Emission control catalyst additives

Technical challenges impact the future of catalyst manufacture

Catalyst manufacturers use slurries containing particles to coat ceramic supports. These slurries are called washcoats and are complex mixtures of ceramics, metal powders and additives dispersed in water. The exact composition of washcoats can vary depending on the manufacturer and the end application, which means the physical and chemical properties of each washcoat formulation are unique.

In recent years, washcoats have become increasingly complex mixtures to achieve new performance and cost efficiency targets. Slurries now have a greater number of particle-particle interactions, making some washcoats harder to process.

By optimising the rheology of catalyst washcoats, our additives can help catalyst manufacturers overcome technical challenges to achieve the following:

How does rheology optimisation of washcoats help?

Rheology optimisation allows catalyst manufacturers to achieve optimal stability, processing, and flow characteristics. Total rheology control can be achieved through careful viscosity and particle suspension control, which is achieved by selecting the right dispersant and rheology additives.

The importance of viscosity control

Viscosity and particle dispersion are essential to catalyst manufacturers to:

Failing to control viscosity and particle size can result in inconsistencies from one batch to the next and can lead to low quality emission control as well as time consuming processing issues; filter clogging, overpressure in the reactor, irregular coating thicknesses are among the issues that catalyst manufacturers could encounter. Thus, controlling viscosity and particle dispersion is crucial.

How is viscosity and particle size controlled in catalyst washcoats today?

Particle size is controlled through precise stirring, milling and dispersion regimes, whilst pH is typically used to balance the electrostability (and therefore viscosity) of the system. Since each particle is charged, pH is controlled and changed via the addition of short organic acids and bases during the washcoat process, making sure that the pH of the dispersion is removed from its isoelectric point, minimising charge interactions within the slurry.

Figure 1: Demonstration that viscosity can vary with changing pH due to the changing electrostability of the system

Whilst viscosity control has been controlled using pH for many years in the catalyst industry, it relies on parameters difficult to control, often resulting in limited physical stability of the dispersion over time.

In addition, as washcoats become increasingly complicated mixtures, the electrostatic repulsions created through pH adjustments can no longer be relied on, as this effect is not enough to overcome particle interactions. Besides, viscosity is also strongly dependant on solids loading, which is often reduced to overcome this limitation, causing further undesirable effects.

For the future of catalyst development, dispersants are therefore necessary for catalyst processing in controlling viscosity and particle size.

Why should catalyst manufacturers also consider rheology modifier additives?

Total rheological control is only obtained by using stabilising rheology modifier additives. Whilst dispersants help with viscosity and particle size, their stabilising effect is limited, especially when other flow characteristics are dominant.

Rheology modifiers are important to impart stability and improve handling of solid slurries during washcoat and extrusion processes and can be used alongside dispersant additives to obtain optimised slurries that are stable and can flow, with even particle distribution.

They can reduce production time, and lower energy consumption during manufacturing.

How do dispersant and rheology modifiers improve catalyst washcoats?

Dispersants: Hypermer™ KD range

Hypermer KD dispersants enhance the compatibility of washcoat particles with water, improving wettability and stabilising the slurry through electro and steric means. The result is a slurry containing minimal agglomerates.

Figure 2: Aggregates are dispersed through the use of a dispersing agent

Without a dispersant, there is a lack of particle-water interactions and particle-particle interactions are energetically favoured. To overcome particle agglomeration, lower solid loading and high shear mixing can be performed, but this translates to increased processing and high water and power consumption. Multiple coating steps may also be required if the washcoat doesn’t coat the monolith support effectively with a lower solid content slurry.

Slurries containing our dispersants are more homogeneous, have a lower viscosity and therefore, can have a higher solids loading without compromising the washcoat quality and processability.

Figure 3: Hypermer dispersants reduce particle size and washcoat viscosity, helping achieve a higher solids loading

New legislation and regulatory requirements drive catalyst development

Emission performance standards for passenger cars and heavy-duty vehicles have been tightened year after year, which affects the entire supply chain of automotive vehicles.

Since 2014, the European Union has had in place the Euro 6 emission standard. After its initial introduction, Euro 6 has undergone incremental revisions, introducing new testing methods such as Real Driving Emission (RDE) testing; a mandatory requirement of new latest Euro 6d standard.

Globally, China will introduce China 6, which are similar to Euro 6 standards, whilst the US already has Euro 6 equivalent standards in place called Tier 3. These emission regulations are met through catalyst development on both a chemical and engineering level.

Despite similar regulations, small variances in how standards are tested has led to differences in the technologies employed to meet regulations in separate regions. The International Council on Clean Transportation (ICCT) highlighted this in 2016 and hypothesised a shift towards a combination of both Lead NOx trap with Selective Catalyst Reduction (SCR) technology as the EU standards were less stringent than the US standard at the time.

As governments continue to set their own emission standards, this will lead to regional variations in the development of new catalyst technologies. But there is no doubt that catalyst technologies will need to continually improve as regulations become more stringent.