Stability of bimetallic Pd–Zn catalysts for the steam reforming of methanol

Density Functional Theory Study of Methanol Steam Reforming on Pt3Sn(111) and the Promotion Effect of a Surface Hydroxy Group

Methanol steam reforming (MSR) is studied on a Pt3Sn surface using the density functional theory (DFT). An MSR network is mapped out, including several reaction pathways. The main pathway proposed is CH3OH + OH → CH3O → CH2O → CH2O + OH → CH2OOH → CHOOH → COOH → COOH + OH → CO2 + H2O. The adsorption strengths of CH3OH, CH2O, CHOOH, H2O and CO2 are relatively weak, while other intermediates are strongly adsorbed on Pt3Sn(111). H2O decomposition to OH is the rate-determining step on Pt3Sn(111). The promotion effect of the OH group is remarkable on the conversions of CH3OH, CH2O and trans-COOH. In particular, the activation barriers of the O-H bond cleavage (e.g., CH3OH → CH3O and trans-COOH → CO2) decrease substantially by ~1 eV because of the involvement of OH. Compared with the case of MSR on Pt(111), the generation of OH from H2O decomposition is more competitive on Pt3Sn(111), and the presence of abundant OH facilitates the combination of CO with OH to generate COOH, which accounts for the improved CO tolerance of the PtSn alloy over pure Pt.

A Swiss-Roll-Type Methanol Mini-Steam Reformer for Hydrogen Generation with High Efficiency and Long-Term Durability

This paper proposes a Swiss-roll-type mini-reformer employing a copper-zinc catalyst for high-efficient SRM process. Although the commercially available copper-zinc catalysts commonly used in cylindrical-type reformers provide decent conversion rates in the short term, their long-term durability still requires improvement, mainly due to temperature variations in the reformer, catalyst loading, and thermal sintering issues. This Swiss-roll-shaped mini-reformer is designed to improve thermal energy preservation/temperature uniformity by using dual spiral channels to improve the long-term durability while maintaining methanol-reforming efficiency. It was fabricated on a copper plate that was 80 mm wide, 80 mm long, and 4 mm high with spiral channels that were 2 mm deep, 4 mm wide, and 350 mm long. To optimize the design and reformer operation, the catalyst porosity, gas hourly speed velocity (GHSV), operation temperature, and fuel feeding rate are investigated. Swiss-roll-type reformers may require higher driving pressures but can provide better thermal energy preservation and temperature uniformity, posing a higher conversion rate for the same amount of catalyst when compared with other geometries. By carefully adjusting the catalyst bed porosity, locations, and catalyst loading amount as well as other conditions, an optimized gas hourly space velocity (GHSV) can be obtained (14,580 mL/g·h) and lead to not only a high conversion rate (96%) and low carbon monoxide generation rate (0.98%) but also a better long-term durability (decay from 96% to 88.12% after 60 h operation time) for SRM processes. The decay rate, 0.13%/h, after 60 h of operation, is five-folds lower than that (0.67%/h, 0.134%/h) of a commercial cylindrical-type fixed-bed reactor with a commercial catalyst.

A Recent Review of Primary Hydrogen Carriers, Hydrogen Production Methods, and Applications

Hydrogen is a promising energy carrier, especially for transportation, owing to its unique physical and chemical properties. Moreover, the combustion of hydrogen gas generates only pure water; thus, its wide utilization can positively affect human society to achieve global net zero CO2 emissions by 2050. This review summarizes the characteristics of the primary hydrogen carriers, such as water, methane, methanol, ammonia, and formic acid, and their corresponding hydrogen production methods. Additionally, state-of-the-art studies and hydrogen energy applications in recent years are also included in this review. In addition, in the conclusion section, we summarize the advantages and disadvantages of hydrogen carriers and hydrogen production techniques and suggest the challenging tasks for future research.

Adsorbed Oxygen Ions and Oxygen Vacancies: Their Concentration and Distribution in Metal Oxide Chemical Sensors and Influencing Role in Sensitivity and Sensing Mechanisms

Oxidation reactions on semiconducting metal oxide (SMOs) surfaces have been extensively worked on in catalysis, fuel cells, and sensors. SMOs engage powerfully in energy-related applications such as batteries, supercapacitors, solid oxide fuel cells (SOFCs), and sensors. A deep understanding of SMO surface and oxygen interactions and defect engineering has become significant because all of the above-mentioned applications are based on the adsorption/absorption and consumption/transportation of adsorbed (physisorbed-chemisorbed) oxygen. More understanding of adsorbed oxygen and oxygen vacancies (VO•,VO••) is needed, as the former is the vital requirement for sensing chemical reactions, while the latter facilitates the replenishment of adsorbed oxygen ions on the surface. We determined the relation between sensor response (sensitivity) and the amounts of adsorbed oxygen ions (O2(ads)−, O(ads), −O2(ads)2−, O(ads)2−), water/hydroxide groups (H2O/OH−), oxygen vacancies (VO•, VO••), and ordinary lattice oxygen ions (Olattice2−) as a function of temperature. During hydrogen (H2) testing, the different oxidation states (W6+, W5+, and W4+) of WO3 were quantified and correlated with oxygen vacancy formation (VO•, VO••). We used a combined application of XPS, UPS, XPEEM-LEEM, and chemical, electrical, and sensory analysis for H2 sensing. The sensor response was extraordinarily high: 424 against H2 at a temperature of 250 °C was recorded and explained on the basis of defect engineering, including oxygen vacancies and chemisorbed oxygen ions and surface stoichiometry of WO3. We established a correlation between the H2 sensing mechanism of WO3, sensor signal magnitude, the amount of adsorbed oxygen ions, and sensor testing temperature. This paper also provides a review of the detection, quantification, and identification of different adsorbed oxygen species. The different surface and bulk-sensitive characterization techniques relevant to analyzing the SMOs-based sensor are tabulated, providing the sensor designer with the chemical, physical, and electronic information extracted from each technique.

Sensitivity Analysis of High-Pressure Methanol—Steam Reformer Using the Condensation Enthalpy of Water Vapor

A methanol–steam reformer (MSR) can safely provide hydrogen-rich fuel for a fuel cell system. Since the operating temperature of an MSR is relatively low, convective heat transfer is typically used to provide thermal energy to the endothermic reactions in the MSR. In this study, the use of phase change heat transfer to provide thermal energy to the endothermic reactions was investigated, which enhanced the temperature uniformity longitudinally along the MSR. ANSYS Fluent® software was used to investigate the performance of the reforming reactions. A comparative analysis using sensible heat and latent heat as the heat supply sources was performed. Using latent heat as a heat source achieved a lesser temperature drop than sensible heat that was under 5.29 K in the outer pipe. Moreover, a sensitivity analysis of methanol–steam-reforming reactions that use phase change heat transfer in terms of the carbon ratio, gas hourly velocity (for the inner and outer pipes of the MSR), inlet temperature (inner and outer pipes), reactor length, and operating pressure (inner pipe) was performed. When the phase change energy of water vapor is used, the wall temperature of the MSR is conveniently controlled and is uniformly distributed along the channel (standard deviation: 0.81 K). Accordingly, the methanol conversion rate of an MSR that uses phase change energy is ~4% higher than that of an MSR that employs convective heat transfer.

Methanol Reforming Processes for Fuel Cell Applications

Hydrogen production through methanol reforming processes has been stimulated over the years due to increasing interest in fuel cell technology and clean energy production. Among different types of methanol reforming, the steam reforming of methanol has attracted great interest as reformate gas stream where high concentration of hydrogen is produced with a negligible amount of carbon monoxide. In this review, recent progress of the main reforming processes of methanol towards hydrogen production is summarized. Different catalytic systems are reviewed for the steam reforming of methanol: mainly copper- and group 8–10-based catalysts, highlighting the catalytic key properties, while the promoting effect of the latter group in copper activity and selectivity is also discussed. The effect of different preparation methods, different promoters/stabilizers, and the formation mechanism is analyzed. Moreover, the integration of methanol steam reforming process and the high temperature–polymer electrolyte membrane fuel cells (HT-PEMFCs) for the development of clean energy production is discussed.

In Situ DRIFTS-MS Methanol Adsorption Study onto Supported NiSn Nanoparticles: Mechanistic Implications in Methanol Steam Reforming

Methanol adsorption over both supported NiSn Nps and analogous NiSn catalyst prepared by impregnation was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to gain insights into the basis of hydrogen production from methanol steam reforming. Different intermediate species such as methoxides with different geometry (bridge and monodentate) and formate species were identified after methanol adsorption and thermal desorption. It is proposed that these species are the most involved in the methanol steam reforming reaction and the major presence of metal-support interface sites in supported NiSn Nps leads to higher production of hydrogen. On the basis of these results, a plausible reaction mechanism was elucidated through the correlation between the thermal stability of these species and the evolution of the effluent gas released. In addition, it was demonstrated that DME is a secondary product generated by condensation of methoxides over the acid sites of alumina support in an acid-catalyzed reaction.

Multifunctional Catalyst Combination for the Direct Conversion of CO2 to Propane

The production of carbon-rich hydrocarbons via CO₂ valorization is essential for the transition to renewable, non-fossil-fuel-based energy sources. However, most of the recent works in the state of the art are devoted to the formation of olefins and aromatics, ignoring the rest of the hydrocarbon commodities that, like propane, are essential to our economy. Hence, in this work, we have developed a highly active and selective PdZn/ZrO₂+SAPO-34 multifunctional catalyst for the direct conversion of CO₂ to propane. Our multifunctional system displays a total selectivity to propane higher than 50% (with 20% CO, 6% C₁, 13% C₂, 10% C₄, and 1% C₅) and a CO₂ conversion close to 40% at 350 °C, 50 bar, and 1500 mL g^-1 h^-1. We attribute these results to the synergy between the intimately mixed PdZn/ZrO₂ and SAPO-34 components that shifts the overall reaction equilibrium, boosting CO₂ conversion and minimizing CO selectivity. Comparison to a PdZn/ZrO₂+ZSM-5 system showed that propane selectivity is further boosted by the topology of SAPO-34. The presence of Pd in the catalyst drives paraffin production via hydrogenation, with more than 99.9% of the products being saturated hydrocarbons, offering very important advantages for the purification of the products.

Identification of C2-C5 products from CO2 hydrogenation over PdZn/TiO2-ZSM-5 hybrid catalysts

The combination of a methanol synthesis catalyst and a solid acid catalyst opens the possibility to obtain olefins or paraffins directly from CO2 and H2 in one step. In this work several PdZn/TiO2-ZSM-5 hybrid catalysts were employed under CO2 hydrogenation conditions (240-360 °C, 20 bar, CO2/N2/H2 = 1 : 1 : 3) for the synthesis of CH3OH, consecutive dehydration to dimethyl ether and further oxygenate conversion to hydrocarbons. No significant changes after 36 h reaction on the methanol synthesis catalyst (PdZn/TiO2) were observed by XRD, XAS or XPS. No olefins were observed, indicating that light olefins undergo further hydrogenation under the reaction conditions, yielding the corresponding alkanes. Increasing the aluminium sites in the zeolites (Si : Al ratio 80 : 1, 50 : 1 and 23 : 1) led to a higher concentration of mild Brønsted acid sites, promoting hydrocarbon chain growth.

Fabrication of PdZn alloy catalysts supported on ZnFe composite oxide for CO2 hydrogenation to methanol

The conversion of CO₂ to methanol is of great significance for providing a means of CO₂ fixation and the development of future fuels. Supported Pd catalysts have been demonstrated to be active for CO₂ hydrogenation to methanol and PdZn alloy plays a key role in this reaction. Therefore, using ZnO-enriched support to increase the amount of nanometric PdZn alloy particles on the surface is an effective strategy to develop ideal catalysts. Herein, we fabricated a PdZn alloy catalyst supported on ZnO-enriched ZnFe₂O₄ spinel for efficient CO₂ hydrogenation to methanol. The amount of formed PdZn alloy and catalyst structure influenced by ZnO concentration on ZnFe₂O₄ were explored to obtain the best Pd-Z₁FO catalyst, which achieves a methanol space-time yield (STY) of 593 gkg_cat^-1h^-1 (12 gg_Pd^-1h^-1) with CO₂ conversion of 14% under reaction conditions of 290 °C, 4.5 MPa and 21600 mLg^-1h^-1. Furthermore, the amount of exposed PdZn alloy sites were measured by using CO-pulse chemisorption and we find a linearity between methanol production rate and PdZn alloy sites.

Tuning Adsorption Energies and Reaction Pathways by Alloying: PdZn versus Pd for CO2 Hydrogenation to Methanol

The tunability offered by alloying different elements is useful to design catalysts with greater activity, selectivity, and stability than single metals. By comparing the Pd(111) and PdZn(111) model catalysts for CO₂ hydrogenation to methanol, we show that intermetallic alloying is a possible strategy to control the reaction pathway from the tuning of adsorbate binding energies. In comparison to Pd, the strong electron-donor character of PdZn weakens the adsorption of carbon-bound species and strengthens the binding of oxygen-bound species. As a consequence, the first step of CO₂ hydrogenation more likely leads to the formate intermediate on PdZn, while the carboxyl intermediate is preferentially formed on Pd. This results in the opening of a pathway from carbon dioxide to methanol on PdZn similar to that previously proposed on Cu. These findings rationalize the superiority of PdZn over Pd for CO₂ conversion into methanol and suggest guidance for designing more efficient catalysts by promoting the proper reaction intermediates.

Pd–Au bimetallic catalysts supported on ZnO for selective 1,3-butadiene hydrogenation

The effect of the ZnO morphology on the properties of Pd–Au bimetallic catalysts has been discussed.

CO2-based hydrogen storage – hydrogen liberation from methanol/water mixtures and from anhydrous methanol

New strategies for the reforming of methanol under mild conditions on the basis of heterogeneous and molecular catalysts have raised the hopes and expectations on this fuel. This contribution will focus on the progress achieved in the production of hydrogen from aqueous and anhydrous methanol with molecular and heterogeneous catalysts. The report entails thermal approaches, as well as light-triggered dehydrogenation reactions. A comparison of the efficiency and mechanistic aspects will be made and principles of catalytic pathways operating in biological systems will be also addressed.

Solvent Free Synthesis of PdZn/TiO2 Catalysts for the Hydrogenation of CO2 to Methanol

Catalytic upgrading of CO₂ to value-added chemicals is an important challenge within the chemical sciences. Of particular interest are catalysts which are both active and selective for the hydrogenation of CO₂ to methanol. PdZn alloy nanoparticles supported on TiO₂ via a solvent-free chemical vapour impregnation method are shown to be effective for this reaction. This synthesis technique is shown to minimise surface contaminants, which are detrimental to catalyst activity. The effect of reductive heat treatments on both structural properties of PdZn/TiO₂ catalysts and rates of catalytic CO₂ hydrogenation are investigated. PdZn nanoparticles formed upon reduction showed high stability towards particle sintering at high reduction temperature with average diameter of 3–6 nm to give 1710 mmol kg⁻¹ h of methanol. Reductive treatment at high temperature results in the formation of ZnTiO₃ as well as PdZn, and gives the highest methanol yield.

Pd–Zn nanocrystals for highly efficient formic acid oxidation

Highly uniform Pd–Zn nanocrystals were facilely fabricated with coexisting noble metals and ascorbic acid, which exhibited superior electrocatalytic activity for formic acid oxidation.

Highly selective PdZn/ZnO catalysts for the methanol steam reforming reaction

Catalysts were prepared by impregnation-based method involving excess Cl⁻ anion addition to the metal chloride precursors which resulted in improved mixing of metals and led to formation of highly ordered PdZn alloys responsible for high catalytic selectivity.

Methanol Steam Reforming: Na Doping of Pt/YSZ Provides Fine Tuning of Selectivity

In this work, we found that sodium doping can be used to improve CO2 selectivity for supported Pt catalyst during methanol steam reforming. These materials are usually very active in the low temperature range; however, they are characterized by high selectivity of CO, which is a poison in downstream polymer electrolyte membrane fuel cells (PEM-FC) application. With Na doping, we found that CO2 selectivity was higher than 90% when 2.5 wt.% of sodium was added to Pt/YSZ. We have speculated that the different product distribution is due to a different reaction pathway being opened for CH3OH decomposition. Methanol decarbonylation was favored when Na was absent or low, while a formate decarboxylation pathway was favored when Na content reached 2.5 wt.%. The proposal is rooted in the observed weakening of the C-H bond of formate, as demonstrated in in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and kinetic isotope effect (KIE) experiments for the water-gas shift reaction conducted at low temperature. When adsorbed methoxy, produced when methanol is dissociatively adsorbed, was converted in the presence of H2O in DRIFTS spectroscopy, formate species were prevalent for a 2% Pt–2.5% Na/YSZ catalyst, while only a minor contribution was observed for 2% Pt/YSZ. Moreover, the formate produced on Na-doped Pt/YSZ exhibited ν(CH) stretching bands at low wavenumber, consistent with C–H bond weakening, thus favoring dehydrogenation (and decarboxylation). It is proposed that when Na is present, formate is likely an intermediate, and because its dehydrogenation is favored, selectivity can be fine-tuned between decarbonylation and decarboxylation based on Na dopant level.

Catalytic Reforming of Oxygenates: State of the Art and Future Prospects

This Review describes recent advances in the design, synthesis, reactivity, selectivity, structural, and electronic properties of the catalysts for reforming of a variety of oxygenates (e.g., from simple monoalcohols to higher polyols, then to sugars, phenols, and finally complicated mixtures like bio-oil). A comprehensive exploration of the structure-activity relationship in catalytic reforming of oxygenates is carried out, assisted by state-of-the-art characterization techniques and computational tools. Critical emphasis has been given on the mechanisms of these heterogeneous-catalyzed reactions and especially on the nature of the active catalytic sites and reaction pathways. Similarities and differences (reaction mechanisms, design and synthesis of catalysts, as well as catalytic systems) in the reforming process of these oxygenates will also be discussed. A critical overview is then provided regarding the challenges and opportunities for research in this area with a focus on the roles that systems of heterogeneous catalysis, reaction engineering, and materials science can play in the near future. This Review aims to present insights into the intrinsic mechanism involved in catalytic reforming and provides guidance to the development of novel catalysts and processes for the efficient utilization of oxygenates for energy and environmental purposes.

How to determine accurate chemical ordering in several nanometer large bimetallic crystallites from electronic structure calculations

The proposed method allows to efficiently determine the atomic arrangement in bimetallic nanoparticles based on electronic structure calculations and unravels the relationship between structural preferences of atoms and binding in nanoalloys.

Chemical and physical properties of binary metallic nanoparticles (nanoalloys) are to a great extent defined by their chemical ordering, i.e. the pattern in which atoms of the two elements are located in a given crystal lattice. The reliable determination of the lowest-energy chemical ordering is a challenge that impedes in-depth studies of several-nm large bimetallic particles. We propose a method to efficiently optimize the chemical ordering based solely on results of electronic structure (density functional) calculations. We show that the accuracy of this method is practically the same as the accuracy of the underlying quantum mechanical approach. This method, due to its simplicity, immediately reveals why one or another chemical ordering is preferred and unravels the nature of the binding within the nanoparticles. For instance, our results provide very intuitive understanding of why gold and silver segregate on low-coordinated sites in Pd₇₀Au₇₀ and Pd₇₀Ag₇₀ particles, while Pd₇₀Cu₇₀ exhibits matryoshka-like structure and Pd₇₀Zn₇₀ features Zn and Pd atoms arranged in layers. To illustrate the power of the new method we optimized the chemical ordering in much larger Pd₇₃₂Au₇₃₁, Pd₇₃₂Ag₇₃₁, Pd₇₃₂Cu₇₃₁, and Pd₇₃₂Zn₇₃₁ nanocrystals, whose size ∼4.4 nm is common for catalytic applications.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

Structural evolution of an intermetallic Pd–Zn catalyst selective for propane dehydrogenation

Formation of PdZn intermetallic nanoalloys selective for propane dehydrogenation tracked using in situ synchrotron XRD.

Low-temperature aqueous-phase reforming of ethanol on bimetallic PdZn catalysts

Bimetallic PdZn catalysts supported on carbon black (CB) and carbon nanotubes (CNTs) were found to be selective for CO-free H₂ production from ethanol at low temperature (250 °C).