Catalyst issues
I. Correct understanding of the concept of catalyst
In 2014, the International Union of Pure and Applied Chemistry (IUPAC) clarified that the term “catalyst” is no longer used for substances that slow down the rate of a chemical reaction. Substances that slow down the rate of a chemical reaction are called inhibitors.
IUPAC defines a catalyst as a substance that increases the rate of a reaction without changing the standard Gibbs free energy of the total reaction. Picture
Role of catalysts (the “four cans” and “four cannots” of catalysts)
1. The “four functions” of catalysts
① Catalysts can only accelerate spontaneous chemical reactions, i.e., chemical reactions when ΔG = ΔH-TΔS < 0.
A catalyst can change the course of a chemical reaction.
③ It can simultaneously reduce the activation energy of both positive and negative reactions by the same magnitude.
④ It can accelerate the rate of chemical reaction, shorten the time of chemical reaction, improve the efficiency of chemical reaction, and increase the yield of products. 2.
2. The “Four Inabilities” of Catalysts
① Catalysts cannot change ΔH.
① Catalysts cannot change ΔH. ② Catalysts cannot change the equilibrium rate of conversion of reactants.
Catalysts cannot change the chemical equilibrium constant.
Catalysts cannot change the direction of chemical equilibrium.
Properties of catalysts
1. Catalysts are selective
The same reactants can be obtained by choosing different catalysts to obtain different products, for example:
① 4NH3 (g) + 5O2 (g) = 4NO (g) + 6H2O (g)
② 4NH3 (g) + 4O2 (g) = 2N2O (g) + 6H2O (g)
③ 4NH3 (g) + 3O2 (g) = 2N2 (g) + 6H2O(g)
Therefore, a catalyst that favors ① should be chosen for nitric acid production.
For parallel reactions, the choice of a suitable catalyst can increase the yield of the target product. We call the reaction that produces the target product the main reaction and the other reactions the side reactions. The selectivity of catalyst is essentially a competition between the chemical reaction rate of the main reaction and that of the side reactions. The faster the rate of the main reaction, the more the main products will be produced, and the less the side products will be produced, so the best way to improve the rate of the chemical reaction and the yield of the main products is to choose the suitable catalyst.
2. Catalysts are monotonous.
Specificity means that the catalyst catalyzes only the main reaction and has no effect on the side reactions.
3. catalyst activity.
For a given catalytic reaction, the lower the activation energy of the chemical reaction drops, the better the catalyst activity is when different catalysts are selected.
Catalyst activity is affected by temperature, pH, and impurities.
There are three types of catalyst deactivation which are chemical deactivation, thermal deactivation and mechanical deactivation.
Chemical deactivation is also called catalyst poisoning, and the substances that make the catalyst poisoned are usually some sulfur, phosphorus, arsenic-containing compounds, as well as halogen compounds, heavy metal compounds and so on. Prevention and treatment is to remove the poison before the reaction, for example, industrial ammonia synthesis of H2S, O2, CO and other impurity gases will make the catalyst poisoning, must be used [Cu (NH3) 2] Ac gas wash.
Most catalysts have an effective use temperature range, beyond which activity is reduced or even completely lost. Mechanical deactivation refers to the destruction of solid catalyst particles as a result of resistance to friction, impact, and other processes. Coking carbon deposits are also a common type of mechanical deactivation, which results in the reduction and clogging of the active center of the catalyst due to carbon deposits covering the active center of the catalyst. Carbon deactivation can be regenerated by charcoal burning, i.e., the catalytic activity can be restored by oxidizing the carbon-containing deposits in the pores of the catalyst and removing them as CO and CO2.
IV. Catalyst Composition
Almost all catalytic reactions contacted in secondary school are a single substance or compound as catalyst, which makes beginners wrongly believe that catalysts are pure substances.
In fact, most of the catalysts in the industry are a mixture of substances.
Generally speaking, catalysts are composed of main catalyst, co-catalyst and carrier.
The main catalyst is the active component of the catalyst, and it is the fundamental substance that plays a catalytic role.
The co-catalyst is the component in the catalyst that has the ability to increase the activity and selectivity of the active component, and to improve the performance of the catalyst in terms of heat resistance, toxicity resistance and lifetime. Co-catalysts are usually categorized into structural co-catalysts and electronic co-catalysts.
2. Catalyst specificity.
Specificity means that the catalyst catalyzes only the main reaction and does not act on side reactions.
3. Catalyst activity.
For a given catalytic reaction, the lower the activation energy of the chemical reaction drops, the better the catalyst activity is when different catalysts are selected.
Catalyst activity is affected by temperature, pH, and impurities.
There are three types of catalyst deactivation which are chemical deactivation, thermal deactivation and mechanical deactivation.
Chemical deactivation is also called catalyst poisoning, and the substances that make the catalyst poisoned are usually some sulfur, phosphorus, arsenic-containing compounds, as well as halogen compounds, heavy metal compounds and so on. Prevention and treatment is to remove the poison before the reaction, for example, industrial ammonia synthesis of H2S, O2, CO and other impurity gases will make the catalyst poisoning, must be used [Cu (NH3) 2] Ac gas wash.
Most catalysts have an effective use temperature range, beyond which activity is reduced or even completely lost. Mechanical deactivation refers to the destruction of solid catalyst particles as a result of resistance to friction, impact, and other processes. Coking carbon deposits are also a common type of mechanical deactivation, which results in the reduction and clogging of the active center of the catalyst due to carbon deposits covering the active center of the catalyst. Carbon deactivation can be regenerated by charcoal burning, i.e., the catalytic activity can be restored by oxidizing the carbon-containing deposits in the pores of the catalyst and removing them as CO and CO2.
IV. Catalyst Composition
Almost all catalytic reactions contacted in secondary school are a single substance or compound as catalyst, which makes beginners wrongly believe that catalysts are pure substances.
In fact, most of the catalysts in the industry are a mixture of substances.
Generally speaking, catalysts are composed of main catalyst, co-catalyst and carrier.
The main catalyst is the active component of the catalyst, and it is the fundamental substance that plays a catalytic role.
The co-catalyst is the component in the catalyst that has the ability to increase the activity and selectivity of the active component, and to improve the performance of the catalyst in terms of heat resistance, toxicity resistance and lifetime. Co-catalysts are usually categorized into structural co-catalysts and electronic co-catalysts.
For example, the catalyst used in the synthesis of NH3 reduces iron powder, which is composed of a mixture of FeO containing 29%~35% and Fe3O4 containing 55%~65%, and active iron microcrystals are formed by the reduction of H2; the co-catalysts are composed of Al2O3 containing 2%~4%, MgO containing 3%~4%, and K2O containing 0.5%~0.8%; the Al2O3 and MgO are structural co-catalysts, which mainly play the role of skeleton, and improve the performance of heat resistance and toxicity resistance and life span. Al2O3 and MgO are structural co-catalysts, which mainly play the roles of skeleton and increase the surface of catalyst, prevent the growth of iron microcrystals and sintering.
K2O is an electron-type co-catalyst, which helps iron to transfer electrons to nitrogen and facilitates nitrogen adsorption and activation.
Carrier is the dispersant, adhesive or support of catalytic active components, and it is the skeleton of loaded active components. Common carriers include zeolite, silica gel, molecular sieve, Al2O3, activated carbon, diatomaceous earth and so on. The catalytic reaction takes place only on the surface of the catalyst, and increasing the surface area of the carrier can improve the catalytic efficiency, which is the reason why porous substances are therefore chosen as carriers.
Catalysts are spherical, columnar and honeycomb.
Six Advanced Placement Examination Points
I. Catalysts and the course of reactions
How do catalysts affect chemical reactions? One familiar example: Cu catalyzes the oxidation of ethanol.
2Cu + O2 = heating = 2CuO
CH3CH2OH + CuO – heating -> CH3CHO + Cu + H2O
The catalyst Cu is first involved in the reaction and then regenerated, it undergoes a course of Cu → CuO → Cu.
ClO2 was prepared from NaClO3, H2O2 and dilute sulphuric acid. the rate of formation of ClO2 was greatly increased by the addition of a small amount of hydrochloric acid at the beginning of the reaction (Cl- catalyzes the reaction). The process may be completed by a two-step reaction, complete it:
② H2O2 + Cl2 === 2Cl- + O2 + 2H+.
[Ans] Cl- is generated from Cl2 in step ②, and it is known that the catalyst Cl- changes from Cl- → Cl2 → Cl-, i.e., Cl- becomes Cl2 in step ①. From the information given in the question and the knowledge of redox, it can be seen that the oxidizing agent is ClO, and ClO2 is produced after the reaction of ClO. ionic equation: 2ClO + 2Cl- + 4H + === 2ClO2↑ + Cl2↑ + 2H2O.
[Answer] 2ClO + 2Cl- + 4H + === 2ClO2↑ + Cl2↑ + 2H2O
II. Catalyst and activation energy, enthalpy change
A catalyst accelerates the rate of a reaction by lowering the activation energy of the reaction. And the better the performance of the catalyst, the lower the activation energy of the reaction.
[Example 2] In the presence of Cu/ZnO catalyst, CO2 and H2 can react in two parallel reactions:
[Answer] 2ClO + 2Cl- + 4H + === 2ClO2↑ + Cl2↑ + 2H2O
Catalyst and activation energy, enthalpy change
A catalyst accelerates the rate of a reaction by lowering the activation energy of the reaction. And the better the performance of the catalyst, the lower the activation energy of the reaction.
[Example 2] In the presence of Cu/ZnO catalyst, CO2 and H2 can react in two parallel reactions:
CO2(g) + 3H2(g) ⇌ CH3OH(g) + H2O(g) I
CO2(g)＋H2(g)⇌CO(g)＋H2O(g) Ⅱ.
A laboratory control CO2 and H2 initial feeding ratio of 1:2.2, in the same pressure, after the same time measured experimental data are shown in the table below. Draw the “reaction process-energy” of reaction I without catalyst, with Cat.1 and with Cat.2 in the graph.
[Remarks] Methanol selectivity: Percentage of CO2 converted that produces methanol.
[Ans] According to the data in the table, we can calculate the percentage of CO2 that produces CH3OH out of the total CO2. 543 K, it is 12.3% × 42.3% ≈ 5.20% with Cat.1, and 10.9% × 72.7% = 7.92% with Cat.2. So Cat.2 has better catalytic selectivity than Cat.1, i.e., the activation energy of the reaction is low in the presence of Cat.2.
[Answer]
[Summarize] When graphing, also note that a catalyst cannot change the total energy of the reactants and products, so the starting and ending points of the curve should be the same. It is also for this reason that a catalyst cannot change the enthalpy change of a reaction.
Catalysts and reaction rates, equilibrium shifts
Catalysts can speed up the rate of reaction and shorten the time required to reach equilibrium. The better the activity of the catalyst, the shorter the time required. However, catalysts cannot shift chemical equilibrium.
[Example 3: A reaction occurs in a closed container: 2NO(g) + 2CO(g) ⇌ N2(g) + 2CO2(g) ΔH < 0. It is shown that increasing the specific surface area of a catalyst can increase the rate of the reaction when the same catalyst of equal mass is used. A student designs the three experiments shown in the table below. Draw the trend curves of c(NO) with time for the three experiments and label the experiments with experiment numbers.
[Ans] Compare experiments I and II first: the specific surface area of the catalyst in experiment II is large, so the reaction rate in experiment II is large; since the catalyst does not affect the equilibrium shift, after reaching equilibrium, the c(NO) in experiments I and II are the same. Then compare Ⅱ and Ⅲ: the temperature of experiment Ⅲ is high, so the reaction rate of experiment Ⅲ is large; because the temperature increases, the equilibrium shifts left, so c(NO) is larger at equilibrium.
[answer].
IV. Temperature and Catalyst Activity
Catalysts have a certain range of activation temperature, too high or too low, will lead to a decrease in the activity of the catalyst. Biocatalyst enzymes, for example, are very sensitive to temperature.
[Example 4] (2017 – gaokao tianjin volume excerpt) H2S and SO2 will bring great harm to the environment and human health, the industry adopts a variety of methods to reduce the emission of these harmful gases …… The principle of biological de-H2S is
H2S + Fe2(SO4)3===S↓ + 2FeSO4 + H2SO4
4FeSO4＋O2＋2H2SO4==2Fe2(SO4)3＋2H2O
The rate at which FeSO4 is oxidized in the presence of Thiobacillus is 5×105 times higher than in the absence of the bacterium ________. If the temperature of the reaction is too high, the rate of the reaction decreases for the following reasons
[Ans] Since Thiobacillus can significantly accelerate the rate of the reaction, it acts as a catalyst for the reaction. When the temperature of the reaction is too high, Thiobacillus loses its catalytic properties due to protein denaturation and the rate of the reaction decreases significantly.
[Answer] Catalyst Thiobacillus loses its catalytic properties due to protein denaturation.
V. Problems with Sources of Catalysts
There is a special class of reactions in chemistry called autocatalytic reactions, in which the products of the reaction have a catalytic effect on the reaction. One of its characteristics is that the rate of reaction is small to begin with, and increases rapidly as the catalytic product accumulates.
[Example 5] Add a certain amount of MnO2 and water to a three-necked flask, stir, and pass in a mixture of SO2 and N2 gas, and the reaction occurs at a constant temperature: MnO2 + H2SO3=== MnSO4 + H2O. If the N2 is replaced by air, the changes in c(Mn2+) and c(SO) with time t are measured as shown in the figure. The reason for the apparent difference in the change in c(Mn2+), c(SO) is that[Answer]
IV. Temperature and Catalyst Activity
Catalysts have a certain range of activation temperature, too high or too low, will lead to a decrease in the activity of the catalyst. Biocatalyst enzymes, for example, are very sensitive to temperature.
[Example 4] (2017 – gaokao tianjin volume excerpt) H2S and SO2 will bring great harm to the environment and human health, the industry adopts a variety of methods to reduce the emission of these harmful gases …… The principle of biological de-H2S is
H2S + Fe2(SO4)3===S↓ + 2FeSO4 + H2SO4
4FeSO4＋O2＋2H2SO4==2Fe2(SO4)3＋2H2O
The rate at which FeSO4 is oxidized in the presence of Thiobacillus is 5×105 times higher than in the absence of the bacterium ________. If the temperature of the reaction is too high, the rate of the reaction decreases for the following reasons
[Ans] Since Thiobacillus can significantly accelerate the rate of the reaction, it acts as a catalyst for the reaction. When the temperature of the reaction is too high, Thiobacillus loses its catalytic properties due to protein denaturation and the rate of the reaction decreases significantly.
[Answer] Catalyst Thiobacillus loses its catalytic properties due to protein denaturation.
V. Problems with Sources of Catalysts
There is a special class of reactions in chemistry called autocatalytic reactions, in which the products of the reaction have a catalytic effect on the reaction. One of its characteristics is that the rate of reaction is small to begin with, and increases rapidly as the catalytic product accumulates.
[Example 5] Add a certain amount of MnO2 and water to a three-necked flask, stir, and pass in a mixture of SO2 and N2 gas, and the reaction occurs at a constant temperature: MnO2 + H2SO3=== MnSO4 + H2O. If the N2 is replaced by air, the changes in c(Mn2+) and c(SO) with time t are measured as shown in the figure. The reason for the apparent difference in the change in c(Mn2+), c(SO) is that
[Ans] “N2 for air” has a side reaction: O2 + 2H2SO3 === 2H2SO4. The slope of the curve representing c(SO) increases gradually, indicating that the rate of production of SO increases gradually. Since the concentration of reactants, pressure, temperature, and contact area remain constant, the factor causing the rate of SO production to accelerate should be the catalyst, and the catalyst is some kind of product, which should be Mn2+.
[answer] Mn2+ catalyzes the reaction between O2 and H2SO3
[Summary] to determine whether a chemical reaction is autocatalytic reaction, the first to exclude the effect of temperature, concentration, pressure, contact area, and then analyze what kind of product has a catalytic effect.
VI. Combined effect of catalyst and temperature on equilibrium
[Example 6] NH3 catalytic reduction of nitrogen oxides technology is currently the most widely used flue gas denitrogenation technology: 4NH3 (g) ＋ 6NO (g) ⇌ 5N2 (g) ＋ 6H2O (g) ΔH < 0. Closed container, in the same time, in the role of catalyst A nitrogen removal rate with the temperature as shown in the figure. Now the experiment is carried out with Catalyst B, which has a slightly weaker catalytic ability. Draw the curve of the nitrogen removal rate with temperature under the action of Catalyst B (disregard the effect of temperature on the activity of the catalyst).
[Analysis] First, analyze why the nitrogen removal rate under Catalyst A increases and then decreases. Below 300 ℃, the higher the temperature, the faster the reaction rate, the more NO is consumed in the same time, so the rate of nitrogen removal increases. Above 300 °C, the rate of denitrification decreases because the temperature increases, the equilibrium shifts in the opposite direction, and less NO is consumed.
Changing to catalyst B involves two variables, catalyst and temperature, which should be considered separately in solving the problem. ① With catalyst B, the rate of nitrogen removal should also increase and then decrease as the temperature increases. Before reaching the highest point of the curve, the catalytic ability of B is weak, and the rate of nitrogen removal at the same temperature should be smaller than that of A. That is to say, the curve drawn should be below A. ③ The equilibrium is reached in the same time under the catalytic of B, then the highest point of the curve should appear at the position higher than 300 ℃. ④ When both A and B reach equilibrium, the equilibrium denitrogenation rate is only related to temperature, so the two curves coincide.
[Answer]
[Summarize and Integrate] Catalysts change the course of a reaction by participating in the chemical reaction, thereby reducing the activation energy, accelerating the reaction rate, and reducing the time required to reach equilibrium. However, catalysts cannot change the enthalpy change or shift equilibrium. The catalytic properties of a catalyst are also affected by external conditions, such as temperature and surface area.
