How to add custom script to the particular Product Category page

in_category() should be used inside The Loop (unless if you pass a post ID as the second parameter like in_category( 1, 1 )) and only for the default/built-in category taxonomy.

For custom taxonomies like the product_cat in your case, you should use has_term(); however, if you’re checking if the current request/page is a taxonomy archive page, you should instead use is_tax():

function wpb_hook_faqtocategory_javascript() {
    if ( is_tax( 'product_cat', 9 ) ) { // 9 is the term ID, but you can use the slug/name
        ?>
        <script type="application/ld+json">

        </script>
        <?php
    }
}

Or in WooCommerce (you’re using WooCommerce, right?), you can use is_product_category(), but you’ll need to know the term slug:

function wpb_hook_faqtocategory_javascript() {
    if ( is_product_category( 'term-slug' ) ) {
        // ... your code here ...
    }
}