization of bulk Pd and Pd (ca. 8% w/w) supported on activated carbon (AC), graphite and graphitic nanofibers (GNF). Catalyst activation has been examined by temperature programmed reduction (TPR) analysis and the activated catalysts analyzed in terms of BET area, TEM, H2 chemisorption/TPD, and XRD measurements. While H2 chemisorption and TEM delivered the same sequence of increasing (surface area weighted) average Pd particle sizes, a significant difference (by up to a factor of 3) in the values obtained from both techniques has been recorded and is attributed to an unwarranted (but widely adopted) assumption of an exclusive H2/Pd adsorption stoichiometry=1/2. It is demonstrated that TEM analysis provides a valid mean particle size once it is established that the associated standard deviation is small and insensitive to additional particle counting. XRD line broadening yielded an essentially equivalent Pd size (20-25 nm) for each supported catalyst. The nature of the hydrogen associated with the supported catalysts has been probed and is shown to comprise of chemisorbed (on Pd), spillover (on the carbon support), and hydride (associated with Pd) species. Physical mixtures of bulk Pd + support (AC, graphite, and GNF) were also considered in order to assess hydrogen spillover by H2 TPD analysis. Generation of spillover hydrogen at room temperature is established where temperatures in excess of 740 K are required for effective desorption from the supported Pd catalysts, i.e., 280 K higher than that required for the desorption of chemisorbed hydrogen. Pd hydride formation (at room temperature) is shown to be reversible with decomposition occurring at ca. 380 K. Taking the hydrodechlorination of chlorobenzene as a test reaction, the capability of Pd hydride to promote a hydrogen scission of C-Cl in the absence of an external supply of H2 is demonstrated with a consequent consumption of the hydride. This catalytic response was entirely recoverable once the Pd hydride was replenished during a subsequent reactivation step.     

Evidence for Pdx(CO)y compound formation on an alumina substrate

Although stable binary Pd carbonyls are unknown in the gas phase, we found strong evidence for a stable carbonyl-like Pd compound on an oxide surface: by in situ vapour deposition of Pd at a rate of 2 × 10¹³ atoms s⁻¹ cm⁻² onto an alumina substrate (90 K) at a pressure of 2 × 10⁻⁶ mbar CO, a binary compound of Pd and CO is formed which is stable up to 190 K. As substrate serves a well-ordered aluminium oxide film grown on a NiAl(110) single crystal surface. The system was characterized under UHV (ultrahigh vacuum) conditions by means of TDS, LEED, UPS and XPS in a coverage range between 1.4 × 10¹⁴ Pd atoms cm⁻² and 1.4 × 10¹⁶ Pd atoms cm⁻². The decomposition at 190 K results in the formation of metallic Pd particles and is accompanied by a sharp and dominant feature in the thermal desorption spectra.     

The mechanism of the hydroalkoxycarbonylation of ethene and alkene-CO copolymerization catalyzed by Pd(II)-diphosphine cations

All the intermediates in the "carboalkoxy" pathway, and their interconversions giving complete catalytic cycles, for palladium-diphosphine-catalyzed hydroalkoxycarbonylation of alkenes, and for alkene-CO copolymerization, have been demonstrated using (31)P{(1)H} and (13)C{(1)H} NMR spectroscopy. The propagation and termination steps of the "hydride" cycles and the crossover between the hydride and carboalkoxy cycles have also been demonstrated, providing the first examples of both cycles, and of chain crossover, being delineated for the same catalyst. Comparison of the propagation and termination steps in the pathways affords new insight into the selectivity-determining steps. Thus, reaction of [Pd(dibpp)(CH(3)CN)(2)](OTf)(2) (dibpp = 1,3-(iBu(2)P)(2)C(3)H(6)) with Et(3)N and CH(3)OH affords [Pd(dibpp)(OCH(3))(CH(3)CN)]OTf, which, on exposure to CO, gives [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf immediately. Labeling studies show the reaction to be readily reversible. However, the back reaction is strongly inhibited by PPh(3), indicating an insertion/deinsertion pathway. Ethene reacts with [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf at 243 K to give [Pd(dibpp){CH(2)CH(2)C(O)OCH(3)}]OTf, that is, there is no intrinsic barrier to alkene insertion into the Pd--C(O)OMe bond, as had been proposed. Instead, termination is proposed to be selectivity determining. Methanolysis of the acyl intermediate [Pd(dibpp){C(O)CH(3)}L]X (L = CO, CH(3)OH; X = CF(3)SO(3) (-) (OTf(-)), CH(3)C(6)H(4)SO(3) (-) (OTs(-))) is required in the hydride cycle to give an ester and occurs at 243 K on the timescale of minutes, whereas methanolysis of the beta chelate, required to give an ester from the carbomethoxy cycle, is slow on a timescale of days, at 298 K. These results suggest that slow methanolysis of the beta chelate, rather than slow insertion of an alkene into the Pd--carboalkoxy bond, as had previously been proposed, is responsible for the dominance of the hydride mechanism in hydroalkoxycarbonylation.     

纳米Cr2O3的制备、表征及催化性能

首次采用溶胶-凝胶法与共沸蒸馏法耦合技术制备了纳米Cr2O3粉体，并运用BET、TEM、XRD、FT-IR、XPS及H2-TPR对其进行表征，同时采用CO2氧化乙烷脱氢制乙烯反应作为探针反应，考察了纳米Cr2O3的催化性能。首次发现纳米ErgO3的FT-IR谱出现了蓝移现象，并且630cm^-1附近的伸缩振动峰强度增强。初步探讨了纳米氧化物的IR蓝移和红移的原因，指出晶型是影响纳米氧化物红外光谱特征的重要因素。实验结果表明纳米Cr2O3上乙烷和CO2转化率均明显高于常规Cr2O3催化剂；在700℃下，乙烷转化率高达77．1％，而乙烯产率达到了58．98％。     

In situ Raman and in situ XRD analysis of PdO reduction and Pd° oxidation supported on γ-Al2O3 catalyst under different atmospheres

Reduction of Pd° and decomposition of palladium oxide supported on γ-alumina were studied at atmospheric pressure under different atmospheres (H(2), CH(4), He) over a 4 wt% Pd/Al(2)O(3) catalyst (mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During temperature programmed tests (reduction, decomposition and oxidation) the crystal domain behaviour of the PdO/Pd° phase was evaluated by in situ Raman spectroscopy and in situ XRD analysis. Under H(2)/N(2), the reduction of small PdO particles (<5 nm) occurs at room temperature, whereas reduction of larger particles (>5 nm) starts at 100 °C and is achieved at 150 °C. Subsequent oxidation in O(2)/N(2) leads to reoxidation of small crystal domain at ambient temperature while oxidation of large particles starts at 300 °C. Under CH(4)/N(2), the small particle reduction occurs between 240 and 250 °C while large particle reduction is fast and occurs between 280 and 290 °C. Subsequent reoxidation of the catalyst reduced in CH(4)/N(2) shows that small and large particle oxidation of Pd° starts also at 300 °C. Under He, no small particle decomposition is observed probably due to strong interactions between particles and support whereas large particle reduction occurs between 700 and 750 °C. After thermal decomposition under He, the oxidation starts at 300 °C. Thus, the reduction phenomenon (small and large crystal domain) depends on the nature of the reducing agent (H(2), CH(4), He). However, whatever the reduction or decomposition treatment or the crystal domain, Pd° oxidation starts at 300 °C and is completed only at temperatures higher than 550 °C. Under lean conditions, with or without water, the palladium consists of reduced sites of palladium (Pd°, Pd(δ+) with δ < 2 or PdO(x) with x < 1) randomly distributed on palladium particles.     

In situ electron microscopy studies of the sintering of palladium nanoparticles on alumina during catalyst regeneration processes.

Sintering of a palladium catalyst supported on alumina (Al2O3) in an oxidizing environment was studied by in situ transmission electron microscopy (TEM). In the case of a fresh catalyst, sintering of Pd particles on an alumina surface in a 500 mTorr steam environment happened via traditional ripening or migration and coalescence mechanisms and was not significant unless heating above 500 degrees C. After the catalyst was used for the hydrogenation of alkynes, TEM coupled with convergent beam electron diffraction and electron energy loss spectroscopy analysis revealed that most of the Pd particles were lifted from the alumina surface by hydrocarbon buildup. This dramatically different morphology totally changed the sintering mechanism of Pd particles during the regeneration process. Catalytic gasification of hydrocarbon around these particles in an oxidizing environment allowed the Pd particles to move around and coalesce with each other at temperatures as low as 350 degrees C. For catalysts heating under 500 mTorr steam at 350 degrees C, steam stripped hydrocarbon catalytically at the beginning, but the reaction stopped after 4 h. Heating in air resulted in both catalytic and noncatalytic stripping of hydrocarbon.     

Vacuum Surface Science Meets Heterogeneous Catalysis: Dehydrogenation of a Liquid Organic Hydrogen Carrier in the Liquid State

Ultrahigh vacuum (UHV) surface science techniques are used to study the heterogeneous catalytic dehydrogenation of a liquid organic hydrogen carrier in its liquid state close to the conditions of real catalysis. For this purpose, perhydrocarbazole (PH), otherwise volatile under UHV, is covalently linked as functional group to an imidazolium cation, forming a non‐volatile ionic liquid (IL). The catalysed dehydrogenation of the PH unit as a function of temperature is investigated for a Pt foil covered by a macroscopically thick PH‐IL film and for Pd particles suspended in the PH‐IL film, and for PH‐IL on Au as inert support. X‐ray photoelectron spectroscopy and thermal desorption spectroscopy allows us to follow in situ the catalysed transition of perhydrocarbazole to carbazole at technical reaction temperatures. The data demonstrate the crucial role of the Pt and Pd catalysts in order to shift the dehydrogenation temperature below the critical temperature of thermal decomposition.