Combinatorial Design of Al2O3 Supported Au Catalysts For Preferential CO Oxidation

Multi-component Au/Al₂O₃ catalysts were designed and tested for PROX reaction using holographic research strategy. On the bases of our previous study Pb has been selected as the main modifier of the Au. In addition to Au and Pb the catalysts library contained V, Ba, Ce, Sm, Ag and Cu resulting in multi component catalysts tailored for PROX reaction. After preparation and testing of 173 catalysts within five generations new catalyst compositions with excellent performance have been obtained. Upon using the best catalyst CO could be removed almost completely and the selectivity of oxygen towards the CO oxidation was around 75%. In the course of catalyst library design it has been revealed that the selection of the objective function (OF) has high impact both on the rate of optimization and the performance of catalysts designed. The complex OF was created from two single desirability functions related to CO conversion and oxygen selectivity towards CO oxidation. In order to maintain high optimization rate there was a need to change the weights of single desirability functions in the course of catalyst library design. The results show that Pb, Sm, Cu and Ag are the key modifiers for PROX reaction under experimental conditions applied.     

Au/Ti-SBA-15 catalysts in CO and preferential (PROX) CO oxidation

The Au/Ti-SBA-15 catalysts were found promising for CO oxidation, including preferential CO oxidation in the presence of H₂ (PROX). The catalytic performance and Au particle size depending on the Ti content: 100% selectivity to CO₂ in PROX at 90% CO conversion was found for the catalysts of the Ti content below 1.32 wt.% Ti.     

Heterogeneous Catalytic Aziridination of Styrene Using Transition-Metal-Exchanged Zeolite Y

Hydrogen for fuel cells can be produced by reforming hydrocarbons. The H₂-rich reformate typically contains about 1 mol% CO which will poison the anode of polymer electrolyte fuel cells. The CO concentration can be reduced by preferential oxidation (PROX) using near-stoichiometric amounts of O₂. The conversion of CO should be over 99% while minimizing oxidation of H₂. Supported Pt catalysts with and without promotion by Ce were compared for the catalytic oxidation of CO by O₂ in a H₂ stream. With unsupported Pt catalysts, selectivity (to CO₂ as opposed to H₂O) was highest at low temperatures and low O₂/CO ratios, however conversion was low. Addition of Ce significantly improved CO conversion under these conditions.     

XAFS Characterization of Pt–Fe/zeolite Catalysts for Preferential Oxidation of CO in Hydrogen Fuel Gases

We have developed a new Pt–Fe/mordenite (Pt–Fe/M) catalyst which shows remarkably high activity and selectivity for the oxidation of CO in H₂-rich gas compared with Pt/M. In the present work, to understand the role and structure of Pt and Fe in the Pt–Fe/M catalyst, the states of metallic components in ion-exchanged, H₂ pre-treated and post-PROX (preferential oxidation of CO) samples have been studied by means of XAFS. It was confirmed that Pt forms the metallic clusters after H₂ pretreatment or the PROX experiment, whereas a large part of Fe exists as oxides even after the H₂ treatment. At post-analysis of the catalysts used for the PROX experiment, an increase in coordination number of Fe–O was observed. Pt clusters in the Pt–Fe(2:1 weight ratio)/M catalyst, which showed the highest PROX performance, were found to have a different electronic structure from the other catalysts. Additionally, preferential CO adsorption onto Pt sites at Pt–Fe/M was clearly demonstrated by infrared spectroscopy analysis in a stream of 1% CO containing H₂. Based on these results, the superior PROX mechanism was discussed.     

Nanoscale fabrication of metal particles and wires using mesoporous silica templates: Electronic properties and catalytic performances in PROX reaction

Pt nanowires and nanoparticles were selectively synthesized in mesoporous silica templates FSM-16 and HMM. The wires and particles were characterized by physicochemical methods. Extracted Pt nanoparticles on HOPG show Coulomb blockade phenomena in STM/STS measurements. Pt nanowires and -particles in FSM-16 shows high catalytic activity and selectivity in preferential oxidation (PROX) of CO in H₂.     

Competition of C-C bond formation and C-H bond formation For acetylene hydrogenation on transition metals: A density functional theory study

Selective hydrogenation of acetylene is an important reaction for production of polymer grade ethylene. The green oil formation has great influence on the selectivity and activity of acetylene selective hydrogenation. This article describes a density functional theory study on the C + H hydrogenation reaction and C + C coupling reaction on the (111) surface of Ag, Cu, Pd, Pt, Rh, and Ir. The activity of acetylene selective hydrogenation is examined by the effective barrier for ethylene formation. A comparison between the reaction barrier of ethylene hydrogenation and desorption is used to identify the selectivity for ethylene formation. The barriers of three pathways for 1,3-butadiene formation suggest that acetylene and vinyl coupling reaction is the favorable pathway. The stability of catalysts is evaluated by the selectivity of 1,3-butadiene, which follows the order of Pt(111) > Ir(111) > Rh(111) > Pd(111) > Cu(111) > Ag(111). Furthermore, the relationship between acetylene adsorption energy and effective barrier of ethylene formation and 1,3-butadiene formation has been established to well understand the catalytic properties of different metals. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1059–1066, 2019     

Electrochemical promotion of the CO2 hydrogenation reaction using thin Rh, Pt and Cu films in a monolithic reactor at atmospheric pressure

A monolithic electropromoted reactor (MEPR) with up to 22 thin Rh/YSZ/Pt or Cu/TiO₂/YSZ/Au plate cells was used to investigate the hydrogenation of CO₂ at atmospheric pressure and temperatures 220–380 °C. The Rh/YSZ/Pt cells lead to CO and CH₄ formation and the open-circuit selectivity to CH₄ is less than 5%. Both positive and negative applied potentials enhance significantly the total hydrogenation rate but the selectivity to CH₄ remains below 12%. The Cu/TiO₂/YSZ/Au cells produce CO, CH₄ and C₂H₄ with selectivities to CH₄ and C₂H₄ up to 80% and 2%. Both positive and negative applied potential significantly enhance the hydrogenation rate and the selectivity to C₂H₄. It was found that the addition of small (0.5 kPa) amounts of CH₃OH in the feed has a pronounced promotional effect on the reaction rate and selectivity of the Cu/TiO₂/YSZ/Au cells. The selective reduction of CO₂ to CH₄ starts at 220 °C (vs 320 °C in absence of CH₃OH) with near 100% CH₄ selectivity at open-circuit and under polarization conditions at temperatures 220–380 °C. The results show the possibility of direct CO₂ conversion to useful products in a MEPR via electrochemical promotion at atmospheric pressure.     

Electrochemical promotion of CO conversion to CO2 in PEM fuel cell PROX reactor

The possibility of electrochemically promoting the water–gas-shift reaction and the CO oxidation reaction in a PEM fuel cell reactor supplied with a methanol reformate mixture was investigated in PEM fuel cells with Pt or Au state-of-the-art E-TEK anodes, in order to explore the use of PEMFC units as preferential oxidation of CO (PROX) reactors. The electropromotion of CO removal was investigated both with air or H₂ fed to the cathode side and also by O₂ bleeding to the anode during normal PEMFC operation. It was found that the catalytic activity of the anode for CO conversion to CO₂ can be modified significantly by varying the catalyst potential. The magnitude of the electrochemical promotion depends strongly on the anodic electrocatalyst (Pt or Au), on the CO concentration of the fuel mixture, on the operating temperature and on the presence of oxygen. The electropromotion effect and the Faradaic efficiency were found to be much higher in CO-rich anode environments.     

Low temperature CO oxidation on Au(111) and the role of adsorbed water

This paper presents results of an investigation of low-temperature CO oxidation and the role of moisture on an atomic oxygen covered Au(111) surface by employing molecular beam scattering techniques under ultrahigh vacuum (UHV) conditions. The effect of atomic oxygen precoverage on CO oxidation was examined at sample temperatures as low as 77 K. Prompt CO₂ production was observed when the CO beam impinges on the sample followed by a rapid decay of CO₂ production in all cases. At oxygen precoverages above 0.5 ML, CO₂ production decreases with increasing oxygen precoverage primarily due to the decrease in CO uptake. CO oxidation at 77 K goes through a precursor mediated reaction mechanism, where CO is in a precursor or trapped state and oxygen atoms are in a chemisorbed state. The role of adsorbed water was studied by using isotopically labeled water [H ₂ ¹⁸ O] to distinguish the oxygen species from that used in oxygen atom exposures [¹⁶O]. Evidence is presented that shows activated water or OH groups formed from water can directly participate in oxidizing CO on an atomic oxygen covered Au(111) surface.     

In/Au(111)和Ir/Au(111)面上巴豆醛选择加氢的机理研究及比较

基于巴豆醛在M/Au（111）合金表面（M=In,Ir）垂直吸附的最稳定吸附结构,采用密度泛函理论对其不完全加氢的反应机理进行探究。从不同加氢机理下各基元反应的活化能、反应热计算以及构型变化分析中可知,巴豆醛在M/Au（111）面上均优先对距离合金表面较近的C＝O进行加氢,且以C为活性中心优先进行加氢为最优机理,其中第1步加氢反应的活化能较高,是该机理的控速步骤。反应物巴豆醛的O原子与合金的掺杂原子M形成较强的化学吸附,提高了M/Au（111）面对C＝O加氢的选择性。巴豆醛按最优机理加氢的基元反应中在In/Au（111）面上最高反应能垒为0.969 eV,比在Ir/Au（111）面的最高反应能垒1.332 eV低,因此认为In/Au合金对其不完全加氢有更好的催化活性。     

Reaction and Surface Characterization Studies of Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts for CO Preferential Oxidation in Reformed Gas

In order to investigate the reasons the activation of a Ru/Al₂O₃ catalyst by heating in a H₂/N₂ mixed gas improves the CO preferential oxidation (PROX) activity, the oxidation state of the Ru on the catalyst surface was studied by using ESCA. As the ratio of Ru(0) to total Ru on the surface was increased, the temperature window of the Ru catalyst, where CO was reduced to below 10 ppm, was expanded to the lower temperature side. The activity of CO oxidation by O₂ of the Ru catalyst at lower temperatures was improved by increasing the ratio of Ru(0). However, the selectivity for CO oxidation hardly varied with the change in the surface Ru(0) ratio at these low temperatures. It is considered that O₂ activation on Ru(0) plays an essential role in CO PROX activity on the Ru catalyst at low temperatures.     

Experimental and Theoretical Study of CO Oxidation on PdAu Catalysts with NO Pulse Effects

The effect of NO on CO oxidation was studied for Pt, Pd and PdAu catalysts. It was found that NO inhibits significantly the CO oxidation reactivity on both Pt and Pd catalysts. On PdAu catalyst, however, the presence of NO resulted in an enhancement of CO oxidation activity. In order to gain an atomistic understanding of this effect, density-function theory (DFT) calculations were performed on the adsorption and reaction properties of NO and CO on these metal surfaces. We have identified that the inhibition effects on Pt and Pd catalysts are due to stronger NO binding, and that the enhanced reactivity on PdAu is due to the reduced NO oxidation barrier on PdAu leading to NO₂ formation.     

Preferential Oxidation of CO in H2-Rich Gases over Co-Promoted Pt-γ-Al2O3 Catalyst

Carbon monoxide is a poison to the Pt anode in proton exchange membrane fuel cell (PEMFC). Preferential oxidation (PROX) is an effective method to reduce CO in hydrogen-rich gas streams to a tolerant level. In the present work, the effect of adding cobalt to Pt/γ-Al₂O₃ on the PROX of CO was investigated. Our results showed that the addition of Co to Pt/γ-Al₂O₃ could not only improve the low-temperature activity but also reduce significantly the loading of Pt in the catalysts. Over the catalyst 3%Co/1%Pt/γ-Al₂O₃ the conversion of CO was close to 100% at 90 °C and space velocity of 8000 mL g^?1 h^?1. In addition, the Co-promoted Pt/γ-Al₂O₃ catalyst showed good resistance to H₂O and CO₂ and could be operated in a wide range of space velocity. At temperatures above 90 °C, the existence of H₂O in the feed increased the conversion and broadened the operating temperature range without worsening the selectivity. When space velocity was changed from 8000 to 80,000 mL g^?1 h^?1 and temperatures was kept between 120 and 160 °C, the conversion of CO was always over 99% and the decrease in O₂ selectivity did not exceed 10%. Furthermore, a strong opposite effect of the ratio of O₂ to CO on the conversion of CO and the selectivity of O₂ was observed. However, at the O₂/CO ratio of 1.0 and temperatures between 120 and 160 °C, a satisfied balance between conversion and selectivity could be obtained.     

Role of Preparation Techniques in the Activity of Au/TiO2 Nanostructures Stabilised on SiO2: CO and Preferential CO Oxidation

Au colloid and titania in different sequence or Au-oxide nano ensembles preformed in hydrosol were deposited on inert amorphous silica or mesoporous SBA-15. It was compared with gold colloid adsorbed on TiO₂ or silica support. The formation of the Au/TiO₂ interface is discussed in terms of surface charges. Preferential CO oxidation in the presence of hydrogen (PROX) has been correlated with the CO oxidation activity and structural properties, perimeter and the influence of the TiO₂ morphology on the catalytic activity has been demonstrated.     

Preferential oxidation of CO over CuO/CeO2 and Pt-Co/Al2O3 catalysts in micro-channel reactors

Micro-channel plates with dimension of 1 mm × 0.3 mm × 48 mm were prepared by chemical etching of stainless steel plates followed by wash coating of CeO₂ and Al₂O₃ on the channels. After coating the support on the plate, Pt, Co, and Cu were added to the plate by incipient wetness method. Reaction experiments of a single reactor showed that the micro-channel reactor coated with CuO/CeO₂ catalyst was highly selective for CO oxidation while the one coated with Pt-Co/Al₂O₃ catalyst was highly active for CO oxidation. The 7-layered reactors coated with two different catalysts were prepared by laser welding and the performances of each reactor were tested in large scale of PROX conditions. The multi-layered reactor coated with Pt-Co/Al₂O₃ catalyst was highly active for PROX and the outlet concentration of CO gradually increased with the O₂/CO ratio due to the oxidation of H₂ which maintained the reactor temperature. The multi-layered reactor coated with CuO/CeO₂ showed lower catalytic activity than that coated with Pt catalyst, but its selectivity was not changed with the increase of O₂/CO ratios due to the high selectivity. In order to combine advantages (high activity and high selectivity) of the two individual catalysts (Pt-Co/Al₂O₃, CuO/CeO₂), a serial reactor was prepared by connecting the two multi-layered micro-channel reactors with different catalysts. The prepared serial reactor exhibited excellent performance for PROX.     

Au–Cu alloy nanoparticles supported on silica gel as catalyst for CO oxidation: Effects of Au/Cu ratios

Au–Cu bimetallic catalysts with Au/Cu ratios ranging from 3/1 to 20/1 were prepared on silica gel support by a two-step method. The catalysts were characterized by ICP, XRD and TEM. The results showed that, irrespective of Au/Cu ratios, all the bimetallic nanoparticles had significantly reduced particle sizes (3.0–3.6 nm) in comparison with monometallic gold catalysts (5.7 nm). Both CO oxidation and PROX reactions were employed to evaluate the catalytic activities of Au–Cu bimetallic catalysts. For CO oxidation, the alloy catalysts show non-monotonic temperature dependence showing a valley in the intermediate temperature range. The catalyst with Au/Cu ratio of 20/1 gave the highest activity at room temperature, but its activity showed the deepest valley with increasing the reaction temperature. On the other hand, the catalyst with Au/Cu ratio of 3/1 exhibited the best performance for PROX reaction. For the Au/Cu ratios investigated, the bimetallic catalysts showed superior performance to monometallic gold catalysts, demonstrating the synergy between gold and copper.     

Preferential oxidation of carbon monoxide over Pt, Au monometallic catalyst, and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate

The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO₂ catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO₂ catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO₂ and H₂O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

Tuning the Performance of Ni‐Based Catalyst by Doping Coinage Metal on Steam Reforming of Methane and Carbon‐Tolerance

Density functional theory (DFT) calculations are employed to investigate the key reactions in steam reforming of methane (SRM) on Ni‐based bimetallic surface alloys, including the dissociation of CH₄ and H₂O, the oxidation of CH by oxygen atom to form formyl (CHO), and the dehydrogenation of CHO to form carbon monoxide (CO). The aim of this investigation is to hunt for an optimal catalyst for SRM, which can inhibit carbon formation while maintaining high activity to the SRM. Coinage metal impurity (Au, Ag, and Cu) doped Ni catalysts have been proven to inhibit carbon deposition. In this work, we focus on investigating the doping effects on some leading processes in SRM. It is found that the coinage metal doping has a little effect on the two‐step dissociation of H₂O, which has a linear correlation between the dissociation barriers and the OH–H coadsorption energies. In addition, the dehydrogenation of CHO is kinetically favorable on all alloy surfaces. However, for the CH oxidation to CHO, only the Ni–Cu surface remains high activity. These results suggest that Ni–Cu bimetallic material is an excellent active carbon‐tolerance SRM catalyst for solid‐oxide fuel cells.     

Pt–Ni/γ-Al2O3 catalyst for the preferential CO oxidation in the hydrogen stream

The preferential CO oxidation (PROX) in the presence of excess hydrogen was studied over Pt–Ni/γ-Al₂O₃. CO chemisorption, X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and temperature-programmed reduction were conducted to characterize active catalysts. The co-impregnated Pt–Ni/γ-Al₂O₃ was superior to Pt/Ni/γ-Al₂O₃ and Ni/Pt/γ-Al₂O₃ prepared by a sequential impregnation of each component on alumina support. The PROX activity was affected by the reductive pretreatment condition. The pre-reduction was essential for the low-temperature PROX activity. As the reduction temperature increased above 423 K, the CO₂ selectivity decreased and the atomic percent of Ni in the bimetallic phase of Pt–Ni increased. This catalyst exhibited the high CO conversion even in the presence of 2% H₂O and 20% CO₂ over a wide reaction temperature. The bimetallic phase of Pt–Ni seems to give rise to high catalytic activity for the PROX in H₂-rich stream.