**1. Introduction**

Volatile organic compounds (VOCs) are a wide group of organic compounds characterized to boiling points less than 250 ◦C at room temperature and at atmospheric pressure [1]. Due to their carcinogenic and toxic nature, most VOCs are considered major causes of air pollution. Indeed, their emission in the environment leads to the formation of secondary dangerous compounds, due to the occurrence of chemical reactions with other airborne pollutants such as NO<sup>x</sup> and SOx, which results in the formation of tropospheric ozone and photochemical smog [2,3]. Long exposure to these pollutants leads to serious problems for human health [4,5]. Global economic and industrial development over the years has caused an exponential increase of anthropogenic VOC emission [6]. VOC discharges include outdoor sources, such as transport, industrial and petrochemical processes, etc., and indoor sources, such as household products, solvents, office materials, cleaning products, domestic cooking, etc. [7]. The emitted VOCs encompass alkanes, paraffins, olefins, aromatics, alcohols, ketones, aldehydes, esters, sulfur/nitrogen-containing VOCs, and halogenated VOCs. Among them, the most common and toxic are benzene, phenol, toluene, styrene, formaldehyde, propylene, and acetone [8], whereas Cl-VOCs and in general halogenated VOCs, due to their inherent stability and toxicity, are also very dangerous [9].

Different technologies have been developed for VOC treatment, and they can be divided into nondestructive and destructive VOC removal. The former include adsorption, membrane separation, and condensation [10–12]. Among these, the adsorption process is considered one of the most efficient treatments, owing to a low energy consumption, relatively low operation cost, and simple

operations for the adsorption/regeneration of the adsorbent [12,13]. With adsorption, it is possible to remove, without the generation of dangerous byproducts, a low/medium concentration of VOCs (<1000 mg/m<sup>3</sup> ) [1]. Regarding destructive (i.e., oxidative) processes, the most commonly used ones are thermal (not-catalytic) combustion and catalytic oxidation, both of which can be applied to treat a medium/high concentration (>5000 mg/m<sup>3</sup> ) of VOCs [14,15]. In particular, catalytic conversion has some advantages compared to thermal incineration; indeed, this process has become more popular than noncatalytic treatments. Catalytic oxidation allows converting VOCs into less toxic substances, such as carbon dioxide and water, in a temperature range much lower than thermo-oxidation [16,17]. Specifically, with catalytic conversion, the operating temperature range is 200–500 ◦C or even lower, whereas in thermal incineration, temperatures are higher (800–1200 ◦C). Lower temperatures permit reducing the production of dioxins and NOx. Furthermore, catalytic oxidation is more versatile and cheaper, especially when it comes to processing low concentrations of organic compounds [18]. In recent years, new technologies have been applied for the elimination of VOCs at low concentrations, namely, the advanced oxidation process (AOP), e.g., photocatalytic degradation, ozone treatment, Fenton oxidation methods [19,20], biodegradation [21], and phytoremediation [22].

Due to the economic and technological advantages of catalytic oxidation, widespread efforts have been committed to the selection of high-performing catalysts for this process. However, considering the large number of organic molecules and the problematic nature of VOCs mixtures, the design and optimization of catalytic materials are challenging tasks. Both noble and transition metals have been widely used as catalysts for either nonhalogenated or halogenated VOCs [23–25]. Notwithstanding their high costs, the supported noble-metal catalysts are widely applied due to their intrinsic features, such as resistance to deactivation, ease of regeneration, and highly catalytic performance [26–28]. These features strictly depend on the synthetic procedure adopted for the preparation of the supported metal catalyst, as well as the type of metal salt precursor, the metal loading, the kind of support, and the particle size [29–31]. Furthermore, VOCs and air/oxygen content, total gas stream rate, and employed reactor (membrane reactor, fixed-bed reactor, etc.) are key parameters that can affect overall catalytic activity [32–34].

In the literature, there are many studies that deeply analyze single or various parameters that influence the final results of catalytic oxidation applied to VOC treatment, including the catalysts used [16,35,36], the nature of VOCs [2,9,37], the combination of different technologies [1,38], the type of reactor [39], or the performances in practical applications [40].

One of the less explored strategies to enhance the catalytic activity of supported noble/transition metal catalysts is the addition of a second metal (noble and/or transition) to the first one.

This work analyzes a little aspect of the VOC catalytic treatment topic: the advantages of using supported bimetallic catalysts with respect to monometallic counterparts, focusing on the morphological, chemicophysical, and textural properties of these peculiar materials, and how these features can influence the catalytic activity.

This review aims to enlarge the scientific panorama about VOC removal through catalytic oxidation, focusing on the bimetallic catalysts, an aspect not yet systematically examined in the literature.
