In heterogeneous catalytic reactions involving solid catalyst and gaseous reactants, the catalysts, most generally used are 

Options

A. metals
B. metal oxides only
C. transition metals only
D. transition metals and transition metal oxides

Answer

To solve the question regarding the most commonly used catalysts in heterogeneous catalytic reactions involving solid catalysts and gaseous reactants, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Heterogeneous Catalysis**:
- Heterogeneous catalysis involves reactions where the catalyst is in a different phase (solid) than the reactants (gaseous). This setup allows for effective interaction between the solid catalyst and gaseous reactants.

2. **Identify Common Types of Catalysts**:
- The question asks about the types of catalysts that are most commonly used in these reactions. Common types include metals, metal oxides, and transition metals.

3. **Focus on Transition Metals**:
- Transition metals, such as palladium (Pd), platinum (Pt), and nickel (Ni), are known for their catalytic properties. They are often used in powdered form to maximize their surface area, which enhances their catalytic activity.

4. **Consider the Properties of Catalysts**:
- Transition metals have high surface areas and are effective in facilitating reactions due to their ability to form various oxidation states and coordinate with reactants.

5. **Conclude with the Most Commonly Used Catalysts**:
- Based on the properties and common usage, the most widely used catalysts in heterogeneous catalytic reactions involving solid catalysts and gaseous reactants are transition metals.

6. **Select the Correct Option**:
- Therefore, the correct answer to the question is "transition metals."

### Final Answer:
The catalysts most generally used in heterogeneous catalytic reactions involving solid catalysts and gaseous reactants are **transition metals**.

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