PdZn catalysts for CO2 hydrogenation to methanol using chemical vapour impregnation (CVI)

The formation of PdZn bimetallic alloys on ZnO, TiO2 and Al2O3 supports was investigated, together with the effect of alloy formation on the CO2 hydrogenation reaction. The chemical vapour impregnation (CVI) method produced PdZn nanoparticles with diameters of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction revealed the changes in the structure of the PdZn alloy that help stabilise formate intermediates during methanol synthesis. PdZn supported on TiO2 exhibits high methanol productivity of 1730 mmol kgcat−1 h−1 that is associated with the high dispersion of the supported PdZn alloy

Effect of Base on the Facile Hydrothermal Preparation of Highly Active IrO_x Oxygen Evolution Catalysts

The efficient electrochemical splitting of water is limited by the anodic oxygen evolution reaction (OER). IrO2 is a potential catalyst with sufficient activity and stability in acidic conditions to be applied in water electrolyzers. The redox properties and structural flexibility of amorphous iridium oxo-hydroxide compared to crystalline rutile-IrO2 are associated with higher catalytic activity for the OER. We prepared IrOx OER catalysts by a simple hydrothermal method varying the alkali metal base (Li2CO3, LiOH, Na2CO3, NaOH, K2CO3, KOH) employed during the synthesis. This work reveals that the surface area, particle morphology, and the concentration of surface hydroxyl groups can be controlled by the base used and greatly influence the catalyst activity and stability for OER. It was found that materials prepared with bases containing lithium cations can lead to amorphous IrOx materials with a significantly lower overpotential (100 mV @ 1.5 mA·cm–2) and increased stability compared to materials prepared with other bases and rutile IrO2. This facile method leads to the synthesis of highly active and stable catalysts which can potentially be applied to larger scale catalyst preparations

CO2 hydrogenation to CH3OH over PdZn catalysts, with reduced CH4 production

Metallic Pd, under CO 2 hydrogenation conditions (> 175 °C, 20 bar in this work), promotes CO formation via the reverse water gas shift (RWGS) reaction. Pd‐based catalysts can show high selectivity to methanol when alloyed with Zn, and PdZn alloy catalysts are commonly reported as a stable alternative to Cu‐based catalysts for the CO 2 hydrogenation to methanol. The production of CH 4 is sometimes reported as a minor by‐product, but nevertheless this can be a major detriment for an industrial process, because methane builds up in the recycle loop, and hence would have to be purged periodically. Thus, it is extremely important to reduce methane production for future green methanol synthesis processes. In this work we have investigated TiO 2 as a support for such catalysts, with Pd, or PdZn deposited by chemical vapour impregnation (CVI). Although titania‐supported PdZn materials show excellent performance, with high selectivity to CH 3 OH + CO, they suffer from methane formation (> 0.01%). However, when ZnTiO 3 is used instead as a support medium for the PdZn alloy, methane production is greatly suppressed. The site for methane production appears to be the TiO 2 , which reduces methanol to methane at anion vacancy sites

Hydrogenation of CO2 to dimethyl ether over brønsted acidic PdZn catalysts

Eschewing the common trend toward use of catalysts composed of Cu, it is reported that PdZn alloys are active for CO2 hydrogenation to oxygenates. It is shown that enhanced CO2 conversion is achievable through the introduction of Brønsted acid sites, which promote dehydration of methanol to dimethyl ether. We report that deposition of PdZn alloy nanoparticles onto the solid acid ZSM-5, via chemical vapor impregnation affords catalysts for the direct hydrogenation of CO2 to DME. This catalyst shows dual functionality; catalyzing both CO2 hydrogenation to methanol and its dehydration to dimethyl in a single catalyst bed, at temperatures of >270 °C. A physically mixed bed comprising 5% Pd 15% Zn/TiO2 and H-ZSM-5 shows a comparably high performance, affording a dimethyl ether synthesis rate of 546 mmol kgcat −1 h−1 at a reaction temperature of 270 °C

Combination of Cu/ZnO methanol synthesis catalysts and ZSM-5 zeolites to produce oxygenates from CO2 and H2

Cu/ZnO methanol catalysts were deposited over several ZSM-5 acid zeolites to directly synthesise oxygenates (methanol and dimethyl ether) from a CO2/H2 feed. Catalysts were prepared by two different preparation methodologies: chemical vapour impregnation (CZZ-CVI) and oxalate gel precipitation (CZZ-OG). Chemical vapour impregnation led to Cu/ZnO being deposited on the zeolite surface, whilst oxalate gel precipitation led to the formation of Cu/ZnO agglomerates. For both sets of catalysts a higher concentration of mild and strong acid sites were produced, compared to the parent ZSM-5 zeolites, and CZZ-CVI had a higher concentration of acid sites compared to CZZ-OG. Nevertheless, CZZ-OG shows considerably higher oxygenate productivity, 1322 mmol Kgcat−1 h−1, compared to 192 mmol Kgcat−1 h−1 over CZZ-CVI (ZSM-5(50), 250 ℃, 20 bar, CO2/H2 = 1/3, 30 ml min−1), which could be assigned to a combination of smaller particle size and enhanced methanol mass transfer within the zeolites

Lithium-directed transformation of amorphous iridium (oxy)hydroxides to produce active water oxidation catalysts

The oxygen evolution reaction (OER) is crucial to future energy systems based on water electrolysis. Iridium oxides are promising catalysts due to their resistance to corrosion under acidic and oxidizing conditions. Highly active iridium (oxy)­hydroxides prepared using alkali metal bases transform into low activity rutile IrO2 at elevated temperatures (>350 °C) during catalyst/electrode preparation. Depending on the residual amount of alkali metals, we now show that this transformation can result in either rutile IrO2 or nano-crystalline Li-intercalated IrO x . While the transition to rutile results in poor activity, the Li-intercalated IrO x has comparative activity and improved stability when compared to the highly active amorphous material despite being treated at 500 °C. This highly active nanocrystalline form of lithium iridate could be more resistant to industrial procedures to produce PEM membranes and provide a route to stabilize the high populations of redox active sites of amorphous iridium (oxy)­hydroxides

The critical role of βPdZn alloy in Pd/ZnO catalysts for the hydrogenation of carbon dioxide to methanol

The rise in atmospheric CO2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO2 as a feedstock. Prime among these is the hydrogenation of CO2 to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO2 hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m2g–1. Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO2 hydrogenation and will aid future catalyst discovery

Preparation of Solid Solution and Layered IrO _x-Ni(OH)₂Oxygen Evolution Catalysts:Toward Optimizing Iridium Efficiency for OER

Minimizing iridium loading in oxygen evolution reaction (OER) catalysts, without impairing electrocatalytic activity and stability is crucial to reduce the cost of water electrolysis. In this work, two Ir0.5Ni0.5Ox mixed oxide catalysts with layered and solid solution morphologies were prepared by modifying a facile hydrothermal methodology. The catalytic OER activity and stability of the Ir–Ni catalyst with a homogeneous distribution (IrNi-HD) was seriously compromised compared to pure IrOx due to the high concentration of surface nickel prone to corrosion under reaction conditions. However, the design of layered IrOx–Ni(OH)x (IrNi-LY) with Ir at the exposed surface allowed a 50% reduction in the molar concentration of the precious metal on the electrode compared to IrOx without impairing the catalytic activity or stability. As a result, IrNi-LY outperformed IrOx in activity when normalized to the Ir mass

CO 2

Metallic Pd, under CO 2 hydrogenation conditions (> 175 °C, 20 bar in this work), promotes CO formation via the reverse water gas shift (RWGS) reaction. Pd‐based catalysts can show high selectivity to methanol when alloyed with Zn, and PdZn alloy catalysts are commonly reported as a stable alternative to Cu‐based catalysts for the CO 2 hydrogenation to methanol. The production of CH 4 is sometimes reported as a minor by‐product, but nevertheless this can be a major detriment for an industrial process, because methane builds up in the recycle loop, and hence would have to be purged periodically. Thus, it is extremely important to reduce methane production for future green methanol synthesis processes. In this work we have investigated TiO 2 as a support for such catalysts, with Pd, or PdZn deposited by chemical vapour impregnation (CVI). Although titania‐supported PdZn materials show excellent performance, with high selectivity to CH 3 OH + CO, they suffer from methane formation (> 0.01%). However, when ZnTiO 3 is used instead as a support medium for the PdZn alloy, methane production is greatly suppressed. The site for methane production appears to be the TiO 2 , which reduces methanol to methane at anion vacancy sites

Hydrogenation of CO₂ to Dimethyl Ether over Brønsted Acidic PdZn Catalysts

Eschewing the common trend toward use of catalysts composed of Cu, it is reported that PdZn alloys are active for CO₂ hydrogenation to oxygenates. It is shown that enhanced CO₂ conversion is achievable through the introduction of Brønsted acid sites, which promote dehydration of methanol to dimethyl ether. We report that deposition of PdZn alloy nanoparticles onto the solid acid ZSM-5, via chemical vapor impregnation affords catalysts for the direct hydrogenation of CO₂ to DME. This catalyst shows dual functionality; catalyzing both CO₂ hydrogenation to methanol and its dehydration to dimethyl in a single catalyst bed, at temperatures of >270 °C. A physically mixed bed comprising 5% Pd 15% Zn/TiO₂ and H-ZSM-5 shows a comparably high performance, affording a dimethyl ether synthesis rate of 546 mmol kg_cat^–1 h^–1 at a reaction temperature of 270 °C