**1. Introduction**

Life on earth inherently depends on the element carbon creating the heart of a myriad of chemical compounds that, together with water and some inorganic compounds, build living matter. Over geologic periods, life has established a dynamic equilibrium of the flows of carbon through the different geo-habitats [1]. With the rise of mankind, this balance has been undermined through the exploitation of vast amounts of fossil resources for generating heat and materials. The carbon dioxide (CO2) emissions from combustion of fossil resources have resulted in rising atmospheric CO2 concentrations and an increasingly evident change in the climate worldwide. Replacing fossil resources that at present make up more than 90% of the energy demand and the feedstock for the chemical industry [2] is one of the most pressing challenges of mankind. All our primary energy demand of annually 12.5 TW a−1 could be covered by harnessing a fraction of the 8405 TW a−1 renewable energy available annually that comprises solar, wind, geothermal, tidal and wave energy [3]. Nevertheless, a sustainable energy supply will be needed for carbonbased compounds in order to close carbon recycle streams. Biomass is a globally

available resource that is considered a suitable alternative feedstock for producing basic chemical building blocks, so-called platform molecules [4], that could substitute the current fossil-based platform chemicals [5].

Biomass largely consists of complex molecules comprising mostly oxygen and other heteroatoms. Lignocellulose, the structural component of plants and the largest fraction of plant biomass, is essentially composed of cellulose, hemicellulose and lignin. Break down of the structure by depolymerisation of the corresponding molecular entities, followed by oxygen removal, yields fuels and platform chemicals for the value-chain of the chemical industry. Sustainable conversion depends on efficient conversion steps obtained ideally via catalytic processes. In this context, the catalytically highly active element ruthenium provides unique properties. Despite ruthenium being counted among the noble metals, it resembles a non-noble metal in many aspects. In metallic form, ruthenium atoms are highly polarisable. Unlike the higher homologue platinum, e.g., that has similar atomic radius, ruthenium has a much higher average electric dipole polarizability. Consequently, distinct catalytic functions can be realised with ruthenium catalysts.

To help readers understand why ruthenium catalysts are so frequently employed in biomass conversion, this chapter will first investigate the properties of ruthenium. Here, the catalytic properties of ruthenium are linked with its propensity to adsorb certain molecular entities. After exploring the interaction of adsorbed molecules with ruthenium surfaces, we will discuss the nature of selected adsorption states, the corresponding binding energies and structures of the adsorption complexes including ordering phenomena observed for molecules co-adsorbed on the ruthenium metal surface. This sets the scene for rational design of catalysts that are specific for the conversion of chemical entities in biomass. Last but not least, we will discuss selected examples for intriguing transformations of biomolecules.

To note here is that this chapter does not aim to comprehensively review the available data on catalysis with ruthenium. Nor does it attempt to summarise all data on the conversion of biomass with ruthenium catalysts. The extensive interest in this field is reflected presently by the more than 800 articles published each year on catalysis with ruthenium, more than 110 of which focus on biomass conversion. Instead, this chapter aims to summarise the catalytic principles governing hydrogenation and hydrogenolysis reactions with heterogeneous ruthenium catalysts with particular focus on applications in biomass conversion. Cited data and papers were selected to exemplify the field and illuminate the discussion.
