is used hereinafter to mean both supported and unsupported catalysts whose catalytically active main component comprises one or more platinum group metals in addition however the catalysts may contain one or more other elements or element compounds for the purpose of modifying the catalyst for example for influencing the selectivity and or activity known processes for regeneration can be used for regenerating both unsupported noble metal blacks and supported suspension catalysts the main difference lies in the fact that the process for regenerating supported catalysts necessarily includes a filtration step for separating the support industrially important hydrogenation reactions for example the hydrogenation stage in the anthraquinone process for the preparation of hydrogen peroxide can be carried out both with the use of unsupported substantially palladium containing catalysts hereinafter known as palladium blacks and with the use of supported catalysts containing substantially palladium as the active main component if suspension catalysts containing noble metals are used an enrichment of various other metals or metal compounds readily occurs during continuous operation this foreign matter is brought into the system with the raw materials in the form of very slight contamination and or stems from abrasion of the material of the plant in which the reaction takes place such an increase in non noble metals in platinum group metal suspension catalysts can be observed during the hydrogenation stage of the so called anthraquinone process for the preparation of hydrogen peroxide if a hydrogenation reactor made of a conventional refined steel is used the iron chromium and nickel content for example rises in the noble metal containing catalyst in order to be able to maintain constant operating conditions in the system it is therefore desirable to keep the content of non platinum metals in the system as a whole constant.