Study on the role of Pt and Pd in Pt–Pd/TiO2 bimetallic catalyst for H2 oxidation at room temperature

The hydrogen safety issue is spotlighted as the hydrogen process is extended. For this reason, we studied catalysts for H₂ oxidation at room temperature to ensure hydrogen safety. Catalysts were prepared by different preparation methods and compared to evaluate the role of Pt and Pd in Pt–Pd/TiO₂ catalysts. The catalytic activity was significantly enhanced when activity metal size was small and it was exposed to catalyst surface to a high Pd ratio. For the 0.1%Pt-0.9%Pd/TiO₂ catalyst, high hydrogen conversion of 90% was obtained under the condition of 0.5% hydrogen injection. To understand the correlation between activity and characteristics of catalyst, the physicochemical characteristics of the various catalysts were investigated by X-ray photoelectron spectroscopy (XPS), temperature-programmed oxidation and reduction (TPOR) and Field Emission-Transmission Electron Microscope (FE-TEM) analysis. From these analysis, it was found that Pt served the role of highly dispersion of active metal (Pt–Pd) and as with increasing Pd ratio of active metal, hydrogen activity was increased, which indicates that hydrogen oxidation had proceeded on the Pd site. Finally, the valence state of the Pd influenced hydrogen oxidation activity of Pt–Pd/TiO₂, which increased with increasing ratio of Pd⁰/Pd_Total.     

Low-temperature CO preferential oxidation in H2-rich stream over iron modified Pd–Cu/hydroxyapatite catalyst

The effect of FeCl₃ addition on the catalytic property of Pd–Cu/hydroxyapatite (Pd–Cu/HAP) for low-temperature CO preferential oxidation (CO-PROX) under H₂-rich condition has been investigated. It can be found that CO conversion of Pd–Cu/HAP rapidly decreases from 56% to 21% within 2 h at 30 °C in the presence of water, however, the Pd–Cu–Fe/HAP with the Fe/Cu atomic ratio of 1:1 presents a stable CO conversion of 40% and CO₂ selectivity of 100% under the same reaction conditions. The characterization results display that the addition of FeCl₃ to Pd–Cu/HAP causes the formation of Fe₂O₃ species, and the strong interaction presents between Fe₂O₃ species and Pd–Cu/HAP. Thus, the Pd⁰ species generated during CO-PROX over Pd–Cu–Fe/HAP can be more easily oxidized than that over Pd–Cu/HAP, which could avoid H₂ adsorption on Pd⁰ species and maintain CO adsorption and activation.     

Coke-resistance over Rh–Ni bimetallic catalyst for low temperature dry reforming of methane

Methane and carbon dioxide can be converted into syngas using the prospective dry reforming of methane technology. Carbon deposition is a major cause of catalyst deactivation in this reaction, especially at low temperature. The superior stability of bimetallic catalysts has made their development more and more appealing. Herein, a series of bimetallic RhNi supported on MgAl₂O₄ catalysts were synthesized and used for low temperature biogas dry reforming. The results demonstrate that the bimetallic RhNi catalyst can convert CH₄ and CO₂ by up to 43% and 52% over at low reaction temperature (600 °C). Moreover, the reaction rate of CH₄ and CO₂ of RhNi–MgAl₂O₄ remains stable during the 20 h long time stability test, most importantly, there was no obviously carbon deposition observed over the spent catalyst. The enhanced coking resistance should be attributed to the addition of a little amount of noble metal Rh can efficiently suppress dissociation of CH_X1 species into carbon, and the high surface areas of MgAl₂O₄ support can also promote the adsorption and activation of carbon dioxide to generate more O1 species. Balancing the rate of methane dissociation and carbon dioxide activation to inhibit the development of carbon deposition is a good strategy, which provides a guidance for design other high performance dry reforming of methane catalysts.     

Combined steam and CO2 reforming of methane over Co–Ce/AC-N catalyst: Effect of preparation methods on catalyst activity and stability

An appropriate preparation method is the essential to improve the catalyst performance. In this study, Co–Ce/AC-N catalysts were fabricated on N-doped activated carbon supports by impregnation, sol-gel, precipitation and mix methods, respectively. It was used to catalyze the combined steam and dry reforming of methane (CSDRM). The effects of different preparation methods on the catalyst performance were investigated by means of N₂ adsorption-desorption, XRD, H₂-TPR, TEM, CO₂-TPD, FTIR and XPS. Compare with the catalysts prepared by other methods, the catalyst prepared by impregnation exhibits a large surface area, high active metal dispersion, and strong metal-support interaction. Meanwhile, it also has strong basic sites and abundant oxygen vacancies. These greatly improve the activity and stability of the catalyst. The conversions of CH₄ and CO₂ at 650 °C were achieved 71.6% and 64.4%, and H₂/CO was retained at 1.5.     

Decomposition of hydrogen iodide over Pt–Ir/C bimetallic catalyst

Decomposition of hydrogen iodide (HI) is one of the key reactions in the sulfur–iodine (S–I) thermochemical water splitting promising for the massive hydrogen production. Much effort has been made to explore the preparation of high performance catalyst for this hydrogen-producing reaction. Although platinum has long been found to be an efficient metallic catalyst, it was prone to agglomerate at elevated temperature resulting in a decrease in the hydrogen yield. A series of bimetallic Pt–Ir/C catalysts were prepared by electroless plating to investigate the effect of Ir/Pt molar ratio on the HI conversion compared with Pt/C and Ir/C catalysts. The physical properties and morphology of the catalysts were characterized by BET, XRD, TEM and ICP-AES. The synergistic effect of platinum and iridium with respect to HI decomposition was confirmed by the fact that the bimetallic Pt–Ir/C-0.77 catalyst with 1 wt% Pt loading and 0.77 wt% Ir loading showed much higher catalytic activity and thermostability compared with Pt/C and Ir/C catalyst. Based on the experimental results obtained, it may be concluded that the bimetallic Pt–Ir/C catalyst was supposed to be a cost-effective and high performance catalyst promising to be employed for the hydrogen production via the S–I thermochemical water splitting cycle.     

Effect of Pt–Pd/C coupled catalyst loading and polybenzimidazole ionomer binder on oxygen reduction reaction in high-temperature PEMFC

Polybenzimidazole (PBI) was studied as an ionomer binder at varying ratios (1–7) in a 20–40 wt% Pt–Pd/C cathode-coupled catalyst layer for the oxygen reduction reaction (ORR) in a high-temperature proton exchange membrane fuel cell (HT-PEMFC). Catalytic activity was examined by CV and LSV, while the properties of the catalysts were characterized by FESEM-EDX, N₂ adsorption–desorption, XRD and FTIR. The results showed that the distribution of metals on the carbon surface, carbon wall thickness and the interaction between ionomer and coupled catalysts affected the ORR performance. The fabricated membrane electrode assembly with 5:95 PBI: 30 wt% Pt–Pd/C catalyst ratio exhibited the best performance and highest durability for HT-PEMFC at 170 °C, yielding a power density of 1.30 Wcm⁻² with 0.02 mg_Pt/cm Pt loading. This performance of ultra-low metal loading of coupled Pt–Pd/C electrocatalyst with PBI binder was comparable to those reported by other studies, highlighting a promising catalyst for fuel cell application.     

Hydrogen feed gas purification over bimetallic Cu–Pd catalysts – Effects of copper precursors on CO oxidation

The bimetallic Pd–Cu catalysts were synthesized from different copper precursors, and their catalytic properties for CO oxidation were investigated. The characterization results indicate that chloride and sulfate precursors lead to formation of the high dispersed Cu metallic particle, while acetate and nitrate precursors lead to formation of the large aggregated Cu particles. The highly dispersed catalysts exhibit better catalytic activities for CO oxidation. In the Pd–Cu catalyst prepared from chloride and sulfate precursors, the Pd and Cu exist in alloy state and can be reduced easily, which might be the reason for the excellent properties for CO oxidation. The increase of Cu content can promote the catalytic activities of catalyst prepared from nitrate precursor. Pd species dispersed as the Cu-rich alloy on catalysts prepared with nitrate precursor, while the Pd species segregated on catalyst surface prepared with chloride precursor.     

Preferential CO oxidation over CuO–CeO2 in excess hydrogen: Effectiveness factors of catalyst particles and temperature window for CO removal

In the preferential oxidation of CO in hydrogen mixtures (PROX), CO and H₂ oxidation occur in parallel on the surface in a porous catalyst. The diffusion of the reactants into the pore structure of the catalyst can affect the catalyst performance significantly, and its effect can be accounted for in terms of the effectiveness factor. Conventional methods for estimating the effectiveness factor are not directly applicable because they have been developed for a single reaction in a catalyst particle. A novel method for a simultaneous estimation of the effectiveness factors of the two reactions was developed in this study. This method is based on the PROX kinetics over a CuO–CeO₂ catalyst and is applicable to the cases where the CO oxidation can be approximated by a first-order reaction and both oxidations are zero-order reactions with respect to the O₂ partial pressure. With the method, the performance of an isothermal PROX reactor was simulated to determine the effects of the feed flow rate, feed composition, reactor temperature and catalyst size on the CO clean-up.     

CuO–CeO2/SiO2 coating on ceramic monolith: Effect of the nature of the catalyst support on CO preferential oxidation in a H2-rich stream

Powder and structured catalysts based on CuO–CeO₂ nanoparticles dispersed on different silica are studied in CO preferential oxidation. Silica of natural origin (Celite) and fumed silica (aerosil), both commercial materials, and synthesized mesoporous SBA-15 with 20, 200 and 650 m²g^-1 respectively, are selected as supports. CuCe/Celite coated on cordierite monolith displays the highest activity, reaching CO conversion above 90% between 140 and 210 °C and more than 99% around 160 °C. The addition of 10% CO₂ and 10% H₂O partially deactivates the monolithic catalyst.The lower surface area of CuCe/Celite favors the contact between CuO and CeO₂ nanoparticles promoting a better interaction of Cu⁺²/Cu⁺ and Ce⁺³/Ce⁺⁴ redox couples. Raman spectroscopy reveals oxygen vacancies and XPS results show high metal lattice surface oxygen concentration and surface enrichment of Cu and Ce which promote the catalytic activity.     

Study of bimetallic Cu–Ni/γ-Al2O3 catalysts for carbon dioxide hydrogenation

The effect of the Cu/Ni ratio on CO₂ hydrogenation at 773 K and 873 K was studied by XRD, TPR, H₂ and CO₂–TPD. There exists strong interaction between Cu and Ni components. At high temperature (773 K and 873 K), the products are CO, CH₄ and H₂O without CH₃OH formation. The Cu/Ni ratio has a significant effect on the conversion and selectivity. Cu favors CO formation while Ni is of benefit in CH₄ formation.     

Combined steam and CO2 reforming of methane (CSCRM) over Ni–Pd/Al2O3 catalyst for syngas formation

In order to syngas formation, combined steam and carbon dioxide reforming of methane (CSCRM) used in the presence of Ni–Pd/Al₂O₃ catalysts, which were synthesized by the sol-gel method. Al₂O₃ supported Ni–Pd catalyst exhibited the appropriate surface area of 176.2 m²/g and high dispersion of NiO phase with an average crystallite size of 11 nm, which was detected on catalyst surface utilizing transmission electron microscopy (TEM). The influence of three independent operating parameters including reaction temperature in the range of 500–1000 °C; (CO₂ + H₂O)/CH₄ ratio, in the range of 1–3 and CO₂/H₂O ratio; in the range of 1–3, were investigated on the responses (i.e., CH₄ conversion, H₂ yield, CO yield, amount of coke formation on the catalyst surface and H₂/CO ratio) in CSCRM by using response surface methodology–central composite design (RSM-CCD) method. The obtained results from ANOVA and the proposed quadratic models could fine forecast the responses. It was seen that the total methane conversion and CO yield was almost accessible at temperatures higher than 850 °C. Moreover, the CO₂/H₂O ratio exhibited no significant effect on the CH₄ conversion, H₂ yield and CO yield of Ni–Pd/Al₂O₃ catalysts in CSCRM reaction. However, the high CO₂/H₂O ratio in inlet feed led to the syngas formation with a low H₂/CO ratio. The results revealed that lower CO₂/H₂O ratio and higher temperature as well as higher (CO₂ + H₂O)/CH₄ ratio help to decrease the coke formation.     

Effect of synthesis method on the oxygen reduction performance of Co–N–C catalyst

In recent years, Co, N co-doped carbon (Co–N–C) materials as oxygen reduction reaction (ORR) catalysts have attracted great attention because of their good ORR stability as well as decent activity. Co-doped zeolitic imidazolate framework-8 (Co@ZIF-8) as the precursor for synthesizing Co–N–C has attracted great interest recently. Co@ZIF-8 synthesis method may affect the properties of the as-synthesized Co@ZIF-8 precursors, which will surely affect the properties and ORR performance of Co@ZIF-8-derived Co–N–C catalysts. Herein, three methods, viz. room-temperature stirring method, reflux method, and hydrothermal method, were used to synthesize Co@ZIF-8 precursors. Physical characterization shows that the synthesis method has a great influence on the textural properties, composition, and graphitization degree of the as-synthesized Co–N–C catalysts. Electrochemical characterization shows that Co–N–C-R synthesized with reflux method exhibits an onset potential (E_onset) of 0.905 V, a half-wave potential (E_1/2) of 0.792 V and a limiting current density (J_L) of 5.85 mA cm^?2 in acidic media, which are higher than those of Co–N–C–S (E_onset = 0.870 V, E_1/2 = 0.770 V, J_L = 4.71 mA cm^?2) and Co–N–C–H (E_onset = 0.892 V, E_1/2 = 0.785 V, J_L = 4.68 mA cm^?2) synthesized with room-temperature stirring method and hydrothermal method, respectively. The better ORR activity observed on Co–N–C-R can be attributed to its larger graphitization degree and larger mesopore volume. Catalytic stability test shows that Co–N–C-R has negligible activity loss after 5000 potential cycles. This work demonstrates that reflux method is a more suitable method for synthesizing Co–N–C catalysts for ORR.     

CO2 hydrogenation on Co/CeO2-δ catalyst: Morphology effect from CeO2 support

The Co/CeO_2-δ catalysts with different morphology structure were prepared for CO₂ catalytic hydrogenation reaction. The physical and chemical properties of these catalysts were characterized by H₂-TPR, XRD, TEM, ICP and H₂-TPD. The characterization results indicated that the different morphology structure of CeO₂ support obviously influence the exposed crystal plane, and then affect the concentration of oxygen vacancies, the metal-support interaction and the dispersion of Co⁰ active species on the CeO₂ support. In addition, the Co ions can be dissolved into the CeO₂ lattice to form Ce–O–Co solid solution, which promotes the formation of Co⁰ active species, oxygen vacancies, and the Ce³⁺-□-Co⁰ structure, thus significantly affects the CO₂ hydrogenation performance of Co/CeO_2-δ catalysts. The exposed {110} and {100} crystal plane of CoCe140 catalyst with nano-rods structure ensure the excellent CO₂ hydrogenation performance, including CO₂ conversion is 48.7%, the CH₄ and CO selectivity are 91.7% and 8.3%, respectively.     

Biogas reforming for hydrogen production over a Ni–Co bimetallic catalyst: Effect of operating conditions

A Ni–Co bimetallic catalyst, Ni–Co/La₂O₃/Al₂O₃, was prepared by conventional incipient wetness impregnation. It shows a high level of activity and excellent stability for biogas reforming. This work examines how operating conditions, such as the reaction temperature, operating pressure, feed ratio, gas hourly space velocity (GHSV), and CO₂ excessive coefficient, affect the catalytic performances of the catalyst. The experimental biogas is simulated with CH₄ and CO₂ at a molar ratio of 1, without any dilute gas. The catalyst was also characterized by XRD, BET, TEM and TG-DSC. In a stability test of 510 h under the conditions of 800 °C, 1 atm, and a GHSV of 6000 ml g_cat⁻¹ h⁻¹, the average coking rate over the catalyst was only about 0.0374 mg g_cat⁻¹ h⁻¹. The experimental results also indicate that the dynamic equilibrium between the deposition and gasification of carbon deposited on the surface of the catalyst can be established during the reaction. The aggregation of metallic Ni/Co and the formation of filamentous carbon over the surface of the catalyst can be inhibited effectively. During the last 50 h of the 510 h stability test, the average conversion of CH₄ and CO₂, the selectivity to H₂ and CO, and ratio of H₂/CO were 95.2%, 96.7%, 95.0%, 98.3%, and 0.96, respectively.     

Effect of Pt: Pd atomic ratio in Pt–Pd/C electrocatalyst-coated membrane on the electrocatalytic activity of ORR in PEM fuel cells

The influence of Pt: Pd atomic ratios (1:2–1:8) on a carbon support upon its suitability as a cathode for a proton exchange membrane (PEM) fuel cell was evaluated at a constant membrane electrocatalyst loading of 0.15 mg/cm². The results clearly demonstrated that the different Pt: Pd atomic ratios had a significant effect on both the electrocatalyst activity and also on the performance in a H₂/O₂ fuel cell. Decreasing the Pt: Pd atomic ratio led to an increase in the particle size of the electrocatalyst but a decrease in the particle dispersion and electrochemical surface area (ESA). With respect to the performance in a PEM fuel cell, decreasing the Pt: Pd atomic ratio led to a decreased exchange current density (j₀), electrocatalytic activity and also mass activity (M_A), but to an increased total resistance (R) of the cell. The maximum activity of the oxygen reduction reaction (ORR) and the peak power (492 mW/cm²) were obtained with an electrocatalyst with a Pt: Pd atomic ratio of 1:2. Finally, the rotating disk electrode (RDE) analysis showed that the mechanism of oxygen reduction on the prepared Pt–Pd/C electrocatalyst involved a four-electron pathway with high oxygen permeability in the Nafion film.     

Reforming of methanol with steam in a micro-reactor with Cu–SiO2 porous catalyst

In the present work, we report the results of a series of experiments for the hydrogen production via steam reforming of methanol with Cu–SiO₂ porous catalyst coated on the internal walls of a micro-reactor with parallel micro-passages. The catalyst was prepared by coating copper and silica nanoparticles on the internal surface of the microchannel via convective flow boiling heat transfer, followed by a calcination procedure at 973 K and therefore, the catalyst does not require any supportive material, which in turn reduced the complexity and cost of the preparation. The experiments were conducted at reactant flow rates of 0.1–0.9 lit/min, operating temperatures of 523–673 K, catalyst loading of 0.25 gr to 1.25 gr and at heat flux value of 500 kW/m². Results of the experiments showed that the methanol conversion can reach 97% at catalyst loading of 1.25 gr. It was also found that with an increase in the gas hourly space velocity (GHSV) of the reactants, the methanol conversion decreases, which was attributed to the decrease in the residence time, the suppression in diffusion of reactants into the pores of the catalyst, and also the decrease in the average film temperature of the reactor. The highest methanol conversion was obtained at gas hourly space velocity of 24,000 ml/(gr.hr) and T = 773 K and for molar ratio of methanol to water of 0.1. The molar ratio of methanol to water also influenced the thermal response of the reactor such that the surface temperature profile of the micro-reactor was more decreased at low methanol/water molar ratios.     

The effective and stable Cu–C@SiO2 catalyst for the syntheses of methanol and ethylene glycol via selective hydrogenation of ethylene carbonate

The co-production of ethylene glycol and methanol via ethylene carbonate hydrogenation derived from CO₂ has attracted great concerns because of the promising chemical utilization of CO₂ in large-scale. Copper-based catalysts are widely concerned in hydrogenation of ester due to the high catalytic efficiency and low cost, but the stability of copper-based catalyst is poor and needs to be further improved. In this study, the modification Cu–C@SiO₂-R catalyst with graphite oxide was prepared by using Cu₃(BTC)₂ as the precursor and ammonia evaporation method, and was applied in ethylene carbonate hydrogenation to synthesis ethylene glycol and methanol. Furthermore, the catalysts were characterized in detail. The results showed that the Cu–C@SiO₂-R catalyst was modified with graphite oxide, the average size of Cu particles was 2.9 nm and Cu particles had good dispersion. In addition, both Cu–C@SiO₂-R and Cu@SiO₂-R catalysts had similar ratio of Cu⁺/(Cu⁰+Cu⁺). In a batch reactor, under 453 K, 5 MPa, 4 h, the catalytic efficiency was 80.0% EC conversion 92.2% EG and 70.8% MeOH selectivity showing excellent catalytic performance capability of Cu–C@SiO₂-R catalyst. In long-term experiment, the Cu–C@SiO₂-R catalyst showed excellent stability after using for 264 h. The activity was 0.63 g_EC g_cat^?1 h^?1, and 100.0% EC conversion 99.9% EG and 85.8% MeOH selectivity could be achieved in a fixed bed. After the long-term experiment, the Cu⁺/(Cu⁺+Cu⁰) ratio in Cu–C@SiO₂-R catalyst kept at around 0.48. In contrast, the Cu⁺/(Cu⁺+Cu⁰) ratio in Cu@SiO₂-R catalyst decreased sharply from 0.48 to 0.38. The stability of the structure and the balance of valence of Cu were considered to be responsible to the stability of Cu–C@SiO₂-R catalyst, because the graphite oxide not only kept the Cu⁺/(Cu⁰+Cu⁺) ratio stability, but also restrained the aggregation of Cu particles and loss of copper. This work provides an in-depth understanding of the stabilization mechanism of Cu and can be a reference for the industrial application of ethylene carbonate hydrogenation.     

Tailoring performance of Co–Pt/MgO–Al2O3 bimetallic aerogel catalyst for methane oxidative carbon dioxide reforming: Effect of Pt/Co ratio

Co–Pt/MgO–Al₂O₃ bimetallic aerogel catalysts were synthesized via a sol-gel combined with supercritical drying method. The catalysts were characterized by XRD, BET, HRTEM, STEM-HAADF, XPS, H₂-TPR, H₂-TPD, TG/DSC, FESEM and their catalytic performances in CH₄ oxidative CO₂ reforming were evaluated. The H₂ spillover effect between Pt and Co enhanced the reducibility of the catalyst, while the strong metal-support interaction (SMSI) effect in the bimetallic aerogel catalysts confined the agglomeration of metal particles. Pt/Co ratio played a key role on the existence of surface metal species, leading to different catalytic performances. The optimal Pt/Co ratio was Pt/Co = 0.02 w/w, on which a 50% higher activity in terms of CH₄ conversion than monometallic Co or Pt aerogel catalysts was obtained. Whereas the impregnated catalyst with an identical composition showed a much lower activity. The Co–Pt aerogel catalysts also showed high resistance to inactive carbon formation. The oxidation temperature of the carbon species deposited on the spent Co–Pt aerogel catalyst was only 275 °C and no filamentous or graphitic carbon was identified, disclosing that the formation of inactive carbon was inhibited due to the synergy between Co and Pt and the SMSI effect.