The word “catalyst” is derived from the Greek word katalyo, which means “to pick up,” or “to unite,” or to annul.”
The concept of the catalyst was first described by chemist Elizabeth Fulham in 1794. She explained the process of catalysis using experiments with oxidation-reduction reactions. However, the first catalyzed chemical reaction was analyzed by Gottlieb Kirchhoff in 1811. He converted starch into sugar by heating it with sulfuric acid.
In this overview article, we have explained different types of catalysts, their working principle, and what is their significance. Let’s start with a basic question.
What Is A Catalyst?
Definition: A catalyst is a substance that speeds up a chemical reaction by interacting with reactants and products. It is not consumed during the reaction. Thus, it can be recovered chemically unchanged at the end of the reaction.
Catalysts usually work by
- Lowering the activation energy of a chemical system (the energy required by a system to undergo a specific chemical reaction) by providing an alternative reaction pathway.
- Altering the reaction’s mechanism, which ultimately changes the nature (as well as energy) of the transition state.
Since a catalyzed reaction has lower activation energy than the corresponding uncatalyzed reaction, it results in a higher reaction rate at the same pressure and temperature and for the same reactant concentrations.
While catalysts react to form temporary intermediates, they do not change the extent or the chemical equilibrium of a reaction (because they affect both forward and reverse reaction rate).
The carbonic anhydrase (enzyme found in red blood cells, pancreatic cells, gastric mucosa, and renal tubules) catalyzes the reaction of carbon dioxide and water to produce carbonic acid. In the body, when carbon dioxide dissolves in water, it remains in chemical equilibrium with carbonic acid.
CO2 + H2O → H2CO3
The enzyme keeps the pH balanced in the body by maintaining the concentration of carbonic acid in the blood and tissues. For example, it creates acid in the stomach lining. The control of bicarbonate ions also affects the water content in the kidney and eyes.
Carbonic anhydrase is one of the fastest enzymes with catalytic rates ranging between 104 and 106 reactions per second. In contrast, the uncatalyzed reaction has a rate of about 0.2 reactions per second — that’s a huge difference.
Energy for the reaction between carbon and water to produce carbonic acid | Credit: Wikimedia
The energy graph shows that the catalyst (carbonic anhydrase) lowers the transition state’s energy for the reaction, which ultimately reduces the activation energy.
However, in both catalyzed and uncatalyzed reactions, the energies of the reactants and products remain the same. Thus, the total energy released during the reaction doesn’t change when the enzyme is added.
This tells us one more important thing: the reaction rate (or the kinetics of a reaction) does not necessarily depend on the thermodynamics of the reaction.
The catalytic activity is measured in moles per second, and its SI unit is katal. One katal means one mole of the catalyst will catalyze one mole of the reaction to create one or more products in one second.
4 Types Of Catalyst
Different types of catalysts are used based on the requirement of chemical reactions. The most commonly used ones are:
1. Positive Catalyst
Positive catalysts increase the rate of a chemical reaction by lowering the activation energy barriers. Since they enable a large number of reaction molecules to turn into products, the percentage of product yield increases significantly.
Vanadium(V) oxide (V2O5) used in the contact process (a method of producing sulfuric acid in the high concentrations) is a perfect example of a positive catalyst. Without this catalyst, the reaction is so slow that virtually no reaction happens in any sensible time.
2. Negative Catalyst
Negative catalysts decrease the rate of reaction by increasing the activation energy barriers, which ultimately results in fewer reactant molecules converting into products.
2H2O2 → 2H2O + O2
Acetanilide, for example, decreases the rate of decomposition of Hydrogen peroxide (H2O2) into water and oxygen. In this case, acetanilide acts as a negative catalyst by slowing down the decomposition of the drug-store hydrogen peroxide solution, inhibiting the reaction, which is catalyzed by heat, light, impurities.
A promoter is a substance the increases the effect of a catalyst. By itself, the promoter has little or no catalytic effect. They can be categorized into two groups based on their purpose.
- The one that facilitates the specific reaction by enhancing the catalyst’s activity.
- The one that suppresses unwanted processes by increasing the selectivity of the catalyst.
For example, in the Haber–Bosch process (which is the main industrial procedure for producing ammonia), traces of molybdenum or mixture of potassium and aluminum oxides act as promoters.
4. Catalyst Inhibitors Or Poisons
Inhibitors decrease the rate of reaction, but they do not introduce a reaction path with higher activation energy. Instead, they remove reaction intermediates such as free radicals, or they deactivate catalysts.
Most inhibitors work by selectively poisoning only specific types of active compounds or by modifying its surface geometry. For example, in catalytic converters for automobiles, the noble metal catalysts are poisoned by lead.
Types Of Catalysis Process and Their Working Principle
The process of speeding up a chemical reaction through a catalyst is called catalysis. This process can be divided into various categories based on the physical state and nature of substances employed in the chemical reaction.
Heterogeneous Catalysis: In this process, the phase of the catalysts differ from that of the reactants or products. Phase distinguishes between not only gas, liquid, and solid components, but also immiscible mixtures such as water and oil.
Typically, heterogeneous catalysis involves gas-phase reactants and solid-phase catalysts. It is a very important process since it enables faster, large-scale production as well as selective product formation.
Homogeneous Catalysis: It refers to reactions where the catalyst is in the same phase as the reactants. In general, homogeneous catalysts are dissolved in a solvent with the substrates.
While they have limited thermal stability, they are more selective than heterogeneous catalysts. Enzymes are good examples of homogeneous catalysts: they are essential for life and harnessed for various industrial processes.
Autocatalysis: In autocatalytic reactions, no specific catalyst is added. Instead, one of the product behaves as a catalyst and accelerates the reaction. This means the reaction accelerates itself or is autocatalyzed.
2MnO4 +16H+ + 5C2O42- → 2Mn2++ 8H2O + 10CO2
One of the simplest examples of such reaction is the oxidation of oxalic acid by potassium permanganate. It produces Mn2+, which catalyzes this very reaction. The rate of the reaction is initially slow, but it gradually increases due to the formation of Mn2+ ions.
In many cases, only small amounts of catalysts are required to make a big difference. The size of catalyst particles plays a crucial role in the way a reaction occurs.
Nanomaterial-based catalysts, for example, greatly enhance catalytic activity and can be easily separated and recycled. Nanowires have unique properties: since they are easier to produce can be precisely handled, they can be used for electrocatalytic purposes.
Silver nanocatalysts have been of great interest in organic synthesis. They have been extensively studied because of their unique reactivity and selectivity. Studies show that silver catalysts produce fewer toxic byproducts compared to traditional manufacturing techniques of plastic and other essential items.
It is well known that catalysts do not trigger a chemical reaction, and neither do they get consumed in a reaction. However, their exact working principle still remains a mystery.
Researchers are still trying to explore the complicated interplay of chemistry, physics, and math that could precisely explain how a catalyst functions. If successful, we can build a far superior catalyst by simulating how different materials might work. The simulation would require testing millions of potential configurations – a job that today’s supercomputers can handle efficiently.
There is a lot of research going on in the field of materials science organometallic chemistry to better understand the mechanisms involved in catalysis.
A large portion of industrial chemicals are produced via catalysis, and most biochemically significant processes are catalyzed as well. In fact, the use of catalysts is so ubiquitous that subareas aren’t readily classified. Below, we have listed some popular areas that involve the extensive utilization of catalysis.
Image credit: Needpix
Nature is the master of catalysis. Enzymes (protein molecules), also referred to as biocatalysts, have the tendency to catalyze many different biological/biochemical reactions. And unlike synthetic catalysts, they are totally green.
Enzymes are used in various food sectors such as dairy, baking, starch processing, brewery, and beverages to process and store food materials. They are often used to improve food properties like taste and digestibility.
Besides food processing, biocatalysts are employed in biosensor components that enable the detection of a wide range of compounds, including toxic and undesired molecules in food. Thus, they play a crucial role in monitoring food safety.
Bulk and Fine Chemicals
Catalytic oxidation is used to produce some largest-scaled chemicals, such as terephthalic acid (from p-xylene), acrylic (from propane or propylene), and nitric acid (from ammonia).
Acid-base catalysis is utilized for producing isocyanates, polyamides, and polyesters. Ziegler-Natta catalyst is often used to prepare bulk polymers from propylene and ethylene.
Catalysis is also effectively used for preparing several fine chemicals. In this case, both heavy industry and specialized processes, such as Heck reaction, are employed to produce complex, single, pure chemical substances in limited quantities.
Refinery Oil Industry | Image credit: Needpix
In the petroleum industry, catalysts are used to transform less valuable cruise oil products like tar and asphalt, into shorter chain products like gasoline, diesel, and kerosene. This process, called cracking, is carried out in large chemical process facilities.
More specifically, catalysis is extensively used in petroleum refining for alkylation, naphtha and steam reforming. It is even used to treat the exhaust from the burning of fossil fuels. Catalytic converters, containing rhodium and platinum, break down harmful byproducts of automobile exhaust.
Furthermore, the anodic and cathodic reactions in fuel cells rely on catalysts. Catalytic heaters use combustible fuel to produce flameless heat.
Image credit: Pixabay
In addition to improving the industrial processes, catalysts also play a direct role in the environment: they could be utilized for the conversion of waste and green raw materials into energy, and the production of clean fuels.
Catalysts could also help in increasing the adoption of renewable energy sources, including the conversion of plant biomass into carbon-neutral liquid fuels. The solar industry is studying different catalysts to use them as a means of energy storage. All in all, catalysts will continue to shape our energy consumption and usage.
At present, various environmental issues are addressed by catalytic processes, such as water and soil remediation, carbon dioxide reduction, decomposition of pollutants for air, and biomass valorization.
Nearly 35% of the global GDP is influenced by catalysts. It is estimated that approximately 90% of all commercial chemical products involve catalysts at some stage of their manufacturing process.
The Haber-Bosch process, for example, utilizes metal-based catalysis to synthesize ammonia. In 2019, more than 230 million metric tons of ammonia were produced worldwide.
According to the Grand View Research, that global catalyst market size will reach $34.1 billion by 2025, with a CAGR (compound annual growth rate) of 4.5% from 2019 to 2015.
The factors driving this market include rising demand for polymers, especially from electronics and automotive industries, and the increasing preference for catalysts due to their reduced environmental emissions and production costs.