请翻译下列文字: A catalyst is a substance that facilitates a chemical reaction. For an outside observer, it seems that the catalyst makes the reaction go faster. As we shall see in Chapter 2, this is an oversimplification. In most cases, the catalyst in fact opens a different, faster reaction pathway. However, at this stage we will use the following general definition. A catalyst is something that makes a reaction go faster, without being consumed in the process. Because the catalyst is not consumed in the process, each catalyst molecule can participate in many consecutive cycles, so we need only a small amount of catalyst relative to the substrate. The substrate/catalyst ratio reflects the catalysts efficiency, which is measured as turnover number or turnover frequency (see below). There are many different types of catalysts. They range from the proton, H + , through Lewis acids, organometallic complexes, organic and inorganic polymers, all the way to enzymes. To simplify things, we divide catalysis into three categories: homogeneous catalysis, heterogeneous catalysis, and biocatalysis. Although the catalysts and the process conditions in each category can be very different, the principles of catalysis are the same. Once you understand these principles, you can apply them in all three categories. The catalyst turnover number (TON) and the turnover frequency (TOF) are two important quantities used for comparing catalyst efficiency. Their definitions, however, vary slightly among the three catalysis fields. In homogeneous catalysis, the TON is the number of cycles that a catalyst can run through before it deactivates, i.e., the number of A molecules that one molecule of catalyst can convert (or turn over) into B molecules. The TOF is simply TON/time, i.e., the number of A molecules that one molecule of catalyst can convert into B molecules in one second, minute, or hour. In heterogeneous catalysis, TON and TOF are often defined per active site, or per gram catalyst. This is because one does not know exactly how many “catalyst molecules” there are on the surface. In biocatalysis, the TON and TOF are defined by the rate measured when all the enzyme molecules are complexed with a reactant, divided by the total enzyme concentration.