Catalysis to produce solar fuels: From the production of hydrogen via water splitting, to hydrogen conversion to methanol by its reaction with CO2

Increasing CO2-anthropogenic concentration in the atmosphere is surpassing sustainable levels and unambiguously jeopardising the global climate. The main share of CO2 emissions corresponds to the production of energy, currently 17 TW per year. By analysing historical trends on population growth and energy consumption, it is expected that the global energy demand will reach 30 TW by 2050, which in turn will aggravate the stress on the environment. In order to alleviate adverse climate-change-related consequences, it was agreed to reduce global CO2 emissions. This could be achieved by shifting the current energy system towards a carbon-free energy vector, hydrogen. Hydrogen can be obtained from the hydrolysis of water using surplus electricity generated from renewables, without contributing to CO2 emissions. The main drawback to switching from a carbon to a hydrogen-based energy system is that H2 is a gas and current technology has evolved around liquid fuels. In order to circumvent this energy transition, hydrogen can be further transformed to liquid fuels via its reaction with CO2. In a polymer electrolyte membrane (PEM) electrolyser, water is oxidised at the anode to oxygen and protons, protons migrate through the membrane to the cathode where they recombine with electrons to form hydrogen. Due to the slow kinetics and multiple reaction steps on the anode compared to the cathode, the oxidation of water to oxygen (or oxygen evolution reaction, OER) is responsible for high overpotentials. Additionally, the anode needs to be made of iridium based catalysts, and because of its low natural abundance, it is essential to develop materials with an efficient Ir usage and to optimise its catalytic activity and stability. This research is divided in two defined themes. The first (chapter 3 to chapter 6) focuses on the optimisation of IrO2 catalysts for the oxygen evolution reaction, necessary half reaction in a PEM water electrolyser for the production of hydrogen. The second (chapter 7) comprises of the optimisation of PdZn catalysts supported on TiO2 for the further transformation of H2, by its reaction with CO2, to solar fuels. In chapters 3 and 4, the effect of the base on the hydrothermal synthesis of unsupported and supported amorphous iridium oxo-hydroxides is studied. The hydrothermal synthesis was chosen because it allowed the synthesis of amorphous IrOx materials without the need for heat treatment at high temperature, thus minimising the possible crystallisation and the concomitant decrease in activity towards OER. It was observed that the base plays an important role in tailoring the morphology, surface area and surface hydroxide concentration of IrOx catalysts, and thus it has a direct effect on the catalytic activity and stability. Specifically, the use of Li2CO3 as a base led to a catalyst with porous morphology, higher surface area and higher hydroxide concentration, which this translated to an improved activity and stability towards OER compared to the state of the art catalyst IrO2·2H2O (Premion, Alfa Aesar). In both chapters, heat treatment was proven to hinder the catalytic activity towards OER, presumably as a result of higher crystallinity, the loss of Ir(III) sites and the decrease in hydroxide concentration. In chapter 5, two different IrO2 crystalline structures (rutile and hollandite) were synthesised, characterised and compared as OER catalysts. In accordance with the literature, the transformation of amorphous iridium oxo-hydroxide, containing Ir(III)/Ir(IV) sites, to crystalline rutile IrO2, made only of Ir(IV) sites, led to a decrease in catalytic activity and stability. However, the presence of Li2CO3 in the amorphous IrOx catalyst led to the formation of hollandite IrO2 instead of rutile IrO2, with lithium as the host cation within the hollandite channels. Apart from the difference in crystallinity, characterisation on hollandite IrO2 indicates that it was closer in nature to amorphous IrOx than to rutile IrO2. The presence of Ir(III) and Ir(IV) was confirmed by XPS, shorter Ir-Ir bond distances and longer Ir-O, compared to rutile IrO2, were observed by EXAFS, and comparable OER activity to IrO2-Li2CO3 was detected by LSV. Additionally, the conversion of amorphous IrO2-Li2CO3 to hollandite IrO2 led to improved stability under OER reaction conditions. In order to use iridium more efficiently and to reduce the iridium loading on the electrode, in chapter 6 IrO2 was diluted with a more abundant and economic metal oxide, nickel or copper oxide. Catalysts with a homogeneous metal distribution and with a core-shell distribution, concentrating iridium at the surface and the non-noble metal oxide at the core, were prepared following a modification of the hydrothermal synthesis method. The synthesis of mixed oxide catalysts with a homogeneous metal distribution led to a decrease in the catalytic activity and the stability of the catalyst, which was proven to be an ineffective synthetic route for considerably decreasing the iridium loading on the electrode. The observed decline in the catalytic performance was attributed to the dissolution of the non-noble metal oxide in contact with the reaction media. However, through a core-shell distribution, IrOx was concentrated on the surface of the catalyst, whilst the non-noble metal remained protected against dissolution inside the nanoparticle core. Following the core-shell synthetic approach, the iridium loading on the electrode was successfully halved without impairing the catalytic activity or stability, compared to pure IrO2-Li2CO3. The second part discussed in chapter 7, studied the optimisation of PdZn/TiO2 catalysts prepared by chemical vapour impregnation (CVI) for the CO2 hydrogenation (pre-reduction at 400 °C, 1 h, reaction at 250 °C, 20 bar, 30 ml·min-1, 60 % H2, 20 % CO2, 20 % N2) to methanol, as a stable alternative to copper catalysts. The first section of the chapter focused on the Pd to Zn molar ratio in the material, maintaining the palladium loading at 5 wt. %. Increasing the Pd:Zn molar ratio from (1:1) to (1:5) led to a greater formation of PdZn alloy, which improved CO2 conversion, but without considerably affecting methanol selectivity. The further addition of zinc, as observed for the catalyst with a Pd:Zn molar ratio of (1:10), led to a decrease in the CO2 conversion. This was presumably caused by zinc blocking active sites when in large excess. The atomic proportion of zinc in the PdZn alloy can vary from 40 at. % to 60 at. %. Hence it could be hypothesised that increasing the pre-reduction temperature could lead to a higher proportion of zinc within the alloy, which in turn can improve methanol selectivity. In general, increasing the pre-reduction temperature from 400 °C to 650 °C led to an increase in the methanol productivity because of improved methanol selectivity, although lower CO2 conversion was observed as a result of particle sintering. However, more interestingly, the CH4 selectivity decreased by one order of magnitude after increasing the pre-reduction treatment from 400 °C to 650 °C, simultaneously with the transformation of ZnO and TiO2 to rhombohedral ZnTiO3. This lead to the hypothesis that the PdZn-TiO2 interphase is responsible for methane production. To prove this hypothesis, PdZn/ZnTiO3 and Pd/ZnTiO3 catalysts were prepared by CVI, after pre-reduction at 400 °C. PdZn alloy formation was confirmed by XRD on both systems, indicating that Zn in the ZnTiO3 phase can migrate out of the lattice to form PdZn. Thus, the PdZn-TiO2 interface was generated in Pd/ZnTiO3 but not in PdZn/ZnTiO3. When tested for CO2 hydrogenation to methanol, the formation of methane on the former catalyst and its absence on the latter corroborated the formulated hypothesis that PdZn-TiO2 acts as the active site for CH4 formation

Correction to: Solvent free synthesis of PdZn/TiO2 catalysts for the hydrogenation of CO2 to methanol

The article Solvent Free Synthesis of PdZn/TiO2 Catalysts for the Hydrogenation of CO2 to Methanol by Hasliza Bahruji, Jonathan Ruiz Esquius, Michael Bowker, Graham Hutchings, Robert D. Armstrong, Wilm Jones was originally published Online First without open access. After publication in volume 61, issue 3–4, pages 144–153, the author decided to opt for Open Choice and to make the article an open access publication. Therefore, the copyright of the article has been changed to ©The Author(s) 2018 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made

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 reaction conditions, yielding the corresponding alkanes. Increasing the aluminium sites in the zeolites (Si:Al ratio 80:1, 50:1 and 23:1) lead to a higher concentration of mild Brønstead acid sites promoting hydrocarbon chain growth

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. </p

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

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

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