7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

A reflection on rational design of catalysts for non-thermal plasma (NTP)
catalysis

Xiaolei Fan1
1
Department of Chemical Engineering, School of Engineering, The University of Manchester
e-mail (speaker): Xiaolei.fan@manchester.ac.uk
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The hybrid NTP-catalytic systems are very promising for
promoting various challenging catalytic chemistries. The
unique activation in gas discharge via electron impact
dissociation and energisation can be efficient to promote
the activation of many stable and/or inert molecules suchas CO2 and N2, being able to drive the desiredtransformation of these molecules in energy efficientmanners. Importantly, being different from theconventional thermocatalysis, induction time of
NTP-catalysis is significantly shorter, making the hybrid
NTP-catalytic system be switched on and off readily.
Hence, NTP-catalysis is compatible with the intermittent
renewable energy sources such as wind and solar power,
being a promising tool to enable the distributed chemicalstorage of non-dispatchable renewable energy. However,to date catalyst design with carefully consideration of thefeatures of NTP activation was less achieved. In thispresentation, we revisited our previous research criticallyto comment on the rational and irrational aspects of thesestudies regarding catalyst design for selected
NTP-catalytic systems, that is, CO2 methanation, dryreforming of methane with CO2, and NH3 synthesis.
Based on the reflection,[1] the following thoughts wereextracted which can be beneficial to colleagues inrelevant fields to finally achieve the rational design ofbespoke catalysts for improving NTP-catalysis.
• NTP activation is different from conventional
thermocatalysis, especially gas phaseconversions. In NTP catalysis, presumably
reactant molecules are activated in gas
discharge, and the resulting reactive species canbe short-lived. Hence, diffusion of such
short-lived reactive species needs to beminimised to allow their interaction with acatalyst surface. Note that this aspect can bevery important in NTP-catalysis since in such asystem the catalyst surface is relatively cold (incomparison with a thermocatalytic system),
which is unlikely to facilitate the dissociation ofreactant molecules. Hence, the accessibility ofcatalytically active sites to plasma-induced
reactive species should be one of the priority
considerations in the catalysts design forNTP-catalysis.
• Regarding catalysis, it means surface reactions
regardless in thermal or NTP systems, and thus
catalyst design for NTP catalysis should bebased on the relevant thermocatalytic
counterparts (e.g., Ru catalyst for NH3 synthesis
and Ni catalyst for reforming reactions).

••••

However, under NTP conditions thetemperature of the catalyst surface can besignificantly lower than that in athermocatalytic counterpart, hence, catalyst
design to ensure the activity and selectivity of a‘cold catalyst surface’ may be beneficial torelevant NTP-catalytic systems.
A good understanding of the reaction
mechanisms and their changes in relation to thevariation of process conditions are important
aspects as well for guiding the catalyst design.
Therefore, relevant mechanistic and kinetic
studies corelating with the catalysts design areneeded to progress our understanding.
Catalyst design for NTP-catalysis cannot accept
the existing experience in thermocatalysis
without reservation. Common catalyst
deactivation mechanisms such as coke
formation and deposition, metal sintering,leaching and fouling, and poisoning of thecatalyst surface might be avoided to someextents under NTP conditions. Therefore,
relevant design strategies to address these issues
might not be appropriate to be adopted by NTP
catalysis. This aspect deserves special attention
to avoid relevant ‘pitfalls’ in future research, i.e.,
do not propose the solutions to address theproblems do not exist in NTP-catalysis.
Considering the aspects above, both the catalyst
architecture and active catalytic sites need to be
engineered carefully for designing bespoke
catalysts for NTP-catalysis. For example, highly
dispersed metallic sites within a porous support
may not be preferred by a NTP-catalytic systemsince (i) plasma discharge cannot be generated
within the micro-/meso-porous catalyst and (ii)plasma-induced energetic species can bequenched during diffusion. Therefore, themetallic sites within cannot be fully utilised.
Plasma discharge can activate reactant
molecules to enable the desired conversions, yet
it can be a double-edged sword to decompose
the target products as well. Hence, if possible,
regarding to the chemistry with this naturecatalyst design also needs to consider this aspect
to avoid the product decomposition andimprove the process efficiency.

References[1] X. FAN* et. al, Catal. Today 419, 114144 (2023)