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.     

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.     

Microscopic models of PdZn alloy catalysts: structure and reactivity in methanol decomposition

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.     

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.     

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.     

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.     

Surface composition of materials used as catalysts for methanol steam reforming: a theoretical study.

PdZn (1:1) alloy is assumed to be the active component of a promising catalyst for methanol steam reforming. Using density functional calculations on periodic supercell slab models, followed by atomistic thermodynamics modeling, we study the chemical composition of the surfaces PdZn(111) and, as a reference, Cu(111) in contact with water and hydrogen at conditions relevant to methanol steam reforming. For the two surfaces, we determine similar maximum adsorption energies for the dissociative adsorption of H(2), O(2), and the molecular adsorption of H(2)O. These reactions are calculated to be exothermic by about -40, -320, and -20 kJ mol(-1), respectively. Using a thermodynamic analysis based on theoretically predicted adsorption energies and vibrational frequencies, we determine the most favorable surface compositions for given pressure windows. However, surface energy plots alone cannot provide quantitative information on individual coverages in a system of coupled adsorption reactions. To overcome this limitation, we employ a kinetic model, from which equilibrium surface coverages of H, O, OH, and H(2)O are derived. We also discuss the sensitivity of our results and the ensuing conclusions with regard to the model surfaces employed and the inaccuracies of our computational method. Our kinetic model predicts surfaces of both materials, PdZn and Cu, to be essentially adsorbate-free already from very low values of the partial pressure of H(2). The model surfaces PdZn(111) and Cu(111) are predicted to be free of water-related adsorbates for a partial H(2) pressure greater than 10(-8) and 10(-5) atm, respectively.     

Pathways for methanol steam reforming involving adsorbed formaldehyde and hydroxyl intermediates on Cu(111): density functional theory studies

Plane-wave density functional theory calculations have been carried out to explore possible pathways in methanol steam reforming (MSR) on Cu(111). We focus on reactions involving the adsorbed formaldehyde intermediate (CH(2)O) produced by methanol decomposition and the surface hydroxyl (OH) species generated by dissociative adsorption of H(2)O. Several possible pathways leading to the H(2) + CO(2) products have been identified. The two most likely pathways involve the formate (CHOO), rather than the carboxyl (COOH), intermediate, and they possess barriers lower than that of the rate-limiting step of MSR, namely the dehydrogenation of adsorbed methoxyl (CH(3)O) species.     

Steam reforming of methanol over Ni, Co, Pd and Pt supported on ZnO

Pd/ZnO and Pt/ZnO exhibited anomalously high selectivity for the steam reforming of methanol, when PdZn and PtZn alloys phases were developed. The selectivity decreased over the catalysts having metal phases.     

Pd/ZnO催化甲醇水蒸气重整理论研究

随着化石能源的日渐枯竭和人们对环境保护的日益重视,发展清洁高效的新能源成为世界各国高度关注的战略课题。甲醇水蒸气重整是生产氢能的有效方法之一,Pd/ZnO催化剂热稳定性好、选择性高,是可能替代Cu/ZnO的催化剂。本文综述了近十年来采用理论方法对Pd/ZnO催化甲醇水蒸气重整制氢机理的研究工作。文章首先论述了催化剂的研究进展,然后对水在单体和聚集状态下在单层及多层平整的和阶梯状的合金表面的吸附和解离进行了总结;接着对甲醇、甲氧基和甲醛在合金表面的吸附和化学反应的热力学和动力学作了介绍;随后基于计算结果,对甲醇反应机理给予了详细的描述。最后对全文进行了总结并对未来的研究作了展望。     

Structured Catalysts Prepared by Electroless Plating Technique onto a Metal Substrate, for a Wall-Type Hydrogen Production System

This article reviews our works on the structured catalysts for a wall-type hydrogen production system including methanol steam reforming (MSR), CO shift reaction (CO SR) and methanol decomposition (MD). The structured catalysts were copper-based, palladium-based and nickel-based catalysts. Such a series of structured catalysts were prepared by the electroless plating technique that is a novel method for preparing a structured type catalyst onto a metal-substrate. The copper-based catalyst exhibited high performance for MSR and CO SR, the palladium-based catalyst high for MSR, and the nickel-based catalyst high for MD. The catalytic properties of these catalysts were affected by the difference of the plating condition and the pretreatment condition prior to the reaction. In the copper-based catalyst, the reforming and shift activities were enhanced by the oxidation treatment. One of the factors of such activity enhancement by the oxidation was thought to be in close proximity existence of copper and zinc atoms. A lot of monodentate-type formate species having high reactivity was formed on the oxidized catalyst, which would be correlated to the activity enhancement. In the palladium-based catalyst, the reforming activity was improved by the continuous reduction treatment followed by the oxidation. Such continuous pretreatment formed the PdZn alloy species thought to be a reforming site in the surface layer. The decomposition performance of the nickel-based catalyst depended on the ratio of the crystallite size of nickel particles to that of aluminum particles. The electronic influence of zinc and phosphorous components incorporated in the plated layer contributed to the improvement of the selectivity of product.     

Dehydrogenation of methanol to methyl formate over supported Ni, Pd and Pt catalysts. Anomalous catalytic functions of PdZn and PtZn alloys

Pd/ZnO and Pt/ZnO exhibited high catalytic performance for the dehydrogenation of methanol to methyl formate upon the formation of PdZn and PtZn alloys.     

Zinc coverage dependent structure of PdZn surface alloy

Catalytic performances of alloy and surface alloy are sensitive to the surface structures and composition. In this paper we present an overall survey of the surface structure of Pd(111) covered with different amount of Zn using Monte Carlo simulations. We demonstrate that the composition of PdZn surface alloy is Zn coverage dependent: the surface concentration of Zn increases with the increase of the deposited Zn. At one or multi-layer of zinc deposited Pd(111), a multilayer 1?:?1 PdZn surface alloy will be formed. Surface alloy islands dominated by palladium are formed at submonolayer Zn coverage. At very low zinc coverage, small palladium ensembles of 3 to 5 Pd atoms exist preferentially on the Pd(111) surface. Our simulated results which are consistent with the pertinent experiments indicate that the unusual high-temperature desorption peak of formaldehyde observed experimentally has likely originated from the small surface ensembles induced by deposited Zn.     

CuO-ZnO-ZrO_2催化甲醇水蒸汽重整反应机理和中间态

应用质谱在线技术,对CuO-ZnO-ZrO2催化甲醇水蒸汽重整(SRM)反应进行程序升温脱附(TPD)和程序升温表面反应(TPSR)研究.结果表明:在反应态催化剂表面,甲醇以分子吸附态形式存在,甲醇水蒸汽重整反应经历甲酸根中间物种.分别用CuO、CuO-ZnO、CuO-ZnO-ZrO2作催化剂,甲醇在气流中的摩尔分数分别高于5.4%、0.37%和0.17%时,甲酸根中间态的分解产物为CO2和H2;而甲醇在气流中的摩尔分数分别低于5.4%、0.37%和0.17%时,甲酸根中间态的分解产物为CO、CO2和H2.     

甲醇水蒸气重整制氢Cu/ZnO/Al2O3催化剂的研究

燃料电池作为一种无污染、高效率的能源引起世界各大汽车公司的广泛关注[1,2]。用于燃料电池的燃料目前研究较多的是氢气,用氢气作燃料存在储存、安全、运输等问题,寻求合适贮氢方法或替代燃料,实现车载制氢是解决问题的办法。甲醇作为液体燃料,因具有高能量密度,低碳含量,以及运输和贮存等优势成为车载制氢的理想燃料,甲醇水蒸气重整制氢反应也成为研究的热点[3～10]。车载制氢对甲醇水蒸气重整制氢反应体系中的产氢速率,氢气和CO的含量都有一定的要求。尤其对CO含量要求更为苛刻,因CO易引起燃料电池阳极催化剂中毒[11,12]。因此,开…     

A MEMS methanol reformer heated by decomposition of hydrogen peroxide

This paper presents the design, fabrication and evaluation of a micro methanol reformer complete with a heat source. The micro system consists of the steam reforming reactor of methanol, the catalytic decomposition reactor of hydrogen peroxide, and a heat exchanger between the two reactors. In the present study, catalytic decomposition of hydrogen peroxide is used as a process to supply heat to the reforming reactor. The decomposition process of hydrogen peroxide produces water vapor and oxygen as a product that can be used efficiently to operate the reformer/PEMFC system. Cu/ZnO was selected as a catalyst for methanol steam reforming and Pt for the decomposition of hydrogen peroxide. Incipient wetness method was used to load catalysts on a porous support. Catalyst loaded supports were inserted in the cavity made on the glass wafer. The performance of the methanol steam reforming system was measured at various test conditions and the optimum operation condition was sought. At the optimum condition, the hydrogen selectivity was 86.4% and the thermal efficiency was 44.8%. The product gas included 74.1% H(2), 24.5% CO(2) and 1.4% CO and the total volume production rate was 23.5 ml min(-1). This amount of hydrogen can produce 1.5 W of power on a typical PEMFC.     

Steam Reforming of Methanol Over Pd-Zn Catalysts

Heating the physical mixtures of Pd and Zn selectively yielded PdZn or Pd_3.9Zn_6.1 alloy. These alloys were highly selective for the steam reforming of methanol.     

ZnO纳米线负载Pd1单原子催化剂催化若干反应性能的考察

将孤立的Pd原子分散到ZnO纳米线(NWs)上作为单原子催化剂(SACs),并考察了它们在若干反应中的催化性能.Pd1/ZnO SAC对甲醇蒸汽重整制氢反应表现出高的活性、稳定性和CO2选择性.该催化剂体系对CO和H2的氧化也具有高活性,但在富氢物料中CO优先氧化反应中的催化剂性能较差,这主要是由于在ZnO负载的Pd1原子上H2氧化的强竞争反应所致.常压下在Pd1/ZnO SAC上就可发生逆水汽变换反应.该系列催化反应测试结果清楚地表明,选择合适金属与载体对开发分子催化转化用单原子催化剂至关重要.     

甲醇水蒸气重整制氢的高效碳纳米管改性Cu/ZnO/Al2O3催化剂

以碳纳米管为助剂,制备用于甲醇水蒸气重整制氢的新型高效Cu/ZnO/Al₂O₃催化剂,并与传统Cu/ZnO/Al₂O₃催化剂在相同条件下的催化性能进行了比较.结果表明,添加适量碳纳米管可显著提高催化剂的低温催化活性和选择性,在大幅度提高产氢速率的同时有效降低了重整产气中CO的含量.SEM和XRD分析证实适量碳纳米管的添加有效促进了Cu/ZnO/Al₂O₃催化剂结构特性的改善,有利于活性铜物种的分散,从而显著提高了催化剂的低温催化性能.     

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.