The aerobic, water-assisted photo-catalytic oxidation of CH4 over platinum-based catalysts in batch vs. continuous reactors -a new route to methanol?

Amonye, Chiamaka

The aerobic, water-assisted photo-catalytic oxidation of CH4 over platinum-based catalysts in batch vs. continuous reactors -a new route to methanol?

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Chemical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

The direct conversion of methane to value-added products such as methanol is often referred to as the ‘holy grail' reaction in catalysis, due to its potential to bypass the energy-intensive syngas route [1], [2]. Several technologies have been investigated for this direct conversion in recent years such as conventional catalytic processes, plasma technologies, membranes technologies, photocatalysis, etc. Yet these processes have not seen commercial implementation due to the thermodynamics of breaking the strong C-H bond (435 kJ/mol) and the tendency of partially oxidised intermediates to undergo further oxidation to COx species [1], [2]. Recent studies have shown that direct methane conversion is feasible over platinumbased catalysts under relatively mild conditions in a trickle bed reactor operating at 220 ⁰C and 30 bar in the presence of liquid water (0 – 0.3 ml/min), with methanol and methoxy methanol at a selectivity of ca. 10% at a methane conversion of 0.6% (and other C1 oxygenated products) [3]. This work highlighted that the direct conversion at relatively low temperatures is possible via weakly adsorbed oxygen species over transition metal surfaces and the site blocking action of water [3][4]. In this study, the direct photocatalytic and thermo-catalytic oxidation of methane was investigated over two platinum-based catalysts (0.5 wt% Pt/TiO2(rutile) and 0.5wt% Pt/WO3) in both batch and continuously operating slurry reactors. It is hypothesized that continuously operating slurry reactors result in higher apparent methanol/C1 oxygenate selectivity in the photocatalytic conversion of methane when compared to the same reaction in batch slurry reactors. Baseline photocatalytic experiments were conducted in a slurry batch reactor operating at 50 ⁰C, 1 atm with UV/visible light. Thereafter, a continuously operating photocatalytic slurry reactor (PSR) was then designed, constructed and commissioned. Experiments spanning 1.5 – 2 days were then conducted on the platinum-based catalysts at 22 bar and varying temperatures (25 – 180 ⁰C). The partial pressures of methane and oxygen were fixed at 0.74 bar and 1.07 bar respectively and UVA (405 nm), blue light (475 nm) and green light (575 nm) were employed as irradiation sources. In the batch reactor, negligible conversion was observed during the dark phase, while irradiation achieved methane consumption rates of 1.89 and 0.95 μmol.gcat-1.min-1 for 0.5 wt% Pt/TiO2(rutile) and 0.5wt% Pt/WO3, respectively. With respect to productivity, deep oxidation products (CO and CO2) dominated over both catalysts, particularly at early reaction times, and product accumulation within the closed reactor generally favoured overoxidation toward thermodynamically stable COx species. Organic products were formed only in minor quantities, with apparent quantum yields for methanol formation remaining low for both catalysts (10-7 – 10-8 %). Although irradiation with UVA and visible light yielded some photocatalytic activity, photon utilisation remained inefficient (~0.011% for Pt/TiO2 and ~0.004% for Pt/WO3), indicating significant underutilization of incident photons due to light attenuation, gas-liquid mass transfer, intrinsic kinetics in the oxidation and reduction reactions and charge carrier recombination [5], [6]. In contrast, the PSR demonstrated steady-state operation across all investigated operating windows but exhibited a fundamentally different limitation regime. Methane conversion remained essentially constant (~0.20-0.23%) across 100 – 180 ⁰C temperature range in the dark and the introduction of UVA, blue and green light irradiation at 180 ⁰C did not enhance conversion of methane consumption rates over either catalyst, despite the sufficient photon supply. The weak temperature dependence and insensitivity to irradiation indicated that under the investigated conditions, the PSR's behaviour was dominated by thermo-catalytic behaviour, rather than photo-catalytic enhancement. It also alludes to system-level constraints, where gas-liquid mass transfer, slurry hydrodynamics, and light attenuation may be governing performance, yielding the observed thermo-catalytic behaviour. This conclusion was further supported by catalyst loading experiments using 0.5 wt% Pt/TiO2(rutile), where an increase in the catalyst loading from 1 g/L to 10 g/L did not result in an increase in conversion, but a reduction in the specific methane consumption rate, this behaviour is consistent with transport and light screening limitations rather than intrinsic kinetic control [5]. Overall, while photocatalytic methane oxidation was demonstrated in both reactor configurations, the performance in the PSR was constrained by transport phenomena rather than photon availability or intrinsic catalyst activity. Future work should focus on reactor-level modifications in the PSR to address the transport limitations identified in this study. Operating within a narrower low-temperature window (<100 ⁰C) under controlled irradiation may provide a clearer separation of thermal and photocatalytic contributions and help identify true optimum operating conditions. In addition, systematic variation of space velocity is required to distinguish intrinsic surface kinetics from mass-transport limitations and assess the influence of time on stream on conversion. Furthermore, alternative photocatalyst supports with stronger visible-light absorption characteristics and reduced electron-hole recombination tendencies could also be beneficial to the improvement of photocatalytic performance under milder operating conditions. iv

Keywords

methane

conversion

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